Publications by Yu Liu
- 2023
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Gate-tunable superconductivity in hybrid InSb–Pb nanowires -
Abstract
- We present a report on hybrid InSb-Pb nanowires that combine high spin-orbit coupling with a high critical field and a large superconducting gap. Material characterization indicates the Pb layer of high crystal quality on the nanowire side facets. Hard induced superconducting gaps and gate-tunable supercurrent are observed in the hybrid nanowires. These results showcase the promising potential of this material combination for a diverse range of applications in hybrid quantum transport devices.
Yan Chen, David van Driel, Charalampos Lampadaris, Sabbir A Khan, Khalifah Alattallah, Lunjie Zeng, Eva Olsson, Tom Dvir, Peter Krogstrup, Yu Liu Journal reference: Appl. Phys. Lett. 123, 082601 (2023) [pdf] DOI: 10.1063/5.0155663
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Supercurrent reversal in ferromagnetic hybrid nanowire Josephson junctions -
Abstract
- We report supercurrent transport measurements in hybrid Josephson junctions comprised of semiconducting InAs nanowires with epitaxial ferromagnetic insulator EuS and superconducting Al coatings. The wires display a hysteretic superconducting window close to the coercivity, away from zero external magnetic field. Using a multi-interferometer setup, we measure the current-phase relation of multiple magnetic junctions and find an abrupt switch between $\pi$ and 0 phases within the superconducting window. We attribute the 0-$\pi$ transition to the discrete flipping of the EuS domains and provide a qualitative theory showing that a sizable exchange field can polarize the junction and lead to the supercurrent reversal. Both $0$ and $\pi$ phases can be realized at zero external field by demagnetizing the wire.
D. Razmadze, R. Seoane Souto, L. Galletti, A. Maiani, Y. Liu, P. Krogstrup, C. Schrade, A. Gyenis, C. M. Marcus, S. Vaitiekėnas [pdf] DOI: 10.1103/PhysRevB.107.L081301 2204.03202v3 [pdf]
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Gate-tunable superconductivity in hybrid InSb–Pb nanowires -
Abstract
- 2022
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Evidence for spin-polarized bound states in semiconductor–superconductor–ferromagnetic-insulator islands -
Abstract
- We report Coulomb blockade transport studies of semiconducting InAs nanowires grown with epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping facets. Comparing experiment to a theoretical model, we associate cotunneling features in even-odd bias spectra with spin-polarized Andreev levels. Results are consistent with zero-field spin splitting exceeding the induced superconducting gap. Energies of subgap states are tunable on either side of zero via electrostatic gates.
S. Vaitiekėnas, R. Seoane Souto, Y. Liu, P. Krogstrup, K. Flensberg, M. Leijnse, C. M. Marcus Journal reference: Phys. Rev. B 105, L041304 (2022) [pdf] DOI: 10.1103/PhysRevB.105.L041304
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Evidence for spin-polarized bound states in semiconductor–superconductor–ferromagnetic-insulator islands -
Abstract
- 2021
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Characteristic interaction potential of black hole molecules from the
microscopic interpretation of Ruppeiner geometry -
Abstract
- Ruppeiner geometry has been found to be a novel promising approach to uncover the microstructure of fluid systems and black holes. In this work, combining with the micro model of the Van der Waals fluid, we shall propose a first microscopic interpretation for the empirical observation of Ruppeiner geometry. Then employing the microscopic interpretation, we disclose the potential microstructure for the anti-de Sitter black hole systems. Of particular interest, we obtain the microscopic interaction potentials for the underlying black hole molecules. This significantly strengthens the study towards to the black hole nature from the viewpoint of the thermodynamics.
Shao-Wen Wei, Yu-Xiao Liu, Robert B. Mann [pdf]
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Bridging Unsupervised and Supervised Depth from Focus via All-in-Focus
Supervision -
Abstract
- Depth estimation is a long-lasting yet important task in computer vision. Most of the previous works try to estimate depth from input images and assume images are all-in-focus (AiF), which is less common in real-world applications. On the other hand, a few works take defocus blur into account and consider it as another cue for depth estimation. In this paper, we propose a method to estimate not only a depth map but an AiF image from a set of images with different focus positions (known as a focal stack). We design a shared architecture to exploit the relationship between depth and AiF estimation. As a result, the proposed method can be trained either supervisedly with ground truth depth, or \emph{unsupervisedly} with AiF images as supervisory signals. We show in various experiments that our method outperforms the state-of-the-art methods both quantitatively and qualitatively, and also has higher efficiency in inference time.
Ning-Hsu Wang, Ren Wang, Yu-Lun Liu, Yu-Hao Huang, Yu-Lin Chang, Chia-Ping Chen, Kevin Jou [pdf]
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Hybrid Neural Fusion for Full-frame Video Stabilization -
Abstract
- Existing video stabilization methods often generate visible distortion or require aggressive cropping of frame boundaries, resulting in smaller field of views. In this work, we present a frame synthesis algorithm to achieve full-frame video stabilization. We first estimate dense warp fields from neighboring frames and then synthesize the stabilized frame by fusing the warped contents. Our core technical novelty lies in the learning-based hybrid-space fusion that alleviates artifacts caused by optical flow inaccuracy and fast-moving objects. We validate the effectiveness of our method on the NUS, selfie, and DeepStab video datasets. Extensive experiment results demonstrate the merits of our approach over prior video stabilization methods.
Yu-Lun Liu, Wei-Sheng Lai, Ming-Hsuan Yang, Yung-Yu Chuang, Jia-Bin Huang [pdf]
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Gluing n-tilting and n-cotilting subcategories -
Abstract
- Recently, Wang, Wei and Zhang define the recollement of extriangulated categories, which is a generalization of both recollement of abelian categories and recollement of triangulated categories. For a recollement $(\mathcal A ,\mathcal B,\mathcal C)$ of extriangulated categories, we show that $n$-tilting (resp. $n$-cotilting) subcategories in $\mathcal A$ and $\mathcal C$ can be glued to get $n$-tilting (resp. $n$-cotilting) subcategories in $\mathcal B$ under certain conditions.
Yu Liu, Panyue Zhou [pdf]
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Silting reduction in extriangulated categories -
Abstract
- We introduce pre-silting and silting subcategories in extriangulated categories and generalize the silting theory in triangulated categories. We prove that the silting reduction $\mathcal B/({\rm thick}\mathcal W)$ of an extriangulated category $\mathcal B$ with respect to a pre-silting subcategory $\mathcal W$ can be realized as a certain subfactor category of $\mathcal B$. This generalizes the result by Iyama-Yang. In particular, for a Gorenstein algebra, we get the relative version of the description of the singularity category due to Happel and Chen-Zhang by this reduction.
Yu Liu, Panyue Zhou, Yu Zhou, Bin Zhu [pdf]
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The newly observed state $D_{s0}(2590)^{+}$ and width of $D^*(2007)^0$ -
Abstract
- We choose the Reduction Formula, PCAC and Low Energy Theory to reduce the $S$ matrix of a OZI allowed two-body strong decay involving a light pseudoscalar, the covariant transition amplitude formula with relativistic wave functions as input is derived. After confirm this method by the decay $D^*(2010)\to D\pi$, we study the state $D^*(2007)$, and the full width $\Gamma_{\rm{th}}(D^*(2007))=53.8\pm0.7$ keV is obtained. Supposing the newly observed $D_{s0}(2590)^{+}$ to be the state $D_s(2^1S_0)^+$, we find its decay width $\Gamma$ is highly sensitive to the $D_{s0}(2590)^{+}$ mass, which result in the meaningless comparison of widths by different models with various input masses. Instead of width, we introduce a model independent quantity $X$ and the ratio $\Gamma/{|{\vec P_f}|^3}$, which are almost mass independent, to give us useful information. The results show that, all the existing theoretical predictions $X_{D_s(2S) \to D^*K}=0.25\sim 0.41$ and $\Gamma/{|{\vec P_f}|^3}=0.81\sim1.77$ MeV$^{-2}$ are much smaller than experimental data $0.585^{+0.015}_{-0.035}$ and $4.54^{+0.25}_{-0.52}$ MeV$^{-2}$. Further compared with $X^{ex}_{D^*(2010) \to D\pi}=0.58$, the current data $X^{ex}_{D_s(2S) \to D^*K}=0.585^{+0.015}_{-0.035}$ is too big to be an reasonable value, so to confirm $D_{s0}(2590)^{+}$ as the state $D_s(2^1S_0)^+$, more experimental studies are needed.
Guo-Li Wang, Wei Li, Tai-Fu Feng, Yue-Long Wang, Yu-Bin Liu [pdf]
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Spatio-temporal Parking Behaviour Forecasting and Analysis Before and
During COVID-19 -
Abstract
- Parking demand forecasting and behaviour analysis have received increasing attention in recent years because of their critical role in mitigating traffic congestion and understanding travel behaviours. However, previous studies usually only consider temporal dependence but ignore the spatial correlations among parking lots for parking prediction. This is mainly due to the lack of direct physical connections or observable interactions between them. Thus, how to quantify the spatial correlation remains a significant challenge. To bridge the gap, in this study, we propose a spatial-aware parking prediction framework, which includes two steps, i.e. spatial connection graph construction and spatio-temporal forecasting. A case study in Ningbo, China is conducted using parking data of over one million records before and during COVID-19. The results show that the approach is superior on parking occupancy forecasting than baseline methods, especially for the cases with high temporal irregularity such as during COVID-19. Our work has revealed the impact of the pandemic on parking behaviour and also accentuated the importance of modelling spatial dependence in parking behaviour forecasting, which can benefit future studies on epidemiology and human travel behaviours.
Shuhui Gong, Xiaopeng Mo, Rui Cao, Yu Liu, Wei Tu, Ruibin Bai [pdf]
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Iterative Self-consistent Parallel Magnetic Resonance Imaging
Reconstruction based on Nonlocal Low-Rank Regularization -
Abstract
- Iterative self-consistent parallel imaging reconstruction (SPIRiT) is an effective self-calibrated reconstruction model for parallel magnetic resonance imaging (PMRI). The joint L1 norm of wavelet coefficients and joint total variation (TV) regularization terms are incorporated into the SPIRiT model to improve the reconstruction performance. The simultaneous two-directional low-rankness (STDLR) in k-space data is incorporated into SPIRiT to realize improved reconstruction. Recent methods have exploited the nonlocal self-similarity (NSS) of images by imposing nonlocal low-rankness of similar patches to achieve a superior performance. To fully utilize both the NSS in Magnetic resonance (MR) images and calibration consistency in the k-space domain, we propose a nonlocal low-rank (NLR)-SPIRiT model by incorporating NLR regularization into the SPIRiT model. We apply the weighted nuclear norm (WNN) as a surrogate of the rank and employ the Nash equilibrium (NE) formulation and alternating direction method of multipliers (ADMM) to efficiently solve the NLR-SPIRiT model. The experimental results demonstrate the superior performance of NLR-SPIRiT over the state-of-the-art methods via three objective metrics and visual comparison.
Ting Pan, Jizhong Duan, Junfeng Wang, Yu Liu [pdf]
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SnowflakeNet: Point Cloud Completion by Snowflake Point Deconvolution
with Skip-Transformer -
Abstract
- Point cloud completion aims to predict a complete shape in high accuracy from its partial observation. However, previous methods usually suffered from discrete nature of point cloud and unstructured prediction of points in local regions, which makes it hard to reveal fine local geometric details on the complete shape. To resolve this issue, we propose SnowflakeNet with Snowflake Point Deconvolution (SPD) to generate the complete point clouds. The SnowflakeNet models the generation of complete point clouds as the snowflake-like growth of points in 3D space, where the child points are progressively generated by splitting their parent points after each SPD. Our insight of revealing detailed geometry is to introduce skip-transformer in SPD to learn point splitting patterns which can fit local regions the best. Skip-transformer leverages attention mechanism to summarize the splitting patterns used in the previous SPD layer to produce the splitting in the current SPD layer. The locally compact and structured point cloud generated by SPD is able to precisely capture the structure characteristic of 3D shape in local patches, which enables the network to predict highly detailed geometries, such as smooth regions, sharp edges and corners. Our experimental results outperform the state-of-the-art point cloud completion methods under widely used benchmarks. Code will be available at https://github.com/AllenXiangX/SnowflakeNet.
Peng Xiang, Xin Wen, Yu-Shen Liu, Yan-Pei Cao, Pengfei Wan, Wen Zheng, Zhizhong Han [pdf]
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A generalized phenomenological model for the magnetic field penetration
and magnetization hysteresis loops of a type-II superconductor -
Abstract
- A generalized phenomenological model for the critical state of type-II superconductors with magnetic field parallel to the superconducting plate is proposed. This model considers the global magnetization including both the equilibrium magnetization from surface screening current and the non-equilibrium magnetization from bulk pinning in a self-consistent way. Our model can be used to simulate the magnetization-hysteresis-loops (MHLs) and flux penetrating process of different type-II superconductors, from low- to high-kappa values. Here we take an optimally doped Ba0.6K0.4Fe2As2 single crystal as a testing example. The model can fit the data quite well and several important parameters can be extracted from the fitting. Thus, the model can be extended to a general case for studying the magnetization and flux penetration in other type-II superconductors.
Wei Xie, Yu-Hao Liu, Hai-Hu Wen [pdf]
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Unified Regularity Measures for Sample-wise Learning and Generalization -
Abstract
- Fundamental machine learning theory shows that different samples contribute unequally both in learning and testing processes. Contemporary studies on DNN imply that such sample difference is rooted on the distribution of intrinsic pattern information, namely sample regularity. Motivated by the recent discovery on network memorization and generalization, we proposed a pair of sample regularity measures for both processes with a formulation-consistent representation. Specifically, cumulative binary training/generalizing loss (CBTL/CBGL), the cumulative number of correct classiffcations of the training/testing sample within training stage, is proposed to quantize the stability in memorization-generalization process; while forgetting/mal-generalizing events, i.e., the mis-classification of previously learned or generalized sample, are utilized to represent the uncertainty of sample regularity with respect to optimization dynamics. Experiments validated the effectiveness and robustness of the proposed approaches for mini-batch SGD optimization. Further applications on training/testing sample selection show the proposed measures sharing the unified computing procedure could benefit for both tasks.
Chi Zhang, Xiaoning Ma, Yu Liu, Le Wang, Yuanqi Su, Yuehu Liu [pdf]
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Unsupervised Learning of Fine Structure Generation for 3D Point Clouds
by 2D Projection Matching -
Abstract
- Learning to generate 3D point clouds without 3D supervision is an important but challenging problem. Current solutions leverage various differentiable renderers to project the generated 3D point clouds onto a 2D image plane, and train deep neural networks using the per-pixel difference with 2D ground truth images. However, these solutions are still struggling to fully recover fine structures of 3D shapes, such as thin tubes or planes. To resolve this issue, we propose an unsupervised approach for 3D point cloud generation with fine structures. Specifically, we cast 3D point cloud learning as a 2D projection matching problem. Rather than using entire 2D silhouette images as a regular pixel supervision, we introduce structure adaptive sampling to randomly sample 2D points within the silhouettes as an irregular point supervision, which alleviates the consistency issue of sampling from different view angles. Our method pushes the neural network to generate a 3D point cloud whose 2D projections match the irregular point supervision from different view angles. Our 2D projection matching approach enables the neural network to learn more accurate structure information than using the per-pixel difference, especially for fine and thin 3D structures. Our method can recover fine 3D structures from 2D silhouette images at different resolutions, and is robust to different sampling methods and point number in irregular point supervision. Our method outperforms others under widely used benchmarks. Our code, data and models are available at https://github.com/chenchao15/2D\_projection\_matching.
Chen Chao, Zhizhong Han, Yu-Shen Liu, Matthias Zwicker [pdf]
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Hierarchical View Predictor: Unsupervised 3D Global Feature Learning
through Hierarchical Prediction among Unordered Views -
Abstract
- Unsupervised learning of global features for 3D shape analysis is an important research challenge because it avoids manual effort for supervised information collection. In this paper, we propose a view-based deep learning model called Hierarchical View Predictor (HVP) to learn 3D shape features from unordered views in an unsupervised manner. To mine highly discriminative information from unordered views, HVP performs a novel hierarchical view prediction over a view pair, and aggregates the knowledge learned from the predictions in all view pairs into a global feature. In a view pair, we pose hierarchical view prediction as the task of hierarchically predicting a set of image patches in a current view from its complementary set of patches, and in addition, completing the current view and its opposite from any one of the two sets of patches. Hierarchical prediction, in patches to patches, patches to view and view to view, facilitates HVP to effectively learn the structure of 3D shapes from the correlation between patches in the same view and the correlation between a pair of complementary views. In addition, the employed implicit aggregation over all view pairs enables HVP to learn global features from unordered views. Our results show that HVP can outperform state-of-the-art methods under large-scale 3D shape benchmarks in shape classification and retrieval.
Zhizhong Han, Xiyang Wang, Yu-Shen Liu, Matthias Zwicker [pdf]
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The microstructure and Ruppeiner geometry of charged anti-de Sitter
black holes in Gauss-Bonnet gravity: from the critical point to the triple
point -
Abstract
- Ruppeiner geometry has been successfully applied in the study of the black hole microstructure by combining with the small-large black hole phase transition. In this paper, we will extend the study to the triple point, where three black hole phases coexist. For the six-dimensional charged Gauss-Bonnet anti-de Sitter black hole, we thoroughly investigate the swallow tail behaviors of the Gibbs free energy and the equal area laws. After obtaining the black hole triple point, we exhibit its phase structures both in pressure-temperature and temperature-horizon radius diagrams. Quite different from the liquid-vapor phase transition, a double peak behavior is present in the temperature-horizon radius phase diagram. Then we construct the Ruppeiner geometry and calculate the corresponding normalized curvature scalar. Near the triple point, we observe multiple negatively divergent behaviors. Positive curvature scalar is observed for the small black hole with high temperature, which indicates that the repulsive interaction dominates among the microstructure. Furthermore, we consider the variation of the curvature scalar along the coexisting intermediate and large black hole curves. Combining with the observation for different fluids, the result suggests that this black hole system behaves more like the argon or methane. Our study provides a first and preliminary step towards understanding black hole microstructure near the triple point, as well as uncovering the particular properties of the Gauss-Bonnet gravity.
Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Real‐time keypoints detection for autonomous recovery of the unmanned ground vehicle -
Abstract
- The combination of a small unmanned ground vehicle (UGV) and a large unmanned carrier vehicle allows more flexibility in real applications such as rescue in dangerous scenarios. The autonomous recovery system, which is used to guide the small UGV back to the carrier vehicle, is an essential component to achieve a seamless combination of the two vehicles. This paper proposes a novel autonomous recovery framework with a low-cost monocular vision system to provide accurate positioning and attitude estimation of the UGV during navigation. First, we introduce a light-weight convolutional neural network called UGV-KPNet to detect the keypoints of the small UGV from the images captured by a monocular camera. UGV-KPNet is computationally efficient with a small number of parameters and provides pixel-level accurate keypoints detection results in real-time. Then, six degrees of freedom pose is estimated using the detected keypoints to obtain positioning and attitude information of the UGV. Besides, we are the first to create a large-scale real-world keypoints dataset of the UGV. The experimental results demonstrate that the proposed system achieves state-of-the-art performance in terms of both accuracy and speed on UGV keypoint detection, and can further boost the 6-DoF pose estimation for the UGV.
Jie Li, Sheng Zhang, Kai Han, Xia Yuan, Chunxia Zhao, Yu Liu [pdf] DOI: 10.1049/iet-ipr.2020.0864 2107.12852v1 [pdf]
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Learning to See Through Obstructions with Layered Decomposition -
Abstract
- We present a learning-based approach for removing unwanted obstructions, such as window reflections, fence occlusions, or adherent raindrops, from a short sequence of images captured by a moving camera. Our method leverages motion differences between the background and obstructing elements to recover both layers. Specifically, we alternate between estimating dense optical flow fields of the two layers and reconstructing each layer from the flow-warped images via a deep convolutional neural network. This learning-based layer reconstruction module facilitates accommodating potential errors in the flow estimation and brittle assumptions, such as brightness consistency. We show that the proposed approach learned from synthetically generated data performs well to real images. Experimental results on numerous challenging scenarios of reflection and fence removal demonstrate the effectiveness of the proposed method.
Yu-Lun Liu, Wei-Sheng Lai, Ming-Hsuan Yang, Yung-Yu Chuang, Jia-Bin Huang [pdf]
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Pulsed characteristic-function measurement of a thermalizing harmonic oscillator -
Abstract
- We present a method for the direct measurement of the Wigner characteristic function of a thermalizing harmonic oscillator that is completely inaccessible for control or measurement. The strategy employs a recently proposed probe-measurement-based scheme [Phys. Rev. Lett. 122, 110406 (2019)] which relies on the pulsed control of a two-level probe. We generalize this scheme to the case of a nonunitary time evolution of the target harmonic oscillator, describing its thermalization through contact to a finite-temperature environment, given in the form of a Lindblad master equation. This generalization is achieved using a superoperator formalism and yields analytical expressions for the direct measurement of the characteristic function, accounting for the decoherence during the measurement process.
Ralf Betzholz, Yu Liu, Jianming Cai Journal reference: Phys. Rev. A 104, 012421 (2021) [pdf] DOI: 10.1103/PhysRevA.104.012421
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Layer Hall effect in a 2D topological axion antiferromagnet -
Abstract
- While ferromagnets have been known and exploited for millennia, antiferromagnets (AFMs) were only discovered in the 1930s. The elusive nature indicates AFMs' unique properties: At large scale, due to the absence of global magnetization, AFMs may appear to behave like any non-magnetic material; However, such a seemingly mundane macroscopic magnetic property is highly nontrivial at microscopic level, where opposite spin alignment within the AFM unit cell forms a rich internal structure. In topological AFMs, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. Here, we study this exciting possibility in an AFM Axion insulator, even-layered MnBi$_2$Te$_4$ flakes, where spatial degrees of freedom correspond to different layers. Remarkably, we report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under no net electric field, even-layered MnBi$_2$Te$_4$ shows no anomalous Hall effect (AHE); However, applying an electric field isolates the response from one layer and leads to the surprising emergence of a large layer-polarized AHE (~50%$\frac{e^2}{h}$). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time $\mathcal{PT}$-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E$\cdot$B, which drives the system between the opposite AFM states. Our results achieve previously unavailable pathways to detect and manipulate the rich internal spatial structure of fully-compensated topological AFMs. The layer-locked Berry curvature represents a first step towards spatial engineering of Berry phase, such as through layer-specific moir\'e potential.
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Biased Tracer Reconstruction with Halo Mass Information -
Abstract
- Plenty of crucial information about our Universe is encoded in the cosmic large-scale structure (LSS). However, the extractions of these information are usually hindered by the nonlinearities of the LSS, which can be largely alleviated by various techniques known as the reconstruction. In realistic applications, the efficiencies of these methods are always degraded by many limiting factors, a quite important one being the shot noise induced by the finite number density of biased matter tracers (i.e., luminous galaxies or dark matter halos) in observations. In this work, we explore the gains of biased tracer reconstruction achieved from halo mass information, which can suppress shot noise component and dramatically improves the cross-correlation between tracer field and dark matter. To this end, we first closely study the clustering biases and the stochasticity properties of halo fields with various number densities under different weighting schemes, i.e., the uniform, mass and optimal weightings. Then, we apply the biased tracer reconstruction method to these different weighted halo fields and investigate how linear bias and observational mass scatter affect the reconstruction performance. Our results demonstrate that halo masses are critical information for significantly improving the performance of biased tracer reconstruction, indicating a great application potential for substantially promoting the precision of cosmological measurements [especially for baryon acoustic oscillations (BAO)] in the ambitious on-going and future galaxy surveys.
Yu Liu, Yu Yu, Baojiu Li Journal reference: The Astrophysical Journal Supplement Series, Volume 254, Issue 1, id.4, 16 pp. (2021) [pdf] DOI: 10.3847/1538-4365/abe868
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$g+ig$ topological superconductivity in the 30$^o$-twisted bilayer
graphene -
Abstract
- Based on our revised perturbational-band theory, we study possible pairing states driven by interaction in the electron-doped quasicrystal 30\degree-twisted bilayer graphene. Our mean-field study on the related t-J model predicts that, the beneath-van-Hove and beyond-van-Hove low doping regimes are covered by the chiral $d+id$ and $g+ig$ topological superconductivities (TSCs) respectively. The $g+ig$-TSC possesses a pairing angular momentum 4, and hence following each effective $C_{12}$- rotation by $\Delta\phi=n\pi/6$, the pairing phase changes $4\Delta\phi$. This intriguing TSC is novel, as it belongs to a special 2D $E_4$- irreducible representation of the effective $D_{12}$ point group unique to this quasicystal and absent on periodic lattices. The Ginzburg-Landau theory suggested that the $g+ig$- TSC originates from the Josephson coupling between the $d+id$ pairings on the two mono-layers.
Yu-Bo Liu, Yongyou Zhang, Wei-Qiang Chen, Fan Yang [pdf]
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Motion of test particle in rotating boson star -
Abstract
- Motion of a test particle plays an important role in understanding the properties of a spacetime. As a new type of the strong gravity system, boson stars could mimic black holes located at the center of galaxies. Studying the motion of a test particle in the spacetime of a rotating boson star will provide the astrophysical observable effects if a boson star is located at the center of a galaxy. In this paper, we investigate the timelike geodesic of a test particle in the background of a rotating boson star with angular number $m=(1, 2, 3)$. With the change of angular number and frequency, a rotating boson star will transform from the low rotating state to the highly relativistic rapidly rotating state, the corresponding Lense-Thirring effects will be more and more significant and it should be studied in detail. By solving the four-velocity of a test particle and integrating the geodesics, we investigate the bound orbits with a zero and nonzero angular momentum. We find that a test particle can stay more longer time in the central region of a boson star when the boson star becomes from low rotating state to highly relativistic rotating state. Such behaviors of the orbits are quite different from the orbits in a Kerr black hole, and the observable effects from these orbits will provide a rule to investigate the astrophysical compact objects in the Galactic center.
Yu-Peng Zhang, Yan-Bo Zeng, Yong-Qiang Wang, Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Ergodic numerical approximation to periodic measures of stochastic differential equations -
Abstract
- In this paper, we consider numerical approximation to periodic measure of a time periodic stochastic differential equations (SDEs) under weakly dissipative condition. For this we first study the existence of the periodic measure $\rho_t$ and the large time behaviour of $\mathcal{U}(t+s,s,x) := \mathbb{E}\phi(X_{t}^{s,x})-\int\phi d\rho_t,$ where $X_t^{s,x}$ is the solution of the SDEs and $\phi$ is a test function being smooth and of polynomial growth at infinity. We prove $\mathcal{U}$ and all its spatial derivatives decay to 0 with exponential rate on time $t$ in the sense of average on initial time $s$. We also prove the existence and the geometric ergodicity of the periodic measure of the discretized semi-flow from the Euler-Maruyama scheme and moment estimate of any order when the time step is sufficiently small (uniform for all orders). We thereafter obtain that the weak error for the numerical scheme of infinite horizon is of the order $1$ in terms of the time step. We prove that the choice of step size can be uniform for all test functions $\phi$. Subsequently we are able to estimate the average periodic measure with ergodic numerical schemes.
Chunrong Feng, Yu Liu, Huaizhong Zhao Journal reference: Journal of Computational and Applied Mathematics, 398 (2021) 113701 [pdf] DOI: 10.1016/j.cam.2021.113701
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Heavy and light jet quenching in different collision systems at the LHC
energies -
Abstract
- Recent experiments have observed large anisotropic collective flows in high multiplicity proton-lead collisions at the Large Hadron Collider (LHC), which indicates the possible formation of mini quark-gluon plasma (QGP) in small collision systems. However, no jet quenching has been confirmed in such small systems so far. To understand this intriguing result, the system size scan experiments have been proposed to bridge the gap between large and small systems. In this work, we perform a systematic study on both heavy and light flavor jet quenching in different collision systems at the LHC energies. Using our state-of-the-art jet quenching model, which combines the next-to-leading-order perturbative QCD framework, a linear Boltzmann transport model and the (3+1)-dimensional viscous hydrodynamics simulation, we provide a good description of nuclear modification factor $R_{\rm AA}$ for charged hadrons and $D$ mesons in central and mid-central Pb+Pb and Xe+Xe collisions measured by CMS collaboration. We further predict the transverse momentum and centrality dependences of $R_{AA}$ for charged hadrons, $D$ and $B$ mesons in Pb+Pb, Xe+Xe, Ar+Ar and O+O collisions at the LHC energies. Our numerical results show a clear system size dependence for both light and heavy flavor hadron $R_{AA}$ across different collision systems. Sizable jet quenching effect is obtained for both heavy and light flavor hadrons in central O+O collisions at the LHC energies. Our study provides a significant bridge for jet quenching from large to small systems, and should be helpful for finding the smallest QGP droplet and the disappearance of QGP in relativistic nuclear collisions.
Yu-Fei Liu, Wen-Jing Xing, Xiang-Yu Wu, Guang-You Qin, Shanshan Cao, Hongxi Xing [pdf]
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Smooth braneworld in 6-dimensional asymptotically AdS spacetime -
Abstract
- In this paper, we investigate a $6$-dimensional smooth thick braneworld model which contains a compact extra dimension and an infinite large one. The braneworld is generated by a real scalar field with a $\phi^6$ potential and the bulk is asymptotically $\text{AdS}_6$ spacetime. The geometry achieves the localization of the free $U(1)$ gauge field, which is a problem in the $5$-dimensional Randall-Sundrum-like models. In addition, we analyze the stability of the braneworld system and the localization of gravitons.
Jun-Jie Wan, Zheng-Quan Cui, Wen-Bin Feng, Yu-Xiao Liu Journal reference: JHEP 05 (2021) 017 [pdf] DOI: 10.1007/JHEP05(2021)017
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Curvature Graph Neural Network -
Abstract
- Graph neural networks (GNNs) have achieved great success in many graph-based tasks. Much work is dedicated to empowering GNNs with the adaptive locality ability, which enables measuring the importance of neighboring nodes to the target node by a node-specific mechanism. However, the current node-specific mechanisms are deficient in distinguishing the importance of nodes in the topology structure. We believe that the structural importance of neighboring nodes is closely related to their importance in aggregation. In this paper, we introduce discrete graph curvature (the Ricci curvature) to quantify the strength of structural connection of pairwise nodes. And we propose Curvature Graph Neural Network (CGNN), which effectively improves the adaptive locality ability of GNNs by leveraging the structural property of graph curvature. To improve the adaptability of curvature to various datasets, we explicitly transform curvature into the weights of neighboring nodes by the necessary Negative Curvature Processing Module and Curvature Normalization Module. Then, we conduct numerous experiments on various synthetic datasets and real-world datasets. The experimental results on synthetic datasets show that CGNN effectively exploits the topology structure information, and the performance is improved significantly. CGNN outperforms the baselines on 5 dense node classification benchmark datasets. This study deepens the understanding of how to utilize advanced topology information and assign the importance of neighboring nodes from the perspective of graph curvature and encourages us to bridge the gap between graph theory and neural networks.
Haifeng Li, Jun Cao, Jiawei Zhu, Yu Liu, Qing Zhu, Guohua Wu [pdf]
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Bimolecular chemistry in the ultracold regime -
Abstract
- Advances in atomic, molecular, and optical (AMO) physics techniques allowed the cooling of simple molecules down to the ultracold regime ($\lesssim$ 1 mK), and opened the opportunities to study chemical reactions with unprecedented levels of control. This review covers recent developments in studying bimolecular chemistry at ultralow temperatures. We begin with a brief overview of methods for producing, manipulating, and detecting ultracold molecules. We then survey experimental works that exploit the controllability of ultracold molecules to probe and modify their long-range interactions. Further combining the use of physical chemistry techniques, such as mass spectrometry and ion imaging, significantly improved the detection of ultracold reactions and enabled explorations of their dynamics in the short-range. We discuss a series of studies on the reaction KRb + KRb $\rightarrow$ K$_2$ + Rb$_2$ initiated below 1 $\mu$K, including the direct observation of a long-lived complex, the demonstration of product rotational state control via conserved nuclear spins, and a test of the statistical model using the complete quantum state distribution of the products.
Yu Liu, Kang-Kuen Ni [pdf] DOI: 10.1146/annurev-physchem-090419-043244 2107.01088v1 [pdf]
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Application of radial basis functions neutral networks in spectral
functions -
Abstract
- The reconstruction of spectral function from correlation function in Euclidean space is a challenging task. In this paper, we employ the Machine Learning techniques in terms of the radial basis functions networks to reconstruct the spectral function from a finite number of correlation data. To test our method, we first generate one type of correlation data using a mock spectral function by mixing several Breit-Wigner propagators. We found that compared with other traditional methods, TSVD, Tikhonov, and MEM, our approach gives a continuous and unified reconstruction for both positive definite and negative spectral function, which is especially useful for studying the QCD phase transition. Moreover, our approach has considerably better performance in the low frequency region. This has advantages for the extraction of transport coefficients which are related to the zero frequency limit of the spectral function. With the mock data generated through a model spectral function of stress energy tensor, we find our method gives a precise and stable extraction of the transport coefficients.
Meng Zhou, Fei Gao, Jingyi Chao, Yu-Xin Liu, Huichao Song [pdf]
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Cycle4Completion: Unpaired Point Cloud Completion using Cycle
Transformation with Missing Region Coding -
Abstract
- In this paper, we present a novel unpaired point cloud completion network, named Cycle4Completion, to infer the complete geometries from a partial 3D object. Previous unpaired completion methods merely focus on the learning of geometric correspondence from incomplete shapes to complete shapes, and ignore the learning in the reverse direction, which makes them suffer from low completion accuracy due to the limited 3D shape understanding ability. To address this problem, we propose two simultaneous cycle transformations between the latent spaces of complete shapes and incomplete ones. The insight of cycle transformation is to promote networks to understand 3D shapes by learning to generate complete or incomplete shapes from their complementary ones. Specifically, the first cycle transforms shapes from incomplete domain to complete domain, and then projects them back to the incomplete domain. This process learns the geometric characteristic of complete shapes, and maintains the shape consistency between the complete prediction and the incomplete input. Similarly, the inverse cycle transformation starts from complete domain to incomplete domain, and goes back to complete domain to learn the characteristic of incomplete shapes. We provide a comprehensive evaluation in experiments, which shows that our model with the learned bidirectional geometry correspondence outperforms state-of-the-art unpaired completion methods.
Xin Wen, Zhizhong Han, Yan-Pei Cao, Pengfei Wan, Wen Zheng, Yu-Shen Liu [pdf]
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PMP-Net: Point Cloud Completion by Learning Multi-step Point Moving
Paths -
Abstract
- The task of point cloud completion aims to predict the missing part for an incomplete 3D shape. A widely used strategy is to generate a complete point cloud from the incomplete one. However, the unordered nature of point clouds will degrade the generation of high-quality 3D shapes, as the detailed topology and structure of discrete points are hard to be captured by the generative process only using a latent code. In this paper, we address the above problem by reconsidering the completion task from a new perspective, where we formulate the prediction as a point cloud deformation process. Specifically, we design a novel neural network, named PMP-Net, to mimic the behavior of an earth mover. It moves each point of the incomplete input to complete the point cloud, where the total distance of point moving paths (PMP) should be shortest. Therefore, PMP-Net predicts a unique point moving path for each point according to the constraint of total point moving distances. As a result, the network learns a strict and unique correspondence on point-level, which can capture the detailed topology and structure relationships between the incomplete shape and the complete target, and thus improves the quality of the predicted complete shape. We conduct comprehensive experiments on Completion3D and PCN datasets, which demonstrate our advantages over the state-of-the-art point cloud completion methods.
Xin Wen, Peng Xiang, Zhizhong Han, Yan-Pei Cao, Pengfei Wan, Wen Zheng, Yu-Shen Liu [pdf]
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A general thermodynamic geometry approach for rotating Kerr anti-de
Sitter black holes -
Abstract
- Combining with the small-large black hole phase transition, the thermodynamic geometry has been well applied to study the microstructure for the charged AdS black hole. In this paper, we extend the geometric approach to the rotating Kerr-AdS black hole and aim to develop a general approach for the Kerr-AdS black hole. Treating the entropy and pressure as the fluctuation coordinates, we construct the Ruppeiner geometry for the Kerr-AdS black hole by making the use of the Christodoulou-Ruffini-like squared-mass formula, which is quite different from the charged case. Employing the empirical observation of the corresponding scalar curvature, we find that, for the near-extremal Kerr-AdS black hole, the repulsive interaction dominates among its microstructure. While for far-from-extremal Kerr-AdS black hole, the attractive interaction dominates. The critical phenomenon is also observed for the scalar curvature. These results uncover the characteristic microstructure of the Kerr-AdS black hole. Such general thermodynamic geometry approach is worth generalizing to other rotating AdS black holes, and more interesting microstructure is expected to be discovered.
Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Quantum phase transition in a non-Hermitian XY spin chain with global complex transverse field -
Abstract
- In this work, we investigate the quantum phase transition in a non-Hermitian XY spin chain. The phase diagram shows that the critical points of Ising phase transition expand into a critical transition zone after introducing a non-Hermitian effect. By analyzing the non-Hermitian gap and long-range correlation function, one can distinguish different phases by means of different gap features and decay properties of correlation function, a tricky problem in traditional XY model. Furthermore, the results reveal the relationship among different regions of the phase diagram, non-Hermitian energy gap and long-range correlation function.
Yu-Guo Liu, Lu Xu, Zhi Li Journal reference: J. Phys.: Condens. Matter 33 295401 (2021) [pdf] DOI: 10.1088/1361-648X/ac00dd
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Resolving Galactic binaries in LISA data using particle swarm optimization and cross-validation -
Abstract
- The space-based gravitational wave (GW) detector LISA is expected to observe signals from a large population of compact object binaries, comprised predominantly of white dwarfs, in the Milky Way. Resolving individual sources from this population against its self-generated confusion noise poses a major data analysis problem. We present an iterative source estimation and subtraction method to address this problem based on the use of particle swarm optimization (PSO). In addition to PSO, a novel feature of the method is the cross-validation of sources estimated from the same data using different signal parameter search ranges. This is found to greatly reduce contamination by spurious sources and may prove to be a useful addition to any multi-source resolution method. Applied to a recent mock data challenge, the method is able to find $O(10^4)$ Galactic binaries across a signal frequency range of $[0.1,15]$ mHz, and, for frequency $\gtrsim 4$ mHz, reduces the residual data after subtracting out estimated signals to the instrumental noise level.
Xue-Hao Zhang, Soumya D. Mohanty, Xiao-Bo Zou, Yu-Xiao Liu Journal reference: Phys. Rev. D 104, 024023 (2021) [pdf] DOI: 10.1103/PhysRevD.104.024023
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Detection of Long-Lived Complexes in Ultracold Atom-Molecule Collisions -
Abstract
- We investigate collisional loss in an ultracold mixture of $^{40}$K$^{87}$Rb molecules and $^{87}$Rb atoms, where chemical reactions between the two species are energetically forbidden. Through direct detection of the KRb$_{2}^{*}$ intermediate complexes formed from atom-molecule collisions, we show that a $1064$ nm laser source used for optical trapping of the sample can efficiently deplete the complex population via photo-excitation, an effect which can explain the universal two-body loss observed in the mixture. By monitoring the time-evolution of the KRb$_{2}^{*}$ population after a sudden reduction in the $1064$ nm laser intensity, we measure the lifetime of the complex ($0.39(6)$ ms), as well as the photo-excitation rate for $1064$ nm light ($0.50(3)$ $\mu$s$^{-1}($kW/cm$^{2})^{-1}$). The observed lifetime is ${\sim}10^{5}$ times longer than recent estimates based on the Rice-Ramsperger-Kassel-Marcus statistical theory, which calls for new insight to explain such a dramatic discrepancy.
Matthew A. Nichols, Yi-Xiang Liu, Lingbang Zhu, Ming-Guang Hu, Yu Liu, Kang-Kuen Ni [pdf]
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New insight on the quark condensate beyond chiral limit -
Abstract
- With analyzing the mass function obtained by solving Dyson-Schwinger Equations, we propose a cut-off independent definition of quark condensate beyond chiral limit. With this well-defined condensate, we then analyze the evolution of the condensate and its susceptibility with the current quark mass. The susceptibility shows a critical mass in the neighborhood of the s-quark current mass, which defines a transition boundary for internal hadron dynamics.
Ling-feng Chen, Zhan Bai, Fei Gao, Yu-xin Liu [pdf]
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FNAS: Uncertainty-Aware Fast Neural Architecture Search -
Abstract
- Reinforcement learning (RL)-based neural architecture search (NAS) generally guarantees better convergence yet suffers from the requirement of huge computational resources compared with gradient-based approaches, due to the rollout bottleneck -- exhaustive training for each sampled generation on proxy tasks. In this paper, we propose a general pipeline to accelerate the convergence of the rollout process as well as the RL process in NAS. It is motivated by the interesting observation that both the architecture and the parameter knowledge can be transferred between different experiments and even different tasks. We first introduce an uncertainty-aware critic (value function) in Proximal Policy Optimization (PPO) to utilize the architecture knowledge in previous experiments, which stabilizes the training process and reduces the searching time by 4 times. Further, an architecture knowledge pool together with a block similarity function is proposed to utilize parameter knowledge and reduces the searching time by 2 times. It is the first to introduce block-level weight sharing in RLbased NAS. The block similarity function guarantees a 100% hitting ratio with strict fairness. Besides, we show that a simply designed off-policy correction factor used in "replay buffer" in RL optimization can further reduce half of the searching time. Experiments on the Mobile Neural Architecture Search (MNAS) search space show the proposed Fast Neural Architecture Search (FNAS) accelerates standard RL-based NAS process by ~10x (e.g. ~256 2x2 TPUv2 x days / 20,000 GPU x hour -> 2,000 GPU x hour for MNAS), and guarantees better performance on various vision tasks.
Jihao Liu, Ming Zhang, Yangting Sun, Boxiao Liu, Guanglu Song, Yu Liu, Hongsheng Li [pdf]
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Thick branes with inner structure in mimetic $f(R)$ gravity -
Abstract
- In this paper, we study the structure and gravitational resonances of thick branes generated by a mimetic scalar field in $f(R)$ gravity. We obtain several typical thick brane solutions for $f(R)=R+\alpha R^2$. To study their stability, we analyze the tensor perturbation of the metric. It is shown that any thick brane model with $df/dR>0$ is stable and the graviton zero mode can be localized on the brane for each solution, which indicates that the four-dimensional Newtonian gravity can be restored. The effect of the parameter $\alpha$ on the gravitational resonances is studied. As a brane splits into multi sub-branes, the effective potential of the tensor perturbation will have an abundant inner structure with multi-wells, and this will lead to new phenomena of the gravitational resonances.
Jing Chen, Wen-Di Guo, Yu-Xiao Liu [pdf]
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Neural-Pull: Learning Signed Distance Functions from Point Clouds by
Learning to Pull Space onto Surfaces -
Abstract
- Reconstructing continuous surfaces from 3D point clouds is a fundamental operation in 3D geometry processing. Several recent state-of-the-art methods address this problem using neural networks to learn signed distance functions (SDFs). In this paper, we introduce \textit{Neural-Pull}, a new approach that is simple and leads to high quality SDFs. Specifically, we train a neural network to pull query 3D locations to their closest points on the surface using the predicted signed distance values and the gradient at the query locations, both of which are computed by the network itself. The pulling operation moves each query location with a stride given by the distance predicted by the network. Based on the sign of the distance, this may move the query location along or against the direction of the gradient of the SDF. This is a differentiable operation that allows us to update the signed distance value and the gradient simultaneously during training. Our outperforming results under widely used benchmarks demonstrate that we can learn SDFs more accurately and flexibly for surface reconstruction and single image reconstruction than the state-of-the-art methods.
Baorui Ma, Zhizhong Han, Yu-Shen Liu, Matthias Zwicker [pdf]
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On the Decycling Number of Bubble-sort Star Graphs -
Abstract
- Bubble-sort star graphs are a combination of star graphs and bubble sort graphs. They are bipartite graphs and also form a family of Cayley graphs. The decycling number of a graph is the minimum number of vertices whose removal from the graph results in an acyclic subgraph. In this paper, we prove the decycling number D(n) of an n-dimensional bubble-sort star graph for n <= 5. We also show D(n) satisfies the inequalities for n >= 6.
Yu-Zhe Liu, Shyue-Ming Tang, Jou-Ming Chang [pdf]
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Sparsity Prior Regularized Q-learning for Sparse Action Tasks -
Abstract
- In many decision-making tasks, some specific actions are limited in their frequency or total amounts, such as "fire" in the gunfight game and "buy/sell" in the stock trading. We name such actions as "sparse action". Sparse action often plays a crucial role in achieving good performance. However, their Q-values, estimated by \emph{classical Bellman update}, usually suffer from a large estimation error due to the sparsity of their samples. The \emph{greedy} policy could be greatly misled by the biased Q-function and takes sparse action aggressively, which leads to a huge sub-optimality. This paper constructs a reference distribution that assigns a low probability to sparse action and proposes a regularized objective with an explicit constraint to the reference distribution. Furthermore, we derive a regularized Bellman operator and a regularized optimal policy that can slow down the propagation of error and guide the agent to take sparse action more carefully. The experiment results demonstrate that our method achieves state-of-the-art performance on typical sparse action tasks.
Jing-Cheng Pang, Tian Xu, Sheng-Yi Jiang, Yu-Ren Liu, Yang Yu [pdf]
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Linking continuum and lattice quark mass functions via an effective
charge -
Abstract
- The quark mass function is computed both by solving the quark propagator Dyson-Schwinger equation and from lattice simulations implementing overlap and Domain-Wall fermion actions for valence and sea quarks, respectively. The results are confronted and seen to produce a very congruent picture, showing a remarkable agreement for the explored range of current-quark masses. The effective running-interaction is based on a process-independent charge rooted on a particular truncation of the Dyson-Schwinger equations in the gauge sector, establishing thus a link from there to the quark sector and inspiring a correlation between the emergence of gluon and hadron masses.
Lei Chang, Yu-Bin Liu, Khépani Raya, J. Rodríguez-Quintero, Yi-Bo Yang [pdf]
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The impact and recovery of asteroid 2018 LA -
Abstract
- The June 2, 2018, impact of asteroid 2018 LA over Botswana is only the second asteroid detected in space prior to impacting over land. Here, we report on the successful recovery of meteorites. Additional astrometric data refine the approach orbit and define the spin period and shape of the asteroid. Video observations of the fireball constrain the asteroid's position in its orbit and were used to triangulate the location of the fireball's main flare over the Central Kalahari Game Reserve. 23 meteorites were recovered. A consortium study of eight of these classifies Motopi Pan as a HED polymict breccia derived from howardite, cumulate and basaltic eucrite, and diogenite lithologies. Before impact, 2018 LA was a solid rock of about 156 cm diameter with high bulk density about 2.85 g/cm3, a relatively low albedo pV about 0.25, no significant opposition effect on the asteroid brightness, and an impact kinetic energy of about 0.2 kt. The orbit of 2018 LA is consistent with an origin at Vesta (or its Vestoids) and delivery into an Earth-impacting orbit via the nu_6 resonance. The impact that ejected 2018 LA in an orbit towards Earth occurred 22.8 +/- 3.8 Ma ago. Zircons record a concordant U-Pb age of 4563 +/- 11 Ma and a consistent 207Pb/206Pb age of 4563 +/- 6 Ma. A much younger Pb-Pb phosphate resetting age of 4234 +/- 41 Ma was found. From this impact chronology, we discuss what is the possible source crater of Motopi Pan and the age of Vesta's Veneneia impact basin.
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QCD phase transition and equation of state of stellar strong interaction matter via Dyson–Schwinger equation approach -
Abstract
- We study the phase structure and phase transition of cold dense QCD matter via the Dyson-Schwinger equation approach. We take the rainbow approximation and the Gaussian-type gluon model. In order to guarantee that the quark number density begins to appear at the nuclear liquid-gas phase transition chemical potential, we propose a chemical potential dependent modification factor for the gluon model. We find that for the iso-symmetric quark matter, the modification reduces the chemical potential of the phase coexistence region of the first--order phase transition. We also implement the relativistic mean field theory to describe the hadron matter, and make use of the Maxwell and Gibbs construction method to study the phase transition of beta--equilibrium and charge neutral matter in compact stars. The results show that the phase transition will not happen in case of the Gaussian--type gluon model without any modification. The results also indicate that the upper boundary of the coexistence region should be larger than the current Nambu solution existing region. We also calculate the mass-radius relation of the compact stars, and find that the hadron-quark phase transition happens at too high chemical potential so that the maximum mass of the compact star is hardly affected by the hadron-quark phase transition.
Zhan Bai, Yu-xin Liu [pdf] DOI: 10.1140/epjc/s10052-021-09423-y 2105.03947v1 [pdf]
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First-principles study of the role of surface in the heavy-fermion compound
-
Abstract
- In the heavy fermion materials, the characteristic energy scales of many exotic strongly correlated phenomena (Kondo effect, magnetic order, superconductivity, etc.) are at milli-electron-volt order, implying that the heavy fermion materials are surface sensitive. Here, we investigate the electronic structures for Si- and Ce-terminated surfaces of CeRh$_2$Si$_2$ by first-principles methods. Our research reveals three notable impacts of surface effects on electronic structures, which are consistent with recent angle-resolved photoemission spectroscopy (ARPES) experiments. Firstly, the relaxation of surface crystal structures changes the relative position of Fermi level, adjusts the dispersion of bands and enhances the Kondo resonance. Secondly, the decrease of the hybridization between the Ce-4$f$ and conduction electrons in the surface layer leads to a weaker Kondo resonance peak and the shift of spin-orbit bands. Thirdly, the variation of crystal electric field around surface Ce atoms affects the splitting of Kondo resonance peaks, and also pushes down the lower-Hubbard bands of surface 4$f$ electrons. Moreover, we find the characteristic of bulk's lower-Hubbard bands, which was overlooked in previous works. Our investigation suggests that these surface effects are potentially important and highlighted in the future researches on properties of strongly correlated materials.
Yue-Chao Wang, Yuan-Ji Xu, Yu Liu, Xing-Jie Han, Xie-Gang Zhu, Yi-feng Yang, Yan Bi, Hai-Feng Liu, Hai-Feng Song Journal reference: Phys. Rev. B 103, 165140 (2021) [pdf] DOI: 10.1103/PhysRevB.103.165140
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Testing the microstructure of
-
Abstract
- Understanding black hole microstructure via the thermodynamic geometry can provide us with more deeper insight into black hole thermodynamics in modified gravities. In this paper, we study the black hole phase transition and Ruppeiner geometry for the $d$-dimensional charged Gauss-Bonnet anti-de Sitter black holes. The results show that the small-large black hole phase transition is universal in this gravity. By reducing the thermodynamic quantities with the black hole charge, we clearly exhibit the phase diagrams in different parameter spaces. Of particular interest is that the radius of the black hole horizon can act as the order parameter to characterize the black hole phase transition. We also disclose that different from the five-dimensional neutral black holes, the charged ones allow the repulsive interaction among its microstructure for small black hole of higher temperature. Another significant difference between them is that the microscopic interaction changes during the small-large black hole phase transition for the charged case, where the black hole microstructure undergoes a sudden change. These results are helpful for peeking into the microstructure of charged black holes in the Gauss-Bonnet gravity.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 104, 024062 (2021) [pdf] DOI: 10.1103/PhysRevD.104.024062
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Dynamics and kinetics of phase transition for Kerr AdS black hole on
free energy landscape -
Abstract
- By treating the black hole event horizon as a stochastic thermal fluctuating variable for small-large black hole phase transition, we investigate the dynamical process of phase transition for the Kerr AdS black holes on free energy landscape. We find that the extremal points of the off-shell Gibbs free energy correspond to physical black holes. For small-large black hole phase transition, the off-shell Gibbs free energy exhibits a double well behavior with the same depth. Contrary to previous research for the dynamics of phase transition for the Kerr-Newman AdS family black holes on free energy landscape, we find that there is a lower bound for the order parameter and the lower bound corresponds to extremal black holes. In particular, the off-shell Gibbs free energy is zero instead of divergent as previous work suggested for vanishing black hole horizon radius, which corresponds to the Gibbs free energy of thermal AdS space. The investigation for the evolution of the probability distribution for the phase transition indicates that the initial stable small (large) black hole tends to switch to stable large (small) black hole. Increasing the temperature along the coexistence curve, the switching process becomes faster and the probability distribution reaches the final stationary Boltzmann distribution at a shorter time. The distribution of the first passage time indicates the time scale of the small-large black hole phase transition, and the peak of the distribution becomes sharper and shifts to the left with the increase of temperature along the coexistence curve. This suggests that a considerable first passage process occurs at a shorter time for higher temperature.
Si-Jiang Yang, Run Zhou, Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Large-Momentum Effective Theory -
Abstract
- Since the parton model was introduced by Feynman more than fifty years ago, we have learned much about the partonic structure of the proton through a large body of high-energy experimental data and dedicated global fits. However, calculating the partonic observables such as parton distribution function (PDFs) from the fundamental theory of strong interactions, QCD, has made limited progress. Recently, the authors have advocated a formalism, large-momentum effective theory (LaMET), through which one can extract parton physics from the properties of the proton travelling at a moderate boost-factor, e.g., $\gamma\sim (2-5)$. The key observation behind this approach is that Lorentz symmetry allows the standard formalism of partons in terms of light-front operators to be replaced by an equivalent one with large-momentum states and time-independent operators of a universality class. With LaMET, the PDFs, generalized PDFs or GPDs, transverse-momentum-dependent PDFs, and light-front wave functions can all be extracted in principle from lattice simulations of QCD (or other non-perturbative methods) through standard effective field theory matching and running. Future lattice QCD calculations with exa-scale computational facilities can help to understand the experimental data related to the hadronic structure, including those from the upcoming Electron-Ion Colliders dedicated to exploring the partonic landscape of the proton. Here we review the progress made in the past few years in development of the LaMET formalism and its applications, particularly on the demonstration of its effectiveness from initial lattice QCD simulations.
Xiangdong Ji, Yu-Sheng Liu, Yizhuang Liu, Jian-Hui Zhang, Yong Zhao [pdf]
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A Novel Unified Stereo Stimuli based Binocular Eye-Tracking System for
Accurate 3D Gaze Estimation -
Abstract
- In addition to the high cost and complex setup, the main reason for the limitation of the three-dimensional (3D) display is the problem of accurately estimating the user's current point-of-gaze (PoG) in a 3D space. In this paper, we present a novel noncontact technique for the PoG estimation in a stereoscopic environment, which integrates a 3D stereoscopic display system and an eye-tracking system. The 3D stereoscopic display system can provide users with a friendly and immersive high-definition viewing experience without wearing any equipment. To accurately locate the user's 3D PoG in the field of view, we build a regression-based 3D eye-tracking model with the eye movement data and stereo stimulus videos as input. Besides, to train an optimal regression model, we also design and annotate a dataset that contains 30 users' eye-tracking data corresponding to two designed stereo test scenes. Innovatively, this dataset introduces feature vectors between eye region landmarks for the gaze vector estimation and a combined feature set for the gaze depth estimation. Moreover, five traditional regression models are trained and evaluated based on this dataset. Experimental results show that the average errors of the 3D PoG are about 0.90~cm on the X-axis, 0.83~cm on the Y-axis, and 1.48~cm$/$0.12~m along the Z-axis with the scene-depth range in 75~cm$/$8~m, respectively.
Sunjing Lin, Yu Liu, Shaochu Wang, Chang Li, Han Wang [pdf]
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Actor-Context-Actor Relation Network for Spatio-Temporal Action
Localization -
Abstract
- Localizing persons and recognizing their actions from videos is a challenging task towards high-level video understanding. Recent advances have been achieved by modeling direct pairwise relations between entities. In this paper, we take one step further, not only model direct relations between pairs but also take into account indirect higher-order relations established upon multiple elements. We propose to explicitly model the Actor-Context-Actor Relation, which is the relation between two actors based on their interactions with the context. To this end, we design an Actor-Context-Actor Relation Network (ACAR-Net) which builds upon a novel High-order Relation Reasoning Operator and an Actor-Context Feature Bank to enable indirect relation reasoning for spatio-temporal action localization. Experiments on AVA and UCF101-24 datasets show the advantages of modeling actor-context-actor relations, and visualization of attention maps further verifies that our model is capable of finding relevant higher-order relations to support action detection. Notably, our method ranks first in the AVA-Kineticsaction localization task of ActivityNet Challenge 2020, out-performing other entries by a significant margin (+6.71mAP). Training code and models will be available at https://github.com/Siyu-C/ACAR-Net.
Junting Pan, Siyu Chen, Mike Zheng Shou, Yu Liu, Jing Shao, Hongsheng Li [pdf]
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Role-Aware Modeling for N-ary Relational Knowledge Bases -
Abstract
- N-ary relational knowledge bases (KBs) represent knowledge with binary and beyond-binary relational facts. Especially, in an n-ary relational fact, the involved entities play different roles, e.g., the ternary relation PlayCharacterIn consists of three roles, ACTOR, CHARACTER and MOVIE. However, existing approaches are often directly extended from binary relational KBs, i.e., knowledge graphs, while missing the important semantic property of role. Therefore, we start from the role level, and propose a Role-Aware Modeling, RAM for short, for facts in n-ary relational KBs. RAM explores a latent space that contains basis vectors, and represents roles by linear combinations of these vectors. This way encourages semantically related roles to have close representations. RAM further introduces a pattern matrix that captures the compatibility between the role and all involved entities. To this end, it presents a multilinear scoring function to measure the plausibility of a fact composed by certain roles and entities. We show that RAM achieves both theoretical full expressiveness and computation efficiency, which also provides an elegant generalization for approaches in binary relational KBs. Experiments demonstrate that RAM outperforms representative baselines on both n-ary and binary relational datasets.
Yu Liu, Quanming Yao, Yong Li [pdf]
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Integrating Information Theory and Adversarial Learning for Cross-modal
Retrieval -
Abstract
- Accurately matching visual and textual data in cross-modal retrieval has been widely studied in the multimedia community. To address these challenges posited by the heterogeneity gap and the semantic gap, we propose integrating Shannon information theory and adversarial learning. In terms of the heterogeneity gap, we integrate modality classification and information entropy maximization adversarially. For this purpose, a modality classifier (as a discriminator) is built to distinguish the text and image modalities according to their different statistical properties. This discriminator uses its output probabilities to compute Shannon information entropy, which measures the uncertainty of the modality classification it performs. Moreover, feature encoders (as a generator) project uni-modal features into a commonly shared space and attempt to fool the discriminator by maximizing its output information entropy. Thus, maximizing information entropy gradually reduces the distribution discrepancy of cross-modal features, thereby achieving a domain confusion state where the discriminator cannot classify two modalities confidently. To reduce the semantic gap, Kullback-Leibler (KL) divergence and bi-directional triplet loss are used to associate the intra- and inter-modality similarity between features in the shared space. Furthermore, a regularization term based on KL-divergence with temperature scaling is used to calibrate the biased label classifier caused by the data imbalance issue. Extensive experiments with four deep models on four benchmarks are conducted to demonstrate the effectiveness of the proposed approach.
Wei Chen, Yu Liu, Erwin M. Bakker, Michael S. Lew [pdf]
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Self-supervised Video Representation Learning by Context and Motion
Decoupling -
Abstract
- A key challenge in self-supervised video representation learning is how to effectively capture motion information besides context bias. While most existing works implicitly achieve this with video-specific pretext tasks (e.g., predicting clip orders, time arrows, and paces), we develop a method that explicitly decouples motion supervision from context bias through a carefully designed pretext task. Specifically, we take the keyframes and motion vectors in compressed videos (e.g., in H.264 format) as the supervision sources for context and motion, respectively, which can be efficiently extracted at over 500 fps on the CPU. Then we design two pretext tasks that are jointly optimized: a context matching task where a pairwise contrastive loss is cast between video clip and keyframe features; and a motion prediction task where clip features, passed through an encoder-decoder network, are used to estimate motion features in a near future. These two tasks use a shared video backbone and separate MLP heads. Experiments show that our approach improves the quality of the learned video representation over previous works, where we obtain absolute gains of 16.0% and 11.1% in video retrieval recall on UCF101 and HMDB51, respectively. Moreover, we find the motion prediction to be a strong regularization for video networks, where using it as an auxiliary task improves the accuracy of action recognition with a margin of 7.4%~13.8%.
Lianghua Huang, Yu Liu, Bin Wang, Pan Pan, Yinghui Xu, Rong Jin [pdf]
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MulDE: Multi-teacher Knowledge Distillation for Low-dimensional Knowledge Graph Embeddings -
Abstract
- Link prediction based on knowledge graph embeddings (KGE) aims to predict new triples to automatically construct knowledge graphs (KGs). However, recent KGE models achieve performance improvements by excessively increasing the embedding dimensions, which may cause enormous training costs and require more storage space. In this paper, instead of training high-dimensional models, we propose MulDE, a novel knowledge distillation framework, which includes multiple low-dimensional hyperbolic KGE models as teachers and two student components, namely Junior and Senior. Under a novel iterative distillation strategy, the Junior component, a low-dimensional KGE model, asks teachers actively based on its preliminary prediction results, and the Senior component integrates teachers' knowledge adaptively to train the Junior component based on two mechanisms: relation-specific scaling and contrast attention. The experimental results show that MulDE can effectively improve the performance and training speed of low-dimensional KGE models. The distilled 32-dimensional model is competitive compared to the state-of-the-art high-dimensional methods on several widely-used datasets.
Kai Wang, Yu Liu, Qian Ma, Quan Z. Sheng [pdf] DOI: 10.1145/3442381.3449898 2010.07152v4 [pdf]
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Dissipative Topological Phase Transition with Strong System-Environment
Coupling -
Abstract
- A primary motivation for studying topological matter regards the protection of topological order from its environment. In this work, we study a topological emitter array coupled to an electromagnetic environment. The photon-emitter coupling produces nonlocal interactions between emitters. Using periodic boundary conditions for all ranges of environment-induced interactions, chiral symmetry inherent to the emitter array is preserved and protects the topological phase. A topological phase transition occurs at a critical photon-emitter coupling which is related to the energy spectrum width of the emitter array. It produces a band touching with parabolic dispersion, distinct to the linear one without considering the environment. Interestingly, the critical point nontrivially changes dissipation rates of edge states, yielding dissipative topological phase transition. In the protected topological phase, edge states suffer from environment-induced dissipation for weak photon-emitter coupling. However, strong coupling leads to dissipationless edge states. Our work presents a way to study topological criticality in open quantum systems.
Wei Nie, Mauro Antezza, Yu-xi Liu, Franco Nori [pdf]
-
High-efficiency Euclidean-based Models for Low-dimensional Knowledge
Graph Embeddings -
Abstract
- Recent knowledge graph embedding (KGE) models based on hyperbolic geometry have shown great potential in a low-dimensional embedding space. However, the necessity of hyperbolic space in KGE is still questionable, because the calculation based on hyperbolic geometry is much more complicated than Euclidean operations. In this paper, based on the state-of-the-art hyperbolic-based model RotH, we develop two lightweight Euclidean-based models, called RotL and Rot2L. The RotL model simplifies the hyperbolic operations while keeping the flexible normalization effect. Utilizing a novel two-layer stacked transformation and based on RotL, the Rot2L model obtains an improved representation capability, yet costs fewer parameters and calculations than RotH. The experiments on link prediction show that Rot2L achieves the state-of-the-art performance on two widely-used datasets in low-dimensional knowledge graph embeddings. Furthermore, RotL achieves similar performance as RotH but only requires half of the training time.
Kai Wang, Yu Liu, Quan Z. Sheng [pdf]
-
Lifelong Person Re-Identification via Adaptive Knowledge Accumulation -
Abstract
- Person ReID methods always learn through a stationary domain that is fixed by the choice of a given dataset. In many contexts (e.g., lifelong learning), those methods are ineffective because the domain is continually changing in which case incremental learning over multiple domains is required potentially. In this work we explore a new and challenging ReID task, namely lifelong person re-identification (LReID), which enables to learn continuously across multiple domains and even generalise on new and unseen domains. Following the cognitive processes in the human brain, we design an Adaptive Knowledge Accumulation (AKA) framework that is endowed with two crucial abilities: knowledge representation and knowledge operation. Our method alleviates catastrophic forgetting on seen domains and demonstrates the ability to generalize to unseen domains. Correspondingly, we also provide a new and large-scale benchmark for LReID. Extensive experiments demonstrate our method outperforms other competitors by a margin of 5.8% mAP in generalising evaluation.
Nan Pu, Wei Chen, Yu Liu, Erwin M. Bakker, Michael S. Lew [pdf]
-
Topology-Enhanced Nonreciprocal Scattering and Photon Absorption in a Waveguide -
Abstract
- Topological matter and topological optics have been studied in many systems, with promising applications in materials science and photonics technology. These advances motivate the study of the interaction between topological matter and light, as well as topological protection in light-matter interactions. In this work, we study a waveguide-interfaced topological atom array. The light-matter interaction is nontrivially modified by topology, yielding novel optical phenomena. We find topology-enhanced photon absorption from the waveguide for large Purcell factor, i.e., $\Gamma/\Gamma_0\gg 1$, where $\Gamma$ and $\Gamma_0$ are the atomic decays to waveguide and environment, respectively. To understand this unconventional photon absorption, we propose a multi-channel scattering approach and study the interaction spectra for edge- and bulk-state channels. We find that, by breaking inversion and time-reversal symmetries, optical anisotropy is enabled for reflection process, but the transmission is isotropic. Through a perturbation analysis of the edge-state channel, we show that the anisotropy in the reflection process originates from the waveguide-mediated non-Hermitian interaction. However, the inversion symmetry in the non-Hermitian interaction makes the transmission isotropic. At a topology-protected atomic spacing, the subradiant edge state exhibits huge anisotropy. Due to the interplay between edge- and bulk-state channels, a large topological bandgap enhances nonreciprocal reflection of photons in the waveguide for weakly broken time-reversal symmetry, i.e., $\Gamma_0/\Gamma\ll 1$, producing complete photon absorption. We show that our proposal can be implemented in superconducting quantum circuits. The topology-enhanced photon absorption is useful for quantum detection. This work shows the potential to manipulate light with topological quantum matter.
Wei Nie, Tao Shi, Franco Nori, Yu-xi Liu Journal reference: Phys. Rev. Applied 15, 044041 (2021) [pdf] DOI: 10.1103/PhysRevApplied.15.044041
-
Boundary effect and dressed states of a giant atom in a topological
waveguide -
Abstract
- The interaction between the quantum emitter and topological photonic system makes the photon behave in exotic ways. We here study the properties of a giant atom coupled to two sites of a one-dimensional topological waveguide, which is described by the Su-Schrieffer-Heeger (SSH) chain. We find that the giant atom can act as an effective boundary and induce the chiral zero modes, which are similar to those in the SSH model with open boundary, for the waveguide under the periodical boundary. Except for the boundary effect, we also find that the giant atom can lift energy degeneracy inside the energy bands of the SSH chain and adjust spatial symmetry of the photon distributions for the states of the dressed giant atom and waveguide. That is, the giant atom can be used to change the properties of the topological environment. Our work may stimulate more studies on the interaction between matter and topological environment.
Weijun Cheng, Zhihai Wang, Yu-xi Liu [pdf]
-
ACDnet: An action detection network for real-time edge computing based on flow-guided feature approximation and memory aggregation -
Abstract
- Interpreting human actions requires understanding the spatial and temporal context of the scenes. State-of-the-art action detectors based on Convolutional Neural Network (CNN) have demonstrated remarkable results by adopting two-stream or 3D CNN architectures. However, these methods typically operate in a non-real-time, ofline fashion due to system complexity to reason spatio-temporal information. Consequently, their high computational cost is not compliant with emerging real-world scenarios such as service robots or public surveillance where detection needs to take place at resource-limited edge devices. In this paper, we propose ACDnet, a compact action detection network targeting real-time edge computing which addresses both efficiency and accuracy. It intelligently exploits the temporal coherence between successive video frames to approximate their CNN features rather than naively extracting them. It also integrates memory feature aggregation from past video frames to enhance current detection stability, implicitly modeling long temporal cues over time. Experiments conducted on the public benchmark datasets UCF-24 and JHMDB-21 demonstrate that ACDnet, when integrated with the SSD detector, can robustly achieve detection well above real-time (75 FPS). At the same time, it retains reasonable accuracy (70.92 and 49.53 frame mAP) compared to other top-performing methods using far heavier configurations. Codes will be available at https://github.com/dginhac/ACDnet.
Yu Liu, Fan Yang, Dominique Ginhac Journal reference: Pattern Recognition Letters, 145 , 118-126, 2021 [pdf] DOI: 10.1016/j.patrec.2021.02.001
-
Phase-Controlled Pathway Interferences and Switchable Fast-Slow Light in a Cavity-Magnon Polariton System -
Abstract
- We study the phase controlled transmission properties in a compound system consisting of a 3D copper cavity and an yttrium iron garnet (YIG) sphere. By tuning the relative phase of the magnon pumping and cavity probe tones, constructive and destructive interferences occur periodically, which strongly modify both the cavity field transmission spectra and the group delay of light. Moreover, the tunable amplitude ratio between pump-probe tones allows us to further improve the signal absorption or amplification, accompanied by either significantly enhanced optical advance or delay. Both the phase and amplitude-ratio can be used to realize in-situ tunable and switchable fast-slow light. The tunable phase and amplitude-ratio lead to the zero reflection of the transmitted light and an abrupt fast-slow light transition. Our results confirm that direct magnon pumping through the coupling loops provides a versatile route to achieve controllable signal transmission, storage, and communication, which can be further expanded to the quantum regime, realizing coherent-state processing or quantum-limited precise measurements.
Jie Zhao, Longhao Wu, Tiefu Li, Yu-xi Liu, Franco Nori, Yulong Liu, Jiangfeng Du Journal reference: Phys. Rev. Applied 15, 024056 (2021) [pdf] DOI: 10.1103/PhysRevApplied.15.024056
-
Sieve Methods in Random Graph Theory -
Abstract
- In this paper, we apply the Turan sieve and the simple sieve developed by R. Murty and the first author to study problems in random graph theory. In particular, we obtain upper and lower bounds on the probability of a graph on n vertices having diameter 2 (or diameter 3 in the case of bipartite graphs) with edge probability p(n) where the edges are chosen independently . An interesting feature revealed in these results is that the Turan sieve and the simple sieve `almost completely' complement each other. As a corollary to our result, we note that the probability of a random graph having diameter 2 approaches 1 as n approaches infinity for constant edge probability p(n)=1/2. This is an appendix of a shorter version of this paper.
Yu-Ru Liu, J. C. Saunders [pdf]
-
Two-dimensional charge order stabilized in clean polytype heterostructures -
Abstract
- Strong evidence suggests that transformative correlated electron behavior may exist only in unrealized clean-limit 2D materials such as 1T-TaS2. Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers impedes correlation-driven quantum behavior. Here we demonstrate a new route to realizing fragile 2D quantum states through epitaxial polytype engineering of van der Waals materials. The isolation of truly 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal-insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces epitaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication.
Suk Hyun Sung, Noah Schnitzer, Steve Novakov, Ismail El Baggari, Xiangpeng Luo, Jiseok Gim, Nguyen Vu, Zidong Li, Todd Brintlinger, Yu Liu, Wenjian Lu, Yuping Sun, Parag Deotare, Kai Sun, Liuyan Zhao, Lena F. Kourkoutis, John T. Heron, Robert Hovden [pdf] DOI: 10.1017/S1431927621003469 2102.09079v1 [pdf]
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A Possible Kilonova Powered by Magnetic Wind from a Newborn Black Hole -
Abstract
- The merger of binary neutron stars (NS-NS) as the progenitor of short Gamma-ray bursts (GRBs) has been confirmed by the discovery of the association of the gravitational wave (GW) event GW170817 with GRB 170817A. However, the merger product of binary NS remains an open question. An X-ray plateau followed by a steep decay ("internal plateau") has been found in some short GRBs, implying that a supra-massive magnetar operates as the merger remnant and then collapses into a newborn black hole (BH) at the end of the plateau. X-ray bump or second-plateau following the "internal plateau" is considered as the expected signature from the fallback accretion onto this newborn BH through Blandford-Znajek mechanism (BZ). At the same time, a nearly isotropic wind driven by Blandford-Paynemechanism (BP) from the newborn BH's disk can produce a bright kilonova. Therefore, the bright kilonova observation for a short GRB with "internal plateau" (and followed by X-ray bump or second-plateau) provides further evidence for this scenario. In this paper, we find that GRB 160821B is a candidate of such a case, and the kilonova emission of GRB 160821B is possibly powered by the BP wind from a newborn BH. Future GW detection of GRB 160821B-like events may provide further support to this scenario, enable us to investigate the properties of the magnetar and the newborn BH, and constrain the equation of state of neutron stars.
Shuai-Bing Ma, Wei Xie, Bin Liao, Bin-Bin Zhang, Hou-Jun Lü, Yu Liu, Wei-Hua Lei [pdf] DOI: 10.3847/1538-4357/abe71b 2010.01338v2 [pdf]
-
Understanding Bounding Functions in Safety-Critical UAV Software -
Abstract
- Unmanned Aerial Vehicles (UAVs) are an emerging computation platform known for their safety-critical need. In this paper, we conduct an empirical study on a widely used open-source UAV software framework, Paparazzi, with the goal of understanding the safety-critical concerns of UAV software from a bottom-up developer-in-the-field perspective. We set our focus on the use of Bounding Functions (BFs), the runtime checks injected by Paparazzi developers on the range of variables. Through an in-depth analysis on BFs in the Paparazzi autopilot software, we found a large number of them (109 instances) are used to bound safety-critical variables essential to the cyber-physical nature of the UAV, such as its thrust, its speed, and its sensor values. The novel contributions of this study are two fold. First, we take a static approach to classify all BF instances, presenting a novel datatype-based 5-category taxonomy with fine-grained insight on the role of BFs in ensuring the safety of UAV systems. Second, we dynamically evaluate the impact of the BF uses through a differential approach, establishing the UAV behavioral difference with and without BFs. The two-pronged static and dynamic approach together illuminates a rarely studied design space of safety-critical UAV software systems.
Xiaozhou Liang, John Henry Burns, Joseph Sanchez, Karthik Dantu, Lukasz Ziarek, Yu David Liu [pdf]
-
Corrigendum to "Hardy Spaces $H_{\mathcal L}^1({\mathbb R}^n)$
Associated to Schrödinger Type Operators $(-Δ)^2+V^2$" [Houston J.
Math 36 (4) (2010), 1067-1095] -
Abstract
- We rectify an incorrect citation of the reference in obtaining the Gaussian upper bound for heat kernels of the Schr\"odinger type operators $(-\Delta)^2+V^2$.
Jun Cao, Yu Liu, Dachun Yang [pdf]
-
Train a One-Million-Way Instance Classifier for Unsupervised Visual
Representation Learning -
Abstract
- This paper presents a simple unsupervised visual representation learning method with a pretext task of discriminating all images in a dataset using a parametric, instance-level classifier. The overall framework is a replica of a supervised classification model, where semantic classes (e.g., dog, bird, and ship) are replaced by instance IDs. However, scaling up the classification task from thousands of semantic labels to millions of instance labels brings specific challenges including 1) the large-scale softmax computation; 2) the slow convergence due to the infrequent visiting of instance samples; and 3) the massive number of negative classes that can be noisy. This work presents several novel techniques to handle these difficulties. First, we introduce a hybrid parallel training framework to make large-scale training feasible. Second, we present a raw-feature initialization mechanism for classification weights, which we assume offers a contrastive prior for instance discrimination and can clearly speed up converge in our experiments. Finally, we propose to smooth the labels of a few hardest classes to avoid optimizing over very similar negative pairs. While being conceptually simple, our framework achieves competitive or superior performance compared to state-of-the-art unsupervised approaches, i.e., SimCLR, MoCoV2, and PIC under ImageNet linear evaluation protocol and on several downstream visual tasks, verifying that full instance classification is a strong pretraining technique for many semantic visual tasks.
Yu Liu, Lianghua Huang, Pan Pan, Bin Wang, Yinghui Xu, Rong Jin [pdf]
-
Two Quasi-periodic Fast-propagating Magnetosonic Wave Events Observed in Active Region NOAA 11167 -
Abstract
- We report a detailed observational study of two quasi-periodic fast-propagating (QFP) magnetosonic wave events occurred on 2011 March 09 and 10, respectively. Interestingly, both the two events have two wave trains (WTs): one main and strong (WT-1) whereas the second appears small and weak (WT-2). Peculiar and common characteristics of the two events are observed, namely: 1) the two QFP waves are accompanied with brightenings during the whole stage of the eruptions; 2) both the two main wave trains are nearly propagating along the same direction; 3) EUV waves are found to be associated with the two events. Investigating various aspects of the target events, we argue that: 1) the second event is accompanied with a flux rope eruption during the whole stage; 2) the second event eruption produces a new filament-like (FL) dark feature; 3) the ripples of the two WT-2 QFP waves seem to result from different triggering mechanisms. Based on the obtained observational results, we propose that the funnel-like coronal loop system is indeed playing an important role in the two WT-1 QFP waves. The development of the second WT-2 QFP wave can be explained as due to the dispersion of the main EUV front. The co-existence of the two events offer thereby a significant opportunity to reveal what driving mechanisms and structures are tightly related to the waves.
Yuhu Miao, Yu Liu, A. Elmhamdi, A. S. Kordi, Y. D. Shen, Rehab Al-Shammari, Khaled Al-Mosabeh, Chaowei Jiang, Ding Yuan [pdf] DOI: 10.3847/1538-4357/ab655f 1912.11792v2 [pdf]
-
Deep Image Retrieval: A Survey -
Abstract
- In recent years a vast amount of visual content has been generated and shared from various fields, such as social media platforms, medical images, and robotics. This abundance of content creation and sharing has introduced new challenges. In particular, searching databases for similar content, i.e.content based image retrieval (CBIR), is a long-established research area, and more efficient and accurate methods are needed for real time retrieval. Artificial intelligence has made progress in CBIR and has significantly facilitated the process of intelligent search. In this survey we organize and review recent CBIR works that are developed based on deep learning algorithms and techniques, including insights and techniques from recent papers. We identify and present the commonly-used benchmarks and evaluation methods used in the field. We collect common challenges and propose promising future directions. More specifically, we focus on image retrieval with deep learning and organize the state of the art methods according to the types of deep network structure, deep features, feature enhancement methods, and network fine-tuning strategies. Our survey considers a wide variety of recent methods, aiming to promote a global view of the field of instance-based CBIR.
Wei Chen, Yu Liu, Weiping Wang, Erwin Bakker, Theodoros Georgiou, Paul Fieguth, Li Liu, Michael S. Lew [pdf]
-
Observing dynamic oscillatory behavior of triple points among black hole
thermodynamic phase transitions -
Abstract
- Understanding the dynamic process of black hole thermodynamic phase transitions at a triple point is a huge challenge. In this letter, we carry out the first investigation of dynamical phase behaviour at a black hole triple point. By numerically solving the Smoluchowski equation near the triple point for a six-dimensional charged Gauss-Bonnet anti-de Sitter black hole, we find that initial small, intermediate, or large black holes can transit to the other two coexistent phases at the triple point, indicating that thermodynamic phase transitions can indeed occur dynamically. More significantly, we observe characteristic weak and strong oscillatory behaviour in this dynamic process, which can be understood from an investigation of the rate of first passage from one phase to another. Our results further an understanding of the dynamic process of black hole thermodynamic phase transitions.
Shao-Wen Wei, Yong-Qiang Wang, Yu-Xiao Liu, Robert B. Mann [pdf]
-
Can the Hyperfine Mass Splitting Formula in Heavy Quarkonia be Applied to the $$B_c$$ System? -
Abstract
- The mass relation ${M_{0^{+}}+3M_{1^{+\prime}}+5M_{2^{+}}= 9M_{1^{+}}}$ miraculously holds for the $P$-wave charmonium $(c\bar{c})$ and bottomonium $(b\bar{b})$ systems with soaring precision. The origin of such relation can be addressed from Quark Models, and have been confirmed experimentally in a limited number of cases. In this connection, we propose $M_{0^{+}}+5M_{2^{+}}=3(M_{1^{+\prime}}+M_{1^{+}})$ as an extension to the $P$-wave $B_{c}$ case. In order to test its applicability, we employ a variety of Quark Model predictions for the $B_c$ mass spectrum. Our numerical analysis confirms such formula is accurate up to very small deviations.
Lei Chang, Muyang Chen, Xue-qian Li, Yu-xin Liu, Khépani Raya [pdf] DOI: 10.1007/s00601-020-01586-w 1912.08339v2 [pdf]
-
Weak deflection angle by electrically and magnetically charged black holes from nonlinear electrodynamics -
Abstract
- Nonlinear electrodynamic (NLED) theories are well-motivated for their extensions to classical electrodynamics in the strong field regime, and have been extensively investigated in seeking for regular black hole solutions. In this paper, we focus on two spherically symmetric and static black hole solutions based on two types of NLED models: the Euler-Heisenberg NLED model and the Bronnikov NLED model, and calculate the weak deflection angle of light by these two black holes with the help of the Gauss-Bonnet theorem. We investigate the effects of the one-loop corrections to quantum electrodynamics on the deflection angle and analyse the behavior of the deflection angle by a regular magnetically charged black hole. It is found that the weak deflection angle of the electrically charged Einstein-Euler-Heisenberg black hole increases with the one-loop corrections and the regular magnetically charged black hole based on the Bronnikov NLED model has a smaller deflection angle than the singular one. Besides, we also calculate the deflection angle of light by the geodesic method for verification. In addition, we discuss the effects of a cold non-magnetized plasma on the deflection angle and find that the deflection angle increases with the plasma parameter.
Qi-Ming Fu, Li Zhao, Yu-Xiao Liu Journal reference: Phys. Rev. D 104, 024033 (2021) [pdf] DOI: 10.1103/PhysRevD.104.024033
-
OpenUVR: an Open-Source System Framework for Untethered Virtual Reality
Applications -
Abstract
- Advancements in heterogeneous computing technologies enable the significant potential of virtual reality (VR) applications. To offer the best user experience (UX), a system should adopt an untethered, wireless-network-based architecture to transfer VR content between the user and the content generator. However, modern wireless network technologies make implementing such an architecture challenging, as VR applications require superior video quality -- with high resolution, high frame rates, and very low latency. This paper presents OpenUVR, an open-source framework that uses commodity hardware components to satisfy the demands of interactive, real-time VR applications. OpenUVR significantly improves UX through a redesign of the system stack and addresses the most time-sensitive issues associated with redundant memory copying in modern computing systems. OpenUVR presents a cross-layered VR datapath to avoid redundant data operations and computation among system components, OpenUVR customizes the network stack to eliminate unnecessary memory operations incurred by mismatching data formats in each layer, and OpenUVR uses feedback from mobile devices to remove memory buffers. Together, these modifications allow OpenUVR to reduce VR application delays to 14.32 ms, meeting the 20 ms minimum latency in avoiding motion sickness. As an open-source system that is fully compatible with commodity hardware, OpenUVR offers the research community an opportunity to develop, investigate, and optimize applications for untethered, high-performance VR architectures.
Alec Rohloff, Zackary Allen, Kung-Min Lin, Joshua Okrend, Chengyi Nie, Yu-Chia Liu, Hung-Wei Tseng [pdf]
-
Quantum versus Classical Regime in Circuit Quantum Acoustodynamics -
Abstract
- We experimentally study a circuit quantum acoustodynamics system, which consists of a superconducting artificial atom, coupled to both a two-dimensional surface acoustic wave resonator and a one-dimensional microwave transmission line. The strong coupling between the artificial atom and the acoustic wave resonator is confirmed by the observation of the vacuum Rabi splitting at the base temperature of dilution refrigerator. We show that the propagation of microwave photons in the microwave transmission line can be controlled by a few phonons in the acoustic wave resonator. Furthermore, we demonstrate the temperature effect on the measurements of the Rabi splitting and temperature induced transitions from high excited dressed states. We find that the spectrum structure of two-peak for the Rabi splitting becomes into those of several peaks, and gradually disappears with the increase of the environmental temperature $T$. The quantum-to-classical transition is observed around the crossover temperature $T_{c}$, which is determined via the thermal fluctuation energy $k_{B}T$ and the characteristic energy level spacing of the coupled system. Experimental results agree well with the theoretical simulations via the master equation of the coupled system at different effective temperatures.
Gang-hui Zeng, Yang Zhang, Aleksey N. Bolgar, Dong He, Bin Li, Xin-hui Ruan, Lan Zhou, Le-Mang Kuang, Oleg V. Astafiev, Yu-xi Liu, Z. H. Peng [pdf]
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Design and characterization of a low-vibration laboratory with cylindrical inertia block geometry -
Abstract
- Many modern nanofabrication and imaging techniques require an ultra-quiet environment to reach optimal resolution. Isolation from ambient vibrations is often achieved by placing the sensitive instrument atop a massive block that floats on air springs and is surrounded by acoustic barriers. Because typical building noise drops off above 120 Hz, it is advantageous to raise the flexural resonance frequencies of the inertia block and instrument far above 120 Hz. However, it can be challenging to obtain a high fundamental frequency of the floating block using a simple rectangular design. Here we design, construct, and characterize a vibration isolation system with a cylindrical inertia block, whose lowest resonance frequency of 249 Hz shows good agreement between finite element analysis simulation and directly measured modes. Our simulations show that a cylindrical design can achieve higher fundamental resonance frequency than a rectangular design of the same mass.
Wenjie Gong, Yu Liu, Wan-Ting Liao, Joseph Gibbons, Jennifer E. Hoffman [pdf] DOI: 10.1063/5.0004964 2101.06815v1 [pdf]
-
Anomalous Hall effect in the weak-itinerant ferrimagnet
-
Abstract
- We carried out a comprehensive study of electronic transport, thermal and thermodynamic properties in FeCr$_2$Te$_4$ single crystals. It exhibits bad-metallic behavior and anomalous Hall effect (AHE) below a weak-itinerant paramagentic-to-ferrimagnetic transition $T_c$ $\sim$ 123 K. The linear scaling between the anomalous Hall resistivity $\rho_{xy}$ and the longitudinal resistivity $\rho_{xx}$ implies that the AHE in FeCr$_2$Te$_4$ is most likely dominated by extrinsic skew-scattering mechanism rather than intrinsic KL or extrinsic side-jump mechanism, which is supported by our Berry phase calculations.
Yu Liu, Hengxin Tan, Zhixiang Hu, Binghai Yan, C. Petrovic Journal reference: Physical Review B 103, 045106 (2021) [pdf] DOI: 10.1103/PhysRevB.103.045106
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Precision test of statistical dynamics with state-to-state ultracold chemistry -
Abstract
- Chemical reactions represent a class of quantum problems that challenge both the current theoretical understanding and computational capabilities. Reactions that occur at ultralow temperatures provide an ideal testing ground for quantum chemistry and scattering theories, as they can be experimentally studied with unprecedented control, yet display dynamics that are highly complex. Here, we report the full product state distribution for the reaction 2KRb $\rightarrow$ K$_2$ + Rb$_2$. Ultracold preparation of the reactants grants complete control over their initial quantum degrees of freedom, while state-resolved, coincident detection of both products enables the measurement of scattering probabilities into all 57 allowed rotational state-pairs. Our results show an overall agreement with a state-counting model based on statistical theory, but also reveal several deviating state-pairs. In particular, we observe a strong suppression of population in the state-pair closest to the exoergicity limit, which we precisely determine to be $9.7711^{+0.0007}_{-0.0005}$ cm$^{-1}$, as a result of the long-range potential inhibiting the escape of products. The completeness of our measurements provides a valuable benchmark for quantum dynamics calculations beyond the current state-of-the-art.
Yu Liu, Ming-Guang Hu, Matthew A. Nichols, Dongzheng Yang, Daiqian Xie, Hua Guo, Kang-Kuen Ni Journal reference: Nature 593, 379-384 (2021) [pdf] DOI: 10.1038/s41586-021-03459-6
-
Electronic properties of InAs/EuS/Al hybrid nanowires -
Abstract
- We study the electronic properties of InAs/EuS/Al heterostructures as explored in a recent experiment [S. Vaitiekenas \emph{et al.}, Nat. Phys. (2020)], combining both spectroscopic results and microscopic device simulations. In particular, we use angle-resolved photoemission spectroscopy to investigate the band bending at the InAs/EuS interface. The resulting band offset value serves as an essential input to subsequent microscopic device simulations, allowing us to map the electronic wave function distribution. We conclude that the magnetic proximity effects at the Al/EuS as well as the InAs/EuS interfaces are both essential to achieve topological superconductivity at zero applied magnetic field. Mapping the topological phase diagram as a function of gate voltages and proximity-induced exchange couplings, we show that the ferromagnetic hybrid nanowire with overlapping Al and EuS layers can become a topological superconductor within realistic parameter regimes, and that the topological phase can be optimized by external gating. Our work highlights the need for a combined experimental and theoretical effort for faithful device simulation.
Chun-Xiao Liu, Sergej Schuwalow, Yu Liu, Kostas Vilkelis, A. L. R. Manesco, P. Krogstrup, Michael Wimmer Journal reference: Phys. Rev. B 104, 014516 (2021) [pdf] DOI: 10.1103/PhysRevB.104.014516
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Energy-level attraction and heating-resistant cooling of mechanical resonators with exceptional points -
Abstract
- We study the energy-level evolution and ground-state cooling of mechanical resonators under a synthetic phononic gauge field. The tunable gauge phase is mediated by the phase difference between the $\mathcal{PT}$- and anti-$\mathcal{PT}$-symmetric mechanical couplings in a multimode optomechanical system. The transmission spectrum then exhibits the asymmetric Fano line shape or double optomechanically induced transparency by modulating the gauge phase. Moreover, the eigenvalues will collapse and become degenerate although the mechanical coupling is continuously increased. Such counterintuitive energy-attraction, instead of anti-crossing, attributes to destructive interferences between $\mathcal{PT}$- and anti-$\mathcal{PT}$-symmetric couplings. We find that the energy-attraction, as well as the accompanied exceptional points (EPs), can be more intuitively observed in the cavity output power spectrum where the mechanical eigenvalues correspond to the peaks. For mechanical cooling, the average phonon occupation number becomes minimum at these EPs. Especially, phonon transport becomes nonreciprocal and even ideally unidirectional at the EPs. Finally, we propose a heating-resistant ground-state cooling based on the nonreciprocal phonon transport, which is mediated by the gauge field. Towards the quantum regime of macroscopic mechanical resonators, most optomechanical systems are ultimately limited by their intrinsic cavity or mechanical heating. Our work revealed that the thermal energy transfer can be blocked by tuning the gauge phase, which supports a promising route to overpass the notorious heating limitations.
Cheng Jiang, Yu-Long Liu, Mika A. Sillanpää Journal reference: Phys. Rev. A 104, 013502 (2021) [pdf] DOI: 10.1103/PhysRevA.104.013502
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Nuclear spin conservation enables state-to-state control of ultracold molecular reactions -
Abstract
- Quantum control of reactive systems has enabled microscopic probes of underlying interaction potentials, the opening of novel reaction pathways, and the alteration of reaction rates using quantum statistics. However, extending such control to the quantum states of reaction outcomes remains challenging. In this work, we realize this goal through the nuclear spin degree of freedom, a result which relies on the conservation of nuclear spins throughout the reaction. Using resonance-enhanced multiphoton ionization spectroscopy to investigate the products formed in bimolecular reactions between ultracold KRb molecules, we find that the system retains a near-perfect memory of the reactants' nuclear spins, manifested as a strong parity preference for the rotational states of the products. We leverage this effect to alter the occupation of these product states by changing the coherent superposition of initial nuclear spin states with an external magnetic field. In this way, we are able to control both the inputs and outputs of a bimolecular reaction with quantum state resolution. The techniques demonstrated here open up the possibilities to study quantum interference between reaction pathways, quantum entanglement between reaction products, and ultracold reaction dynamics at the state-to-state level.
Ming-Guang Hu, Yu Liu, Matthew A. Nichols, Lingbang Zhu, Goulven Quéméner, Olivier Dulieu, Kang-Kuen Ni Journal reference: Nat. Chem. 13, 435 (2021) [pdf] DOI: 10.1038/s41557-020-00610-0
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A gridded establishment dataset as a proxy for economic activity in China -
Abstract
- Measuring the geographical distribution of economic activity plays a key role in scientific research and policymaking. However, previous studies and data on economic activity either have a coarse spatial resolution or cover a limited time span, and the high-resolution characteristics of socioeconomic dynamics are largely unknown. Here, we construct a dataset on the economic activity of mainland China, the gridded establishment dataset (GED), which measures the volume of establishments at a 0.01$^{\circ}$ latitude by 0.01$^{\circ}$ longitude scale. Specifically, our dataset captures the geographically based opening and closing of approximately 25.5 million firms that registered in mainland China over the period 2005-2015. The characteristics of fine granularity and long-term observability give the GED a high application value. The dataset not only allows us to quantify the spatiotemporal patterns of the establishments, urban vibrancy and socioeconomic activity, but also helps us uncover the fundamental principles underlying the dynamics of industrial and economic development.
Lei Dong, Xiaohui Yuan, Meng Li, Carlo Ratti, Yu Liu Journal reference: Sci Data 8, 5 (2021) [pdf] DOI: 10.1038/s41597-020-00792-9
-
Mass dependence of pseudocritical temperature in mean field approximation -
Abstract
- We restrict our computation in the mean field approximation which could lead to a clear critical behavior. We analyze the scaling behavior with different shape of interaction kernel by considering different dressed-gluon models. The critical exponent we obtained is consistent with that in the $3D$ $\textrm{O}(4)$ universality class. The size of critical region is up to $m_{0}^{} \le 2\sim 4\;$MeV in this mean field approximation which sets naturally an upper bound of the critical region since the fluctuations beyond mean-field usually diminish the critical region. Besides, we analyze the possible percentage of the maximum chiral susceptibility and pion mass range at which the chiral phase transition temperature is independent of the current quark mass. The results show that the percentage and the pion mass range depend on the details of interaction kernel, which differs in gluon models.
Zhan Bai, Lei Chang, Jingyi Chao, Fei Gao, Yu-xin Liu Journal reference: Phys. Rev. D 104, 014005 (2021) [pdf] DOI: 10.1103/PhysRevD.104.014005
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From $n$-exangulated categories to $n$-abelian categories -
Abstract
- Herschend-Liu-Nakaoka introduced the notion of $n$-exangulated categories. It is not only a higher dimensional analogue of extriangulated categories defined by Nakaoka-Palu, but also gives a simultaneous generalization of $n$-exact categories in the sense of Jasso and $(n+2)$-angulated in the sense of Geiss-Keller-Oppermann. Let $\mathscr C$ be an $n$-exangulated category with enough projectives and enough injectives, and $\mathscr X$ a cluster tilting subcategory of $\mathscr C$. In this article, we show that the quotient category $\mathscr C/\mathscr X$ is an $n$-abelian category. This extends a result of Zhou-Zhu for $(n+2)$-angulated categories. Moreover, it highlights new phenomena when it is applied to $n$-exact categories.
Yu Liu, Panyue Zhou [pdf]
-
Identification of time-varying signals in quantum systems -
Abstract
- The identification of time-varying \textit{in situ} signals is crucial for characterizing the dynamics of quantum processes occurring in highly isolated environments. Under certain circumstances, they can be identified from time-resolved measurements via Ramsey interferometry experiments, but only with very special probe systems can the signals be explicitly read out, and a theoretical analysis is lacking on whether the measurement data are sufficient for unambiguous identification. In this paper, we formulate this problem as the invertibility of the underlying quantum input-output system, and derive the algebraic identifiability criterion and the algorithm for numerically identifying the signals. The criterion and algorithm can be applied to both closed and open quantum systems, and their effectiveness is demonstrated by numerical examples.
Xi Cao, Yu-xi Liu, Rebing Wu Journal reference: Phys. Rev. A 103, 022612 (2021) [pdf] DOI: 10.1103/PhysRevA.103.022612
-
Gorenstein dimension of abelian categories -
Abstract
- Let C be triangulated category and X a cluster tilting subcategory of C. Koenig and Zhu showed that the quotient category C/X is Gorenstein of Gorenstein dimension at most one. The notion of an extriangulated category was introduced by Nakaoka and Palu as a simultaneous generalization of exact categories and triangulated categories. Now let C be extriangulated category with enough projectives and enough injectives, and X a cluster tilting subcategory of C. In this article, we show that under certain conditions the quotient category C/X is Gorenstein of Gorenstein dimension at most one. As an application, this result generalizes work by Koenig and Zhu.
Yu Liu, Panyue Zhou [pdf]
-
Characteristic interaction potential of black hole molecules from the
microscopic interpretation of Ruppeiner geometry -
Abstract
- 2020
-
Covariant spin kinetic theory I: collisionless limit -
Abstract
- We develop a covariant kinetic theory for massive fermions in curved spacetime and external electromagnetic field based on quantum field theory. We derive four coupled semi-classical kinetic equations accurate at $O(\hbar)$, which describe the transports of particle number and spin degrees of freedom. The relation with the chiral kinetic theory is discussed. As an application, we study the spin polarization in the presence of finite Riemann curvature and electromagnetic field in both local and global equilibrium states.
Yu-Chen Liu, Kazuya Mameda, Xu-Guang Huang Journal reference: Chin. Phys. C 44, 094101 (2020) [pdf] DOI: 10.1088/1674-1137/44/9/094101
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Prediction of spin polarized Fermi arcs in quasiparticle interference in CeBi -
Abstract
- We predict that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path $(0, 0, k_z)$ at $15$ meV} above and $100$ meV below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at $E=15$ meV and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin-polarized but in opposite directions, which can be detected by spin-polarized ARPES measurements. Our theoretical study of quasiparticle interference (QPI) for a nonmagnetic impurity at the Bi site also reveals several features related to the Fermi arcs. Specifically, we predict that the spin polarization of the Fermi arcs leads to a bifurcation-shaped feature only in the spin-dependent QPI spectrum, serving as a fingerprint of the Weyl nodes.
Zhao Huang, Christopher A. Lane, Chao Cao, Guo-Xiang Zhi, Yu Liu, Christian Matt, Brinda Kuthanazhi, Paul C. Canfield, Dmitry Yarotski, A. J. Taylor, Jian-Xin Zhu Journal reference: Phys. Rev. B 102, 235167 (2020) [pdf] DOI: 10.1103/PhysRevB.102.235167
-
Simulation and Control of Deformable Autonomous Airships in Turbulent
Wind -
Abstract
- Abstract. Fixed wing and multirotor UAVs are common in the field of robotics. Solutions for simulation and control of these vehicles are ubiquitous. This is not the case for airships, a simulation of which needs to address unique properties, i) dynamic deformation in response to aerodynamic and control forces, ii) high susceptibility to wind and turbulence at low airspeed, iii) high variability in airship designs regarding placement, direction and vectoring of thrusters and control surfaces. We present a flexible framework for modeling, simulation and control of airships, based on the Robot operating system (ROS), simulation environment (Gazebo) and commercial off the shelf (COTS) electronics, both of which are open source. Based on simulated wind and deformation, we predict substantial effects on controllability, verified in real world flight experiments. All our code is shared as open source, for the benefit of the community and to facilitate lighter-than-air vehicle (LTAV) research. https://github.com/robot-perception-group/airship_simulation
Eric Price, Yu Tang Liu, Michael J. Black, Aamir Ahmad [pdf]
-
Universal Urban Spreading Pattern of COVID-19 and Its Underlying
Mechanism -
Abstract
- Currently, the global situation of COVID-19 is aggravating, pressingly calling for efficient control and prevention measures. Understanding spreading pattern of COVID-19 has been widely recognized as a vital step for implementing non-pharmaceutical measures. Previous studies investigated such an issue in large-scale (e.g., inter-country or inter-state) scenarios while urban spreading pattern still remains an open issue. Here, we fill this gap by leveraging the trajectory data of 197,808 smartphone users (including 17,808 anonymous confirmed cases) in 9 cities in China. We find a universal spreading pattern in all cities: the spatial distribution of confirmed cases follows a power-law-like model and the spreading centroid is time-invariant. Moreover, we reveal that human mobility in a city drives the spatialtemporal spreading process: long average travelling distance results in a high growth rate of spreading radius and wide spatial diffusion of confirmed cases. With such insight, we adopt Kendall model to simulate urban spreading of COVID-19 that can well fit the real spreading process. Our results unveil the underlying mechanism behind the spatial-temporal urban evolution of COVID-19, and can be used to evaluate the performance of mobility restriction policies implemented by many governments and to estimate the evolving spreading situation of COVID-19.
-
Dynamic State Estimation for Power System Control and Protection -
Abstract
- Dynamic state estimation (DSE) accurately tracks the dynamics of a power system and provides the evolution of the system state in real-time. This paper focuses on the control and protection applications of DSE, comprehensively presenting different facets of control and protection challenges arising in modern power systems. It is demonstrated how these challenges are effectively addressed with DSE-enabled solutions. As precursors to these solutions, reformulation of DSE considering both synchrophasor and sampled value measurements and comprehensive comparisons of DSE and observers have been presented. The usefulness and necessity of DSE based solutions in ensuring system stability, reliable protection and security, and resilience by revamping of control and protection methods are shown through examples, practical applications, and suggestions for further development.
-
Spin-polarized imaging of strongly interacting fermions in the
ferrimagnetic state of Weyl candidate CeBi -
Abstract
- CeBi has an intricate magnetic phase diagram whose fully-polarized state has recently been suggested as a Weyl semimetal, though the role of $f$ states in promoting strong interactions has remained elusive. Here we focus on the less-studied, but also time-reversal symmetry-breaking ferrimagnetic phase of CeBi, where our density functional theory (DFT) calculations predict additional Weyl nodes near the Fermi level $E_\mathrm{F}$. We use spin-polarized scanning tunneling microscopy and spectroscopy to image the surface ferrimagnetic order on the itinerant Bi $p$ states, indicating their orbital hybridization with localized Ce $f$ states. We observe suppression of this spin-polarized signature at $E_\mathrm{F}$, coincident with a Fano line shape in the conductance spectra, suggesting the Bi $p$ states partially Kondo screen the $f$ magnetic moments, and this $p-f$ hybridization causes strong Fermi-level band renormalization. The $p$ band flattening is supported by our quasiparticle interference (QPI) measurements, which also show band splitting in agreement with DFT, painting a consistent picture of a strongly interacting magnetic Weyl semimetal.
Christian E. Matt, Yu Liu, Harris Pirie, Nathan C. Drucker, Na Hyun Jo, Brinda Kuthanazhi, Zhao Huang, Christopher Lane, Jian-Xin Zhu, Paul C. Canfield, Jennifer E. Hoffman [pdf]
-
Fine-Grained 3D Shape Classification With Hierarchical Part-View Attention -
Abstract
- Fine-grained 3D shape classification is important for shape understanding and analysis, which poses a challenging research problem. However, the studies on the fine-grained 3D shape classification have rarely been explored, due to the lack of fine-grained 3D shape benchmarks. To address this issue, we first introduce a new 3D shape dataset (named FG3D dataset) with fine-grained class labels, which consists of three categories including airplane, car and chair. Each category consists of several subcategories at a fine-grained level. According to our experiments under this fine-grained dataset, we find that state-of-the-art methods are significantly limited by the small variance among subcategories in the same category. To resolve this problem, we further propose a novel fine-grained 3D shape classification method named FG3D-Net to capture the fine-grained local details of 3D shapes from multiple rendered views. Specifically, we first train a Region Proposal Network (RPN) to detect the generally semantic parts inside multiple views under the benchmark of generally semantic part detection. Then, we design a hierarchical part-view attention aggregation module to learn a global shape representation by aggregating generally semantic part features, which preserves the local details of 3D shapes. The part-view attention module hierarchically leverages part-level and view-level attention to increase the discriminability of our features. The part-level attention highlights the important parts in each view while the view-level attention highlights the discriminative views among all the views of the same object. In addition, we integrate a Recurrent Neural Network (RNN) to capture the spatial relationships among sequential views from different viewpoints. Our results under the fine-grained 3D shape dataset show that our method outperforms other state-of-the-art methods.
Xinhai Liu, Zhizhong Han, Yu-Shen Liu, Matthias Zwicker [pdf] DOI: 10.1109/TIP.2020.3048623 2005.12541v2 [pdf]
-
Tensor perturbations and thick branes in higher-dimensional f(R) gravity -
Abstract
- We study brane worlds in an anisotropic higher-dimensional spacetime within the context of $f(R)$ gravity. Firstly, we demonstrate that this spacetime with a concrete metric ansatz is stable against linear tensor perturbations under certain conditions. Moreover, the Kaluza-Klein modes of the graviton are analyzed. Secondly, we investigate thick brane solutions in six dimensions and their properties. We further exhibit two sets of solutions for thick branes. At last, the effective potential of the Kaluza-Klein modes of the graviton is discussed for the two solved $f(R)$ models in higher dimensions.
Zheng-Quan Cui, Zi-Chao Lin, Jun-Jie Wan, Yu-Xiao Liu, Li Zhao Journal reference: JHEP 12 (2020) 130 [pdf] DOI: 10.1007/JHEP12(2020)130
-
A Semi-Lagrangian Computation of Front Speeds of G-equation in ABC and
Kolmogorov Flows with Estimation via Ballistic Orbits -
Abstract
- The Arnold-Beltrami-Childress (ABC) flow and the Kolmogorov flow are three dimensional periodic divergence free velocity fields that exhibit chaotic streamlines. We are interested in front speed enhancement in G-equation of turbulent combustion by large intensity ABC and Kolmogorov flows. First, we give a quantitative construction of the ballistic orbits of ABC and Kolmogorov flows, namely those with maximal large time asymptotic speeds in a coordinate direction. Thanks to the optimal control theory of G-equation (a convex but non-coercive Hamilton-Jacobi equation), the ballistic orbits serve as admissible trajectories for front speed estimates. To study the tightness of the estimates, we compute front speeds of G-equation based on a semi-Lagrangian (SL) scheme with Strang splitting and weighted essentially non-oscillatory (WENO) interpolation. The SL scheme is stable when the ratio of time step and spatial grid size (in the propagation direction) is smaller than a positive constant independent of the flow intensity. Numerical results show that the front speed growth rate in terms of the flow intensity may approach the analytical bounds from the ballistic orbits.
Chou Kao, Yu-Yu Liu, Jack Xin [pdf]
-
Gaussian Estimates for Heat Kernels of Higher Order Schrödinger
Operators with Potentials in Generalized Schechter Classes -
Abstract
- Let $m\in\mathbb N$, $P(D):=\sum_{|\alpha|=2m}(-1)^m a_\alpha D^\alpha$ be a $2m$-order homogeneous elliptic operator with real constant coefficients on $\mathbb{R}^n$, and $V$ a measurable function on $\mathbb{R}^n$. In this article, the authors introduce a new generalized Schechter class concerning $V$ and show that the higher order Schr\"odinger operator $\mathcal{L}:=P(D)+V$ possesses a heat kernel that satisfies the Gaussian upper bound and the H\"older regularity when $V$ belongs to this new class. The Davies--Gaffney estimates for the associated semigroup and their local versions are also given. These results pave the way for many further studies on the analysis of $\mathcal{L}$.
Jun Cao, Yu Liu, Dachun Yang, Chao Zhang [pdf]
-
Hereditary cotorsion pairs on extriangulated subcategories -
Abstract
- Let $\mathcal B$ be an extriangulated category with enough projectives and enough injectives. We define a proper $m$-term subcategory $\mathcal G$ on $\mathcal B$, which is an extriangulated subcategory. Then we give a correspondence between cotorsion pairs on $\mathcal G$, support $\tau$-tilting subcategories on an abelian quotient of $\mathcal G$ when $m=2$. If such $\mathcal G$ is induced by a hereditary cotorsion pair, then we give a correspondence between cotorsion pairs on $\mathcal G$ and intermediate cotorsion pairs on $\mathcal B$ under certain assumptions. At last, we study an important property of such extriangulated subcategory $\mathcal G$.
Yu Liu, Panyue Zhou [pdf]
-
High Fermi velocities and small cyclotron masses in LaAlGe -
Abstract
- We report quantum oscillation measurements of LaAlGe, a Lorentz-violating type-II Weyl semimetal with tilted Weyl cones. Very small quasiparticle masses and very high Fermi velocities were detected at the Fermi surface. Whereas three main frequencies have been observed, angular dependence of two Fermi surface sheets indicates possible two-dimensional (2D) character despite the absence of the 2D structural features such as van der Waals bonds. Such conducting states may offer a good platform for low-dimensional polarized spin current in magnetic RAlGe (R = Ce, Pr) materials.
Zhixiang Hu, Qianheng Du, Yu Liu, D. Graf, C. Petrovic Journal reference: Applied Physics Letters 117, 224410 (2020) - Editor's pick scilight [pdf] DOI: 10.1063/5.0035445
-
SPU-Net: Self-Supervised Point Cloud Upsampling by Coarse-to-Fine
Reconstruction with Self-Projection Optimization -
Abstract
- The task of point cloud upsampling aims to acquire dense and uniform point sets from sparse and irregular point sets. Although significant progress has been made with deep learning models, they require ground-truth dense point sets as the supervision information, which can only trained on synthetic paired training data and are not suitable for training under real-scanned sparse data. However, it is expensive and tedious to obtain large scale paired sparse-dense point sets for training from real scanned sparse data. To address this problem, we propose a self-supervised point cloud upsampling network, named SPU-Net, to capture the inherent upsampling patterns of points lying on the underlying object surface. Specifically, we propose a coarse-to-fine reconstruction framework, which contains two main components: point feature extraction and point feature expansion, respectively. In the point feature extraction, we integrate self-attention module with graph convolution network (GCN) to simultaneously capture context information inside and among local regions. In the point feature expansion, we introduce a hierarchically learnable folding strategy to generate the upsampled point sets with learnable 2D grids. Moreover, to further optimize the noisy points in the generated point sets, we propose a novel self-projection optimization associated with uniform and reconstruction terms, as a joint loss, to facilitate the self-supervised point cloud upsampling. We conduct various experiments on both synthetic and real-scanned datasets, and the results demonstrate that we achieve comparable performance to the state-of-the-art supervised methods.
Xinhai Liu, Xinchen Liu, Zhizhong Han, Yu-Shen Liu [pdf]
-
Fast Accurate Beam and Channel Tracking for Two-Dimensional Phased Antenna Arrays -
Abstract
- The sparsity and the severe attenuation of millimeter-wave (mmWave) channel imply that highly directional communication is needed. The narrow beam produced by large array requires accurate alignment, which is difficult to achieve when serving fast-moving users. In this paper, we focus on accurate two-dimensional (2D) beam and channel tracking problem aiming at minimizing exploration overhead and tracking error. Using a typical frame structure with periodic exploration and communication, a proven minimum overhead of exploration is provided first. Then tracking algorithms are designed for three types of channels with different dynamic properties. It is proved that the algorithms for quasi-static channels and channels in Dynamic Case I are optimal in approaching the minimum Cramer-Rao lower bound (CRLB). The computational complexity of our algorithms is analyzed showing their efficiency, and simulation results verify their advantages in both tracking error and tracking speed.
Yu Liu, Jiahui Li, Xiujun Zhang, Shidong Zhou Journal reference: in IEEE Access, vol. 8, pp. 209844-209877, 2020 [pdf] DOI: 10.1109/ACCESS.2020.3038699
-
Overcharging a Reissner-Nordstrom Taub-NUT regular black hole -
Abstract
- The destruction of a regular black hole event horizon might provide us the possibility to access regions inside black hole event horizon. This paper investigates the possibility of overcharging a charged Taub-NUT regular black hole via the scattering of a charged field and the absorption of a charged particle, respectively. For the charged scalar field scattering, both the near-extremal and extremal charged Taub-NUT regular black holes cannot be overcharged. For the test charged particle absorption, the result shows that the event horizon of the extremal charged Taub-NUT regular black hole still exists while the event horizon of the near-extremal one can be destroyed. However, if the charge and energy cross the event horizon in a continuous path, the near-extremal charged Taub-NUT regular black hole might not be overcharged.
Wen-Bin Feng, Si-Jiang Yang, Qin Tan, Jie Yang, Yu-Xiao Liu [pdf]
-
Understanding the mesoscopic scaling patterns within cities -
Abstract
- Understanding quantitative relationships between urban elements is crucial for a wide range of applications. The observation at the macroscopic level demonstrates that the aggregated urban quantities (e.g., gross domestic product) scale systematically with population sizes across cities, also known as urban scaling laws. However, at the mesoscopic level, we lack an understanding of whether the simple scaling relationship holds within cities, which is a fundamental question regarding the spatial origin of scaling in urban systems. Here, by analyzing four extensive datasets covering millions of mobile phone users and urban facilities, we investigate the scaling phenomena within cities. We find that the mesoscopic infrastructure volume and socioeconomic activity scale sub- and super-linearly with the active population, respectively. For a same scaling phenomenon, however, the exponents vary in cities of similar population sizes. To explain these empirical observations, we propose a conceptual framework by considering the heterogeneous distributions of population and facilities, and the spatial interactions between them. Analytical and numerical results suggest that, despite the large number of complexities that influence urban activities, the simple interaction rules can effectively explain the observed regularity and heterogeneity in scaling behaviors within cities.
Lei Dong, Zhou Huang, Jiang Zhang, Yu Liu Journal reference: Sci Rep 10, 21201 (2020) [pdf] DOI: 10.1038/s41598-020-78135-2
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End-To-End Quantum Machine Learning Implemented with Controlled Quantum Dynamics -
Abstract
- Toward quantum machine learning deployed on imperfect near-term intermediate-scale quantum (NISQ) processors, the entire physical implementation of should include as less as possible hand-designed modules with only a few ad-hoc parameters to be determined. This work presents such a hardware-friendly end-to-end quantum machine learning scheme that can be implemented with imperfect near-term intermediate-scale quantum (NISQ) processors. The proposal transforms the machine learning task to the optimization of controlled quantum dynamics, in which the learning model is parameterized by experimentally tunable control variables. Our design also enables automated feature selection by encoding the raw input to quantum states through agent control variables. Comparing with the gate-based parameterized quantum circuits, the proposed end-to-end quantum learning model is easy to implement as there are only few ad-hoc parameters to be determined. Numerical simulations on the benchmarking MNIST dataset demonstrate that the model can achieve high performance using only 3-5 qubits without downsizing the dataset, which shows great potential for accomplishing large-scale real-world learning tasks on NISQ processors.arning models. The scheme is promising for efficiently performing large-scale real-world learning tasks using intermediate-scale quantum processors.
Re-Bing Wu, Xi Cao, Pinchen Xie, Yu-xi Liu Journal reference: Phys. Rev. Applied 14, 064020 (2020) [pdf] DOI: 10.1103/PhysRevApplied.14.064020
-
Air temperature and humidity during the solar eclipses of 26 December
2019 and of 21 June 2020 in Saudi Arabia and in other eclipses with similar
environments -
Abstract
- We report air temperature and humidity changes during the two solar eclipses of 26 December 2019, and of 21 June 2020, respectively, in the cities of Al-Hofuf and Riyadh in Saudi Arabia. During the December eclipse the Sun rose already eclipsed (91.53% of the area covered) while the June eclipse, although also annular in other places of the Arabian Peninsula, was just partial at Riyadh (area covered 72.80%). This difference apparently affected the observed response on the recorded variables of temperature, relative humidity (RH) and vapor pressure (VP) in the two events. Change in these variables went unnoticed for the first eclipse since it was within the natural variability of the day; yet for the other, they showed clearly some trend alterations, which we analyze and discuss. A decrease in temperature of 3.2 {\deg}C was detected in Riyadh; however, RH and VP showed an oscillation that we explain in the light of a similar effect reported in other eclipses. We found a time lag of about 15 min measured from the eclipse central phase in this city. We made an inspection of related fluctuations and dynamics from the computed rates of the temporal variation of temperature and RH. Trying to identify the influence of solar eclipses in similar environments we have made a broad inter-comparison with other observations of these variables in the Near East, northern Africa and in the United States. We compare our results with results obtained by other authors working with the December eclipse but in the United Arab Emirates and Oman, which showed dissimilar results. These inter-comparisons show how effectively the lower atmosphere can respond to a solar eclipse within a desert environment and others similar. As a preamble, a historical revision of temperature and humidity in the context of eclipse meteorology is also included.
Marcos A. Penaloza-Murillo, Abouazza Elmhamdi, Jay M. Pasachoff, Michael T. Roman, Yu Liu, Z. A. Al-Mostafa, A. H. Maghrabi, H. A. Al-Trabulsy [pdf]
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Equivalence of solutions between the four-dimensional novel and regularized EGB theories in a cylindrically symmetric spacetime -
Abstract
- Recently, a novel four-dimensional Einstein-Gauss-Bonnet (EGB) theory was presented to bypass the Lovelock's theorem and to give nontrivial effects on the four-dimensional local gravity. The main mechanism is to introduce a redefinition $\alpha\rightarrow\alpha/(D-4)$ and to take the limit $D\rightarrow4$. However, this theory does not have standard four-dimensional field equations. Some regularization procedures are then proposed to address this problem [arXiv:2003.11552, arXiv:2003.12771, arXiv:2004.08362, arXiv:2004.09472, arXiv:2004.10716]. The resultant regularized four-dimensional EGB theory has the same on-shell action as the original theory. Thus it is expected that the novel four-dimensional EGB theory is equivalent to its regularized version. However, the equivalence of these two theories is symmetry-dependent. In this paper, we test the equivalence in a cylindrically symmetric spacetime. The well-defined field equations of the two theories are obtained, with which our follow-up analysis shows that they are equivalent in such spacetime. Cylindrical cosmic strings are then considered as specific examples of the metric. Three sets of solutions are obtained and the corresponding string mass densities are evaluated. The results reveal how the Gauss-Bonnet term in four dimensions contributes to the string geometry in the new theory.
Zi-Chao Lin, Ke Yang, Shao-Wen Wei, Yong-Qiang Wang, Yu-Xiao Liu Journal reference: Eur. Phys. J. C 80, 1033 (2020) [pdf] DOI: 10.1140/epjc/s10052-020-08612-5
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Desires and Motivation: The Computational Rule, the Underlying Neural
Circuitry, and the Relevant Clinical Disorders -
Abstract
- As organism is a dissipative system. The process from multi desires to exclusive motivation is of great importance among all sensory-action loops. In this paper we argued that a proper Desire-Motivation model should be a continuous dynamic mapping from the dynamic desire vector to the sparse motivation vector. Meanwhile, it should at least have specific stability and adjustability of motivation intensity. Besides, the neuroscience evidences suggest that the Desire-Motivation model should have dynamic information acquisition and should be a recurrent neural network. A five-equation model is built based on the above arguments, namely the Recurrent Gating Desire-Motivation (RGDM) model. Additionally, a heuristic speculation based on the RGDM model about corresponding brain regions is carried out. It believes that the tonic and phasic firing of ventral tegmental area dopamine neurons should execute the respective and collective feedback functions of recurrent processing. The analysis about the RGMD model shows the expectations about individual personality from three dimensions, namely stability, intensity, and motivation decision speed. These three dimensions can be combined and create eight different personalities, which is correspondent to Jung's personality structure theorem. Furthermore, the RGDM model can be used to predict three different brand-new types of depressive disorder with different phenotypes. Moreover, it can also explain several other psychiatry disorders from new perspectives.
Yu Liu, Yinghong Zhao, Mo Chen [pdf]
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DASNet: Dual Attentive Fully Convolutional Siamese Networks for Change Detection in High-Resolution Satellite Images -
Abstract
- Change detection is a basic task of remote sensing image processing. The research objective is to identity the change information of interest and filter out the irrelevant change information as interference factors. Recently, the rise of deep learning has provided new tools for change detection, which have yielded impressive results. However, the available methods focus mainly on the difference information between multitemporal remote sensing images and lack robustness to pseudo-change information. To overcome the lack of resistance of current methods to pseudo-changes, in this paper, we propose a new method, namely, dual attentive fully convolutional Siamese networks (DASNet) for change detection in high-resolution images. Through the dual-attention mechanism, long-range dependencies are captured to obtain more discriminant feature representations to enhance the recognition performance of the model. Moreover, the imbalanced sample is a serious problem in change detection, i.e. unchanged samples are much more than changed samples, which is one of the main reasons resulting in pseudo-changes. We put forward the weighted double margin contrastive loss to address this problem by punishing the attention to unchanged feature pairs and increase attention to changed feature pairs. The experimental results of our method on the change detection dataset (CDD) and the building change detection dataset (BCDD) demonstrate that compared with other baseline methods, the proposed method realizes maximum improvements of 2.1\% and 3.6\%, respectively, in the F1 score. Our Pytorch implementation is available at https://github.com/lehaifeng/DASNet.
Jie Chen, Ziyang Yuan, Jian Peng, Li Chen, Haozhe Huang, Jiawei Zhu, Yu Liu, Haifeng Li Journal reference: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 2020 [pdf] DOI: 10.1109/JSTARS.2020.3037893
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Transverse-momentum-dependent parton distribution functions from large-momentum effective theory -
Abstract
- We show that transverse-momentum-dependent parton distribution functions (TMDPDFs), important non-perturbative quantities for describing the properties of hadrons in high-energy scattering processes such as Drell-Yan and semi-inclusive deep-inelastic scattering with observed small transverse momentum, can be obtained from Euclidean QCD calculations in the framework of large-momentum effective theory (LaMET). We present a LaMET factorization of the Euclidean quasi-TMDPDFs in terms of the physical TMDPDFs and off-light-cone soft function at leading order in $1/P^z$ expansion, with the perturbative matching coefficient satisfying a renormalization group equation. We also discuss the implementation in lattice QCD with finite-length gauge links as well as the rapidity-regularization-independent factorization for Drell-Yan cross section.
Xiangdong Ji, Yizhuang Liu, Yu-Sheng Liu [pdf] DOI: 10.1016/j.physletb.2020.135946 1911.03840v2 [pdf]
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Novel dual relation and constant in Hawking-Page phase transitions -
Abstract
- Universal relations and constants have important applications in understanding a physical theory. In this article, we explore this issue for Hawking-Page phase transitions in Schwarzschild anti-de Sitter black holes. We find a novel exact dual relation between the minimum temperature of the ($d$+1)-dimensional black hole and the Hawking-Page phase transition temperature in $d$ dimensions, reminiscent of the holographic principle. Furthermore, we find that the normalized Ruppeiner scalar curvature is a universal constant at the Hawking-Page transition point. Since the Ruppeiner curvature can be treated as an indicator of the intensity of the interactions amongst black hole microstructures, we conjecture that this universal constant denotes an interaction threshold, beyond which a virtual black hole becomes a real one. This new dual relation and universal constant are fundamental in understanding Hawking-Page phase transitions, and might have new important applications in the black hole physics in the near future.
Shao-Wen Wei, Yu-Xiao Liu, Robert B. Mann Journal reference: Phys. Rev. D 102, 104011 (2020) [pdf] DOI: 10.1103/PhysRevD.102.104011
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Visual Localization Under Appearance Change: Filtering Approaches -
Abstract
- A major focus of current research on place recognition is visual localization for autonomous driving. In this scenario, as cameras will be operating continuously, it is realistic to expect videos as an input to visual localization algorithms, as opposed to the single-image querying approach used in other visual localization works. In this paper, we show that exploiting temporal continuity in the testing sequence significantly improves visual localization - qualitatively and quantitatively. Although intuitive, this idea has not been fully explored in recent works. To this end, we propose two filtering approaches to exploit the temporal smoothness of image sequences: i) filtering on discrete domain with Hidden Markov Model, and ii) filtering on continuous domain with Monte Carlo-based visual localization. Our approaches rely on local features with an encoding technique to represent an image as a single vector. The experimental results on synthetic and real datasets show that our proposed methods achieve better results than state of the art (i.e., deep learning-based pose regression approaches) for the task on visual localization under significant appearance change. Our synthetic dataset and source code are made publicly available at https://sites.google.com/view/g2d-software/home and https://github.com/dadung/Visual-Localization-Filtering.
Anh-Dzung Doan, Yasir Latif, Tat-Jun Chin, Yu Liu, Shin-Fang Ch'ng, Thanh-Toan Do, Ian Reid [pdf]
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A Search for the Guest Star Associated with Swift J1818-5937 -
Abstract
- We searched the possible historical records for the young magnetar Swift J1818.0-1607, and found a guest star in AD 1798 that might be associated with it.
Yu Liu, Yuan-Chuan Zou [pdf] DOI: 10.3847/2515-5172/abbad9 2009.12806v2 [pdf]
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$τ$-tilting theory in abelian categories -
Abstract
- Let $\mathcal A$ be a Hom-finite abelian category with enough projectives. In this note, we show that any covariantly finite $\tau$-rigid subcategory is contained in a support $\tau$-tilting subcategory. We also show that support $\tau$-tilting subcategories are in bijection with certain finitely generated torsion classes. Some applications of our main results are also given.
Yu Liu, Panyue Zhou [pdf]
-
Explorable Tone Mapping Operators -
Abstract
- Tone-mapping plays an essential role in high dynamic range (HDR) imaging. It aims to preserve visual information of HDR images in a medium with a limited dynamic range. Although many works have been proposed to provide tone-mapped results from HDR images, most of them can only perform tone-mapping in a single pre-designed way. However, the subjectivity of tone-mapping quality varies from person to person, and the preference of tone-mapping style also differs from application to application. In this paper, a learning-based multimodal tone-mapping method is proposed, which not only achieves excellent visual quality but also explores the style diversity. Based on the framework of BicycleGAN, the proposed method can provide a variety of expert-level tone-mapped results by manipulating different latent codes. Finally, we show that the proposed method performs favorably against state-of-the-art tone-mapping algorithms both quantitatively and qualitatively.
Chien-Chuan Su, Ren Wang, Hung-Jin Lin, Yu-Lun Liu, Chia-Ping Chen, Yu-Lin Chang, Soo-Chang Pei [pdf]
-
On the Exploration of Incremental Learning for Fine-grained Image
Retrieval -
Abstract
- In this paper, we consider the problem of fine-grained image retrieval in an incremental setting, when new categories are added over time. On the one hand, repeatedly training the representation on the extended dataset is time-consuming. On the other hand, fine-tuning the learned representation only with the new classes leads to catastrophic forgetting. To this end, we propose an incremental learning method to mitigate retrieval performance degradation caused by the forgetting issue. Without accessing any samples of the original classes, the classifier of the original network provides soft "labels" to transfer knowledge to train the adaptive network, so as to preserve the previous capability for classification. More importantly, a regularization function based on Maximum Mean Discrepancy is devised to minimize the discrepancy of new classes features from the original network and the adaptive network, respectively. Extensive experiments on two datasets show that our method effectively mitigates the catastrophic forgetting on the original classes while achieving high performance on the new classes.
Wei Chen, Yu Liu, Weiping Wang, Tinne Tuytelaars, Erwin M. Bakker, Michael Lew [pdf]
-
Characterizing and Comparing COVID-19 Misinformation Across Languages,
Countries and Platforms -
Abstract
- Misinformation/disinformation about COVID-19 has been rampant on social media around the world. In this study, we investigate COVID-19 misinformation/ disinformation on social media in multiple languages - Farsi (Persian), Chinese, and English, about multiple countries - Iran, China, and the United States (US), and on multiple platforms such as Twitter, Facebook, Instagram, Weibo, and WhatsApp. Misinformation, especially about a global pandemic, is a global problem yet it is common for studies of COVID-19 misinformation on social media to focus on a single language, like English, a single country, like the US, or a single platform, like Twitter. We utilized opportunistic sampling to compile 200 specific items of viral and yet debunked misinformation across these languages, countries and platforms emerged between January 1 and August 31. We then categorized this collection based both on the topics of the misinformation and the underlying roots of that misinformation. Our multi-cultural and multilingual team observed that the nature of COVID-19 misinformation on social media varied in substantial ways across different languages/countries depending on the cultures, beliefs/religions, popularity of social media, types of platforms, freedom of speech and the power of people versus governments. We observe that politics is at the root of most of the collected misinformation across all three languages in this dataset. We further observe the different impact of government restrictions on platforms and platform restrictions on content in Iran, China, and the US and their impact on a key question of our age: how do we control misinformation without silencing the voices we need to hold governments accountable?
Golshan Madraki, Isabella Grasso, Jacqueline Otala, Yu Liu, Jeanna Matthews [pdf]
-
Lattice QCD Calculations of Transverse-Momentum-Dependent Soft Function through Large-Momentum Effective Theory -
Abstract
- The transverse-momentum-dependent (TMD) soft function is a key ingredient in QCD factorization of Drell-Yan and other processes with relatively small transverse momentum. We present a lattice QCD study of this function at moderately large rapidity on a 2+1 flavor CLS dynamic ensemble with $a=0.098$ fm. We extract the rapidity-independent (or intrinsic) part of the soft function through a large-momentum-transfer pseudo-scalar meson form factor and its quasi-TMD wave function using leading-order factorization in large-momentum effective theory. We also investigate the rapidity-dependent part of the soft function---the Collins-Soper evolution kernel---based on the large-momentum evolution of the quasi-TMD wave function.
Qi-An Zhang, Jun Hua, Yikai Huo, Xiangdong Ji, Yizhuang Liu, Yu-Sheng Liu, Maximilian Schlemmer, Andreas Schäfer, Peng Sun, Wei Wang, Yi-Bo Yang Journal reference: Phys. Rev. Lett. 125, 192001 (2020) [pdf] DOI: 10.1103/PhysRevLett.125.192001
-
Adaptive Subspace Sampling for Class Imbalance Processing-Some
clarifications, algorithm, and further investigation including applications
to Brain Computer Interface -
Abstract
- Kohonen's Adaptive Subspace Self-Organizing Map (ASSOM) learns several subspaces of the data where each subspace represents some invariant characteristics of the data. To deal with the imbalance classification problem, earlier we have proposed a method for oversampling the minority class using Kohonen's ASSOM. This investigation extends that study, clarifies some issues related to our earlier work, provides the algorithm for generation of the oversamples, applies the method on several benchmark data sets, and makes application to three Brain Computer Interface (BCI) applications. First we compare the performance of our method using some benchmark data sets with several state-of-the-art methods. Finally, we apply the ASSOM-based technique to analyze the three BCI based applications using electroencephalogram (EEG) datasets. These tasks are classification of motor imagery , drivers' fatigue states, and phases of migraine. Our results demonstrate the effectiveness of the ASSOM-based meth od in dealing with imbalance classification problem.
Chin-Teng Lin, Kuan-Chih Huang, Yu-Ting Liu, Yang-Yin Lin, Tsung-Yu Hsieh, Nikhil R. Pal, Shang-Lin Wu, Chieh-Ning Fang, Zehong Cao [pdf]
-
Most lithium-rich low-mass evolved stars revealed as red clump stars by asteroseismology and spectroscopy -
Abstract
- Lithium has confused scientists for decades at almost each scale of the universe. Lithium-rich giants are peculiar stars with lithium abundances over model prediction. A large fraction of lithium-rich low-mass evolved stars are traditionally supposed to be red giant branch (RGB) stars. Recent studies, however, report that red clump (RC) stars are more frequent than RGB. Here, we present a uniquely large systematic study combining the direct asteroseismic analysis with the spectroscopy on the lithium-rich stars. The majority of lithium-rich stars are confirmed to be RCs, whereas RGBs are minor. We reveal that the distribution of lithium-rich RGBs steeply decline with the increasing lithium abundance, showing an upper limit around 2.6 dex, whereas the Li abundances of RCs extend to much higher values. We also find that the distributions of mass and nitrogen abundance are notably different between RC and RGB stars. These findings indicate that there is still unknown process that significantly affects surface chemical composition in low-mass stellar evolution.
Hong-Liang Yan, Yu-Tao Zhou, Xianfei Zhang, Yaguang Li, Qi Gao, Jian-Rong Shi, Gang Zhao, Wako Aoki, Tadafumi Matsuno, Yan Li, Xiao-Dong Xu, Haining Li, Ya-Qian Wu, Meng-Qi Jin, Benoît Mosser, Shao-Lan Bi, Jian-Ning Fu, Kaike Pan, Takuma Suda, Yu-Juan Liu, Jing-Kun Zhao, Xi-Long Liang [pdf] DOI: 10.1038/s41550-020-01217-8 2010.02106v1 [pdf]
-
Magnetic mixed valent semimetal
-
Abstract
- We report discovery of new antiferromagnetic semimetal EuZnSb$_2$, obtained and studied in the form of single crystals. Electric resistivity, magnetic susceptibility and heat capacity indicate antiferromagnetic order of Eu with $T_N$ = 20 K. The effective moment of Eu$^{2+}$ inferred from the magnetization and specific heat measurement is 3.5 $\mu_B$, smaller than the theoretical value of Eu$^{2+}$ due to presence of both Eu$^{3+}$ and Eu$^{2+}$. Magnetic field-dependent resistivity measurements suggest dominant quasi two dimensional Fermi surfaces whereas the first-principle calculations point to the presence of Dirac fermions. Therefore, EuZnSb$_2$ could represent the first platform to study the interplay of dynamical charge fluctuations, localized magnetic 4$f$ moments and Dirac states with Sb orbital character.
Aifeng Wang, Sviatoslav Baranets, Yu Liu, Xiao Tong, E. Stavitski, Jing Zhang, Yisheng Chai, Wei-Guo Yin, Svilen Bobev, C. Petrovic Journal reference: Physical Review Research 2, 033462 (2020) [pdf] DOI: 10.1103/PhysRevResearch.2.033462
-
Quantum optimal control using phase-modulated driving fields -
Abstract
- Quantum optimal control represents a powerful technique to enhance the performance of quantum experiments by engineering the controllable parameters of the Hamiltonian. However, the computational overhead for the necessary optimization of these control parameters drastically increases as their number grows. We devise a novel variant of a gradient-free optimal-control method by introducing the idea of phase-modulated driving fields, which allows us to find optimal control fields efficiently. We numerically evaluate its performance and demonstrate the advantages over standard Fourier-basis methods in controlling an ensemble of two-level systems showing an inhomogeneous broadening. The control fields optimized with the phase-modulated method provide an increased robustness against such ensemble inhomogeneities as well as control-field fluctuations and environmental noise, with one order of magnitude less of average search time. Robustness enhancement of single quantum gates is also achieved by the phase-modulated method. Under environmental noise, an XY-8 sequence constituted by optimized gates prolongs the coherence time by $50\%$ compared with standard rectangular pulses in our numerical simulations, showing the application potential of our phase-modulated method in improving the precision of signal detection in the field of quantum sensing.
Jiazhao Tian, Haibin Liu, Yu Liu, Pengcheng Yang, Ralf Betzholz, Ressa S. Said, Fedor Jelezko, Jianming Cai Journal reference: Phys. Rev. A 102, 043707 (2020) [pdf] DOI: 10.1103/PhysRevA.102.043707
-
Probing the Partonic Degrees of Freedom in High-Multiplicity
-
Abstract
- We investigate the role of partonic degrees of freedom in high multiplicity p-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV carried out at the Large Hadron Collider (LHC) by studying the production and collective flow of identified hadrons at intermediate $p_T$ via the coalescence of soft and hard partons generated from viscous hydrodynamics (VISH2+1) and the energy loss model (LBT), respectively. We find that combining the intermediate $p_T$ hadrons from the coalescence with the low $p_T$ hadrons from hydrodynamics and high $p_T$ hadrons from the jet fragmentation, our Hydro-Coal-Frag model provides a nice description of the measured $p_T$-spectra and the differential elliptic flow $v_2(p_T)$ of pions, kaons and protons over the $p_T$ range from 0 to 6 GeV. We further demonstrate the necessity of including the quark coalescence contribution to reproduce the experimentally observed approximate number of constituent quark scaling of hadron $v_2$ at intermediate $p_T$. Our results thus indicate the importance of partonic degrees of freedom and also hint at the possible formation of quark-gluon plasma in high multiplicity p+Pb collisions at the LHC.
Wenbin Zhao, Che Ming Ko, Yu-Xin Liu, Guang-You Qin, Huichao Song Journal reference: Phys. Rev. Lett. 125, 072301 (2020) [pdf] DOI: 10.1103/PhysRevLett.125.072301
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Commands 4 Autonomous Vehicles (C4AV) Workshop Summary -
Abstract
- The task of visual grounding requires locating the most relevant region or object in an image, given a natural language query. So far, progress on this task was mostly measured on curated datasets, which are not always representative of human spoken language. In this work, we deviate from recent, popular task settings and consider the problem under an autonomous vehicle scenario. In particular, we consider a situation where passengers can give free-form natural language commands to a vehicle which can be associated with an object in the street scene. To stimulate research on this topic, we have organized the \emph{Commands for Autonomous Vehicles} (C4AV) challenge based on the recent \emph{Talk2Car} dataset (URL: https://www.aicrowd.com/challenges/eccv-2020-commands-4-autonomous-vehicles). This paper presents the results of the challenge. First, we compare the used benchmark against existing datasets for visual grounding. Second, we identify the aspects that render top-performing models successful, and relate them to existing state-of-the-art models for visual grounding, in addition to detecting potential failure cases by evaluating on carefully selected subsets. Finally, we discuss several possibilities for future work.
Thierry Deruyttere, Simon Vandenhende, Dusan Grujicic, Yu Liu, Luc Van Gool, Matthew Blaschko, Tinne Tuytelaars, Marie-Francine Moens [pdf]
-
Image Retrieval for Structure-from-Motion via Graph Convolutional
Network -
Abstract
- Conventional image retrieval techniques for Structure-from-Motion (SfM) suffer from the limit of effectively recognizing repetitive patterns and cannot guarantee to create just enough match pairs with high precision and high recall. In this paper, we present a novel retrieval method based on Graph Convolutional Network (GCN) to generate accurate pairwise matches without costly redundancy. We formulate image retrieval task as a node binary classification problem in graph data: a node is marked as positive if it shares the scene overlaps with the query image. The key idea is that we find that the local context in feature space around a query image contains rich information about the matchable relation between this image and its neighbors. By constructing a subgraph surrounding the query image as input data, we adopt a learnable GCN to exploit whether nodes in the subgraph have overlapping regions with the query photograph. Experiments demonstrate that our method performs remarkably well on the challenging dataset of highly ambiguous and duplicated scenes. Besides, compared with state-of-the-art matchable retrieval methods, the proposed approach significantly reduces useless attempted matches without sacrificing the accuracy and completeness of reconstruction.
Shen Yan, Yang Pen, Shiming Lai, Yu Liu, Maojun Zhang [pdf]
-
Topological approach to derive the global Hawking temperature of (massive) BTZ black hole -
Abstract
- In this paper, we study the Hawking temperature of the BTZ black hole based on the purely topological method proposed by Robson, Villari, and Biancalana (RVB) [Phys. Rev. D 99, 044042 (2019)]. The Hawking temperature of the charged rotating BTZ black hole can be accurately derived by this topological method. We also calculate the Hawking temperature of the BTZ black hole in massive gravity. Because the metric function of the BTZ black hole in massive gravity has a mass term, the corresponding Hawking temperature cannot be derived unless an integral constant is added.
Yu-Peng Zhang, Shao-Wen Wei, Yu-Xiao Liu [pdf] DOI: 10.1016/j.physletb.2020.135788 2009.07704v1 [pdf]
-
Superconducting Order from Local Disorder -
Abstract
- In all Fe superconductors the maximal $T_c$ correlates with the average anion height above the Fe plane, i.e. with the geometry of the FeAs$_4$ or FeCh$_4$ (Ch = Te, Se, S) tetrahedron. By synthesizing FeSe$_{1-x}$S$_x$ (0 $\leq$ x $\leq$ 1) single crystal alloys and by performing a series of experiments we find that $T_c$ does scale with the average anion height for $x$ in the presence of nematic order and near FeS, whereas superconductivity changes for all other $x$ track local crystallographic disorder and disorder-related scattering. Our findings demonstrate the strong coupling between disorder and $T_c$ as $x$ is tuned beyond the nematic critical point (NCP) and provide evidence of a $T_c$ tuning mechanism related to local bond disorder.
-
Ground state cooling of magnomechanical resonator in PT-symmetric cavity
magnomechanical system at room temperature -
Abstract
- We propose to realize the ground state cooling of magnomechanical resonator in a parity-time (PT)-symmetric cavity magnomechanical system composed of a loss ferromagnetic sphere and a gain microwave cavity. In the scheme, the magnomechanical resonator can be cooled close to its ground state via the magnomechanical interaction, and it is found that the cooling effect in PT-symmetric system is much higher than that in non-PT-symmetric system. Resorting to the magnetic force noise spectrum, we investigate the final mean phonon number with experimentally feasible parameters and find surprisingly that the ground state cooling of magnomechanical resonator can be directly achieved at room temperature. Furthermore, we also illustrate that the ground state cooling can be flexibly controlled via the external magnetic field.
Zhi-Xin Yang, Liang Wang, Yu-Mu Liu, Dong-Yang Wang, Cheng-Hua Bai, Shou Zhang, Hong-Fu Wang [pdf]
-
Holographic flows with scalar self-interaction toward the Kasner
universe -
Abstract
- Considering a thermal state of the dual CFT with a uniform deformation by a scalar operator, we study a holographic renormalization group flow at nonzero temperature in the bulk described by the Einstein-scalar field theory with the self-interaction term $\lambda \phi^4$ in asymptotic anti-de Sitter spacetime. We show that the holographic flow with the self-interaction term could run smoothly through the event horizon of a black hole and deform the Schwarzschild singularity to a Kasner universe at late times. Furthermore, we also study the effect of the scalar self-interaction on the deformed near-singularity Kasner exponents and the relationship between entanglement velocity and Kasner singularity exponents at late times.
Yong-Qiang Wang, Yan Song, Qian Xiang, Shao-Wen Wei, Tao Zhu, Yu-Xiao Liu [pdf]
-
Dynamic properties of thermodynamic phase transition for
five-dimensional neutral Gauss-Bonnet AdS black hole on free energy landscape -
Abstract
- Understanding the dynamic process of the thermodynamic phase transition can provide the deep insight into the black hole microscopic properties and structures. We in this paper study the dynamic properties of the stable small-large black hole phase transition for the five-dimensional neutral Gauss-Bonnet AdS black hole. Firstly, by using the first law of black holes, we prove that the extremal points of the free energy on the landscape denote the real black hole solutions satisfying the field equations. The local maximal and minimal points correspond to local unstable and stable black hole states, respectively. Especially, on the free energy landscape, the wells of the coexistence small and large black holes have the same depth. Then we investigate the probability evolution governed by the Fokker-Planck equation. Due to the thermal fluctuation, we find that the small (large) black hole state can transit to the large (small) black hole state. Furthermore, the first passage time is calculated. For each temperature, a single peak is presented, which suggests that there is a considerable fraction of the first passage events taking place at short time. And the higher the temperature is, the faster decrease of the probability is. These results will uncover some intriguing dynamic properties of the stable small-large black hole phase transition in modified gravity.
Shao-Wen Wei, Yu-Xiao Liu, Yong-Qiang Wang [pdf]
-
Three-dimensional Fermi surface and small effective masses in Mo
-
Abstract
- We report Fermi surface characteristics of Mo8Ga41, a two-gap superconductor with critical temperature Tc = 10 K, obtained from quantum oscillation measurements. Four major frequencies have been observed with relatively small quasiparticle masses. Angular dependence of major frequencies indicates three-dimensional Fermi surface sheets. This argues for a relatively isotropic superconducting state and, given its relatively high Tc, shows that a search for materials in this class could be of interest for superconducting wire applications.
Zhixiang Hu, D. Graf, Yu Liu, C. Petrovic Journal reference: Applied Physics Letters 116, 202601 (2020) [pdf] DOI: 10.1063/5.0005177
-
Testing the nature of Gauss-Bonnet gravity by four-dimensional rotating
black hole shadow -
Abstract
- The recent discover of the novel four-dimensional static and spherically symmetric Gauss-Bonnet black hole provides a promising bed to test the Gauss-Bonnet gravity by using the astronomical observation [Phys. Rev. Lett. 124, 081301 (2020)]. In this paper, we first obtain the rotating Gauss-Bonnet black hole solution by using the Newman-Janis algorithm, and then study the shadow cast by non-rotating and rotating Gauss-Bonnet black holes. The result indicates that positive Gauss-Bonnet coupling parameter shrinks the shadow, while negative one enlarges it. Meanwhile, both the distortion and ratio of two diameters of the shadow are found to increase with the coupling parameter for certain spin. Comparing with the Kerr black hole, the shadow gets more distorted for positive coupling parameter, and less distorted for negative one. Furthermore, we calculate angular diameter of the shadow by making use of the observation of M87*. The result indicates that negative dimensionless Gauss-Bonnet coupling parameter in (-4.5, 0) is more favored. Therefore, our result gives a first constraint to the Gauss-Bonnet gravity. We believe further study on the four-dimensional rotating black hole will shed new light on the Gauss-Bonnet gravity.
Shao-Wen Wei, Yu-Xiao Liu [pdf]
-
Discriminability Distillation in Group Representation Learning -
Abstract
- Learning group representation is a commonly concerned issue in tasks where the basic unit is a group, set, or sequence. Previously, the research community tries to tackle it by aggregating the elements in a group based on an indicator either defined by humans such as the quality and saliency, or generated by a black box such as the attention score. This article provides a more essential and explicable view. We claim the most significant indicator to show whether the group representation can be benefited from one of its element is not the quality or an inexplicable score, but the discriminability w.r.t. the model. We explicitly design the discrimiability using embedded class centroids on a proxy set. We show the discrimiability knowledge has good properties that can be distilled by a light-weight distillation network and can be generalized on the unseen target set. The whole procedure is denoted as discriminability distillation learning (DDL). The proposed DDL can be flexibly plugged into many group-based recognition tasks without influencing the original training procedures. Comprehensive experiments on various tasks have proven the effectiveness of DDL for both accuracy and efficiency. Moreover, it pushes forward the state-of-the-art results on these tasks by an impressive margin.
Manyuan Zhang, Guanglu Song, Hang Zhou, Yu Liu [pdf]
-
Valence band electronic structure of the van der Waals ferromagnetic insulators: VI$$_3$$ and CrI$$_3$$ -
Abstract
- Ferromagnetic van der Waals (vdW) insulators are of great scientific interest for their promising applications in spintronics. It has been indicated that in the two materials within this class, CrI$_3$ and VI$_3$, the magnetic ground state, the band gap, and the Fermi level could be manipulated by varying the layer thickness, strain or doping. To understand how these factors impact the properties, a detailed understanding of the electronic structure would be required. However, the experimental studies of the electronic structure of these materials are still very sparse. Here, we present the detailed electronic structure of CrI$_3$ and VI$_3$ measured by angle-resolved photoemission spectroscopy (ARPES). Our results show a band-gap of the order of 1 eV, sharply contrasting some theoretical predictions such as Dirac half-metallicity and metallic phases, indicating that the intra-atomic interaction parameter (U) and spin-orbit coupling (SOC) were not properly accounted for in the calculations. We also find significant differences in the electronic properties of these two materials, in spite of similarities in their crystal structure. In CrI$_3$, the valence band maximum is dominated by the I 5{\it p}, whereas in VI$_3$ it is dominated by the V 3{\it d} derived states. Our results represent valuable input for further improvements in the theoretical modeling of these systems.
Asish K. Kundu, Yu Liu, C. Petrovic, T. Valla Journal reference: Scientific Reports 10 (2020) 15602 [pdf] DOI: 10.1038/s41598-020-72487-5
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Complete Strain Mapping of Nanosheets of Tantalum Disulfide -
Abstract
- Quasi-two-dimensional (quasi-2D) materials hold promise for future electronics because of their unique band structures that result in electronic and mechanical properties sensitive to crystal strains in all three dimensions. Quantifying crystal strain is a prerequisite to correlating it with the performance of the device, and calls for high resolution but spatially resolved rapid characterization methods. Here we show that using fly-scan nano X-ray diffraction we can accomplish a tensile strain sensitivity below 0.001% with a spatial resolution of better than 80 nm over a spatial extent of 100 $\mu$m on quasi 2D flakes of 1T-TaS2. Coherent diffraction patterns were collected from a $\sim$ 100 nm thick sheet of 1T-TaS2 by scanning 12keV focused X-ray beam across and rotating the sample. We demonstrate that the strain distribution around micron and sub-micron sized 'bubbles' that are present in the sample may be reconstructed from these images. The experiments use state of the art synchrotron instrumentation, and will allow rapid and non-intrusive strain mapping of thin film samples and electronic devices based on quasi 2D materials.
-
Complementary Boundary Generator with Scale-Invariant Relation Modeling
for Temporal Action Localization: Submission to ActivityNet Challenge 2020 -
Abstract
- This technical report presents an overview of our solution used in the submission to ActivityNet Challenge 2020 Task 1 (\textbf{temporal action localization/detection}). Temporal action localization requires to not only precisely locate the temporal boundaries of action instances, but also accurately classify the untrimmed videos into specific categories. In this paper, we decouple the temporal action localization task into two stages (i.e. proposal generation and classification) and enrich the proposal diversity through exhaustively exploring the influences of multiple components from different but complementary perspectives. Specifically, in order to generate high-quality proposals, we consider several factors including the video feature encoder, the proposal generator, the proposal-proposal relations, the scale imbalance, and ensemble strategy. Finally, in order to obtain accurate detections, we need to further train an optimal video classifier to recognize the generated proposals. Our proposed scheme achieves the state-of-the-art performance on the temporal action localization task with \textbf{42.26} average mAP on the challenge testing set.
Haisheng Su, Jinyuan Feng, Hao Shao, Zhenyu Jiang, Manyuan Zhang, Wei Wu, Yu Liu, Hongsheng Li, Junjie Yan [pdf]
-
Multispinon excitations in the spin S=1/2 antiferromagnetic Heisenberg
model -
Abstract
- With the commutation relations of the spin operators, we first write out the equations of motion of the spin susceptibility and related correlation functions that have a hierarchical structure, then under the "soft cut-off" approximation, we give a set of equations of motion of spin susceptibilities for a spin S=1/2 antiferromagnetic Heisenberg model, that is independent of whether or not the system has a long range order in the low energy/temperature limit. Applying for a chain, a square lattice and a honeycomb lattice, respectively, we obtain the upper and the lowest boundaries of the low-lying excitations by solving this set of equations. For a chain, the upper and the lowest boundaries of the low-lying excitations are the same as that of the exact ones obtained by the Bethe ansatz, where the elementary excitations are the spinon pairs. For a square lattice, the spin wave excitation (magnons) resides in the region close to the lowest boundary of the low-lying excitations, and the multispinon excitations take place in the high energy region close to the upper boundary of the low-lying excitations. For a honeycomb lattice, we have one kind of "mode" of the low-lying excitation. The present results obey the Lieb-Schultz-Mattis theorem, and they are also consistent with recent neutron scattering observations and numerical simulations for a square lattice.
Yu-Liang Liu [pdf]
-
Learning Camera-Aware Noise Models -
Abstract
- Modeling imaging sensor noise is a fundamental problem for image processing and computer vision applications. While most previous works adopt statistical noise models, real-world noise is far more complicated and beyond what these models can describe. To tackle this issue, we propose a data-driven approach, where a generative noise model is learned from real-world noise. The proposed noise model is camera-aware, that is, different noise characteristics of different camera sensors can be learned simultaneously, and a single learned noise model can generate different noise for different camera sensors. Experimental results show that our method quantitatively and qualitatively outperforms existing statistical noise models and learning-based methods.
Ke-Chi Chang, Ren Wang, Hung-Jin Lin, Yu-Lun Liu, Chia-Ping Chen, Yu-Lin Chang, Hwann-Tzong Chen [pdf]
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Gravitational resonances on f(T)-branes -
Abstract
- In this work, we investigate the gravitational resonances in various $f(T)$-brane models with the warp factor $\text{e}^{A(y)}=\tanh\big(k(y+b)\big)-\tanh\big(k(y-b)\big)$. For three kinds of $f(T)$, we give the solutions to the system. Besides, we consider the tensor perturbation of vielbein and obtain the effective potentials by the Kaluza-Klein (KK) decomposition. Then, we analyze what kind of effective potential can produce the gravitational resonances. Effects of different parameters on the gravitational resonances are analysed. The lifetimes of the resonances could be long enough as the age of our universe in some ranges of the parameters. This indicates that the gravitational resonances might be considered as one of the candidates of dark matter. Combining the current experimental observations, we constrain the parameters for these brane models.
Qin Tan, Wen-Di Guo, Yu-Peng Zhang, Yu-Xiao Liu [pdf] DOI: 10.1140/epjc/s10052-021-09162-0 2008.08440v1 [pdf]
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Phase transition and microstructures of five-dimensional charged Gauss-Bonnet-AdS black holes in the grand canonical ensemble -
Abstract
- In this paper, we study the small-large black hole phase transition and construct the Ruppeiner geometry for the five-dimensional charged Gauss-Bonnet-AdS black hole in the grand canonical ensemble. By making use of the equal area law, we obtain the analytical coexistence curve of the small and large black holes. Then the phase diagrams are examined. We also calculate the change of the thermodynamic volume during the small-large phase transition, which indicates that there exists a sudden change among the black hole microstructures. The corresponding normalized scalar curvature of the Ruppeiner geometry is also calculated. Combing with the empirical observation of scalar curvature, we find that for low electric potential, the attractive interaction dominates among the microstructures, while a high electric potential produces repulsive interactions. In the reduced parameter space, we observe that only attractive interaction is allowed when the coexistence region is excluded. The normalized scalar curvature also admits a critical exponent 2 and a universal constant $-\frac{1}{8}$. In particular, the value of the normalized scalar curvature keeps the same along the coexistence small and large black hole curves. So in the grand canonical ensemble, the interaction can keep constant at the phase transition where the black hole microstructures change. These results disclose the intriguing microstructures for the charged AdS black hole in the Gauss-Bonnet gravity.
Run Zhou, Yu-Xiao Liu, Shao-Wen Wei [pdf] DOI: 10.1103/PhysRevD.102.124015 2008.08301v1 [pdf]
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Three-dimensional Ising ferrimagnetism of Cr-Fe-Cr trimers in
-
Abstract
- We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide FeCr$_2$Te$_4$ that undergoes a ferrimagnetic transition below $T_c$ $\sim$ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagentic transition. Critical exponents $\beta = 0.30(1)$ with $T_c = 122.4(5)$ K, $\gamma = 1.22(1)$ with $T_c = 122.8(1)$ K, and $\delta = 4.24(2)$ at $T_c$ $\sim$ 123 K suggest that the spins approach three-dimensional Ising ($\beta$ = 0.325, $\gamma$ = 1.24, and $\delta$ = 4.82) model coupled with the attractive long-range interactions between spins that decay as $J(r)\approx r^{-4.88}$. Our results suggest that the ferrimagnetism in FeCr$_2$Te$_4$ is due to itinerant ferromagnetism among the antiferromagnetically coupled Cr-Fe-Cr trimers.
Yu Liu, R. J. Koch, Zhixiang Hu, Niraj Aryal, Eli Stavitski, Xiao Tong, Klaus Attenkofer, E. S. Bozin, Weiguo Yin, C. Petrovic Journal reference: Phys. Rev. B 102, 085158 (2020) [pdf] DOI: 10.1103/PhysRevB.102.085158
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Dark information in black hole with mimetic dark matter -
Abstract
- It has been shown that the nonthermal spectrum of Hawking radiation will lead to information-carrying correlations between emitted particles in the radiation. The mutual information carried by such correlations can not be locally observed and hence is dark. With dark information, the black hole information is conserved. In this paper, we look for the spherically symmetric black hole solution in the background of dark matter in mimetic gravity and investigate the radiation spectrum and dark information of the black hole. The black hole has a similar spacetime structure to the Schwarzschild case, while its horizon radius is decreased by the dark matter. By using the statistical mechanical method, the nonthermal radiation spectrum is calculated. This radiation spectrum is very different from the Schwarzschild case at its last stage because of the effect of the dark matter. The mimetic dark matter reduces the lifetime of the black hole but increases the dark information of the Hawking radiation.
Yu-Xiao Liu, Yu-Han Ma, Yong-Qiang Wang, Shao-Wen Wei, Chang-Pu Sun [pdf]
-
Room-Temperature Skyrmion Thermopower in Fe3Sn2 -
Abstract
- We present first room-temperature thermoelectric signature of the skyrmion lattice. This was observed in Fe3Sn2, a Kagome Dirac crystal with massive Dirac fermions that features high-temperature skyrmion phase. The room-temperature skyrmion lattice shows magnetic-field dependence of the wavevector whereas thermopower is dominated by the electronic diffusion mechanism, allowing for the skyrmionic bubble lattice detection. Our results pave the way for the future skyrmion-based devices based on the manipulation of the thermal gradient.
Qianheng Du, Myung-Geun Han, Yu Liu, Weijun Ren, Yimei Zhu, Cedomir Petrovic [pdf]
-
Single Cell Transcriptome Research in Human Placenta -
Abstract
- Human placenta is a complex and heterogeneous organ interfacing between the mother and the fetus that supports fetal development. Alterations to placental structural components are associated with various pregnancy complications. To reveal the heterogeneity among various placenta cell types in normal and diseased placentas, as well as elucidate molecular interactions within a population of placental cells, a new genomics technology called single cell RNA-Seq (or scRNA-seq) has been employed in the last couple of years. Here we review the principles of scRNA-seq technology, and summarize the recent human placenta studies at scRNA-seq level across gestational ages as well as in pregnancy complications such as preterm birth and preeclampsia. We list the computational analysis platforms and resources available for the public use. Lastly, we discuss the future areas of interest for placenta single cell studies, as well as the data analytics needed to accomplish them.
Hui Li, Qianhui Huang, Yu Liu, Lana X Garmire [pdf]
-
Dual Gaussian-based Variational Subspace Disentanglement for
Visible-Infrared Person Re-Identification -
Abstract
- Visible-infrared person re-identification (VI-ReID) is a challenging and essential task in night-time intelligent surveillance systems. Except for the intra-modality variance that RGB-RGB person re-identification mainly overcomes, VI-ReID suffers from additional inter-modality variance caused by the inherent heterogeneous gap. To solve the problem, we present a carefully designed dual Gaussian-based variational auto-encoder (DG-VAE), which disentangles an identity-discriminable and an identity-ambiguous cross-modality feature subspace, following a mixture-of-Gaussians (MoG) prior and a standard Gaussian distribution prior, respectively. Disentangling cross-modality identity-discriminable features leads to more robust retrieval for VI-ReID. To achieve efficient optimization like conventional VAE, we theoretically derive two variational inference terms for the MoG prior under the supervised setting, which not only restricts the identity-discriminable subspace so that the model explicitly handles the cross-modality intra-identity variance, but also enables the MoG distribution to avoid posterior collapse. Furthermore, we propose a triplet swap reconstruction (TSR) strategy to promote the above disentangling process. Extensive experiments demonstrate that our method outperforms state-of-the-art methods on two VI-ReID datasets.
Nan Pu, Wei Chen, Yu Liu, Erwin M. Bakker, Michael S. Lew [pdf]
-
Comparative Analyses of Plasma Properties and Composition in Two Types of Small-scale Interplanetary Flux-ropes -
Abstract
- The origin of small-scale interplanetary magnetic flux-ropes (SIMFRs) and the relationship between SIMFRs and magnetic clouds (MCs) are still controversial. In this study, two populations of SMIFRs were collected, i.e., SIMFRs originating from the Sun (SIMFR-SUN) and those originating from the solar wind (SIMFR-SW). We defined the SIMFR-SUN (SIMFR-SW) as the SMIFRs that include (exclude) the counter-streaming suprathermal electrons and stay away from (close to) the heliospheric current sheet. After fitting with force-free flux-rope model, 52 SIMFR-SUN and 57 SIMFR-SW events observed by Advanced Composition Explorer (ACE) from 1998 February to 2011 August were qualified. Using the approach of relating the measurements to their spatial position within the flux-ropes, a comparative survey of plasma and composition characteristics inside the two populations of SIMFRs is presented. Results show that the two populations of SIMFRs have apparent differences. Compared with SIMFR-SW, SIMFR-SUN are MC-like, featuring lower central proton density, higher Vrad, higher low-FIP element abundances, higher and more fluctuate average ion charge-states and the ion chargestate ratios which are related to the heating in low corona. In addition, for the ion charge-state distributions inside SIMFR-SUN, the sunward side is higher than earthward, which might be caused by the flare heating during eruption. Moreover, both SIMFR-SUN and MCs show anti-correlation between plasma beta and He/P trend. These characteristics indicate that SIMFR-SUN and MCs are very likely to have the identical origination. This study supports the two-source origin of SIMFRs, i.e., the solar corona and the solar wind.
Jin Huang, Yu Liu, Jihong Liu, Yuandeng Shen [pdf] DOI: 10.3847/2041-8213/abac18 2008.02256v1 [pdf]
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Cooper instability and superconductivity on the Penrose lattice -
Abstract
- Bulk SC has recently been observed in the Al-Zn-Mg QC. To settle the several fundamental issues on the SC on the QC, we perform a systematic study on an attractive Hubbard model on the Penrose lattice. The first issue is the Cooper instability under an infinitesimal attractive interaction on the QC without a Fermi surface. We start from the two-electron problem outside the filled Fermi-sea, where we analytically prove that an infinitesimal Hubbard attraction can lead to the Cooper instability as long as the density of state is nonzero at the Fermi level, which provides the basis for the SC on the QC. Our numerical results yield that the Cooper pairing always takes place between a time-reversal partner, satisfying the Anderson's theorem. On this basis, we perform a MF study on the system, at both the zero and finite temperatures. The MF study also shows that an arbitrarily weak attraction can lead to the pairing order, with the resulting pairing state well described by the BCS theory, and the thermal dynamic behaviors well consistent with experiment results. The second issue is about the superfluid density on the QC without translational symmetry. It's clarified that although the normal state of the system locates at the critical point of the metal-insulator transition, the pairing state exhibits real SC, carrying finite superfluid density that can be verified by the Meissner effect. Further more, our study reveals a fundamental difference between the SC on the periodic lattice and that on the QC: while the paramagnetic superfluid density in the former case vanishes at zero temperature, that in the latter case is nonzero due to the lack of translational symmetry, reflecting the consumption of superfluid density from the scattering by the non-periodic structure. These properties of the SC on the Penrose lattice revealed here are universal for all QCs.
Yongyou Zhang, Yu-Bo Liu, Ye Cao, Wei-Qiang Chen, Fan Yang [pdf]
-
AirCapRL: Autonomous Aerial Human Motion Capture using Deep
Reinforcement Learning -
Abstract
- In this letter, we introduce a deep reinforcement learning (RL) based multi-robot formation controller for the task of autonomous aerial human motion capture (MoCap). We focus on vision-based MoCap, where the objective is to estimate the trajectory of body pose and shape of a single moving person using multiple micro aerial vehicles. State-of-the-art solutions to this problem are based on classical control methods, which depend on hand-crafted system and observation models. Such models are difficult to derive and generalize across different systems. Moreover, the non-linearity and non-convexities of these models lead to sub-optimal controls. In our work, we formulate this problem as a sequential decision making task to achieve the vision-based motion capture objectives, and solve it using a deep neural network-based RL method. We leverage proximal policy optimization (PPO) to train a stochastic decentralized control policy for formation control. The neural network is trained in a parallelized setup in synthetic environments. We performed extensive simulation experiments to validate our approach. Finally, real-robot experiments demonstrate that our policies generalize to real world conditions. Video Link: https://bit.ly/38SJfjo Supplementary: https://bit.ly/3evfo1O
Rahul Tallamraju, Nitin Saini, Elia Bonetto, Michael Pabst, Yu Tang Liu, Michael J. Black, Aamir Ahmad [pdf]
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Magnon Blockade in a PT‐Symmetric‐Like Cavity Magnomechanical System -
Abstract
- We investigate the magnon blockade effect in a parity-time (PT) symmetric-like three-mode cavity magnomechanical system involving the magnon-photon and magnon-phonon interactions. In the broken and unbroken PT-symmetric regions, we respectively calculate the second-order correlation function analytically and numerically and further determine the optimal value of detuning. By adjusting different system parameters, we study the different blockade mechanisms and find that the perfect magnon blockade effect can be observed under the weak parameter mechanism. Our work paves a way to achieve the magnon blockade in experiment.
Liang Wang, Zhi-Xin Yang, Yu-Mu Liu, Cheng-Hua Bai, Dong-Yang Wang, Shou Zhang, Hong-Fu Wang Journal reference: Annalen der Physik 2020, 2000028 [pdf] DOI: 10.1002/andp.202000028
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Light charged pion in ultra-strong magnetic field -
Abstract
- In this work, the mass of charged pions is investigated in the presence of background magnetic fields stronger than the energy scale of QCD. We introduce an anomaly magnetic momentum term in the Dirac equation and obtain the quark propagator as consequence. We find a novel finite Landau level, denoted as $tl$LL, becoming dominant rather than the conventional lowest Landau level. We examine that, due to the shifting of Landau level, it drives a mass decreasing around $eB\sim 0.8~\mathrm{GeV}^2$ for charged pions and their masses drastically limit to the neutral one at ultra-strong magnetic field, $eB\sim 1.6~\mathrm{GeV}^2$, which is consistent with the recent lattice simulation.
Jingyi Chao, Yu-Xin Liu, Lei Chang [pdf]
-
Deep Reinforcement Learning for Dynamic Spectrum Sensing and Aggregation
in Multi-Channel Wireless Networks -
Abstract
- In this paper, the problem of dynamic spectrum sensing and aggregation is investigated in a wireless network containing N correlated channels, where these channels are occupied or vacant following an unknown joint 2-state Markov model. At each time slot, a single cognitive user with certain bandwidth requirement either stays idle or selects a segment comprising C (C < N) contiguous channels to sense. Then, the vacant channels in the selected segment will be aggregated for satisfying the user requirement. The user receives a binary feedback signal indicating whether the transmission is successful or not (i.e., ACK signal) after each transmission, and makes next decision based on the sensing channel states. Here, we aim to find a policy that can maximize the number of successful transmissions without interrupting the primary users (PUs). The problem can be considered as a partially observable Markov decision process (POMDP) due to without full observation of system environment. We implement a Deep Q-Network (DQN) to address the challenge of unknown system dynamics and computational expenses. The performance of DQN, Q-Learning, and the Improvident Policy with known system dynamics is evaluated through simulations. The simulation results show that DQN can achieve near-optimal performance among different system scenarios only based on partial observations and ACK signals.
Yunzeng Li, Wensheng Zhang, Cheng-Xiang Wang, Jian Sun, Yu Liu [pdf]
-
Lifting heptagon symbols to functions -
Abstract
- Seven-point amplitudes in planar ${\cal N}=4$ super-Yang-Mills theory have previously been constructed through four loops using the Steinmann cluster bootstrap, but only at the level of the symbol. We promote these symbols to actual functions, by specifying their first derivatives and boundary conditions on a particular two-dimensional surface. To do this, we impose branch-cut conditions and construct the entire heptagon function space through weight six. We plot the amplitudes on a few lines in the bulk Euclidean region, and explore the properties of the heptagon function space under the coaction associated with multiple polylogarithms.
Lance J. Dixon, Yu-Ting Liu [pdf] DOI: 10.1007/JHEP10(2020)031 2007.12966v1 [pdf]
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Born–Infeld black holes in 4D Einstein–Gauss–Bonnet gravity -
Abstract
- A novel four-dimensional Einstein-Gauss-Bonnet gravity was formulated by D. Glavan and C. Lin [Phys. Rev. Lett. 124, 081301 (2020)], which is intended to bypass the Lovelock's theorem and to yield a non-trivial contribution to the four-dimensional gravitational dynamics. However, the validity and consistency of this theory has been called into question recently. We study a static and spherically symmetric black hole charged by a Born-Infeld electric field in the novel four-dimensional Einstein-Gauss-Bonnet gravity. It is found that the black hole solution still suffers the singularity problem, since particles incident from infinity can reach the singularity. It is also demonstrated that the Born-Infeld charged black hole may be superior to the Maxwell charged black hole to be a charged extension of the Schwarzschild-AdS-like black hole in this new gravitational theory. Some basic thermodynamics of the black hole solution is also analyzed. Besides, we regain the black hole solution in the regularized four-dimensional Einstein-Gauss-Bonnet gravity proposed by H. L\"u and Y. Pang [arXiv:2003.11552].
Ke Yang, Bao-Min Gu, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Eur. Phys. J. C 80, 662 (2020) [pdf] DOI: 10.1140/epjc/s10052-020-8246-6
-
Learning Where to Focus for Efficient Video Object Detection -
Abstract
- Transferring existing image-based detectors to the video is non-trivial since the quality of frames is always deteriorated by part occlusion, rare pose, and motion blur. Previous approaches exploit to propagate and aggregate features across video frames by using optical flow-warping. However, directly applying image-level optical flow onto the high-level features might not establish accurate spatial correspondences. Therefore, a novel module called Learnable Spatio-Temporal Sampling (LSTS) has been proposed to learn semantic-level correspondences among adjacent frame features accurately. The sampled locations are first randomly initialized, then updated iteratively to find better spatial correspondences guided by detection supervision progressively. Besides, Sparsely Recursive Feature Updating (SRFU) module and Dense Feature Aggregation (DFA) module are also introduced to model temporal relations and enhance per-frame features, respectively. Without bells and whistles, the proposed method achieves state-of-the-art performance on the ImageNet VID dataset with less computational complexity and real-time speed. Code will be made available at https://github.com/jiangzhengkai/LSTS.
Zhengkai Jiang, Yu Liu, Ceyuan Yang, Jihao Liu, Peng Gao, Qian Zhang, Shiming Xiang, Chunhong Pan [pdf]
-
Explaining Deep Neural Networks using Unsupervised Clustering -
Abstract
- We propose a novel method to explain trained deep neural networks (DNNs), by distilling them into surrogate models using unsupervised clustering. Our method can be applied flexibly to any subset of layers of a DNN architecture and can incorporate low-level and high-level information. On image datasets given pre-trained DNNs, we demonstrate the strength of our method in finding similar training samples, and shedding light on the concepts the DNNs base their decisions on. Via user studies, we show that our model can improve the user trust in model's prediction.
Yu-han Liu, Sercan O. Arik [pdf]
-
Anisotropic magnetocaloric effect and critical behavior in
-
Abstract
- We report anisotropic magnetocaloric effect and critical behavior in quasi-one-dimensional ferromagnetic CrSbSe$_3$ single crystal. The maximum magnetic entropy change $-\Delta S_M^{max}$ is 2.16 J kg$^{-1}$ K$^{-1}$ for easy $a$ axis (2.03 J kg$^{-1}$ K$^{-1}$ for hard $b$ axis) and the relative cooling power $RCP$ is 163.1 J kg$^{-1}$ for easy $a$ axis (142.1 J kg$^{-1}$ for hard $b$ axis) near $T_c$ with a magnetic field change of 50 kOe. The magnetocrystalline anisotropy constant $K_u$ is estimated to be 148.5 kJ m$^{-3}$ at 10 K, decreasing to 39.4 kJ m$^{-3}$ at 70 K. The rescaled $\Delta S_M(T,H)$ curves along all three axes collapse onto a universal curve, respectively, confirming the second order ferromagnetic transition. Further critical behavior analysis around $T_c \sim$ 70 K gives that the critical exponents $\beta$ = 0.26(1), $\gamma$ = 1.32(2), and $\delta$ = 6.17(9) for $H\parallel a$, while $\beta$ = 0.28(2), $\gamma$ = 1.02(1), and $\delta$ = 4.14(16) for $H\parallel b$. The determined critical exponents suggest that the anisotropic magnetic coupling in CrSbSe$_3$ is strongly dependent on orientations of the applied magnetic field.
Yu Liu, Zhixiang Hu, C. Petrovic Journal reference: Phys. Rev. B 102, 014425 (2020) [pdf] DOI: 10.1103/PhysRevB.102.014425
-
Anisotropic magnetocaloric effect and critical behavior in
-
Abstract
- We report anisotropic magnetocaloric effect and magnetic critical behavior in van der Waals crystal CrCl$_3$. The maximum magnetic entropy change $-\Delta S_M^{max} \sim 14.6$ J kg$^{-1}$ K$^{-1}$ and the relative cooling power $RCP \sim 340.3$ J kg$^{-1}$ near $T_c$ with a magnetic field change of 5 T are much larger when compared to CrI$_{3}$ or CrBr$_{3}$. The rescaled $\Delta S_M(T,H)$ curves collapse onto a universal curve, confirming the second order ferromagnetic transition. Further critical behavior analysis around $T_c$ presents a set of critical exponents $\beta$ = 0.28(1) with $T_c$ = 19.4(2) K, $\gamma$ = 0.89(1) with $T_c$ = 18.95(8) K, and $\delta$ = 4.6(1) at $T_c$ = 19 K, which are close to those of theoretical tricritical mean field model.
Yu Liu, C. Petrovic Journal reference: Phys. Rev. B 102, 014424 (2020) [pdf] DOI: 10.1103/PhysRevB.102.014424
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DRWR: A Differentiable Renderer without Rendering for Unsupervised 3D
Structure Learning from Silhouette Images -
Abstract
- Differentiable renderers have been used successfully for unsupervised 3D structure learning from 2D images because they can bridge the gap between 3D and 2D. To optimize 3D shape parameters, current renderers rely on pixel-wise losses between rendered images of 3D reconstructions and ground truth images from corresponding viewpoints. Hence they require interpolation of the recovered 3D structure at each pixel, visibility handling, and optionally evaluating a shading model. In contrast, here we propose a Differentiable Renderer Without Rendering (DRWR) that omits these steps. DRWR only relies on a simple but effective loss that evaluates how well the projections of reconstructed 3D point clouds cover the ground truth object silhouette. Specifically, DRWR employs a smooth silhouette loss to pull the projection of each individual 3D point inside the object silhouette, and a structure-aware repulsion loss to push each pair of projections that fall inside the silhouette far away from each other. Although we omit surface interpolation, visibility handling, and shading, our results demonstrate that DRWR achieves state-of-the-art accuracies under widely used benchmarks, outperforming previous methods both qualitatively and quantitatively. In addition, our training times are significantly lower due to the simplicity of DRWR.
Zhizhong Han, Chao Chen, Yu-Shen Liu, Matthias Zwicker [pdf]
-
Relative rigid subcategories and $τ$-tilting theory -
Abstract
- Let $\mathcal B$ be an extriangulated category with enough projectives $\mathcal P$ and enough injectives $\mathcal I$, and let $\mathcal R$ be a contravariantly finite rigid subcategory of $\mathcal B$ which contains $\mathcal P$. We have an abelian quotient category $\mathcal H/\mathcal R\subseteq \mathcal B/\mathcal R$ which is equivalent ${\rm mod}(\mathcal R/\mathcal P)$. In this article, we find a one-to-one correspondence between support $\tau$-tilting (resp. $\tau$-rigid) subcategories of $\mathcal H/\mathcal R$ and maximal relative rigid (resp. relative rigid) subcategories of $\mathcal H$, and show that support tilting subcategories in $\mathcal H/\mathcal R$ is a special kind of support $\tau$-tilting subcategories. We also study the relation between tilting subcategories of $\mathcal B/\mathcal R$ and cluster tilting subcategories of $\mathcal B$ when $\mathcal R$ is cluster tilting.
Yu Liu, Panyue Zhou [pdf]
-
Generalizing Tensor Decomposition for N-ary Relational Knowledge Bases -
Abstract
- With the rapid development of knowledge bases (KBs), link prediction task, which completes KBs with missing facts, has been broadly studied in especially binary relational KBs (a.k.a knowledge graph) with powerful tensor decomposition related methods. However, the ubiquitous n-ary relational KBs with higher-arity relational facts are paid less attention, in which existing translation based and neural network based approaches have weak expressiveness and high complexity in modeling various relations. Tensor decomposition has not been considered for n-ary relational KBs, while directly extending tensor decomposition related methods of binary relational KBs to the n-ary case does not yield satisfactory results due to exponential model complexity and their strong assumptions on binary relations. To generalize tensor decomposition for n-ary relational KBs, in this work, we propose GETD, a generalized model based on Tucker decomposition and Tensor Ring decomposition. The existing negative sampling technique is also generalized to the n-ary case for GETD. In addition, we theoretically prove that GETD is fully expressive to completely represent any KBs. Extensive evaluations on two representative n-ary relational KB datasets demonstrate the superior performance of GETD, significantly improving the state-of-the-art methods by over 15\%. Moreover, GETD further obtains the state-of-the-art results on the benchmark binary relational KB datasets.
Yu Liu, Quanming Yao, Yong Li [pdf]
-
Kohn-Luttinger Mechanism Driven Exotic Topological Superconductivity on the Penrose Lattice -
Abstract
- The Kohn-Luttinger mechanism for unconventional superconductivity (SC) driven by weak repulsive electron-electron interactions on a periodic lattice is generalized to the quasicrystal (QC) via a real-space perturbative approach. The repulsive Hubbard model on the Penrose lattice is studied as an example, on which a classification of the pairing symmetries is performed and a pairing phase diagram is obtained. Two remarkable properties of these pairing states are revealed, due to the combination of the presence of the point-group symmetry and the lack of translation symmetry on this lattice. Firstly, the spin and spacial angular momenta of a Cooper pair is de-correlated: for each pairing symmetry, both spin-singlet and spin-triplet pairings are possible even in the weak-pairing limit. Secondly, the pairing states belonging to the 2D irreducible representations of the $D_5$ point group can be time-reversal-symmetry-breaking topological SCs carrying spontaneous bulk super current and spontaneous vortices. These two remarkable properties are general for the SCs on all QCs, and are rare on periodic lattices. Our work starts the new area of unconventional SCs driven by repulsive interactions on the QC.
Ye Cao, Yongyou Zhang, Yu-Bo Liu, Cheng-Cheng Liu, Wei-Qiang Chen, Fan Yang Journal reference: Phys. Rev. Lett. 125, 017002 (2020) [pdf] DOI: 10.1103/PhysRevLett.125.017002
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Spectrum and rearrangement decays of tetraquark states with four different flavors -
Abstract
- We have systematically investigated the mass spectrum and rearrangement decay properties of the exotic tetraquark states with four different flavors using a color-magnetic interaction model. Their masses are estimated by assuming that the $X(4140)$ is a $cs\bar{c}\bar{s}$ tetraquark state and their decay widths are obtained by assuming that the Hamiltonian for decay is a constant. According to the adopted method, we find that the most stable states are probably the isoscalar $bs\bar{u}\bar{d}$ and $cs\bar{u}\bar{d}$ with $J^P=0^+$ and $1^+$. The width for most unstable tetraquarks is about tens of MeVs, but that for unstable $cu\bar{s}\bar{d}$ and $cs\bar{u}\bar{d}$ can be around 100 MeV. For the $X(5568)$, our method cannot give consistent mass and width if it is a $bu\bar{s}\bar{d}$ tetraquark state. For the $I(J^P)=0(0^+),0(1^+)$ double-heavy $T_{bc}=bc\bar{u}\bar{d}$ states, their widths can be several MeVs.
Jian-Bo Cheng, Shi-Yuan Li, Yan-Rui Liu, Yu-Nan Liu, Zong-Guo Si, Tao Yao Journal reference: Phys. Rev. D 101, 114017 (2020) [pdf] DOI: 10.1103/PhysRevD.101.114017
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Flavor dependence of the thermal dissociations of vector and axial-vector mesons -
Abstract
- The in-medium behavior of ground-state $q\bar{q}$ mesons, where $q \in \{u,d,s,c\}$, in vector and axial-vector channels is studied based on the spectral analysis for mesonic correlators at finite temperature and zero chemical potential. We first compute the correlators by solving the quark gap equations and the inhomogeneous Bethe-Salpeter equations in the rainbow-ladder approximation. Using a phenomenological ansatz, the spectral functions are extracted by fitting the correlators. By analyzing the evolution of the spectral functions with the temperature, we obtain the dissociation temperatures of mesons and discuss their relations to the critical temperature of the chiral symmetry restoration. The results show a pattern of the flavor dependence of the thermal dissociation of the mesons.
Ling-feng Chen, Si-Xue Qin, Yu-xin Liu Journal reference: Phys. Rev. D 102, 054015 (2020) [pdf] DOI: 10.1103/PhysRevD.102.054015
-
1st place solution for AVA-Kinetics Crossover in AcitivityNet Challenge
2020 -
Abstract
- This technical report introduces our winning solution to the spatio-temporal action localization track, AVA-Kinetics Crossover, in ActivityNet Challenge 2020. Our entry is mainly based on Actor-Context-Actor Relation Network. We describe technical details for the new AVA-Kinetics dataset, together with some experimental results. Without any bells and whistles, we achieved 39.62 mAP on the test set of AVA-Kinetics, which outperforms other entries by a large margin. Code will be available at: https://github.com/Siyu-C/ACAR-Net.
Siyu Chen, Junting Pan, Guanglu Song, Manyuan Zhang, Hao Shao, Ziyi Lin, Jing Shao, Hongsheng Li, Yu Liu [pdf]
-
Formal Foundations of Continuous Graph Processing -
Abstract
- With the growing need for online and iterative graph processing, software systems that continuously process large-scale graphs become widely deployed. With optimizations inherent as part of their design, these systems are complex, and have unique features beyond conventional graph processing. This paper describes CG Calculus, the first semantic foundation for continuous graph processing. The calculus captures the essential behavior of both the backend graph processing engine and the frontend application, with a focus on two essential features: temporal locality optimization (TLO) and incremental operation processing (IOP). A key design insight is that the operations continuously applied to the graph can be captured by a semantics defined over the operation stream flowing through the graph nodes. CG Calculus is a systematic study on the correctness of building continuous graph processing systems and applications. The most important result is result determinism: despite significant non-deterministic executions introduced by TLO and IOP, the results produced by CG Calculus are the same as conventional graph processing without TLO or IOP. The metatheory of CG Calculus is mechanized in Coq.
Philip Dexter, Yu David Liu, Kenneth Chiu [pdf]
-
Kondo scenario of the γ-α phase transition in single
crystalline Cerium thin films -
Abstract
- The physical mechanism driving the $\gamma$-$\alpha$ phase transition of face-centre-cubic (fcc) cerium (Ce) remains controversial until now. In this work, high quality single crystalline fcc-Ce thin films were grown on Graphene/6$H$-SiC(0001) substrate, and explored by XRD and ARPES measurement. XRD spectra showed a clear $\gamma$-$\alpha$ phase transition at $T_{\gamma-\alpha}\approx$ 50 K, which is retarded by strain effect from substrate comparing with $T_{\gamma-\alpha}$ (about 140 K) of the bulk Ce metal. However, APRES spectra did not show any signature of $\alpha$-phase emerging in the surface-layer from 300 K to 17 K, which implied that $\alpha$-phase might form at the bulk-layer of our Ce thin films. Besides, an evident Kondo dip near Fermi energy was observed in the APRES spectrum at 80 K, indicting the formation of Kondo singlet states in $\gamma$-Ce. Furthermore, the DFT+DMFT calculations were performed to simulate the electronic structures and the theoretical spectral functions agreed well with the experimental ARPES spectra. In $\gamma$-Ce, the behavior of the self-energy's imaginary part at low frequency not only confirmed that the Kondo singlet states emerged at $T_{\rm KS} \geq 80$ K, but also implied that they became coherent states at a lower characteristic temperature ($T_{\rm coh}\sim 40$ K) due to the indirect RKKY interaction among $f$-$f$ electrons. Besides, $T_{\rm coh}$ from the theoretical simulation was close to $T_{\gamma-\alpha}$ from the XRD spectra. These issues suggested that the Kondo scenario might play an important role in the $\gamma$-$\alpha$ phase transition of cerium thin films.
-
TMD soft function from large-momentum effective theory -
Abstract
- We study Euclidean formulations of the transverse-momentum-dependent (TMD) soft function, which is a cross section for soft gluon radiations involving color charges moving in two conjugate lightcone directions in quantum chromodynamics. We show it is related to a special form factor of a pair of color sources traveling with nearly-lightlike velocities, which can be matched to TMD physical observables in semi-inclusive deep-inelastic scattering and Drell-Yan process in the framework of large momentum effective theory. It can also be extracted by combining a large-momentum form factor of light meson and its leading TMD wave function. These formulations are useful for initiating nonperturbative calculations of this useful quantity.
Xiangdong Ji, Yizhuang Liu, Yu-Sheng Liu [pdf] DOI: 10.1016/j.nuclphysb.2020.115054 1910.11415v3 [pdf]
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Constraint on the radius of five-dimensional dS spacetime with GW170817 and GRB 170817A -
Abstract
- The recent detections of the gravitational wave (GW) event GW170817 and its electromagnetic counterpart GRB 170817A produced by a binary neutron star (NS) merger is a new milestone of multimessenger astronomy. The time interval between these two signals has attracted widespread attention from physicists. In the braneworld scenario, GWs could propagate through the bulk while electromagnetic waves (EMWs) are bounded on the brane, i.e., our Universe. Therefore, the trajectories of GWs and EMWs may follow different pathes. If GWs and EMWs are originated simultaneously from the same source on the brane, they are expected to arrive at the observer successively. Consequently, the time delay between GW170817 and GRB 170817A may carry the information of the extra dimension. In this paper, we try to investigate the phenomenon in the context of a five-dimensional dS ($\text{dS}_5$) spacetime. We first study two special Universe models, i.e., de Sitter and Einstein-de Sitter models, and calculate the gravitation horizon radius in each case. For the real Universe, we then consider the $\Lambda$CDM model. Our results show that for the de Sitter model of the Universe, the $\text{dS}_5$ radius could not contribute to the time delay. With the data of the observation, we constrain the $\text{dS}_5$ radius to $\ell\gtrsim7.5\times10^{2}\,\text{Tpc}$ for the Einstein-de Sitter model and $\ell\gtrsim2.4\times10^{3}\,\text{Tpc}$ for the $\Lambda$CDM model. After considering the uncertainty in the source redshift and the time-lags given by different astrophysical processes of the binary NS merger, we find that our constraints are not sensitive to the redshift in the range of (0.005, 0.01) and the time-lag in the range of (-100s, 1.734s).
Zi-Chao Lin, Hao Yu, Yu-Xiao Liu Journal reference: Phys.Rev.D 101 (2020) 10, 104058 [pdf] DOI: 10.1103/PhysRevD.101.104058
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PulseGAN: Learning to generate realistic pulse waveforms in remote
photoplethysmography -
Abstract
- Remote photoplethysmography (rPPG) is a non-contact technique for measuring cardiac signals from facial videos. High-quality rPPG pulse signals are urgently demanded in many fields, such as health monitoring and emotion recognition. However, most of the existing rPPG methods can only be used to get average heart rate (HR) values due to the limitation of inaccurate pulse signals. In this paper, a new framework based on generative adversarial network, called PulseGAN, is introduced to generate realistic rPPG pulse signals through denoising the chrominance signals. Considering that the cardiac signal is quasi-periodic and has apparent time-frequency characteristics, the error losses defined in time and spectrum domains are both employed with the adversarial loss to enforce the model generating accurate pulse waveforms as its reference. The proposed framework is tested on the public UBFC-RPPG database in both within-database and cross-database configurations. The results show that the PulseGAN framework can effectively improve the waveform quality, thereby enhancing the accuracy of HR, the heart rate variability (HRV) and the interbeat interval (IBI). The proposed method achieves the best performance compared to the denoising autoencoder (DAE) and CHROM, with the mean absolute error of AVNN (the average of all normal-to-normal intervals) improving 20.85% and 41.19%, and the mean absolute error of SDNN (the standard deviation of all NN intervals) improving 20.28% and 37.53%, respectively, in the cross-database test. This framework can be easily extended to other existing deep learning based rPPG methods, which is expected to expand the application scope of rPPG techniques.
Rencheng Song, Huan Chen, Juan Cheng, Chang Li, Yu Liu, Xun Chen [pdf]
-
Kink-antikink collision in a Lorentz-violating ϕ4 model -
Abstract
- In this work, kink-antikink collision in a two-dimensional Lorentz-violating $\phi^4$ model is considered. It is shown that the Lorentz-violating term in the proposed model does not affect the structure of the linear perturbation spectrum of the standard $\phi^4$ model, and thus there exists only one vibrational mode. The Lorentz-violating term impacts, however, the frequency and spatial wave function of the vibrational mode. As a consequence, the well-known results on $\phi^4$ kink-antikink collision will also change. Collisions of kink-antikink pairs with different values of initial velocities and Lorentz-violating parameters are simulated using the Fourier spectral method. Our results indicate that models with larger Lorentz-violating parameters would have smaller critical velocities $v_c$ and smaller widths of bounce windows. Interesting fractal structures existing in the curves of maximal energy densities of the scalar field are also found.
Haobo Yan, Yuan Zhong, Yu-Xiao Liu, Kei-ichi Maeda [pdf] DOI: 10.1016/j.physletb.2020.135542 2004.13329v2 [pdf]
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Lattice study of two-photon decay widths for scalar and pseudo-scalar charmonium -
Abstract
- In this exploratory study, two photon decay widths of pseudo-scalar ($\eta_c$) and scalar ($\chi_{c0}$) charmonium are computed using two ensembles of $N_f=2$ twisted mass lattice QCD gauge configurations. The simulation is performed two lattice ensembles with lattice spacings $a=0.067$ fm with size $32^3\times{64}$ and $a=0.085$ fm with size $24^3\times{48}$, respectively. The results for the decay widths for the two charmonia are obtained which are in the right ballpark however smaller than the experimental ones. Possible reasons for these discrepancies are discussed.
Ying Chen, Ming Gong, Ning Li, Chuan Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Yu Meng, Chao Xiong, Ke-Long Zhang [pdf] DOI: 10.1088/1674-1137/44/8/083108 2003.09817v2 [pdf]
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Anomalous chiral transports and spin polarization in heavy-ion collisions -
Abstract
- Relativistic heavy-ion collisions create hot quark-gluon plasma as well as very strong electromagnetic (EM) and fluid vortical fields. The strong EM field and vorticity can induce intriguing macroscopic quantum phenomena such as chiral magnetic, chiral separation, chiral electric separation, and chiral vortical effects as well as the spin polarization of hadrons. These phenomena provide us with experimentally feasible means to study the nontrivial topological sector of quantum chromodynamics, the possible parity violation of strong interaction at high temperature, and the subatomic spintronics of quark-gluon plasma. These studies, both in theory and in experiments, are strongly connected with other subfields of physics such as condensed matter physics, astrophysics, and cold atomic physics, and thus form an emerging interdisciplinary research area. We give an introduction to the aforementioned phenomena induced by the EM field and vorticity and an overview of the current status of their experimental research in heavy-ion collisions. We also briefly discuss spin hydrodynamics as well as chiral and spin kinetic theories.
Yu-Chen Liu, Xu-Guang Huang Journal reference: Nucl. Sci. Tech. 31, 56 (2020) [pdf] DOI: 10.1007/s41365-020-00764-z
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Point Cloud Completion by Skip-attention Network with Hierarchical
Folding -
Abstract
- Point cloud completion aims to infer the complete geometries for missing regions of 3D objects from incomplete ones. Previous methods usually predict the complete point cloud based on the global shape representation extracted from the incomplete input. However, the global representation often suffers from the information loss of structure details on local regions of incomplete point cloud. To address this problem, we propose Skip-Attention Network (SA-Net) for 3D point cloud completion. Our main contributions lie in the following two-folds. First, we propose a skip-attention mechanism to effectively exploit the local structure details of incomplete point clouds during the inference of missing parts. The skip-attention mechanism selectively conveys geometric information from the local regions of incomplete point clouds for the generation of complete ones at different resolutions, where the skip-attention reveals the completion process in an interpretable way. Second, in order to fully utilize the selected geometric information encoded by skip-attention mechanism at different resolutions, we propose a novel structure-preserving decoder with hierarchical folding for complete shape generation. The hierarchical folding preserves the structure of complete point cloud generated in upper layer by progressively detailing the local regions, using the skip-attentioned geometry at the same resolution. We conduct comprehensive experiments on ShapeNet and KITTI datasets, which demonstrate that the proposed SA-Net outperforms the state-of-the-art point cloud completion methods.
Xin Wen, Tianyang Li, Zhizhong Han, Yu-Shen Liu [pdf]
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Different asymptotic behaviors of thick branes in mimetic gravity -
Abstract
- In this paper, thick branes generated by mimetic scalar field with Lagrange multiplier formulation are investigated. We give three typical thick brane background solutions with different asymptotic behaviors and show that all the solutions are stable under tensor perturbations. The effective potentials of the tensor perturbations exhibit as volcano potential, P\"{o}schl-Teller potential, and harmonic oscillator potential for the three background solutions, respectively. All the tensor zero modes (massless gravitons) of the three cases can be localized on the brane. We also calculate the corrections to the Newtonian potential. On a large scale, the corrections to the Newtonian potential can be ignored. While on a small scale, the correction from the volcano-like potential is more pronounced than the other two cases. Combining the latest results of short-range gravity experiments that the usual Newtonian potential $\propto1/r$ holds down to a length scale at $52\mu$m, we get the constraint on the scale parameter as $k\gtrsim 10^{-4}$eV, and constraint on the corresponding five-dimensional fundamental scale as $bM_\ast \gtrsim10^5$TeV.
Tao-Tao Sui, Yu-Peng Zhang, Bao-Min Gu, Yu-Xiao Liu [pdf]
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Bond-breaking induced Lifshitz transition in robust Dirac semimetal VAI
-
Abstract
- Topological electrons in semimetals are usually vulnerable to chemical doping and environment change, which restricts their potential application in future electronic devices. In this paper we report that the type-II Dirac semimetal $\mathbf{VAl_3}$ hosts exceptional, robust topological electrons which can tolerate extreme change of chemical composition. The Dirac electrons remain intact even after a substantial part of V atoms have been replaced in the $\mathbf{V_{1-x}Ti_xAl_3}$ solid solutions. This Dirac semimetal state ends at $x=0.35$ where a Lifshitz transition to $p$-type trivial metal occurs. The V-Al bond is completely broken in this transition as long as the bonding orbitals are fully depopulated by the holes donated from Ti substitution. In other words, the Dirac electrons in $\mathbf{VAl_3}$ are protected by the V-Al bond whose molecular orbital is their bonding gravity center. Our understanding on the interrelations among electron count, chemical bond and electronic properties in topological semimetals suggests a rational approach to search robust, chemical-bond-protected topological materials.
Yiyuan Liu, Yu-Fei Liu, Xin Gui, Cheng Xiang, Hui-bin Zhou, Chuang-Han Hsu, HsinLin, Tay-RongChang, WeiweiXie, ShuangJia [pdf] DOI: 10.1073/pnas.1917697117 2005.07970v1 [pdf]
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Frobenius $n$-exangulated categories -
Abstract
- Herschend-Liu-Nakaoka introduced the notion of $n$-exangulated categories as higher dimensional analogues of extriangulated categories defined by Nakaoka-Palu. The class of $n$-exangulated categories contains $n$-exact categories and $(n+2)$-angulated categories as examples. In this article, we introduce a notion of Frobenius $n$-exangulated categories which are a generalization of Frobenius $n$-exact categories. We show that the stable category of a Frobenius $n$-exangulated category is an $(n+2)$-angulated category. As an application, this result generalizes the work by Jasso. We provide a class of $n$-exangulated categories which are neither $n$-exact categories nor $(n+2)$-angulated categories. Finally, we discuss an application of the main results and give some examples illustrating it.
Yu Liu, Panyue Zhou [pdf]
-
Excited states of holographic superconductors -
Abstract
- In this paper we re-investigate the model of the anti-de Sitter gravity coupled to Maxwell and charged scalar fields, which has been studied as the gravitational dual to a superconductor for a long time since the famous work [Phys.\ Rev.\ Lett.\ {\bf 101}, 031601 (2008)]. By numerical method, we present a novel family of solutions of holographical superconductor with excited states, and find there exists a lower critical temperature in the corresponding excited state. Moreover, we study the condensate and conductivity in the excited states. It is very interesting that the conductivity $\sigma$ of each excited state has an additional pole in $\text{Im}[\sigma]$ and a delta function in $\text{Re}[\sigma]$ arising at the low temperature inside the gap, which is just the evidence of the existence of excited states.
Yong-Qiang Wang, Tong-Tong Hu, Yu-Xiao Liu, Jie Yang, Li Zhao [pdf] DOI: 10.1007/JHEP06(2020)013 1910.07734v2 [pdf]
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Extended thermodynamics and microstructures of four-dimensional charged Gauss-Bonnet black hole in AdS space -
Abstract
- The discovery of new four-dimensional black hole solutions presents a new approach to understand the Gauss-Bonnet gravity in low dimensions. In this paper, we test the Gauss-Bonnet gravity by studying the phase transition and microstructures for the four-dimensional charged AdS black hole. In the extended phase space, where the cosmological constant and the Gauss-Bonnet coupling parameter are treated as thermodynamic variables, we find that the thermodynamic first law and the corresponding Smarr formula are satisfied. Both in the canonical ensemble and grand canonical ensemble, we observe the small-large black hole phase transition, which is similar to the case of the van der Walls fluid. This phase transition can also appear in the neutral black hole system. Furthermore, we construct the Ruppeiner geometry, and find that besides the attractive interaction, the repulsive interaction can also dominate among the microstructures for the small black hole with high temperature in a charged or neutral black hole system. This is quite different from the five-dimensional neutral black hole, for which only dominant attractive interaction can be found. The critical behaviors of the normalized scalar curvature are also examined. These results will shed new light into the characteristic property of four-dimensional Gauss-Bonnet gravity.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 101, 104018 (2020) [pdf] DOI: 10.1103/PhysRevD.101.104018
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Modeling Event Propagation via Graph Biased Temporal Point Process -
Abstract
- Temporal point process is widely used for sequential data modeling. In this paper, we focus on the problem of modeling sequential event propagation in graph, such as retweeting by social network users, news transmitting between websites, etc. Given a collection of event propagation sequences, conventional point process model consider only the event history, i.e. embed event history into a vector, not the latent graph structure. We propose a Graph Biased Temporal Point Process (GBTPP) leveraging the structural information from graph representation learning, where the direct influence between nodes and indirect influence from event history is modeled respectively. Moreover, the learned node embedding vector is also integrated into the embedded event history as side information. Experiments on a synthetic dataset and two real-world datasets show the efficacy of our model compared to conventional methods and state-of-the-art.
Weichang Wu, Huanxi Liu, Xiaohu Zhang, Yu Liu, Hongyuan Zha [pdf]
-
A First Look at Commercial 5G Performance on Smartphones -
Abstract
- We conduct to our knowledge a first measurement study of commercial 5G performance on smartphones by closely examining 5G networks of three carriers (two mmWave carriers, one mid-band carrier) in three U.S. cities. We conduct extensive field tests on 5G performance in diverse urban environments. We systematically analyze the handoff mechanisms in 5G and their impact on network performance. We explore the feasibility of using location and possibly other environmental information to predict the network performance. We also study the app performance (web browsing and HTTP download) over 5G. Our study consumes more than 15 TB of cellular data. Conducted when 5G just made its debut, it provides a "baseline" for studying how 5G performance evolves, and identifies key research directions on improving 5G users' experience in a cross-layer manner. We have released the data collected from our study (referred to as 5Gophers) at https://fivegophers.umn.edu/www20.
Arvind Narayanan, Eman Ramadan, Jason Carpenter, Qingxu Liu, Yu Liu, Feng Qian, Zhi-Li Zhang Journal reference: Proceedings of The Web Conference 2020 (WWW'20) [pdf] DOI: 10.1145/3366423.3380169
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A novel multimodal approach for hybrid brain-computer interface -
Abstract
- Brain-computer interface (BCI) technologies have been widely used in many areas. In particular, non-invasive technologies such as electroencephalography (EEG) or near-infrared spectroscopy (NIRS) have been used to detect motor imagery, disease, or mental state. It has been already shown in literature that the hybrid of EEG and NIRS has better results than their respective individual signals. The fusion algorithm for EEG and NIRS sources is the key to implement them in real-life applications. In this research, we propose three fusion methods for the hybrid of the EEG and NIRS-based brain-computer interface system: linear fusion, tensor fusion, and $p$th-order polynomial fusion. Firstly, our results prove that the hybrid BCI system is more accurate, as expected. Secondly, the $p$th-order polynomial fusion has the best classification results out of the three methods, and also shows improvements compared with previous studies. For a motion imagery task and a mental arithmetic task, the best detection accuracy in previous papers were 74.20\% and 88.1\%, whereas our accuracy achieved was 77.53\% and 90.19\% . Furthermore, unlike complex artificial neural network methods, our proposed methods are not as computationally demanding.
Zhe Sun, Zihao Huang, Feng Duan, Yu Liu [pdf]
-
Cost-effectiveness Analysis of Antiepidemic Policies and Global
Situation Assessment of COVID-19 -
Abstract
- With a two-layer contact-dispersion model and data in China, we analyze the cost-effectiveness of three types of antiepidemic measures for COVID-19: regular epidemiological control, local social interaction control, and inter-city travel restriction. We find that: 1) intercity travel restriction has minimal or even negative effect compared to the other two at the national level; 2) the time of reaching turning point is independent of the current number of cases, and only related to the enforcement stringency of epidemiological control and social interaction control measures; 3) strong enforcement at the early stage is the only opportunity to maximize both antiepidemic effectiveness and cost-effectiveness; 4) mediocre stringency of social interaction measures is the worst choice. Subsequently, we cluster countries/regions into four groups based on their control measures and provide situation assessment and policy suggestions for each group.
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Donaldson-Thomas theory of quantum Fermat quintic threefolds I -
Abstract
- In this paper, we study non-commutative projective schemes whose associated non-commutative graded algebras are finite over their centers. We study their moduli spaces of stable sheaves, and construct a symmetric obstruction theory in the Calabi-Yau-3 case. This allows us to define Donaldson-Thomas type invariants. We also discuss the simplest examples, called quantum Fermat quintic threefolds.
Yu-Hsiang Liu [pdf]
-
Donaldson-Thomas theory of quantum Fermat quintic threefolds II -
Abstract
- This paper is a continuation of author's previous work arXiv:1911.07949, where we defined Donaldson-Thomas invariants of quantum Fermat threefolds. In this paper, we study the generic quantum Fermat threefold. We give explicit local models for Hilbert schemes of points as quivers with potential, and compute degree zero Donaldson-Thomas invariants. The result is expressed in terms of certain colored plane partitions.
Yu-Hsiang Liu [pdf]
-
Change Detection in Heterogeneous Optical and SAR Remote Sensing Images Via Deep Homogeneous Feature Fusion -
Abstract
- Change detection in heterogeneous remote sensing images is crucial for disaster damage assessment. Recent methods use homogenous transformation, which transforms the heterogeneous optical and SAR remote sensing images into the same feature space, to achieve change detection. Such transformations mainly operate on the low-level feature space and may corrupt the semantic content, deteriorating the performance of change detection. To solve this problem, this paper presents a new homogeneous transformation model termed deep homogeneous feature fusion (DHFF) based on image style transfer (IST). Unlike the existing methods, the DHFF method segregates the semantic content and the style features in the heterogeneous images to perform homogeneous transformation. The separation of the semantic content and the style in homogeneous transformation prevents the corruption of image semantic content, especially in the regions of change. In this way, the detection performance is improved with accurate homogeneous transformation. Furthermore, we present a new iterative IST (IIST) strategy, where the cost function in each IST iteration measures and thus maximizes the feature homogeneity in additional new feature subspaces for change detection. After that, change detection is accomplished accurately on the original and the transformed images that are in the same feature space. Real remote sensing images acquired by SAR and optical satellites are utilized to evaluate the performance of the proposed method. The experiments demonstrate that the proposed DHFF method achieves significant improvement for change detection in heterogeneous optical and SAR remote sensing images, in terms of both accuracy rate and Kappa index.
Xiao Jiang, Gang Li, Yu Liu, Xiao-Ping Zhang, You He [pdf] DOI: 10.1109/JSTARS.2020.2983993 2004.03830v1 [pdf]
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Spinning test particle in four-dimensional Einstein-Gauss-Bonnet Black
Hole -
Abstract
- In this paper, we investigate the motion of a classical spinning test particle orbiting around a static spherically symmetric black hole in a novel four-dimensional Einstein-Gauss-Bonnet gravity [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)]. We find that the effective potential of a spinning test particle in the background of the black hole has two minima when the Gauss-Bonnet coupling parameter $\alpha$ is nearly in a special range $-6.1<\alpha/M^2<-2$ ($M$ is the mass of the black hole), which means such particle can be in two separate orbits with the same spin angular momentum and orbital angular momentum. We also investigate the innermost stable circular orbits of the spinning test particle and find that the effect of the particle spin on the the innermost stable circular is similar to the case of the four-dimensional black hole in general relativity.
Yu-Peng Zhang, Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Anisotropic Convolutional Networks for 3D Semantic Scene Completion -
Abstract
- As a voxel-wise labeling task, semantic scene completion (SSC) tries to simultaneously infer the occupancy and semantic labels for a scene from a single depth and/or RGB image. The key challenge for SSC is how to effectively take advantage of the 3D context to model various objects or stuffs with severe variations in shapes, layouts and visibility. To handle such variations, we propose a novel module called anisotropic convolution, which properties with flexibility and power impossible for the competing methods such as standard 3D convolution and some of its variations. In contrast to the standard 3D convolution that is limited to a fixed 3D receptive field, our module is capable of modeling the dimensional anisotropy voxel-wisely. The basic idea is to enable anisotropic 3D receptive field by decomposing a 3D convolution into three consecutive 1D convolutions, and the kernel size for each such 1D convolution is adaptively determined on the fly. By stacking multiple such anisotropic convolution modules, the voxel-wise modeling capability can be further enhanced while maintaining a controllable amount of model parameters. Extensive experiments on two SSC benchmarks, NYU-Depth-v2 and NYUCAD, show the superior performance of the proposed method. Our code is available at https://waterljwant.github.io/SSC/
Jie Li, Kai Han, Peng Wang, Yu Liu, Xia Yuan [pdf]
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Synchronization in PT-symmetric optomechanical resonators -
Abstract
- Synchronization has great impacts in various fields such as self-clocking, communication, neural networks, etc. Here we present a mechanism of synchronization for two mechanical modes in two coupled optomechanical resonators by introducing the so-called PT-symmetric structure. It is shown that the degree of synchronization between the two far-off-resonant mechanical modes can be increased by decreasing the coupling strength between the two optomechanical resonators. Additionally, when we consider the stochastic noises in the optomechanical resonators, we find that more noises can enhance the degree of synchronization of the system under particular parameter regime. Our results open up the new dimension of research for PT-symmetric systems and synchronization.
Chang-long Zhu, Yu-long Liu, Lan Yang, Yu-xi Liu, Jing Zhang [pdf]
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Learning to See Through Obstructions -
Abstract
- We present a learning-based approach for removing unwanted obstructions, such as window reflections, fence occlusions or raindrops, from a short sequence of images captured by a moving camera. Our method leverages the motion differences between the background and the obstructing elements to recover both layers. Specifically, we alternate between estimating dense optical flow fields of the two layers and reconstructing each layer from the flow-warped images via a deep convolutional neural network. The learning-based layer reconstruction allows us to accommodate potential errors in the flow estimation and brittle assumptions such as brightness consistency. We show that training on synthetically generated data transfers well to real images. Our results on numerous challenging scenarios of reflection and fence removal demonstrate the effectiveness of the proposed method.
Yu-Lun Liu, Wei-Sheng Lai, Ming-Hsuan Yang, Yung-Yu Chuang, Jia-Bin Huang [pdf]
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Single-Image HDR Reconstruction by Learning to Reverse the Camera
Pipeline -
Abstract
- Recovering a high dynamic range (HDR) image from a single low dynamic range (LDR) input image is challenging due to missing details in under-/over-exposed regions caused by quantization and saturation of camera sensors. In contrast to existing learning-based methods, our core idea is to incorporate the domain knowledge of the LDR image formation pipeline into our model. We model the HDRto-LDR image formation pipeline as the (1) dynamic range clipping, (2) non-linear mapping from a camera response function, and (3) quantization. We then propose to learn three specialized CNNs to reverse these steps. By decomposing the problem into specific sub-tasks, we impose effective physical constraints to facilitate the training of individual sub-networks. Finally, we jointly fine-tune the entire model end-to-end to reduce error accumulation. With extensive quantitative and qualitative experiments on diverse image datasets, we demonstrate that the proposed method performs favorably against state-of-the-art single-image HDR reconstruction algorithms.
Yu-Lun Liu, Wei-Sheng Lai, Yu-Sheng Chen, Yi-Lung Kao, Ming-Hsuan Yang, Yung-Yu Chuang, Jia-Bin Huang [pdf]
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Higgs boson decay h → Zγ and muon magnetic dipole moment in the μνSSM -
Abstract
- To solve the $\mu$ problem and generate three tiny neutrino masses in the MSSM, the $\mu$ from $\nu$ Supersymmetric Standard Model ($\mu\nu$SSM) introduces three singlet right-handed neutrino superfields, which lead to the mixing of the Higgs doublets with the sneutrinos. The mixing affects the lightest Higgs boson mass and the Higgs couplings. The present observed 95\% CL upper limit on signal strength of the 125 GeV Higgs boson decay $h\rightarrow Z\gamma$ is 6.6, which still is plenty of space to prove the existence of new physics. In this work, we investigate the signal strength of the 125 GeV Higgs boson decay channel $h\rightarrow Z\gamma$ in the $\mu\nu$SSM. Besides, we consider the two-loop electroweak corrections of muon anomalous magnetic dipole moment (MDM) in the model, which also make important contributions compared with one-loop electroweak corrections.
Chang-Xin Liu, Hai-Bin Zhang, Jin-Lei Yang, Shu-Min Zhao, Yu-Bin Liu, Tai-Fu Feng Journal reference: JHEP04(2020)002 [pdf] DOI: 10.1007/JHEP04(2020)002
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DPGN: Distribution Propagation Graph Network for Few-shot Learning -
Abstract
- Most graph-network-based meta-learning approaches model instance-level relation of examples. We extend this idea further to explicitly model the distribution-level relation of one example to all other examples in a 1-vs-N manner. We propose a novel approach named distribution propagation graph network (DPGN) for few-shot learning. It conveys both the distribution-level relations and instance-level relations in each few-shot learning task. To combine the distribution-level relations and instance-level relations for all examples, we construct a dual complete graph network which consists of a point graph and a distribution graph with each node standing for an example. Equipped with dual graph architecture, DPGN propagates label information from labeled examples to unlabeled examples within several update generations. In extensive experiments on few-shot learning benchmarks, DPGN outperforms state-of-the-art results by a large margin in 5% $\sim$ 12% under supervised setting and 7% $\sim$ 13% under semi-supervised setting. Code will be released.
Ling Yang, Liangliang Li, Zilun Zhang, Xinyu Zhou, Erjin Zhou, Yu Liu [pdf]
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Spin-Triplet Excitonic Insulator: The Case of Semihydrogenated Graphene -
Abstract
- While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a graphone, however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in graphone, which offers yet another test by experiment.
Zeyu Jiang, Wenkai Lou, Yu Liu, Yuanchang Li, Haifeng Song, Kai Chang, Wenhui Duan, Shengbai Zhang Journal reference: Phys. Rev. Lett. 124, 166401 (2020) [pdf] DOI: 10.1103/PhysRevLett.124.166401
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On the relation between relative rigid and support tilting -
Abstract
- Let B be an extriangulated category with enough projectives and enough injectives. Let C be a fully rigid subcategory of B which admits a twin cotorsion pair ((C,K),(K,D)). The quotient category B/K is abelian, we assume that it is hereditary and has finite length. In this article, we study the relation between support tilting subcategories of B/K and maximal relative rigid subcategories of B. More precisely, we show that the image of any cluster tilting subcategory of B is support tilting in B/K and any support tilting subcategory in B/K can be lifted to a unique relative maximal rigid subcategory in B. We also give a bijection between these two classes of subcategories if C is generated by an object.
Yu Liu, Panyue Zhou [pdf]
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Vortex-Meissner phase transition induced by a two-tone-drive-engineered artificial gauge potential in the fermionic ladder constructed by superconducting qubit circuits -
Abstract
- We propose to periodically modulate the onsite energy via two-tone drives, which can be furthermore used to engineer artificial gauge potential. As an example, we show that the fermionic ladder model penetrated with effective magnetic flux can be constructed by superconducting flux qubits using such two-tone-drive-engineered artificial gauge potential. In this superconducting system, the single-particle ground state can range from vortex phase to Meissner phase due to the competition between the interleg coupling strength and the effective magnetic flux. We also present the method to experimentally measure the chiral currents by the single-particle Rabi oscillations between adjacent qubits. In contrast to previous methods of generating artifical gauge potential, our proposal does not need the aid of auxiliary couplers and in principle remains valid only if the qubit circuit maintains enough anharmonicity. The fermionic ladder model with effective magnetic flux can also be interpreted as one-dimensional spin-orbit-coupled model, which thus lay a foundation towards the realization of quantum spin Hall effect.
Yan-Jun Zhao, Xun-Wei Xu, Hui Wang, Yu-xi Liu, Wu-Ming Liu [pdf] DOI: 10.1103/PhysRevA.102.053722 2003.10638v1 [pdf]
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A Statistical Study of the Plasma and Composition Distribution inside Magnetic Clouds: 1998–2011 -
Abstract
- A comprehensive analysis of plasma and composition characteristics inside magnetic clouds (MCs) observed by the Advanced Composition Explorer (ACE) spacecraft from 1998 February to 2011 August is presented. The results show that MCs have specific interior structures, and MCs of different speeds show differences in composition and structure. Compared with the slow MCs, fast MCs have enhanced mean charge states of iron, oxygen, silicon, magnesium, $\mathrm{O^{7+}/O^{6+}}$, $\mathrm{C^{6+}/C^{5+}}$, $\mathrm{C^{6+}/C^{4+}}$ and $\mathrm{Fe^{\geq16+}/Fe_{total}}$ values. For ionic species in fast MCs, a higher atomic number represents a greater enhancement of mean charge state than slow MCs. We also find that both the fast and slow MCs display bimodal structure distribution in the mean iron charge state ($\mathrm{\langle Q\rangle Fe}$), which suggests that the existence of flux rope prior to the eruption is common. Furthermore, the $\mathrm{\langle Q\rangle Fe} $, $\mathrm{Fe^{\geq16+}/Fe_{total}}$, and $\mathrm{O^{7+}/O^{6+}}$ ratio distribution inside fast MCs have the feature that the posterior peak is higher than the anterior one. This result agrees with the "standard model" for CME/flares, by which magnetic reconnection occurs beneath the flux rope, thereby ionizing the ions of the posterior part of flux rope sufficiently by high-energy electron collisions or by direct heating in the reconnection region.
Jin Huang, Yu Liu, Hengqiang Feng, Ake Zhao, Z. Z. Abidin, Yuandeng Shen, Oloketuyi Jacob [pdf] DOI: 10.3847/1538-4357/ab7a28 2003.09965v1 [pdf]
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LRC-Net: Learning Discriminative Features on Point Clouds by Encoding
Local Region Contexts -
Abstract
- Learning discriminative feature directly on point clouds is still challenging in the understanding of 3D shapes. Recent methods usually partition point clouds into local region sets, and then extract the local region features with fixed-size CNN or MLP, and finally aggregate all individual local features into a global feature using simple max pooling. However, due to the irregularity and sparsity in sampled point clouds, it is hard to encode the fine-grained geometry of local regions and their spatial relationships when only using the fixed-size filters and individual local feature integration, which limit the ability to learn discriminative features. To address this issue, we present a novel Local-Region-Context Network (LRC-Net), to learn discriminative features on point clouds by encoding the fine-grained contexts inside and among local regions simultaneously. LRC-Net consists of two main modules. The first module, named intra-region context encoding, is designed for capturing the geometric correlation inside each local region by novel variable-size convolution filter. The second module, named inter-region context encoding, is proposed for integrating the spatial relationships among local regions based on spatial similarity measures. Experimental results show that LRC-Net is competitive with state-of-the-art methods in shape classification and shape segmentation applications.
Xinhai Liu, Zhizhong Han, Fangzhou Hong, Yu-Shen Liu, Matthias Zwicker [pdf]
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Neutrino effects on the morphology of cosmic large-scale structure -
Abstract
- In this work, we propose a powerful probe of neutrino effects on the large-scale structure (LSS) of the Universe, i.e., Minkowski functionals (MFs). The morphology of LSS can be fully described by four MFs. This tool, with strong statistical power, is robust to various systematics and can comprehensively probe all orders of N-point statistics. By using a pair of high-resolution N-body simulations, for the first time, we comprehensively studied the subtle neutrino effects on the morphology of LSS. For an ideal LSS survey of volume $\sim1.73$ Gpc$^3$/$h^3$, neutrino signals are mainly detected from void regions with a significant level up to $\thicksim 10\sigma$ and $\thicksim 300\sigma$ for CDM and total matter density fields, respectively. This demonstrates its enormous potential for much improving the neutrino mass constraint in the data analysis of up-coming ambitious LSS surveys.
Yu Liu, Yu Yu, Hao-Ran Yu, Pengjie Zhang Journal reference: Phys.Rev.D 101,063515 (2020) [pdf] DOI: 10.1103/PhysRevD.101.063515
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Saturating the quantum Cramér-Rao bound and measuring the related
quantum Fisher information in a nitrogen-vacancy center in diamond -
Abstract
- The quantum Cram\'er-Rao bound sets a fundamental limit on the accuracy of unbiased parameter estimation in quantum systems, relating the uncertainty in determining a parameter to the inverse of the quantum Fisher information. We experimentally demonstrate near saturation of the quantum Cram\'er-Rao bound in the phase estimation of a solid-state spin system, provided by a nitrogen-vacancy center in diamond. This is achieved by comparing the experimental uncertainty in phase estimation with an independent measurement of the related quantum Fisher information. The latter is finely extracted from coherent dynamical responses of the system under weak parametric modulations, without performing any quantum-state tomography. Our method offers a versatile and powerful tool to explore the fundamental role of the quantum Fisher information in quantum technologies.
Yu Liu, Min Yu, Pengcheng Yang, Musang Gong, Qingyun Cao, Shaoliang Zhang, Haibin Liu, Markus Heyl, Tomoki Ozawa, Nathan Goldman, Jianming Cai [pdf]
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Rotate-and-Render: Unsupervised Photorealistic Face Rotation from
Single-View Images -
Abstract
- Though face rotation has achieved rapid progress in recent years, the lack of high-quality paired training data remains a great hurdle for existing methods. The current generative models heavily rely on datasets with multi-view images of the same person. Thus, their generated results are restricted by the scale and domain of the data source. To overcome these challenges, we propose a novel unsupervised framework that can synthesize photo-realistic rotated faces using only single-view image collections in the wild. Our key insight is that rotating faces in the 3D space back and forth, and re-rendering them to the 2D plane can serve as a strong self-supervision. We leverage the recent advances in 3D face modeling and high-resolution GAN to constitute our building blocks. Since the 3D rotation-and-render on faces can be applied to arbitrary angles without losing details, our approach is extremely suitable for in-the-wild scenarios (i.e. no paired data are available), where existing methods fall short. Extensive experiments demonstrate that our approach has superior synthesis quality as well as identity preservation over the state-of-the-art methods, across a wide range of poses and domains. Furthermore, we validate that our rotate-and-render framework naturally can act as an effective data augmentation engine for boosting modern face recognition systems even on strong baseline models.
Hang Zhou, Jihao Liu, Ziwei Liu, Yu Liu, Xiaogang Wang [pdf]
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1st Place Solutions for OpenImage2019 -- Object Detection and Instance
Segmentation -
Abstract
- This article introduces the solutions of the two champion teams, `MMfruit' for the detection track and `MMfruitSeg' for the segmentation track, in OpenImage Challenge 2019. It is commonly known that for an object detector, the shared feature at the end of the backbone is not appropriate for both classification and regression, which greatly limits the performance of both single stage detector and Faster RCNN \cite{ren2015faster} based detector. In this competition, we observe that even with a shared feature, different locations in one object has completely inconsistent performances for the two tasks. \textit{E.g. the features of salient locations are usually good for classification, while those around the object edge are good for regression.} Inspired by this, we propose the Decoupling Head (DH) to disentangle the object classification and regression via the self-learned optimal feature extraction, which leads to a great improvement. Furthermore, we adjust the soft-NMS algorithm to adj-NMS to obtain stable performance improvement. Finally, a well-designed ensemble strategy via voting the bounding box location and confidence is proposed. We will also introduce several training/inferencing strategies and a bag of tricks that give minor improvement. Given those masses of details, we train and aggregate 28 global models with various backbones, heads and 3+2 expert models, and achieves the 1st place on the OpenImage 2019 Object Detection Challenge on the both public and private leadboards. Given such good instance bounding box, we further design a simple instance-level semantic segmentation pipeline and achieve the 1st place on the segmentation challenge.
Yu Liu, Guanglu Song, Yuhang Zang, Yan Gao, Enze Xie, Junjie Yan, Chen Change Loy, Xiaogang Wang [pdf]
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KPNet: Towards Minimal Face Detector -
Abstract
- The small receptive field and capacity of minimal neural networks limit their performance when using them to be the backbone of detectors. In this work, we find that the appearance feature of a generic face is discriminative enough for a tiny and shallow neural network to verify from the background. And the essential barriers behind us are 1) the vague definition of the face bounding box and 2) tricky design of anchor-boxes or receptive field. Unlike most top-down methods for joint face detection and alignment, the proposed KPNet detects small facial keypoints instead of the whole face by in a bottom-up manner. It first predicts the facial landmarks from a low-resolution image via the well-designed fine-grained scale approximation and scale adaptive soft-argmax operator. Finally, the precise face bounding boxes, no matter how we define it, can be inferred from the keypoints. Without any complex head architecture or meticulous network designing, the KPNet achieves state-of-the-art accuracy on generic face detection and alignment benchmarks with only $\sim1M$ parameters, which runs at 1000fps on GPU and is easy to perform real-time on most modern front-end chips.
Guanglu Song, Yu Liu, Yuhang Zang, Xiaogang Wang, Biao Leng, Qingsheng Yuan [pdf]
-
Revisiting the Sibling Head in Object Detector -
Abstract
- The ``shared head for classification and localization'' (sibling head), firstly denominated in Fast RCNN~\cite{girshick2015fast}, has been leading the fashion of the object detection community in the past five years. This paper provides the observation that the spatial misalignment between the two object functions in the sibling head can considerably hurt the training process, but this misalignment can be resolved by a very simple operator called task-aware spatial disentanglement (TSD). Considering the classification and regression, TSD decouples them from the spatial dimension by generating two disentangled proposals for them, which are estimated by the shared proposal. This is inspired by the natural insight that for one instance, the features in some salient area may have rich information for classification while these around the boundary may be good at bounding box regression. Surprisingly, this simple design can boost all backbones and models on both MS COCO and Google OpenImage consistently by ~3% mAP. Further, we propose a progressive constraint to enlarge the performance margin between the disentangled and the shared proposals, and gain ~1% more mAP. We show the \algname{} breaks through the upper bound of nowadays single-model detector by a large margin (mAP 49.4 with ResNet-101, 51.2 with SENet154), and is the core model of our 1st place solution on the Google OpenImage Challenge 2019.
Guanglu Song, Yu Liu, Xiaogang Wang [pdf]
-
Search to Distill: Pearls are Everywhere but not the Eyes -
Abstract
- Standard Knowledge Distillation (KD) approaches distill the knowledge of a cumbersome teacher model into the parameters of a student model with a pre-defined architecture. However, the knowledge of a neural network, which is represented by the network's output distribution conditioned on its input, depends not only on its parameters but also on its architecture. Hence, a more generalized approach for KD is to distill the teacher's knowledge into both the parameters and architecture of the student. To achieve this, we present a new Architecture-aware Knowledge Distillation (AKD) approach that finds student models (pearls for the teacher) that are best for distilling the given teacher model. In particular, we leverage Neural Architecture Search (NAS), equipped with our KD-guided reward, to search for the best student architectures for a given teacher. Experimental results show our proposed AKD consistently outperforms the conventional NAS plus KD approach, and achieves state-of-the-art results on the ImageNet classification task under various latency settings. Furthermore, the best AKD student architecture for the ImageNet classification task also transfers well to other tasks such as million level face recognition and ensemble learning.
Yu Liu, Xuhui Jia, Mingxing Tan, Raviteja Vemulapalli, Yukun Zhu, Bradley Green, Xiaogang Wang [pdf]
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SeqXY2SeqZ: Structure Learning for 3D Shapes by Sequentially Predicting
1D Occupancy Segments From 2D Coordinates -
Abstract
- Structure learning for 3D shapes is vital for 3D computer vision. State-of-the-art methods show promising results by representing shapes using implicit functions in 3D that are learned using discriminative neural networks. However, learning implicit functions requires dense and irregular sampling in 3D space, which also makes the sampling methods affect the accuracy of shape reconstruction during test. To avoid dense and irregular sampling in 3D, we propose to represent shapes using 2D functions, where the output of the function at each 2D location is a sequence of line segments inside the shape. Our approach leverages the power of functional representations, but without the disadvantage of 3D sampling. Specifically, we use a voxel tubelization to represent a voxel grid as a set of tubes along any one of the X, Y, or Z axes. Each tube can be indexed by its 2D coordinates on the plane spanned by the other two axes. We further simplify each tube into a sequence of occupancy segments. Each occupancy segment consists of successive voxels occupied by the shape, which leads to a simple representation of its 1D start and end location. Given the 2D coordinates of the tube and a shape feature as condition, this representation enables us to learn 3D shape structures by sequentially predicting the start and end locations of each occupancy segment in the tube. We implement this approach using a Seq2Seq model with attention, called SeqXY2SeqZ, which learns the mapping from a sequence of 2D coordinates along two arbitrary axes to a sequence of 1D locations along the third axis. SeqXY2SeqZ not only benefits from the regularity of voxel grids in training and testing, but also achieves high memory efficiency. Our experiments show that SeqXY2SeqZ outperforms the state-ofthe-art methods under widely used benchmarks.
Zhizhong Han, Guanhui Qiao, Yu-Shen Liu, Matthias Zwicker [pdf]
-
Universal thermodynamic relations with constant corrections for rotating AdS black holes -
Abstract
- In [Phys. Rev. Lett. 124, 101103 (2020)], a universal relation between corrections to entropy and extremality was proposed. The relation was also found to exactly hold for the four-dimensional charged AdS black hole. In this paper, we extend the study to the rotating BTZ and Kerr-AdS black holes when a constant correction to General Relativity is considered for the first time. The entropy and extremality bound are calculated, and they have a closely dependent behavior with the coupling parameter of the constant correction. We confirm the universal relation for the rotating AdS black holes. Furthermore, taking into consideration of the shift of the angular momentum, we confirm one more new universal relation for the rotating cases. In particular, we state a conjecture on a universal relation, which gives a universal conjecture relation between the shifted thermodynamic quantities for arbitrary black hole background. We believe that these universal relations will shed new light on the region of the quantum gravity.
Shao-Wen Wei, Ke Yang, Yu-Xiao Liu [pdf] DOI: 10.1016/j.nuclphysb.2020.115279 2003.06785v1 [pdf]
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Hearts of cotorsion pairs are functor categories over cohearts -
Abstract
- We study hearts of cotorsion pairs in triangulated and exact categories.We give a sufficient and necessary condition when the hearts have enough projectives. We also show in such condition they are equivalent to functor categories over cohearts of the cotorsion pairs.
Yu Liu [pdf]
-
Top-1 Solution of Multi-Moments in Time Challenge 2019 -
Abstract
- In this technical report, we briefly introduce the solutions of our team 'Efficient' for the Multi-Moments in Time challenge in ICCV 2019. We first conduct several experiments with popular Image-Based action recognition methods TRN, TSN, and TSM. Then a novel temporal interlacing network is proposed towards fast and accurate recognition. Besides, the SlowFast network and its variants are explored. Finally, we ensemble all the above models and achieve 67.22\% on the validation set and 60.77\% on the test set, which ranks 1st on the final leaderboard. In addition, we release a new code repository for video understanding which unifies state-of-the-art 2D and 3D methods based on PyTorch. The solution of the challenge is also included in the repository, which is available at https://github.com/Sense-X/X-Temporal.
Manyuan Zhang, Hao Shao, Guanglu Song, Yu Liu, Junjie Yan [pdf]
-
An Empirical “High-confidence” Candidate Zone for
-
Abstract
- In the third catalog of active galactic nuclei detected by the $Fermi$ Large Area Telescope Clean (3LAC) sample, there are 402 blazars candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lacs) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the Fermi sample. By studying the 3LAC sample, we found some critical values of $\gamma$-ray photon spectral index ($\Gamma_{\rm ph}$), variability index (VI) and radio flux (${\rm F_R}$) of the sources separate known FSRQs and BL Lac objects. We further utilize those values to defined an empirical "high-confidence" candidate zone that can be used to classify the BCUs. Within such a zone ($\Gamma_{\rm ph}<2.187$, log${\rm F_R}<2.258$ and ${ \rm logVI <1.702}$), we found that 120 BCUs can be classified BL Lac candidates with a higher degree of confidence (with a misjudged rate $<1\%$). Our results suggest that an empirical "high confidence" diagnosis is possible to distinguish the BL Lacs from the Fermi observations based on only on the direct observational data of $\Gamma_{\rm ph}$, VI and ${\rm F_R}$.
Shi-Ju Kang, Kerui Zhu, Jianchao Feng, Qingwen Wu, Bin-Bin Zhang, Yue Yin, Fei-Fei Wang, Yu Liu, Tian-Yuan Zheng [pdf] DOI: 10.3847/1538-4357/ab722d 2003.05942v1 [pdf]
-
Transparent Gatable Superconducting Shadow Junctions -
Abstract
- Gate tunable junctions are key elements in quantum devices based on hybrid semiconductor-superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single crystalline InAs, InSb and $\mathrm{InAs_{1-x}Sb_x}$ nanowires with epitaxial superconductors and in-situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in-situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high $I_\mathrm{C} R_\mathrm{N}$, close to the KO$-$2 limit. This study demonstrates a promising engineering path towards reliable gate-tunable superconducting qubits.
-
Bandgap-assisted quantum control of topological edge states in a cavity -
Abstract
- Quantum matter with exotic topological order has potential applications in quantum computation. However, in present experiments, the manipulations on topological states are still challenging. We here propose an architecture for optical control of topological matter. We consider a topological superconducting qubit array with Su-Schrieffer-Heeger (SSH) Hamiltonian which couples to a microwave cavity. Based on parity properties of the topological qubit array, we propose an optical spectroscopy method to observe topological phase transition, i.e., edge-to-bulk transition. This new method can be achieved by designing cavity-qubit couplings. A main purpose of this work is to understand how topological phase transition affects light-matter interaction. We find that topological bandgap plays an essential role on this issue. In topological phase, the resonant vacuum Rabi splitting of degenerate edge states coupling to the cavity field is protected from those of bulk states by the bandgap. In dispersive regime, the cavity induced coupling between edge states is dominant over couplings between edge and bulk states, due to the topological bandgap. As a result, quantum interference between topological edge states occures and enables single-photon transport through boundaries of the topological qubit array. Our work may pave a way for topological quantum state engineering.
Wei Nie, Yu-xi Liu Journal reference: Phys. Rev. Research 2, 012076 (2020) [pdf] DOI: 10.1103/PhysRevResearch.2.012076
-
New insights into thermodynamics and microstructure of AdS black holes -
Abstract
- Recently, black hole thermodynamics and phase transition have been studied in the extended phase space. Besides the VdW-like phase transition, more interesting phase transitions were found. More interestingly, combining with the thermodynamic geometry, the microstructure of black holes was investigated.In this paper, we give a brief review of recent progress on this subject.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Science Bulletin 65 (2020) 259-261 [pdf] DOI: 10.1016/j.scib.2019.11.020
-
Intriguing microstructures of five-dimensional neutral Gauss-Bonnet AdS black hole -
Abstract
- In this paper, we analytically study the phase structure and construct the Ruppeiner geometry in the extended phase space for the five-dimensional neutral Gauss-Bonnet AdS black hole. Through calculating the scalar curvature of the Ruppeiner geometry and combining the phase transition, we show that the attractive interaction is dominant in the microstructure of the black hole system. More significantly, there is an intriguing property that the normalized scalar curvature has the same expression for the saturated small and large black hole curves. This implies that although the microstructure is different before and after the small-large black hole phase transition, the interaction between the microscopic constituents keeps unchanged. These results are quite valuable on further understanding the microstructure of the AdS black hole in modified gravity.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys.Lett. B 803, 135287 (2020) [pdf] DOI: 10.1016/j.physletb.2020.135287
-
High-temperature Anomalous Hall Effect in Transition Metal
Dichalcogenide-Ferromagnetic Insulator Heterostructure -
Abstract
- Integration of transition metal dichalcogenides (TMDs) on ferromagnetic materials (FM) may yield fascinating physics and promise for electronics and spintronic applications. In this work, high-temperature anomalous Hall effect (AHE) in the TMD ZrTe2 thin film using heterostructure approach by depositing it on ferrimagnetic insulator YIG (Y3Fe5O12, yttrium iron garnet) is demonstrated. In this heterostructure, significant anomalous Hall effect can be observed at temperatures up to at least 400 K, which is a record high temperature for the observation of AHE in TMDs, and the large RAHE is more than one order of magnitude larger than those previously reported value in topological insulators or TMDs based heterostructures. The magnetization of interfacial reaction-induced ZrO2 between YIG and ZrTe2 is believed to play a crucial role for the induced high-temperature anomalous Hall effect in the ZrTe2. These results reveal a promising system for the room-temperature spintronic device applications, and it may also open a new avenue toward introducing magnetism to TMDs and exploring the quantum AHE at higher temperatures considering the prediction of nontrivial topology in ZrTe2.
Sheung Mei Ng, Hui Chao Wang, Yu Kuai Liu, Hon Fai Wong, Hei Man Yau, Chun Hung Suen, Ze Han Wu, Chi Wah Leung, Ji Yan Dai [pdf]
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Quantifying urban areas with multi-source data based on percolation theory -
Abstract
- Quantifying urban areas is crucial for addressing associated urban issues such as environmental and sustainable problems. Remote sensing data, especially the nighttime light images, have been widely used to delineate urbanized areas across the world. Meanwhile, some emerging urban data, such as volunteered geographical information (e.g., OpenStreetMap) and social sensing data (e.g., mobile phone and social media), have also shown great potential in revealing urban boundaries and dynamics. However, consistent and robust methods to quantify urban areas from these multi-source data have remained elusive. Here, we propose a percolation-based method to extract urban areas from these multi-source urban data. We derive the optimal urban/non-urban threshold by considering the critical nature of urban systems with the support of the percolation theory. Furthermore, we apply the method with three open-source datasets - population, road, and nighttime light - to 28 countries. We show that the proposed method captures the similar urban characteristics in terms of urban areas from multi-source data, and Zipf's law holds well in most countries. The accuracy of the derived urban areas by different datasets has been validated with the Landsat-based reference data in 10 cities, and the accuracy can be further improved through data fusion ($\kappa=0.69-0.85$, mean $\kappa=0.78$). Our study not only provides an efficient method to quantify urban areas with open-source data, but also deepens the understanding of urban systems and sheds some light on multi-source data fusion in geographical fields.
Wenpu Cao, Lei Dong, Lun Wu, Yu Liu Journal reference: Remote Sensing of Environment, (2020) [pdf] DOI: 10.1016/j.rse.2020.111730
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Photo-excitation of long-lived transient intermediates in ultracold reactions -
Abstract
- Controlling the pathways and outcomes of reactions is a broadly pursued goal in chemistry. In gas phase reactions, this is typically achieved by manipulating the properties of the reactants, including their translational energy, orientation, and internal quantum state. In contrast, here we influence the pathway of a reaction via its intermediate complex, which is generally too short-lived to be affected by external processes. In particular, the ultracold preparation of potassium-rubidium (KRb) reactants leads to a long-lived intermediate complex (K$_2$Rb$_2^*$), which allows us to steer the reaction away from its nominal ground-state pathway onto a newly identified excited-state pathway using a laser source at 1064 nm, a wavelength commonly used to confine ultracold molecules. Furthermore, by monitoring the change in the complex population after the sudden removal of the excitation light, we directly measure the lifetime of the complex to be $360 \pm 30$ ns, in agreement with our calculations based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical theory. Our results shed light on the origin of the two-body loss widely observed in ultracold molecule experiments. Additionally, the long complex lifetime, coupled with the observed photo-excitation pathway, opens up the possibility to spectroscopically probe the structure of the complex with high resolution, thus elucidating the reaction dynamics.
Yu Liu, Ming-Guang Hu, Matthew A. Nichols, David D. Grimes, Tijs Karman, Hua Guo, Kang-Kuen Ni Journal reference: Nature Physics, volume 16, pages 1132-1136 (2020) [pdf] DOI: 10.1038/s41567-020-0968-8
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3D Gated Recurrent Fusion for Semantic Scene Completion -
Abstract
- This paper tackles the problem of data fusion in the semantic scene completion (SSC) task, which can simultaneously deal with semantic labeling and scene completion. RGB images contain texture details of the object(s) which are vital for semantic scene understanding. Meanwhile, depth images capture geometric clues of high relevance for shape completion. Using both RGB and depth images can further boost the accuracy of SSC over employing one modality in isolation. We propose a 3D gated recurrent fusion network (GRFNet), which learns to adaptively select and fuse the relevant information from depth and RGB by making use of the gate and memory modules. Based on the single-stage fusion, we further propose a multi-stage fusion strategy, which could model the correlations among different stages within the network. Extensive experiments on two benchmark datasets demonstrate the superior performance and the effectiveness of the proposed GRFNet for data fusion in SSC. Code will be made available.
Yu Liu, Jie Li, Qingsen Yan, Xia Yuan, Chunxia Zhao, Ian Reid, Cesar Cadena [pdf]
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Hyperspectral Classification Based on 3D Asymmetric Inception Network
with Data Fusion Transfer Learning -
Abstract
- Hyperspectral image(HSI) classification has been improved with convolutional neural network(CNN) in very recent years. Being different from the RGB datasets, different HSI datasets are generally captured by various remote sensors and have different spectral configurations. Moreover, each HSI dataset only contains very limited training samples and thus it is prone to overfitting when using deep CNNs. In this paper, we first deliver a 3D asymmetric inception network, AINet, to overcome the overfitting problem. With the emphasis on spectral signatures over spatial contexts of HSI data, AINet can convey and classify the features effectively. In addition, the proposed data fusion transfer learning strategy is beneficial in boosting the classification performance. Extensive experiments show that the proposed approach beat all of the state-of-art methods on several HSI benchmarks, including Pavia University, Indian Pines and Kennedy Space Center(KSC). Code can be found at: https://github.com/UniLauX/AINet.
Haokui Zhang, Yu Liu, Bei Fang, Ying Li, Lingqiao Liu, Ian Reid [pdf]
-
The Accurate Modification of Tunneling Radiation of Fermions with
Arbitrary Spin in Kerr-de Sitter Black Hole Space-time -
Abstract
- The quantum tunneling radiation of fermions with arbitrary spin at the event horizon of Kerr-de Sitter black hole is accurately modified by using the dispersion relation proposed in the study of string theory and quantum gravitational theory. The derived tunneling rate and temperature at the black hole horizons are analyzed and studied.
Bei Sha, Zhi-E Liu, Xia Tan, Yu-Zhen Liu, Jie Zhang [pdf]
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Accurate correction of arbitrary spin fermion quantum tunneling from non-stationary Kerr-de Sitter black hole based on corrected Lorentz dispersion relation -
Abstract
- According to a corrected dispersion relation proposed in the study on string theory and quantum gravity theory, Rarita-Schwinger equation has been precisely modified, which results in a Rarita-Schwinger-Hamilton-Jacobi equation, and through which, the characteristics of arbitrary spin fermions quantum tunneling radiation from non-stationary Kerr-de Sitter black hole are researched. A series of accurately corrected physical quantities such as surface gravity, chemical potential, tunneling probability and Hawking temperature that describe the properties of the black hole are derived. This research has enriched the research methods and made precision of the research contents of black hole physics.
Bei Sha, Zhi-E Liu, Yu-Zhen Liu, Xia Tan, Jie Zhang, Shu-Zheng Yang [pdf] DOI: 10.1088/1674-1137/abb4d6 2002.03368v1 [pdf]
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On the Fairness of Name-Based Rationing System for Purchases of Masks
Policy -
Abstract
- In this paper, mathematical model and condition are built for the analysis of fairness of name-based rationing system for purchases of masks policy announced and launched in Taiwan.
Yu-Ting Liu [pdf]
-
Long-Range Gesture Recognition Using Millimeter Wave Radar -
Abstract
- Millimeter wave (mmWave) based gesture recognition technology provides a good human computer interaction (HCI) experience. Prior works focus on the close-range gesture recognition, but fall short in range extension, i.e., they are unable to recognize gestures more than one meter away from considerable noise motions. In this paper, we design a long-range gesture recognition model which utilizes a novel data processing method and a customized artificial Convolutional Neural Network (CNN). Firstly, we break down gestures into multiple reflection points and extract their spatial-temporal features which depict gesture details. Secondly, we design a CNN to learn changing patterns of extracted features respectively and output the recognition result. We thoroughly evaluate our proposed system by implementing on a commodity mmWave radar. Besides, we also provide more extensive assessments to demonstrate that the proposed system is practical in several real-world scenarios.
Yu Liu, Yuheng Wang, Haipeng Liu, Anfu Zhou, Jianhua Liu, Ning Yang [pdf]
-
Lorentz symmetry violation and the tunneling radiation of fermions with
spin $1/2$ for Kerr Anti-de-Sitter black hole -
Abstract
- We studied the correction of the quantum tunneling radiation of fermions with spin $1/2$ in Kerr Anti-de-Sitter black hole. First, the dynamic equation of spin $1/2$ fermions was corrected using Lorentz's violation theory. Second, the new expressions of the fermions quantum tunneling rate, the Hawking temperature of the black hole and the entropy of the black hole were obtained according to the corrected fermions dynamic equation. At last, some comments are made on the results of our work.
Zhi-E Liu, Xia Tan, Yu-Zhen Liu, Bei Sha, Jie Zhang, Shu-Zheng Yang [pdf]
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Site testing campaign for the Large Optical/infrared Telescope of China: overview -
Abstract
- The Large Optical/infrared Telescope (LOT) is a ground-based 12m diameter optical/infrared telescope which is proposed to be built in the western part of China in the next decade. Based on satellite remote sensing data, along with geographical, logistical and political considerations, three candidate sites were chosen for ground-based astronomical performance monitoring. These sites include: Ali in Tibet, Daocheng in Sichuan, and Muztagh Ata in Xinjiang. Up until now, all three sites have continuously collected data for two years. In this paper, we will introduce this site testing campaign, and present its monitoring results obtained during the period between March 2017 and March 2019.
Lu Feng, Jin-Xin Hao, Zi-Huang Cao, Jin-Min Bai, Ji Yang, Xu Zhou, Yong-Qiang Yao, Jin-Liang Hou, Yong-Heng Zhao, Yu Liu, Teng-Fei Song, Li-Yong Liu, Jia Yin, Hua-Lin Chen, Chong Pei, Ali Esamdin, Lu Ma, Chun-Hai Bai, Peng Wei, Jing Xu, Guang-Xin Pu, Guo-Jie Feng, Xuan Zhang, Liang Ming, Abudusaimaitijiang Yisikandee, Jian-Rong Shi, Jian Li, Yuan Tian, Zheng Wang, Xia Wang, Xiao-Jun Jiang, Jian-Feng Wang, Jian-Feng Tian, Yan-Jie Xue, Jian-Sheng Chen, Jing-Yao Hu, Zhi-Xia Shen, Yun-Ying Jiang [pdf] DOI: 10.1088/1674-4527/20/6/80 2001.11378v2 [pdf]
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MVDLite: A Light-weight Model View Definition Representation with Fast
Validation for Building Information Model -
Abstract
- Model View Definition (MVD) is the standard methodology to define the exchange requirements and data constraints for Building Information Model (BIM). In this paper, MVDLite is proposed as a novel light-weight representation for MVD. Compared with mvdXML, MVDLite is more concise and could be used in more flexible scenarios. MVDLite introduces a "rule chain" structure to combine the subgraph templates and value constants, based on which a fast MVD validation algorithm is proposed. It is also compatible with the current mvdXML format, and supports bi-directional conversion with mvdXML. A case study is provided to show the workflow for developing an enterprise-level MVD based on MVDLite, and its applications in MVD validation and partial model extraction. The outperforming experimental results show that our method is much faster than the state-of-the-art methods on large real-world models.
Han Liu, Ge Gao, Hehua Zhang, Yu-Shen Liu, Yan Song, Ming Gu [pdf]
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Depth Based Semantic Scene Completion with Position Importance Aware
Loss -
Abstract
- Semantic Scene Completion (SSC) refers to the task of inferring the 3D semantic segmentation of a scene while simultaneously completing the 3D shapes. We propose PALNet, a novel hybrid network for SSC based on single depth. PALNet utilizes a two-stream network to extract both 2D and 3D features from multi-stages using fine-grained depth information to efficiently captures the context, as well as the geometric cues of the scene. Current methods for SSC treat all parts of the scene equally causing unnecessary attention to the interior of objects. To address this problem, we propose Position Aware Loss(PA-Loss) which is position importance aware while training the network. Specifically, PA-Loss considers Local Geometric Anisotropy to determine the importance of different positions within the scene. It is beneficial for recovering key details like the boundaries of objects and the corners of the scene. Comprehensive experiments on two benchmark datasets demonstrate the effectiveness of the proposed method and its superior performance. Models and Video demo can be found at: https://github.com/UniLauX/PALNet.
Yu Liu, Jie Li, Xia Yuan, Chunxia Zhao, Roland Siegwart, Ian Reid, Cesar Cadena [pdf]
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Number of constituent quark scaling of elliptic flows in high multiplicity p-Pb collisions at
-
Abstract
- We briefly summarize our recent study on the number of constituent quark (NCQ) scaling of hadron elliptic flows in high multiplicity p-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV. With the inclusion of hadron production via the quark coalescence model at intermediate $p_T$, the viscous hydrodynamics at low $p_T$, and jet fragmentation at high $p_T$, our $Hydro-Coal-Frag$ model provides a nice description of the $p_T$-spectra and differential elliptic flow $v_2(p_T)$ of pions, kaons and protons over the $p_T$ range from 0 to 6 GeV. Our results demonstrate that including the quark coalescence is essential for reproducing the observed approximate NCQ scaling of hadron $v_2$ at intermediate $p_T$ in experiments, indicating strongly the existence of partonic degrees of freedom and the formation of quark-gluon plasma in high multiplicity p--Pb collisions at the LHC.
Wenbin Zhao, Che Ming Ko, Yu-Xin Liu, Guang-You Qin, Huichao Song [pdf] DOI: 10.1016/j.nuclphysa.2020.121876 2001.10689v1 [pdf]
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Topologically Protected Quantum Coherence in a Superatom -
Abstract
- Exploring the properties and applications of topological quantum states is essential to better understand topological matter. Here, we theoretically study a quasi-one-dimensional topological atom array. In the low-energy regime, the atom array is equivalent to a topological superatom. Driving the superatom in a cavity, we study the interaction between light and topological quantum states. We find that the edge states exhibit topology-protected quantum coherence, which can be characterized from the photon transmission. This quantum coherence helps us to find a superradiance-subradiance transition, and we also study its finite-size scaling behavior. The superradiance-subradiance transition also exists in symmetry-breaking systems. More importantly, it is shown that the quantum coherence of the subradiant edge state is robust to random noises, allowing the superatom to work as a topologically protected quantum memory. We suggest a relevant experiment with three-dimensional circuit QED. Our study may have applications in quantum computation and quantum optics based on topological edge states.
Wei Nie, Z. H. Peng, Franco Nori, Yu-xi Liu Journal reference: Phys. Rev. Lett. 124, 023603 (2020) [pdf] DOI: 10.1103/PhysRevLett.124.023603
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Collision of two kinks with inner structure -
Abstract
- In this work, we study kink collisions in a scalar field model with scalar-kinetic coupling. This model supports kink/antikink solutions with inner structure in the energy density. The collision of two such kinks is simulated by using the Fourier spectral method. We numerically calculate how the critical velocity and the widths of the first three two bounce windows vary with the model parameters. After that, we report some interesting collision results including two-bion escape final states, kink-bion-antikink intermediate states and kink or antikink intertwined final states. These results show that kinks with inner structure in the energy density have similar properties as those of the double kinks.
Yuan Zhong, Xiao-Long Du, Zhou-Chao Jiang, Yu-Xiao Liu, Yong-Qiang Wang Journal reference: JHEP02(2020)153 [pdf] DOI: 10.1007/JHEP02(2020)153
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Low-Complexity LSTM Training and Inference with FloatSD8 Weight
Representation -
Abstract
- The FloatSD technology has been shown to have excellent performance on low-complexity convolutional neural networks (CNNs) training and inference. In this paper, we applied FloatSD to recurrent neural networks (RNNs), specifically long short-term memory (LSTM). In addition to FloatSD weight representation, we quantized the gradients and activations in model training to 8 bits. Moreover, the arithmetic precision for accumulations and the master copy of weights were reduced from 32 bits to 16 bits. We demonstrated that the proposed training scheme can successfully train several LSTM models from scratch, while fully preserving model accuracy. Finally, to verify the proposed method's advantage in implementation, we designed an LSTM neuron circuit and showed that it achieved significantly reduced die area and power consumption.
Yu-Tung Liu, Tzi-Dar Chiueh [pdf]
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Temporal Interlacing Network -
Abstract
- For a long time, the vision community tries to learn the spatio-temporal representation by combining convolutional neural network together with various temporal models, such as the families of Markov chain, optical flow, RNN and temporal convolution. However, these pipelines consume enormous computing resources due to the alternately learning process for spatial and temporal information. One natural question is whether we can embed the temporal information into the spatial one so the information in the two domains can be jointly learned once-only. In this work, we answer this question by presenting a simple yet powerful operator -- temporal interlacing network (TIN). Instead of learning the temporal features, TIN fuses the two kinds of information by interlacing spatial representations from the past to the future, and vice versa. A differentiable interlacing target can be learned to control the interlacing process. In this way, a heavy temporal model is replaced by a simple interlacing operator. We theoretically prove that with a learnable interlacing target, TIN performs equivalently to the regularized temporal convolution network (r-TCN), but gains 4% more accuracy with 6x less latency on 6 challenging benchmarks. These results push the state-of-the-art performances of video understanding by a considerable margin. Not surprising, the ensemble model of the proposed TIN won the $1^{st}$ place in the ICCV19 - Multi Moments in Time challenge. Code is made available to facilitate further research at https://github.com/deepcs233/TIN
Hao Shao, Shengju Qian, Yu Liu [pdf]
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Weak cosmic censorship conjecture for a Kerr-Taub-NUT black hole with a test scalar field and particle -
Abstract
- Motivated by the recent researches of black holes with NUT charge, we investigate the validity of the weak cosmic censorship conjecture for Kerr-Taub-NUT black hole with a test massive scalar field and a test particle, respectively. For the scalar field scattering gedanken experiment, we consider an infinitesimal time interval process. The result shows that both extremal and near-extremal Kerr-Taub-NUT black holes cannot be over-spun. For the test particle thought experiment, the study suggests that extremal Kerr-Taub-NUT black hole cannot be over-spun; while near-extremal Kerr-Taub-NUT black hole can be over-spun. By comparing the two methods, the results indicate the time interval for particles crossing the black hole horizon might be important for consideration of the weak cosmic censorship conjecture.
Si-Jiang Yang, Jing Chen, Jun-Jie Wan, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 101, 064048 (2020) [pdf] DOI: 10.1103/PhysRevD.101.064048
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Generalized geometrical coupling for vector field localization on thick brane in asymptotic anti–de Sitter spacetime -
Abstract
- It is known that a five-dimensional free vector field $A_{M}$ cannot be localized on Randall-Sundrum (RS)-like thick branes, namely, the thick branes embedded in asymptotic Anti-de Sitter (AdS) spacetime. In order to localize a vector field on the RS-like thick brane, an extra coupling term should be introduced. In this paper, we generalize the geometrical coupling mechanism by adding two mass terms ($\alpha Rg^{MN}A_{M}A_{N}+\beta R^{MN}A_{M}A_{N}$) into the action. We decompose the fundamental vector field $A_{M}$ into three parts: transverse vector part $\hat{A}_{\mu}$, scalar parts $\phi$ and $A_{5}$. Then, we find that the transverse vector part $\hat{A}_{\mu}$ decouples from the scalar parts. In order to eliminate the tachyonic modes of $\hat{A}_{\mu}$, the two coupling parameters $\alpha$ and $\beta$ should satisfy a relation. Combining the restricted condition, we can get a combination parameter as $\gamma=\frac{3}{2}\pm\sqrt{1+12\alpha}$. Only if $\gamma>1/2$, the zero mode of $\hat{A}_{\mu}$ can be localized on the RS-like thick brane. We also investigate the resonant character of the vector part $\hat{A}_{\mu}$ for the general RS-like thick brane with the warp factor $A(z)=-\ln(1+k^2z^2)/2$ by choosing the relative probability method. The result shows that, only for $\gamma>3$, the massive resonant Kaluza-Klein modes can exist. The number of resonant Kaluza-Klein states increases with the combination parameter $\gamma$, and the lifetime of the first resonant state can be long enough as the age of our universe. This indicates that the vector resonances might be considered as one of the candidates of dark matter.
Tao-Tao Sui, Wen-Di Guo, Qun-Ying Xie, Yu-Xiao Liu Journal reference: Phys. Rev. D 101, 055031 (2020) [pdf] DOI: 10.1103/PhysRevD.101.055031
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Abstract
- We predict the masses of the lowlying $B_c$ mesons with $J^P = 0^-,\,1^-,\,0^+,\,1^+,\,2^+$, using a flavor dependent interaction pattern which gives an unified successful description of the light, heavy-light and heavy mesons and is also appliable to the radial excited heavy mesons. The errors are controlled carefully. With the errors from the RL approximation subduced, our predictions are consistent with the lQCD and quark model results, which supports strongly that the flavor dependent interaction pattern is reasonable. Our predictions provide significant guides to the experiment search of the $B_c$ mesons.
Muyang Chen, Lei Chang, Yu-xin Liu Journal reference: Phys. Rev. D 101, 056002 (2020) [pdf] DOI: 10.1103/PhysRevD.101.056002
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Heavy-light mesons beyond the ladder approximation -
Abstract
- The heavy-light mesons are studied within the framework of Dyson-Schwinger equations of QCD. Inspired by the axial-vector Ward-Takahashi identity resulting from the chiral symmetry, we propose a truncation scheme beyond the ladder approximation without introducing any additional parameter. For the pseudoscalar and vector heavy-light mesons, the obtained mass spectrum has the level of relative errors at $5\%$ compared with experimental data and lattice-QCD results. For the leptonic decay constants, our results are comparable with those from experiments and/or lattice QCD. For some channels, the discrepancies are sizable but significantly smaller than those using the equal spacing rule. The truncation scheme proposed in this work is simple and could be improved and applied to study other open flavor hadrons including both mesons and baryons.
Pianpian Qin, Si-xue Qin, Yu-xin Liu Journal reference: Phys. Rev. D 101, 114014 (2020) [pdf] DOI: 10.1103/PhysRevD.101.114014
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Unpolarized isovector quark distribution function from lattice QCD: A systematic analysis of renormalization and matching -
Abstract
- We present a detailed Lattice QCD study of the unpolarized isovector quark Parton Distribution Function (PDF) using large-momentum effective theory framework. We choose a quasi-PDF defined by a spatial correlator which is free from mixing with other operators of the same dimension. In the lattice simulation, we use a Gaussian-momentum-smeared source at $M_\pi=356$ MeV and $P_z \in \{1.8,2.3\}$ GeV. To control the systematics associated with the excited states, we explore {five different source-sink separations}. The nonperturbative renormalization is conducted in a regularization-independent momentum subtraction scheme, and the matching between the renormalized quasi-PDF and $\bar{\rm MS}$ PDF is calculated based on perturbative QCD up to one-loop order. Systematic errors due to renormalization and perturbative matching are also analyzed in detail. Our results for lightcone PDF are in reasonable agreement with the latest phenomenological analysis.
Yu-Sheng Liu, Jiunn-Wei Chen, Yi-Kai Huo, Luchang Jin, Maximilian Schlemmer, Andreas Schäfer, Peng Sun, Wei Wang, Yi-Bo Yang, Jian-Hui Zhang, Qi-An Zhang, Kuan Zhang, Yong Zhao Journal reference: Phys. Rev. D 101, 034020 (2020) [pdf] DOI: 10.1103/PhysRevD.101.034020
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Quantum numbers of the pentaquark states ${{\rm{P}}}_{{\rm{c}}}^{+}$ via symmetry analysis -
Abstract
- We investigate the quantum numbers of the pentaquark states $\textrm{P}_{\textrm{c}}^{+}$, which are composed of four (three flavors) quarks and an antiquark, by analyzing their inherent nodal structure in this paper. Assuming that the four quarks form a tetrahedron or a square, and the antiquark locates at the center of the four quark cluster, we determine the nodeless structure of the states with orbital angular moment $L \leq 3$, and in turn, the accessible low-lying states. Since the inherent nodal structure depends only on the inherent geometric symmetry, we propose the quantum numbers $J^{P}$ of the low-lying pentaquark states $\textrm{P}_{c}^{+}$ may be ${\frac{3}{2}}^{-}$, ${\frac{5}{2}}^{-} $, ${\frac{3}{2}}^{+}$, ${\frac{5}{2}}^{+} $, independent of dynamical models.
Chong-yao Chen, Muyang Chen, Yu-xin Liu Journal reference: Commun. Theor. Phys. 72, 125202 (2020) [pdf] DOI: 10.1088/1572-9494/abb7cd
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Critical behavior and magnetocaloric effect in
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Abstract
- Layered van der Waals ferromagnets are promising candidates for designing new spintronic devices. Here we investigated the critical properties and magnetocaloric effect connected with ferromagnetic transition in layered van der Waals VI$_3$ single crystals. The critical exponents $\beta = 0.244(5)$ with a critical temperature $T_c = 50.10(2)$ K and $\gamma = 1.028(12)$ with $T_c = 49.97(5)$ K are obtained from the modified Arrott plot, whereas $\delta = 5.24(2)$ is obtained from a critical isotherm analysis at $T_c = 50$ K. The magnetic entropy change $-\Delta S_M(T,H)$ features a maximum at $T_c$, i.e., $-\Delta S_M^{max} \sim$ 2.64 (2.27) J kg$^{-1}$ K$^{-1}$ with out-of-plane (in-plane) field change of 5 T. This is consistent with $-\Delta S_M^{max}$ $\sim$ 2.80 J kg$^{-1}$ K$^{-1}$ deduced from heat capacity and the corresponding adiabatic temperature change $\Delta T_{ad}$ $\sim$ 0.96 K with out-of-plane field change of 5 T. The critical analysis suggests that the ferromagnetic phase transition in VI$_3$ is situated close to a three- to two-dimensional critical point. The rescaled $\Delta S_M(T,H)$ curves collapse onto a universal curve, confirming a second-order type of the magnetic transition and reliability of the obtained critical exponents.
Yu Liu, Milinda Abeykoon, C. Petrovic Journal reference: Phys. Rev. Research 2, 013013 (2020) [pdf] DOI: 10.1103/PhysRevResearch.2.013013
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Generation of macroscopic entangled cat states in a longitudinally coupled cavity-QED model -
Abstract
- Macroscopic entangled cat states not only are significant in the demonstration of the fundamentals of quantum physics, but also have wide applications in modern quantum technologies such as continuous-variable quantum information processing and quantum metrology. Here we propose a scheme for generation of macroscopic entangled cat states in a molecular cavity-QED system, which is composed of an organic molecule (including electronic and vibrational states) coupled to a single-mode cavity field. By simultaneously modulating the resonance frequencies of the molecular vibration and the cavity field, the molecular vibrational displacement can be enhanced significantly and hence macroscopic entangled cat states between the molecular vibrational mode and the cavity mode can be created. We also study quantum coherence effects in the generated states by calculating the joint Wigner function and the degree of entanglement. The dissipation effects are included by considering the state generation in the open-system case. Our results will pave the way to the study of quantum physics and quantum chemistry in molecular cavity-QED systems.
Jian Huang, Yu-Hong Liu, Jin-Feng Huang, Jie-Qiao Liao Journal reference: Phys. Rev. A 101, 043841 (2020) [pdf] DOI: 10.1103/PhysRevA.101.043841
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New insight about the effective restoration of
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Abstract
- The effective restoration of the U_{A}(1) symmetry is revisited by implementing the functional renormalization group approach combining with the 2+1 flavor Polyakov-loop quark-meson model. A temperature-dependent 't Hooft term is taken to imitate the restoration of the U_{A}(1) symmetry. Order parameters, meson spectrum and mixing angles, especially the pressure and the entropy density of the system are calculated to explore the effects of different U_{A}(1) symmetry restoration patterns. We show then that the temperature for the restoration of the U_{A}(1) symmetry is much higher than that for the chiral symmetry SU_{A}(3).
Xiang Li, Wei-jie Fu, Yu-xin Liu Journal reference: Phys. Rev. D 101, 054034 (2020) [pdf] DOI: 10.1103/PhysRevD.101.054034
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Quantum Sensing with a Single-Qubit Pseudo-Hermitian System -
Abstract
- Quantum sensing exploits fundamental features of quantum system to achieve highly efficient measurement of physical quantities. Here, we propose a strategy to realize a single-qubit pseudo-Hermitian sensor from a dilated two-qubit Hermitian system. The pseudo-Hermitian sensor exhibits divergent susceptibility in dynamical evolution that does not necessarily involve exceptional point. We demonstrate its potential advantages to overcome noises that cannot be averaged out by repetitive measurements. The proposal is feasible with the state-of-art experimental capability in a variety of qubit systems, and represents a step towards the application of non-Hermitian physics in quantum sensing.
Yaoming Chu, Yu Liu, Haibin Liu, Jianming Cai Journal reference: Phys. Rev. Lett. 124, 020501 (2020) [pdf] DOI: 10.1103/PhysRevLett.124.020501
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Observation of Anti-
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Abstract
- As the counterpart of PT symmetry, abundant phenomena and potential applications of anti-PT symmetry have been predicted or demonstrated theoretically. However, experimental realization of the coupling required in the anti-PT symmetry is difficult. Here, by coupling two YIG spheres to a microwave cavity, the large cavity dissipation rate makes the magnons coupled dissipatively with each other, thereby obeying a two-dimensional anti-PT Hamiltonian. In terms of the magnon-readout method, a new method adopted here, we demonstrate the validity of our method in constructing an anti-PT system and present the counterintuitive level attraction process. Our work provides a new platform to explore the anti-PT symmetry properties and paves the way to study multi-magnoncavity-polariton systems.
Jie Zhao, Yulong Liu, Longhao Wu, Chang-Kui Duan, Yu-xi Liu, Jiangfeng Du Journal reference: Phys. Rev. Applied 13, 014053 (2020) [pdf] DOI: 10.1103/PhysRevApplied.13.014053
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2D honeycomb borophene oxide: a promising anode material offering super high capacity for Li/Na-ion batteries -
Abstract
- Rational design of novel two-dimensional (2D) electrode materials with high capacity is crucial for the further development of Li-ion and Na-ion batteries. Herein, based on first-principles calculations, we systemically investigate Li and Na storage behaviors in the recently discovered 2D topological nodal-loop metal - the honeycomb borophene oxide (h-B2O). We show that h-B2O is an almost ideal anode material. It has good conductivity before and after Li/Na adsorption, fast ion diffusion with diffusion barrier less than 0.5 eV, low open-circuit voltage (less than 1 V), and small lattice change (less than 6.2%) during intercalation. Most remarkably, its theoretical storage capacity is extremely high, reaching up to 2137 mAh/g for Li and 1425 mAh/g for Na. Its Li storage capacity is more than six times higher than graphite (372 mAh/g), and is actually the highest among all 2D materials discovered to date. Our results strongly suggest that 2D h-B2O is an exceedingly promising anode material for both Li- and Na-ion batteries with super high capacity.
Junping Hu, Chengyong Zhong, Weikang Wu, Ning Liu, Yu Liu, Shengyuan A. Yang, Chuying Ouyang Journal reference: J. Phys.: Condens. Matter 32 (2020) 065001 [pdf] DOI: 10.1088/1361-648X/ab4f4d
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Excited
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Abstract
- We study the most recently observed excited $B_{c}$ states with the Dyson-Schwinger equation and the Bethe-Salpeter equation approach of continuum QCD. The obtained $M_{B^+_{c}(2S)}=6.813(16)\text{GeV}$, $M_{B^{*+}_{c}(2S)}=6.841(18)\text{GeV}$ and the mass splitting $M_{B_c^+(2S)}-M^{\text{rec}}_{B_c^{*+}(2S)} \approx 0.039 \text{GeV}$ agree with the observations very well. Moreover we predict the leptonic decay constant $f_{B^+_{c}(2S)}=-0.165(10)\text{GeV}$, $f_{B^{*+}_{c}(2S)}=-0.161(7)\text{GeV}$ respectively.
Lei Chang, Muyang Chen, Yu-xin Liu Journal reference: Phys. Rev. D 102, 074010 (2020) [pdf] DOI: 10.1103/PhysRevD.102.074010
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Imaging emergent heavy Dirac fermions of a topological Kondo insulator -
Abstract
- Kondo insulators are primary candidates in the search for strongly correlated topological quantum phases, which may host topological order, fractionalization, and non-Abelian statistics. Within some Kondo insulators, the hybridization gap is predicted to protect a nontrivial topological invariant and to harbor emergent heavy Dirac fermion surface modes. We use high-energy-resolution spectroscopic imaging in real and momentum space on the Kondo insulator, SmB$_6$. On cooling through $T^*_{\Delta}\approx$ 35 K we observe the opening of an insulating gap that expands to $\Delta\approx$ 10 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching $m^* = (410\pm20)m_e$. We thus demonstrate existence of a strongly correlated topological Kondo insulator phase hosting the heaviest known Dirac fermions.
Harris Pirie, Yu Liu, A. Soumyanarayanan, Pengcheng Chen, Yang He, M. M. Yee, P. F. S. Rosa, J. D. Thompson, Dae-Jeong Kim, Z. Fisk, Xiangfeng Wang, J. Paglione, Dirk K. Morr, M. H. Hamidian, Jennifer E. Hoffman Journal reference: Nat. Phys. 16, 52 (2020) [pdf] DOI: 10.1038/s41567-019-0700-8
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A modification of Faddeev–Popov approach free from Gribov ambiguity -
Abstract
- We propose a modified version of the Faddeev-Popov quantization approach for non-Abelian gauge field theory to avoid the Gribov ambiguity. We show that by means of introducing a new method to insert the correct identity into the Yang-Mills generating functional and considering the identity generated by an integral through a subgroup of the gauge group, the problem of the Gribov ambiguity can be removed naturally. Meanwhile by handling the absolute value of the Faddeev-Popov determinant with the method introduced by Williams and collaborators, we lift the Jacobian determinant together with the absolute value and obtain a local Lagrangian. The new Lagrangian have a nilpotent symmetry which can be viewed as an analogue of the BRST symmetry.
Chong-yao Chen, Fei Gao, Yu-xin Liu Journal reference: Commun. Theor. Phys. 72, 125201 (2020) [pdf] DOI: 10.1088/1572-9494/abb7cb
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Zero-bias peaks at zero magnetic field in ferromagnetic hybrid nanowires -
Abstract
- We report transport measurements and tunneling spectroscopy in hybrid nanowires with epitaxial layers of superconducting Al and the ferromagnetic insulator EuS, grown on semiconducting InAs nanowires. In devices where the Al and EuS covered facets overlap, we infer a remanent effective Zeeman field of order 1 T, and observe stable zero-bias conductance peaks in tunneling spectroscopy into the end of the nanowire, consistent with topological superconductivity at zero applied field. Hysteretic features in critical current and tunneling spectra as a function of applied magnetic field support this picture. Nanowires with non-overlapping Al and EuS covered facets do not show comparable features. Topological superconductivity in zero applied field allows new device geometries and types of control.
S. Vaitiekėnas, Y. Liu, P. Krogstrup, C. M. Marcus [pdf] DOI: 10.1038/s41567-020-1017-3 2004.02226v1 [pdf]
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Covariant spin kinetic theory I: collisionless limit -
Abstract
- 2019
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The Sariçiçek howardite fall in Turkey: Source crater of
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Abstract
- The Sari\c{c}i\c{c}ek howardite meteorite shower consisting of 343 documented stones occurred on 2 September 2015 in Turkey and is the first documented howardite fall. Cosmogenic isotopes show that Sari\c{c}i\c{c}ek experienced a complex cosmic ray exposure history, exposed during ~12-14 Ma in a regolith near the surface of a parent asteroid, and that an ca.1 m sized meteoroid was launched by an impact 22 +/- 2 Ma ago to Earth (as did one third of all HED meteorites). SIMS dating of zircon and baddeleyite yielded 4550.4 +/- 2.5 Ma and 4553 +/- 8.8 Ma crystallization ages for the basaltic magma clasts. The apatite U-Pb age of 4525 +/- 17 Ma, K-Ar age of ~3.9 Ga, and the U,Th-He ages of 1.8 +/- 0.7 and 2.6 +/- 0.3 Ga are interpreted to represent thermal metamorphic and impact-related resetting ages, respectively. Petrographic, geochemical and O-, Cr- and Ti- isotopic studies confirm that Sari\c{c}i\c{c}ek belongs to the normal clan of HED meteorites. Petrographic observations and analysis of organic material indicate a small portion of carbonaceous chondrite material in the Sari\c{c}i\c{c}ek regolith and organic contamination of the meteorite after a few days on soil. Video observations of the fall show an atmospheric entry at 17.3 +/- 0.8 kms-1 from NW, fragmentations at 37, 33, 31 and 27 km altitude, and provide a pre-atmospheric orbit that is the first dynamical link between the normal HED meteorite clan and the inner Main Belt. Spectral data indicate the similarity of Sari\c{c}i\c{c}ek with the Vesta asteroid family spectra, a group of asteroids stretching to delivery resonances, which includes (4) Vesta. Dynamical modeling of meteoroid delivery to Earth shows that the disruption of a ca.1 km sized Vesta family asteroid or a ~10 km sized impact crater on Vesta is required to provide sufficient meteoroids <4 m in size to account for the influx of meteorites from this HED clan.
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Low-Temperature Thermopower in CoSbS -
Abstract
- We report giant thermopower S = 2.5 mV/K in CoSbS single crystals, a material that shows strong high-temperature thermoelectric performance when doped with Ni or Se. Changes of low temperature thermopower induced by magnetic field point to mechanism of electronic diffusion of carriers in the heavy valence band. Intrinsic magnetic susceptibility is consistent with the Kondo- Insulator-like accumulation of electronic states around the gap edges. This suggests that giant thermopower stems from temperature-dependent renormalization of the non-interacting bands and buildup of the electronic correlations on cooling.
Qianheng Du, Milinda Abeykoon, Yu Liu, G. Kotliar, C. Petrovic Journal reference: Physical Review Letters 123, 076602 (2019) [pdf] DOI: 10.1103/PhysRevLett.123.076602
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Probing ultracold chemistry using ion spectrometry -
Abstract
- Rapid progress in atomic, molecular, and optical (AMO) physics techniques enabled the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. In particular, both the external and internal quantum degrees of freedom of the reactant atoms and molecules are controlled, allowing studies that explored the role of the long range potential in ultracold reactions. The kinetics of these reactions have typically been determined using the loss of reactants as proxies. To extend such studies into the short-range, we developed an experimental apparatus that combines the production of quantum-state-selected ultracold KRb molecules with ion mass and kinetic energy spectrometry, and directly observed KRb + KRb reaction intermediates and products [Science, 2019, 366, 1111]. Here, we present the apparatus in detail. For future studies that aim for detecting the quantum states of the reaction products, we demonstrate a photodissociation based scheme to calibrate the ion kinetic energy spectrometer at low energies.
Yu Liu, David D. Grimes, Ming-Guang Hu, Kang-Kuen Ni Journal reference: Phys. Chem. Chem. Phys., 2020,22, 4861-4874 [pdf] DOI: 10.1039/C9CP07015J
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Ruppeiner geometry, phase transitions, and the microstructure of charged AdS black holes -
Abstract
- Originally considered for van der Waals fluids and charged black holes [Phys. Rev. Lett. 123, 071103 (2019)], we extend and generalize our approach to higher-dimensional charged AdS black holes. Beginning with thermodynamic fluctuations, we construct the line element of the Ruppeiner geometry and obtain a universal formula for the scalar curvature $R$. We first review the thermodynamics of a van der Waals fluid and calculate the coexistence and spinodal curves. From this we are able to clearly display the phase diagram. Notwithstanding the invalidity of the equation of state in the coexistence phase regions, we find that the scalar curvature is always negative for the van der Waals fluid, indicating that attractive interactions dominate amongst the fluid microstructures. Along the coexistence curve, the scalar curvature $R$ decreases with temperature, and goes to negative infinity at a critical temperature. We then numerically study the critical phenomena associated with the scalar curvature. We next consider four-dimensional charged AdS black holes. Vanishing of the heat capacity at constant volume yields a divergent scalar curvature. In order to extract the corresponding information, we define a new scalar curvature that has behaviour similar to that of a van der Waals fluid. We analytically confirm that at the critical point of the small/large black hole phase transition, the scalar curvature has a critical exponent 2, and $R(1-\tilde{T})^{2}C_{v}=1/8$, the same as that of a van der Waals fluid. However we also find that the scalar curvature can be positive for the small charged AdS black hole, implying that repulsive interactions dominate among the black hole microstructures. We then generalize our study to higher-dimensional charged AdS black holes.
Shao-Wen Wei, Yu-Xiao Liu, Robert B. Mann Journal reference: Phys. Rev. D 100, 124033 (2019) [pdf] DOI: 10.1103/PhysRevD.100.124033
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Photon sphere and reentrant phase transition of charged Born-Infeld-AdS black holes -
Abstract
- In this paper, we extend the study of the relationship between the photon sphere and the thermodynamic phase transition, especially the reentrant phase transition, to this black hole background. According to the number of the thermodynamic critical points, the black hole systems are divided into four cases with different values of Born-Infeld parameter b, where the black hole systems can have no phase transition, reentrant phase transition, or Van der Waals-like phase transition. For these different cases, we obtain the corresponding phase structures in pressure-temperature diagram and temperature-specific volume diagram. The tiny differences between these cases are clearly displayed. On the other hand, the radius rps and the minimal impact parameter ups of the photon sphere are calculated via the effective potential of the radial motion of photons. For different cases, rps and ups are found to have different behaviors. In particular, with the increase of rps or ups, the temperature possesses a decrease-increase-decrease-increase behavior for fixed pressure if there exists the reentrant phase transition. While for fixed temperature, the pressure will show an increase-decrease-increase-decrease behavior instead. These behaviors are quite different from that of the Van der Waals-like phase transition. Near the critical point, the changes of rps and ups among the black hole phase transition confirm an universal critical exponent 12. Therefore, all the results indicate that, for the charged Born-Infeld-AdS black holes, not only the Van der Waals-like phase transition, but also the reentrant phase transition can be reflected through the photon sphere.
Yu-Meng Xu, Hui-Min Wang, Yu-Xiao Liu, Shao-Wen Wei Journal reference: Phys. Rev. D 100, 104044 (2019) [pdf] DOI: 10.1103/PhysRevD.100.104044
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Quantum Fourier Transform in Oscillating Modes -
Abstract
- Quantum Fourier transform (QFT) is a key ingredient of many quantum algorithms. In typical applications such as phase estimation, a considerable number of ancilla qubits and gates are used to form a Hilbert space large enough for high-precision results. Qubit recycling reduces the number of ancilla qubits to one, but it is only applicable to semi-classical QFT and requires repeated measurements and feedforward within the coherence time of the qubits. In this work, we explore a novel approach based on resonators that forms a high-dimensional Hilbert space for the realization of QFT. By employing the perfect state-transfer method, we map an unknown multi-qubit state to a single resonator, and obtain the QFT state in the second oscillator through cross-Kerr interaction and projective measurement. A quantitive analysis shows that our method allows for high-dimensional and fully-quantum QFT employing the state-of-the-art superconducting quantum circuits. This paves the way for implementing various QFT related quantum algorithms.
Qi-Ming Chen, Frank Deppe, Re-Bing Wu, Luyan Sun, Yu-xi Liu, Yuki Nojiri, Stefan Pogorzalek, Michael Renger, Matti Partanen, Kirill G. Fedorov, Achim Marx, Rudolf Gross [pdf]
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A Bridge from Euclidean Nonperturbative approach to Minkowskian
Distribution Functions -
Abstract
- We give out a simple way to connect the parton distribution functions defined in Minkowskian space and the nonperturbative QCD methods grounded in Euclidean space (e.g., lattice QCD(LQCD), Dyson-Schwinger (DS) equations, functional renormalization group (FRG) approach) in this work. We combine the MIT bag model with the DS equation approach to calculate the longitudinal distribution function, transverse distribution function and scalar distribution function in a proton at renormalization point $\mu = 2\,\text{GeV}$. We look then insight into the dressed effects on the axial, tensor and scalar charges in a nucleon to some extent. This method can be regard as a new bridge between the Euclidean non-perturbative approaches and the Minkowskian space physics.
Langtian Liu, Lei Chang, Yu-xin Liu [pdf]
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Direct observation of bimolecular reactions of ultracold KRb molecules -
Abstract
- Femtochemistry techniques have been instrumental in accessing the short time scales necessary to probe transient intermediates in chemical reactions. Here we take the contrasting approach of prolonging the lifetime of an intermediate by preparing reactant molecules in their lowest ro-vibronic quantum state at ultralow temperatures, thereby drastically reducing the number of exit channels accessible upon their mutual collision. Using ionization spectroscopy and velocity-map imaging of a trapped gas of potassium-rubidium molecules at a temperature of 500~nK, we directly observe reactants, intermediates, and products of the reaction $^{40}$K$^{87}$Rb + $^{40}$K$^{87}$Rb $\rightarrow$ K$_2$Rb$^*_2$ $\rightarrow$ K$_2$ + Rb$_2$. Beyond observation of a long-lived energy-rich intermediate complex, this technique opens the door to further studies of quantum-state resolved reaction dynamics in the ultracold regime.
Ming-Guang Hu, Yu Liu, David D. Grimes, Yen-Wei Lin, Andrei H. Gheorghe, Romain Vexiau, Nadia Bouloufa-Maafa, Olivier Dulieu, Till Rosenband, Kang-Kuen Ni Journal reference: Science 366, 1111-1115 (2019) [pdf] DOI: 10.1126/science.aay9531
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Optimization of heterogeneous ternary Li3PO4-Li3BO3-Li2SO4 mixture for
Li-ion conductivity by machine learning -
Abstract
- Mixing heterogeneous Li-ion conductive materials is one of potential ways to enhance the Li-ion conductivity more than that of the parent materials. However, the development of the mixtures had not exhibited significant progress because it is a formidable task to cover the vast possible composition of the parent materials using traditional ways. Here, we introduce a fashion based on machine learning to optimize the composition ratio of ternary Li3PO4-Li3BO3-Li2SO4 mixture for its Li-ion conductivity. According to our results, the optimum composition of the ternary mixture system is 25:14:61 (Li3PO4: Li3BO3: Li2SO4 in mol%), whose Li-ion conductivity is measured as 4.9 x 10E-4 S/cm at 300 {\deg}C. Our X-ray structure analysis indicates that Li-ion conductivity in the mixing systems is enhanced by virtue of the coexistence of two or more phases. Although the mechanism enhancing Li-ion conductivity is not simple, our results demonstrate the effectiveness of machine learning for the development of materials.
Kenji Homma, Yu Liu, Masato Sumita, Ryo Tamura, Naoki Fushimi, Junichi Iwata, Koji Tsuda, Chioko Kaneta [pdf]
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Magnetic anisotropy and entropy change in trigonal
-
Abstract
- We present a comprehensive investigation on anisotropic magnetic and magnetocaloric properties of the quasi-two-dimensional weak itinerant ferromagnet trigonal Cr$_{0.62}$Te single crystals. Magnetic-anisotropy-induced satellite transition $T^*$ is observed at low fields applied parallel to the $ab$ plane below $T_c$. The $T^*$ is featured by an anomalous magnetization downturn, similar to that in structurally related CrI$_3$, and shows a monotonous shift towards lower temperature with increasing field. Magnetocrystalline anisotropy is also reflected in magnetic entropy change $\Delta S_M(T,H)$ and relative cooling power RCP. Given the high $T_c$, Cr$_{0.62}$Te crystals are materials of interest for nanofabrication in basic science and applied technology.
Yu Liu, Milinda Abeykoon, Eli Stavitski, Klaus Attenkofer, C. Petrovic Journal reference: Physical Review B 100, 245114 (2019) [pdf] DOI: 10.1103/PhysRevB.100.245114
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Local electric field induced spin photocurrent in ReS2 -
Abstract
- Circular polarized photocurrent is observed near the electrodes on a few-layer ReS2sample at room temperature. For both electrodes, the spatial distribution of the circular polarized photocurrent shows a feature of two wings, with one positive and the other negative. We suggest that this phenomenon arises from the inverse spin Hall effect due to local electric field near the electrode. Bias voltage that modulates this field further controls the sign and magnitude of the inverse spin Hall effect photocurrent. Our research shows that electric field near electrodes has a significant impact on spin transmission operation, hence it could be taken into account for manufacturing spintronic devices in future.
Yang Zhang, Yu Wang, Yu Liu, Xiao-Lin Zeng, Jing Wu, Jin ling Yu, Tian-Jun Cao, Shi-Jun Liang, Yong-Hai Chen [pdf]
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Motion deviation of test body induced by spin and cosmological constant in extreme mass ratio inspiral binary system -
Abstract
- The future space-borne detectors will provide the possibility to detect gravitational waves emitted from extreme mass ratio inspirals of stellar-mass compact objects into supermassive black holes. It is natural to expect that the spin of the compact object and cosmological constant will affect the orbit of the inspiral process and hence lead to the considerable phase shift of the corresponding gravitational waves. In this paper, we investigate the motion of a spinning test particle in the spinning black hole background with a cosmological constant and give the order of motion deviation induced by the particle's spin and the cosmological constant by considering the corresponding innermost stable circular orbit. By taking the neutron star or kerr black hole as the small body, the deviations of the innermost stable circular orbit parameters induced by the particle's spin and cosmological constant are given. Our results show that the deviation induced by particle's spin is much larger than that induced by cosmological constant when the test particle locates not very far away from the black hole, the accumulation of phase shift during the inspiral from the cosmological constant can be ignored when compared to the one induced by the particle's spin. However when the test particle locates very far away from the black hole, the impact from the cosmological constant will increase dramatically. Therefore the accumulation of phase shift for the whole process of inspiral induced by the cosmological constant and the particle's spin should be handled with caution.
Yu-Peng Zhang, Shao-Wen Wei, Pau Amaro-Seoane, Jie Yang, Yu-Xiao Liu Journal reference: Eur.Phys.J. C79 (2019) 856 [pdf] DOI: 10.1140/epjc/s10052-019-7334-y
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Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr
-
Abstract
- Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally-packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Our observation of topologically-nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically-thin insulators for magneto-electronic and quantum devices.
Myung-Geun Han, Joseph A. Garlow, Yu Liu, Huiqin Zhang, Jun Li, Donald DiMarzio, Mark Knight, Cedomir Petrovic, Deep Jariwala, Yimei Zhu Journal reference: Nano Letters, 2019 [pdf] DOI: 10.1021/acs.nanolett.9b02849
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Excited states of holographic superconductors with backreaction -
Abstract
- In this paper we investigate the model of the anti-de Sitter gravity coupled to a Maxwell field and a free, complex scalar field, and construct a fully back-reacted holographic model of superconductor with excited states. With the fixed charge $q$, there exist a series of excited states of holographic superconductor with the corresponding critical chemical potentials. The condensates as functions of the temperature for the two operators $\mathcal{O}_1$ and $\mathcal{O}_2$ of excited states are also studied. For the optical conductivity in the excited states, we find that there exist the additional peaks in the imaginary and real parts of the conductivity. Moreover, the number of peaks corresponding to $n$-th excited state is equal to $n$.
Yong-Qiang Wang, Hong-Bo Li, Yu-Xiao Liu, Yin Zhong [pdf]
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A game method for improving the interpretability of convolution neural
network -
Abstract
- Real artificial intelligence always has been focused on by many machine learning researchers, especially in the area of deep learning. However deep neural network is hard to be understood and explained, and sometimes, even metaphysics. The reason is, we believe that: the network is essentially a perceptual model. Therefore, we believe that in order to complete complex intelligent activities from simple perception, it is necessary to con-struct another interpretable logical network to form accurate and reasonable responses and explanations to external things. Researchers like Bolei Zhou and Quanshi Zhang have found many explanatory rules for deep feature extraction aimed at the feature extraction stage of convolution neural network. However, although researchers like Marco Gori have also made great efforts to improve the interpretability of the fully connected layers of the network, the problem is also very difficult. This paper firstly analyzes its reason. Then a method of constructing logical network based on the fully connected layers and extracting logical relation between input and output of the layers is proposed. The game process between perceptual learning and logical abstract cognitive learning is implemented to improve the interpretable performance of deep learning process and deep learning model. The benefits of our approach are illustrated on benchmark data sets and in real-world experiments.
Jinwei Zhao, Qizhou Wang, Fuqiang Zhang, Wanli Qiu, Yufei Wang, Yu Liu, Guo Xie, Weigang Ma, Bin Wang, Xinhong Hei [pdf]
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Phase-controlled single-photon nonreciprocal transmission in a one-dimensional waveguide -
Abstract
- We study the controllable single-photon scattering via a one-dimensional waveguide which is coupled to a two-level emitter and a single-mode cavity simultaneously. The emitter and the cavity are also coupled to each other and form a three-level system with cyclic transitions within the zero- and single-excitation subspaces. As a result, the phase of emitter-cavity coupling strength serves as a sensitive control parameter. When the emitter and cavity locate at the same point of the waveguide, we demonstrate the Rabi splitting and quasidark-state--induced perfect transmission for the incident photons. More interestingly, when they locate at different points of the waveguide, a controllable nonreciprocal transmission can be realized and the non-reciprocity is robust to the weak coupling between the system and environment. Furthermore, we demonstrate that our theoretical model is experimentally feasible with currently available technologies.
Zhihai Wang, Lei Du, Yong Li, Yu-xi Liu Journal reference: Phys. Rev. A 100, 053809 (2019) [pdf] DOI: 10.1103/PhysRevA.100.053809
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Multilayered Kelvin–Helmholtz Instability in the Solar Corona -
Abstract
- The Kelvin-Helmholtz (KH) instability is commonly found in many astrophysical, laboratory, and space plasmas. It could mix plasma components of different properties and convert dynamic fluid energy from large scale structure to smaller ones. In this study, we combined the ground-based New Vacuum Solar Telescope (NVST) and the Solar Dynamic Observatories (SDO) / Atmospheric Imaging Assembly (AIA) to observe the plasma dynamics associated with active region 12673 on 09 September 2017. In this multi-temperature view, we identified three adjacent layers of plasma flowing at different speeds, and detected KH instabilities at their interfaces. We could unambiguously track a typical KH vortex and measure its motion. We found that the speed of this vortex suddenly tripled at a certain stage. This acceleration was synchronized with the enhancements in emission measure and average intensity of the 193 \AA{} data. We interpret this as evidence that KH instability triggers plasma heating. The intriguing feature in this event is that the KH instability observed in the NVST channel was nearly complementary to that in the AIA 193 \AA{}. Such a multi-thermal energy exchange process is easily overlooked in previous studies, as the cold plasma component is usually not visible in the extreme ultraviolet channels that are only sensitive to high temperature plasma emissions. Our finding indicates that embedded cold layers could interact with hot plasma as invisible matters. We speculate that this process could occur at a variety of length scales and could contribute to plasma heating.
Ding Yuan, Yuandeng Shen, Yu Liu, Xueshang Feng, Rony Keppens [pdf] DOI: 10.3847/2041-8213/ab4bcd 1910.05710v1 [pdf]
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A Coding-free Software Framework of Developing Web Data Management
Systems -
Abstract
- More and more enterprises recently intend to deploy data management systems in the cloud. Due to the professionalism of software development, it has still been difficult for non-programmers to develop this kind of systems, even a small one. However, the development of SaaS brings forth the more feasibility of coding-free software development than before. Based on the SaaS architecture, this paper presents a set of theory and method for coding-free construction of a data management system, on which our contributions involve in a practical application platform, a set of construction method and a set of interface on data exchange. By abstracting the common features of data management systems, we design a universal web platform to quickly generate and publish customized system instances. Moreover, we propose a kind of method to develop a data management system using a specific requirements table in spreadsheet. The corresponding platform maps the requirements table into a system instance through parsing the table model and implementing the objective system in the running stage. Finally, we implement the proposed framework and deploy it on web. The empirical result demonstrates the feasibility and availability of the coding-free method in developing web data management systems.
Can Yang, Shiying Pan, Runmin Li, Yu Liu, Lizhang Peng [pdf]
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Gradient Information Guided Deraining with A Novel Network and
Adversarial Training -
Abstract
- In recent years, deep learning based methods have made significant progress in rain-removing. However, the existing methods usually do not have good generalization ability, which leads to the fact that almost all of existing methods have a satisfied performance on removing a specific type of rain streaks, but may have a relatively poor performance on other types of rain streaks. In this paper, aiming at removing multiple types of rain streaks from single images, we propose a novel deraining framework (GRASPP-GAN), which has better generalization capacity. Specifically, a modified ResNet-18 which extracts the deep features of rainy images and a revised ASPP structure which adapts to the various shapes and sizes of rain streaks are composed together to form the backbone of our deraining network. Taking the more prominent characteristics of rain streaks in the gradient domain into consideration, a gradient loss is introduced to help to supervise our deraining training process, for which, a Sobel convolution layer is built to extract the gradient information flexibly. To further boost the performance, an adversarial learning scheme is employed for the first time to train the proposed network. Extensive experiments on both real-world and synthetic datasets demonstrate that our method outperforms the state-of-the-art deraining methods quantitatively and qualitatively. In addition, without any modifications, our proposed framework also achieves good visual performance on dehazing.
Yinglong Wang, Haokui Zhang, Yu Liu, Qinfeng Shi, Bing Zeng [pdf]
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Semiconductor–Ferromagnetic Insulator–Superconductor Nanowires: Stray Field and Exchange Field -
Abstract
- Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of semiconductor - ferromagnetic insulator - superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite InAs / rock-salt EuS interfaces as well as rock-salt EuS / face-centered cubic Al interfaces. Because of the magnetic anisotropy originating from the nanowire shape, the magnetic structure of the EuS phase are easily tuned into single magnetic domains. This effect efficiently ejects the stray field lines along the nanowires. With tunnel spectroscopy measurements of the density of states, we show the material has a hard induced superconducting gap, and magnetic hysteretic evolution which indicates that the magnetic exchange fields are not negligible. These hybrid nanowires fulfil key material requirements for serving as a platform for spin-based quantum applications, such as scalable topological quantum computing.
Yu Liu, Saulius Vaitiekenas, Sara Marti-Sanchez, Christian Koch, Sean Hart, Zheng Cui, Thomas Kanne, Sabbir A. Khan, Rawa Tanta, Shivendra Upadhyay, Martin Espineira Cachaza, Charles M. Marcus, Jordi Arbiol, Kathryn A. Moler, Peter Krogstrup [pdf] DOI: 10.1021/acs.nanolett.9b04187 1910.03364v1 [pdf]
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How to improve the interpretability of kernel learning -
Abstract
- In recent years, machine learning researchers have focused on methods to construct flexible and interpretable prediction models. However, an interpretability evaluation, a relationship between generalization performance and an interpretability of the model and a method for improving the interpretability have to be considered. In this paper, a quantitative index of the interpretability is proposed and its rationality is proved, and equilibrium problem between the interpretability and the generalization performance is analyzed. Probability upper bound of the sum of the two performances is analyzed. For traditional supervised kernel machine learning problem, a universal learning framework is put forward to solve the equilibrium problem between the two performances. The condition for global optimal solution based on the framework is deduced. The learning framework is applied to the least-squares support vector machine and is evaluated by some experiments.
Jinwei Zhao, Qizhou Wang, Yufei Wang, Yu Liu, Zhenghao Shi, Xinhong Hei [pdf]
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Meta Learning with Differentiable Closed-form Solver for Fast Video
Object Segmentation -
Abstract
- This paper tackles the problem of video object segmentation. We are specifically concerned with the task of segmenting all pixels of a target object in all frames, given the annotation mask in the first frame. Even when such annotation is available this remains a challenging problem because of the changing appearance and shape of the object over time. In this paper, we tackle this task by formulating it as a meta-learning problem, where the base learner grasping the semantic scene understanding for a general type of objects, and the meta learner quickly adapting the appearance of the target object with a few examples. Our proposed meta-learning method uses a closed form optimizer, the so-called "ridge regression", which has been shown to be conducive for fast and better training convergence. Moreover, we propose a mechanism, named "block splitting", to further speed up the training process as well as to reduce the number of learning parameters. In comparison with the-state-of-the art methods, our proposed framework achieves significant boost up in processing speed, while having very competitive performance compared to the best performing methods on the widely used datasets.
Yu Liu, Lingqiao Liu, Haokui Zhang, Hamid Rezatofighi, Ian Reid [pdf]
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Null geodesics, quasinormal modes, and thermodynamic phase transition for charged black holes in asymptotically flat and dS spacetimes -
Abstract
- The numerical study indicates that there exists a relation between the quasinormal modes and the Davies point for a black hole. In this paper, we analytically study this relation for the charged Reissner-Nordstr\"{o}m black holes in asymptotically flat and dS spacetimes. In the eikonal limit, the angular velocity $\Omega$ and the Lyapunov exponent $\lambda$ of the photon sphere, respectively, corresponding to the real and imaginary parts of the quasinormal modes are obtained from the null geodesics. Both in asymptotically flat and dS spacetimes, we observe the spiral-like shapes in the complex quasinormal mode plane. However, the starting point of the shapes do not coincide with the Davies point. Nevertheless, we find a new relation that the Davies point exactly meet the maximum of the temperature $T$ in the $T$-$\Omega$ and $T$-$\lambda$ planes. In higher dimensional asymptotically flat spacetime, even there is no the spiral-like shape, such relation still holds. Therefore, we provide a new relation between the black hole thermodynamics and dynamics. Applying this relation, we can test the black hole thermodynamic property by the quasinormal modes.
Shao-Wen Wei, Yu-Xiao Liu [pdf] DOI: 10.1088/1674-1137/abae54 1909.11911v1 [pdf]
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Reduced thermodynamics and an exact phase transition of five-dimensional Kerr-AdS black holes with equal spin parameters -
Abstract
- Multi-spinning higher dimensional Kerr-AdS black holes admit the stable small-large black hole phase transition of van der Waals type. In this paper, we study the exact critical phenomena and phase structure in five-dimensional spacetime. First, we examine the thermodynamic laws in the reduced parameter space and find that they are quite different from the conventional thermodynamic laws. Then based on the reduced laws, the phase structure in different parameter spaces is investigated. The stable and metastable black hole phases are clearly displayed. We present a highly accurate fitting formula for the coexistence curve of small and large black hole phases. Using this fitting formula, we examine the critical exponents when the black hole system approaches the critical point along the coexistence curve. Moreover, employing the dimensional analysis and symmetry analysis, we also give a numerical study of the critical point for the unequal spinning black holes. These results are very useful on further understanding the microstructure of the multi-spinning black holes in higher dimensional spacetime.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 100, 064004 (2019) [pdf] DOI: 10.1103/PhysRevD.100.064004
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Curvature radius and Kerr black hole shadow -
Abstract
- We consider applications of the curvature radius of a Kerr black hole shadow and propose three new approaches to simultaneously determine the black hole spin and inclination angle of the observer. The first one uses only two symmetric characteristic points, i.e., the top and the bottom points of the shadow, and is the smallest amount of data employed to extract information about spin and inclination angle amongst all current treatments. The second approach shows that only measuring the curvature radius at the characteristic points can also yield the black hole spin and the inclination angle. The observables used in the third approach have large changes to the spin and the inclination angle, which may give us a more accurate way to determine these parameters. Moreover, by modeling the supermassive black hole M87* with a Kerr black hole, we calculate the angular size for these curvature radii of the shadow. Some novel properties are found and analyzed. The results may shine new light on the relationship between the curvature radius and the black hole shadow, and provide several different approaches to test the nature of the black hole through the shadow.
Shao-Wen Wei, Yuan-Chuan Zou, Yu-Xiao Liu, Robert B. Mann Journal reference: JCAP 1908 (2019) 030 [pdf] DOI: 10.1088/1475-7516/2019/08/030
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Stable halogen 2D materials: the case of iodine and astatine -
Abstract
- Two-dimensional (2D) materials have wide applications towards electronic devices, energy storages, and catalysis, et al. So far, most of the pure element 2D materials are composed of group IIIA,IVA, and VA elements. Beyond the scope, the orbit hybrid configuration becomes a key fact to influence 2D structure stably. Here we show a sp2d3 hybridization in the outmost electrons with O-shell for Iodine and P-shell for astatine element, builds up triangle configuration (beta-type) to form 2D structures beta-iodiene and beta-astatiene. Each atom is connected by pi bonds, and surrounded by 6 atoms. The pi bonds become possible, and band gap approaches zero because of interaction of unpaired single electron to each atom, depending on reducing bond length. By applying compression strain or spin orbit coupling (SOC), the Dirac points or topological nontrivial points can be available in the beta-iodiene and beta-astatiene. Our discovery has paved a new way to construction of 2D materials.
Xinyue Zhang, Yu Liu, Qingsong Huang [pdf] DOI: 10.1088/1361-648X/ab87cf 1909.11643v1 [pdf]
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Blind Channel Estimation and Data Detection with Unknown Modulation and
Coding Scheme -
Abstract
- This paper investigates a complete blind receiver approach in an unknown multipath fading channel, which has multiple tasks including blind channel estimation, noise power estimation, modulation classification, channel coding recognition, and data detection. Each of these tasks has been sufficiently studied in the literature. However, to the best of our knowledge, this overall problem has not been investigated previously. This paper is the first attempt to address this overall problem jointly. We propose a complete blind receiver approach that jointly estimates the unknown channel state information and noise power, recognizes the unknown modulation and coding scheme, detects the data of interest, and thus named BERD receiver. Another merit of the proposed BERD receiver is that it can be implemented for both a single receiver and multiple receivers, which ensures successful estimation, recognition, and detection for such an extremely difficult problem. In addition, numerical results show the performance of the proposed receiver in three folds: a) the BERD receiver outperforms the linear minimum mean squared error (LMMSE) pilot-based channel estimator by over 3.5 dB at the MSE of 0.01; b) the correct modulation/coding recognition performance of the BERD receiver is within 0.3 dB as close to the recognition benchmark when the perfect channel state information (CSI) is available; c) the BERD receiver is within 0.5 dB at the bit error rate of 0.001 compared to the benchmark when the modulation, the channel coding, and the CSI are perfectly known. Finally, the BERD receiver finds many applications in both civilian and military scenarios, such as the interference cancelation in spectrum sharing, real-time signal interception, and processing in electronic warfare operations, automatic recognition of a detect signal in software-defined radio.
Yu Liu, Fanggang Wang [pdf]
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Fast Low-rank Metric Learning for Large-scale and High-dimensional Data -
Abstract
- Low-rank metric learning aims to learn better discrimination of data subject to low-rank constraints. It keeps the intrinsic low-rank structure of datasets and reduces the time cost and memory usage in metric learning. However, it is still a challenge for current methods to handle datasets with both high dimensions and large numbers of samples. To address this issue, we present a novel fast low-rank metric learning (FLRML) method.FLRML casts the low-rank metric learning problem into an unconstrained optimization on the Stiefel manifold, which can be efficiently solved by searching along the descent curves of the manifold.FLRML significantly reduces the complexity and memory usage in optimization, which makes the method scalable to both high dimensions and large numbers of samples.Furthermore, we introduce a mini-batch version of FLRML to make the method scalable to larger datasets which are hard to be loaded and decomposed in limited memory. The outperforming experimental results show that our method is with high accuracy and much faster than the state-of-the-art methods under several benchmarks with large numbers of high-dimensional data. Code has been made available at https://github.com/highan911/FLRML
Han Liu, Zhizhong Han, Yu-Shen Liu, Ming Gu [pdf]
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G4.8+6.2, a possible kilonova remnant? -
Abstract
- Kilonova explosions typically release $\sim 10^{50-51}$ erg in kinetic energy, which is sufficient to constitute a kilonova remnant (KNR); however, it has not yet been confirmed. In this work, we investigate the probable association between G4.8+6.2 and the guest star of AD 1163, which is recorded by the Korea ancient astronomers. Although the evidence available is insufficient to draw a definite conclusion, it is at least theoretically self-consistent that the guest star of AD 1163 was a historical kilonova associated with G4.8+6.2, considering the possible short visible timescale of AD 1163, the relatively high Galactic latitude of G4.8+6.2, and that G4.8+6.2 is spatially coincident with the guest star of AD 1163. Further observation of G4.8+6.2 is needed to test our hypothesis. If our interpretation is correct, our results indicate that young KNRs should have a large diameter and low surface brightness, unlike other young supernova remnants.
Yu Liu, Yuan-Chuan Zou, Bing Jiang, He Gao, Shuai-Bing Ma, Bin Liao [pdf] DOI: 10.1093/mnrasl/slz141 1909.05714v1 [pdf]
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Boosting Throughput and Efficiency of Hardware Spiking Neural
Accelerators using Time Compression Supporting Multiple Spike Codes -
Abstract
- Spiking neural networks (SNNs) are the third generation of neural networks and can explore both rate and temporal coding for energy-efficient event-driven computation. However, the decision accuracy of existing SNN designs is contingent upon processing a large number of spikes over a long period. Nevertheless, the switching power of SNN hardware accelerators is proportional to the number of spikes processed while the length of spike trains limits throughput and static power efficiency. This paper presents the first study on developing temporal compression to significantly boost throughput and reduce energy dissipation of digital hardware SNN accelerators while being applicable to multiple spike codes. The proposed compression architectures consist of low-cost input spike compression units, novel input-and-output-weighted spiking neurons, and reconfigurable time constant scaling to support large and flexible time compression ratios. Our compression architectures can be transparently applied to any given pre-designed SNNs employing either rate or temporal codes while incurring minimal modification of the neural models, learning algorithms, and hardware design. Using spiking speech and image recognition datasets, we demonstrate the feasibility of supporting large time compression ratios of up to 16x, delivering up to 15.93x, 13.88x, and 86.21x improvements in throughput, energy dissipation, the tradeoffs between hardware area, runtime, energy, and classification accuracy, respectively based on different spike codes on a Xilinx Zynq-7000 FPGA. These results are achieved while incurring little extra hardware overhead.
Changqing Xu, Wenrui Zhang, Yu Liu, Peng Li [pdf]
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Tensor stability in Born-Infeld determinantal gravity -
Abstract
- We consider the transverse-traceless tensor perturbation of a spatial flat homogeneous and isotropic spacetime in Born-Infeld determinantal gravity, and investigate the evolution of the tensor mode for two solutions in the early universe. For the first solution where the initial singularity is replaced by a regular geometric de Sitter inflation of infinite duration, the evolution of the tensor mode is stable for the parameter spaces $\alpha<-1$, $\omega\geq-1/3$ and $\alpha=-1$, $\omega>0$. For the second solution where the initial singularity is replaced by a primordial brusque bounce, which suffers a sudden singularity at the bouncing point, the evolution of the tensor mode is stable for all regions of the parameter space. Our calculation suggests that the tensor evolution can hold stability in large parameter spaces, which is a remarkable property of Born-Infeld determinantal gravity. We also constrain the theoretical parameter $|\lambda|\geq 10^{-38} \text{m}^{-2}$ by resorting to the current bound on the speed of the gravitational waves.
Ke Yang, Yu-Peng Zhang, Yu-Xiao Liu Journal reference: Eur. Phys. J. C (2019) 79: 736 [pdf] DOI: 10.1140/epjc/s10052-019-7253-y
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Towards Flops-constrained Face Recognition -
Abstract
- Large scale face recognition is challenging especially when the computational budget is limited. Given a \textit{flops} upper bound, the key is to find the optimal neural network architecture and optimization method. In this article, we briefly introduce the solutions of team 'trojans' for the ICCV19 - Lightweight Face Recognition Challenge~\cite{lfr}. The challenge requires each submission to be one single model with the computational budget no higher than 30 GFlops. We introduce a searched network architecture `Efficient PolyFace' based on the Flops constraint, a novel loss function `ArcNegFace', a novel frame aggregation method `QAN++', together with a bag of useful tricks in our implementation (augmentations, regular face, label smoothing, anchor finetuning, etc.). Our basic model, `Efficient PolyFace', takes 28.25 Gflops for the `deepglint-large' image-based track, and the `PolyFace+QAN++' solution takes 24.12 Gflops for the `iQiyi-large' video-based track. These two solutions achieve 94.198\% @ 1e-8 and 72.981\% @ 1e-4 in the two tracks respectively, which are the state-of-the-art results.
Yu Liu, Guanglu Song, Manyuan Zhang, Jihao Liu, Yucong Zhou, Junjie Yan [pdf]
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Point2SpatialCapsule: Aggregating Features and Spatial Relationships of Local Regions on Point Clouds Using Spatial-Aware Capsules -
Abstract
- Learning discriminative shape representation directly on point clouds is still challenging in 3D shape analysis and understanding. Recent studies usually involve three steps: first splitting a point cloud into some local regions, then extracting corresponding feature of each local region, and finally aggregating all individual local region features into a global feature as shape representation using simple max pooling. However, such pooling-based feature aggregation methods do not adequately take the spatial relationships between local regions into account, which greatly limits the ability to learn discriminative shape representation. To address this issue, we propose a novel deep learning network, named Point2SpatialCapsule, for aggregating features and spatial relationships of local regions on point clouds, which aims to learn more discriminative shape representation. Compared with traditional max-pooling based feature aggregation networks, Point2SpatialCapsule can explicitly learn not only geometric features of local regions but also spatial relationships among them. It consists of two modules. To resolve the disorder problem of local regions, the first module, named geometric feature aggregation, is designed to aggregate the local region features into the learnable cluster centers, which explicitly encodes the spatial locations from the original 3D space. The second module, named spatial relationship aggregation, is proposed for further aggregating clustered features and the spatial relationships among them in the feature space using the spatial-aware capsules developed in this paper. Compared to the previous capsule network based methods, the feature routing on the spatial-aware capsules can learn more discriminative spatial relationships among local regions for point clouds, which establishes a direct mapping between log priors and the spatial locations through feature clusters.
Xin Wen, Zhizhong Han, Xinhai Liu, Yu-Shen Liu [pdf] DOI: 10.1109/TIP.2020.3019925 1908.11026v1 [pdf]
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Thickness-dependent magnetic order in CrI3 single crystals -
Abstract
- Two-dimensional (2D) materials with intrinsic ferromagnetism provide unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI$_3$, showing long-range magnetic order in monolayer with the Curie temperature ($T_c$) of 45 K. Here we study detailed evolution of magnetic transition and magnetic critical properties in response to systematic reduction in crystal thickness down to 50 nm. Bulk $T_c$ of 61 K is gradually suppressed to 57 K, however, the satellite transition at $T^*$ = 45 K is observed layer-independent at fixed magnetic field of 1 kOe. The origin of $T^*$ is proposed to be a crossover from pinning to depinning of magnetic domain walls. The reduction of thickness facilitates a field-driven metamagnetic transition around 20 kOe with out-of-plane field, in contrast to the continuous changes with in-plane field. The critical analysis around $T_c$ elucidates the mean-field type interactions in microscale-thick CrI$_3$.
Yu Liu, Lijun Wu, Xiao Tong, Jun Li, Jing Tao, Yimei Zhu, C. Petrovic Journal reference: Scientific Reports 9, 13599 (2019) [pdf] DOI: 10.1038/s41598-019-50000-x
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Anisotropic magnetocaloric effect in Fe3−xGeTe2 -
Abstract
- We present a comprehensive study on anisotropic magnetocaloric porperties of the van der Waals weak-itinerant ferromagnet Fe$_{3-x}$GeTe$_2$ that features gate-tunable room-temperature ferromagnetism in few-layer device. Intrinsic magnetocrystalline anisotropy is observed to be temperature-dependent and most likely favors the long-range magnetic order in thin Fe$_{3-x}$GeTe$_2$ crsytal. The magnetic entropy change $-\Delta S_M$ also reveals an anisotropic characteristic between $H // ab$ and $H // c$, which could be well scaled into a universal curve. The peak value $-\Delta S_M^{max}$ of 1.20 J kg$^{-1}$ K$^{-1}$ and the corresponding adiabatic temperature change $\Delta T_{ad}$ of 0.66 K are deduced from heat capacity with out-of-plane field change of 5 T. By fitting of the field-dependent parameters of $-\Delta S_M^{max}$ and the relative cooling power RCP, it gives $-\Delta S_M^{max} \propto H^n$ with $n = 0.603(6)$ and $RCP \propto H^m$ with $m = 1.20(1)$ when $H // c$. Given the high and tunable $T_c$, Fe$_{3-x}$GeTe$_2$ crystals are of interest for fabricating the heterostructure-based spintronics device.
Yu Liu, Jun Li, Jing Tao, Yimei Zhu, C. Petrovic Journal reference: Scientific Reports 9, 13233 (2019) [pdf] DOI: 10.1038/s41598-019-49654-4
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Nonreciprocal transition between two nondegenerate energy levels -
Abstract
- Stimulated emission and absorption are two fundamental processes of light-matter interaction, and the coefficients of the two processes should be equal in general. However, we will describe a generic method to realize significant difference between the stimulated emission and absorption coefficients of two nondegenerate energy levels, which we refer to as nonreciprocal transition. As a simple implementation, a cyclic three-level atom system, comprising two nondegenerate energy levels and one auxiliary energy level, is employed to show nonreciprocal transition via a combination of synthetic magnetism and reservoir engineering. Moreover, a single-photon nonreciprocal transporter is proposed using two one dimensional semi-infinite coupled-resonator waveguides connected by an atom with nonreciprocal transition effect. Our work opens up a route to design atom-mediated nonreciprocal devices in a wide range of physical systems.
Xun-Wei Xu, Yan-Jun Zhao, Hui Wang, Ai-Xi Chen, Yu-xi Liu [pdf]
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Fractional heat semigroups on metric measure spaces with finite
densities and applications to fractional dissipative equations -
Abstract
- Let $(\mathbb M, d,\mu)$ be a metric measure space with upper and lower densities: $$ \begin{cases} |||\mu|||_{\beta}:=\sup_{(x,r)\in \mathbb M\times(0,\infty)} \mu(B(x,r))r^{-\beta}<\infty;\\ \mathbb \mu(b(x,r))r^{-\beta^{\star}} m\times(0,\infty)} |||\mu|||_{\beta^{\star}}:="\inf_{(x,r)\in">0, \end{cases} $$ where $\beta, \beta^{\star}$ are two positive constants which are less than or equal to the Hausdorff dimension of $\mathbb M$. Assume that $p_t(\cdot,\cdot)$ is a heat kernel on $\mathbb M$ satisfying Gaussian upper estimates and $\mathcal L$ is the generator of the semigroup associated with $p_t(\cdot,\cdot)$. In this paper, via a method independent of Fourier transform, we establish the decay estimates for the kernels of the fractional heat semigroup $\{e^{-t \mathcal{L}^{\alpha}}\}_{t>0}$ and the operators $\{{\mathcal{L}}^{\theta/2} e^{-t \mathcal{L}^{\alpha}}\}_{t>0}$, respectively. By these estimates, we obtain the regularity for the Cauchy problem of the fractional dissipative equation associated with $\mathcal L$ on $(\mathbb M, d,\mu)$. Moreover, based on the geometric-measure-theoretic analysis of a new $L^p$-type capacity defined in $\mathbb{M}\times(0,\infty)$, we also characterize a nonnegative Randon measure $\nu$ on $\mathbb M\times(0,\infty)$ such that $R_\alpha L^p(\mathbb M)\subseteq L^q(\mathbb M\times(0,\infty),\nu)$ under $(\alpha,p,q)\in (0,1)\times(1,\infty)\times(1,\infty)$, where $u=R_\alpha f$ is the weak solution of the fractional diffusion equation $(\partial_t+ \mathcal{L}^\alpha)u(t,x)=0$ in $\mathbb M\times(0,\infty)$ subject to $u(0,x)=f(x)$ in $\mathbb M$.
Jizheng Huang, Pengtao Li, Yu Liu, Shaoguang Shi [pdf]
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Capacity & Perimeter from $α$-Hermite Bounded Variation -
Abstract
- Let $\mathcal{H}_{\alpha}=\Delta-(\alpha-1)|x|^{\alpha}$ be an $[1,\infty)\ni\alpha$-Hermite operator for the hydrogen atom located at the origin in $\mathbb R^d$. In this paper, we are motivated by the classical case $\alpha=1$ to investigate the space of functions with $\alpha$-{\it Hermite Bounded Variation} and its functional capacity and geometrical perimeter.
Jizheng Huang, Pengtao Li, Yu Liu [pdf]
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In defense of OSVOS -
Abstract
- As a milestone for video object segmentation, one-shot video object segmentation (OSVOS) has achieved a large margin compared to the conventional optical-flow based methods regarding to the segmentation accuracy. Its excellent performance mainly benefit from the three-step training mechanism, that are: (1) acquiring object features on the base dataset (i.e. ImageNet), (2) training the parent network on the training set of the target dataset (i.e. DAVIS-2016) to be capable of differentiating the object of interest from the background. (3) online fine-tuning the interested object on the first frame of the target test set to overfit its appearance, then the model can be utilized to segment the same object in the rest frames of that video. In this paper, we argue that for the step (2), OSVOS has the limitation to 'overemphasize' the generic semantic object information while 'dilute' the instance cues of the object(s), which largely block the whole training process. Through adding a common module, video loss, which we formulate with various forms of constraints (including weighted BCE loss, high-dimensional triplet loss, as well as a novel mixed instance-aware video loss), to train the parent network in the step (2), the network is then better prepared for the step (3), i.e. online fine-tuning on the target instance. Through extensive experiments using different network structures as the backbone, we show that the proposed video loss module can improve the segmentation performance significantly, compared to that of OSVOS. Meanwhile, since video loss is a common module, it can be generalized to other fine-tuning based methods and similar vision tasks such as depth estimation and saliency detection.
Yu Liu, Yutong Dai, Anh-Dzung Doan, Lingqiao Liu, Ian Reid [pdf]
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Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS
Interfaces: Band Alignment and Magnetic Structure -
Abstract
- Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field and local proximitized magnetic exchange. In this work, we present lattice matched hybrid epitaxy of semiconductor - ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure as well as their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the bandgap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs. Induced moments in the adjacent InAs layers were not detected although our ab initio calculations indicate a small exchange field in the InAs layer. This work presents a step towards realizing high quality semiconductor - ferromagnetic insulator hybrids, which is a critical requirement for development of various quantum and spintronic applications without external magnetic fields.
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Minimax Estimation of Large Precision Matrices with Bandable Cholesky
Factor -
Abstract
- Last decade witnesses significant methodological and theoretical advances in estimating large precision matrices. In particular, there are scientific applications such as longitudinal data, meteorology and spectroscopy in which the ordering of the variables can be interpreted through a bandable structure on the Cholesky factor of the precision matrix. However, the minimax theory has still been largely unknown, as opposed to the well established minimax results over the corresponding bandable covariance matrices. In this paper, we focus on two commonly used types of parameter spaces, and develop the optimal rates of convergence under both the operator norm and the Frobenius norm. A striking phenomenon is found: two types of parameter spaces are fundamentally different under the operator norm but enjoy the same rate optimality under the Frobenius norm, which is in sharp contrast to the equivalence of corresponding two types of bandable covariance matrices under both norms. This fundamental difference is established by carefully constructing the corresponding minimax lower bounds. Two new estimation procedures are developed: for the operator norm, our optimal procedure is based on a novel local cropping estimator targeting on all principle submatrices of the precision matrix while for the Frobenius norm, our optimal procedure relies on a delicate regression-based thresholding rule. Lepski's method is considered to achieve optimal adaptation. We further establish rate optimality in the nonparanormal model. Numerical studies are carried out to confirm our theoretical findings.
Yu Liu, Zhao Ren [pdf]
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Charged AdS black hole heat engines -
Abstract
- We study the heat engine by a $d$-dimensional charged anti-de Sitter black hole by making a comparison between the small-large black hole phase transition and the liquid-vapour phase transition of water. With the help of the first law and equal-area law, we obtain an exact formula for the efficiency of a black hole engine modeled with a Rankine cycle with or without a back pressure mechanism. When the low temperature is fixed, both the heat and work decreases with the high temperature $T_{1}$. And the efficiency increases with $T_{1}$, while decreases with the charge $q$. For a Rankine cycle with a back pressure mechanism, we find that both the maximum work and efficiency can be approached at the high temperature $T_{1}$. In the reduced parameter space, it also confirms the similar result. Moreover, we observe that the work and efficiency of the black hole heat engine rapidly increase with the number of spacetime dimensions. Thus higher-dimensional charged anti-de Sitter black hole can act as a more efficient power plant producing the mechanical work, and might be a possible source of the power gamma rays and ultrahigh-energy cosmic rays.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Nucl. Phys. B 946, 114700 (2019) [pdf] DOI: 10.1016/j.nuclphysb.2019.114700
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Repulsive Interactions and Universal Properties of Charged Anti–de Sitter Black Hole Microstructures -
Abstract
- The Ruppeiner geometry of thermodynamic fluctuations provides a powerful diagnostic of black hole microstructures. We investigate this for charged AdS black holes and find that while an attractive microstructure interaction dominates for most parameter ranges, a weak repulsive interaction dominates for small black holes of high temperature. This unique property distinguishes the black hole system from that of a Van der Waals fluid, where only attractive microstructure interactions are found. We also find two other novel universal properties for charged black holes. One is that the repulsive interaction is independent of the black hole charge and temperature. The other is that the behavior of the Ruppeiner curvature scalar near criticality is characterized by a dimensionless constant that is identical to that for a Van der Waals fluid, providing us with new insight into black hole microstructures.
Shao-Wen Wei, Yu-Xiao Liu, Robert B. Mann Journal reference: Phys. Rev. Lett. 123, 071103 (2019) [pdf] DOI: 10.1103/PhysRevLett.123.071103
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Stereoscopic Observations of an Erupting Mini-filament-driven Two-sided-loop Jet and the Applications for Diagnosing a Filament Magnetic Field -
Abstract
- The ubiquitous solar jets or jet-like activities are generally regarded as an important source of energy and mass input to the upper solar atmosphere and the solar wind. However, questions about their triggering and driving mechanisms are not completely understood. By taking advantage of high temporal and high spatial resolution stereoscopic observations taken by the Solar Dynamic Observatory (SDO) and the Solar Terrestrial Relations Observatory (STEREO), we report an intriguing two-sided-loop jet occurred on 2013 June 02, which was dynamically associated with the eruption of a mini-filament below an overlying large filament, and two distinct reconnection processes are identified during the formation stage. The SDO observations reveals that the two-sided-loop jet showed a concave shape with a projection speed of about 80 - 136. From the other view angle, the STEREO ahead observations clearly showed that the trajectory of the two arms of the two-sided-loop were along the cavity magnetic field lines hosting the large filament. Contrary to the well-accepted theoretical model, the present observation sheds new light on our understanding of the formation mechanism of two-sided-loop jets. Moreover, the eruption of the two-sided-loop jet not only supplied mass to the overlying large filament, but also provided a rare opportunity to diagnose the magnetic structure of the overlying large filament via the method of three-dimensional reconstruction.
Yuandeng Shen, Zhining Qu, Ding Yuan, Huadong Chen, Yadan Duan, Chengrui Zhou, Zehao Tang, Jin Huang, Yu Liu [pdf] DOI: 10.3847/1538-4357/ab3a4d 1908.03660v2 [pdf]
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The analytical description of a doped Mott insulator -
Abstract
- With the hierarchical Green's function approach, we study a doped Mott insulator described with the Hubbard model by analytically solving the equations of motion of an one-particle Green's function and related multiple-point correlation functions, and find that the separation of the spin and charge degrees of freedom of the electrons is an intrinsic character of the doped Mott insulator. For enough of large on-site repulsive Coulomb interaction, we show that the spectral weight of the one-particle Green's function is proportional to the hole doping concentration that is mainly produced by the charge fluctuation of electrons, while the excitation spectrum of the electrons is composed of two parts: one is contributed by the spin fluctuation of the electrons which is proportional to the hole doping concentration, and another one is coming from the coupling between the charge and spin fluctuations of the electrons that takes the maximum at undoping. All of these low energy/temperature physical properties originate from the strong on-site Coulomb interaction. The present results are consistent with the spectroscopy observations of the cuprate superconductors, and the numerical calculations in normal state above pseudogap regime.
Yu-Liang Liu [pdf] DOI: 10.1142/S0217979219503557 1908.04453v1 [pdf]
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A coupled-channel lattice study of the resonance-like structure
-
Abstract
- In this exploratory study, near-threshold scattering of $D$ and $\bar{D}^*$ meson is investigated using lattice QCD with $N_f=2+1+1$ twisted mass fermion configurations. The calculation is performed within the coupled-channel L\"uscher's finite-size formalism. The study focuses on the channel with $I^G(J^{PC})=1^+(1^{+-})$ where the resonance-like structure $Z_c(3900)$ was discovered. We first identify the most relevant two channels of the problem and the lattice study is performed within the two-channel scattering model. Combined with a two-channel Ross-Shaw theory, scattering parameters are extracted from the energy levels by solving the generalized eigenvalue problem. Our results on the scattering length parameters suggest that, at the particular lattice parameters that we studied, the best fitted parameters do not correspond to a peak behavior in the elastic scattering cross section near the threshold. Furthermore, within the zero-range Ross-Shaw theory, the scenario of a narrow resonance close to the threshold is disfavored beyond $3\sigma$ level.
Ting Chen, Ying Chen, Ming Gong, Chuan Liu, Liuming Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Markus Werner, Jian-Bo Zhang Journal reference: Chin.Phys. C43 (2019) no.10, 103103 [pdf] DOI: 10.1088/1674-1137/43/10/103103
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Exploiting temporal consistency for real-time video depth estimation -
Abstract
- Accuracy of depth estimation from static images has been significantly improved recently, by exploiting hierarchical features from deep convolutional neural networks (CNNs). Compared with static images, vast information exists among video frames and can be exploited to improve the depth estimation performance. In this work, we focus on exploring temporal information from monocular videos for depth estimation. Specifically, we take the advantage of convolutional long short-term memory (CLSTM) and propose a novel spatial-temporal CSLTM (ST-CLSTM) structure. Our ST-CLSTM structure can capture not only the spatial features but also the temporal correlations/consistency among consecutive video frames with negligible increase in computational cost. Additionally, in order to maintain the temporal consistency among the estimated depth frames, we apply the generative adversarial learning scheme and design a temporal consistency loss. The temporal consistency loss is combined with the spatial loss to update the model in an end-to-end fashion. By taking advantage of the temporal information, we build a video depth estimation framework that runs in real-time and generates visually pleasant results. Moreover, our approach is flexible and can be generalized to most existing depth estimation frameworks. Code is available at: https://tinyurl.com/STCLSTM
Haokui Zhang, Chunhua Shen, Ying Li, Yuanzhouhan Cao, Yu Liu, Youliang Yan [pdf]
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Practical scheme from QCD to phenomena via Dyson-Schwinger equations -
Abstract
- We deliver a new scheme to compute the quark propagator and the quark-gluon interaction vertex through the coupled Dyson-Schwinger equations (DSEs) of QCD. We take the three-gluon vertex into account in our calculations, and implement the gluon propagator and the running coupling function fitted by the solutions of their respective DSEs. We obtain the momentum and current mass dependence of the quark propagator and the quark-gluon vertex, and the chiral quark condensate which agrees with previous results excellently. We also compute the quark-photon vertex within this scheme and give the anomalous chromo- and electro-magnetic moment of quark. The obtained results also agree with previous ones very well. These applications manifest that the new scheme is realistic and then practical for explaining the QCD-related phenomena.
Can Tang, Fei Gao, Yu-xin Liu Journal reference: Phys. Rev. D 100, 056001 (2019) [pdf] DOI: 10.1103/PhysRevD.100.056001
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A Critical Review on the Electromigration Effect, the Electroplastic Effect, and Perspectives on the Effects of Electric Current Upon Alloy Phase Stability -
Abstract
- The electronic interconnections in the state-of-the-art integrated circuit manufacturing have been scaled down to the micron or sub-micron scale. This results in a dramatic increase in the current density passing through interconnections, which means that the electromigration (EM) effect plays a significant role in the reliability of products. Although thorough studies and reviews of EM effects have been continuously conducted in the past 60 years, some parts of EM theories lack clear elucidation of the electric current-induced non-directional effects, including electric current-induced phase equilibrium changes. This review article is intended to provide a broad picture of electric current-induced lattice stability changes and to summarize the existing literature on EM-related phenomena, EM-related theoretical models, and relevant effects of the electroplastic (EP) effect in order to lead to a better understanding of electric current-induced effects on materials. This article also posits that EM is either part of the EP effect or shares the intrinsic electric current-induced plastic deformation associated with the EP effect. This concept appears to contribute to the missing parts of the EM theories.
Yu-chen Liu, Shih-kang Lin Journal reference: JOM (2019) [pdf] DOI: 10.1007/s11837-019-03661-y
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Valence transition in topological Kondo insulator -
Abstract
- We investigate the valence transition in three-dimensional topological Kondo insulator through slave-boson analysis of periodic Anderson model. By including the effect of intra-atomic Coulomb correlation $U_{fc}$ between conduction and local electrons, we find a first-order valence transition from Kondo region to mixed valence upon ascending of local level above a critical $U_{fc}$, and this valence transition usually occurs very close to or simultaneously with a topological transition. Near the parameter region of zero-temperature valence transition, rise of temperature can generate a thermal valence transition from mixed valence to Kondo region, accompanied by a first-order topological transition. Remarkably, above a critical $U_{fc}$ which is considerable smaller than that generating paramagnetic valence transition, the original continuous antiferromagnetic transition is shifted to first order one, at which a discontinuous valence shift takes place. Upon increased $U_{fc}$, the paramagnetic valence transition approaches then converges with the first-order antiferromagnetic transition, leaving an significant valence shift on the magnetic boundary. The continuous antiferromagnetic transition, first-order antiferromagnetic transition, paramagnetic valence transition and topological transitions are all summarized in a global phase diagram. Our proposed exotic transition processes can help to understand the thermal valence variation as well as the valence shift around the pressure-induced magnetic transition in topological Kondo insulator candidates and in other heavy-fermion systems.
Jia-Tao Zhuang, Xiao-Jun Zheng, Zhi-Yong Wang, Xing Ming, Huan Li, Yu Liu, Hai-Feng Song [pdf] DOI: 10.1088/1361-648X/ab4625 1908.00913v1 [pdf]
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L2G Auto-encoder -
Abstract
- Auto-encoder is an important architecture to understand point clouds in an encoding and decoding procedure of self reconstruction. Current auto-encoder mainly focuses on the learning of global structure by global shape reconstruction, while ignoring the learning of local structures. To resolve this issue, we propose Local-to-Global auto-encoder (L2G-AE) to simultaneously learn the local and global structure of point clouds by local to global reconstruction. Specifically, L2G-AE employs an encoder to encode the geometry information of multiple scales in a local region at the same time. In addition, we introduce a novel hierarchical self-attention mechanism to highlight the important points, scales and regions at different levels in the information aggregation of the encoder. Simultaneously, L2G-AE employs a recurrent neural network (RNN) as decoder to reconstruct a sequence of scales in a local region, based on which the global point cloud is incrementally reconstructed. Our outperforming results in shape classification, retrieval and upsampling show that L2G-AE can understand point clouds better than state-of-the-art methods.
Xinhai Liu, Zhizhong Han, Xin Wen, Yu-Shen Liu, Matthias Zwicker [pdf] DOI: 10.1145/3343031.3350960 1908.00720v1 [pdf]
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Scalable Place Recognition Under Appearance Change for Autonomous
Driving -
Abstract
- A major challenge in place recognition for autonomous driving is to be robust against appearance changes due to short-term (e.g., weather, lighting) and long-term (seasons, vegetation growth, etc.) environmental variations. A promising solution is to continuously accumulate images to maintain an adequate sample of the conditions and incorporate new changes into the place recognition decision. However, this demands a place recognition technique that is scalable on an ever growing dataset. To this end, we propose a novel place recognition technique that can be efficiently retrained and compressed, such that the recognition of new queries can exploit all available data (including recent changes) without suffering from visible growth in computational cost. Underpinning our method is a novel temporal image matching technique based on Hidden Markov Models. Our experiments show that, compared to state-of-the-art techniques, our method has much greater potential for large-scale place recognition for autonomous driving.
Anh-Dzung Doan, Yasir Latif, Tat-Jun Chin, Yu Liu, Thanh-Toan Do, Ian Reid [pdf]
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ShapeCaptioner: Generative Caption Network for 3D Shapes by Learning a
Mapping from Parts Detected in Multiple Views to Sentences -
Abstract
- 3D shape captioning is a challenging application in 3D shape understanding. Captions from recent multi-view based methods reveal that they cannot capture part-level characteristics of 3D shapes. This leads to a lack of detailed part-level description in captions, which human tend to focus on. To resolve this issue, we propose ShapeCaptioner, a generative caption network, to perform 3D shape captioning from semantic parts detected in multiple views. Our novelty lies in learning the knowledge of part detection in multiple views from 3D shape segmentations and transferring this knowledge to facilitate learning the mapping from 3D shapes to sentences. Specifically, ShapeCaptioner aggregates the parts detected in multiple colored views using our novel part class specific aggregation to represent a 3D shape, and then, employs a sequence to sequence model to generate the caption. Our outperforming results show that ShapeCaptioner can learn 3D shape features with more detailed part characteristics to facilitate better 3D shape captioning than previous work.
Zhizhong Han, Chao Chen, Yu-Shen Liu, Matthias Zwicker [pdf]
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Matching generalized parton quasidistributions in the RI/MOM scheme -
Abstract
- Within the framework of large momentum effective theory (LaMET), genenaralized parton distributions (GPDs) can be extracted from lattice calculations of quasi-GPDs through a perturbative matching relation, up to power corrections that are suppressed by the hadron momentum. In this paper, we focus on isovector quark GPDs, including the unpolarized, longitudinally and transversely polarized cases, and present the one-loop matching that connects the quasi-GPDs renormalized in a regularization-independent momentum subtraction (RI/MOM) scheme to the GPDs in MS scheme. We find that the matching coefficient is independent of the momentum transfer squared. As a consequence, the matching for the quasi-GPD with zero skewness is the same as that for the quasi-PDF. Our results provide a crucial input for the determination of quark GPDs from lattice QCD using LaMET.
Yu-Sheng Liu, Wei Wang, Ji Xu, Qi-An Zhang, Jian-Hui Zhang, Shuai Zhao, Yong Zhao Journal reference: Phys. Rev. D 100, 034006 (2019) [pdf] DOI: 10.1103/PhysRevD.100.034006
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Multi-Angle Point Cloud-VAE: Unsupervised Feature Learning for 3D Point
Clouds from Multiple Angles by Joint Self-Reconstruction and Half-to-Half
Prediction -
Abstract
- Unsupervised feature learning for point clouds has been vital for large-scale point cloud understanding. Recent deep learning based methods depend on learning global geometry from self-reconstruction. However, these methods are still suffering from ineffective learning of local geometry, which significantly limits the discriminability of learned features. To resolve this issue, we propose MAP-VAE to enable the learning of global and local geometry by jointly leveraging global and local self-supervision. To enable effective local self-supervision, we introduce multi-angle analysis for point clouds. In a multi-angle scenario, we first split a point cloud into a front half and a back half from each angle, and then, train MAP-VAE to learn to predict a back half sequence from the corresponding front half sequence. MAP-VAE performs this half-to-half prediction using RNN to simultaneously learn each local geometry and the spatial relationship among them. In addition, MAP-VAE also learns global geometry via self-reconstruction, where we employ a variational constraint to facilitate novel shape generation. The outperforming results in four shape analysis tasks show that MAP-VAE can learn more discriminative global or local features than the state-of-the-art methods.
Zhizhong Han, Xiyang Wang, Yu-Shen Liu, Matthias Zwicker [pdf]
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Thick brane in reduced Horndeski theory -
Abstract
- Horndeski theory is the most general scalar-tensor theory retaining second-order field equations, although the action includes higher-order terms. This is achieved by a special choice of coupling constants. In this paper, we investigate thick brane system in reduced Horndeski theory, especially the effect of the non-minimal derivative coupling on thick brane. First, the equations of motion are presented and a set of analytic background solutions are obtained. Then, to investigate the stability of the background scalar profile, we present a novel canonically normalized method, and show that although the original background scalar field is unstable, the canonical one is stable. The stability of the thick brane under tensor perturbation is also considered. It is shown that the tachyon is absent and the graviton zero mode can be localized on the brane. The localized graviton zero mode recovers the four-dimensional Newtonian potential and the presence of the non-minimal derivative coupling results in a splitting of its wave function. The correction of the massive graviton KK modes to the Newtonian potential is also analyzed briefly.
Qi-Ming Fu, Hao Yu, Li Zhao, Yu-Xiao Liu Journal reference: Phys. Rev. D 100, 124057 (2019) [pdf] DOI: 10.1103/PhysRevD.100.124057
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Relative rigid objects in extriangulated categories -
Abstract
- In this paper, we study a close relationship between relative cluster tilting theory in extriangulated categories and tau-tilting theory in module categories. Our main results show that relative rigid objects are in bijection with $\tau$-rigid pairs, and also relative maximal rigid objects with support tau-tilting pairs under some assumptions. These results generalize their work by Adachi-Iyama-Reiten, Yang-Zhu and Fu-Geng-Liu. Finally, we introduce mutation of relative maximal rigid objects and show that any basic relative almost maximal rigid object has exactly two non-isomorphic indecomposable complements.
Yu Liu, Panyue Zhou [pdf]
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Lecture Notes in Computer Science -
Abstract
- Pinterest is a popular Web application that has over 250 million active users. It is a visual discovery engine for finding ideas for recipes, fashion, weddings, home decoration, and much more. In the last year, the company adopted Semantic Web technologies to create a knowledge graph that aims to represent the vast amount of content and users on Pinterest, to help both content recommendation and ads targeting. In this paper, we present the engineering of an OWL ontology---the Pinterest Taxonomy---that forms the core of Pinterest's knowledge graph, the Pinterest Taste Graph. We describe modeling choices and enhancements to WebProt\'eg\'e that we used for the creation of the ontology. In two months, eight Pinterest engineers, without prior experience of OWL and WebProt\'eg\'e, revamped an existing taxonomy of noisy terms into an OWL ontology. We share our experience and present the key aspects of our work that we believe will be useful for others working in this area.
Rafael S. Gonçalves, Matthew Horridge, Rui Li, Yu Liu, Mark A. Musen, Csongor I. Nyulas, Evelyn Obamos, Dhananjay Shrouty, David Temple [pdf] DOI: 10.1007/978-3-030-30796-7_26 1907.02106v1 [pdf]
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Exploring effective charge in electromigration using machine learning -
Abstract
- The effective charge of an element is a parameter characterizing the electromgration effect, which can determine the reliability of interconnection in electronic technologies. In this work, machine learning approaches were employed to model the effective charge (z*) as a linear function of physically meaningful elemental properties. Average 5-fold (leave-out-alloy-group) cross-validation yielded root-mean-square-error divided by whole data set standard deviation (RMSE/$\sigma$) values of 0.37 $\pm$ 0.01 (0.22 $\pm$ 0.18), respectively, and $R^2$ values of 0.86. Extrapolation to z* of totally new alloys showed limited but potentially useful predictive ability. The model was used in predicting z* for technologically relevant host-impurity pairs.
Yu-chen Liu, Benjamin Afflerbach, Ryan Jacobs, Shih-kang Lin, Dane Morgan Journal reference: MRS Communications, 1-9 (2019) [pdf] DOI: 10.1557/mrc.2019.63
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Super Fast Beam and Channel Tracking in 2D Phased Antenna Arrays -
Abstract
- Millimeter wave (mmWave) is an attractive candidate for high-speed mobile communications in the future. However, due to the propagation characteristics of mmWave, beam and and and and alignment becomes a key challenge for serving users with fast moving speeds. In this paper, we develop a joint beam and channel tracking algorithm that can track beams from the horizontal and vertical directions by using two-dimensional (2D) phased antenna arrays. A general sequence of optimal trial beamforming parameters is obtained to achieve the minimum Cramer-Rao lower bound (CRLB) of joint beam and channel tracking asymptotically as antenna number grows to infinity. This sequence is proved to be asymptotically optimal in different conditions, e.g., channel coefficients, path directions, and antenna array sizes. We prove that the proposed algorithm converges to the minimum CRLB in static scenarios. Simulation results show that our algorithm outperforms several existing algorithms in tracking accuracy and speed band.
Yu Liu, Jiahui Li, Yin Sun, Shidong Zhou [pdf]
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Lattice dynamics and phase transitions in
-
Abstract
- We present Raman spectroscopy measurements of the van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$, together with lattice dynamics. Four out of eight Raman active modes are observed and assigned, in agreement with numerical calculations. The energies and line-widths of the observed modes display an unconventional temperature dependence at about 150 K and 220 K, followed by the nonmonotonic evolution of the Raman continuum. Whereas the former can be related to the magnetic phase transition, the origin of the latter anomaly remains an open question.
A. Milosavljević, A. Šolajić, S. Djurdjić-Mijin, J. Pešić, B. Višić, Yu Liu, C. Petrovic, N. Lazarević, Z. V. Popović Journal reference: Phys. Rev. B 99, 214304, Published 17 June 2019 [pdf] DOI: 10.1103/PhysRevB.99.214304
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Implementing Black Hole as Efficient Power Plant -
Abstract
- Treating the black hole molecules as working substance and considering its phase structure, we study the black hole heat engine by a charged anti-de Sitter black hole. In the reduced temperature-entropy chart, it is found that the work, heat, and efficiency of the engine are free of the black hole charge. Applying the Rankine cycle with or without a back pressure mechanism to the black hole heat engine, the compact formula for the efficiency is obtained. And the heat, work and efficiency are worked out. The result shows that the black hole engine working along the Rankine cycle with a back pressure mechanism has a higher efficiency. This provides a novel and efficient mechanism to produce the useful mechanical work, and such black hole heat engine may act as a possible energy source for the high energy astrophysical phenomena near the black hole.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Commun. Theor. Phys. 71, 711 (2019) [pdf] DOI: 10.1088/0253-6102/71/6/711
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Quasinormal modes and van der Waals-like phase transition of charged AdS black holes in Lorentz symmetry breaking massive gravity -
Abstract
- Using the quasinormal modes of a massless scalar perturbation, we investigate the small/large black hole phase transition in the Lorentz symmetry breaking massive gravity. We mainly focus on two issues: i) the sign change of slope of the quasinormal mode frequencies in the complex-$\omega$ diagram; ii) the behaviors of the imaginary part of the quasinormal mode frequencies along the isobaric or isothermal processes. For the first issue, our result shows that, at low fixed temperature or pressure, the phase transition can be probed by the sign change of slope. While increasing the temperature or pressure to some certain values near the critical point, there will appear the deflection point, which indicates that such method may not be appropriate to test the phase transition. In particular, the behavior of the quasinormal mode frequencies for the small and large black holes tend to the same at the critical point. For the second issue, it is shown that the non-monotonic behavior is observed only when the small/large black hole phase transition occurs. Therefore, this property can provide us with an additional method to probe the phase transition through the quasinormal modes.
Bin Liang, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Int. J. Mod. Phys. D 28, 1950113 (2019) [pdf] DOI: 10.1142/S021827181950113X
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From Caesar Cipher to Unsupervised Learning: A New Method for Classifier
Parameter Estimation -
Abstract
- Many important classification problems, such as object classification, speech recognition, and machine translation, have been tackled by the supervised learning paradigm in the past, where training corpora of parallel input-output pairs are required with high cost. To remove the need for the parallel training corpora has practical significance for real-world applications, and it is one of the main goals of unsupervised learning. Recently, encouraging progress in unsupervised learning for solving such classification problems has been made and the nature of the challenges has been clarified. In this article, we review this progress and disseminate a class of promising new methods to facilitate understanding the methods for machine learning researchers. In particular, we emphasize the key information that enables the success of unsupervised learning - the sequential statistics as the distributional prior in the labels. Exploitation of such sequential statistics makes it possible to estimate parameters of classifiers without the need of paired input-output data. In this paper, we first introduce the concept of Caesar Cipher and its decryption, which motivated the construction of the novel loss function for unsupervised learning we use throughout the paper. Then we use a simple but representative binary classification task as an example to derive and describe the unsupervised learning algorithm in a step-by-step, easy-to-understand fashion. We include two cases, one with Bigram language model as the sequential statistics for use in unsupervised parameter estimation, and another with a simpler Unigram language model. For both cases, detailed derivation steps for the learning algorithm are included. Further, a summary table compares computational steps of the two cases in executing the unsupervised learning algorithm for learning binary classifiers.
Yu Liu, Li Deng, Jianshu Chen, Chang Wen Chen [pdf]
-
Constraints on generalized Eddington-inspired Born-Infeld branes -
Abstract
- The Palatini $f(|\hat{\Omega}|)$ gravity is a generalized theory of the Eddington-inspired Born-Infeld gravity, where $\Omega_{~N}^{K}\equiv\delta_{~N}^{K}+bg^{KL}R_{LN}(\Gamma)$ is an auxiliary tensor constructed with the spacetime metric $g$ and independent connection $\Gamma$. In this paper, we study $f(|\hat{\Omega}|)$ theory with $f(|\hat{\Omega}|)=|\hat{\Omega}|^{\frac{1}{2}+n}$ in the thick brane scenario and give some constraints on the brane model. We finally found an analytic solution of the thick brane generated by a single scalar field. The behavior of the negative energy density denotes the localization of the thick brane at the origin of the extra dimension. In our braneworld, the warp factor is divergent at the boundary of the extra dimension while the brane system is asymptotically anti$-$de Sitter. It is shown that the tensor perturbation of the brane is stable and the massless graviton is localized on the thick brane. Therefore, the effective Einstein-Hilbert action on the brane can be rebuilt in the low-energy approximation. According to the recent test of the gravitational inverse-square law, we give some constraints on the $f(|\hat{\Omega}|)$ brane.
Zi-Chao Lin, Ke Yang, Yu-Peng Zhang, Jian Wang, Yu-Xiao Liu Journal reference: Phys. Rev. D 99, 084020 (2019) [pdf] DOI: 10.1103/PhysRevD.99.084020
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Geodesics and periodic orbits in Kehagias-Sfetsos black holes in deformed Hor̆ava-Lifshitz gravity -
Abstract
- The motion of a massive test particle around a Kehagias-Sfetsos black hole in deformed Ho\u{r}ava-Lifshitz gravity is studied. Employing the effective potential, the marginally bound orbits and the innermost stable circular orbits are analyzed. For the marginally bound orbits, their radius and angular momentum decrease with the parameter $\omega$ of the gravity. For the innermost stable circular orbits, the energy and angular momentum also decrease with $\omega$. Based on these results, we investigate the periodic orbits in the Kehagias-Sfetsos black holes. It is found that the apsidal angle parameter increases with the particle energy, while decreases with the angular momentum. Moreover, compared to the Schwarzschild black hole, the periodic orbits in Kehagias-Sfetsos black holes always have lower energy. These results provide us a possible way to distinguish the Kehagias-Sfetsos black hole in deformed Ho\u{r}ava-Lifshitz gravity from the Schwarzschild black hole.
Shao-Wen Wei, Jie Yang, Yu-Xiao Liu Journal reference: Phys. Rev. D 99, 104016 (2019) [pdf] DOI: 10.1103/PhysRevD.99.104016
-
Gravitational Waves and Extra Dimensions: A Short Review -
Abstract
- We give a brief review on the recent development of gravitational waves in extra-dimensional theories of gravity. Studying extra-dimensional theories with gravitational waves provides a new way to constrain extra dimensions. After a flash look at the history of gravitational waves and a brief introduction to several major extra-dimensional theories, we focus on the sources and spectra of gravitational waves in extra-dimensional theories. It is shown that one can impose limits on the size of extra dimensions and the curvature of the universe by researching the propagations of gravitational waves and the corresponding electromagnetic waves. Since gravitational waves can propagate throughout the bulk, how the amplitude of gravitational waves decreases determines the number of extra dimensions for some models. In addition, we also briefly present some other characteristics of gravitational waves in extra-dimensional theories.
Hao Yu, Zi-Chao Lin, Yu-Xiao Liu Journal reference: Commun. Theor. Phys. 71 (2019) 991-1006 [pdf] DOI: 10.1088/0253-6102/71/8/991
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Parts4Feature: Learning 3D Global Features from Generally Semantic Parts
in Multiple Views -
Abstract
- Deep learning has achieved remarkable results in 3D shape analysis by learning global shape features from the pixel-level over multiple views. Previous methods, however, compute low-level features for entire views without considering part-level information. In contrast, we propose a deep neural network, called Parts4Feature, to learn 3D global features from part-level information in multiple views. We introduce a novel definition of generally semantic parts, which Parts4Feature learns to detect in multiple views from different 3D shape segmentation benchmarks. A key idea of our architecture is that it transfers the ability to detect semantically meaningful parts in multiple views to learn 3D global features. Parts4Feature achieves this by combining a local part detection branch and a global feature learning branch with a shared region proposal module. The global feature learning branch aggregates the detected parts in terms of learned part patterns with a novel multi-attention mechanism, while the region proposal module enables locally and globally discriminative information to be promoted by each other. We demonstrate that Parts4Feature outperforms the state-of-the-art under three large-scale 3D shape benchmarks.
Zhizhong Han, Xinhai Liu, Yu-Shen Liu, Matthias Zwicker [pdf]
-
3DViewGraph: Learning Global Features for 3D Shapes from A Graph of
Unordered Views with Attention -
Abstract
- Learning global features by aggregating information over multiple views has been shown to be effective for 3D shape analysis. For view aggregation in deep learning models, pooling has been applied extensively. However, pooling leads to a loss of the content within views, and the spatial relationship among views, which limits the discriminability of learned features. We propose 3DViewGraph to resolve this issue, which learns 3D global features by more effectively aggregating unordered views with attention. Specifically, unordered views taken around a shape are regarded as view nodes on a view graph. 3DViewGraph first learns a novel latent semantic mapping to project low-level view features into meaningful latent semantic embeddings in a lower dimensional space, which is spanned by latent semantic patterns. Then, the content and spatial information of each pair of view nodes are encoded by a novel spatial pattern correlation, where the correlation is computed among latent semantic patterns. Finally, all spatial pattern correlations are integrated with attention weights learned by a novel attention mechanism. This further increases the discriminability of learned features by highlighting the unordered view nodes with distinctive characteristics and depressing the ones with appearance ambiguity. We show that 3DViewGraph outperforms state-of-the-art methods under three large-scale benchmarks.
Zhizhong Han, Xiyang Wang, Chi-Man Vong, Yu-Shen Liu, Matthias Zwicker, C. L. Philip Chen [pdf]
-
Modulating quantum Fisher information of qubit in dissipative cavity by coupling strength -
Abstract
- By using the non-Markovian master equation, we investigate the effect of the cavity and the environment on the quantum Fisher information (QFI) of an atom qubit system in a dissipation cavity. We obtain the formulae of QFI for two different initial states and analyze the effect of the atom-cavity coupling and the cavity-reservoir coupling on the QFI. The results show that the dynamic behavior of the QFI is obviously dependent on the initial atomic states, the atom-cavity coupling and the cavity-reservoir coupling. The stronger the atom-cavity coupling, the quicker the QFI oscillates and the slower the QFI reduces. Especially, the QFI will tend to a stable value not zero if the atom-cavity coupling is large enough. On the other hand, the smaller the cavity-reservoir coupling, the stronger the non-Markovian effect, the slower the QFI decay. In other words, choosing the best parameter can improve the accuracy of parameter estimation. In addition, the physical explanation of the dynamic behavior of the QFI is given by means of the QFI flow.
Danping Lin, Yu Liu, Hong-Mei Zou Journal reference: Chin. Phys. B . 2018, 27(11): 110303 [pdf] DOI: 10.1088/1674-1056/27/11/110303
-
Superposition of macroscopically squeezed states with enhanced squeezing
in cavity optomechanical system at single-photon level -
Abstract
- We propose an efficient approach to generate the superposed macroscopically squeezed states with enhanced squeezing in a two-mode optomechanical system. This can be achieved by introducing a sinusoidal modulation to either the cavity frequencies or the coupling strengths between two cavity modes. The squeezement of the oscillator can be significantly enhanced to 12.16 dB with single photon, once the relative ratio of coupling strength is optimized under proper conditions. Further enhanced squeezing can be obtained by carefully adjusting the system parameters. In terms of the Wigner quasi-probability distribution, we show the squeezed error ellipses and interference fringes of the Yurke-Stoler-type squeezed states, denoting the squeezing and superposition properties. Our state generation scheme show reliable performance and robust resistance to finite environmental fluctuations, which implies applications for both fundamental interest and practical value.
Sai-Nan Huai, Wei Nie, Yun-bo Zhang, Yu-xi Liu [pdf]
-
PRSim -
Abstract
- {\it SimRank} is a classic measure of the similarities of nodes in a graph. Given a node $u$ in graph $G =(V, E)$, a {\em single-source SimRank query} returns the SimRank similarities $s(u, v)$ between node $u$ and each node $v \in V$. This type of queries has numerous applications in web search and social networks analysis, such as link prediction, web mining, and spam detection. Existing methods for single-source SimRank queries, however, incur query cost at least linear to the number of nodes $n$, which renders them inapplicable for real-time and interactive analysis. { This paper proposes \prsim, an algorithm that exploits the structure of graphs to efficiently answer single-source SimRank queries. \prsim uses an index of size $O(m)$, where $m$ is the number of edges in the graph, and guarantees a query time that depends on the {\em reverse PageRank} distribution of the input graph. In particular, we prove that \prsim runs in sub-linear time if the degree distribution of the input graph follows the power-law distribution, a property possessed by many real-world graphs. Based on the theoretical analysis, we show that the empirical query time of all existing SimRank algorithms also depends on the reverse PageRank distribution of the graph.} Finally, we present the first experimental study that evaluates the absolute errors of various SimRank algorithms on large graphs, and we show that \prsim outperforms the state of the art in terms of query time, accuracy, index size, and scalability.
Zhewei Wei, Xiaodong He, Xiaokui Xiao, Sibo Wang, Yu Liu, Xiaoyong Du, Ji-Rong Wen [pdf] DOI: 10.1145/3299869.3319873 1905.02354v1 [pdf]
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Extracting human emotions at different places based on facial expressions and spatial clustering analysis -
Abstract
- The emergence of big data enables us to evaluate the various human emotions at places from a statistic perspective by applying affective computing. In this study, a novel framework for extracting human emotions from large-scale georeferenced photos at different places is proposed. After the construction of places based on spatial clustering of user generated footprints collected in social media websites, online cognitive services are utilized to extract human emotions from facial expressions using the state-of-the-art computer vision techniques. And two happiness metrics are defined for measuring the human emotions at different places. To validate the feasibility of the framework, we take 80 tourist attractions around the world as an example and a happiness ranking list of places is generated based on human emotions calculated over 2 million faces detected out from over 6 million photos. Different kinds of geographical contexts are taken into consideration to find out the relationship between human emotions and environmental factors. Results show that much of the emotional variation at different places can be explained by a few factors such as openness. The research may offer insights on integrating human emotions to enrich the understanding of sense of place in geography and in place-based GIS.
Yuhao Kang, Qingyuan Jia, Song Gao, Xiaohuan Zeng, Yueyao Wang, Stephan Angsuesser, Yu Liu, Xinyue Ye, Teng Fei Journal reference: Transactions in GIS, Year 2019, Volume 23, Issue 3 [pdf] DOI: 10.1111/tgis.12552
-
Chiral kinetic theory in curved spacetime -
Abstract
- Many-body systems with chiral fermions exhibit anomalous transport phenomena originated from quantum anomalies. Based on quantum field theory, we derive the kinetic theory for chiral fermions interacting with an external electromagnetic field and a background curved geometry. The resultant framework respects the covariance under the U(1) gauge, local Lorentz, and diffeomorphic transformations. It is particularly useful to study the gravitational or non-inertial effects for chiral systems. As the first application, we study the chiral dynamics in a rotating coordinate and clarify the roles of the Coriolis force and spin-vorticity coupling in generating the chiral vortical effect (CVE). We also show that the CVE is an intrinsic phenomenon of a rotating chiral fluid, and thus independent of observer's frame.
Yu-Chen Liu, Lan-Lan Gao, Kazuya Mameda, Xu-Guang Huang Journal reference: Phys. Rev. D 99, 085014 (2019) [pdf] DOI: 10.1103/PhysRevD.99.085014
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RGBD Based Dimensional Decomposition Residual Network for 3D Semantic
Scene Completion -
Abstract
- RGB images differentiate from depth images as they carry more details about the color and texture information, which can be utilized as a vital complementary to depth for boosting the performance of 3D semantic scene completion (SSC). SSC is composed of 3D shape completion (SC) and semantic scene labeling while most of the existing methods use depth as the sole input which causes the performance bottleneck. Moreover, the state-of-the-art methods employ 3D CNNs which have cumbersome networks and tremendous parameters. We introduce a light-weight Dimensional Decomposition Residual network (DDR) for 3D dense prediction tasks. The novel factorized convolution layer is effective for reducing the network parameters, and the proposed multi-scale fusion mechanism for depth and color image can improve the completion and segmentation accuracy simultaneously. Our method demonstrates excellent performance on two public datasets. Compared with the latest method SSCNet, we achieve 5.9% gains in SC-IoU and 5.7% gains in SSC-IOU, albeit with only 21% network parameters and 16.6% FLOPs employed compared with that of SSCNet.
Jie Li, Yu Liu, Dong Gong, Qinfeng Shi, Xia Yuan, Chunxia Zhao, Ian Reid [pdf]
-
Experimental Quantum-enhanced Cryptographic Remote Control -
Abstract
- The Internet of Things (IoT), as a cutting-edge integrated cross-technology, promises to informationize people's daily lives, while being threatened by continuous challenges of eavesdropping and tampering. The emerging quantum cryptography, harnessing the random nature of quantum mechanics, may also enable unconditionally secure control network, beyond the applications in secure communications. Here, we present a quantum-enhanced cryptographic remote control scheme that combines quantum randomness and one-time pad algorithm for delivering commands remotely. We experimentally demonstrate this on an unmanned aircraft vehicle (UAV) control system. We precharge quantum random number (QRN) into controller and controlee before launching UAV, instead of distributing QRN like standard quantum communication during flight. We statistically verify the randomness of both quantum keys and the converted ciphertexts to check the security capability. All commands in the air are found to be completely chaotic after encryption, and only matched keys on UAV can decipher those commands precisely. In addition, the controlee does not response to the commands that are not or incorrectly encrypted, showing the immunity against interference and decoy. Our work adds true randomness and quantum enhancement into the realm of secure control algorithm in a straightforward and practical fashion, providing a promoted solution for the security of artificial intelligence and IoT.
Xiao-Ling Pang, Lu-Feng Qiao, Ke Sun, Yu Liu, Ai-Lin Yang, Xian-Min Jin [pdf]
-
AIP Conference Proceedings -
Abstract
- We investigate the chemical potential and baryon number density of the hadron-quark phase transition in neutron star matter. The hadron matter is described with relativistic mean field theory, and the quark matter is described with the Dyson-Schwinger equation approach of QCD. In order to study the first-order phase transition, we develop the sound speed interpolation scheme to construct the equation of state in the middle density region where the hadron phase and quark phase coexist. The phase transition chemical potential is constrained with the maximum mass, the tidal deformability and the radius of neutrons stars. And the most probable value of the phase transition chemical potential is found.
Zhan Bai, Yu-xin Liu [pdf] DOI: 10.1063/1.5117820 1904.01978v3 [pdf]
-
Talking Face Generation by Adversarially Disentangled Audio-Visual
Representation -
Abstract
- Talking face generation aims to synthesize a sequence of face images that correspond to a clip of speech. This is a challenging task because face appearance variation and semantics of speech are coupled together in the subtle movements of the talking face regions. Existing works either construct specific face appearance model on specific subjects or model the transformation between lip motion and speech. In this work, we integrate both aspects and enable arbitrary-subject talking face generation by learning disentangled audio-visual representation. We find that the talking face sequence is actually a composition of both subject-related information and speech-related information. These two spaces are then explicitly disentangled through a novel associative-and-adversarial training process. This disentangled representation has an advantage where both audio and video can serve as inputs for generation. Extensive experiments show that the proposed approach generates realistic talking face sequences on arbitrary subjects with much clearer lip motion patterns than previous work. We also demonstrate the learned audio-visual representation is extremely useful for the tasks of automatic lip reading and audio-video retrieval.
Hang Zhou, Yu Liu, Ziwei Liu, Ping Luo, Xiaogang Wang [pdf]
-
Knowledge Distillation via Route Constrained Optimization -
Abstract
- Distillation-based learning boosts the performance of the miniaturized neural network based on the hypothesis that the representation of a teacher model can be used as structured and relatively weak supervision, and thus would be easily learned by a miniaturized model. However, we find that the representation of a converged heavy model is still a strong constraint for training a small student model, which leads to a high lower bound of congruence loss. In this work, inspired by curriculum learning we consider the knowledge distillation from the perspective of curriculum learning by routing. Instead of supervising the student model with a converged teacher model, we supervised it with some anchor points selected from the route in parameter space that the teacher model passed by, as we called route constrained optimization (RCO). We experimentally demonstrate this simple operation greatly reduces the lower bound of congruence loss for knowledge distillation, hint and mimicking learning. On close-set classification tasks like CIFAR100 and ImageNet, RCO improves knowledge distillation by 2.14% and 1.5% respectively. For the sake of evaluating the generalization, we also test RCO on the open-set face recognition task MegaFace.
Xiao Jin, Baoyun Peng, Yichao Wu, Yu Liu, Jiaheng Liu, Ding Liang, Junjie Yan, Xiaolin Hu [pdf]
-
Gravitational resonances in mimetic thick branes -
Abstract
- In this work, we investigate gravitational resonances in both single and double mimetic thick branes, which can provide a new way to detect the extra dimension. For the single brane model, we apply the relative probability proposed in [Phys. Rev. D. 80 (2009) 065019]. For the double brane model, we investigate the resonances quasi-localized on the double brane, on the sub-branes and between the sub-branes, respectively. To investigate the resonances quasi-localized on the double brane, we introduce two different definitions of the relative probability and find that the corresponding mass spectra of gravitational resonances are almost the same. For the gravitational resonances quasi-localized on sub-branes and between the sub-branes, the influence of the distance between the two sub-branes and the thickness of the sub-branes are analyzed and new features are found in both cases.
Yi Zhong, Yu-Peng Zhang, Wen-Di Guo, Yu-Xiao Liu Journal reference: JHEP 1904 (2019) 154 [pdf] DOI: 10.1007/JHEP04(2019)154
-
Adversarial Cross-Modal Retrieval via Learning and Transferring
Single-Modal Similarities -
Abstract
- Cross-modal retrieval aims to retrieve relevant data across different modalities (e.g., texts vs. images). The common strategy is to apply element-wise constraints between manually labeled pair-wise items to guide the generators to learn the semantic relationships between the modalities, so that the similar items can be projected close to each other in the common representation subspace. However, such constraints often fail to preserve the semantic structure between unpaired but semantically similar items (e.g. the unpaired items with the same class label are more similar than items with different labels). To address the above problem, we propose a novel cross-modal similarity transferring (CMST) method to learn and preserve the semantic relationships between unpaired items in an unsupervised way. The key idea is to learn the quantitative similarities in single-modal representation subspace, and then transfer them to the common representation subspace to establish the semantic relationships between unpaired items across modalities. Experiments show that our method outperforms the state-of-the-art approaches both in the class-based and pair-based retrieval tasks.
Xin Wen, Zhizhong Han, Xinyu Yin, Yu-Shen Liu [pdf]
-
Measuring road network topology vulnerability by Ricci curvature -
Abstract
- Describing the basic properties of road network systems, such as their robustness, vulnerability, and reliability, has been a very important research topic in the field of urban transportation. Current research mainly uses several statistical indicators of complex networks to analyze the road network systems. However, these methods are essentially node-based. These node-based methods are more concerned with the number of connections between nodes, and lack of consideration for interactions. So, this leads to the well-known node paradox problem, and their ability of characterizing the local and intrinsic properties of a network is weak. From the perspective of network intrinsic geometry, this paper proposes a method for measuring road network vulnerability using a discrete Ricci curvature, which can identify the key sections of a road network and indicate its fragile elements. The results show that our method performs better than complex network statistics on measuring the vulnerability of a road network. Additionally, it can characterize the evolution of the road network vulnerability among different periods of time in the same city through our method. Finally, we compare our method with the previous method of centrality and show the different between them. This article provides a new perspective on a geometry to analyze the vulnerability of a road network and describes the inherent nature of the vulnerability of a road system from a new perspective. It also contributes to enriching the analytical methods of complex road networks.
Lei Gao, Xingquan Liu, Yu Liu, Pu Wang, Min Deng, Qing Zhu, Haifeng Li Journal reference: Physica A: Statistical Mechanics and its Applications 2019 [pdf] DOI: 10.1016/j.physa.2019.121071
-
Eta decay and muonic puzzles -
Abstract
- New physics motivated by muonic puzzles (proton radius and muon $g-2$ discrepancies) is studied. Using a light scalar boson $\phi$, assuming Yukawa interactions, accounts for these muonic puzzles simultaneously. Our previous work limits the existence of such a scalar boson's mass $m_\phi$ from about 160 keV to 60 MeV. We improve this result by including the influence of all of the possible particles that couple to the $\phi$ in computing the decay rate. Doing this involves including the strong interaction physics, involving quarks, necessary to compute the $\eta\pi\phi$ vertex function. The Nambu-Jona-Lasinio model, which accounts for the spontaneous symmetry breaking that yields the constituent mass is employed to represent the relevant strong-interaction physics. We use the $\eta\pi\phi$ vertex function to reanalyze the electron beam dump experiments. The result is that the allowed range of $m_\phi$ lies between about 160 keV and 3.5 MeV. This narrow range represents an inviting target for ruling out or discovering this scalar boson. A possible UV completion of our phenomenological model is discussed.
Yu-Sheng Liu, Ian C. Cloët, Gerald A. Miller [pdf] DOI: 10.1016/j.nuclphysb.2019.114638 1805.01028v2 [pdf]
-
Weak cosmic censorship conjecture in Kerr black holes of modified gravity -
Abstract
- By neglecting the effects of self-force and radiation, we investigate the possibility of destroying the Kerr-MOG black hole through the point particle absorption process. Using the instability of event horizon and equation of particle motion, we get the upper and lower energy bounds allowed for a matter particle to produce the naked singularity. We find that the energy gap always exists between the upper and lower energy bounds for both extremal and near-extremal black holes, which means some tailored particles can actually lead to the violation of the weak cosmic censorship conjecture. However, when considering the effect of the adiabatic process, the result shows that the Kerr-MOG black hole gets more stable instead of a naked singularity, and thus the weak cosmic censorship conjecture can be restored at some level.
Bin Liang, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Mod. Phys. Lett. A 34, 1950037 (2019) [pdf] DOI: 10.1142/S0217732319500378
-
Thermodynamics of
-
Abstract
- We study the thermodynamics of QCD system under external magnetic field via the 2+1 flavor Polyakov-loop quark-meson model. To incorporate quantum and thermal fluctuations, the functional renormalization group approach is implemented in our work. Pressure, entropy density, magnetic susceptibility and other thermodynamic quantities are calculated and analyzed to investigate the effect of magnetic field on the QCD system. The calculated results are in reasonable agreement with lattice QCD simulations and perturbation theory. We then give an intuitive picture for the response of QCD system to the magnetic field.
Xiang Li, Wei-jie Fu, Yu-xin Liu Journal reference: Phys. Rev. D 99, 074029 (2019) [pdf] DOI: 10.1103/PhysRevD.99.074029
-
Correlation Congruence for Knowledge Distillation -
Abstract
- Most teacher-student frameworks based on knowledge distillation (KD) depend on a strong congruent constraint on instance level. However, they usually ignore the correlation between multiple instances, which is also valuable for knowledge transfer. In this work, we propose a new framework named correlation congruence for knowledge distillation (CCKD), which transfers not only the instance-level information, but also the correlation between instances. Furthermore, a generalized kernel method based on Taylor series expansion is proposed to better capture the correlation between instances. Empirical experiments and ablation studies on image classification tasks (including CIFAR-100, ImageNet-1K) and metric learning tasks (including ReID and Face Recognition) show that the proposed CCKD substantially outperforms the original KD and achieves state-of-the-art accuracy compared with other SOTA KD-based methods. The CCKD can be easily deployed in the majority of the teacher-student framework such as KD and hint-based learning methods.
Baoyun Peng, Xiao Jin, Jiaheng Liu, Shunfeng Zhou, Yichao Wu, Yu Liu, Dongsheng Li, Zhaoning Zhang [pdf]
-
Conditional Adversarial Generative Flow for Controllable Image Synthesis -
Abstract
- Flow-based generative models show great potential in image synthesis due to its reversible pipeline and exact log-likelihood target, yet it suffers from weak ability for conditional image synthesis, especially for multi-label or unaware conditions. This is because the potential distribution of image conditions is hard to measure precisely from its latent variable $z$. In this paper, based on modeling a joint probabilistic density of an image and its conditions, we propose a novel flow-based generative model named conditional adversarial generative flow (CAGlow). Instead of disentangling attributes from latent space, we blaze a new trail for learning an encoder to estimate the mapping from condition space to latent space in an adversarial manner. Given a specific condition $c$, CAGlow can encode it to a sampled $z$, and then enable robust conditional image synthesis in complex situations like combining person identity with multiple attributes. The proposed CAGlow can be implemented in both supervised and unsupervised manners, thus can synthesize images with conditional information like categories, attributes, and even some unknown properties. Extensive experiments show that CAGlow ensures the independence of different conditions and outperforms regular Glow to a significant extent.
Rui Liu, Yu Liu, Xinyu Gong, Xiaogang Wang, Hongsheng Li [pdf]
-
Determining Hadron-Quark Phase Transition Chemical Potential via
Astronomical Observations -
Abstract
- We propose a scheme to determine the chemical potential and baryon number density of the hadron-quark phase transition in cold dense strong interaction matter (compact star matter). The hadron matter is described with the relativistic mean field theory, and the quark matter is described with the Dyson-Schwinger equation approach of QCD. To study the first-order phase transition, we take the sound speed as the interpolation objective to construct the equation of state in the middle density region. With the maximum mass, the tidal deformability and the radius of neutron stars being taken as calibration quantities, the phase transition chemical potential is constrained to a quite small range. And the most probable value of the phase transition chemical potential is found.
Zhan Bai, Yu-xin Liu [pdf]
-
Pulsed Quantum-State Reconstruction of Dark Systems -
Abstract
- We propose a novel strategy to reconstruct the quantum state of dark systems, i.e., degrees of freedom that are not directly accessible for measurement or control. Our scheme relies on the quantum control of a two-level probe that exerts a state-dependent potential on the dark system. Using a sequence of control pulses applied to the probe makes it possible to tailor the information one can obtain and, for example, allows us to reconstruct the density operator of a dark spin as well as the Wigner characteristic function of a harmonic oscillator. Because of the symmetry of the applied pulse sequence, this scheme is robust against slow noise on the probe. The proof-of-principle experiments are readily feasible in solid-state spins and trapped ions.
Yu Liu, Jiazhao Tian, Ralf Betzholz, Jianming Cai Journal reference: Phys. Rev. Lett. 122, 110406 (2019) [pdf] DOI: 10.1103/PhysRevLett.122.110406
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Multipath-enabled private audio with noise -
Abstract
- We address the problem of privately communicating audio messages to multiple listeners in a reverberant room using a set of loudspeakers. We propose two methods based on emitting noise. In the first method, the loudspeakers emit noise signals that are appropriately filtered so that after echoing along multiple paths in the room, they sum up and descramble to yield distinct meaningful audio messages only at specific focusing spots, while being incoherent everywhere else. In the second method, adapted from wireless communications, we project noise signals onto the nullspace of the MIMO channel matrix between the loudspeakers and listeners. Loudspeakers reproduce a sum of the projected noise signals and intended messages. Again because of echoes, the MIMO nullspace changes across different locations in the room. Thus, the listeners at focusing spots hear intended messages, while the acoustic channel of an eavesdropper at any other location is jammed. We show, using both numerical and real experiments, that with a small number of speakers and a few impulse response measurements, audio messages can indeed be communicated to a set of listeners while ensuring negligible intelligibility elsewhere.
Anadi Chaman, Yu-Jeh Liu, Jonah Casebeer, Ivan Dokmanić [pdf]
-
Possible correlations between gamma-ray burst and its host galaxy offset -
Abstract
- We collected the information of 304 gamma-ray bursts (GRBs) from the literature, and analyzed the correlations among the host galaxy offsets (the distance from the site of the GRB to the center of its host galaxy), $T_{\rm 90,i}$ (the duration $T_{\rm 90}$ in rest-frame), $T_{\rm R45,i}$ (the duration $T_{\rm R45}$ in rest-frame), $E_{\rm \gamma,iso}$ (the isotropic equivalent energy), $L_{\rm \gamma,iso}$ ($=E_{\rm \gamma,iso}/T_{\rm 90,i}$, the isotropic equivalent luminosity) and $L_{\rm pk}$ (peak luminosity). We found that $ T_{\rm 90,i}$, $ T_{\rm R45,i}$, $ E_{\rm \gamma,iso}$, $L_{\rm pk}$ have negative correlation with $ {\rm offset}$, which is consistent with origin of short GRBs (SGRBs) and long GRBs (LGRBs). On separate analysis, we found similar results for $\log E_{\rm \gamma,iso}$ - $\log {\rm (offset)}$ and $\log L_{\rm pk}$ - $\log {\rm (offset)}$ relations in case of SGRBs only, while no obvious relation for LGRBs. There is no correlations between offset and $L_{\rm \gamma,iso}$. We also put the special GRB 170817A {and GRB 060218A} on the plots. {The two GRBs both have low luminosity and small offset.} In the $ \log ({\rm offset})- \log T_{\rm 90,i}$ plot, we found GRB 170817A locates in between the two regions of SGRBs and LGRBs and it is the outlier in the $ {\rm offset}-E_{\rm \gamma, iso}$, $ {\rm offset}-L_{\rm \gamma, iso}$ and $ {\rm offset}-L_{\rm pk}$ plots. Together with GRB 060218A being an outlier in all plots, it indicates the speciality of GRBs 170817A and 060218A, and might imply more subgroups of the GRB samples.
Fei-Fei Wang, Yuan-Chuan Zou, Yu Liu, Bin Liao, Reetanjali Moharana Journal reference: 2018JHEAp..18...21W [pdf] DOI: 10.1016/j.jheap.2018.03.001
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Dressed quark tensor vertex and nucleon tensor charge -
Abstract
- We construct the quark-antiquark scattering kernels of Bethe-Salpeter equation from the quark self-energy directly under two specific forms of quark-gluon vertices. The quark dressed tensor vertex is then calculated within this consistent framework and rainbow-ladder(RL) approximation. After employing a simplified nucleon model, the nucleon tensor charge can be defined with the tensor vertex. We then compute the tensor charge with the bare tensor vertex and the dressed vertices obtained in this framework and in RL approximation. The obtained results are consistent with the lattice QCD calculations. We also find that typically the gluon dressing effects suppress the nucleon tensor charge compared to the bare tensor vertex, by about $23\%$ for RL approximation, and turn to be about $13\%$ in this framework.
Langtian Liu, Lei Chang, Yu-xin Liu Journal reference: Phys. Rev. D 99, 074013 (2019) [pdf] DOI: 10.1103/PhysRevD.99.074013
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Abelian quotients associated with fully rigid subcategories -
Abstract
- In this article, we study the Gorenstein property of abelian quotient categories induced by fully rigid subcategories on an exact category B. We also study when d-cluster tilting subcategories become fully rigid. We show that the quotient abelian category induced by such d-cluster tilting subcategories are hereditary.
Yu Liu [pdf]
-
A New Model to Predict Optimum Conditions for Growth of 2D Materials on a Substrate -
Abstract
- Very recently we developed an efficient method to calculate the free energy of 2D materials on substrates and achieved high calculation precision for graphene or $\gamma$-graphyne on copper substrates. In the present work, the method was further confirmed to be accurate by molecular dynamic simulations of silicene on Ag substrate using empirical potential and was applied to predict the optimum conditions based on \emph{ab initio} calculations for silicene growth on Ag (110) and Ag (111) surface, which are in good agreement with previous experimental observations.
Yu-Peng Liu, Bo-Yuan Ning, Le-Cheng Gong, Tsu-Chien Weng, Xi-Jing Ning Journal reference: Nanomaterials, 9(2019) 978 [pdf] DOI: 10.3390/nano9070978
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A Comprehensive Statistical Study of Gamma-Ray Bursts -
Abstract
- In order to obtain an overview of the gamma-ray bursts (GRBs), we need a full sample. In this paper, we collected 6289 GRBs (from GRB 910421 to GRB 160509A) from the literature, including prompt emission, afterglow and host galaxy properties. We hope to use this large sample to reveal the intrinsic properties of GRB. We have listed all the data in machine readable tables, including the properties of the GRBs, correlation coefficients and linear regression results of two arbitrary parameters, and linear regression results of any three parameters. These machine readable tables could be used as a data reservoir for further studies on the classifications or correlations. One may find some intrinsic properties from these statistical results. With this comprehensive table, it is possible to find relations between different parameters, and to classify the GRBs into different kinds of sub-groups. With the completion, it may reveal the nature of GRBs and may be used as tools like pseudo-redshift indicators, standard candles, etc. All the machine readable data and statistical results are available on the website of the journal.
Feifei Wang, Yuan-Chuan Zou, Fuxiang Liu, Bin Liao, Yu Liu, Yating Chai, Lei Xia [pdf] DOI: 10.3847/1538-4357/ab0a86 1902.05489v1 [pdf]
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Intrinsic curvature and topology of shadows in Kerr spacetime -
Abstract
- From the viewpoint of differential geometry and topology, we investigate the characterization of the shadows in a Kerr spacetime. Two new quantities, the length of the shadow boundary and the local curvature radius are introduced. Each shadow can be uniquely determined by these two quantities. For the black hole case, the result shows that we can constrain the black hole spin and the angular coordinate of the observer only by measuring the maximum and minimum of the curvature radius. While for the naked singularity case, we adopt the length parameter and the maximum of the curvature radius. This technique is completely independent of the coordinate system and the location of the shadow, and is expected to uniquely determine the parameters of the spacetime. Moreover, we propose a topological covariant quantity to measure and distinguish different topological structures of the shadows.
Shao-Wen Wei, Yu-Xiao Liu, Robert B. Mann Journal reference: Phys. Rev. D 99, 041303 (2019) [pdf] DOI: 10.1103/PhysRevD.99.041303
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Probing the relationship between the null geodesics and thermodynamic phase transition for rotating Kerr-AdS black holes -
Abstract
- In this paper, we aim to examine the relationship between the unstable circular photon orbit and the thermodynamic phase transition for a rotating Kerr-AdS black hole. On one side, we give a brief review of the phase transition for the Kerr-AdS black hole. The coexistence curve and the metastable curve corresponding to the phase transition are clearly shown. On the other side, we calculate the radius and the angular momentum of the unstable circular orbits for a photon by analyzing the effective potential. Then combining these two sides, we find the following results. i) The radius and the angular momentum of the unstable circular photon orbits demonstrate the non-monotonic behaviors when the thermodynamic phase transition takes place. So from the behavior of the circular orbit, one can determine whether there exists a thermodynamic phase transition. ii) The difference of the radius or the angular momentum for the coexistence small and large black holes can be treated as an order parameter to describe the phase transition. And near the critical point, it has a critical exponent of $\frac{1}{2}$. iii) The temperature and pressure corresponding to the extremal points of the radius or the angular momentum of the unstable circular photon orbit completely agree with that of the metastable curves from the thermodynamic side. Thus, these results confirm the relationship between the geodesics and thermodynamic phase transition for the Kerr-AdS black hole. Therefore, on one hand, we are allowed to probe the thermodynamic phase transition from the gravity side. On the other hand, the signature of the strong gravitational effect can also be revealed from the black hole thermodynamics.
Shao-Wen Wei, Yu-Xiao Liu, Yong-Qiang Wang Journal reference: Phys. Rev. D 99, 044013 (2019) [pdf] DOI: 10.1103/PhysRevD.99.044013
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A New Model to Predict Optimum Conditions for Growth of 2D Materials on a Substrate -
Abstract
- A method was developed to calculate the free energy of 2D materials on substrates and was demonstrated by the system of graphene and {\gamma}-graphyne on copper substrate. The method works at least 3 orders faster than state-of-the-art algorithms, and the accuracy was tested by molecular dynamics simulations, showing that the precision for calculations of the internal energy achieves up to 0.03% in a temperature range from 100 to 1300K. As expected, the calculated the free energy of a graphene sheet on Cu (111) or Ni (111) surface in a temperature range up to 3000K is always smaller than the one of a {\gamma}-graphyne sheet with the same number of C atoms, which is consistent with the fact that growth of graphene on the substrates is much easier than {\gamma}-graphyne.
Yu-Peng Liu, Bo-Yuan Ning, Le-Cheng Gong, Tsu-Chien Weng, Xi-Jing Ning Journal reference: Nanomaterials, 9(2019) 978 [pdf] DOI: 10.3390/nano9070978
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Pentaquark states with the
$$QQQq\bar{q}$$
Q
Q
Q
q
q
¯
configuration in a simple model -
Abstract
- We discuss the mass splittings for the $S$-wave triply heavy pentaquark states with the $QQQq\bar{q}$ $(Q=b,c;q=u,d,s)$ configuration which is a mirror structure of $Q\bar{Q}qqq$. The latter configuration is related with the nature of $P_c(4380)$ observed by the LHCb Collaboration. The considered pentaquark masses are roughly estimated with a simple method. One finds that such states are probably not narrow even if they do exist. This leaves room for molecule interpretation for a state around the low-lying threshold of a doubly heavy baryon and a heavy-light meson, e.g. $\Xi_{cc}D$, if it were observed. As a by product, we conjecture that upper limits for the masses of the conventional triply heavy baryons can be determined by the masses of the conventional doubly heavy baryons.
Shi-Yuan Li, Yan-Rui Liu, Yu-Nan Liu, Zong-Guo Si, Jing Wu Journal reference: Eur. Phys. J. C 79, 87 (2019) [pdf] DOI: 10.1140/epjc/s10052-019-6589-7
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First Unambiguous Imaging of Large-scale Quasi-periodic Extreme-ultraviolet Wave or Shock -
Abstract
- We report the first unambiguous quasi-periodic large-scale extreme-ultraviolet (EUV) wave or shock that was detected by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. During the whip-like unwinding eruption of a small filament on 2012 April 24, multiple consecutive large-scale wavefronts emanating from AR11467 were observed simultaneously along the solar surface and a closed transequatorial loop system. In the meantime, an upward propagating dome-shaped wavefront was also observed, whose initial speed and deceleration are about 1392 km/s and 1.78 km/s^2, respectively. Along the solar surface, the quasi-peridoic wavefronts had a period of about 163 +/- 21 seconds and propagated at a nearly constant speed of 747 +/- 26 km/s; they interacted with active region AR11469 and launched a sympathetic upward propagating secondary EUV wave. The wavefronts along the loop system propagated at a speed of 897 km/s, and they were reflected back at the southern end of the loop system at a similar speed. In addition to the propagating waves, a standing kink wave was also present in the loop system simultaneously. Periodicity analysis reveals that the period of the wavefronts was consistent with that of the unwinding helical structures of the erupting filament. Based on these observational facts, we propose that the observed quasi-periodic EUV wavefronts were most likely excited by the periodic unwinding motion of the filament helical structures. In addition, two different seismological methods are applied to derive the magnetic field strength of the loop system, and for the first time the reliability of these inversion techniques are tested with the same magnetic structure.
Yuandeng Shen, P. F. Chen, Ying D. Liu, Kazunari Shibata, Zehao Tang, Yu Liu [pdf] DOI: 10.3847/1538-4357/ab01dd 1901.08199v1 [pdf]
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Rate Distortion For Model Compression: From Theory To Practice -
Abstract
- The enormous size of modern deep neural networks makes it challenging to deploy those models in memory and communication limited scenarios. Thus, compressing a trained model without a significant loss in performance has become an increasingly important task. Tremendous advances has been made recently, where the main technical building blocks are parameter pruning, parameter sharing (quantization), and low-rank factorization. In this paper, we propose principled approaches to improve upon the common heuristics used in those building blocks, namely pruning and quantization. We first study the fundamental limit for model compression via the rate distortion theory. We bring the rate distortion function from data compression to model compression to quantify this fundamental limit. We prove a lower bound for the rate distortion function and prove its achievability for linear models. Although this achievable compression scheme is intractable in practice, this analysis motivates a novel model compression framework. This framework provides a new objective function in model compression, which can be applied together with other classes of model compressor such as pruning or quantization. Theoretically, we prove that the proposed scheme is optimal for compressing one-hidden-layer ReLU neural networks. Empirically, we show that the proposed scheme improves upon the baseline in the compression-accuracy tradeoff.
Weihao Gao, Yu-Han Liu, Chong Wang, Sewoong Oh [pdf]
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Fine tuning the hydrophobicity of counter‐anions to tailor pore size in porous all‐poly(ionic liquid) membranes -
Abstract
- Charged porous polymer membranes (CPMs) emerging as a multifunctional platform for diverse applications in chemistry, materials science, and biomedicine have been attracting widespread attention. Fabrication of CPMs in a controllable manner is of particular significance for optimizing their function and maximizing practical values. Herein, we report the fabrication of CPMs exclusively from poly(ionic liquid)s (PILs), and their pore size and wettability were precisely tailored by rational choice of the counteranions. Specifically, stepwise subtle increase in hydrophobicity of the counteranions by extending the length of fluorinated alkyl substituents, i.e. from bis(trifluoromethane sulfonyl)imide (Tf2N) to bis(pentafluoroethane sulfonyl)imide (Pf2N) and bis(heptafluoropropane sulfonyl)imide (Hf2N), decreases the average pore size gradually from 1546 nm to 157 nm and 77 nm, respectively. Meanwhile, their corresponding water contact angles increased from 90 degree to 102 degree and 120o. The exquisite control over the porous architectures and surface wettability of CPMs by systematic variation of the anion's hydrophobicity provides a solid proof of the impact of the PIL anions on CPMs' structure.
Zhiping Jiang, Yu-ping Liu, Yue Shao, Peng Zhao, Jiayin Yuan, Hong Wang Journal reference: polymer international, 2019, 10.1002/pi.5764 [pdf] DOI: 10.1002/pi.5764
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Direct evaluation of the force constant matrix in quantum Monte Carlo -
Abstract
- We develop a formalism to directly evaluate the matrix of force constants within a Quantum Monte Carlo calculation. We utilize the matrix of force constants to accurately relax the positions of atoms in molecules and determine their vibrational modes, using a combination of Variational and Diffusion Monte Carlo. The computed bond lengths differ by less than 0.007{\AA} from the experimental results for all four tested molecules. For hydrogen and hydrogen chloride, we obtain fundamental vibrational frequencies within 0.1% of experimental results and ~10 times more accurate than leading computational methods. For carbon dioxide and methane, the vibrational frequency obtained is on average within 1.1% of the experimental result, which is at least 3 times closer than results using Restricted Hartree-Fock and Density Functional Theory with a Perdew-Burke-Ernzerhof (PBE) functional and comparable or better than Density Functional Theory with a semi-empirical functional.
Yu Yang Fredrik Liu, Bartholomew Andrews, Gareth J. Conduit Journal reference: J. Chem. Phys., 150(3):034104, Jan 2019 [pdf] DOI: 10.1063/1.5070138
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Learning Pairwise Relationship for Multi-object Detection in Crowded
Scenes -
Abstract
- As the post-processing step for object detection, non-maximum suppression (GreedyNMS) is widely used in most of the detectors for many years. It is efficient and accurate for sparse scenes, but suffers an inevitable trade-off between precision and recall in crowded scenes. To overcome this drawback, we propose a Pairwise-NMS to cure GreedyNMS. Specifically, a pairwise-relationship network that is based on deep learning is learned to predict if two overlapping proposal boxes contain two objects or zero/one object, which can handle multiple overlapping objects effectively. Through neatly coupling with GreedyNMS without losing efficiency, consistent improvements have been achieved in heavily occluded datasets including MOT15, TUD-Crossing and PETS. In addition, Pairwise-NMS can be integrated into any learning based detectors (Both of Faster-RCNN and DPM detectors are tested in this paper), thus building a bridge between GreedyNMS and end-to-end learning detectors.
Yu Liu, Lingqiao Liu, Hamid Rezatofighi, Thanh-Toan Do, Qinfeng Shi, Ian Reid [pdf]
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Anisotropic magnetic entropy change in
-
Abstract
- Intrinsic, two-dimensional (2D) ferromagnetic semiconductors are an important class of materials for spintronics applications. Cr$_2$X$_2$Te$_6$ (X = Si and Ge) semiconductors show 2D Ising-like ferromagnetism, which is preserved in few-layer devices. The maximum magnetic entropy change associated with the critical properties around the ferromagnetic transition for Cr$_2$Si$_2$Te$_6$ $-\Delta S_M^{max} \sim$ 5.05 J kg$^{-1}$ K$^{-1}$ is much larger than $-\Delta S_M^{max} \sim$ 2.64 J kg$^{-1}$ K$^{-1}$ for Cr$_2$Ge$_2$Te$_6$ with an out-of-plane field change of 5 T. The rescaled $-\Delta S_M(T,H)$ curves collapse onto a universal curve independent of temperature and field for both materials. This indicates similar critical behavior and 2D Ising magnetism, confirming the magnetocrystalline anisotropy that could preserve the long-range ferromagnetism in few-layers of Cr$_2$X$_2$Te$_6$.
Yu Liu, C. Petrovic Journal reference: Physical Review MATERIALS 3, 014001 (2019) [pdf] DOI: 10.1103/PhysRevMaterials.3.014001
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Host Galaxies of Type Ic and Broad-lined Type Ic Supernovae from the Palomar Transient Factory: Implications for Jet Production -
Abstract
- Unlike the ordinary supernovae (SNe) some of which are hydrogen and helium deficient (called Type Ic SNe), broad-lined Type Ic SNe (SNe Ic-bl) are very energetic events, and all SNe coincident with bona fide long duration gamma-ray bursts (LGRBs) are of Type Ic-bl. Understanding the progenitors and the mechanism driving SN Ic-bl explosions vs those of their SNe Ic cousins is key to understanding the SN-GRB relationship and jet production in massive stars. Here we present the largest set of host-galaxy spectra of 28 SNe Ic and 14 SN Ic-bl, all discovered before 2013 by the same untargeted survey, namely the Palomar Transient Factory (PTF). We carefully measure their gas-phase metallicities, stellar masses (M*s) and star-formation rates (SFRs) by taking into account recent progress in the metallicity field and propagating uncertainties correctly. We further re-analyze the hosts of 10 literature SN-GRBs using the same methods and compare them to our PTF SN hosts with the goal of constraining their progenitors from their local environments by conducting a thorough statistical comparison, including upper limits. We find that the metallicities, SFRs and M*s of our PTF SN Ic-bl hosts are statistically comparable to those of SN-GRBs, but significantly lower than those of the PTF SNe Ic. The mass-metallicity relations as defined by the SNe Ic-bl and SN-GRBs are not significantly different from the same relations as defined by the SDSS galaxies, in contrast to claims by earlier works. Our findings point towards low metallicity as a crucial ingredient for SN Ic-bl and SN-GRB production since we are able to break the degeneracy between high SFR and low metallicity. We suggest that the PTF SNe Ic-bl may have produced jets that were choked inside the star or were able break out of the star as unseen low-luminosity or off-axis GRBs.
Maryam Modjaz, Federica B. Bianco, Magdalena Siwek, Shan Huang, Daniel A. Perley, David Fierroz, Yu-Qian Liu, Iair Arcavi, Avishay Gal-Yam, Nadia Blagorodnova, Bradley S. Cenko, Alexei V. Filippenko, Mansi M. Kasliwal, S. R. Kulkarni, Steve Schulze, Kirsty Taggart, Weikang Zhen [pdf] DOI: 10.3847/1538-4357/ab4185 1901.00872v1 [pdf]
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Excited Kerr black holes with scalar hair -
Abstract
- In the context of complex scalar field coupled to Einstein gravity theory, we present a novel family of solutions of Kerr black holes with excited-state scalar hair inspired by the work of Herdeiro and Radu in [Phys.\ Rev.\ Lett.\ {\bf 112}, 221101 (2014)], which can be regarded as numerical solutions of rotating compact objects with excited scalar hair, including boson stars and black holes. In contrast to Kerr black holes with ground state scalar hair, we find that the first-excited Kerr black holes with scalar hair have two types of nodes, including radial $n_r=1$ and angular $n_\theta=1$ nodes. Moreover, in the case of radial nodes the curves of the mass versus the frequency form nontrivial loops, and in the case of angular nodes the curves can be divided into two kinds: closed and open loops. We also study the dependence of the horizon area on angular momentum and Hawking temperature.
Yong-Qiang Wang, Yu-Xiao Liu, Shao-Wen Wei Journal reference: Phys. Rev. D 99, 064036 (2019) [pdf] DOI: 10.1103/PhysRevD.99.064036
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Matching the meson quasidistribution amplitude in the RI/MOM scheme -
Abstract
- The $x$-dependence of light-cone distribution amplitude (LCDA) can be directly calculated from a quasi distribution amplitude (DA) in lattice QCD within the framework of large-momentum effective theory (LaMET). In this paper, we study the one-loop renormalization of the quasi-DA in the regularization-independent momentum subtraction (RI/MOM) scheme. The renormalization factor for the quasi parton distribution function can be used to renormalize the quasi-DA provided that they are implemented on lattice and in perturbation theory in the same manner. We derive the one-loop matching coefficient that matches quasi-DA in the RI/MOM scheme onto LCDA in the $\overline{\rm MS}$ scheme. Our result provides the crucial step to extract the LCDAs from lattice matrix elements of quasi-DAs.
Yu-Sheng Liu, Wei Wang, Ji Xu, Qi-An Zhang, Shuai Zhao, Yong Zhao Journal reference: Phys. Rev. D 99, 094036 (2019) [pdf] DOI: 10.1103/PhysRevD.99.094036
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Magnetic and structural properties of the iron oxychalcogenides
-
Abstract
- We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se). Elastic neutron scattering reveals that $M$ = S and Se have similar nuclear structures at room and low temperatures. We find that both materials obtain antiferromagnetic ordering at a Neel temperature $T_{N}$ 90.1 $\pm$ 0.16 K and 107.2 $\pm$ 0.06 K for $M$= Se and S, respectively. The magnetic arrangements of $M$ = S, Se are obtained through Rietveld refinement. We find the order parameter exponent $\beta$ to be 0.129 $\pm$ 0.006 for $M$ = Se and 0.133 $\pm$ 0.007 for $M$ = S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen exchange, the nature of the magnetic interactions is similar in these materials.
B. Freelon, Z. Yamani, Ian Swainson, R. Flauca, Yu Hao Liu, L. Craco, M. S. Laad, Meng Wang, Jiaqi Chen, R. J. Birgeneau, Minghu Fang Journal reference: Phys. Rev. B 99, 024109 (2019) [pdf] DOI: 10.1103/PhysRevB.99.024109
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Double-peak specific heat and spin freezing in the spin-2 triangular lattice antiferromagnet
-
Abstract
- We report the properties of a triangular lattice iron-chalcogenide antiferromagnet FeAl$_{2}$Se$_{4}$. The spin susceptibility reveals a significant antiferromagnetic interaction with a Curie-Weiss temperature {\Theta}$_{CW}$ ~ -200K and a spin-2 local moment. Despite a large spin and a large |{\Theta}$_{CW}$|, the low-temperature behaviors are incompatible with conventional classical magnets. No long-range order is detected down to 0.4K. Similar to the well-known spin-1 magnet NiGa$_{2}$S$_{4}$, the specific heat of FeAl$_{2}$Se$_{4}$ exhibits an unusual double-peak structure and a T$^{2}$ power law at low temperatures, which are attributed to the underlying quadrupolar spin correlations and the Halperin-Saslow modes, respectively. The spin freezing occurs at ~ 14K, below which the relaxation dynamics is probed by the ac susceptibility. Our results are consistent with the early theory for the spin-1 system with Heisenberg and biquadratic spin interactions. We argue that the early proposal of the quadrupolar correlation and gauge glass dynamics may be well extended to FeAl$_{2}$Se$_{4}$. Our results provide useful insights about the magnetic properties of frustrated quantum magnets with high spins.
Kunkun Li, Shifeng Jin, Jiangang Guo, Yanping Xu, Yixi Su, Erxi Feng, Yu Liu, Shengqiang Zhou, Tianping Ying, Shiyan Li, Ziqiang Wang, Gang Chen, Xiaolong Chen Journal reference: Phys. Rev. B 99, 054421 (2019) [pdf] DOI: 10.1103/PhysRevB.99.054421
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Finite temperature physics of 1D topological Kondo insulator: Stable Haldane phase, emergent energy scale and beyond -
Abstract
- We have studied the one-dimensional $p$-wave periodic Anderson model at finite temperature with the help of the numerically exact determinant quantum Monte Carlo simulation. It is found that the topological Haldane phase established for ground-state is still stable against small thermal fluctuation and its characteristic edge magnetization develops at low temperature. Moreover, the saturated low-$T$ spin structure factor and the $\frac{1}{T}$-law of susceptibility are useful to detect the free edge spin moment, which may be relevant for experimental explorations. We have also comparatively studied the conventional $s$-wave periodic Anderson model, which helps us identify an emergent energy scale $T_{cr}$. $T_{cr}$ signals a crossover into interesting low-$T$ regime and seems to be the expected Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling. Finally, the collective Kondo screening effect has been examined and it is heavily reduced at boundary, which may give a fruitful playground for novel physics beyond the well-established Haldane state and topological band insulators.
Yin Zhong, Yu Liu, Qin Wang, Ke Liu, Hai-Feng Song, Hong-Gang Luo Journal reference: Front. Phys. 14(2), 23602 (2019) [pdf] DOI: 10.1007/s11467-018-0868-x
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The Sariçiçek howardite fall in Turkey: Source crater of
-
Abstract
- 2018
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Proton Isovector Helicity Distribution on the Lattice at Physical Pion Mass -
Abstract
- We present a state-of-the-art calculation of the isovector quark helicity Bjorken-$x$ distribution in the proton using lattice-QCD ensembles at the physical pion mass. We compute quasi-distributions at proton momenta $P_z \in \{2.2, 2.6, 3.0\}$~GeV on the lattice, and match them systematically to the physical parton distribution using large-momentum effective theory (LaMET). We reach an unprecedented precision through high statistics in simulations, large-momentum proton matrix elements, and control of excited-state contamination. The resulting distribution with combined statistical and systematic errors is in agreement with the latest phenomenological analysis of the spin-dependent experimental data; in particular, $\Delta \bar{u}(x)>\Delta \bar{d}(x)$.
Huey-Wen Lin, Jiunn-Wei Chen, Xiangdong Ji, Luchang Jin, Ruizi Li, Yu-Sheng Liu, Yi-Bo Yang, Jian-Hui Zhang, Yong Zhao Journal reference: Phys.Rev.Lett. 121 (2018) no.24, 242003 [pdf] DOI: 10.1103/PhysRevLett.121.242003
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T-GCN: A Temporal Graph Convolutional Network for Traffic Prediction -
Abstract
- Accurate and real-time traffic forecasting plays an important role in the Intelligent Traffic System and is of great significance for urban traffic planning, traffic management, and traffic control. However, traffic forecasting has always been considered an open scientific issue, owing to the constraints of urban road network topological structure and the law of dynamic change with time, namely, spatial dependence and temporal dependence. To capture the spatial and temporal dependence simultaneously, we propose a novel neural network-based traffic forecasting method, the temporal graph convolutional network (T-GCN) model, which is in combination with the graph convolutional network (GCN) and gated recurrent unit (GRU). Specifically, the GCN is used to learn complex topological structures to capture spatial dependence and the gated recurrent unit is used to learn dynamic changes of traffic data to capture temporal dependence. Then, the T-GCN model is employed to traffic forecasting based on the urban road network. Experiments demonstrate that our T-GCN model can obtain the spatio-temporal correlation from traffic data and the predictions outperform state-of-art baselines on real-world traffic datasets. Our tensorflow implementation of the T-GCN is available at https://github.com/lehaifeng/T-GCN.
Ling Zhao, Yujiao Song, Chao Zhang, Yu Liu, Pu Wang, Tao Lin, Min Deng, Haifeng Li Journal reference: IEEE Transactions on Intelligent Transportation Systems-2019 [pdf] DOI: 10.1109/TITS.2019.2935152
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Joint Beam and Channel Tracking for Two-Dimensional Phased Antenna
Arrays -
Abstract
- Analog beamforming is a low-cost architecture for millimeter-wave (mmWave) mobile communications. However, it has two disadvantages for serving fast mobility users: (i) the mmWave beam in the wireless channel and the beam steered by analog beamforming have small angular spreads which are difficult to align with each other and (ii) the receiver can only observe the mmWave channel in one beam direction and rely on beam-probing algorithms to check other directions. In this paper, we develop a beam probing and tracking algorithm that can efficiently track fast-moving mmWave beams in three-dimensional (3D) space. This algorithm has several salient features: (1) fading channel supportive: it can simultaneously track the channel coefficient and two-dimensional (2D) beam direction in fading channel environments; (2) low probing overhead: it achieves the minimum probing requirement for joint beam and channel tracking; (3) fast tracking speed and high tracking accuracy: its tracking error converges to the minimum Cramer-Rao lower bound (CRLB) in static scenarios in theory and it outperforms several existing tracking algorithms with lower tracking error and faster tracking speed in simulations.
Yu Liu, Jiahui Li, Yin Sun, Shidong Zhou [pdf]
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Quantized Scalar Fields in Curved Spacetime Background of Thick Brane -
Abstract
- In this paper, we adopt the method of quantum fields in curved spacetime to quantize a free scalar matter field in the braneworld background whose warped factor is of the form that could generate P\"{o}schl-Teller potential. Then we consider the interaction between the scalar field $\phi(x)$ and a classical scalar source $\rho(x)$ with the form $\mathcal{H}_I=\sqrt{-g}\tilde{g}\rho(x)\phi(x)$. The corresponding S-matrix is given and the number of particles generated by the source is obtained. Furthermore, we get the {particle numbers} in the cases that only ground state mode of the field could be detected and both the ground and first exited states could be detected, respectively. Finally, by the particle number density we define, we show how the extra dimension makes a difference specifically by the excited modes.
Jian Wang, Yu-Xiao Liu [pdf]
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Pyramid Network with Online Hard Example Mining for Accurate Left Atrium
Segmentation -
Abstract
- Accurately segmenting left atrium in MR volume can benefit the ablation procedure of atrial fibrillation. Traditional automated solutions often fail in relieving experts from the labor-intensive manual labeling. In this paper, we propose a deep neural network based solution for automated left atrium segmentation in gadolinium-enhanced MR volumes with promising performance. We firstly argue that, for this volumetric segmentation task, networks in 2D fashion can present great superiorities in time efficiency and segmentation accuracy than networks with 3D fashion. Considering the highly varying shape of atrium and the branchy structure of associated pulmonary veins, we propose to adopt a pyramid module to collect semantic cues in feature maps from multiple scales for fine-grained segmentation. Also, to promote our network in classifying the hard examples, we propose an Online Hard Negative Example Mining strategy to identify voxels in slices with low classification certainties and penalize the wrong predictions on them. Finally, we devise a competitive training scheme to further boost the generalization ability of networks. Extensively verified on 20 testing volumes, our proposed framework achieves an average Dice of 92.83% in segmenting the left atria and pulmonary veins.
Cheng Bian, Xin Yang, Jianqiang Ma, Shen Zheng, Yu-An Liu, Reza Nezafat, Pheng-Ann Heng, Yefeng Zheng [pdf]
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$n$-exangulated categories -
Abstract
- For each positive integer $n$ we introduce the notion of $n$-exangulated categories as higher dimensional analogues of extriangulated categories defined by Nakaoka-Palu. We characterize which $n$-exangulated categories are $n$-exact in the sense of Jasso and which are $(n+2)$-angulated in the sense of Geiss-Keller-Oppermann. For extriangulated categories with enough projectives and injectives we introduce the notion of $n$-cluster tilting subcategories and show that under certain conditions such $n$-cluster tilting subcategories are $n$-exangulated.
Martin Herschend, Yu Liu, Hiroyuki Nakaoka [pdf]
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U(1) gauge vector field on a codimension-2 brane -
Abstract
- In this paper, we obtain a gauge invariant effective action for a bulk massless $U(1)$ gauge vector field on a brane with codimension two by using a general Kaluza-Klein (KK) decomposition for the field. It suggests that there exist two types of scalar KK modes to keep the gauge invariance of the action for the massive vector KK modes. Both the vector and scalar KK modes can be massive. The masses of the vector KK modes $m^{(n)}$ contain two parts, $m_{1}^{(n)}$ and $m_{2}^{(n)}$, due to the existence of the two extra dimensions. The masses of the two types of scalar KK modes $m_{\phi}^{(n)}$ and $m_{\varphi}^{(n)}$ are related to the vector ones, i.e., $m_{\phi}^{(n)}=m_{1}^{(n)}$ and $m_{\varphi}^{(n)}=m_{2}^{(n)}$. Moreover, we derive two Schr\"{o}dinger-like equations for the vector KK modes, for which the effective potentials are just the functions of the warp factor.
Chun-E Fu, Yuan Zhong, Yu-Xiao Liu [pdf] DOI: 10.1007/JHEP01(2019)021 1810.02081v3 [pdf]
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Localization of gravitino field on f(R)-thick branes -
Abstract
- In this paper, we consider the localization of a five-dimensional gravitino field on $f(R)$-thick branes. We obtain the coupled chiral equations of the Kaluza-Klein (KK) modes of gravitinos with the gauge condition $\Psi_z=0$. The chiral equations of a gravitino's KK modes are found to be almost identical to those of the Dirac fermion. However, their chiralities are exactly opposite. The chiral KK modes of gravitinos could be localized in some types of $f(R)$-thick branes on introducing a coupling term. We investigate the localization of a gravitino on three types of $f(R)$-thick branes through a Yukawa-like coupling term with background scalar fields. It has been shown that all the KK modes of gravitinos cannot be localized in the pure geometric $f(R)$-thick branes by adding a five-dimensional gravitino mass term. However, for the $f(R)$-thick branes generated by one or two background scalar fields, only the left- or right-handed zero mode could be localized in the branes, and the massive KK resonant modes are the same for both left- and right-handed gravitinos despite their opposing chiralities. All these results are consistent with those of the five-dimensional Dirac fermion except their chiralities, which may be an important sign to distinguish the gravitino field and the Dirac fermion field.
Xiang-Nan Zhou, Yun-Zhi Du, Hao Yu, Yu-Xiao Liu Journal reference: Sci.China Phys.Mech.Astron. 61 (2018) no.11, 110411 [pdf] DOI: 10.1007/s11433-018-9246-2
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RGB-D Based Action Recognition with Light-weight 3D Convolutional
Networks -
Abstract
- Different from RGB videos, depth data in RGB-D videos provide key complementary information for tristimulus visual data which potentially could achieve accuracy improvement for action recognition. However, most of the existing action recognition models solely using RGB videos limit the performance capacity. Additionally, the state-of-the-art action recognition models, namely 3D convolutional neural networks (3D-CNNs) contain tremendous parameters suffering from computational inefficiency. In this paper, we propose a series of 3D light-weight architectures for action recognition based on RGB-D data. Compared with conventional 3D-CNN models, the proposed light-weight 3D-CNNs have considerably less parameters involving lower computation cost, while it results in favorable recognition performance. Experimental results on two public benchmark datasets show that our models can approximate or outperform the state-of-the-art approaches. Specifically, on the RGB+D-NTU (NTU) dataset, we achieve 93.2% and 97.6% for cross-subject and cross-view measurement, and on the Northwestern-UCLA Multiview Action 3D (N-UCLA) dataset, we achieve 95.5% accuracy of cross-view.
Haokui Zhang, Ying Li, Peng Wang, Yu Liu, Chunhua Shen [pdf]
-
How far from automatically interpreting deep learning -
Abstract
- In recent years, deep learning researchers have focused on how to find the interpretability behind deep learning models. However, today cognitive competence of human has not completely covered the deep learning model. In other words, there is a gap between the deep learning model and the cognitive mode. How to evaluate and shrink the cognitive gap is a very important issue. In this paper, the interpretability evaluation, the relationship between the generalization performance and the interpretability of the model and the method for improving the interpretability are concerned. A universal learning framework is put forward to solve the equilibrium problem between the two performances. The uniqueness of solution of the problem is proved and condition of unique solution is obtained. Probability upper bound of the sum of the two performances is analyzed.
Jinwei Zhao, Qizhou Wang, Yufei Wang, Xinhong Hei, Yu Liu [pdf]
-
Anomalous Hall effect in the trigonal
-
Abstract
- We report anomalous Hall effect (AHE) and transport properties of trigonal Cr$_5$Te$_8$ (tr-Cr$_5$Te$_8$) single crystals. The electrical resistivity as well as the Seebeck coefficient shows a clear kink at the paramagnetic-ferromagnetic transition of tr-Cr$_5$Te$_8$, which is also confirmed by the heat capacity measurement. The scaling behavior between anomalous Hall resistivity $\rho^A_{xy}$ and longitudinal resistivity $\rho_{xx}$ is linear below $T_c$. Further analysis suggests that the AHE in tr-Cr$_5$Te$_8$ is dominated by the skew-scattering mechanism rather than the intrinsic or extrinsic side-jump mechanism.
Yu Liu, C. Petrovic Journal reference: Physical Review B 98, 195122 (2018) [pdf] DOI: 10.1103/PhysRevB.98.195122
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Point2Sequence: Learning the Shape Representation of 3D Point Clouds
with an Attention-based Sequence to Sequence Network -
Abstract
- Exploring contextual information in the local region is important for shape understanding and analysis. Existing studies often employ hand-crafted or explicit ways to encode contextual information of local regions. However, it is hard to capture fine-grained contextual information in hand-crafted or explicit manners, such as the correlation between different areas in a local region, which limits the discriminative ability of learned features. To resolve this issue, we propose a novel deep learning model for 3D point clouds, named Point2Sequence, to learn 3D shape features by capturing fine-grained contextual information in a novel implicit way. Point2Sequence employs a novel sequence learning model for point clouds to capture the correlations by aggregating multi-scale areas of each local region with attention. Specifically, Point2Sequence first learns the feature of each area scale in a local region. Then, it captures the correlation between area scales in the process of aggregating all area scales using a recurrent neural network (RNN) based encoder-decoder structure, where an attention mechanism is proposed to highlight the importance of different area scales. Experimental results show that Point2Sequence achieves state-of-the-art performance in shape classification and segmentation tasks.
Xinhai Liu, Zhizhong Han, Yu-Shen Liu, Matthias Zwicker [pdf]
-
Linear stability of f(R, ϕ, X) thick branes: tensor perturbations -
Abstract
- We explore thick branes in $f(R,\phi,X)$ gravity. We obtain the linear tensor perturbation equation of $f(R,\phi,X)$ branes and show that the branes are stable against the tensor perturbations under the condition of $\frac{\partial f(R,\phi,X)}{\partial R}>0$. In order to obtain thick brane solutions of the fourth-order field equations in this theory, we employ the reconstruction technique. We get exact solutions of the specific $f(R,\phi,X)$ thick brane generated by a non-canonical scalar field. It is shown that the zero mode of the graviton for the thick brane is localized under certain conditions. This implies that the four-dimensional Newtonian potential is recovered on the brane. The effects of the Kaluza-Klein modes of the graviton for the $f(R,\phi,X)$ thick brane are also discussed.
Zheng-Quan Cui, Yu-Xiao Liu, Bao-Min Gu, Li Zhao Journal reference: JHEP 11 (2018) 083 [pdf] DOI: 10.1007/JHEP11(2018)083
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Gradient Harmonized Single-stage Detector -
Abstract
- Despite the great success of two-stage detectors, single-stage detector is still a more elegant and efficient way, yet suffers from the two well-known disharmonies during training, i.e. the huge difference in quantity between positive and negative examples as well as between easy and hard examples. In this work, we first point out that the essential effect of the two disharmonies can be summarized in term of the gradient. Further, we propose a novel gradient harmonizing mechanism (GHM) to be a hedging for the disharmonies. The philosophy behind GHM can be easily embedded into both classification loss function like cross-entropy (CE) and regression loss function like smooth-$L_1$ ($SL_1$) loss. To this end, two novel loss functions called GHM-C and GHM-R are designed to balancing the gradient flow for anchor classification and bounding box refinement, respectively. Ablation study on MS COCO demonstrates that without laborious hyper-parameter tuning, both GHM-C and GHM-R can bring substantial improvement for single-stage detector. Without any whistles and bells, our model achieves 41.6 mAP on COCO test-dev set which surpasses the state-of-the-art method, Focal Loss (FL) + $SL_1$, by 0.8.
Buyu Li, Yu Liu, Xiaogang Wang [pdf]
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Learning to Measure Change: Fully Convolutional Siamese Metric Networks
for Scene Change Detection -
Abstract
- A critical challenge problem of scene change detection is that noisy changes generated by varying illumination, shadows and camera viewpoint make variances of a scene difficult to define and measure since the noisy changes and semantic ones are entangled. Following the intuitive idea of detecting changes by directly comparing dissimilarities between a pair of features, we propose a novel fully Convolutional siamese metric Network(CosimNet) to measure changes by customizing implicit metrics. To learn more discriminative metrics, we utilize contrastive loss to reduce the distance between the unchanged feature pairs and to enlarge the distance between the changed feature pairs. Specifically, to address the issue of large viewpoint differences, we propose Thresholded Contrastive Loss (TCL) with a more tolerant strategy to punish noisy changes. We demonstrate the effectiveness of the proposed approach with experiments on three challenging datasets: CDnet, PCD2015, and VL-CMU-CD. Our approach is robust to lots of challenging conditions, such as illumination changes, large viewpoint difference caused by camera motion and zooming. In addition, we incorporate the distance metric into the segmentation framework and validate the effectiveness through visualization of change maps and feature distribution. The source code is available at https://github.com/gmayday1997/ChangeDet.
Enqiang Guo, Xinsha Fu, Jiawei Zhu, Min Deng, Yu Liu, Qing Zhu, Haifeng Li [pdf]
-
Y^2Seq2Seq: Cross-Modal Representation Learning for 3D Shape and Text by
Joint Reconstruction and Prediction of View and Word Sequences -
Abstract
- A recent method employs 3D voxels to represent 3D shapes, but this limits the approach to low resolutions due to the computational cost caused by the cubic complexity of 3D voxels. Hence the method suffers from a lack of detailed geometry. To resolve this issue, we propose Y^2Seq2Seq, a view-based model, to learn cross-modal representations by joint reconstruction and prediction of view and word sequences. Specifically, the network architecture of Y^2Seq2Seq bridges the semantic meaning embedded in the two modalities by two coupled `Y' like sequence-to-sequence (Seq2Seq) structures. In addition, our novel hierarchical constraints further increase the discriminability of the cross-modal representations by employing more detailed discriminative information. Experimental results on cross-modal retrieval and 3D shape captioning show that Y^2Seq2Seq outperforms the state-of-the-art methods.
Zhizhong Han, Mingyang Shang, Xiyang Wang, Yu-Shen Liu, Matthias Zwicker [pdf]
-
View Inter-Prediction GAN: Unsupervised Representation Learning for 3D
Shapes by Learning Global Shape Memories to Support Local View Predictions -
Abstract
- In this paper we present a novel unsupervised representation learning approach for 3D shapes, which is an important research challenge as it avoids the manual effort required for collecting supervised data. Our method trains an RNN-based neural network architecture to solve multiple view inter-prediction tasks for each shape. Given several nearby views of a shape, we define view inter-prediction as the task of predicting the center view between the input views, and reconstructing the input views in a low-level feature space. The key idea of our approach is to implement the shape representation as a shape-specific global memory that is shared between all local view inter-predictions for each shape. Intuitively, this memory enables the system to aggregate information that is useful to better solve the view inter-prediction tasks for each shape, and to leverage the memory as a view-independent shape representation. Our approach obtains the best results using a combination of L_2 and adversarial losses for the view inter-prediction task. We show that VIP-GAN outperforms state-of-the-art methods in unsupervised 3D feature learning on three large scale 3D shape benchmarks.
Zhizhong Han, Mingyang Shang, Yu-Shen Liu, Matthias Zwicker [pdf]
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Mobility-aware Caching Scheduling for Fog Computing in mmWave Band -
Abstract
- As an extension of cloud computing, fog computing at the edge of networks provides low latency, location awareness, and real-time interactions. At the same time, millimeter wave (mmWave) communications are able to provide directional multi-gigabit transmission rates with large available bandwidth. Based on the user mobile trajectories in a region, several activity hotspots that users pass by frequently can be obtained. By caching popular content at the edge nodes near the hotspots, users can download the cached content directly at a short distance, and the user experience can be significantly improved. Considering multiple hotspots in a region, how to efficiently schedule the transmission for the caching at edge nodes becomes a key problem. In this paper, we focus on the problem of mobility aware transmission scheduling for caching at edge nodes near hotspots, and utilize multi-hop relaying and concurrent transmissions to achieve better performance. After formulating the optimal scheduling problem as a stochastic nonlinear mixed integer program, we propose a mobility aware caching scheduling scheme, called MHRC (Multi-Hop Relaying based Caching), where multi-hop D2D paths are established for edge nodes, and concurrent transmissions are exploited in the scheduling of caching at edge nodes. Extensive performance evaluation demonstrates MHRC achieves more than 1x higher expected cached data amount compared with state-of-the-art schemes.
Yong Niu, Yu Liu, Yong Li, Zhangdui Zhong, Bo Ai, Pan Hui [pdf]
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Probesim -
Abstract
- Single-source and top-$k$ SimRank queries are two important types of similarity search in graphs with numerous applications in web mining, social network analysis, spam detection, etc. A plethora of techniques have been proposed for these two types of queries, but very few can efficiently support similarity search over large dynamic graphs, due to either significant preprocessing time or large space overheads. This paper presents ProbeSim, an index-free algorithm for single-source and top-$k$ SimRank queries that provides a non-trivial theoretical guarantee in the absolute error of query results. ProbeSim estimates SimRank similarities without precomputing any indexing structures, and thus can naturally support real-time SimRank queries on dynamic graphs. Besides the theoretical guarantee, ProbeSim also offers satisfying practical efficiency and effectiveness due to several non-trivial optimizations. We conduct extensive experiments on a number of benchmark datasets, which demonstrate that our solutions significantly outperform the existing methods in terms of efficiency and effectiveness. Notably, our experiments include the first empirical study that evaluates the effectiveness of SimRank algorithms on graphs with billion edges, using the idea of pooling.
Yu Liu, Bolong Zheng, Xiaodong He, Zhewei Wei, Xiaokui Xiao, Kai Zheng, Jiaheng Lu [pdf] DOI: 10.14778/3151113.3151115 1709.06955v2 [pdf]
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Describing the ADD model in a warped geometry -
Abstract
- We propose a new description of the (4+N)-dimensional Arkani-Hamed-Dimopoulos-Dvali (ADD) model in a (4+1)-dimensional warped geometry to solve the gauge hierarchy problem. It has the same KK spectrum as in the ADD model and recovers its phenomenons that do not involve the interaction among the graviton KK modes. There is no hierarchy between the fundamental length and the size of the extra dimension. An explicit realization is constructed in the nonlocal gravity theory to give a warped description of the six-dimensional ADD model. Remarkably, the equivalent number N of the extra dimensions in this description may be non-integral, which provides a new mechanism to escape the experimental constrains.
Bin Guo, Yu-Xiao Liu, Ke Yang, Xin-He Meng [pdf]
-
Fault Diagnosis and Bad Data Detection of Power Transmission Network - A
Time Domain Approach -
Abstract
- Fault analysis and bad data are often processed in separate manners. In this paper it is proved that fault as well as bad current measurement data can be modeled as control failure for the power transmission network and any fault on the transmission line can be treated as multiple bad data. Subsequently a linear observer theory is designed in order to identify the fault type and bad data simultaneously. The state space model based observer theory allows a particular failure mode manifest itself as residual which remains in a fixed direction. Moreover coordinate transformation is performed to allow the residual for each failure mode to generate specific geometry characteristic in separate output dimensions. The design approach based on the observer theory is presented in this paper. The design allows 1) bad data detection for current measurement, and 2) fault location, and fault resistance estimation (as a byproduct) where the fault location accuracy is not affected by fault resistance. However it loses freedom in designing the eigenvalues in the excessive subspace. While the theoretical framework is general, the analysis and design are dedicated to transmission lines.
Zhenyu Tan, Yu Liu, Hongbo Sun, Bai Cui [pdf]
-
Recovery of Saturated $γ$ Signal Waveforms by Artificial Neural
Networks -
Abstract
- Particle may sometimes have energy outside the range of radiation detection hardware so that the signal is saturated and useful information is lost. We have therefore investigated the possibility of using an Artificial Neural Network (ANN) to restore the saturated waveforms of $\gamma$ signals. Several ANNs were tested, namely the Back Propagation (BP), Simple Recurrent (Elman), Radical Basis Function (RBF) and Generalized Radial Basis Function (GRBF) neural networks (NNs) and compared with the fitting method based on the Marrone model. The GBRFNN was found to perform best.
Yu Liu, Jing-Jun Zhu, Neil Roberts, Ke-Ming Chen, Yu-Lu Yan, Shuang-Rong Mo, Peng Gu, Hao-Yang Xing [pdf]
-
Nucleon Transversity Distribution at the Physical Pion Mass from Lattice
QCD -
Abstract
- We report a state-of-the-art lattice calculation of the isovector quark transversity distribution of the proton at the physical pion mass. Within the framework of large-momentum effective theory (LaMET), we compute the transversity quasi-distributions using clover valence fermions on 2+1+1-flavor (up/down, strange, charm) HISQ-lattice configurations with boosted proton momenta as large as 3.0~GeV. The relevant lattice matrix elements are nonperturbatively renormalized in regularization-independent momentum-subtraction (RI/MOM) scheme and systematically matched to the physical transversity distribution. With high statistics, large proton momenta and meticulous control of excited-state contamination, we provide the best theoretical prediction for the large-$x$ isovector quark transversity distribution, with better precision than the most recent global analyses of experimental data. Our result also shows that the sea quark asymmetry in the proton transversity distribution is consistent with zero, which has been assumed in all current global analyses.
Yu-Sheng Liu, Jiunn-Wei Chen, Luchang Jin, Ruizi Li, Huey-Wen Lin, Yi-Bo Yang, Jian-Hui Zhang, Yong Zhao [pdf]
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Braneworld in
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Abstract
- In this paper, we investigate the braneworld scenario in $f(T)$ gravity with a $K$-field as the background field. We consider various different specific forms of $f(T)$ gravity and $K$-field, and find a general way to construct the braneworld model. Based on our solutions, the split of branes is investigated. Besides, the stability of the braneworld is studied by investigating the tensor perturbation of the vielbein.
Jian Wang, Wen-Di Guo, Zi-Chao Lin, Yu-Xiao Liu Journal reference: Phys. Rev. D 98, 084046 (2018) [pdf] DOI: 10.1103/PhysRevD.98.084046
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Hierarchy problem and new warped extra dimension -
Abstract
- In this paper, we propose a new mechanism with warped extra dimension to solve the hierarchy problem, which is parallel to the Randall-Sundrum (RS) brane scenario. Different from the RS scenario, the fundamental scale is TeV scale and the four-dimensional Planck scale is generated from the exponential warped extra dimension at size of a few TeV$^{-1}$. The experimental consequences of this scenario are very different from that of the RS scenario. In the explicit realization in the nonlocal gravity theory, there is a tower of spin-2 excitations with mass gap $10^{-4}\text{eV}$ and they are coupled with the gravitational scale to the standard model particles. We further discuss the possible generalizations in other modified gravity theories. The experimental consequences are similar to $(4+N)$-dimensional large extra dimension but $N$ can be a non-integer, which satisfies the experimental constraints more easily than the integer large extra dimension model.
Bin Guo, Yu-Xiao Liu, Ke Yang, Shao-Wen Wei Journal reference: Phys. Rev. D 98, 085022 (2018) [pdf] DOI: 10.1103/PhysRevD.98.085022
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Modeling granular material segregation using a combined finite element
method and advection-diffusion-segregation equation model -
Abstract
- A two-dimensional, transient, multi-scale modeling approach is presented for predicting the magnitude and rate of percolation segregation for binary mixtures of granular material in a rotating drum and conical hopper. The model utilizes finite element method simulations to determine the bulk-level granular velocity field, which is then combined with particle-level diffusion and segregation correlations using the advection-diffusion-segregation equation. The utility of this modelling approach is demonstrated by predicting segregation patterns in a rotating drum and during the discharge of conical hoppers with different geometries. The model exhibits good quantitative accuracy in predicting DEM and experimental segregation data reported in the literature for cohesionless granular materials. Moreover, since the numerical approach does not directly model individual particles, it is expected to scale well to systems of industrial scale.
Yu Liu, Marcial Gonzalez, Carl Wassgren [pdf]
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Learning to Calibrate Quantum Control Pulses by Iterative Deconvolution -
Abstract
- In experimental control of quantum systems, the precision is often hindered by imperfect applied electronics that distort control pulses delivered to target quantum devices. To mitigate such error, the deconvolution method is commonly used for compensating the distortion via an identified convolutional model. However, its effectiveness is limited by model inaccuracies (e.g., imprecise parameters or unmodeled distortion dynamics). In this paper, we propose a learning-based scheme to eliminate the residual calibration error by repeatedly applying the deconvolution operations. The resulting iterative deconvolution method is shown to be able to correct both linear and nonlinear model errors to the highest precision allowed by available finite sampling rates. The calibration error induced by finite sampling rates is also analyzed, from which we propose that the inter-sampling error can be suppressed by actively introducing nonlinear components in the control electronics.
Xi Cao, Bing Chu, Haijin Ding, Luyan Sun, Yu-xi Liu, Rebing Wu [pdf]
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Thermoelectric studies of
-
Abstract
- We report thermoelectric properties of Ir$_{1-x}$Rh$_x$Te$_2$ ($0 \leqslant x \leqslant 0.3$) alloy series where superconductivity at low temperatures emerges as the high-temperature structural transition ($T_s$) is suppressed. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the anionic substitution of Se into Te, in which the structural transition is more stable with Se substitution. Rh substitution results in a slight reduction of lattice parameters and in an increase of number of carriers per unit cell. Weak-coupled BCS superconductivity in Ir$_{0.8}$Rh$_{0.2}$Te$_2$ that emerges at low temperature ($T_c^{zero}$ = 2.45 K) is most likely driven by electron-phonon coupling rather than dimer fluctuations mediated pairing.
Yu Liu, $^{1}$ Hechang Lei, $^{1}$ Kefeng Wang, $^{1}$ Milinda Abeykoon, $^{2}$ J. B. Warren, $^{3}$ Emil Bozin, $^{1}$, C. Petrovic$^{1}$ Journal reference: Physical Review B 98, 094519 (2018) [pdf] DOI: 10.1103/PhysRevB.98.094519
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Magnetic and topological transitions in three-dimensional topological Kondo insulator -
Abstract
- By using an extended slave-boson method, we draw a global phase diagram summarizing both magnetic phases and paramagnetic (PM) topological insulating phases (TI$_s$) in three-dimensional topological Kondo insulator (TKI). By including electron hopping (EH) up to third neighbor, we identify four strong topological insulating (STI) phases and two weak topological insulating (WTI) phases, then the PM phase diagrams characterizing topological transitions between these TI$_s$ are depicted as functions of EH, $f$-electron energy level and hybridization constant. We also find an insulator-metal transition from a STI phase which has surface Fermi rings and spin textures in qualitative agreement to TKI candidate SmB$_6$. In weak hybridization regime, antiferromagnetic (AF) order naturally arises in the phase diagrams, and depending on how the magnetic boundary crosses the PM topological transition lines, AF phases are classified into AF topological insulator (AFTI) and non-topological AF insulator (nAFI), according to their $\mathcal{Z}_2$ indices. In two small regions of parameter space, two distinct topological transition processes between AF phases occur, leading to two types of AFTI, showing distinguishable surface dispersions around their Dirac points.
Huan Li, Zhi-Yong Wang, Xiao-Jun Zheng, Yu Liu, Yin Zhong [pdf] DOI: 10.1088/0256-307X/35/12/127501 1809.09867v1 [pdf]
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$\mathcal{Z}_2$ classification for a novel antiferromagnetic topological insulating phase in three-dimensional topological Kondo insulator -
Abstract
- Antiferromagnetic topological insulator (AFTI) is a topological matter that breaks time-reversal symmetry. Since its proposal, explorations of AFTI in strong-correlated systems are still lacking. In this paper, we show for the first time that a novel AFTI phase can be realized in three-dimensional topological Kondo insulator (TKI). In a wide parameter region, the ground states of TKI undergo a second-order transition to antiferromagnetic insulating phases which conserve a combined symmetry of time reversal and a lattice translation, allowing us to derive a $\mathcal{Z}_2$-classification formula for these states. By calculating the $\mathcal{Z}_2$ index, the antiferromagnetic insulating states are classified into (AFTI) or non-topological antiferromagnetic insulator (nAFI) in different parameter regions. On the antiferromagnetic surfaces in AFTI, we find topologically protected gapless Dirac cones inside the bulk gap, leading to metallic Fermi rings exhibiting helical spin texture with weak spin-momentum locking. Depending on model parameters, the magnetic transitions take place either between AFTI and strong topological insulator, or between nAFI and weak topological insulator. By varying some model parameters, we find a topological transition between AFTI and nAFI, driving by closing of bulk gap. Our work may account for the pressure-induced magnetism in TKI compound SmB$_6$, and helps to explore richer AFTI phases in heavy-fermion systems as well as in other strong-correlated systems.
Huan Li, Yin Zhong, Yu Liu, Hong-Gang Luo, Hai-Feng Song Journal reference: Journal of Physics: Condensed Matter (2018) [pdf] DOI: 10.1088/1361-648X/aae17b
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Abelian categories arising from cluster tilting subcategories II: quotient functors -
Abstract
- In this paper, we consider a kind of ideal quotient of an extriangulated category such that the ideal is the kernel of a functor from this extriangulated category to an abelian category. We study a condition when the functor is dense and full, in another word, the ideal quotient becomes abelian. Moreover, a new equivalent characterization of cluster-tilting subcategories is given by applying homological methods according to this functor. As an application, we show that in a connected 2-Calabi-Yau triangulated category B, a functorially finite, extension closed subcategory T of B is cluster tilting if and only if B/T is an abelian category.
Yu Liu, Panyue Zhou [pdf] DOI: 10.1017/prm.2019.42 1809.06597v1 [pdf]
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Cocktails, but no party: multipath-enabled private audio -
Abstract
- We describe a private audio messaging system that uses echoes to unscramble messages at a few predetermined locations in a room. The system works by splitting the audio into short chunks and emitting them from different loudspeakers. The chunks are filtered so that as they echo around the room, they sum to noise everywhere except at a few chosen focusing spots where they exactly reproduce the intended messages. Unlike in the case of standard personal audio zones, the proposed method renders sound outside the focusing spots unintelligible. Our method essentially depends on echoes: the room acts as a mixing system such that at given points we get the desired output. Finally, we only require a modest number of loudspeakers and only a few impulse response measurements at points where the messages should be delivered. We demonstrate the effectiveness of the proposed method via objective quantitative metrics as well as informal listening experiments in a real room.
Yu-Jeh Liu, Jonah Casebeer, Ivan Dokmanić [pdf]
-
Glueball spectrum from
-
Abstract
- The lowest-lying glueballs are investigated in lattice QCD using $N_f=2$ clover Wilson fermion on anisotropic lattices. We simulate at two different and relatively heavy quark masses, corresponding to physical pion mass of $m_\pi\sim 938$ MeV and $650$ MeV. The quark mass dependence of the glueball masses have not been investigated in the present study. Only the gluonic operators built from Wilson loops are utilized in calculating the corresponding correlation functions. In the tensor channel, we obtain the ground state mass to be 2.363(39) GeV and 2.384(67) GeV at $m_\pi\sim 938$ MeV and $650$ MeV, respectively. In the pseudoscalar channel, when using the gluonic operator whose continuum limit has the form of $\epsilon_{ijk}TrB_iD_jB_k$, we obtain the ground state mass to be 2.573(55) GeV and 2.585(65) GeV at the two pion masses. These results are compatible with the corresponding results in the quenched approximation. In contrast, if we use the topological charge density as field operators for the pseudoscalar, the masses of the lowest state are much lighter (around 1GeV) and compatible with the expected masses of the flavor singlet $q\bar{q}$ meson. This indicates that the operator $\epsilon_{ijk}TrB_iD_jB_k$ and the topological charge density couple rather differently to the glueball states and $q\bar{q}$ mesons. The observation of the light flavor singlet pseudoscalar meson can be viewed as the manifestation of effects of dynamical quarks. In the scalar channel, the ground state masses extracted from the correlation functions of gluonic operators are determined to be around 1.4-1.5 GeV, which is close to the ground state masses from the correlation functions of the quark bilinear operators. In all cases, the mixing between glueballs and conventional mesons remains to be further clarified in the future.
Wei Sun, Long-Cheng Gui, Ying Chen, Ming Gong, Chuan Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Jian-Bo Zhang Journal reference: Chin.Phys. C42 (2018) no.9, 093103 [pdf] DOI: 10.1088/1674-1137/42/9/093103
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Abelian Categories Arising from Cluster Tilting Subcategories -
Abstract
- For a triangulated category T, if C is a cluster-tilting subcategory of T, then the quotient category T\C is an abelian category. Under certain conditions, the converse also holds. This is an very important result of cluster-tilting theory, due to Koenig-Zhu and Beligiannis. Now let B be a suitable extriangulated category, which is a simultaneous generalization of triangulated categories and exact categories. We introduce the notion of pre-cluster tilting subcategory C of B, which is a generalization of cluster tilting subcategory. We show that C is cluster tilting if and only if B/C is abelian.
Yu Liu, Panyue Zhou [pdf] DOI: 10.1007/s10485-019-09590-w 1809.02315v1 [pdf]
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Critical behavior and magnetocaloric effect in
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Abstract
- The critical properties and magnetocaloric effect of semiconducting ferrimagnet Mn$_3$Si$_2$Te$_6$ single crystals have been investigated by bulk magnetization and heat capacity around $T_c$. Critical exponents $\beta = 0.41\pm0.01$ with a critical temperature $T_c = 74.18\pm0.08$ K and $\gamma = 1.21\pm0.02$ with $T_c = 74.35\pm0.05$ K are deduced by the Kouvel-Fisher plot, whereas $\delta = 4.29\pm0.05(3.40\pm0.02)$ is obtained by a critical isotherm analysis at $T = 74(75)$ K. The magnetic exchange distance is found to decay as $J(r)\approx r^{-4.79}$, which lies between the mean-field and 3D Heisenberg models. Moreover, the magnetic entropy change $-\Delta S_M$ features a maximum at $T_c$, i.e., $-\Delta S_M^{max} \sim$ 2.53(1.67) J kg$^{-1}$ K$^{-1}$ with in-plane(out-of-plane) field change of 5 T, confirming large magnetic anisotropy. The heat capacity measurement further gives $-\Delta S_M^{max}$ $\sim$ 2.94 J kg$^{-1}$ K$^{-1}$ and the corresponding adiabatic temperature change $\Delta T_{ad}$ $\sim$ 1.14 K with out-of-plane field change of 9 T.
Yu Liu, C. Petrovic Journal reference: Physical Review B 98, 064423 (2018) [pdf] DOI: 10.1103/PhysRevB.98.064423
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Observation of Floquet Raman Transition in a Driven Solid-State Spin System -
Abstract
- We experimentally observe Floquet Raman transitions in the weakly driven solid state spin system of nitrogen-vacancy center in diamond. The periodically driven spin system simulates a two-band Wannier-Stark ladder model, and allows us to observe coherent spin state transfer arising from Raman transition mediated by Floquet synthetic levels. It also leads to the prediction of analog photon-assisted Floquet Raman transition and dynamical localisation in a driven two-level quantum system. The demonstrated rich Floquet dynamics offers new capabilities to achieve effective Floquet coherent control of a quantum system with potential applications in various types of quantum technologies based on driven quantum dynamics. In particular, the Floquet-Raman system may be used as a quantum simulator for the physics of periodically driven systems.
Zijun Shu, Yu Liu, Qingyun Cao, Pengcheng Yang, Shaoliang Zhang, Martin B. Plenio, Fedor Jelezko, Jianming Cai Journal reference: Phys. Rev. Lett. 121, 210501 (2018) [pdf] DOI: 10.1103/PhysRevLett.121.210501
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Modelling Irregular Spatial Patterns using Graph Convolutional Neural
Networks -
Abstract
- The understanding of geographical reality is a process of data representation and pattern discovery. Former studies mainly adopted continuous-field models to represent spatial variables and to investigate the underlying spatial continuity/heterogeneity in the regular spatial domain. In this article, we introduce a more generalized model based on graph convolutional neural networks (GCNs) that can capture the complex parameters of spatial patterns underlying graph-structured spatial data, which generally contain both Euclidean spatial information and non-Euclidean feature information. A trainable semi-supervised prediction framework is proposed to model the spatial distribution patterns of intra-urban points of interest(POI) check-ins. This work demonstrates the feasibility of GCNs in complex geographic decision problems and provides a promising tool to analyze irregular spatial data.
Di Zhu, Yu Liu [pdf]
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Knowledge Graph Embedding with Entity Neighbors and Deep Memory Network -
Abstract
- Knowledge Graph Embedding (KGE) aims to represent entities and relations of knowledge graph in a low-dimensional continuous vector space. Recent works focus on incorporating structural knowledge with additional information, such as entity descriptions, relation paths and so on. However, common used additional information usually contains plenty of noise, which makes it hard to learn valuable representation. In this paper, we propose a new kind of additional information, called entity neighbors, which contain both semantic and topological features about given entity. We then develop a deep memory network model to encode information from neighbors. Employing a gating mechanism, representations of structure and neighbors are integrated into a joint representation. The experimental results show that our model outperforms existing KGE methods utilizing entity descriptions and achieves state-of-the-art metrics on 4 datasets.
Kai Wang, Yu Liu, Xiujuan Xu, Dan Lin [pdf]
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Localization of five-dimensional Elko spinors with non-minimal coupling on thick branes -
Abstract
- It has been found that the zero mode of a five-dimensional Elko spinor could be localized on branes by introducing a Yukawa-type coupling between the Elko spinor and the background scalar field or the Ricci scalar. However, the Yukawa-type coupling is not appropriate for all brane models. In this paper, we explore other localization mechanism for the Elko spinor by introducing the non-minimal coupling $f(\phi)\mathfrak{L}_{Elko}$ between the five-dimensional Elko spinor and the background scalar field. We give the general expressions of the Elko zero mode and the function $f(\phi)$. Through two thick brane models and three concrete examples, we show that the Elko zero mode can be localized on the branes by this new mechanism. This provides us more possibilities of localizing the Elko zero mode.
Xiang-Nan Zhou, Yun-Zhi Du, Hua-Zhen Zhao, Yu-Xiao Liu Journal reference: Eur.Phys.J. C 78 (2018) 493 [pdf] DOI: 10.1140/epjc/s10052-018-5971-1
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Improved Techniques for Learning to Dehaze and Beyond: A Collective
Study -
Abstract
- Here we explore two related but important tasks based on the recently released REalistic Single Image DEhazing (RESIDE) benchmark dataset: (i) single image dehazing as a low-level image restoration problem; and (ii) high-level visual understanding (e.g., object detection) of hazy images. For the first task, we investigated a variety of loss functions and show that perception-driven loss significantly improves dehazing performance. In the second task, we provide multiple solutions including using advanced modules in the dehazing-detection cascade and domain-adaptive object detectors. In both tasks, our proposed solutions significantly improve performance. GitHub repository URL is: https://github.com/guanlongzhao/dehaze
Yu Liu, Guanlong Zhao, Boyuan Gong, Yang Li, Ritu Raj, Niraj Goel, Satya Kesav, Sandeep Gottimukkala, Zhangyang Wang, Wenqi Ren, Dacheng Tao [pdf]
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Merger estimates for rotating Kerr black holes in modified gravity -
Abstract
- In this paper, we explore the signatures of non-rotating and rotating black hole mergers in the matter-free modified gravity. First, we solve the unstable circular null orbits and the innermost stable circular timelike orbits via the geodesic motion. The characteristic quantities of these orbits are systematically analyzed by varying the black hole spin and the scalar field parameter of the gravity. Then based on it, we study the ringdown modes from the light ring/quasinormal modes correspondence. The final spins of the merged black holes are also estimated with the Buonanno-Kidder-Lehner recipe. Several black hole merging cases are investigated in detail. All these results show that the black hole mergers are closely dependent of the scalar field parameter of the gravity.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 98, 024042 (2018) [pdf] DOI: 10.1103/PhysRevD.98.024042
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Generalized Su-Schrieffer-Heeger model in one dimensional optomechanical
arrays -
Abstract
- We propose an implementation of a generalized Su-Schrieffer-Heeger (SSH) model based on optomechanical arrays. The topological properties of the generalized SSH model depend on the effective optomechanical interactions enhanced by strong driving optical fields. Three phases including one trivial and two distinct topological phases are found in the generalized SSH model. The phase transition can be observed by turning the strengths and phases of the effective optomechanical interactions via adjusting the external driving fields. Moreover, four types of edge states can be created in generalized SSH model of an open chain under single-particle excitation, and the dynamical behaviors of the excitation in the open chain are related to the topological properties under the periodic boundary condition. We show that the edge states can be pumped adiabatically along the optomechanical arrays by periodically modulating the amplitude and frequency of the driving fields. The generalized SSH model based on the optomechanical arrays provides us a tunable platform to engineer topological phases for photons and phonons, which may have potential applications in controlling the transport of photons and phonons.
Xun-Wei Xu, Yan-Jun Zhao, Hui Wang, Ai-Xi Chen, Yu-xi Liu [pdf]
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Coronal EUV, QFP, and kink waves simultaneously launched during the course of jet–loop interaction -
Abstract
- We present the observations of an extreme ultraviolet (EUV) wave, a quasi-periodic fast-propagating (QFP) magnetosonic wave, and a kink wave that were simultaneously associated with the impingement of a coronal jet upon a group of coronal loops. After the interaction, the coronal loop showed obvious kink oscillation that had a period of about 428 seconds. In the meantime, a large-scale EUV wave and a QFP wave are observed on the west of the interaction position. It is interesting that the QFP wave showed refraction effect during its passing through two strong magnetic regions. The angular extent, speed, and lifetime of the EUV (QFP) wave were about 140 (40) degree, 423 (322) km/s, and 6 (26) minutes, respectively. It is measured that the period of the QFP wave was about 390 +/- 100 second. Based on the observational analysis results, we propose that the kink wave was probably excited by the interaction of the jet; the EUV was probably launched by the sudden expansion of the loop system due to the impingement of the coronal jet; and the QFP wave was possibly formed through the dispersive evolution of the disturbance caused by the jet-loop interaction.
Yuandeng Shen, Zehao Tang, Hongbo Li, Yu Liu [pdf] DOI: 10.1093/mnrasl/sly127 1807.09533v1 [pdf]
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Combinatorial Multi-Armed Bandit with General Reward Functions -
Abstract
- In this paper, we study the stochastic combinatorial multi-armed bandit (CMAB) framework that allows a general nonlinear reward function, whose expected value may not depend only on the means of the input random variables but possibly on the entire distributions of these variables. Our framework enables a much larger class of reward functions such as the $\max()$ function and nonlinear utility functions. Existing techniques relying on accurate estimations of the means of random variables, such as the upper confidence bound (UCB) technique, do not work directly on these functions. We propose a new algorithm called stochastically dominant confidence bound (SDCB), which estimates the distributions of underlying random variables and their stochastically dominant confidence bounds. We prove that SDCB can achieve $O(\log{T})$ distribution-dependent regret and $\tilde{O}(\sqrt{T})$ distribution-independent regret, where $T$ is the time horizon. We apply our results to the $K$-MAX problem and expected utility maximization problems. In particular, for $K$-MAX, we provide the first polynomial-time approximation scheme (PTAS) for its offline problem, and give the first $\tilde{O}(\sqrt T)$ bound on the $(1-\epsilon)$-approximation regret of its online problem, for any $\epsilon>0$.
Wei Chen, Wei Hu, Fu Li, Jian Li, Yu Liu, Pinyan Lu [pdf]
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Evidence of spin-phonon coupling in
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Abstract
- We present the Raman scattering results on layered 2D semiconducting ferromagnetic compound CrSiTe$_3$. Four Raman active modes, predicted by symmetry, have been observed and assigned. The experimental results are supported by DFT calculations. The self-energies of the $A_g^3$ and the $E_g^3$ symmetry modes exhibit unconventional temperature evolution around 180 K. In addition, the doubly degenerate $E_g^3$ mode shows clear change of asymmetry in the same temperature region. The observed behavior is consistent with the presence of the previously reported short-range magnetic order and the strong spin-phonon coupling.
A. Milosavljević, A. Šolajić, J. Pešić, Yu Liu, C. Petrovic, N. Lazarević, Z. V. Popović Journal reference: Phys. Rev. B 98, 104306, (2018) [pdf] DOI: 10.1103/PhysRevB.98.104306
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Cross-correlation between photons and phonons in quadratically coupled optomechanical systems -
Abstract
- We study photon, phonon statistics and the cross-correlation between photons and phonons in a quadratically coupled optomechanical system. Photon blockade, phonon blockade and strongly anticorrelated photons and phonons can be observed in the same parameter regime with the effective nonlinear coupling between the optical and mechanical modes, enhanced by a strong optical driving field. Interestingly, an optimal value of the effective nonlinear coupling strength for the photon blockade is not within the strong nonlinear coupling regime. This abnormal phenomenon results from the destructive interference between different paths for two-photon excitation in the optical mode with a moderate effective nonlinear coupling strength. Further more, we show that phonon (photon) pairs and correlated photons and phonons can be generated in the strong nonlinear coupling regime with a proper detuning between the weak mechanical driving field and mechanical mode. Our results open up a way to generate anticorrelated and correlated photons and phonons, which may have important applications in quantum information processing.
Xun-Wei Xu, Hai-Quan Shi, Ai-Xi Chen, Yu-xi Liu Journal reference: Phys. Rev. A 98, 013821 (2018) [pdf] DOI: 10.1103/PhysRevA.98.013821
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Abstract
- Presented here is decryst, a software suite for structure determination from powder diffraction, which uses the direct space method, and is able to apply anti-bump constraints automatically and efficiently during the procedure of global optimisation using the crystallographic collision detection algorithm in arXiv:1708.03180. decryst employs incremental computation in its global optimisation cycles, which results in dramatic performance enhancement; it is also designed with parallel and distributed computing in mind, allowing for even better performance by simultaneous use of multiple processors. decryst is free and open source software, and can be obtained at https://gitlab.com/CasperVector/decryst/; it strives to be simple yet flexible, in the hope that the underlying techniques could be adopted in more crystallographic applications.
Yu Liu Journal reference: J. Appl. Cryst. 2018, 51(4), 1237-1243 [pdf] DOI: 10.1107/S160057671800804X
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Mapping and Measuring Large-scale Photonic Correlation with
Single-photon Imaging -
Abstract
- Quantum correlation and its measurement are essential in exploring fundamental quantum physics problems and developing quantum enhanced technologies. Quantum correlation may be generated and manipulated in different spaces, which demands different measurement approaches corresponding to position, time, frequency and polarization of quantum particles. In addition, after early proof-of-principle demonstrations, it is of great demand to measure quantum correlation in a Hilbert space large enough for real quantum applications. When the number of modes goes up to several hundreds, it becomes economically unfeasible for single-mode addressing and also extremely challenging for processing correlation events with hardware. Here we present a general and large-scale measurement approach of Correlation on Spatially-mapped Photon-Level Image (COSPLI). The quantum correlations in other spaces are mapped into the position space and are captured by single-photon-sensitive imaging system. Synthetic methods are developed to suppress noises so that single-photon registrations can be faithfully identified in images. We eventually succeed in retrieving all the correlations with big-data technique from tens of millions of images. We demonstrate our COSPLI by measuring the joint spectrum of parametric down-conversion photons. Our approach provides an elegant way to observe the evolution results of large-scale quantum systems, representing an innovative and powerful tool added into the platform for boosting quantum information processing.
Ke Sun, Jun Gao, Ming-Ming Cao, Zhi-Qiang Jiao, Yu Liu, Zhan-Ming Li, Eilon Poem, Andreas Eckstein, Ruo-Jing Ren, Xiao-Ling Pang, Hao Tang, Ian A. Walmsley, Xian-Min Jin [pdf]
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EUV Waves Driven by the Sudden Expansion of Transequatorial Loops Caused by Coronal Jets -
Abstract
- We present two events to study the driving mechanism of extreme-ultraviolet (EUV) waves that are not associated with coronal mass ejections (CMEs), by using high resolution observations taken by the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory. Observational results indicate that the observed EUV waves were accompanied by ares and coronal jets, but without CMEs that were regarded as the driver of most EUV waves in previous studies. In the first case, it is observed that a coronal jet ejected along a transequatorial loop system at a plane-of-the-sky (POS) speed of $335 \pm 22$ km/s, in the meantime, an arc-shaped EUV wave appeared on the eastern side of the loop system. In addition, the EUV wave further interacted with another interconnecting loop system and launched a fast propagating (QFP) magnetosonic wave along the loop system, which had a period of 200 s and a speed of $388 \pm 65$ km/s, respectively. In the second case, we also observed a coronal jet ejected at a POS speed of $282 \pm 44$ km/s along a transequatorial loop system and the generation of bright EUV wave on the eastern side of the loop system. Based on the observational results, we propose that the observed EUV waves on the eastern side of the transequatorial loop systems are fast-mode magnetosonic waves, and they were driven by the sudden lateral expansion of the transequatorial loop systems due to the direct impingement of the associated coronal jets, while the QFP wave in the first case formed due to the dispersive evolution of the disturbance caused by the interaction between the EUV wave and the interconnecting coronal loops. It is noted that EUV waves driven by sudden loop expansions have shorter lifetimes than those driven by CMEs.
Yuandeng Shen, Zehao Tang, Yuhu Miao, Jiangtao Su, Yu Liu Journal reference: ApJL, 2018, 860, L8 [pdf] DOI: 10.3847/2041-8213/aac8dd
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Zoom Out-and-In Network with Map Attention Decision for Region Proposal
and Object Detection -
Abstract
- In this paper, we propose a zoom-out-and-in network for generating object proposals. A key observation is that it is difficult to classify anchors of different sizes with the same set of features. Anchors of different sizes should be placed accordingly based on different depth within a network: smaller boxes on high-resolution layers with a smaller stride while larger boxes on low-resolution counterparts with a larger stride. Inspired by the conv/deconv structure, we fully leverage the low-level local details and high-level regional semantics from two feature map streams, which are complimentary to each other, to identify the objectness in an image. A map attention decision (MAD) unit is further proposed to aggressively search for neuron activations among two streams and attend the most contributive ones on the feature learning of the final loss. The unit serves as a decisionmaker to adaptively activate maps along certain channels with the solely purpose of optimizing the overall training loss. One advantage of MAD is that the learned weights enforced on each feature channel is predicted on-the-fly based on the input context, which is more suitable than the fixed enforcement of a convolutional kernel. Experimental results on three datasets, including PASCAL VOC 2007, ImageNet DET, MS COCO, demonstrate the effectiveness of our proposed algorithm over other state-of-the-arts, in terms of average recall (AR) for region proposal and average precision (AP) for object detection.
Hongyang Li, Yu Liu, Wanli Ouyang, Xiaogang Wang [pdf]
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Beyond Trade-off: Accelerate FCN-based Face Detector with Higher
Accuracy -
Abstract
- Fully convolutional neural network (FCN) has been dominating the game of face detection task for a few years with its congenital capability of sliding-window-searching with shared kernels, which boiled down all the redundant calculation, and most recent state-of-the-art methods such as Faster-RCNN, SSD, YOLO and FPN use FCN as their backbone. So here comes one question: Can we find a universal strategy to further accelerate FCN with higher accuracy, so could accelerate all the recent FCN-based methods? To analyze this, we decompose the face searching space into two orthogonal directions, `scale' and `spatial'. Only a few coordinates in the space expanded by the two base vectors indicate foreground. So if FCN could ignore most of the other points, the searching space and false alarm should be significantly boiled down. Based on this philosophy, a novel method named scale estimation and spatial attention proposal ($S^2AP$) is proposed to pay attention to some specific scales and valid locations in the image pyramid. Furthermore, we adopt a masked-convolution operation based on the attention result to accelerate FCN calculation. Experiments show that FCN-based method RPN can be accelerated by about $4\times$ with the help of $S^2AP$ and masked-FCN and at the same time it can also achieve the state-of-the-art on FDDB, AFW and MALF face detection benchmarks as well.
Guanglu Song, Yu Liu, Ming Jiang, Yujie Wang, Junjie Yan, Biao Leng [pdf]
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Thick brane in mimetic $f(T)$ gravity -
Abstract
- We apply the mimetic $f(T)$ theory into the thick brane model. We take the Lagrange multiplier formulation of the action and get the corresponding field equations of motion. Considering the mimetic field as a single-kink or a double-kink, we find solutions for different kinds of $f(T)$. Besides, we investigate the stability of the mimetic $f(T)$ brane by considering the tensor perturbations of the vielbein. Localization problem is also studied and it is shown that the four-dimensional gravity can be recovered for all the solutions. The effects of the torsion show that for the polynomial form of $f(T)$, the zero mode has a split compared with that of $f(T)=T$, but situations for the exponential form of $f(T)$ is similar to that of $f(T)=T$.
Wen-Di Guo, Yi Zhong, Ke Yang, Tao-Tao Sui, Yu-Xiao Liu [pdf]
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Formation and eruption of a double-decker filament triggered by micro-bursts and recurrent jets in the filament channel -
Abstract
- We present the observations of a double-decker filament to study its formation, triggering, and eruption physics. It is observed that the double-decker filament was formed by splitting of an original single filament. During the splitting process, intermittent bright point bursts are observed in the filament channel, which resulted in the generation of the upper filament branch. The eruption of the newly formed double-decker filament was possibly triggered by two recurrent two-sided loop jets in the filament channel and the continuous mass unloading from the upper filament body. The interaction between the first jet and the filament directly resulted in the unstable of the lower branch and the fast rising phase of the upper branch. The second jet occurred at the same site about three hours after the first one, which further disturbed and accelerated the rising of the lower filament branch. It is interesting that the rising lower branch overtook the upper one, and then the two branches probably merged into one filament. Finally, the whole filament erupted violently and caused a large-scale coronal mass ejection, leaving behind a pair of flare ribbons and two dimming regions on the both sides of the filament channel. We think that the intermittent bursts may directly result in the rearrangement of the filament magnetic field and therefore the formation of the double-decker filament, then the recurrent jets further caused the fully eruption of the entire filament system. The study provides convincing evidence for supporting the scenario that a double-decker filament can be formed by splitting a single filament into two branches.
Zhanjun Tian, Yuandeng Shen, Yu Liu [pdf] DOI: 10.1016/j.newast.2018.05.005 1805.12314v1 [pdf]
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Homologous Large-amplitude Nonlinear Fast-mode Magnetosonic Waves Driven by Recurrent Coronal Jets -
Abstract
- The detailed observational analysis of a homologous Extreme-ultraviolet (EUV) wave event is presented to study the driving mechanism and the physical property of the EUV waves, combining high resolution data taken by the Solar Dynamics Observatory and the Solar TErrestrial RElations Observatory. It is observed that four homologous EUV waves originated from the same active region AR11476 within about one hour, and the time separations between consecutive waves were of 8 - 20 minutes. The waves showed narrow arc-shaped wavefronts and propagated in the same direction along a large-scale transequatorial loop system at a speed of 648 - 712 km/s and a deceleration of 0.985 - 1.219 km/s2. The EUV waves were accompanied by weak flares, coronal jets, and radio type III bursts, in which the EUV waves were delayed with respect to the start times of the radio type III bursts and coronal jets about 2 - 13 and 4 - 9 minutes, respectively. Different to previous studies of homologous EUV waves, no coronal mass ejections were found in the present event. Based on the observational results and the close temporal the spatial relationship between the EUV waves and the coronal jets, for the first time, we propose that the observed homologous EUV waves were large-amplitude nonlinear fast-mode magnetosonic waves or shocks driven by the associated recurrent coronal jets, resemble the generation mechanism of a piston shock in a tube. In addition, it is found that the recurrent jets were tightly associated with the alternating flux cancellation and emergence in the eruption source region and radio type III bursts.
Yuandeng Shen, Yu Liu, Ying D. Liu, Jiangtao Su, Zehao Tang, Yuhu Miao [pdf] DOI: 10.3847/1538-4357/aac9be 1805.12303v1 [pdf]
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Domain wall brane in a reduced Born–Infeld-f(T) theory -
Abstract
- The Born-Infeld $f(T)$ theory is reduced from the Born-Infeld determinantal gravity in Weitzenb\"ock spacetime. We investigate a braneworld scenario in this theory and obtain an analytic domain wall solution by utilizing the first-order formalism. The model is stable against the linear tensor perturbation. It is shown that the massless graviton is localized on the brane, but the continuous massive gravitons are non-localized and will generate a tiny correction with the behavior of ${1}/{(k r)^{3}}$ to the Newtonian potential. The four-dimensional teleparallel gravity is recovered as an effective infrared theory on the brane. As a physical application, we consider the (quasi-)localization property of spin-1/2 Dirac fermion in this model.
Ke Yang, Wen-Di Guo, Zi-Chao Lin, Yu-Xiao Liu Journal reference: Phys. Lett. B 782 (2018) 170-175 [pdf] DOI: 10.1016/j.physletb.2018.05.017
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LOCALIZATIONS OF THE HEARTS OF COTORSION PAIRS -
Abstract
- In this article, we study localizations of hearts of cotorsion pairs (U,V) where U is rigid on an extriangulated category B. The hearts of such cotorsion pairs are equivalent to the functor categories over the stable category of U. Inspired by Marsh and Palu, we consider the mutation of U that induces a cotorsion pair (U',V'). Generally speaking, the hearts of (U,V) and (U',V') are not equivalent to each other, but we will give a generalized pseudo-Morita equivalence between certain localizations of their hearts.
Yu Liu [pdf] DOI: 10.1017/S0017089519000284 1705.00278v5 [pdf]
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Anisotropic magnetocaloric effect in single crystals of
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Abstract
- We report a systematic investigation of dc magnetization and ac susceptibility, as well as anisotropic magnetocaloric effect in bulk CrI$_3$ single crystals. A second-stage magnetic transition was observed just below the Curie temperature $T_c$, indicating a two-step magnetic ordering. The low temperature thermal demagnetization could be well fitted by the spin-wave model rather than the single-particle model, confirming its localized magnetism. The maximum magnetic entropy change $-\Delta S_M^{max} \sim 5.65$ J kg$^{-1}$ K$^{-1}$ and the corresponding adiabatic temperature change $\Delta T_{ad} \sim 2.34$ K are achieved from heat capacity analysis with the magnetic field up to 9 T. Anisotropy of $\Delta S_M(T,H)$ was further investigated by isothermal magnetization, showing that the difference of $-\Delta S_M^{max}$ between the $ab$ plane and the $c$ axis reaches a maximum value $\sim$ 1.56 J kg$^{-1}$ K$^{-1}$ with the field change of 5 T. With the scaling analysis of $\Delta S_M$, the rescaled $\Delta S_M(T,H)$ curves collapse onto a universal curve, indicating a second-order type of the magnetic transition. Furthermore, the $-\Delta S_M^{max}$ follows the power law of $H^n$ with $n = 0.64(1)$, and the relative cooling power RCP depends on $H^m$ with $m = 1.12(1)$.
Yu Liu, C. Petrovic Journal reference: Physical Review B 97, 174418 (2018) [pdf] DOI: 10.1103/PhysRevB.97.174418
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Photon orbits and thermodynamic phase transition of
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Abstract
- We study the relationship between the null geodesics and thermodynamic phase transition for the charged AdS black hole. In the reduced parameter space, we find that there exist non-monotonic behaviors of the photon sphere radius and the minimum impact parameter for the pressure below its critical value. The study also shows that the changes of the photon sphere radius and the minimum impact parameter can serve as order parameters for the small-large black hole phase transition. In particular, these changes have an universal exponent of $\frac{1}{2}$ near the critical point for any dimension $d$ of spacetime. These results imply that there may exist universal critical behavior of gravity near the thermodynamic critical point of the black hole system.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 97, 104027 (2018) [pdf] DOI: 10.1103/PhysRevD.97.104027
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Lattice Calculation of Parton Distribution Function from LaMET at
Physical Pion Mass with Large Nucleon Momentum -
Abstract
- We present a lattice-QCD calculation of the unpolarized isovector parton distribution function (PDF) using ensembles at the physical pion mass with large proton boost momenta $P_z \in \{2.2,2.6,3.0\}$~GeV within the framework of large-momentum effective theory (LaMET). In contrast to our previous physical-pion PDF result, we increase the statistics significantly, double the boost momentum, increase the investment in excited-state contamination systematics, and switch to $\gamma_t$ operator to avoid mixing with scalar matrix elements. We use four source-sink separations in our analysis to control the systematics associated with excited-state contamination. The one-loop LaMET matching corresponding to the new operator is calculated and applied to our lattice data. We detail the systematics that affect PDF calculations, providing guidelines to improve the precision of future lattice PDF calculations. We find our final parton distribution to be in reasonable agreement with the PDF provided by the latest phenomenological analysis.
Jiunn-Wei Chen, Luchang Jin, Huey-Wen Lin, Yu-Sheng Liu, Yi-Bo Yang, Jian-Hui Zhang, Yong Zhao [pdf]
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Transferable Semi-supervised Semantic Segmentation -
Abstract
- The performance of deep learning based semantic segmentation models heavily depends on sufficient data with careful annotations. However, even the largest public datasets only provide samples with pixel-level annotations for rather limited semantic categories. Such data scarcity critically limits scalability and applicability of semantic segmentation models in real applications. In this paper, we propose a novel transferable semi-supervised semantic segmentation model that can transfer the learned segmentation knowledge from a few strong categories with pixel-level annotations to unseen weak categories with only image-level annotations, significantly broadening the applicable territory of deep segmentation models. In particular, the proposed model consists of two complementary and learnable components: a Label transfer Network (L-Net) and a Prediction transfer Network (P-Net). The L-Net learns to transfer the segmentation knowledge from strong categories to the images in the weak categories and produces coarse pixel-level semantic maps, by effectively exploiting the similar appearance shared across categories. Meanwhile, the P-Net tailors the transferred knowledge through a carefully designed adversarial learning strategy and produces refined segmentation results with better details. Integrating the L-Net and P-Net achieves 96.5% and 89.4% performance of the fully-supervised baseline using 50% and 0% categories with pixel-level annotations respectively on PASCAL VOC 2012. With such a novel transfer mechanism, our proposed model is easily generalizable to a variety of new categories, only requiring image-level annotations, and offers appealing scalability in real applications.
Huaxin Xiao, Yunchao Wei, Yu Liu, Maojun Zhang, Jiashi Feng [pdf]
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PAD-Net: A Perception-Aided Single Image Dehazing Network -
Abstract
- In this work, we investigate the possibility of replacing the $\ell_2$ loss with perceptually derived loss functions (SSIM, MS-SSIM, etc.) in training an end-to-end dehazing neural network. Objective experimental results suggest that by merely changing the loss function we can obtain significantly higher PSNR and SSIM scores on the SOTS set in the RESIDE dataset, compared with a state-of-the-art end-to-end dehazing neural network (AOD-Net) that uses the $\ell_2$ loss. The best PSNR we obtained was 23.50 (4.2% relative improvement), and the best SSIM we obtained was 0.8747 (2.3% relative improvement.)
Yu Liu, Guanlong Zhao [pdf]
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Polaronic transport and thermoelectricity in
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Abstract
- We report a study of Co-doped berthierite Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ ($x$ = 0, 0.1, and 0.2). The alloy series of Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ crystallize in an orthorhombic structure with the Pnma space group, similar to FeSb$_2$, and show semiconducting behavior. The large discrepancy between activation energy for conductivity, $E_\rho$ (146 $\sim$ 270 meV), and thermopower, $E_S$ (47 $\sim$ 108 meV), indicates the polaronic transport mechanism. Bulk magnetization and heat capacity measurements of pure FeSb$_2$S$_4$ ($x$ = 0) exhibit a broad antiferromagnetic (AFM) transition ($T_N$ = 46 K) followed by an additional weak transition ($T^*$ = 50 K). Transition temperatures ($T_N$ and $T^*$) slightly decrease with increasing Co content $x$. This is also reflected in the thermal conductivity measurement, indicating strong spin-lattice coupling. Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ shows relatively high value of thermopower (up to $\sim$ 624 $\mu$V K$^{-1}$ at 300 K) and thermal conductivity much lower when compared to FeSb$_{2}$, a feature desired for potential applications based on FeSb$_{2}$ materials.
Yu Liu, Chang-Jong Kang, Eli Stavitski, Qianheng Du, Klaus Attenkofer, G. Kotliar, C. Petrovic Journal reference: Physical Review B 97, 155202 (2018) [pdf] DOI: 10.1103/PhysRevB.97.155202
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Anomalous Hall effect in the van der Waals bonded ferromagnet
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Abstract
- We report anomalous Hall effect (AHE) in single crystals of quasi-two-dimensional Fe$_{3-x}$GeTe$_2$ ($x \approx 0.36$) ferromagnet grown by the flux method which induces defects on Fe site and bad metallic resistivity. Fe K-edge x-ray absorption spectroscopy was measured to provide information on local atomic environment in such crystals. The dc and ac magnetic susceptibility measurements indicate a second-stage transition below 119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A linear scaling behavior between the modified anomalous Hall resistivity $\rho_{xy}/\mu_0H_{eff}$ and longitudinal resistivity $\rho_{xx}^2M/\mu_0H_{eff}$ implies that the AHE in Fe$_{3-x}$GeTe$_2$ should be dominated by the intrinsic Karplus-Luttinger mechanism rather than the extrinsic skew-scattering and side-jump mechanisms. The observed deviation in the linear-M Hall conductivity $\sigma_{xy}^A$ below 30 K is in line with its transport characteristic at low temperatures, implying the scattering of conduction electrons due to magnetic disorder and the evolution of the Fermi surface induced by possible spin-reorientation transition.
Yu Liu, Eli Stavitski, Klaus Attenkofer, C. Petrovic Journal reference: Physical Review B 97, 165415 (2018) [pdf] DOI: 10.1103/PhysRevB.97.165415
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Exploring Disentangled Feature Representation Beyond Face Identification -
Abstract
- This paper proposes learning disentangled but complementary face features with minimal supervision by face identification. Specifically, we construct an identity Distilling and Dispelling Autoencoder (D2AE) framework that adversarially learns the identity-distilled features for identity verification and the identity-dispelled features to fool the verification system. Thanks to the design of two-stream cues, the learned disentangled features represent not only the identity or attribute but the complete input image. Comprehensive evaluations further demonstrate that the proposed features not only maintain state-of-the-art identity verification performance on LFW, but also acquire competitive discriminative power for face attribute recognition on CelebA and LFWA. Moreover, the proposed system is ready to semantically control the face generation/editing based on various identities and attributes in an unsupervised manner.
Yu Liu, Fangyin Wei, Jing Shao, Lu Sheng, Junjie Yan, Xiaogang Wang [pdf]
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Synthesizing exceptional points with three resonators -
Abstract
- In non-Hermitian coulped-resonator networks, the eigenvectors of degenerate eigenmodes may become parallel due to the singularity at so-called Exceptional Points (EP). To exploit the parametric sensitivity at EPs, an important problem is, given an arbitrary set of coupled resonators, how to generate a desired EP by properly coupling them together. This paper provides the solution for the case of three resonators. We show that all physically admissible EPs can be realized with either weakly coupled linear networks or strongly coupled circular networks, and the latter type of EPs has not been reported in the literature. Each admissible EP eigenvalue can be realized by two and only two resonator networks, and the formulas for calculating the required coupling constants are provided. The characteristics of these EPs are illustrated by the change of transmission spectra near them, which verify the enhanced sensitivity induced by the singularity of EPs.
Re-Bing Wu, Yu Zheng, Qi-Ming Chen, Yu-xi Liu Journal reference: Phys. Rev. A 98, 033817 (2018) [pdf] DOI: 10.1103/PhysRevA.98.033817
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Kinks in higher derivative scalar field theory -
Abstract
- We study static kink configurations in a type of two-dimensional higher derivative scalar field theory whose Lagrangian contains second-order derivative terms of the field. The linear fluctuation around arbitrary static kink solutions is analyzed. We find that, the linear spectrum can be described by a supersymmetric quantum mechanics problem, and the criteria for stable static solutions can be given analytically. We also construct a superpotential formalism for finding analytical static kink solutions. Using this formalism we first reproduce some existed solutions and then offer a new solution. The properties of our solution is studied and compared without those preexisted. We also show the possibility in constructing twinlike model in the higher derivative theory, and give the consistency conditions for twinlike models corresponding to the canonical scalar field theory.
Yuan Zhong, Rong-Zhen Guo, Chun-E Fu, Yu-Xiao Liu Journal reference: Phys. Lett. B 782 (2018) 346-352 [pdf] DOI: 10.1016/j.physletb.2018.05.048
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Field effect enhancement in buffered quantum nanowire networks -
Abstract
- III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.
Filip Krizek, Joachim E. Sestoft, Pavel Aseev, Sara Marti-Sanchez, Saulius Vaitiekenas, Lucas Casparis, Sabbir A. Khan, Yu Liu, Tomas Stankevic, Alexander M. Whiticar, Alexandra Fursina, Frenk Boekhout, Rene Koops, Emanuele Uccelli, Leo P. Kouwenhoven, Charles M. Marcus, Jordi Arbiol, Peter Krogstrup Journal reference: Phys. Rev. Materials 2, 093401 (2018) [pdf] DOI: 10.1103/PhysRevMaterials.2.093401
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Modeling granular material blending in a rotating drum using a finite
element method and advection-diffusion equation multi-scale model -
Abstract
- A multi-scale model is presented for predicting the magnitude and rate of powder blending in a rotating drum blender. The model combines particle diffusion coefficient correlations from the literature with advective flow field information from blender finite element method simulations. The multi-scale model predictions for overall mixing and local concentration variance closely match results from discrete element method (DEM) simulations for a rotating drum, but take only hours to compute as opposed to taking days of computation time for the DEM simulations. Parametric studies were performed using the multi-scale model to investigate the influence of various parameters on mixing behavior. The multi-scale model is expected to be more amenable to predicting mixing in complex geometries and scale more efficiently to industrial-scale blenders than DEM simulations or analytical solutions.
Yu Liu, Marcial Gonzalez, Carl Wassgren [pdf]
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Stable Palatini
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Abstract
- We consider the static domain wall braneworld scenario constructed from the Palatini formalism $f(\mathcal{R})$ theory. We check the self-consistency under scalar perturbations. By using the scalar-tensor formalism we avoid dealing with the higher-order equations. We develop the techniques to deal with the coupled system. We show that under some conditions, the scalar perturbation simply oscillates with time, which guarantees the stability. We also discuss the localization condition of the scalar mode by analyzing the effective potential and the fifth dimensional profile of the scalar mode. We apply these results to an explicit example, and show that only some of the solutions allow for stable scalar perturbations. These stable solutions also give nonlocalizable massless mode. This is important for reproducing a viable four-dimensional gravity.
Bao-Min Gu, Yu-Xiao Liu, Yuan Zhong Journal reference: Phys. Rev. D 98, 024027 (2018) [pdf] DOI: 10.1103/PhysRevD.98.024027
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Tuning the coupling between superconducting resonators with collective qubits -
Abstract
- By simultaneously coupling multiple two-level artificial atoms to two superconducting resonators, we design a quantum switch that tunes the resonator-resonator coupling strength from zero to a large value proportional to the number of qubits. This process is implemented by engineering the qubits into different subradiant states, where the microwave photons decay from different qubits destructively interfere with each other such that the resonator-resonator coupling strength keeps stable in an open environment. Based on a three-step control scheme, we switch the coupling strength among different values within nanoseconds without changing the transition frequency of the qubits. We also apply the quantum switch to a network of superconducting resonators, and demonstrate its potential applications in quantum simulation and quantum information storage and processing.
Qi-Ming Chen, Re-Bing Wu, Luyan Sun, Yu-xi Liu Journal reference: Phys. Rev. A 98, 042328 (2018) [pdf] DOI: 10.1103/PhysRevA.98.042328
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Mechanically modulated emission spectra and blockade of polaritons in a hybrid semiconductor-optomechanical system -
Abstract
- We study a hybrid semiconductor-optomechanical system, which consists of a cavity with an oscillating mirror made by semiconducting materials or with a semiconducting membrane inside. The cavity photons and the excitons in the oscillating mirror or semiconducting membrane form into polaritons. And correspondingly, the optomechanical interaction between the cavity photons and the mirror or membrane is changed into the polariton-mechanical interaction. We theoretically study the eigenenergies and eigenfunctions of this tripartite hybrid system with the generalized rotating-wave approximation. We show that the emission spectrum of polariton mode is modulated by the mechanical resonator. We also study the mechanical effect on the statistical properties of the polariton when the cavity is driven by a weak classical field. This work provides a detailed description of the rich nonlinearity owing to the competition between parametric coupling and three-wave mixing interaction concerning the polariton modes and the phonon mode. It also offers a way to operate the photons, phonons and excitons, e.g., detect the properties of mechanical resonator through the fine spectra of the polaritons or control the transmission of light in the integrated semiconducting-optomechanical platform.
Sai-Nan Huai, Yu-Long Liu, Yunbo Zhang, Yu-xi Liu Journal reference: Phys. Rev. A 98, 033825 (2018) [pdf] DOI: 10.1103/PhysRevA.98.033825
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Three-dimensional magnetic critical behavior in
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Abstract
- CrI$_3$ is a promising candidate for the van der Waals bonded ferromagnetic devices since its ferromagnetism can be maintained upon exfoliating of bulk crystals down to single layer. In this work we studied critical properties of bulk CrI$_3$ single crystals around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta$ = 0.260(4) with a critical temperature $T_c$ = 60.05(13) K and $\gamma$ = 1.136(6) with $T_c$ = 60.43(4) K are obtained by the Kouvel-Fisher method, whereas $\delta$ = 5.32(2) is obtained by a critical isotherm analysis at $T_c$ = 60 K. The critical exponents determined in bulk CrI$_3$ single crystals suggest a three-dimensional long-range magnetic coupling with the exchange distance decaying as $J(r)\approx r^{-4.69}$.
Yu Liu, C. Petrovic Journal reference: Physical Review B 97, 014420 (2018) [pdf] DOI: 10.1103/PhysRevB.97.014420
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Observation of unusual optical band structure of CH3NH3PbI3 perovskite
single crystal -
Abstract
- Extensive efforts have been undertaken on the photoelectric physics of hybrid organolead halide perovskites to unveil the reason for the attractive photovoltaic performance. Yet, the resulting evidences are far from being fully conclusive. Herein, we provide another direct support for this issue. In addition to the observation on the conventional band edge at 1.58 eV that presents a blueshift toward temperature increase, interestingly, we also observe an unusual optical band edge at 1.48 eV in CH3NH3PbI3 perovskite single crystals. Contrary to the conventional band edge, this one shows an obvious redshift toward the enhancement in temperature, in agreement with the Varshni relation. More interestingly, the unusual band edge exhibits a series of obvious absorption and photocurrent signals, but the according photoluminescence signals are not observable. This indicates that this band edge is particularly beneficial for the photovoltaic effect due to the inhibited radiative recombination. The kinetics on photo-involved charge transition and transfer are investigated using the pump-probe photoconductivity technique, and a changeable band structure model was proposed.
Wei Huang, Shizhong Yue, Yu Liu, Laipan Zhu, Peng Jin, Qing Wu, Yang Zhang, Yanan Chen, Kong Liu, Ping Liang, Shengchun Qu, Zhijie Wang, Yonghai Chen [pdf]
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Dispersively formed quasi-periodic fast magnetosonic wavefronts due to the eruption of a nearby mini-filament -
Abstract
- The observational analysis is performed to study the excitation mechanism and the propagation properties of a quasi-periodic fast-propagating (QFP) magnetosonic wave. The QFP wave was associated with the eruption of a nearby mini-filament and a small B4 GOES flare, which may indicate that the generation of a QFP wave do not need too much flare energy. The propagation of the QFP wave was along a bundle of funnel-shaped open loops with a speed of about 1100+/-78, and an acceleration of -2.2+/-1.1. Periodicity analysis indicates that the periods of the QFP wave are 43+/-6, 79+/-18 second. For the first time, we find that the periods of the QFP wave and the accompanying flare are inconsistent, which is different from the findings as reported in previous studies. We propose that the present QFP wave was possibly caused by the mechanism of dispersive evolution of an initially broadband disturbance resulted from the nearby mini-filament eruption.
Yuandeng Shen, Tengfei Song, Yu Liu [pdf] DOI: 10.1093/mnrasl/sly044 1803.01125v1 [pdf]
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Absence of Dirac states in
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Abstract
- We report magnetotransport properties of BaZnBi$_{2}$ single crystals. Whereas electronic structure features Dirac states, such states are removed from the Fermi level by spin-orbit coupling (SOC) and consequently electronic transport is dominated by the small hole and electron pockets. Our results are consistent with three dimensional (3D) but also with quasi two dimensional (2D) portions of the Fermi surface. The spin-orbit coupling-induced gap in Dirac states is much larger when compared to isostructural SrMnBi$_{2}$. This suggests that not only long range magnetic order but also mass of the alkaline earth atoms A in ABX$_{2}$ (A = alkaine earth, B = transition metal and X=Bi/Sb) are important for the presence of low-energy states obeying the relativistic Dirac equation at the Fermi surface
Weijun Ren, Aifeng Wang, D. Graf, Yu Liu, Zhidong Zhang, Wei-Guo Yin, C. Petrovic Journal reference: Phys. Rev. B 97, 035147 (2018) [pdf] DOI: 10.1103/PhysRevB.97.035147
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Electromagnetically Induced Transparency in Circuit Quantum Electrodynamics with Nested Polariton States -
Abstract
- Electromagnetically induced transparency (EIT) is a signature of quantum interference in an atomic three-level system. By driving the dressed cavity-qubit states of a two-dimensional circuit QED system, we generate a set of polariton states in the nesting regime. The lowest three energy levels are utilized to form the $\Lambda$-type system. EIT is observed and verified by Akaike's information criterion based testing. Negative group velocities up to $-0.52\pm0.09$ km/s are obtained based on the dispersion relation in the EIT transmission spectrum.
Junling Long, H. S. Ku, Xian Wu, Xiu Gu, Russell E. Lake, Mustafa Bal, Yu-xi Liu, David P. Pappas Journal reference: Phys. Rev. Lett. 120, 083602 (2018) [pdf] DOI: 10.1103/PhysRevLett.120.083602
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Stability of braneworlds with non-minimally coupled multi-scalar fields -
Abstract
- Linear stability of braneworld models constructed with multi-scalar fields is very different from that of single-scalar field models. It is well known that both the tensor and scalar perturbation equations of the later can always be written as a supersymmetric Schr\"{o}dinger equation, so it can be shown that the perturbations are stable at linear level. However, in general it is not true for multi-scalar field models and especially there is no effective method to deal with the stability problem of the scalar perturbations for braneworld models constructed with non-minimally coupled multi-scalar fields. In this paper we present a method to investigate the stability of such braneworld models. It is easy to find that the tensor perturbations are stable. For the stability problem of the scalar perturbations, we present a systematic covariant approach. The covariant quadratic order action and the corresponding first-order perturbed equations are derived. By introducing the orthonormal bases in field space and making the Kaluza-Klein decomposition, we show that the Kaluza-Klein modes of the scalar perturbations satisfy a set of coupled Schr\"{o}dinger-like equations, with which the stability of the scalar perturbations and localization of the scalar zero modes can be analyzed according to nodal theorem. The result depends on the explicit models. For superpotential derived barane models, the scalar perturbations are stable, but there exist normalizable scalar zero modes, which will result in unaccepted fifth force on the brane. We also use this method to analyze the $f(R)$ braneworld model with an explicit solution and find that the scalar perturbations are stable and the scalar zero modes can not be localized on the brane, which ensure that there is no extra long-range force and the Newtonian potential on the brane can be recovered.
Feng-Wei Chen, Bao-Min Gu, Yu-Xiao Liu Journal reference: Eur.Phys.J. C78 (2018) 131 [pdf] DOI: 10.1140/epjc/s10052-018-5613-7
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Colour Confinement: a Dynamical Phenomenon of QCD -
Abstract
- We study in this Letter the origin of the confinement in QCD by analyzing the colour charge of physics states. We derive the colour charge operator in QCD and compare it with the electromagnetic charge operator in QED. It shows that the two charges have very similar structure, but the dynamical properties of the gauge fields are different. The difference between the behaviours of the gauge boson propagator at zero momentum for QCD and that for QED guarantees that there occurs colour confinement in QCD but there is no confinement in QED. We give then a universal relation between the confinement and the dynamical property of QCD and reveals the origin of the colour confinement, which can be demonstrated as the dynamical effect of QCD or more explicitly the dynamical mass generation of the gluon.
Fei Gao, Chong-yao Chen, Yu-xin Liu [pdf]
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Recurrent Scale Approximation for Object Detection in CNN -
Abstract
- Since convolutional neural network (CNN) lacks an inherent mechanism to handle large scale variations, we always need to compute feature maps multiple times for multi-scale object detection, which has the bottleneck of computational cost in practice. To address this, we devise a recurrent scale approximation (RSA) to compute feature map once only, and only through this map can we approximate the rest maps on other levels. At the core of RSA is the recursive rolling out mechanism: given an initial map at a particular scale, it generates the prediction at a smaller scale that is half the size of input. To further increase efficiency and accuracy, we (a): design a scale-forecast network to globally predict potential scales in the image since there is no need to compute maps on all levels of the pyramid. (b): propose a landmark retracing network (LRN) to trace back locations of the regressed landmarks and generate a confidence score for each landmark; LRN can effectively alleviate false positives caused by the accumulated error in RSA. The whole system can be trained end-to-end in a unified CNN framework. Experiments demonstrate that our proposed algorithm is superior against state-of-the-art methods on face detection benchmarks and achieves comparable results for generic proposal generation. The source code of RSA is available at github.com/sciencefans/RSA-for-object-detection.
Yu Liu, Hongyang Li, Junjie Yan, Fangyin Wei, Xiaogang Wang, Xiaoou Tang [pdf]
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ZOOpt: Toolbox for Derivative-Free Optimization -
Abstract
- Recent advances of derivative-free optimization allow efficient approximating the global optimal solutions of sophisticated functions, such as functions with many local optima, non-differentiable and non-continuous functions. This article describes the ZOOpt (https://github.com/eyounx/ZOOpt) toolbox that provides efficient derivative-free solvers and are designed easy to use. ZOOpt provides a Python package for single-thread optimization, and a light-weighted distributed version with the help of the Julia language for Python described functions. ZOOpt toolbox particularly focuses on optimization problems in machine learning, addressing high-dimensional, noisy, and large-scale problems. The toolbox is being maintained toward ready-to-use tool in real-world machine learning tasks.
Yu-Ren Liu, Yi-Qi Hu, Hong Qian, Yang Yu, Chao Qian [pdf]
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Mathematical modeling reveals spontaneous emergence of self-replication in chemical reaction systems -
Abstract
- Explaining the origin of life requires us to explain how self-replication arises. To be specific, how can a self-replicating entity develop spontaneously from a chemical reaction system in which no reaction is self-replicating? Previously proposed mathematical models either supply an explicit framework for a minimal living system or only consider catalyzed reactions, and thus fail to provide a comprehensive theory. We set up a general model for chemical reaction systems that properly accounts for energetics, kinetics and the conservation law. We find that (1) some systems are collectively-catalytic where reactants are transformed into end products with the assistance of intermediates (as in the citric acid cycle), while some others are self-replicating where different parts replicate each other and the system self-replicates as a whole (as in the formose reaction); (2) many alternative chemical universes often contain one or more such systems; (3) it is possible to construct a self-replicating system where the entropy of some parts spontaneously decreases, in a manner similar to that discussed by Schr\"odinger; (4) complex self-replicating molecules can emerge spontaneously and relatively easily from simple chemical reaction systems through a sequence of transitions. Together these results start to explain the origins of prebiotic evolution.
Yu Liu, David Sumpter Journal reference: Journal of Biological Chemistry, 293 (49): 18854-18863, 2018 [pdf] DOI: 10.1074/jbc.RA118.003795
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The artificial ecosystem: number soup (part II) -
Abstract
- This paper is a follow-up work about the artificial ecosystem model: number soup (Liu and Sumpter, J. Royal Soc. Interface, 2017). It elaborates more details about this model and points out future directions.
Yu Liu [pdf]
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Vacuum-induced Autler-Townes splitting in a superconducting artificial atom -
Abstract
- We experimentally study a vacuum-induced Autler-Townes doublet in a superconducting three-level artificial atom strongly coupled to a coplanar waveguide resonator and simultaneously to a transmission line. The Autler-Townes splitting is observed in the reflection spectrum from the three-level atom in a transition between the ground state and the second excited state when the transition between the two excited states is resonant with a resonator. By applying a driving field to the resonator, we observe a change in the regime of the Autler-Townes splitting from quantum (vacuum-induced) to classical (with many resonator photons). Furthermore, we show that the reflection of propagating microwaves in a transmission line could be controlled by different frequency single photons in a resonator.
Z. H. Peng, J. H. Ding, Y. Zhou, L. L. Ying, Z. Wang, L. Zhou, L. M. Kuang, Yu-xi Liu, O. Astafiev, J. S. Tsai Journal reference: Phys. Rev. A 97, 063809 (2018) [pdf] DOI: 10.1103/PhysRevA.97.063809
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Scalar particle production in a simple Horndeski theory -
Abstract
- The scalar particle production through a scalar field non-minimally coupled with geometry is investigated in the context of a spatially homogeneous and isotropic universe. In this paper, in order to study the evolution of particle production over time in the case of analytical solutions, we focus on a simple Horndeski theory. We first suppose that the universe is dominated by a scalar field and derive the energy conservation condition. Then from the thermodynamic point of view, the macroscopic non-conservation of the scalar field energy-momentum tensor can be explained as an irreversible production of the scalar particles. Based on the explanation, we obtain a scalar particle production rate and the corresponding entropy. Finally, since the universe, in general, could be regarded as a closed system satisfying the laws of thermodynamics, we naturally impose some thermodynamic constraints on it. The thermodynamic properties of the universe can provide additional constraints on the simple Horndeski theory.
Hao Yu, Wen-Di Guo, Ke Yang, Yu-Xiao Liu Journal reference: Phys. Rev. D 97, 083524 (2018) [pdf] DOI: 10.1103/PhysRevD.97.083524
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Applicability of Kerker preconditioning scheme to the self-consistent density functional theory calculations of inhomogeneous systems -
Abstract
- Kerker preconditioner, based on the dielectric function of homogeneous electron gas, is designed to accelerate the self-consistent field (SCF) iteration in the density functional theory (DFT) calculations. However, question still remains regarding its applicability to the inhomogeneous systems. In this paper, we develop a modified Kerker preconditioning scheme which captures the long-range screening behavior of inhomogeneous systems thus improve the SCF convergence. The effectiveness and efficiency is shown by the tests on long-z slabs of metals, insulators and metal-insulator contacts. For situations without a priori knowledge of the system, we design the a posteriori indicator to monitor if the preconditioner has suppressed charge sloshing during the iterations. Based on the a posteriori indicator, we demonstrate two schemes of the self-adaptive configuration for the SCF iteration.
Yuzhi Zhou, Han Wang, Yu Liu, Xingyu Gao, Haifeng Song Journal reference: Phys. Rev. E 97, 033305 (2018) [pdf] DOI: 10.1103/PhysRevE.97.033305
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Revisiting the equation of state of hybrid stars in the Dyson-Schwinger equation approach to QCD -
Abstract
- We investigate the equation of state(EoS) and the effect of the hadron-quark phase transition of strong interaction matter in compact stars. The hadron matter is described with the relativistic mean field theory,and the quark matter is described with the Dyson-Schwinger equation approach of QCD. The complete EoS of the hybrid star matter is constructed with not only the Gibbs construction but also the 3-window interpolation. The mass-radius relation of hybrid stars is also investigated. We find that, although the EoSs of both the hadron matter with hyperon and $\Delta$-baryon and the quark matter are generally softer than that of the nucleon matter, the 3-window interpolation construction may provide an EoS stiff enough for a hybrid star with mass exceeding 2$M_{\odot}^{}$ and, in turn, solve the so called "hyperon puzzle".
Zhan Bai, Huan Chen, Yu-xin Liu Journal reference: Phys. Rev. D 97, 023018 (2018) [pdf] DOI: 10.1103/PhysRevD.97.023018
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Thick branes with inner structure in mimetic gravity -
Abstract
- In this paper, thick branes generated by mimetic scalar field are investigated. Three typical thick brane models are constructed and the linear tensor and scalar perturbations are analyzed. These branes have different inner structures, some of which are absent in general relativity. For each brane model, the solution is stable under both tensor and scalar perturbations. The tensor zero modes are localized on the branes, while the scalar perturbations do not propagate and they are not localized on the brane. As the branes split into multi sub-branes for specific parameters, the potentials of the tensor perturbations also split into multi-wells, and this may lead to new phenomenon in the resonance of the tensor perturbation and the localization of matter fields.
Yi Zhong, Yuan Zhong, Yu-Peng Zhang, Yu-Xiao Liu Journal reference: Eur.Phys.J. C78 (2018) 45 [pdf] DOI: 10.1140/epjc/s10052-018-5527-4
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Innermost stable circular orbit of spinning particle in charged spinning black hole background -
Abstract
- In this paper we investigate the innermost stable circular orbit (ISCO) for a classical spinning test particle in the background of Kerr-Newman black hole. It is shown that the orbit of the spinning particle is related to the spin of the test particle. The motion of the spinning test particle will be superluminal if its spin is too large. We give an additional condition by considering the superluminal constraint for the ISCO in the black hole backgrounds. We obtain numerically the relations between the ISCO and the properties of the black holes and the test particle. It is found that the radius of the ISCO for a spinning test particle is smaller than that of a non-spinning test particle in the black hole backgrounds.
Yu-Peng Zhang, Shao-Wen Wei, Wen-Di Guo, Tao-Tao Sui, Yu-Xiao Liu Journal reference: Phys. Rev. D 97, 084056 (2018) [pdf] DOI: 10.1103/PhysRevD.97.084056
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Localization of gravitino field on branes -
Abstract
- In this paper, we investigate the localization of a bulk gravitino field on the scalar-tensor branes and compare the result with that in the Randall-Sundrum-1 (RS1) model. The coupled chiral equations for the Kaluza-Klein (KK) modes of the gravitino field are obtained by fixing the gauge $\Psi_5=0$ and using the chiral KK decompositions. It is shown that, in the RS1 model for the left- and right-handed zero modes of the gravitino field, only one of them can be localized near one brane. For the massive modes, both chiral modes survive and the lower KK modes are localized near the IR brane from the four-dimensional physical coordinate point of view. However, for the scalar-tensor brane model, the localization of the gravitino chiral zero modes depends on the coupling parameter $\lambda$, and they will be not localized around anyone brane within a certain range of the parameter $\lambda$, which is quite different from the RS1 model. Furthermore, we also give the corresponding mass spectra of the massive KK gravitinos in the scalar-tensor model.
Yun-Zhi Du, Li Zhao, Xiang-Nan Zhou, Yi Zhong, Yu-Xiao Liu Journal reference: Annals Phys. 388 (2018) 69-88 [pdf] DOI: 10.1016/j.aop.2017.10.021
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The Effect of Massive Neutrinos on the Position of Cold Dark Matter Halo: Revealed via the Delaunay Triangulation Void -
Abstract
- Using cosmological $N$-body simulation which coevolves cold dark matter (CDM) and neutrino particles, we discover the local effect of massive neutrinos on the spatial distribution of CDM halos, reflected on properties of the Delaunay Triangulation (DT) voids. Smaller voids are generally in regions with higher neutrino abundance and so their surrounding halos are impacted by a stronger neutrino free streaming. This makes the voids larger (surrounding halos being washed outward the void center). On the contrary, larger voids are generally in regions with lower neutrino abundance and so their surrounding halos are less impacted by neutrino free streaming, making the voids smaller (surrounding halos being squeezed toward the void center). This characteristic change of the spatial distribution of the halos suppresses the 2-point correlation function of halos on scales $\sim$ 1 Mpc$/h$ and significantly skews the number function of the DT voids, which serve as measurable neutrino effects in current or future galaxy surveys.
Jian Qin, Yu Liang, Cheng Zhao, Hao-Ran Yu, Yu Liu, Tong-Jie Zhang Journal reference: The Astrophysical Journal, Volume 862, Number 1 (2018) [pdf] DOI: 10.3847/1538-4357/aacbd2
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Linearization of a warped
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Abstract
- Without using conformal transformation, a simple type of five-dimensional $f(R)-$brane model is linearized directly in its higher-order frame. In this paper, the linearization is conducted in the equation of motion approach. We first derive all the linear perturbation equations without specifying a gauge condition. Then by taking the curvature gauge we derive the master equations of the linear perturbations. We show that these equations are equivalent to those obtained in the quadratical action approach [Phys. Rev. D 95 (2017) 104060], except the vector sector, in which a constraint equation can be obtained in the equation of motion approach but absent in the quadratical action approach. Our work sets an example on how to linearize higher-order theories without using conformal transformation, and might be useful for studying more complicated theories.
Yuan Zhong, Ke Yang, Yu-Xiao Liu Journal reference: Phys. Rev. D 97, 044032 (2018) [pdf] DOI: 10.1103/PhysRevD.97.044032
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Temperature effect on shear and bulk viscosities of QCD matter -
Abstract
- We investigate the temperature dependence of the shear and bulk viscosities and their ratios to the entropy density via a continuum QCD approach. We calculate the pion mass and decay constant in the framework of Dyson-Schwinger equations of QCD and the pion thermal width by combining with Roy equations. We obtain then the variation behaviors of the viscosities, especially a novel feature of the bulk viscosity, with respect to temperature.
Fei Gao, Yu-xin Liu Journal reference: Phys. Rev. D 97, 056011 (2018) [pdf] DOI: 10.1103/PhysRevD.97.056011
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Cosmological twinlike models with multi scalar fields -
Abstract
- We consider cosmological models driven by several canonical or noncanonical scalar fields. We show how the superpotential method enables one to construct twinlike models for a particular canonical model from some noncanonical ones. We conclude that it is possible to construct twinlike models for multi-field cosmological models, even when the spatial curvature is nonzero. This work extends the discussions of [D. Bazeia and J. D. Dantas, Phys. Rev. D, 85 (2012) 067303] to cases with multi scalar fields and with non-vanished spatial curvature, by using a different superpotential method.
Yuan Zhong, Chun-E Fu, Yu-Xiao Liu Journal reference: Sci. China Phys. Mech. Astron. (2018) 61: 90411 [pdf] DOI: 10.1007/s11433-018-9194-7
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Proton Isovector Helicity Distribution on the Lattice at Physical Pion Mass -
Abstract
- 2017
-
Optimal occlusion uniformly partitions red blood cells fluxes within a
microvascular network -
Abstract
- In animals, gas exchange between blood and tissues occurs in narrow vessels, whose diameter is comparable to that of a red blood cell. Red blood cells must deform to squeeze through these narrow vessels, transiently blocking or occluding the vessels they pass through. Although the dynamics of vessel occlusion have been studied extensively, it remains an open question why microvessels need to be so narrow. We study occlusive dynamics within a model microvascular network: the embryonic zebrafish trunk. We show that pressure feedbacks created when red blood cells enter the finest vessels of the trunk act together to uniformly partition red blood cells through the microvasculature. Using mathematical models as well as direct observation, we show that these occlusive feedbacks are tuned throughout the trunk network to prevent the vessels closest to the heart from short-circuiting the network. Thus occlusion is linked with another open question of microvascular function: how are red blood cells delivered at the same rate to each micro-vessel? Our analysis shows that tuning of occlusive feedbacks increase the total dissipation within the network by a factor of 11, showing that uniformity of flows rather than minimization of transport costs may be prioritized by the microvascular network.
Shyr-Shea Chang, Shenyinying Tu, Kyung In Baek, Andrew Pietersen, Yu-Hsiu Liu, Van Savage, Sheng-Ping L. Hwang, Tzung K. Hsiai, Marcus Roper [pdf]
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Critical behavior of the van der Waals bonded ferromagnet
-
Abstract
- The critical properties of the single-crystalline van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$ were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. The Fe$_{3-x}$GeTe$_2$ single crystals grown by self-flux method with Fe deficiency $x \approx 0.36$ exhibit bulk FM ordering below $T_c = 152$ K. The M\"{o}ssbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents $\beta = 0.372(4)$ with a critical temperature $T_c = 151.25(5)$ K and $\gamma = 1.265(15)$ with $T_c = 151.17(12)$ K are obtained by the Kouvel-Fisher method whereas $\delta = 4.50(1)$ is obtained by a critical isotherm analysis at $T_c = 151$ K. These critical exponents obey the Widom scaling relation $\delta = 1+\gamma/\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m = f_\pm(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg ($d = 3, n = 3$) spins coupled with the attractive long-range interactions between spins that decay as $J(r)\approx r^{-(3+\sigma)}$ with $\sigma=1.89$.
Yu Liu, V. N. Ivanovski, C. Petrovic Journal reference: Physical Review B 96, 144429 (2017) [pdf] DOI: 10.1103/PhysRevB.96.144429
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Critical behavior of the quasi-two-dimensional weak itinerant ferromagnet trigonal chromium telluride
-
Abstract
- The critical properties of flux-grown single-crystalline quasi-two-dimensional weak itinerant ferromagnet Cr$_{0.62}$Te were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. Critical exponents $\beta = 0.315(7)$ with a critical temperature $T_c = 230.6(3)$ K and $\gamma = 1.81(2)$ with $T_c = 229.1(1)$ K are obtained by the Kouvel-Fisher method whereas $\delta = 6.35(4)$ is obtained by a critical isotherm analysis at $T_c = 230$ K. With these obtained exponents, the magnetization-field-temperature curves collapse into two independent curves following a single scaling equation $M|\frac{T-T_c}{T_c}|^{-\beta} = f_\pm(H|\frac{T-T_c}{T_c}|^{-\beta\delta})$ around $T_c$, suggesting the reliability of the obtained exponents. Additionally, the determined exponents of Cr$_{0.62}$Te exhibit an Ising-like behavior with a change from short-range order to long-range order in the nature of magnetic interaction and with an extension from 2D to 3D on cooling through $T_c$.
Yu Liu, C. Petrovic Journal reference: Physical Review B 96, 134410 (2017) [pdf] DOI: 10.1103/PhysRevB.96.134410
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Complexity growth rates for AdS black holes in massive gravity and f(R) gravity -
Abstract
- The "complexity = action" duality states that the quantum complexity is equal to the action of the stationary AdS black holes within the Wheeler-DeWitt patch at late time approximation. We compute the action growth rates of the neutral and charged black holes in massive gravity and the neutral, charged and Kerr-Newman black holes in $f(R)$ gravity to test this conjecture. Besides, we investigate the effects of the massive graviton terms, higher derivative terms and the topology of the black hole horizon on the complexity growth rate.
Wen-Di Guo, Shao-Wen Wei, Yan-Yan Li, Yu-Xiao Liu Journal reference: Eur.Phys.J. C77 (2017) 904 [pdf] DOI: 10.1140/epjc/s10052-017-5466-5
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A Quasi-periodic Fast-propagating Magnetosonic Wave Associated with the Eruption of a Magnetic Flux Rope -
Abstract
- Using high temporal and high spatial resolution observations taken by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, we present the detailed observational analysis of a high quality quasi-periodic fast- propagating (QFP) magnetosonic wave that was associated with the eruption of a magnetic flux rope and a GOES C5.0 flare. For the first time, we find that the QFP wave lasted during the entire flare lifetime rather than only the rising phase of the accompanying flare as reported in previous studies. In addition, the propagation of the different parts of the wave train showed different kinematics and morphologies. For the southern (northern) part, the speed, duration, intensity variation are about 875 +/- 29 (1485 +/- 233) km/s, 45 (60) minutes, and 4% (2%), and the pronounced periods of them are 106 +/- 12 and 160 +/- 18 (75 +/- 10 and 120 +/- 16) seconds, respectively. It is interesting that the northern part of the wave train showed obvious refraction effect when they pass through a region of strong magnetic field. Periodicity analysis result indicates that all the periods of the QFP wave can be found in the period spectrum of the accompanying flare, suggesting their common physical origin. We propose that the quasi-periodic nonlinear magnetohydrodynamics process in the magnetic reconnection that produces the accompanying flare should be important for exciting of QFP wave, and the different magnetic distribution along different paths can account for the different speeds and morphology evolution of the wave fronts.
Yuandeng Shen, Yu Liu, Tengfei Song, Zhanjun Tian [pdf] DOI: 10.3847/1538-4357/aaa3ff 1712.09045v1 [pdf]
-
Region-based Quality Estimation Network for Large-scale Person
Re-identification -
Abstract
- One of the major restrictions on the performance of video-based person re-id is partial noise caused by occlusion, blur and illumination. Since different spatial regions of a single frame have various quality, and the quality of the same region also varies across frames in a tracklet, a good way to address the problem is to effectively aggregate complementary information from all frames in a sequence, using better regions from other frames to compensate the influence of an image region with poor quality. To achieve this, we propose a novel Region-based Quality Estimation Network (RQEN), in which an ingenious training mechanism enables the effective learning to extract the complementary region-based information between different frames. Compared with other feature extraction methods, we achieved comparable results of 92.4%, 76.1% and 77.83% on the PRID 2011, iLIDS-VID and MARS, respectively. In addition, to alleviate the lack of clean large-scale person re-id datasets for the community, this paper also contributes a new high-quality dataset, named "Labeled Pedestrian in the Wild (LPW)" which contains 7,694 tracklets with over 590,000 images. Despite its relatively large scale, the annotations also possess high cleanliness. Moreover, it's more challenging in the following aspects: the age of characters varies from childhood to elderhood; the postures of people are diverse, including running and cycling in addition to the normal walking state.
Guanglu Song, Biao Leng, Yu Liu, Congrui Hetang, Shaofan Cai [pdf]
-
Crack detection in beam structures with a novel Laplace based Wavelet
Finite Element method -
Abstract
- Beam structure is one of the most widely used structures in mechanical engineering and civil engineering. Ultrasonic guided wave based crack identification is one of the most important and accepted approaches applied to detect unseen small flaws in structures. Numerical simulations of ultrasonic guided wave propagation have caught more and more attention due to the fast development of hardware and software in the last few years. From all the numerical simulation methods, wavelet based finite element method has been proved to be one of the most efficient methods due to its better spatial resolution, which means it needs fewer elements to get the same accuracy and it can improve the calculation cost significantly. However, it needs a very small time interval. Laplace transform can easily convert the time domain into a frequency domain and then revert it back to a time domain. Laplace transform has thus the advantage of finding better results with a very large time interval. which can save a lot of time cost. This paper will present an innovative method combining Laplace transform and the B-spline wavelet on interval (BSWI) finite element method. This novel method allows to get results with the same accuracy and with a significantly lower time cost, which would not only decrease the total number of elements in the structure but also increase the time integration interval. The numerical Laplace transform and BSWI finite element will be introduced. Moreover, this innovative method is applied to simulate the ultrasonic wave propagation in a beam structure in different materials. Numerical examples for crack identification in beam structures have been studied for verification.
Shuaifang Zhang, Dongsheng Li, Wei Shen, Xiwen Zhang, Yu Liu [pdf]
-
Device-to-Device Communications Enabled Energy Efficient Multicast Scheduling in mmWave Small Cells -
Abstract
- To keep pace with the rapid growth of mobile traffic demands, dense deployment of small cells in millimeter wave (mmWave) bands has become a promising candidate for next generation wireless communication systems. With a greatly increased data rate from huge bandwidth of mmWave communications, energy consumption should be mitigated for higher energy efficiency. Due to content popularity, many content-based mobile applications can be supported by the multicast service. mmWave communications exploit directional antennas to overcome high path loss, and concurrent transmissions can be enabled for better multicast service. On the other hand, device-to-device (D2D) communications in physical proximity should be exploited to improve multicast performance. In this paper, we propose an energy efficient multicast scheduling scheme, referred to as EMS, which utilizes both D2D communications and concurrent transmissions to achieve high energy efficiency. In EMS, a D2D path planning algorithm establishes multi-hop D2D transmission paths, and a concurrent scheduling algorithm allocates the links on the D2D paths into different pairings. Then the transmission power of links is adjusted by the power control algorithm. Furthermore, we theoretically analyze the roles of D2D communications and concurrent transmissions in reducing energy consumption. Extensive simulations under various system parameters demonstrate the superior performance of EMS in terms of energy consumption compared with the state-of-the-art schemes. Furthermore, we also investigate the choice of the interference threshold to optimize network performance.
Yong Niu, Yu Liu, Yong Li, Xinlei Chen, Zhangdui Zhong, Zhu Han Journal reference: IEEE Transactions on Communications, 2017 [pdf] DOI: 10.1109/TCOMM.2017.2773529
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The hierarchical Green’s function approach to the two-dimensional Hubbard model -
Abstract
- By introducing multipe-site correlation functions, we propose a hierarchical Green function approach, and apply it to study the characteristic properties of a 2D square lattice Hubbard model by solving the equation of motions of a one-particle Green function and related multipe-site correlation functions. Under a cut-off approximation and taking the Fourier representation of multipe-site correlation functions, we obtain an analytical expression of one-particle Green function with static correlation functions. Then we calculate the spectral density function of electrons, and obtain that besides two main peaks corresponding to the lower and upper Hubbard bands in the spectral density function, there emerge some novel states between these two main peaks, and the total spectral weight of these emerged states is proportional to the hole doping concentration . Meanwhile, there also emerge some collective modes related to possible charge/spin density wave and/or electronic pairing density wave ordering states. This calculation is completely consistent with the spectroscopy observations of the cuprate superconductors in normal states. On the other hand, the appearence of the static correlation functions in the one-particle Green function can be used to describe the intertwined orders observed in the normal state of the cuprate superconductors.
Yu-Liang Liu [pdf] DOI: 10.1142/S0217979218502582 1712.02002v1 [pdf]
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Wave analysis in one dimensional structures with a wavelet finite
element model and precise integration method -
Abstract
- Numerical simulation of ultrasonic wave propagation provides an efficient tool for crack identification in structures, while it requires a high resolution and expensive time calculation cost in both time integration and spatial discretization. Wavelet finite element model provides a highorder finite element model and gives a higher accuracy on spatial discretization, B-Spline wavelet interval (BSWI) has been proved to be one of the most commonly used wavelet finite element model with the advantage of getting the same accuracy but with fewer element so that the calculation cost is much lower than traditional finite element method and other high-order element methods. Precise Integration Method provides a higher resolution in time integration and has been proved to be a stable time integration method with a much lower cut-off error for same and even smaller time step. In this paper, a wavelet finite element model combined with precise integration method is presented for the numerical simulation of ultrasonic wave propagation and crack identification in 1D structures. Firstly, the wavelet finite element based on BSWI is constructed for rod and beam structures. Then Precise Integrated Method is introduced with application for the wave propagation in 1D structures. Finally, numerical examples of ultrasonic wave propagation in rod and beam structures are conducted for verification. Moreover, crack identification in both rod and beam structures are studied based on the new model.
Shuaifang Zhang, Dongdong He, Dongsheng Li, Zhifeng Zhang, Yu Liu, Wei Shen [pdf]
-
Baryon Acoustic Oscillation detections from the clustering of massive
halos and different density region tracers in TianNu simulation -
Abstract
- The Baryon Acoustic Oscillations (BAO) refer to the ripples of material density in the Universe. As the most direct density tracers in the universe, galaxies have been commonly used in studies of BAO peak detection. The spatial number density of galaxies, to a certain extent, reflects the distribution of the material density of our Universe. Using galaxies as matter tracers, we can construct more overlapping empty spheres (DT voids) than the matter tracers, via Delaunay Triangulation technique. We show that their radii excellently reflect the galaxy number density round them, and they can serve as reliable different density region tracers. Using the data from an unprecedented large-scale $N$-body simulation "TianNu", we conduct some fundamental statistical studies and clustering analysis of the DT voids. We discuss in detail the representative features of two-point correlation functions of different DT void populations. We show that the peak, the position of which corresponds to the average radius of data samples, is the most representative feature of the two-point correlation function of the DT voids. In addition, we also construct another voids, the disjoint voids, and investigate their some statistical properties and clustering properties. And we find that the occupied space of all disjoint voids accounts for about $45\%$ of the volume of the simulation box, regardless of the number density of mock galaxies. We also investigate the BAO detections based on different tracers, i.e. mock galaxies, low-density region tracers, and high-density region tracers respectively. Our results show that BAO intensities detected by low/high-density region tracers are enhanced significantly compared to the BAO detection by mock galaxies, for the mock galaxy catalogue with the number density of $7.52\times10^{-5}$ $h^3$ Mpc$^{-3}$.
Yu Liu, Yu Liang, Hao-Ran Yu, Cheng Zhao, Jian Qin, Tong-Jie Zhang [pdf]
-
Time-independent Green's Function of a Quantum Simple Harmonic
Oscillator System and Solutions with Additional Generic Delta-Function
Potentials -
Abstract
- The one-dimensional time-independent Green's function $G_0$ of a quantum simple harmonic oscillator system ($V_0(x)=m \omega^2 x^2/2$) can be obtained by solving the equation directly. It has a compact expression, which gives correct eigenvalues and eigenfunctions easily. The Green's function $G$ with an additional delta-function potential can be obtained readily. The same technics of solving the Green's function $G_0$ can be used to solve the eigenvalue problem of the simple harmonic oscillator with an generic delta-function potential at an arbitrary site, i.e. $V_1(x)\propto \delta(x-a)$. The Wronskians play an important and interesting role in the above studies. Furthermore, the approach can be easily generalized to solve the quantum system of a simple harmonic oscillator with two or more generic delta-function potentials. We give the solutions of the case with two additional delta-functions for illustration.
Chun-Khiang Chua, Yu-Tsai Liu, Gwo-Guang Wong [pdf]
-
Topological edge states and pumping in a chain of coupled
superconducting qubits -
Abstract
- Topological insulators have inspired the study with various quantum simulators. Exploiting the tunability of the qubit frequency and qubit-qubit coupling, we show that a superconducting qubit chain can simulate various topological band models. When the system is restricted to the single-spin excitation subspace, the Su-Schrieffer-Heeger (SSH) model can be equivalently simulated by alternating the coupling strength between neighboring qubits. The existence of topological edge states in this qubit chain is demonstrated in the quench dynamics after the first qubit is excited. This excitation propagates along the chain where the qubit-qubit coupling is homogeneous. In contrast, in our qubit chain, the spin-up state localizes at the first qubit and the rest qubits remain in the spin-down state. We further show that the spin-up state can be transported along the chain by modulating the coupling strengths and the qubit frequencies. This demonstrates adiabatic pumping based on the Rice-Mele model. Moreover, we also discuss possible ways to construct other topological models with different topological phenomena within the current technology of superconducting qubits.
Xiu Gu, Shu Chen, Yu-xi Liu [pdf]
-
Efficient single-photon frequency conversion in the microwave domain using superconducting quantum circuits -
Abstract
- We present an approach to achieve efficient single-photon frequency conversion in the microwave domain based on coherent control in superconducting quantum circuits, which consist of a driven artificial atom coupled to a semi-infinite transmission line. Using the full quantum-mechanical method, we analyze the single-photon scattering process in this system and find that single-photon frequency up- or down-conversion with efficiency close to unity can be achieved by adjusting the parameters of the control field applied to the artificial atom. We further show that our approach is experimentally feasible in currently available superconducting flux qubit circuits.
W. Z. Jia, Y. W. Wang, Yu-xi Liu Journal reference: Physical Review A 96, 053832 (2017) [pdf] DOI: 10.1103/PhysRevA.96.053832
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On a Small-scale EUV Wave: The Driving Mechanism and the Associated Oscillating Filament -
Abstract
- We present observations of a small-scale Extreme-ultraviolet (EUV) wave that was associated with a mini-filament eruption and a GOES B1.9 micro-fare in the quiet Sun region. The initiation of the event was due to the photospheric magnetic emergence and cancellation in the eruption source region, which first caused the ejection of a small plasma ejecta, then the ejecta impacted on a nearby mini-filament and thereby led to the filament's eruption and the associated fare. During the filament eruption, an EUV wave at a speed of 182 { 317 km/s was formed ahead of an expanding coronal loop, which propagated faster than the expanding loop and showed obvious deceleration and refection during the propagation. In addition, the EUV wave further resulted in the transverse oscillation of a remote filament whose period and damping time are 15 and 60 minutes, respectively. Based on the observational results, we propose that the small-scale EUV wave should be a fast-mode magnetosonic wave that was driven by the the expanding coronal loop. Moreover, with the application of filament seismology, it is estimated that the radial magnetic field strength is about 7 Gauss. The observations also suggest that small-scale EUV waves associated with miniature solar eruptions share similar driving mechanism and observational characteristics with their large-scale counterparts.
Yuandeng Shen, Yu Liu, Zhanjun Tian, Zhining Qu Journal reference: The Astrophysical Journal, 2017, 851, 101 [pdf] DOI: 10.3847/1538-4357/aa9af0
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Rethinking Feature Discrimination and Polymerization for Large-scale
Recognition -
Abstract
- Feature matters. How to train a deep network to acquire discriminative features across categories and polymerized features within classes has always been at the core of many computer vision tasks, specially for large-scale recognition systems where test identities are unseen during training and the number of classes could be at million scale. In this paper, we address this problem based on the simple intuition that the cosine distance of features in high-dimensional space should be close enough within one class and far away across categories. To this end, we proposed the congenerous cosine (COCO) algorithm to simultaneously optimize the cosine similarity among data. It inherits the softmax property to make inter-class features discriminative as well as shares the idea of class centroid in metric learning. Unlike previous work where the center is a temporal, statistical variable within one mini-batch during training, the formulated centroid is responsible for clustering inner-class features to enforce them polymerized around the network truncus. COCO is bundled with discriminative training and learned end-to-end with stable convergence. Experiments on five benchmarks have been extensively conducted to verify the effectiveness of our approach on both small-scale classification task and large-scale human recognition problem.
Yu Liu, Hongyang Li, Xiaogang Wang [pdf]
-
Microwave photonics with superconducting quantum circuits -
Abstract
- In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.
Xiu Gu, Anton Frisk Kockum, Adam Miranowicz, Yu-xi Liu, Franco Nori Journal reference: Physics Reports 718-719, 1-102 (2017) [pdf] DOI: 10.1016/j.physrep.2017.10.002
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MLBench: How Good Are Machine Learning Clouds for Binary Classification
Tasks on Structured Data? -
Abstract
- We conduct an empirical study of machine learning functionalities provided by major cloud service providers, which we call machine learning clouds. Machine learning clouds hold the promise of hiding all the sophistication of running large-scale machine learning: Instead of specifying how to run a machine learning task, users only specify what machine learning task to run and the cloud figures out the rest. Raising the level of abstraction, however, rarely comes free - a performance penalty is possible. How good, then, are current machine learning clouds on real-world machine learning workloads? We study this question with a focus on binary classication problems. We present mlbench, a novel benchmark constructed by harvesting datasets from Kaggle competitions. We then compare the performance of the top winning code available from Kaggle with that of running machine learning clouds from both Azure and Amazon on mlbench. Our comparative study reveals the strength and weakness of existing machine learning clouds and points out potential future directions for improvement.
Yu Liu, Hantian Zhang, Luyuan Zeng, Wentao Wu, Ce Zhang [pdf]
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Tuning of few-electron states and optical absorption anisotropy in GaAs quantum rings -
Abstract
- The electronic and optical properties of a GaAs quantum ring (QR) with few electrons in the presence of the Rashba spin-orbit interaction (RSOI) and the Dresselhaus spin-orbit interaction (DSOI) have been investigated theoretically. Configuration interaction (CI) method is employed to calculate the eigenvalues and eigenstates of the multiple-electron QR accurately. Our numerical results demonstrate that the symmetry breaking induced by the RSOI and DSOI leads to an anisotropic distribution of multi-electron states. The Coulomb interaction offers additional modulation of the electron distribution and thus the optical absorption indices in the quantum rings. By tuning the magnetic/electric fields and/or electron numbers in a quantum ring, one can change its optical property significantly. Our theory provides a new way to control multi-electron states and optical properties of QR by hybrid modulations or by electrical means only.
Zhenhua Wu, Jian Li, J. Li, H. Yin, Yu Liu [pdf] DOI: 10.1039/C7CP05675C 1608.07034v2 [pdf]
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Heptavalent symmetric graphs with solvable stabilizers admitting
vertex-transitive non-abelian simple groups -
Abstract
- A graph $\Gamma$ is said to be symmetric if its automorphism group $\rm Aut(\Gamma)$ acts transitively on the arc set of $\Gamma$. In this paper, we show that if $\Gamma$ is a finite connected heptavalent symmetric graph with solvable stabilizer admitting a vertex-transitive non-abelian simple group $G$ of automorphisms, then either $G$ is normal in $\rm Aut(\Gamma)$, or $\rm Aut(\Gamma)$ contains a non-abelian simple normal subgroup $T$ such that $G\leq T$ and $(G,T)$ is explicitly given as one of $11$ possible exception pairs of non-abelian simple groups. Furthermore, if $G$ is regular on the vertex set of $\Gamma$ then the exception pair $(G,T)$ is one of $7$ possible pairs, and if $G$ is arc-transitive then the exception pair $(G,T)=(A_{17},A_{18})$ or $(A_{35},A_{36})$.
Jia-Li Du, Yan-Quan Feng, Yu-Qin Liu [pdf]
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Towards diluted magnetism in TaAs -
Abstract
- Magnetism in Weyl semimetals is desired to investigate the interaction between the magnetic moments and Weyl fermions, e.g. to explore anomalous quantum Hall phenomena. Here we demonstrate that proton irradiation is an effective tool to induce ferromagnetism in the Weyl semimetal TaAs. The intrinsic magnetism is observed with a transition temperature above room temperature. The magnetic moments from d states are found to be localized around Ta atoms. Further, the first-principles calculations indicate that the d states localized on the nearest-neighbor Ta atoms of As vacancy sites are responsible for the observed magnetic moments and the long-ranged magnetic order. The results show the feasibility of inducing ferromagnetism in Weyl semimetals so that they may facilitate the applications of this material in spintronics.
Yu Liu, Zhilin Li, Liwei Guo, Xiaolong Chen, Ye Yuan, Chi Xu, René Hübner, Shavkat Akhmadaliev, Arkady V. Krasheninnikov, Alpha T. N'Diaye, Elke Arenholz, Manfred Helm, Shengqiang Zhou Journal reference: Phys. Rev. Materials 1, 044203 (2017) [pdf] DOI: 10.1103/PhysRevMaterials.1.044203
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Scalar perturbations of Eddington-inspired Born-Infeld braneworld -
Abstract
- We consider the scalar perturbations of Eddington-inspired Born-Infeld braneworld models in this paper. The dynamical equation for the physical propagating degree of freedom $\xi(x^\mu,y)$ is achieved by using the Arnowitt-Deser-Misner decomposition method: $F_1(y) {\partial_y^2 \xi} + F_2(y){\partial_y \xi } + {\partial^{\mu}\partial_{\mu}}\xi=0$. We conclude that the solution is tachyonic-free and stable under scalar perturbations for $F_1(y)>0$ but unstable for $F_1(y)< 0$. The stability of a known analytic domain wall solution with the warp factor given by $a(y)= \text{sech}^{\frac{3}{{4p}}}(ky)$ is analyzed and it is shown that only the solution for $0<p<\sqrt{8/3}$ is="is" stable.
Ke Yang, Yu-Xiao Liu, Bin Guo, Xiao-Long Du Journal reference: Phys. Rev. D 96, 064039 (2017) [pdf] DOI: 10.1103/PhysRevD.96.064039
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Numerical approximation of random periodic solutions of stochastic differential equations -
Abstract
- In this paper, we discuss the numerical approximation of random periodic solutions (r.p.s.) of stochastic differential equations (SDEs) with multiplicative noise. We prove the existence of the random periodic solution as the limit of the pull-back flow when the starting time tends to $-\infty$ along the multiple integrals of the period. As the random periodic solution is not explicitly constructible, it is useful to study the numerical approximation. We discretise the SDE using the Euler-Maruyama scheme and moldiflied Milstein scheme. Subsequently we obtain the existence of the random periodic solution as the limit of the pull-back of the discretised SDE. We prove that the latter is an approximated random periodic solution with an error to the exact one at the rate of $\sqrt {\Delta t}$ in the mean-square sense in Euler-Maruyama method and $\Delta t$ in the Milstein method. We also obtain the weak convergence result for the approximation of the periodic measure.
Chunrong Feng, Yu Liu, Huaizhong Zhao Journal reference: Z. Angew. Math. Phys. (2017) 68:119 [pdf] DOI: 10.1007/s00033-017-0868-7
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Real-time detection and resolution of atom bumping in crystallographic models -
Abstract
- A basic principle in crystal structure determination is that there should be proper distances between adjacent atoms. Therefore, detection of atom bumping is of fundamental significance in structure determination, especially in the direct space method where crystallographic models are just randomly generated. Presented in this article is an algorithm that detects atom bonding in a unit cell based on the sweep and prune algorithm of axis-aligned bounding boxes (AABBs) and running in $O(n \log n)$ time bound, where $n$ is the total number of atoms in the unit cell. This algorithm only needs the positions of individual atoms in the unit cell and does not require any prior knowledge of existing bonds, and is thus suitable for modelling of inorganic crystals where the bonding relations are often unknown a priori. With this algorithm, computation routines requiring bonding information, eg. anti-bumping and computation of coordination numbers and valences, can be performed efficiently. As an example application, an evaluation function for atom bumping is proposed, which can be used for real-time elimination of crystallographic models with unreasonably short bonds during the procedure of global optimisation in the direct space method.
Yu Liu Journal reference: Acta. Cryst. 2017, A73(5), 414-422 [pdf] DOI: 10.1107/S2053273317011548
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Large magnetoresistance in the type-II Weyl semimetal
-
Abstract
- We report magnetotransport study on type-II Weyl semimetal WP$_2$ single crystals. Magnetoresistance (MR) exhibits a nonsaturating $H^{n}$ field dependence (14,300\% at 2 K and 9 T) whereas systematic violation of Kohler's rule was observed. Quantum oscillations reveal a complex multiband electronic structure. The cyclotron effective mass close to the mass of free electron m$_e$ was observed in quantum oscillations along $b$-axis, while reduced effective mass of about 0.5$m_e$ was observed in $a$-axis quantum oscillations, suggesting Fermi surface anisotropy. Temperature dependence of the resistivity shows a large upturn that cannot be explained by the multi-band magnetoresistance of conventional metals. Even though crystal structure of WP$_{2}$ is not layered as in transition metal dichalcogenides, quantum oscillations suggest partial two-dimensional character.
Aifeng Wang, D. Graf, Yu Liu, Qianheng Du, Jiabao Zheng, Hechang Lei, C. Petrovic Journal reference: Phys. Rev. B 96, 121107 (2017) [pdf] DOI: 10.1103/PhysRevB.96.121107
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The radiation mechanism of fast radio bursts -
Abstract
- Fast radio bursts are radio transients observed mainly around 1.5 GHz. Their peak frequency decreases at a rate of 100 ~ 500 MHz/s and some of them have a broader pulse with an exponentially decaying tail. Common assumptions for fast radio bursts include a dispersion effect resulting in the peak frequency drifting and a scattering effect resulting in pulse broadening. These assumptions attribute the abnormally large dispersion measure and scattering measure to the environmental medium of the host galaxy. Here we show that the radiation of fast radio bursts can be explained as an undulator radiation and the large dispersion measure can be due to a motion effect mainly from the rotation of the source which is probably variable stars. In our scenario, the pulse broadening is near-field effects and the pulse itself represents a Fresnel diffraction pattern sweeping the observer. Our work is the first analysis of properties of fast radio bursts in the context of a special mechanism of the radiation instead of a special propagation environment of the radiation.
Qiwu Song, Yu Huang, Hengqiang Feng, Lei Yang, Tuanhui Zhou, Qingyu Luo, Tengfei Song, Xuefei Zhang, Yu Liu, Guangli Huang [pdf]
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A universal constraint on the infrared behavior of the ghost propagator in QCD -
Abstract
- With proposing a unified description of the fields variation at the level of generating functional, we obtain a new identity for the quark-gluon interaction vertex based on gauge symmetry, which is similar to the Slavnov-Taylor Identities(STIs) based on the Becchi-Rouet-Stora-Tyutin transformation. With these identities, we find that in Landau gauge, the dressing function of the ghost propagator approaches to a constant as its momentum goes to zero, which provides a strong constraint on the infrared behaviour of ghost propagator.
Fei Gao, Can Tang, Yu-xin Liu [pdf] DOI: 10.1016/j.physletb.2017.09.069 1708.07031v1 [pdf]
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Nonreciprocal single-photon frequency converter via multiple semi-infinite coupled-resonator waveguides -
Abstract
- We propose to construct a nonreciprocal single-photon frequency converter via multiple semi-infinite coupled-resonator waveguides (CRWs). We first demonstrate that the frequency of a single photon can be converted nonreciprocally through two CRWs, which are coupled indirectly by optomechanical interactions with two nondegenerate mechanical modes. Based on such nonreciprocity, two different single-photon circulators are proposed in the T-shaped waveguides consisting of three semi-infinite CRWs, which are coupled in pairwise by optomechanical interactions. One circulator is proposed by using two nondegenerate mechanical modes and the other one is proposed by using three nondegenerate mechanical modes. Nonreciprocal single-photon frequency conversion is induced by breaking the time-reversal symmetry, and the optimal conditions for nonreciprocal frequency conversion are obtained. These proposals can be used to realize nonreciprocal frequency conversion of single photons in any two distinctive waveguides with different frequencies and they can allow for dynamic control of the direction of frequency conversion by tuning the phases of external driving lasers, which may have versatile applications in hybrid quantum networks.
Xun-Wei Xu, Ai-Xi Chen, Yong Li, Yu-xi Liu Journal reference: Phys. Rev. A 96, 053853 (2017) [pdf] DOI: 10.1103/PhysRevA.96.053853
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Introduction to Extra Dimensions and Thick Braneworlds -
Abstract
- In this review, we give a brief introduction on the aspects of some extra dimension models and the five-dimensional thick brane models in extended theories of gravity. First, we briefly introduce the Kaluza-Klein theory, the domain wall model, the large extra dimension model, and the warped extra dimension models. Then some thick brane solutions in extended theories of gravity are reviewed. Finally, localization of bulk matter fields on thick branes is discussed.
Yu-Xiao Liu [pdf]
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Possible Beyond the Standard Model Physics Motivated by Muonic Puzzles -
Abstract
- Recent measurements of the proton radius using the Lamb shift in muonic hydrogen are troublingly discrepant with values extracted from hydrogen spectroscopy and electron-proton scattering experiments. This discrepancy, which differs by more than five standard deviations, may be a signal of new physics caused by a violation of lepton universality. Another candidate for a new physics signal is the muon anomalous magnetic moment. The measurement at BNL differs from the standard model prediction by at least three standard deviations. Motivated by these two puzzles, first we use polarized lepton-nucleon elastic scattering to search for a new scalar boson, and furthermore we suggest new measurements of the nucleon form factors. Next, we display a method to analyze the beam dump experiments without using approximation on phase space, and we use it to constrain all possible new spin-0 and spin-1 particles, and a variety of other measurements to study the possibility of the new physics. Finally, assuming a new scalar boson can solve the two puzzles simultaneously, we present a general model-independent analysis and constrain the existence of the new physics.
Yu-Sheng Liu [pdf]
-
Successive Two-sided Loop Jets Caused by Magnetic Reconnection between Two Adjacent Filamentary Threads -
Abstract
- We present observational analysis of two successive two-sided loop jets observed by the ground-based New Vacuum Solar Telescope (NVST) and the space-borne Solar Dynamics Observatory ( SDO). The two successive two-sided loop jets manifested similar evolution process and both were associated with the interaction of two small-scale adjacent filamentary threads, magnetic emerging and cancellation processes at the jet's source region. High temporal and high spatial resolution observations reveal that the two adjacent ends of the two filamentary threads are rooted in opposite magnetic polarities within the source region. The two threads approached to each other, and then an obvious brightening patch is observed at the interaction position. Subsequently, a pair of hot plasma ejections are observed heading to opposite directions along the paths of the two filamentary threads, and with a typical speed of two-sided loop jets of the order 150 km/s. Close to the end of the second jet, we report the formation of a bright hot loop structure at the source region, which suggests the formation of new loops during the interaction. Based on the observational results, we propose that the observed two-sided loop jets are caused by the magnetic reconnection between the two adjacent filamentary threads, largely different from the previous scenario that a two-sided loop jet is generated by magnetic reconnection between an emerging bipole and the overlying horizontal magnetic fields.
Zhanjun Tian, Yu Liu, Yuandeng Shen, Abouazza Elmhamdi, Jiangtao Su, Ying D. Liu, Ayman. S. Kordi Journal reference: The Astrophysical Journal, 2017, 845, 94 [pdf] DOI: 10.3847/1538-4357/aa8095
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Vacuum induced transparency and photon number resolved Autler-Townes
splitting in a three-level system -
Abstract
- We study the absorption spectrum of a probe field by a {\Lambda}-type three-level system, which is coupled to a quantized control field through the two upper energy levels. The probe field is applied to the ground and the second excited states. When the quantized control field is in vacuum, we derive a threshold condition to discern vacuum induced transparency (VIT) and vacuum induced Autler- Townes splitting (ATS). We also find that the parameter change from VIT to vacuum induced ATS is very similar to that from broken PT symmetry to PT symmetry. Moreover, we find the photon number resolved spectrum in the parameter regime of vacuum induced ATS when the mean photon number of the quantized control field is changed from zero (vacuum) to a finite number. However, there is no photon number resolved spectrum in the parameter regime of VIT even that the quantized control field contains the finite number of photons. Finally, we further discuss possible experimental realization.
Jiang-Hao Ding, Sai-nan Huai, Hou Ian, Yu-xi Liu [pdf]
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Energy-localization-enhanced ground-state cooling of a mechanical resonator from room temperature in optomechanics using a gain cavity -
Abstract
- When a gain system is coupled to a loss system, the energy usually flows from the gain system to the loss one. We here present a counterintuitive theory for the ground-state cooling of the mechanical resonator in optomechanical system via a gain cavity. The energy flows first from the mechanical resonator into the loss cavity, then into the gain cavity, and finally localizes there. The energy localization in the gain cavity dramatically enhances the cooling rate of the mechanical resonator. Moreover, we show that unconventional optical spring effect, e.g., giant frequency shift and optically induced damping of the mechanical resonator, can be realized. Those feature a pre-cooling free ground-state cooling, i.e., the mechanical resonator in thermal excitation at room temperature can directly be cooled to its ground state. This cooling approach has the potential application for fundamental tests of quantum physics without complicated cryogenic setups.
Yu-Long Liu, Yu-xi Liu [pdf] DOI: 10.1103/PhysRevA.96.023812 1609.02722v2 [pdf]
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Topological phase in 1D topological Kondo insulator: Z2 topological insulator, Haldane-like phase and Kondo breakdown -
Abstract
- We have simulated a half-filled $1D$ $p$-wave periodic Anderson model with numerically exact projector quantum Monte Carlo technique, and the system is indeed located in the Haldane-like state as detected in previous works on the $p$-wave Kondo lattice model, though the soluble non-interacting limit corresponds to the conventional $Z_{2}$ topological insulator. The site-resolved magnetization in an open boundary system and strange correlator for the periodic boundary have been used to identify the mentioned topological states. Interestingly, the edge magnetization in the Haldane-like state is not saturated to unit magnetic moment due to the intrinsic charge fluctuation in our periodic Anderson-like model, which is beyond the description of the Kondo lattice-like model in existing literature. The finding here underlies the correlation driven topological state in this prototypical interacting topological state of matter and naive use of non-interacting picture should be taken care. Moreover, no trace of the surface Kondo breakdown at zero temperature is observed and it is suspected that frustration-like interaction may be crucial in inducing such radical destruction of Kondo screening. The findings here may be relevant to our understanding of interacting topological materials like topological Kondo insulator candidate SmB$_{6}$.
Yin Zhong, Yu Liu, Hong-Gang Luo Journal reference: Eur. Phys. J. B (2017) 90: 147 [pdf] DOI: 10.1140/epjb/e2017-80102-0
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Analyzing the Largest Spectroscopic Data Set of Hydrogen-poor Super-luminous Supernovae -
Abstract
- Super-luminous supernovae (SLSNe) are tremendously luminous explosions whose power sources and progenitors are highly debated. Broad-lined SNe Ic (SNe Ic-bl) are the only type of SNe that are connected with long-duration gamma ray bursts (GRBs). Studying the spectral similarity and difference between the populations of hydrogen-poor SLSNe (SLSNe Ic) and of hydrogen-poor stripped-envelope core-collapse SNe, in particular SNe Ic and SNe Ic-bl, can provide crucial observations to test predictions of theories based on various power source models and progenitor models. In this paper, we collected all of the published optical spectra of 32 SLSNe Ic, 21 SNe Ic-bl, as well as 17 SNe Ic, quantified their spectral features, constructed average spectra, and compared them in a systematic way using new tools we have developed. We find that SLSNe Ic and SNe Ic-bl, including those connected with GRBs, have comparable widths for their spectral features and average absorption velocities at all phases. Thus, our findings strengthen the connection between SLSNe Ic and GRBs. In particular, SLSNe Ic have average \FeII absorption velocities of $-15,000 +/- 2,600$ \kms~at 10 days after peak, which are higher than those of SNe Ic by $\sim7,000$ \kms~on average. SLSNe Ic also have significantly broader \FeII lines than SNe Ic. Moreover, we find that such high absorption and width velocities of SLSNe Ic may be hard to explain by the interaction model, and none of 13 SLSNe Ic with measured absorption velocities spanning over 10 days has a convincing flat velocity-evolution, which is inconsistent with the magnetar model in one dimension. Lastly, we compare SN 2011kl, the first SN connected with an ultra-long GRB, with the mean spectrum of SLSNe Ic and of SNe Ic-bl.
Yu-Qian Liu, Maryam Modjaz, Federica B. Bianco [pdf] DOI: 10.3847/1538-4357/aa7f74 1612.07321v2 [pdf]
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Scale-Aware Face Detection -
Abstract
- Convolutional neural network (CNN) based face detectors are inefficient in handling faces of diverse scales. They rely on either fitting a large single model to faces across a large scale range or multi-scale testing. Both are computationally expensive. We propose Scale-aware Face Detector (SAFD) to handle scale explicitly using CNN, and achieve better performance with less computation cost. Prior to detection, an efficient CNN predicts the scale distribution histogram of the faces. Then the scale histogram guides the zoom-in and zoom-out of the image. Since the faces will be approximately in uniform scale after zoom, they can be detected accurately even with much smaller CNN. Actually, more than 99% of the faces in AFW can be covered with less than two zooms per image. Extensive experiments on FDDB, MALF and AFW show advantages of SAFD.
Zekun Hao, Yu Liu, Hongwei Qin, Junjie Yan, Xiu Li, Xiaolin Hu [pdf]
-
Magnetotransport properties of
-
Abstract
- We report magnetotransport and de Haas-van Alphen (dHvA) effect studies on MoP$_2$ single crystals, predicted to be type-2 Weyl semimetal with four pairs of robust Weyl points located below the Fermi level and long Fermi arcs. The temperature dependence of resistivity shows a peak before saturation, which does not move with magnetic field. Large nonsaturating magnetoresistance (MR) was observed, and the field dependence of MR exhibits a crossover from semicalssical weak-field $B^2$ dependence to the high-field linear-field dependence, indicating the presence of Dirac linear energy dispersion. In addition, systematic violation of Kohler's rule was observed, consistent with multiband electronic transport. Strong spin-orbit coupling (SOC) splitting has an effect on dHvA measurements whereas the angular-dependent dHvA orbit frequencies agree well with the calculated Fermi surface. The cyclotron effective mass $\sim$ 1.6$m_e$ indicates the bands might be trivial, possibly since the Weyl points are located below the Fermi level. Interestingly, quasi-two dimensional(2D) band structure is observed even though the crystal structure of MoP$_2$ is not layered.
Aifeng Wang, D. Graf, Yu Liu, C. Petrovic Journal reference: Phys. Rev. B 96, 195107 (2017) [pdf] DOI: 10.1103/PhysRevB.96.195107
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Critical behavior of quasi-two-dimensional semiconducting ferromagnet
-
Abstract
- The critical properties of the single-crystalline semiconducting ferromagnet CrGeTe$_3$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta = 0.200\pm0.003$ with critical temperature $T_c = 62.65\pm0.07$ K and $\gamma = 1.28\pm0.03$ with $T_c = 62.75\pm0.06$ K are obtained by the Kouvel-Fisher method whereas $\delta = 7.96\pm0.01$ is obtained by the critical isotherm analysis at $T_c = 62.7$ K. These critical exponents obey the Widom scaling relation $\delta = 1+\gamma/\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m = f_\pm(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The determined exponents match well with those calculated from the results of renormalization group approach for a two-dimensional Ising system coupled with long-range interaction between spins decaying as $J(r)\approx r^{-(d+\sigma)}$ with $\sigma=1.52$.
Yu Liu, C. Petrovic Journal reference: Phys. Rev. B 96, 054406 (2017) [pdf] DOI: 10.1103/PhysRevB.96.054406
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Linearization of a warped
-
Abstract
- The linearization of a type of $f(R)$ gravity is studied directly in the higher-order frame for an arbitrary five-dimensional warped space-time background. The quadratic actions of the normal modes of the scalar, vector, and tensor perturbations are derived by taking the curvature gauge, under which the linear perturbation of the scalar curvature is zero, and all the perturbation equations reduce to second order. By comparing our results to those obtained in the Einstein frame, we find that the quadratic actions of the normal modes are equivalent for these two frames.
Yuan Zhong, Yu-Xiao Liu Journal reference: Phys. Rev. D 95, 104060 (2017) [pdf] DOI: 10.1103/PhysRevD.95.104060
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Hidden-charm Pentaquark Production at
-
Abstract
- We study one possible production mechanism for hidden charm pentaquark in e+e- collision, where it is produced via a color-octet c \bar c pair fragmentation. The pentaquark production at B factory energy is dominated by e+e- to c \bar c g to Pc + X. At Z^0 pole, for the pentaquark production, there are several partonic processes playing significant role. Our results show that it is possible to search for the direct pentaquark production signal at e+e- colliders, which is important to understand the properties of pentaquark.
Shi-Yuan Li, Yan-Rui Liu, Yu-Nan Liu, Zong-Guo Si, Xiao-Feng Zhang [pdf] DOI: 10.1088/0253-6102/69/3/291 1706.04765v1 [pdf]
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Four-fermion interactions and the chiral symmetry breaking in an external magnetic field -
Abstract
- We investigate the chiral symmetry and its spontaneous breaking at finite temperature and in an external magnetic field with four-fermion interactions of different channels. Quantum and thermal fluctuations are included within the functional renormalization group approach, and properties of the set of flow equations for different couplings, such as its fixed points, are discussed. It is found that external parameters, e.g. the temperature and the external magnetic field and so on, do not change the structure of the renormalization group flows for the couplings. The flow strength is found to be significantly dependent on the route and direction in the plane of couplings of different channels. Therefore, the critical temperature for the chiral phase transition shows a pronounced dependence on the direction as well. Given fixed initial ultraviolet couplings, the critical temperature increases with the increasing magnetic field, viz., the magnetic catalysis is observed with initial couplings fixed.
Wei-jie Fu, Yu-xin Liu Journal reference: Phys. Rev. D 96, 074019 (2017) [pdf] DOI: 10.1103/PhysRevD.96.074019
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Unsupervised Sequence Classification using Sequential Output Statistics -
Abstract
- We consider learning a sequence classifier without labeled data by using sequential output statistics. The problem is highly valuable since obtaining labels in training data is often costly, while the sequential output statistics (e.g., language models) could be obtained independently of input data and thus with low or no cost. To address the problem, we propose an unsupervised learning cost function and study its properties. We show that, compared to earlier works, it is less inclined to be stuck in trivial solutions and avoids the need for a strong generative model. Although it is harder to optimize in its functional form, a stochastic primal-dual gradient method is developed to effectively solve the problem. Experiment results on real-world datasets demonstrate that the new unsupervised learning method gives drastically lower errors than other baseline methods. Specifically, it reaches test errors about twice of those obtained by fully supervised learning.
Yu Liu, Jianshu Chen, Li Deng [pdf]
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Phononic Josephson oscillation and self-trapping with two-phonon exchange interaction -
Abstract
- We propose a bosonic Josephson junction (BJJ) in two nonlinear mechanical resonator coupled through two-phonon exchange interaction induced by quadratic optomechanical couplings. The nonlinear dynamic equations and effective Hamiltonian are derived to describe behaviors of the BJJ. We show that the BJJ can work in two different dynamical regimes: Josephson oscillation and macroscopic self-trapping. The system can transfer from one regime to the other one when the self-interaction and asymmetric parameters exceed their critical values. We predict that a transition from Josephson oscillation to macroscopic self-trapping can be induced by the phonon damping in the asymmetric BJJs. Our results opens up a way to demonstrate BJJ with two-phonon exchange interaction and can be applied to other systems, such as the optical and microwave systems.
Xun-Wei Xu, Ai-Xi Chen, Yu-xi Liu Journal reference: Phys. Rev. A 96, 023832 (2017) [pdf] DOI: 10.1103/PhysRevA.96.023832
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Critical phenomena and chemical potential of a charged AdS black hole -
Abstract
- We study the thermodynamics and the chemical potential for a five-dimensional charged AdS black hole by treating the cosmological constant as the number of colors $N$ in the boundary gauge theory and its conjugate quantity as the associated chemical potential $\mu$. It is found that there exists a small-large black hole phase transition. The critical phenomena are investigated in the $N^{2}$-$\mu$ chart. In particular, in the reduced parameter space, all the thermodynamic quantities can be rescaled with the black hole charge such that these reduced quantities are charge-independent. Then we obtain the coexistence curve and the phase diagram. The latent heat is also numerically calculated. Moreover, the heat capacity and the thermodynamic scalar are studied. The result indicates that the information of the first-order black hole phase transition is encoded in the heat capacity and scalar. However, the phase transition point cannot be directly calculated with them. Nevertheless, the critical point linked to a second-order phase transition can be determined by either the heat capacity or the scalar. In addition, we calculate the critical exponents of the heat capacity and the scalar for the saturated small and large black holes near the critical point.
Shao-Wen Wei, Bin Liang, Yu-Xiao Liu Journal reference: Phys. Rev. D 96, 124018 (2017) [pdf] DOI: 10.1103/PhysRevD.96.124018
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Interaction between magnetic moments and itinerant carriers in
-
Abstract
- Elucidating the interaction between magnetic moments and itinerant carriers is an important step to spintronic applications. Here, we investigate magnetic and transport properties in d0 ferromagnetic SiC single crystals prepared by postimplantation pulsed laser annealing. Magnetic moments are contributed by the p states of carbon atoms, but their magnetic circular dichroism is different from that in semi-insulating SiC samples. The anomalous Hall effect and negative magnetoresistance indicate the influence of d0 spin order on free carriers. The ferromagnetism is relatively weak in N-implanted SiC compared with that in Al-implanted SiC after annealing. The results suggest that d0 magnetic moments and itinerant carriers can interact with each other, which will facilitate the development of SiC spintronic devices with d0 ferromagnetism.
Yu Liu, Ye Yuan, Fang Liu, Roman Boettger, Wolfgang Anwand, Yutian Wang, Anna Semisalova, Alexey N. Ponomaryov, Xia Lu, Alpha T. N'Diaye, Elke Arenholz, Viton Heera, Wolfgang Skorupa, Manfred Helm, Shengqiang Zhou Journal reference: Phys. Rev. B 95, 195309 (2017) [pdf] DOI: 10.1103/PhysRevB.95.195309
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Investigating the topological structure of quenched lattice QCD with overlap fermions using a multi-probing approximation -
Abstract
- The topological charge density and topological susceptibility are determined by multi-probing approximation using overlap fermions in quenched SU(3) gauge theory. Then we investigate the topological structure of the quenched QCD vacuum, and compare it with results from the all-scale topological density, the results are consistent. Random permuted topological charge density is used to check whether these structures represent underlying ordered properties. Pseudoscalar glueball mass is extracted from the two-point correlation function of the topological charge density. We study $3$ ensembles of different lattice spacing $a$ with the same lattice volume $16^{3}\times32$, the results are compatible with the results of all-scale topological charge density, and the topological structures revealed by multi-probing are much closer to all-scale topological charge density than that by eigenmode expansion.
You-Hao Zou, Jian-Bo Zhang, Guang-Yi Xiong, Ying Chen, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma Journal reference: Chin. Phys. C, No. 10 (2017) [pdf] DOI: 10.1088/1674-1137/41/10/103104
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Validity of the Weizsäcker-Williams approximation and the analysis of beam dump experiments: Production of an axion, a dark photon, or a new axial-vector boson -
Abstract
- Beam dump experiments have been used to search for new particles, $\phi$, with null results interpreted in terms of limits on masses $m_\phi$ and coupling constants $\epsilon$. However these limits have been obtained by using approximations [including the Weizs\"{a}cker-Williams (WW) approximation] or Monte-Carlo simulations. We display methods to obtain the cross section and the resulting particle production rates without using approximations on the phase space integral or Monte-Carlo simulations. In our previous work we examined the case of the new scalar boson production; in this paper we explore all possible new spin-0 and spin-1 particles. We show that the approximations cannot be used to obtain accurate values of cross sections. The corresponding exclusion plots differ by substantial amounts when seen on a linear scale. Furthermore, a new region ($m_\phi<2m_e$) of parameter space can be explored without using one of the common approximations, $m_\phi\gg m_e$. We derive new expressions for the three photon decays of dark photon and four photon decays of new axial-vector bosons. As a result, the production cross section and exclusion region of different low mass ($m_\phi<2m_e$) bosons are very different. Moreover, our method can be used as a consistency check for Monte-Carlo simulations.
Yu-Sheng Liu, Gerald A. Miller Journal reference: Phys. Rev. D 96, 016004 (2017) [pdf] DOI: 10.1103/PhysRevD.96.016004
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Black hole entropy emission property -
Abstract
- In this work, we examine the entropy emission property of black holes. When the greybody factor is considered, it is found that Schwarzschild black hole is a one-dimensional entropy emitter, which is independent of the spacetime dimension and the spin of the emitted quanta. However, when generalized to other black holes with two or more parameters, the result shows that the one-dimensional entropy emission property will be violated. Thus our result implies that not all black holes behave as one-dimensional entropy emitters.
Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Quality Aware Network for Set to Set Recognition -
Abstract
- This paper targets on the problem of set to set recognition, which learns the metric between two image sets. Images in each set belong to the same identity. Since images in a set can be complementary, they hopefully lead to higher accuracy in practical applications. However, the quality of each sample cannot be guaranteed, and samples with poor quality will hurt the metric. In this paper, the quality aware network (QAN) is proposed to confront this problem, where the quality of each sample can be automatically learned although such information is not explicitly provided in the training stage. The network has two branches, where the first branch extracts appearance feature embedding for each sample and the other branch predicts quality score for each sample. Features and quality scores of all samples in a set are then aggregated to generate the final feature embedding. We show that the two branches can be trained in an end-to-end manner given only the set-level identity annotation. Analysis on gradient spread of this mechanism indicates that the quality learned by the network is beneficial to set-to-set recognition and simplifies the distribution that the network needs to fit. Experiments on both face verification and person re-identification show advantages of the proposed QAN. The source code and network structure can be downloaded at https://github.com/sciencefans/Quality-Aware-Network.
Yu Liu, Junjie Yan, Wanli Ouyang [pdf]
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Learning Deep Features via Congenerous Cosine Loss for Person
Recognition -
Abstract
- Person recognition aims at recognizing the same identity across time and space with complicated scenes and similar appearance. In this paper, we propose a novel method to address this task by training a network to obtain robust and representative features. The intuition is that we directly compare and optimize the cosine distance between two features - enlarging inter-class distinction as well as alleviating inner-class variance. We propose a congenerous cosine loss by minimizing the cosine distance between samples and their cluster centroid in a cooperative way. Such a design reduces the complexity and could be implemented via softmax with normalized inputs. Our method also differs from previous work in person recognition that we do not conduct a second training on the test subset. The identity of a person is determined by measuring the similarity from several body regions in the reference set. Experimental results show that the proposed approach achieves better classification accuracy against previous state-of-the-arts.
Yu Liu, Hongyang Li, Xiaogang Wang [pdf]
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Exposing strangeness: Projections for kaon electromagnetic form factors -
Abstract
- A continuum approach to the kaon and pion bound-state problems is used to reveal their electromagnetic structure. For both systems, when used with parton distribution amplitudes appropriate to the scale of the experiment, Standard Model hard-scattering formulae are accurate to within 25% at momentum transfers $Q^2 \approx 8\,$GeV$^2$. There are measurable differences between the distribution of strange and normal matter within the kaons, e.g. the ratio of their separate contributions reaches a peak value of $1.5$ at $Q^2 \approx 6\,$GeV$^2$. Its subsequent $Q^2$-evolution is accurately described by the hard scattering formulae. Projections for kaon and pion form factors at timelike momenta beyond the resonance region are also presented. These results and projections should prove useful in planning next-generation experiments.
Fei Gao, Lei Chang, Yu-Xin Liu, Craig D. Roberts, Peter C. Tandy Journal reference: Phys. Rev. D 96, 034024 (2017) [pdf] DOI: 10.1103/PhysRevD.96.034024
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Single-photon nonreciprocal transport in one-dimensional coupled-resonator waveguides -
Abstract
- We study the transport of a single photon in two coupled one-dimensional semi-infinite coupled-resonator waveguides (CRWs), in which both end sides are coupled to a dissipative cavity. We demonstrate that a single photon can transfer from one semi-infinite CRW to the other nonreciprocally. Based on such nonreciprocity, we further construct a three-port single-photon circulator by a T-shaped waveguide, in which three semi-infinite CRWs are pairwise mutually coupled to each other. The single-photon nonreciprocal transport is induced by the breaking of the time-reversal symmetry and the optimal conditions for these phenomena are obtained analytically. The CRWs with broken time-reversal symmetry will open up a kind of quantum devices with versatile applications in quantum networks.
Xun-Wei Xu, Ai-Xi Chen, Yong Li, Yu-xi Liu Journal reference: Phys. Rev. A 95, 063808 (2017) [pdf] DOI: 10.1103/PhysRevA.95.063808
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First principles calculations of the interface properties of a-Al
-
Abstract
- Al2O3 is a potential dielectric material for metal-oxide-semiconductor (MOS) devices. Al2O3 films deposited on semiconductors usually exhibit amorphous due to lattice mismatch. Compared to two-dimensional graphene, MoS2 is a typical semiconductor, therefore, it has more extensive application. The amorphous-Al2O3/MoS2 (a-Al2O3/MoS2) interface has attracted people's attention because of its unique properties. In this paper, the interface behaviors of a-Al2O3/MoS2 under non-strain and biaxial strain are investigated by first principles calculations based on density functional theory (DFT). First of all, the generation process of a-Al2O3 sample is described, which is calculated by molecular dynamics and geometric optimization. Then, we introduce the band alignment method, and calculate band offset of a-Al2O3/MoS2 interface. It is found that the valence band offset (VBO) and conduction band offset (CBO) change with the number of MoS2 layers. The dependence of leakage current on the band offset is also illustrated. At last, the band structure of monolayer MoS2 under biaxial strain is discussed. The biaxial strain is set in the range from -6% to 6% with the interval of 2%. Impact of the biaxial strain on the band alignment is investigated.
Li-Bin Shi, Ming-Biao Li, Xiao-Ming Xiu, Xu-Yang Liu, Kai-Cheng Zhang, Yu-Hui Liu, Chun-Ran Li, Hai-Kuan Dong [pdf] DOI: 10.1063/1.4983815 1702.06498v1 [pdf]
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Zoom Out-and-In Network with Recursive Training for Object Proposal -
Abstract
- In this paper, we propose a zoom-out-and-in network for generating object proposals. We utilize different resolutions of feature maps in the network to detect object instances of various sizes. Specifically, we divide the anchor candidates into three clusters based on the scale size and place them on feature maps of distinct strides to detect small, medium and large objects, respectively. Deeper feature maps contain region-level semantics which can help shallow counterparts to identify small objects. Therefore we design a zoom-in sub-network to increase the resolution of high level features via a deconvolution operation. The high-level features with high resolution are then combined and merged with low-level features to detect objects. Furthermore, we devise a recursive training pipeline to consecutively regress region proposals at the training stage in order to match the iterative regression at the testing stage. We demonstrate the effectiveness of the proposed method on ILSVRC DET and MS COCO datasets, where our algorithm performs better than the state-of-the-arts in various evaluation metrics. It also increases average precision by around 2% in the detection system.
Hongyang Li, Yu Liu, Wanli Ouyang, Xiaogang Wang [pdf]
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Validity of the Weizsäcker-Williams approximation and the analysis of beam dump experiments: Production of a new scalar boson -
Abstract
- Beam dump experiments have been used to search for new particles with null results interpreted in terms of limits on masses $m_\phi$ and coupling constants $\epsilon$. However these limits have been obtained by using approximations [including the Weizs\"{a}cker-Williams (WW) approximation] or Monte-Carlo simulations. We display methods, using a new scalar boson as an example, to obtain the cross section and the resulting particle production numbers without using approximations or Monte-Carlo simulations. We show that the approximations cannot be used to obtain accurate values of cross sections. The corresponding exclusion plots differ by substantial amounts when seen on a linear scale. In the event of a discovery, we generate pseudodata (assuming given values of $m_\phi$ and $\epsilon$) in the currently allowed regions of parameter space. The use of approximations to analyze the pseudodata for the future experiments is shown to lead to considerable errors in determining the parameters. Furthermore, a new region of parameter space can be explored without using one of the common approximations, $m_\phi\gg m_e$. Our method can be used as a consistency check for Monte-Carlo simulations.
Yu-Sheng Liu, David McKeen, Gerald A. Miller Journal reference: Phys. Rev. D 95, 036010 (2017) [pdf] DOI: 10.1103/PhysRevD.95.036010
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Probing extra dimension through gravitational wave observations of compact binaries and their electromagnetic counterparts -
Abstract
- The future gravitational wave (GW) observations of compact binaries and their possible electromagnetic counterparts may be used to probe the nature of the extra dimension. It is widely accepted that gravitons and photons are the only two completely confirmed objects that can travel along null geodesics in our four-dimensional space-time. However, if there exist extra dimensions and only GWs can propagate freely in the bulk, the causal propagations of GWs and electromagnetic waves (EMWs) are in general different. In this paper, we study null geodesics of GWs and EMWs in a five-dimensional anti-de Sitter space-time in the presence of the curvature of the universe. We show that for general cases the horizon radius of GW is longer than EMW within equal time. Taking the GW150914 event detected by the Advanced Laser Interferometer Gravitational-Wave Observatory and the X-ray event detected by the Fermi Gamma-ray Burst Monitor as an example, we study how the curvature $k$ and the constant curvature radius $l$ affect the horizon radii of GW and EMW in the de Sitter and Einstein-de Sitter models of the universe. This provides an alternative method for probing extra dimension through future GW observations of compact binaries and their electromagnetic counterparts.
Hao Yu, Bao-Min Gu, Fa Peng Huang, Yong-Qiang Wang, Xin-He Meng, Yu-Xiao Liu Journal reference: JCAP 1702 (2017) 039 [pdf] DOI: 10.1088/1475-7516/2017/02/039
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Hearts of twin Cotorsion pairs on extriangulated categories -
Abstract
- In this article, we study the heart of a cotorsion pairs on an exact category and a triangulated category in a unified meathod, by means of the notion of an extriangulated category. We prove that the heart is abelian, and construct a cohomological functor to the heart. If the extriangulated category has enough projectives, this functor gives an equivalence between the heart and the category of coherent functors over the coheart modulo projectives. We also show how an n-cluster tilting subcategory of an extriangulated category gives rise to a family of cotorsion pairs with equivalent hearts.
Yu Liu, Hiroyuki Nakaoka [pdf]
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Fermion localization mechanism with derivative geometrical coupling on branes -
Abstract
- In order to localize fermions on branes with codimension one, one usually introduces the Yukawa coupling between fermions and background scalar fields or the recently proposed derivative fermion-scalar coupling in [Phys. Rev. D 89 (2014) 086001]. In this paper, we explore the coupling between a spinor field $\Psi$ and the scalar curvature of spacetime $R$, $\eta\Psi\Gamma^M\partial_M F(R)\gamma^5\bar{\Psi}$ with $F(R)$ a function of $R$, to investigate localization of the fermion. Because of $Z_2$ symmetry of the extra dimension, the new coupling mechanism proposed here can easily deal with the problem encountered in the Yukawa coupling with even background scalar fields. More importantly, the new mechanism will also work for the branes without background scalar fields. By investigating three examples, we find that fermions can be localized on the branes with the new mechanism.
Yan-Yan Li, Yu-Peng Zhang, Wen-Di Guo, Yu-Xiao Liu Journal reference: Phys. Rev. D 95, 115003 (2017) [pdf] DOI: 10.1103/PhysRevD.95.115003
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Photon emission of extremal Kerr–Newman black holes -
Abstract
- In this paper, we deal with the null geodesics extending from the near-horizon region out to a distant observatory in an extremal Kerr-Newman black hole background. In particular, using the matched asymptotic expansion method, we analytically solve the null geodesics near the superradiant bound in the form of algebraic equations. For the case that the photon trajectories are limited in the equatorial plane, the shifts in the azimuthal angle and time are obtained.
Shao-Wen Wei, Bao-Min Gu, Yong-Qiang Wang, Yu-Xiao Liu Journal reference: Eur. Phys. J. C 77 (2017) 128 [pdf] DOI: 10.1140/epjc/s10052-017-4699-7
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Full linear perturbations and localization of gravity on f(R, T) brane -
Abstract
- We study the thick brane world system constructed in the recently proposed $f(R,T)$ theories of gravity, with $R$ the Ricci scalar and $T$ the trace of the energy-momentum tensor. We try to get the analytic background solutions and discuss the full linear perturbations, especially the scalar perturbations. We compare how the brane world model is modified with that of general relativity coupled to a canonical scalar field. It is found that some more interesting background solutions are allowed, and only the scalar perturbation mode is modified. There is no tachyon state exists in this model and only the massless tensor mode can be localized on the brane, which recovers the effective four-dimensional gravity. These conclusions hold provided that two constraints on the original formalism of the action are satisfied.
Bao-Min Gu, Yu-Peng Zhang, Hao Yu, Yu-Xiao Liu Journal reference: Eur. Phys. J. C 77 (2017) 115 [pdf] DOI: 10.1140/epjc/s10052-017-4666-3
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Ultracold Molecular Assembly -
Abstract
- Chemical reactions can be surprisingly efficient at ultracold temperatures ( < 1mK) due to the wave nature of atoms and molecules. The study of reactions in the ultracold regime is a new research frontier enabled by cooling and trapping techniques developed in atomic and molecular physics. In addition, ultracold molecular gases that offer diverse molecular internal states and large electric dipolar interactions are sought after for studies of strongly interacting many-body quantum physics. Here we propose a new approach for producing ultracold molecules in the absolute internal and motional quantum ground state, where single molecules are assembled one by one from individual atoms. The scheme involves laser cooling, optical trapping, Raman sideband cooling, and coherent molecular state transfer. As a crucial initial step, we demonstrate quantum control of constituent atoms, including 3D ground-state cooling of a single Cs atom, in a simple apparatus. As laser technology advances to shorter wavelengths, additional atoms will be amenable to laser-cooling, allowing more diverse, and eventually more complex, molecules to be assembled with full quantum control.
Lee R. Liu, Jessie T. Zhang, Yichao Yu, Nicholas R. Hutzler, Yu Liu, Till Rosenband, Kang-Kuen Ni [pdf]
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First law of black hole mechanics in variable background fields -
Abstract
- It is well known that in general theories of gravity with the diffeomorphism symmetry, the black hole entropy is a Noether charge. But what will happen if the symmetry is explicitly broken? By investigating the covariant first law of black hole mechanics with background fields, we show that the Noether entropy is still applicable due to the local nature of the black hole entropy. Moreover, motivated by the proposal that the cosmological constant behaves as a thermodynamic variable, we allow the non-dynamical background fields to be varied. To illustrate this general formalism, we study a generic static black brane in the massive gravity. Using the first law and the scaling argument, we obtain two Smarr formulas. We show that both of them can be retrieved without relying on the first law, hence providing a self-consistent check of the theory.
Shao-Feng Wu, Xian-Hui Ge, Yu-Xiao Liu [pdf] DOI: 10.1007/s10714-017-2246-3 1602.08661v4 [pdf]
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Identifying the QCD Phase Transitions via the Gravitational Wave
Frequency -
Abstract
- We investigate the nonradial oscillations of newly born neutron stars (NSs) and strange quark stars (SQSs). This is done with the relativistic nuclear field theory with hyperon degrees of freedom employed to describe the equation of state for the stellar matter in NSs, and with both the MIT bag model and the Nambu--Jona-Lasinio model adopted to construct the configurations of the SQSs. We find that the gravitational-mode ($g$-mode) eigenfrequencies of newly born SQSs are about one order of magnitude lower than those of NSs, which is independent of models implemented to describe the equation of state for the strange quark matter. Meanwhile the eigenfrequencies of the other modes of nonradial oscillations, e.g., fundamental ($f$)- and pressure ($p$)-modes, are much larger than those of the $g$-mode. In the light of the first direct observation of gravitational waves, it is promising to employ the gravitational waves to identify the QCD phase transition in high density strong interaction matter.
Wei-jie Fu, Zhan Bai, Yu-xin Liu [pdf]
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Realization of microwave amplification, attenuation, and frequency conversion using a single three-level superconducting quantum circuit -
Abstract
- Using different configurations of applied strong driving and weak probe fields, we find that only a single three-level superconducting quantum circuit (SQC) is enough to realize amplification, attenuation and frequency conversion of microwave fields. Such a three-level SQC has to possess $\Delta$-type cyclic transitions. Different from the parametric amplification (attenuation) and frequency conversion in nonlinear optical media, the real energy levels of the three-level SQC are involved in the energy exchange when these processes are completed. We quantitatively discuss the effects of amplification (attenuation) and the frequency conversion for different types of driving fields. The optimal points are obtained for achieving the maximum amplification (attenuation) and conversion efficiency. Our study provides a new method to amplify (attenuate) microwave, realize frequency conversion, and also lay a foundation for generating single or entangled microwave photon states using a single three-level SQC.
Yan-Jun Zhao, Jiang-Hao Ding, Z. H. Peng, Yu-xi Liu Journal reference: Phys. Rev. A 95, 043806 (2017) [pdf] DOI: 10.1103/PhysRevA.95.043806
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Two-photon transition form factor of
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Abstract
- The two photon transition of $\bar{c}c$ quarkonia are studied within a covariant approach based on the consistent truncation scheme of the quantum chromodynamics Dyson-Schwinger equation for the quark propagator and the Bethe--Salpeter equation for the mesons. We find the decay widths of $\eta_{c}^{} \to \gamma\gamma$ and $\chi_{c0,2}^{} \to \gamma\gamma$ in good agreement with experimental data. The obtained transition form factor of $\eta_{c}^{} \to \gamma\gamma^{\ast}$ for a wide range of space-like photon momentum transfer squared is also in agreement with the experimental findings of the BABAR experiment. As a by-product, the decay widths of $\eta_{b}^{},\chi_{b0,2}^{} \to \gamma\gamma$ and the transition form factor of $\eta_{b}^{}, \chi_{c0,b0}^{} \to\gamma\gamma^{\ast}$ are predicted, which await for experimental test.
Jing Chen, Minghui Ding, Lei Chang, Yu-xin Liu Journal reference: Phys. Rev. D 95, 016010 (2017) [pdf] DOI: 10.1103/PhysRevD.95.016010
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Simulating heavy fermion physics in optical lattice: Periodic Anderson model with harmonic trapping potential -
Abstract
- Periodic Anderson model (PAM), where local electron orbitals interplay with itinerant electronic carriers, plays an essential role in our understanding on heavy fermion materials. Motivated by recent proposal of simulating Kondo lattice model (KLM) in terms of alkaline-earth metal atoms, we make a further step toward simulation of PAM, which includes crucial charge/valence fluctuation of local f-electron beyond purely low-energy spin fluctuation in KLM. To realize PAM, transition induced by suitable laser between electronic excited and ground state of alkaline-earth metal atoms ($^{1}S_{0}$$\rightleftharpoons$$^{3}P_{0}$) is introduced, and it leads to effective hybridization between local electron and conduction electron in PAM. Generally, the $SU(N)$ version of PAM can be realized by our proposal, which gives a unique opportunity to detect large-$N$ physics without complexity in realistic materials. In the present work, high temperature physical feature of standard ($SU(2)$) PAM with harmonic trapping potential is detailed analyzed by quantum Monte Carlo and dynamic mean-field theory. Indications for near-future experiments are provided. We expect our theoretical proposal and (hopefully) forthcoming experiment will deepen our understanding on heavy fermion systems and at the same time triggers further studies on related Mott physics, quantum criticality and non-trivial topology in both inhomogeneous and nonequilibrium realm.
Yin Zhong, Yu Liu, Hong-Gang Luo Journal reference: Front. Phys. 12(5), 127502 (2017) [pdf] DOI: 10.1007/s11467-017-0690-x
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Controllable optical response by modifying the gain and loss of a mechanical resonator and cavity mode in an optomechanical system -
Abstract
- We theoretically study a strongly-driven optomechanical system which consists of a passive optical cavity and an active mechanical resonator. When the optomechanical coupling strength is varied, phase transitions, which are similar those observed in $\mathcal{PT}$-symmetric systems, are observed. We show that the optical transmission can be controlled by changing the gain of the mechanical resonator and loss of the optical cavity mode. Especially, we find that: (i) for balanced gain and loss, optical amplification and absorption can be tuned by changing the optomechanical coupling strength through a control field; (ii) for unbalanced gain and loss, even with a tiny mechanical gain, both optomechanically-induced transparency and anomalous dispersion can be observed around a critical point, which exhibits an ultra-long group delay. The time delay $\tau$ can be optimized by regulating the optomechanical coupling strength through the control field and improved up to several orders of magnitude ($\tau\sim2$ $\mathrm{ms}$) compared to that of conventional optomechanical systems ($\tau\sim1$ $\mu\mathrm{s}$). The presence of mechanical gain makes the group delay more robust to environmental perturbations. Our proposal provides a powerful platform to control light transport using a $\mathcal{PT}$-symmetric-like optomechanical system.
Yu-Long Liu, Rebing Wu, Jing Zhang, Şahin Kaya Özdemir, Lan Yang, Franco Nori, Yu-xi Liu Journal reference: Phys. Rev. A 95, 013843 (2017) [pdf] DOI: 10.1103/PhysRevA.95.013843
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Quantum feedback: Theory, experiments, and applications -
Abstract
- The control of individual quantum systems is now a reality in a variety of physical settings. Feedback control is an important class of control methods because of its ability to reduce the effects of noise. In this review we give an introductory overview of the various ways in which feedback may be implemented in quantum systems, the theoretical methods that are currently used to treat it, the experiments in which it has been demonstrated to-date, and its applications. In the last few years there has been rapid experimental progress in the ability to realize quantum measurement and control of mesoscopic systems. We expect that the next few years will see further rapid advances in the precision and sophistication of feedback control protocols realized in the laboratory.
Jing Zhang, Yu-xi Liu, Re-Bing Wu, Kurt Jacobs, Franco Nori Journal reference: Physics Reports 679, 1-60 (2017) [pdf] DOI: 10.1016/j.physrep.2017.02.003
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Optimal occlusion uniformly partitions red blood cells fluxes within a
microvascular network -
Abstract
- 2016
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Channel Measurements and Models for High-Speed Train Wireless
Communication Systems in Tunnel Scenarios: A Survey -
Abstract
- The rapid developments of high-speed trains (HSTs) introduce new challenges to HST wireless communication systems. Realistic HST channel models play a critical role in designing and evaluating HST communication systems. Due to the length limitation, bounding of tunnel itself, and waveguide effect, channel characteristics in tunnel scenarios are very different from those in other HST scenarios. Therefore, accurate tunnel channel models considering both large-scale and small-scale fading characteristics are essential for HST communication systems. Moreover, certain characteristics of tunnel channels have not been investigated sufficiently. This article provides a comprehensive review of the measurement campaigns in tunnels and presents some tunnel channel models using various modeling methods. Finally, future directions in HST tunnel channel measurements and modeling are discussed.
Yu Liu, Ammar Ghazal, Cheng-Xiang Wang, Xiaohu Ge, Yang Yang, Yapei Zhang [pdf]
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Phonon blockade in a nanomechanical resonator resonantly coupled to a qubit -
Abstract
- We study phonon statistics in a nanomechanical resonator (NAMR) which is resonantly coupled to a qubit. We find that there are two different mechanisms for phonon blockade in such a resonantly coupled NAMR-qubit system. One is due to the strong anharmonicity of the NAMR-qubit system with large coupling strength; the other one is due to the destructive interference between different paths for two-phonon excitation in the NAMR-qubit system with a moderate coupling strength. We find that the phonon blockade is fragile towards thermal mode occupations and can only be observed for NAMR being at ultracold effective temperature. In order to enlarge the mean phonon number for strong phonon antibunching with a moderate NAMR-qubit coupling strength, we assume that two external driving fields are applied to the NAMR and qubit, respectively. In this case, we find that the phonon blockades under two mechanisms can appear at the same frequency regime by optimizing the strength ratio and phase difference of the two external driving fields.
Xun-Wei Xu, Ai-Xi Chen, Yu-xi Liu Journal reference: Phys. Rev. A 94, 063853 (2016) [pdf] DOI: 10.1103/PhysRevA.94.063853
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Superconducting Order from Disorder in 2H-TaSe$_{2-x}$S$_{x}$
(0$\leq$x$\leq$2) -
Abstract
- We report on the emergence of robust superconducting order in single crystal alloys of 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2) . The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe$_{2}$ and TaS$_{2}$. The evolution of superconducting critical temperature T$_{c} (x)$ correlates with the full width at half maximum of the Bragg peaks and with the linear term of the high temperature resistivity. The conductivity of the crystals near the middle of the alloy series is higher or similar than that of either one of the end members 2H-TaSe$_{2}$ and/or 2H-TaS$_{2}$. It is known that in these materials superconductivity (SC) is in close competition with charge density wave (CDW) order. We interpret our experimental findings in a picture where disorder tilts this balance in favor of superconductivity by destroying the CDW order.
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Adiabatic quantum parameter amplification for generic robust quantum
sensing -
Abstract
- Quantum enhanced sensing provides a powerful tool for the precise measurement of physical parameters that is applicable in many areas of science and technology. The achievable gain in sensitivity is largely limited by the influence of noise and decoherence. Here, we propose a paradigm of adiabatic quantum parameter amplification to overcome this limitation. We demonstrate that it allows to achieve generic robust quantum sensing, namely it is robust against noise that may even acting on the same degree of freedom as the field. Furthermore, the proposal achieves entanglement-free Heisenberg limit sensitivity that surpasses the limit of classical statistics.
Yu Liu, Zijun Shu, Martin B. Plenio, Jianming Cai [pdf]
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Charged spinning black holes as accelerators of spinning particles -
Abstract
- It is well known that some black holes can act as accelerators for particles without spin. Recently, there are some works considering collision of two spinning particles in the background of Schwarzschild and Kerr black holes and it was shown that {the center-of-mass energy of the test particles is related to the spin}. In this paper we extend the results to some more general cases. We consider Kerr-Newman black holes as accelerators for spinning particles. We derive the center-of-mass energy of the spinning particles and use numerical method to investigate how the center-of-mass energy is affected by the properties of the black holes and spinning particles.
Yu-Peng Zhang, Bao-Min Gu, Shao-Wen Wei, Jie Yang, Yu-Xiao Liu Journal reference: Phys. Rev. D 94, 124017 (2016) [pdf] DOI: 10.1103/PhysRevD.94.124017
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On the Exploration of Convolutional Fusion Networks for Visual
Recognition -
Abstract
- Despite recent advances in multi-scale deep representations, their limitations are attributed to expensive parameters and weak fusion modules. Hence, we propose an efficient approach to fuse multi-scale deep representations, called convolutional fusion networks (CFN). Owing to using 1$\times$1 convolution and global average pooling, CFN can efficiently generate the side branches while adding few parameters. In addition, we present a locally-connected fusion module, which can learn adaptive weights for the side branches and form a discriminatively fused feature. CFN models trained on the CIFAR and ImageNet datasets demonstrate remarkable improvements over the plain CNNs. Furthermore, we generalize CFN to three new tasks, including scene recognition, fine-grained recognition and image retrieval. Our experiments show that it can obtain consistent improvements towards the transferring tasks.
Yu Liu, Yanming Guo, Michael S. Lew [pdf]
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Distinct surface and bulk charge density waves in ultrathin
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Abstract
- We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.
Rui He, Junichi Okamoto, Zhipeng Ye, Gaihua Ye, Heidi Anderson, Xia Dai, Xianxin Wu, Jiangping Hu, Yu Liu, Wenjian Lu, Yuping Sun, Abhay N. Pasupathy, Adam W. Tsen Journal reference: Phys. Rev. B 94, 201108 (2016) [pdf] DOI: 10.1103/PhysRevB.94.201108
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Bayesian extraction of the parton distribution amplitude from the Bethe–Salpeter wave function -
Abstract
- We propose a new numerical method to compute parton distribution amplitude(PDA) from the Euclidean Bethe-Salpeter wave function. The essential step is to extract the weight function in the Nakanishi representation of the Bethe-Salpeter wave function in Euclidean space, which is an ill-posed inversion problem, via the maximum entropy method(MEM). The Nakanishi weight function as well as the corresponding light-front PDA can be well determined. We confirm the previous works on PDA computation therein the different method has been performed.
Fei Gao, Lei Chang, Yu-xin Liu [pdf] DOI: 10.1016/j.physletb.2017.04.077 1611.03560v1 [pdf]
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POI: Multiple Object Tracking with High Performance Detection and
Appearance Feature -
Abstract
- Detection and learning based appearance feature play the central role in data association based multiple object tracking (MOT), but most recent MOT works usually ignore them and only focus on the hand-crafted feature and association algorithms. In this paper, we explore the high-performance detection and deep learning based appearance feature, and show that they lead to significantly better MOT results in both online and offline setting. We make our detection and appearance feature publicly available. In the following part, we first summarize the detection and appearance feature, and then introduce our tracker named Person of Interest (POI), which has both online and offline version.
Fengwei Yu, Wenbo Li, Quanquan Li, Yu Liu, Xiaohua Shi, Junjie Yan [pdf]
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Direct calculation of the linear thermal expansion coefficients of
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Abstract
- Using density-functional perturbation theory and the Gr\"uneisen formalism, we directly calculate the linear thermal expansion coefficients (TECs) of a hexagonal bulk system MoS$_2$ in the crystallographic $a$ and $c$ directions. The TEC calculation depends critically on the evaluation of a temperature-dependent quantity $I_i(T)$, which is the integral of the product of heat capacity and $\Gamma_i(\nu)$, of frequency $\nu$ and strain type $i$, where $\Gamma_i(\nu)$ is the phonon density of states weighted by the Gr\"uneisen parameters. We show that to determine the linear TECs we may use minimally two uniaxial strains in the $z$ direction, and either the $x$ or $y$ direction. However, a uniaxial strain in either the $x$ or $y$ direction drastically reduces the symmetry of the crystal from a hexagonal one to a base-centered orthorhombic one. We propose to use an efficient and accurate symmetry-preserving biaxial strain in the $xy$ plane to derive the same result for $\Gamma(\nu)$. We highlight that the Gr\"uneisen parameter associated with a biaxial strain may not be the same as the average of Gr\"uneisen parameters associated with two separate uniaxial strains in the $x$ and $y$ directions due to possible preservation of degeneracies of the phonon modes under a biaxial deformation. Large anisotropy of TECs is observed where the linear TEC in the $c$ direction is about $1.8$ times larger than that in the $a$ or $b$ direction at high temperatures. Our theoretical TEC results are compared with experiment. The symmetry-preserving approach adopted here may be applied to a broad class of two lattice-parameter systems such as hexagonal, trigonal, and tetragonal systems, which allows many complicated systems to be treated on a first-principles level.
Chee Kwan Gan, Yu Yang Fredrik Liu Journal reference: Physical Review B v. 94 (2016) p. 134303 [pdf] DOI: 10.1103/PhysRevB.94.134303
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Crafting GBD-Net for Object Detection -
Abstract
- The visual cues from multiple support regions of different sizes and resolutions are complementary in classifying a candidate box in object detection. Effective integration of local and contextual visual cues from these regions has become a fundamental problem in object detection. In this paper, we propose a gated bi-directional CNN (GBD-Net) to pass messages among features from different support regions during both feature learning and feature extraction. Such message passing can be implemented through convolution between neighboring support regions in two directions and can be conducted in various layers. Therefore, local and contextual visual patterns can validate the existence of each other by learning their nonlinear relationships and their close interactions are modeled in a more complex way. It is also shown that message passing is not always helpful but dependent on individual samples. Gated functions are therefore needed to control message transmission, whose on-or-offs are controlled by extra visual evidence from the input sample. The effectiveness of GBD-Net is shown through experiments on three object detection datasets, ImageNet, Pascal VOC2007 and Microsoft COCO. This paper also shows the details of our approach in wining the ImageNet object detection challenge of 2016, with source code provided on \url{https://github.com/craftGBD/craftGBD}.
Xingyu Zeng, Wanli Ouyang, Junjie Yan, Hongsheng Li, Tong Xiao, Kun Wang, Yu Liu, Yucong Zhou, Bin Yang, Zhe Wang, Hui Zhou, Xiaogang Wang [pdf]
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Pure spin current and perfect valley filter by designed separation of the chiral states in two-dimensional honeycomb lattices -
Abstract
- We propose a realization of pure spin currents and perfect valley filter based on a quantum anomalous Hall insulator, around which edge states with up-spin and down-spin circulate. By applying staggered sublattice potential on the strips along the edges of sample, the edge states with down spin can be pushed into the inner boundaries of the strips while the other edge states with up spin remain on the outer boundaries, resulting in spatially separated chiral states with perfect spin polarization. Moreover, a valley filter, which is immune to short-range and smooth long-range scatterers, can be engineered by additionally applying boundary potentials on the outmost lattices of the sample. We also find that the boundary potential can be used to control the size effect induced oscillation of the inner chiral states. The connection of the boundary potential to size effect is revealed.
Da-Ping Liu, Zhi-Ming Yu, Yu-Liang Liu Journal reference: Phys. Rev. B. 94, 155112 (2016) [pdf] DOI: 10.1103/PhysRevB.94.155112
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Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS
-
Abstract
- Charge density waves (CDW) and their concomitant periodic lattice distortions (PLD) govern the electronic properties in many layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly-commensurate (NC) and commensurate (C) phases in thin exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one bond length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.
Robert Hovden, Adam W. Tsen, Pengzi Liu, Benjamin H. Savitzky, Ismail El Baggari, Yu Liu, Wenjian Lu, Yuping Sun, Philip Kim, Abhay N. Pasupathy, Lena F. Kourkoutis [pdf] DOI: 10.1073/pnas.1606044113 1609.09486v1 [pdf]
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Interface effect in QCD phase transitions via Dyson-Schwinger equation approach -
Abstract
- With the chiral susceptibility criterion we obtain the phase diagram of strong-interaction matter in terms of temperature and chemical potential in the framework of Dyson-Schwinger equations (DSEs) of QCD.After calculating the pressure and some other thermodynamic properties of the matter in the DSE method, we get the phase diagram in terms of temperature and baryon number density. We also obtain the interface tension and the interface entropy density to describe the inhomogeneity of the two phases in the coexistence region of the first order phase transition. After including the interface effect, we find that the total entropy density of the system increases in both the deconfinement (dynamical chiral symmetry restoration) and the hadronization (dynamical chiral symmetry breaking) processes of the first order phase transitions and thus solve the entropy puzzle in the hadronization process.
Fei Gao, Yu-xin Liu Journal reference: Phys. Rev. D 94, 094030 (2016) [pdf] DOI: 10.1103/PhysRevD.94.094030
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Localization and mass spectrum of q -form fields on branes -
Abstract
- In this paper, we investigate localization of a bulk massless q-form field on codimension-one brane by using a new Kaluza-Klein (KK) decomposition, for which there are two types of KK modes for the bulk q-form field, the q-form and (q-1)-form modes. The first modes may be massive or massless while the second ones are all masselss. These two types fo KK modes satisfy twy Schrodinger-like equations. For a five-dimensional brane model with a finite dimension, the spectrum of a bulk 3-form field on the brane consists of some massive bound 3-form KK modes as well as some massless bound 2-form ones with different configuration along the extra dimension. These 2-form modes are different form those obtained from a bulk 2-form field. For a five-dimensional degenerated Bloch brane model with an infinite extra dimension, some massive 3-form resonant KK modes and corresponding massless 2-form resonant ones are obtained for a bulk 3-form field.
Chun-E Fu, Yuan Zhong, Qun-Ying Xie, Yu-Xiao Liu Journal reference: Physics Letters B 757 (2016) 180 [pdf] DOI: 10.1016/j.physletb.2016.03.069
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COMPARISON OF DIVERSITY OF TYPE IIB SUPERNOVAE WITH ASYMMETRY IN CASSIOPEIA A USING LIGHT ECHOES -
Abstract
- We compare the diversity of spectral line velocities in a large sample of type IIb supernovae (SNe IIb) with the expected asphericity in the explosion, as measured from the light echoes of Cassiopeia A (Cas A), which was a historical galactic SN IIb. We revisit the results of Rest et al. (2011a), who used light echoes to observe Cas A from multiple lines of sight and hence determine its asphericity, as seen in the velocity of three spectral lines (He I $\lambda$5876, H$\alpha$ and the Ca II NIR triplet). We confirm and improve on this measurement by reproducing the effect of the light echoes in the spectra of several extragalactic SNe IIb found in the literature as well as mean SN IIb spectra recently created by Liu et al. (2016), and comparing these to the observed light echo spectra of Cas A, including their associated uncertainties. In order to quantify the accuracy of this comparison we smooth the light echo spectra of Cas A using Gaussian processes and use a Monte Carlo method to measure the absorption velocities of these three features in the spectra. We then test the hypothesis that the diversity of ejecta velocities seen in SNe IIb can be explained by asphericity. We do this by comparing the range of velocities seen in the different light echoes, and hence different lines of sight, of Cas A to that seen in the population of SNe IIb. We conclude that these two ranges are of the same order and thus asphericity could be enough to explain the diversity in the expansion velocity alone.
Kieran Finn, Federica B. Bianco, Maryam Modjaz, Yu-Qian Liu, Armin Rest [pdf] DOI: 10.3847/0004-637X/830/2/73 1605.03186v2 [pdf]
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Three-dimensional bulk electronic structure of the Kondo lattice CeIn3
revealed by photoemission -
Abstract
- We show the three-dimensional electronic structure of the Kondo lattice CeIn3 using soft x-ray angle resolved photoemission spectroscopy in the paramagnetic state. For the first time, we have directly observed the three-dimensional topology of the Fermi surface of CeIn3 by photoemission. The Fermi surface has a complicated hole pocket centred at the {\Gamma}-Z line and an elliptical electron pocket centred at the R point of the Brillouin zone. Polarization and photon-energy dependent photoemission results both indicate the nearly localized nature of the 4f electrons in CeIn3, consistent with the theoretical prediction by means of the combination of density functional theory and single-site dynamical meanfield theory. Those results illustrate that the f electrons of CeIn3, which is the parent material of CeMIn5 compounds, are closer to the localized description than the layered CeMIn5 compounds.
Yun Zhang, Haiyan Lu, Xiegang Zhu, Shiyong Tan, Qin Liu, Qiuyun Chen, Wei Feng, Donghua Xie, Lizhu Luo, Yu Liu, Haifeng Song, Zhengjun Zhang, Xinchun Lai [pdf]
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Electrophobic Scalar Boson and Muonic Puzzles -
Abstract
- A new scalar boson which couples to the muon and proton can simultaneously solve the proton radius puzzle and the muon anomalous magnetic moment discrepancy. Using a variety of measurements, we constrain the mass of this scalar and its couplings to the electron, muon, neutron, and proton. Making no assumptions about the underlying model, these constraints and the requirement that it solve both problems limit the mass of the scalar to between about 100 keV and 100 MeV. We identify two unexplored regions in the coupling constant-mass plane. Potential future experiments and their implications for theories with mass-weighted lepton couplings are discussed.
Yu-Sheng Liu, David McKeen, Gerald A. Miller Journal reference: Phys. Rev. Lett. 117, 101801 (2016) [pdf] DOI: 10.1103/PhysRevLett.117.101801
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Time-dependent scalar fields in modified gravities in a stationary spacetime -
Abstract
- Most no-hair theorems involve the assumption that the scalar field is independent of time. Recently in [Phys. Rev. D90 (2014) 041501(R)] the existence of time-dependent scalar hair outside a stationary black hole in general relativity was ruled out. We generalize this work to modified gravities and non-minimally coupled scalar field with an additional assumption that the spacetime is axisymmetric. It is shown that in higher-order gravity such as metric $f(R)$ gravity the time-dependent scalar hair doesn't exist. While in Palatini $f(R)$ gravity and non-minimally coupled case the time-dependent scalar hair may exist.
Yi Zhong, Bao-Min Gu, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Eur.Phys.J. C 76 (2016) 377 [pdf] DOI: 10.1140/epjc/s10052-016-4225-3
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Spatial interferences in the electron transport of heavy-fermion materials -
Abstract
- The scanning tunneling microscopy/spectroscopy and the point contact spectroscopy represent one of the major progresses in recent heavy fermion research. Both have revealed important information on the composite nature of the emergent heavy electron quasiparticles. However, a detailed and thorough microscopic understanding of the similarities and differences in the underlying physical processes of these techniques is still lacking. Here we study the electron transport in the normal state of the periodic Anderson lattice by using the Keldysh nonequilibrium Green's function technique. In addition to the well-known Fano interference between the conduction and f-electron channels, our results further reveal the effect of spatial interference between different spatial paths at the interface on the differential conductance and their interesting interplay with the band features such as the hybridization gap and the Van Hove singularity. We find that the spatial interference leads to a weighted average in the momentum space for the electron transport and could cause suppression of the electronic band features under certain circumstances. In particular, it reduces the capability of probing the f-electron spectral weight near the edges of the hybridization gap for large interface depending on the Fermi surface of the lead. Our results indicates an intrinsic inefficiency of the point contact spectroscopy in probing the f-electrons.
Shu-feng Zhang, Yu Liu, Hai-Feng Song, Yi-feng Yang Journal reference: Phys. Rev. B 94, 085124 (2016) [pdf] DOI: 10.1103/PhysRevB.94.085124
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ANALYZING THE LARGEST SPECTROSCOPIC DATA SET OF STRIPPED SUPERNOVAE TO IMPROVE THEIR IDENTIFICATIONS AND CONSTRAIN THEIR PROGENITORS -
Abstract
- Using the largest spectroscopic dataset of stripped-envelope core-collapse supernovae (stripped SNe), we present a systematic investigation of spectral properties of Type IIb SNe (SNe IIb), Type Ib SNe (SNe Ib), and Type Ic SNe (SNe Ic). Prior studies have been based on individual objects or small samples. Here, we analyze 227 spectra of 14 SNe IIb, 258 spectra of 21 SNe Ib, and 207 spectra of 17 SNe Ic based on the stripped SN dataset of Modjaz et al. (2014) and other published spectra of individual SNe. Each SN in our sample has a secure spectroscopic ID, a date of $V$-band maximum light, and multiple spectra at different phases. We analyze these spectra as a function of subtype and phase in order to improve the SN identification scheme and constrain the progenitors of different kinds of stripped SNe. By comparing spectra of SNe IIb with those of SNe Ib, we find that the strength of H$\alpha$ can be used to quantitatively differentiate between these two subtypes at all epochs. Moreover, we find a continuum in observational properties between SNe IIb and Ib. We address the question of hidden He in SNe Ic by comparing our observations with predictions from various models that either include hidden He or in which He has been burnt. Our results favor the He-free progenitor models for SNe Ic. Finally, we construct continuum-divided average spectra as a function of subtype and phase to quantify the spectral diversity of the different types of stripped SNe.
Yu-Qian Liu, Maryam Modjaz, Federica B. Bianco, Or Graur Journal reference: ApJ Volume 827, Number 2, 2016 [pdf] DOI: 10.3847/0004-637X/827/2/90
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Phase diagram of the Kondo-Heisenberg model on honeycomb lattice with geometrical frustration -
Abstract
- We calculated the phase diagram of the Kondo-Heisenberg model on two-dimensional honeycomb lattice with both nearest-neighbor and next-nearest-neighbor antiferromagnetic spin exchanges, to investigate the interplay between RKKY and Kondo interactions at presence of magnetic frustration. Within a mean-field decoupling technology in slave-fermion representation, we derived the zero-temperature phase diagram as a function of Kondo coupling $J_k$ and frustration strength $Q$. The geometrical frustration can destroy the magnetic order, driving the original antiferromagnetic (AF) phase to non-magnetic valence bond state (VBS). In addition, we found two distinct VBS. As $J_k$ is increased, a phase transition from AF to Kondo paramagnetic (KP) phase occurs, without the intermediate phase coexisting AF order with Kondo screening found in square lattice systems. In the KP phase, the enhancement of frustration weakens the Kondo screening effect, resulting in a phase transition from KP to VBS. We also found a process to recover the AF order from VBS by increasing $J_k$ in a wide range of frustration strength. Our work may provide deeper understanding for the phase transitions in heavy-fermion materials, particularly for those exhibiting triangular frustration.
Huan Li, Hai-Feng Song, Yu Liu [pdf] DOI: 10.1209/0295-5075/116/37005 1607.05395v1 [pdf]
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QCD phase transitions via a refined truncation of Dyson-Schwinger equations -
Abstract
- We investigate both the chiral and deconfinement phase transitions of QCD matter in a refined scheme of Dyson-Schwinger equations, which have been shown to be successful in giving the meson mass spectrum and matching the interaction with the results from ab initio computation. We verify the equivalence of the chiral susceptibility criterion with different definitions for the susceptibility and confirm that the chiral susceptibility criterion is efficient to fix not only the chiral phase boundary but also the critical end point (CEP), especially when one could not have the effective thermodynamical potential. We propose a generalized Schwinger function criterion for the confinement. We give the phase diagram of both phase transitions and show that in the refined scheme the position of the CEP shifts to lower chemical potential and higher temperature. Based on our calculation and previous results of the chemical freeze out conditions, we propose that the CEP locates in the states of the matter generated by the Au--Au collisions with $\sqrt{s_{NN}^{}}=9\sim15$ GeV.
Fei Gao, Yu-xin Liu Journal reference: Phys. Rev. D 94, 076009 (2016) [pdf] DOI: 10.1103/PhysRevD.94.076009
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Two-photon decays of $$\eta _c$$ η c from lattice QCD -
Abstract
- We present an exploratory lattice study for the two-photon decay of $\eta_c$ using $N_f=2$ twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration. Two different lattice spacings of $a=0.067$fm and $a=0.085$fm are used in the study, both of which are of physical size of 2$fm$. The decay widths are found to be $1.025(5)$KeV for the coarser lattice and $1.062(5)$KeV for the finer lattice respectively where the errors are purely statistical. A naive extrapolation towards the continuum limit yields $\Gamma\simeq 1.122(14)$KeV which is smaller than the previous quenched result and most of the current experimental results. Possible reasons are discussed.
Ting Chen, Ying Chen, Ming Gong, Yu-Hong Lei, Ning Li, Chuan Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Wei-Feng Qiu, Zhan-Lin Wang, Jian-Bo Zhang [pdf] DOI: 10.1140/epjc/s10052-016-4212-8 1602.00076v4 [pdf]
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Lattice study of
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Abstract
- In this exploratory lattice study, low-energy near threshold scattering of the $(\bar{D}_1 D^{*})^\pm$ meson system is analyzed using lattice QCD with $N_f=2$ twisted mass fermion configurations. Both s-wave ($J^P=0^-$) and p-wave ($J^P=1^+$) channels are investigated. It is found that the interaction between the two charmed mesons is attractive near the threshold in both channels. This calculation provides some hints in the searching of resonances or bound states around the threshold of $(\bar{D}_1 D^{*})^\pm$ system.
Ting Chen, Ying Chen, Ming Gong, Yu-Hong Lei, Ning Li, Chuan Liu, Yu-Bin Liu, Zhao-Feng Liu, Jian-Ping Ma, Zhan-Lin Wang, Jian-Bo Zhang Journal reference: Phys. Rev. D 93, 114501 (2016) [pdf] DOI: 10.1103/PhysRevD.93.114501
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Understanding Doping, Vacancy, Lattice Stability, and Superconductivity in K
-
Abstract
- Metal-intercalated iron selenides are a class of superconductors that have received much attention but are less understood in comparison with their FeAs-based counterparts. Here, the controversial issues such as Fe vacancy, the real phase responsible for superconductivity, and lattice stability have been addressed based on first-principles calculations. New insights into the distinct features in terms of carrier doping have been revealed.
Yu Liu, Gang Wang, Tianping Ying, Xiaofang Lai, Shifeng Jin, Ning Liu, Jiangping Hu, Xiaolong Chen Journal reference: Advanced Science 1600098 (2016) [pdf] DOI: 10.1002/advs.201600098
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Large range modification of exciton species in monolayer WS_2 -
Abstract
- Unconventional emissions from exciton and trion in monolayer WS2 are studied by photoexcitation. Excited by 532nm laser beam, the carrier species in the monolayer WS2 are affected by the excess electrons escaping from photoionization of donor impurity, the concentration of which varies with different locations of the sample. Simply increasing the excitation power at room temperature, the excess electron and thus the intensity ratio of excited trion and exciton can be continuously tuned over a large range from 0.1 to 7.7. Furthermore, this intensity ratio can also be manipulated by varying temperature. However, in this way the resonance energy of the exciton and trion show red-shifts with increasing temperature due to electron-phonon coupling. The binding energy of the trion is determined to be ~23meV and independent to temperature, indicating strong Coulomb interaction of carriers in such 2D materials.
Ke Wei, Yu Liu, Hang Yang, Xiangai Cheng, Tian Jiang Journal reference: Appl. Opt. 55(23), 6251-6255 (2016) [pdf] DOI: 10.1364/AO.55.006251
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Pure geometric thick f(R)-branes: stability and localization of gravity -
Abstract
- We study two exactly solvable five-dimensional thick brane world models in pure metric $f(R)$ gravity. Working in the Einstein frame, we show that these solutions are stable against small linear perturbations, including the tensor, vector, and scalar modes. For both models, the corresponding gravitational zero mode is localized on the brane, which leads to the four-dimensional Newton's law; while the massive modes are nonlocalized and only contribute a small correction to the Newton's law at a large distance.
Yuan Zhong, Yu-Xiao Liu Journal reference: Eur. Phys. J. C76 (2016) 321 [pdf] DOI: 10.1140/epjc/s10052-016-4163-0
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Superconductivity and Charge Density Wave in ZrTe3−xSex -
Abstract
- Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0\leq}x\leq0.1$) increases up to 4 K plateau for $0.04$$\leq$$x$$\leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.
Xiangde Zhu, Wei Ning, Lijun Li, Langsheng Ling, Ranran Zhang, Jinglei Zhang, Kefeng Wang, Yu Liu, Li Pi, Yongchang Ma, Haifeng Du, Minglian Tian, Yuping Sun, Cedomir Petrovic, Yuheng Zhang Journal reference: Scientific Reports 6, 26974 (2016) [pdf] DOI: 10.1038/srep26974
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Storytelling of Photo Stream with Bidirectional Multi-thread Recurrent
Neural Network -
Abstract
- Visual storytelling aims to generate human-level narrative language (i.e., a natural paragraph with multiple sentences) from a photo streams. A typical photo story consists of a global timeline with multi-thread local storylines, where each storyline occurs in one different scene. Such complex structure leads to large content gaps at scene transitions between consecutive photos. Most existing image/video captioning methods can only achieve limited performance, because the units in traditional recurrent neural networks (RNN) tend to "forget" the previous state when the visual sequence is inconsistent. In this paper, we propose a novel visual storytelling approach with Bidirectional Multi-thread Recurrent Neural Network (BMRNN). First, based on the mined local storylines, a skip gated recurrent unit (sGRU) with delay control is proposed to maintain longer range visual information. Second, by using sGRU as basic units, the BMRNN is trained to align the local storylines into the global sequential timeline. Third, a new training scheme with a storyline-constrained objective function is proposed by jointly considering both global and local matches. Experiments on three standard storytelling datasets show that the BMRNN model outperforms the state-of-the-art methods.
Yu Liu, Jianlong Fu, Tao Mei, Chang Wen Chen [pdf]
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Comment on "Exact Classification of Landau-Majorana-Stuckelberg-Zener
Resonances by Floquet Determinants" -
Abstract
- In a recent Letter [S. Ganeshan, E. Barnes, and S. Das Sarma, Phys. Rev. Lett. 111, 130405 (2013)], Ganeshan et al. present a general framework to classify the resonance structure of Landau-Majorana-Stuckelberg-Zener interferometry into three basic categories distinguished by whether these resonances correspond to periodic or nonperiodic quantum evolution. In this Comment, we show that their identification of the real resonances in the regime of small drive amplitude is incorrect.
Qing-Wei Wang, Yu-Liang Liu [pdf]
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An efficient and compact quantum switch for quantum circuits -
Abstract
- The engineering of quantum devices has reached the stage where we now have small scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway. However as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses (in principle on/off ratio higher than 100 dB). This has important implementations in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling processing. Our approach suggests a new way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors without the need for auxiliary circuits and yet be free of the frequency crowding problems.
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On The Birch and Swinnerton-Dyer Conjecture for CM Elliptic Curves over
$\BQ$ -
Abstract
- For CM elliptic curve over rational field with analytic rank one, for any potential good ordinary prime p, not dividing the number of roots of unity in the complex multiplication field, we show the p-part of its Shafarevich-Tate group has order predicted by the Birch and Swinnerton-Dyer conjecture.
Yongxiong Li, Yu Liu, Ye Tian [pdf]
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Phase diagram and thermal properties of strong-interaction matter -
Abstract
- We introduce a novel procedure for computing the (mu,T)-dependent pressure in continuum QCD; and therefrom obtain a complex phase diagram and predictions for thermal properties of the system, providing the in-medium behaviour of the trace anomaly, speed of sound, latent heat and heat capacity.
Fei Gao, Jing Chen, Yu-Xin Liu, Si-Xue Qin, Craig D. Roberts, Sebastian M. Schmidt Journal reference: Phys. Rev. D 93, 094019 (2016) [pdf] DOI: 10.1103/PhysRevD.93.094019
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Properties of mesons in a strong magnetic field -
Abstract
- By extending the $\Phi$-derivable approach in Nambu-Jona-Lasinio model to finite magnetic field we calculate the properties of pion, $\sigma$ and $\rho$ mesons in a magnetic field at finite temperature in not only the quark-antiquark bound state scheme but also the pion-pion scattering resonant state scenario. Our calculation results manifest that the masses of $\pi^{0}$ and $\sigma$ meson can be nearly degenerate at the pseudo-critical temperature which increases with increasing the magnetic field strength, and the $\pi^{\pm}$ mass ascends suddenly at almost the same critical temperature. While the $\rho$ mesons' masses decrease with the temperature but increase with the magnetic field strength. We also check the Gell-Mann-Oakes-Renner relation and find that the relation can be violated obviously with increasing the temperature, and the effect of the magnetic field becomes pronounced around the critical temperature. With different criteria, we analyze the effect of the magnetic field on the chiral phase transition and find that the pseudo-critical temperature of the chiral phase cross, $T_{c}^{\chi}$, is always enhanced by the magnetic field. Moreover our calculations indicate that the $\rho$ mesons will get melted as the chiral symmetry has not yet been restored, but the $\sigma$ meson does not disassociate even at very high temperature. Particularly, it is the first to show that there does not exist vector meson condensate in the QCD vacuum in the pion-pion scattering scheme.
Rui Zhang, Wei-jie Fu, Yu-xin Liu Journal reference: Eur. Phys. J. C 76, 307 (2016) [pdf] DOI: 10.1140/epjc/s10052-016-4123-8
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Gravitational resonances on f(R)-brane -
Abstract
- In this paper, we investigate various $f(R)$-brane models and compare their gravitational resonance structures with the corresponding general relativity (GR)-branes. {Starting from some known GR-brane solutions}, we derive thick $f(R)$-brane solutions such that the metric, scalar field, and scalar potential coincide with those of the corresponding GR-branes. {We find that for branes generated by a single or several canonical scalar fields, there is no obvious distinction between the GR-branes and corresponding $f(R)$-branes in terms of gravitational resonance structure.} Then we discuss the branes generated by K-fields. In this case, there could exist huge differences between GR-branes and $f(R)$-branes.
Hao Yu, Yuan Zhong, Bao-Min Gu, Yu-Xiao Liu Journal reference: Eur. Phys. J. C76 (2016) no.4, 195 [pdf] DOI: 10.1140/epjc/s10052-016-4039-3
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Spectrum and Bethe-Salpeter amplitudes of Ω baryons from lattice QCD -
Abstract
- The $\Omega$ baryons with $J^P=3/2^\pm, 1/2^\pm$ are studied on the lattice in the quenched approximation. Their mass levels are ordered as $M_{3/2^+}<m_{3/2^-}\approx m_{1/2^-}<m_{1/2^+}$,="M_{1/2^-}<M_{1/2^+}$," as="as" is="is" expected="expected" from="from" the="the" constituent="constituent" quark="quark" model.="model." the="the" mass="mass" values="values" are="are" also="also" close="close" to="to" those="those" of="of" the="the" four="four" $\omega$="$\Omega$" states="states" observed="observed" in="in" experiments,="experiments," respectively.="respectively." we="We" calculate="calculate" the="the" bethe-salpeter="Bethe-Salpeter" amplitudes="amplitudes" of="of" $\omega(3/2^+)$="$\Omega(3/2^+)$" and="and" $\omega(1/2^+)$="$\Omega(1/2^+)$" and="and" find="find" there="there" is="is" a="a" radial="radial" node="node" for="for" the="the" $\omega(1/2^+)$="$\Omega(1/2^+)$" bethe-salpeter="Bethe-Salpeter" amplitude,="amplitude," which="which" may="may" imply="imply" that="that" $\omega(1/2^+)$="$\Omega(1/2^+)$" is="is" an="an" orbital="orbital" excitation="excitation" of="of" $\omega$="$\Omega$" baryons="baryons" as="as" a="a" member="member" of="of" the="the" $(d,l_n^p)="(70,0_2^+)$" supermultiplet="supermultiplet" in="in" the="the" $su(6)\bigotimes="$SU(6)\bigotimes" o(3)$="O(3)$" quark="quark" model="model" description.="description." our="Our" results="results" are="are" helpful="helpful" for="for" identifying="identifying" the="the" quantum="quantum" number="number" of="of" experimentally="experimentally" observed="observed" $\omega$="$\Omega$" states.
Jian Liang, Wei Sun, Ying Chen, Wei-Feng Chiu, Ming Gong, Chuan Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Jian-Bo Zhang Journal reference: Chin. Phys. C 40 (2016) 041001 [pdf] DOI: 10.1088/1674-1137/40/4/041001
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Analytical and exact critical phenomena of
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Abstract
- In the extended phase space, the $d$-dimensional singly spinning Kerr-AdS black holes exhibit the van der Waals's phase transition and reentrant phase transition. Since the black hole system is a single characteristic parameter thermodynamic system, we show that the form of the critical point can be uniquely determined by the dimensional analysis. When $d=4$, we get the analytical critical point. The coexistence curve and phase diagrams are obtained. The result shows that the fitting form of the coexistence curve in the reduced parameter space is independent of the angular momentum. When $d=5$---$9$, the exact critical points are numerically solved. It demonstrates that when $d\geq6$, there are two critical points. However, the small one does not participate in the phase transition. Moreover, the exact critical reentrant phase transition points are also obtained. All the critical points are obtained without any approximation.
Shao-Wen Wei, Peng Cheng, Yu-Xiao Liu Journal reference: Phys. Rev. D 93, 084015 (2016) [pdf] DOI: 10.1103/PhysRevD.93.084015
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Topology Discovery for Linear Wireless Networks With Application to Train Backbone Inauguration -
Abstract
- A train backbone network consists of a sequence of nodes arranged in a linear topology. A key step that enables communication in such a network is that of topology discovery, or train inauguration, whereby nodes learn in a distributed fashion the physical topology of the backbone network. While the current standard for train inauguration assumes wired links between adjacent backbone nodes, this work investigates the more challenging scenario in which the nodes communicate wirelessly. The key motivations for this desired switch from wired topology discovery to wireless one are the flexibility and capability for expansion and upgrading of a wireless backbone. The implementation of topology discovery over wireless channels is made difficult by the broadcast nature of the wireless medium, and by fading and interference. A novel topology discovery protocol is proposed that overcomes these issues and requires relatively minor changes to the wired standard. The protocol is shown via analysis and numerical results to be robust to the impairments caused by the wireless channel including interference from other trains.
Yu Liu, Jianghua Feng, Osvaldo Simeone, Jun Tang, Zheng Wen, Alexander M. Haimovich, MengChu Zhou [pdf] DOI: 10.1109/TITS.2015.2512605 1503.00184v3 [pdf]
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Modulation Classification for MIMO-OFDM Signals via Approximate Bayesian Inference -
Abstract
- The problem of modulation classification for a multiple-antenna (MIMO) system employing orthogonal frequency division multiplexing (OFDM) is investigated under the assumption of unknown frequency-selective fading channels and signal-to-noise ratio (SNR). The classification problem is formulated as a Bayesian inference task, and solutions are proposed based on Gibbs sampling and mean field variational inference. The proposed methods rely on a selection of the prior distributions that adopts a latent Dirichlet model for the modulation type and on the Bayesian network formalism. The Gibbs sampling method converges to the optimal Bayesian solution and, using numerical results, its accuracy is seen to improve for small sample sizes when switching to the mean field variational inference technique after a number of iterations. The speed of convergence is shown to improve via annealing and random restarts. While most of the literature on modulation classification assume that the channels are flat fading, that the number of receive antennas is no less than that of transmit antennas, and that a large number of observed data symbols are available, the proposed methods perform well under more general conditions. Finally, the proposed Bayesian methods are demonstrated to improve over existing non-Bayesian approaches based on independent component analysis and on prior Bayesian methods based on the `superconstellation' method.
Yu Liu, Osvaldo Simeone, Alexander M. Haimovich, Wei Su [pdf] DOI: 10.1109/TVT.2016.2537791 1509.03353v2 [pdf]
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Resonance spectrum of a bulk fermion on branes -
Abstract
- It is known that there are two mechanisms for localizing a bulk fermion on a brane, one is the well-known Yukawa coupling and the other is the new coupling proposed in [Phys. Rev. D 89, 086001 (2014)]. In this paper, we investigate localization and resonance spectrum of a bulk fermion on the same branes with the two localization mechanisms. It is found that both the two mechanisms can result in a volcano-like effective potential of the fermion Kaluza-Klein modes. The left-chiral fermion zero mode can be localized on the brane and there exist some discrete massive fermion Kaluza-Klein modes that quasilocalized on the brane (also called fermion resonances). The number of the fermion resonances increases linearly with the coupling parameter.
Yu-Peng Zhang, Yun-Zhi Du, Wen-Di Guo, Yu-Xiao Liu Journal reference: Phys. Rev. D 93, 065042 (2016) [pdf] DOI: 10.1103/PhysRevD.93.065042
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Critical phenomena in the extended phase space of Kerr-Newman-AdS black holes -
Abstract
- Treating the cosmological constant as a thermodynamic pressure, we investigate the critical behavior of a Kerr-Newman-AdS black hole system. The critical points for the van der Waals like phase transition are numerically solved. The highly accurate fitting formula for them is given and is found to be dependent of the charge $Q$ and angular momentum $J$. In the reduced parameter space, we find that the temperature, Gibbs free energy, and coexistence curve depend only on the dimensionless angular momentum-charge ratio $\epsilon=J/Q^2$ rather than $Q$ and $J$. Moreover, when varying $\epsilon$ from 0 to $\infty$, the coexistence curve will continuously change from that of the Reissner-Nordstr\"{o}m-AdS black hole to the Kerr-AdS black hole. These results may guide us to study the critical phenomena for other thermodynamic systems with two characteristic parameters.
Peng Cheng, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 94, 024025 (2016) [pdf] DOI: 10.1103/PhysRevD.94.024025
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New localization mechanism and Hodge duality for
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Abstract
- In this paper, we investigate the problem of localization and the Hodge duality for a $q-$form field on a $p-$brane with codimension one. By a general Kaluza-Klein (KK) decomposition without gauge fixing, we obtain two Schr\"{o}dinger-like equations for two types of KK modes of the bulk $q-$form field, which determine the localization and mass spectra of these KK modes. It is found that there are two types of zero modes (the $0-$level modes): a $q-$form zero mode and a $(q-1)-$form one, which cannot be localized on the brane at the same time. For the $n-$level KK modes, there are two interacting KK modes, a massive $q-$form KK mode and a massless $(q-1)-$form one. By analyzing gauge invariance of the effective action and choosing a gauge condition, the $n-$level massive $q-$form KK mode decouples from the $n-$level massless $(q-1)-$form one. It is also found that the Hodge duality in the bulk naturally becomes two dualities on the brane. The first one is the Hodge duality between a $q-$form zero mode and a $(p-q-1)-$form one, or between a $(q-1)-$form zero mode and a $(p-q)-$form one. The second duality is between two group KK modes: one is an $n-$level massive $q-$form KK mode with mass $m_n$ and an $n-$level massless $(q-1)-$form mode; another is an $n-$level $(p-q)-$form one with the same mass $m_n$ and an $n-$level massless $(p-q-1)-$form mode. Because of the dualities, the effective field theories on the brane for the KK modes of the two dual bulk form fields are physically equivalent.
Chun-E Fu, Yu-Xiao Liu, Heng Guo, Sheng-Li Zhang Journal reference: Phys. Rev. D 93, 064007 (2016) [pdf] DOI: 10.1103/PhysRevD.93.064007
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Intrinsic diamagnetism in the Weyl semimetal TaAs -
Abstract
- We investigate the magnetic properties of TaAs, a prototype Weyl semimetal. TaAs crystals show weak diamagnetism with magnetic susceptibility of about -7 * 10^{-7} emu/(g*Oe) at 5 K. A general feature is the appearance of a minimum at around 185 K in magnetization measurements as a function of temperature. No phase transition is observed in the temperature range between 5 K and 400 K. The magnetic properties indicate that the intrinsic Fermi level in TaAs is not located at the Weyl nodes, in agreement with the theory prediction.
Yu Liu, Zhilin Li, Liwei Guo, Xiaolong Chen, Ye Yuan, Fang Liu, Slawomir Prucnal, Manfred Helm, Shengqiang Zhou Journal reference: Journal of Magnetism and Magnetic Materials, 408, 73-76 (2016) [pdf] DOI: 10.1016/j.jmmm.2016.01.100
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Nonreciprocal conversion between microwave and optical photons in electro-optomechanical systems -
Abstract
- We propose to demonstrate nonreciprocal conversion between microwave and optical photons in an electro-optomechanical system where a microwave mode and an optical mode are coupled indirectly via two non-degenerate mechanical modes. The nonreciprocal conversion is obtained in the broken time-reversal symmetry regime, where the conversion of photons from one frequency to the other is enhanced for constructive quantum interference while the conversion in the reversal direction is suppressed due to destructive quantum interference. It is interesting that the nonreciprocal response between the microwave and optical modes in the electro-optomechanical system appears at two different frequencies with opposite directions. The proposal can be used to realize nonreciprocal conversion between photons of any two distinctive modes with different frequencies. Moreover, the electro-optomechanical system can also be used to construct a three-port circulator for three optical modes with distinctively different frequencies by adding an auxiliary optical mode coupled to one of the mechanical modes.
Xun-Wei Xu, Yong Li, Ai-Xi Chen, Yu-xi Liu Journal reference: Phys. Rev. A 93, 023827 (2016) [pdf] DOI: 10.1103/PhysRevA.93.023827
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Polariton states in circuit QED for electromagnetically induced transparency -
Abstract
- Electromagnetically induced transparency (EIT) has been extensively studied in various systems. However, it is not easy to observe in superconducting quantum circuits (SQCs), because the Rabi frequency of the strong controlling field corresponding to EIT is limited by the decay rates of the SQCs. Here, we show that EIT can be achieved by engineering decay rates in a superconducting circuit QED system through a classical driving field on the qubit. Without such a driving field, the superconducting qubit and the cavity field are approximately decoupled in the large detuning regime, and thus the eigenstates of the system are approximately product states of the cavity field and qubit states. However, the driving field can strongly mix these product states and so-called polariton states can be formed. The weights of the states for the qubit and cavity field in the polariton states can be tuned by the driving field, and thus the decay rates of the polariton states can be changed. We choose a three-level system with $\Lambda$-type transitions in such a driven circuit QED system, and demonstrate how EIT and ATS can be realized in this compound system. We believe that this study will be helpful for EIT experiments using SQCs.
Xiu Gu, Sai-nan Huai, Franco Nori, Yu-xi Liu Journal reference: Phys. Rev. A 93, 063827 (2016) [pdf] DOI: 10.1103/PhysRevA.93.063827
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Structural, optical and electrical properties of Sb2Te3 films prepared by pulsed laser deposition -
Abstract
- Detecting regional spatial structures based on spatial interactions is crucial in applications ranging from urban planning to traffic control. In the big data era, various movement trajectories are available for studying spatial structures. This research uses large scale Shanghai taxi trip data extracted from GPS-enabled taxi trajectories to reveal traffic flow patterns and urban structure of the city. Using the network science methods, 15 temporally stable regions reflecting the scope of people's daily travels are found using community detection method on the network built from short trips, which represent residents' daily intra-urban travels and exhibit a clear pattern. In each region, taxi traffic flows are dominated by a few 'hubs' and 'hubs' in suburbs impact more trips than 'hubs' in urban areas. Land use conditions in urban regions are different from those in suburban areas. Additionally, 'hubs' in urban area associate with office buildings and commercial areas more, whereas residential land use is more common in suburban hubs. The taxi flow structures and land uses reveal the polycentric and layered concentric structure of Shanghai. Finally, according to the temporal variations of taxi flows and the diversity levels of taxi trip lengths, we explore the total taxi traffic properties of each region and proved the city structure we find. External trips across regions also take large proportion of the total traffic in each region, especially in suburbs. The results could help transportation policy making and shed light on the way to reveal urban structures with big data.
Xi Liu, Li Gong, Yongxi Gong, Yu Liu Journal reference: Journal of Transport Geography, 2015, 43, 78-90 [pdf] DOI: 10.1016/j.jcrysgro.2015.01.022
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Hybrid metric-Palatini brane system -
Abstract
- It is known that the metric and Palatini formalisms of gravity theories have their own interesting features but also suffer from some different drawbacks. Recently, a novel gravity theory called hybrid metric-Palatini gravity was put forward to cure or improve their individual deficiencies. The action of this gravity theory is a hybrid combination of the usual Einstein-Hilbert action and a $f(\mathcal{R})$ term constructed by the Palatini formalism. Interestingly, it seems that the existence of a light and long-range scalar field in this gravity may modify the cosmological and galactic dynamics without conflicting with the laboratory and Solar System tests. In this paper we focus on the tensor perturbation of thick branes in this novel gravity theory. We consider two models as examples, namely, the thick branes constructed by a background scalar field and by pure gravity. The thick branes in both models have no inner structure. However, the graviton zero mode in the first model has inner structure when the parameter in this model is larger than its critical value. We find that the effective four-dimensional gravity can be reproduced on the brane for both models. Moreover, the stability of both brane systems against the tensor perturbation can also be ensured.
Qi-Ming Fu, Li Zhao, Yu-Xiao Liu Journal reference: Phys. Rev. D 94, 024020 (2016) [pdf] DOI: 10.1103/PhysRevD.94.024020
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Performance Analysis of Relay Selection With Enhanced Dynamic
Decode-and-Forward and Network Coding in Two-Way Relay Channels -
Abstract
- In this paper, we adopt the relay selection (RS) protocol proposed by Bletsas, Khisti, Reed and Lippman (2006) with Enhanced Dynamic Decode-and-Forward (EDDF) and network coding (NC) system in a two-hop two-way multi-relay network. All nodes are single-input single-output (SISO) and half-duplex, i.e., they cannot transmit and receive data simultaneously. The outage probability is analyzed and we show comparisons of outage probability with various scenarios under Rayleigh fading channel. Our results show that the relay selection with EDDF and network coding (RS-EDDF&NC) scheme has the best performance in the sense of outage probability upon the considered decode-and-forward (DF) relaying if there exist sufficiently relays. In addition, the performance loss is large if we select a relay at random. This shows the importance of relay selection strategies.
Wei-Cheng Liu, Yu-Chen Liu [pdf]
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Half exact functors associated with cotorsion pairs on exact categories -
Abstract
- In the previous article "Hearts of twin cotorsion pairs on exact categories", we introduced the notion of the heart for any cotorsion pair on an exact category with enough projectives and injectives, and showed that it is an abelian category. In this paper, we construct a half exact functor from the exact category to the heart. This is analog of the construction of Abe and Nakaoka for triangulated categories. We will also use this half exact functor to find out a sufficient condition when two different hearts are equivalent.
Yu Liu [pdf]
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A PAIR OF GIANT PLANETS AROUND THE EVOLVED INTERMEDIATE-MASS STAR HD 47366: MULTIPLE CIRCULAR ORBITS OR A MUTUALLY RETROGRADE CONFIGURATION -
Abstract
- We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at Okayama Astrophysical Observatory, Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81+-0.13M_sun, a radius of 7.30+-0.33R_sun, and solar metallicity. The planetary system is composed of two giant planets with minimum mass of 1.75^{+0.20}_{-0.17}Mjup and 1.86^{+0.16}_{-0.15}Mjup, orbital period of 363.3^{+2.5}_{-2.4} d and 684.7^{+5.0}_{-4.9} d, and eccentricity of 0.089^{+0.079}_{-0.060} and 0.278^{+0.067}_{-0.094}, respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ~0.15, which is just within 1.4sigma of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160 degree.
Bun'ei Sato, Liang Wang, Yu-Juan Liu, Gang Zhao, Masashi Omiya, Hiroki Harakawa, Makiko Nagasawa, Robert A. Wittenmyer, Paul Butler, Nan Song, Wei He, Fei Zhao, Eiji Kambe, Kunio Noguchi, Hiroyasu Ando, Hideyuki Izumiura, Norio Okada, Michitoshi Yoshida, Yoichi Takeda, Yoichi Itoh, Eiichiro Kokubo, Shigeru Ida [pdf] DOI: 10.3847/0004-637X/819/1/59 1601.04417v1 [pdf]
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Heegner Points on Modular Curves -
Abstract
- In this paper, we study the Heegner points on more general modular curves other than $X_0(N)$, which generalizes Gross' work "Heegner points on $X_0(N)$". The explicit Gross-Zagier formula and the Euler system property are stated in this case. Using such kind of Heegner points, we construct certain families of quadratic twists of $X_0(36)$, with the ranks of Mordell-Weil groups being zero and one respectively, and show that the $2$-part of their BSD conjectures hold.
Li Cai, Yihua Chen, Yu Liu [pdf]
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Tunable multiphonon blockade in coupled nanomechanical resonators -
Abstract
- A single phonon in a nonlinear nanomechanical resonator (NAMR) can block the excitation of a second phonon [Phys. Rev. A 82, 032101 (2010)]. This intrinsically quantum effect is called phonon blockade, and is an analog of Coulomb blockade and photon blockade. Here we predict tunable multiphonon blockade in coupled nonlinear NAMRs, where nonlinearity is induced by two-level systems (TLSs) assuming dispersive (far off-resonance) interactions. Specifically, we derive an effective Kerr-type interaction in a hybrid system consisting of two nonlinearly-interacting NAMRs coupled to two TLSs and driven by classical fields. The interaction between a given NAMR and a TLS is described by a Jaynes-Cummings-like model. We show that by properly tuning the frequency of the driving fields one can induce various types of phonon blockade, corresponding to the entangled phonon states of either two qubits, qutrit and quartit, or two qudits. Thus, a $k$-phonon Fock state (with $k=1,2,3$) can impede the excitation of more phonons in a given NAMR, which we interpret as a $k$-phonon blockade (or, equivalently, phonon tunneling). Our results can be explained in terms of resonant transitions in the Fock space and via phase-space interference using the $s$-parametrized Cahill-Glauber quasiprobability distributions including the Wigner function. We study the nonclassicality, entanglement, and dimensionality of the blockaded phonon states during both dynamics and in the stationary limits.
Adam Miranowicz, Jiri Bajer, Neill Lambert, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 93, 013808 (2016) [pdf] DOI: 10.1103/PhysRevA.93.013808
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Cosmological evolution of generalized non-local gravity -
Abstract
- We construct a class of generalized non-local gravity (GNLG) model which is the modified theory of general relativity (GR) obtained by adding a term $m^{2n-2} R\Box^{-n}R$ to the Einstein-Hilbert action. Concretely, we not only study the gravitational equation for the GNLG model by introducing auxiliary scalar fields, but also analyse the classical stability and examine the cosmological consequences of the model for different exponent $n$. We find that the half of the scalar fields are always ghost-like and the exponent $n$ must be taken even number for a stable GNLG model. Meanwhile, the model spontaneously generates three dominant phases of the evolution of the universe, and the equation of state parameters turn out to be phantom-like. Furthermore, we clarify in another way that exponent $n$ should be even numbers by discuss the spherically symmetric static solutions in Newtonian gauge. It is worth stressing that the results given by us can include ones in refs. [28, 34] as the special case of $n=2$.
Xue Zhang, Ya-Bo Wu, Song Li, Yu-Chen Liu, Bo-Hai Chen, Yun-Tian Chai, Shuang Shu Journal reference: JCAP07(2016)003 [pdf] DOI: 10.1088/1475-7516/2016/07/003
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Tensor perturbations of
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Abstract
- We explore the tensor perturbation of the $f(T)$ brane embedded in an AdS$_5$ spacetime. With the transverse-traceless condition, we get the tensor perturbation equation of the $f(T)$ brane and show that the stability of this brane system can be ensured. In addition, we take $ f(T)=T+\alpha T^2$ as an example to analyse the localization problem of the graviton zero mode. It is shown that the graviton zero mode can be localized on the brane.
Wen-Di Guo, Qi-Ming Fu, Yu-Peng Zhang, Yu-Xiao Liu Journal reference: Phys. Rev. D 93, 044002 (2016) [pdf] DOI: 10.1103/PhysRevD.93.044002
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Metrology with
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Abstract
- We propose and analyze a new approach based on parity-time ($\mathcal{PT}$) symmetric microcavities with balanced gain and loss to enhance the performance of cavity-assisted metrology. We identify the conditions under which $\mathcal{PT}$-symmetric microcavities allow to improve sensitivity beyond what is achievable in loss-only systems. We discuss its application to the detection of mechanical motion, and show that the sensitivity is significantly enhanced in the vicinity of the transition point from unbroken- to broken-$\mathcal{PT}$ regimes. We believe that our results open a new direction for $\mathcal{PT}$-symmetric physical systems and it may find use in ultra-high precision metrology and sensing.
Zhong-Peng Liu, Jing Zhang, Şahin Kaya Özdemir, Bo Peng, Hui Jing, Xin-You Lü, Chun-Wen Li, Lan Yang, Franco Nori, Yu-xi Liu Journal reference: Phys. Rev. Lett. 117, 110802 (2016) [pdf] DOI: 10.1103/PhysRevLett.117.110802
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Mode coupling and photon antibunching in a bimodal cavity containing a dipole quantum emitter -
Abstract
- We study the effect of mode-coupling on a single-photon device in which a dipole-quantum-emitter (DQE) is embedded in a bimodal whispering-gallery-mode cavity (WGMC). A scatterer is used to induce mode coupling between counter-clockwise and clockwise propagating light fields, which interact with the DQE. In contrast to models for the interaction between a DQE and a (one-mode or two-mode) cavity field, we find that strong photon antibunching can occur even for a weak DQE-field coupling and large dephasing of the DQE, when mode coupling is introduced. We also find that mode coupling can make the device robust against either the frequency mismatch between cavity modes and the DQE or the coupling strength mismatch between the DQE and each mode in the two-mode cavity. Moreover, we find that these mismatches can be used to generate better antibunching in the weak DQE-field coupling regime. Our study shows that mode coupling in a bimodal cavity is very important for the realization of a good single-photon device.
Yu-Long Liu, Guan-Zhong Wang, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 93, 013856 (2016) [pdf] DOI: 10.1103/PhysRevA.93.013856
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Direct evidence of interaction-induced Dirac cones in a monolayer silicene/Ag(111) system -
Abstract
- Silicene, analogous to graphene, is a one-atom-thick two-dimensional crystal of silicon which is expected to share many of the remarkable properties of graphene. The buckled honeycomb structure of silicene, along with its enhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spin-split states in silicene are tunable with external fields. Although the low-energy Dirac cone states lie at the heart of all novel quantum phenomena in a pristine sheet of silicene, the question of whether or not these key states can survive when silicene is grown or supported on a substrate remains hotly debated. Here we report our direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved photoemission measurements on silicene(3x3)/Ag(111), we reveal the presence of six pairs of Dirac cones on the edges of the first Brillouin zone of Ag(111), other than expected six Dirac cones at the K points of the primary silicene(1x1) Brillouin zone. Our result shows clearly that the unusual Dirac cone structure originates not from the pristine silicene alone but from the combined effect of silicene(3x3) and the Ag(111) substrate. This study identifies the first case of a new type of Dirac Fermion generated through the interaction of two different constituents. Our observation of Dirac cones in silicene/Ag(111) opens a new materials platform for investigating unusual quantum phenomena and novel applications based on two-dimensional silicon systems.
Ya Feng, Defa Liu, Baojie Feng, Xu Liu, Lin Zhao, Zhuojin Xie, Yan Liu, Aiji Liang, Cheng Hu, Yong Hu, Shaolong He, Guodong Liu, Jun Zhang, Chuangtian Chen, Zuyan Xu, Lan Chen, Kehui Wu, Yu-Tzu Liu, Hsin Lin, Zhi-Quan Huang, Chia-Hsiu Hsu, Feng-Chuan Chuang, Arun Bansil, X. J. Zhou Journal reference: PNAS 113(51), 14656-14661 (2016) [pdf] DOI: 10.1073/pnas.1613434114
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Quantum simulation of pairing Hamiltonians with nearest-neighbor-interacting qubits -
Abstract
- Although a universal quantum computer is still far from reach, the tremendous advances in controllable quantum devices, in particular with solid-state systems, make it possible to physically implement "quantum simulators". Quantum simulators are physical setups able to simulate other quantum systems efficiently that are intractable on classical computers. Based on solid-state qubit systems with various types of nearest-neighbor interactions, we propose a complete set of algorithms for simulating pairing Hamiltonians. Fidelity of the target states corresponding to each algorithm is numerically studied. We also compare algorithms designed for different types of experimentally available Hamiltonians and analyze their complexity. Furthermore, we design a measurement scheme to extract energy spectra from the simulators. Our simulation algorithms might be feasible with state-of-the-art technology in solid-state quantum devices.
Zhixin Wang, Xiu Gu, Lian-Ao Wu, Yu-xi Liu Journal reference: Phys. Rev. A 93, 062301 (2016) [pdf] DOI: 10.1103/PhysRevA.93.062301
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Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate
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Abstract
- We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 $\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.
Nasser Alidoust, Chang Liu, Su-Yang Xu, Ilya Belopolski, Tongfei Qi, Minggang Zeng, Madhab Neupane, Guang Bian, Yu-Tzu Liu, Stephen D. Wilson, Hsin Lin, Arun Bansil, Gang Cao, M. Zahid Hasan Journal reference: Phys. Rev. B 93, 245132 (2016) [pdf] DOI: 10.1103/PhysRevB.93.245132
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Channel Measurements and Models for High-Speed Train Wireless
Communication Systems in Tunnel Scenarios: A Survey -
Abstract
- 2015
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Phase-dependent optical response properties in an optomechanical system by coherently driving the mechanical resonator -
Abstract
- We explore theoretically the optical response properties in an optomechanical system under electromagneti- cally induced transparency condition but with the mechanical resonator being driven by an additional coherent field. In this configuration, more complex quantum coherent and interference phenomena occur. In partic- ular, we find that the probe transmission spectra depend on the total phase of the applied fields. Our study also provides an efficient way to control propagation of amplification.
W. Z. Jia, L. F. Wei, Yong Li, Yu-xi Liu Journal reference: Phys. Rev. A 91, 043843 (2015) [pdf] DOI: 10.1103/PhysRevA.91.043843
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Correlated Emission Lasing in Harmonic Oscillators Coupled via a Single Three-Level Artificial Atom -
Abstract
- A single superconducting artificial atom provides a unique basis for coupling electromagnetic fields and photons hardly achieved with a natural atom. Bringing a pair of harmonic oscillators into resonance with transitions of the three-level atom converts atomic spontaneous processes into correlated emission dynamics. We demonstrate two-mode correlated emission lasing on harmonic oscillators coupled via the fully controllable three-level artificial atom. Correlation of two different color emissions reveals itself as equally narrowed linewiths and quench of their mutual phase-diffusion. The mutual linewidth is more than four orders of magnitude narrower than the Schawlow-Townes limit. The interference between the different color lasing fields demonstrates the two-mode fields are strongly correlated.
Z. H. Peng, Yu-xi Liu, J. T. Peltonen, T. Yamamoto, J. S. Tsai, O. Astafiev Journal reference: Phys. Rev. Lett. 115, 223603 (2015) [pdf] DOI: 10.1103/PhysRevLett.115.223603
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Leading-twist parton distribution amplitudes of S-wave heavy-quarkonia -
Abstract
- The leading-twist parton distribution amplitudes (PDAs) of ground-state $^1S_0$ and $^3S_1$ $c\bar c$- and $b\bar b$-quarkonia are calculated using a symmetry-preserving continuum treatment of the meson bound-state problem which unifies the properties of these heavy-quark systems with those of light-quark bound-states, including QCD's Goldstone modes. Analysing the evolution of $^1S_0$ and $^3S_1$ PDAs with current-quark mass, $\hat m_q$, increasing away from the chiral limit, it is found that in all cases there is a value of $\hat m_q$ for which the PDA matches the asymptotic form appropriate to QCD's conformal limit and hence is insensitive to changes in renormalisation scale, $\zeta$. This mass lies just above that associated with the $s$-quark. At current-quark masses associated with heavy-quarkonia, on the other hand, the PDAs are piecewise convex-concave-convex. They are much narrower than the asymptotic distribution on a large domain of $\zeta$; but nonetheless deviate noticeably from $\varphi_{Q\bar Q}(x) = \delta(x-1/2)$, which is the result in the static-quark limit. There are also material differences between $^1S_0$ and $^3S_1$ PDAs, and between the PDAs for different vector-meson polarisations, which vanish slowly with increasing $\zeta$. An analysis of moments of the root-mean-square relative-velocity, $\langle v^{2m}\rangle$, in $^1S_0$ and $^3S_1$ systems reveals that $\langle v^4\rangle$-contributions may be needed in order to obtain a reliable estimate of matrix elements using such an expansion, especially for processes involving heavy pseudoscalar quarkonia.
Minghui Ding, Fei Gao, Lei Chang, Yu-Xin Liu, Craig D. Roberts [pdf] DOI: 10.1016/j.physletb.2015.11.075 1511.04943v1 [pdf]
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Fine Magnetic Structure and Origin of Counter-Streaming Mass Flows in a
Quiescent Solar Prominence -
Abstract
- We present high-resolution observations of a quiescent solar prominence which was consisted of a vertical and a horizontal foot encircled by an overlying spine, and counter-streaming mass flows were ubiquitous in the prominence. While the horizontal foot and the spine were connecting to the solar surface, the vertical foot was suspended above the solar surface and supported by a semicircular bubble structure. The bubble first collapsed and then reformed at a similar height, finally, it started to oscillate for a long time. We find that the collapsing and oscillation of the bubble boundary were tightly associated with a flare-like feature located at the bottom of the bubble. Based on the observational results, we propose that the prominence should be composed of an overlying horizontal spine encircling a low-lying horizontal and a vertical foot, in which the horizontal foot was consisted of shorter field lines running partially along the spine and with the both ends connecting to the solar surface, while the vertical foot was consisted of piling-up dips due to the sagging of the spine fields and supported by a bipolar magnetic system formed by parasitic polarities (i.e., the bubble). The upflows in the vertical foot were possibly caused by the magnetic reconnection at the separator between the bubble and the overlying dips, which intruded into the persistent downflow field and formed the picture of counter-streaming mass flows. In addition, the counter-streaming flows in the horizontal foot were possibly caused by the imbalanced pressure at the both ends.
Yuandeng Shen, Yu Liu, Ying D. Liu, P. F. Chen, Jiangtao Su, Zhi Xu, Zhong Liu [pdf]
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Ghost Cities Analysis Based on Positioning Data in China -
Abstract
- Real estate projects are developed excessively in China in this decade. Many new housing districts are built, but they far exceed the actual demand in some cities. These cities with a high housing vacancy rate are called ghost cities. The real situation of vacant housing areas in China has not been studied in previous research. This study, using Baidu positioning data, presents the spatial distribution of the vacant housing areas in China and classifies cities with a large vacant housing area as cities or tourism sites. To the best of our knowledge, it is the first time that we detected and analyzed the ghost cities in China at such fine scale. To understand the human dynamic in ghost cities, we select one city and one tourism sites as cases to analyze the features of human dynamics. This study illustrates the capability of big data in sensing our cities objectively and comprehensively.
Guanghua Chi, Yu Liu, Zhengwei Wu, Haishan Wu [pdf]
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Defect-induced magnetism in SiC: Interplay between ferromagnetism and paramagnetism -
Abstract
- Defect-induced ferromagnetism has triggered a lot of investigations and controversies. The major issue is that the induced ferromagnetic signal is so weak that it can sufficiently be accounted for by trace contamination. To resolve this issue, we studied the variation of the magnetic properties of SiC after neutron irradiation with fluence covering four orders of magnitude. A large paramagnetic component has been induced and scales up with defect concentration, which can be well accounted for by uncoupled divacancies. However, the ferromagnetic contribution is still weak and only appears in the low fluence range of neutrons or after annealing treatments. First-principles calculations hint towards a mutually exclusive role of the concentration of defects: Defects favor spin polarization at the expense of magnetic interaction. Combining both experimental and first-principles calculation results, the defect-induced ferromagnetism can be understood as a local effect which cannot be scaled up with the volume. Therefore, our investigation answers the long-standing question why the defect-induced ferromagnetic signal is weak.
Yutian Wang, Yu Liu, Elke Wendler, René Hübner, Wolfgang Anwand, Gang Wang, Xuliang Chen, Wei Tong, Zhaorong Yang, Frans Munnik, Gregor Bukalis, Xiaolong Chen, Sibylle Gemming, Manfred Helm, Shengqiang Zhou Journal reference: Phys. Rev. B 92, 174409 (2015) [pdf] DOI: 10.1103/PhysRevB.92.174409
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Topological susceptibility near T c in SU(3) gauge theory -
Abstract
- Topological charge susceptibility $\chi_{t}$ for pure gauge SU(3) theory at finite temperature is studied using anisotropic lattices. The over-improved stout-link smoothing method is utilized to calculate the topological charge. Near the phase transition point we find a rapid declining behavior for $\chi_{t}$ with values decreasing from $(188(1)\mathrm{MeV})^{4}$ to $(67(3)\mathrm{MeV})^{4}$ as the temperature increased from zero temperature to $1.9T_{c}$ which demonstrates the existence of topological excitations far above $T_{c}$. The 4th order cumulant $c_4$ of topological charge, as well as the ratio $c_4/\chi_t$ are also investigated. Results of $c_4$ show step-like behavior near $T_c$ while the ratio at high temperature agrees with the value as predicted by the diluted instanton gas model.
Guang-Yi Xiong, Jian-Bo Zhang, Ying Chen, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma [pdf] DOI: 10.1016/j.physletb.2015.10.085 1508.07704v2 [pdf]
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Chemical Freeze-out Parameters via a Non-perturbative QCD Approach -
Abstract
- By analyzing the calculated baryon number susceptibility ratios ${\chi_{1}^{B}}/{\chi_{2}^{B}}$ and ${\chi_{3}^{B}}/{\chi_{1}^{B}}$ in two-flavor system via the Dyson-Schwinger equation approach of QCD, we determine the chemical freeze-out temperature and baryon chemical potential in cases of both thermodynamic limit and finite size. We calculate the center-of-mass energy dependence of the ${\chi_{4}^{B}}/{\chi_{2}^{B}}\, (\kappa \sigma^{2})$ at the freeze-out line and find an excellent agreement with experimental data in $\sqrt{S_{NN}^{}} \geq 19.6\,$GeV region when taking into account the finite size effect. Our calculations indicate that the $\kappa \sigma^{2}$ exhibits a non-monotonic behavior in lower collision energy region.
Jing Chen, Fei Gao, Yu-xin Liu [pdf]
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Matching the linear spectra of twinlike defects -
Abstract
- Twinlike defects refer to topological defect solutions of some apparently different field models that share the same defect configuration and the same energy density. Usually, one can distinguish twinlike defects in terms of their linear spectra, but in some special cases twinlike defects even share the same linear spectrum. In this paper, we derive the algebraic conditions for two twinlike defects to share identical linear spectrum from the viewpoint of the normal modes of the linear fluctuations. We also extend our discussion to braneworld models, where gravity plays an important role.
Yuan Zhong, Yu-Xiao Liu Journal reference: Class.Quant.Grav. 32 (2015) 165002 [pdf] DOI: 10.1088/0264-9381/32/16/165002
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Giant nonlinearity via breaking parity-time symmetry: A route to low-threshold phonon diodes -
Abstract
- Nonreciprocal devices that permit wave transmission in only one direction are indispensible in many fields of science including, e.g., electronics, optics, acoustics, and thermodynamics. Manipulating phonons using such nonreciprocal devices may have a range of applications such as phonon diodes, transistors, switches, etc. One way of achieving nonreciprocal phononic devices is to use materials with strong nonlinear response to phonons. However, it is not easy to obtain the required strong mechanical nonlinearity, especially for few-phonon situations. Here, we present a general mechanism to amplify nonlinearity using $\mathcal{PT}$-symmetric structures, and show that an on-chip micro-scale phonon diode can be fabricated using a $\mathcal{PT}$-symmetric mechanical system, in which a lossy mechanical-resonator with very weak mechanical nonlinearity is coupled to a mechanical resonator with mechanical gain but no mechanical nonlinearity. When this coupled system transits from the $\mathcal{PT}$-symmetric regime to the broken-$\mathcal{PT}$-symmetric regime, the mechanical nonlinearity is transferred from the lossy resonator to the one with gain, and the effective nonlinearity of the system is significantly enhanced. This enhanced mechanical nonlinearity is almost lossless because of the gain-loss balance induced by the $\mathcal{PT}$-symmetric structure. Such an enhanced lossless mechanical nonlinearity is then used to control the direction of phonon propagation, and can greatly decrease (by over three orders of magnitude) the threshold of the input-field intensity necessary to observe the unidirectional phonon transport. We propose an experimentally realizable lossless low-threshold phonon diode of this type. Our study opens up new perspectives for constructing on-chip few-phonon devices and hybrid phonon-photon components.
Jing Zhang, Bo Peng, Sahin Kaya Ozdemir, Yu-xi Liu, Hui Jing, Xin-you Lu, Yu-long Liu, Lan Yang, Franco Nori Journal reference: Phys. Rev. B 92, 115407 (2015) [pdf] DOI: 10.1103/PhysRevB.92.115407
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Electron emission of stripping foil and collimation system for CSNS/RCS -
Abstract
- For the Rapid Cycling Synchrotron of the China Spallation Neutron Source (CSNS/RCS), the electron emission plays an important role in the accelerator limitation. The interactions between the proton beam and the stripping foil were studied, and the electron scattering processes were simulated by the ORBIT and FLUKA codes. Then, the electron energy distribution and the electron yielding rate can be given. Furthermore, the interactions between the proton beam and the collimation system were studied, and the electron scattering processes were simulated. Then, the energy distribution of the primary electron emission can be given and the yielding rate of the primary electron can be obtained.
Ming-Yang Huang, Yu-Dong Liu, Na Wang, Sheng Wang [pdf]
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Null Geodesics and Gravitational Lensing in a Nonsingular Spacetime -
Abstract
- In this paper, the null geodesics and gravitational lensing in a nonsingular spacetime are investigated. According to the nature of the null geodesics, the spacetime is divided into several cases. In the weak deflection limit, we find the influence of the nonsingularity parameter $q$ on the positions and magnifications of the images is negligible. In the strong deflection limit, the coefficients and observables for the gravitational lensing in a nonsingular black hole background and a weakly nonsingular spacetime are obtained. Comparing these results, we find that, in a weakly nonsingular spacetime, the relativistic images have smaller angular position and relative magnification, but larger angular separation than that of a nonsingular black hole. These results might offer a way to probe the spacetime nonsingularity parameter and put a bound on it by the astronomical instruments in the near future.
Shao-Wen Wei, Yu-Xiao Liu, Chun-E Fu Journal reference: Adv.High Energy Phys. 2015 (2015) 454217 [pdf] DOI: 10.1155/2015/454217
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Comment on "Integrability of the Rabi Model" -
Abstract
- Using Hill's determinant method we show that the set of Judd's solutions is only a subset of all the eigenvalues with the form $E_n=n\omega-g^2/\omega$ in the spectrum of the Rabi model. Therefore Braak's solution of the quantum Rabi model is not complete.
Qing-Wei Wang, Yu-Liang Liu [pdf]
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Insight into the Microscopic Structure of an AdS Black Hole from a Thermodynamical Phase Transition -
Abstract
- Comparing with an ordinary thermodynamic system, we investigate the possible microscopic structure of a charged anti-de Sitter black hole completely from the thermodynamic viewpoint. The number density of the black hole molecules is introduced to measure the microscopic degrees of freedom of the black hole. We found that the number density suffers a sudden change accompanied by a latent heat when the black hole system crosses the small-large black hole coexistence curve, while when the system passes the critical point, it encounters a second-order phase transition with a vanishing latent heat due to the continuous change of the number density. Moreover, the thermodynamic scalar curvature suggests that there is a weak attractive interaction between two black hole molecules. These phenomena might cast new insight into the underlying microscopic structure of a charged anti-de Sitter black hole.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. Lett. 115, 111302 (2015) [pdf] DOI: 10.1103/PhysRevLett.115.111302
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Engineering entangled microwave photon states through multiphoton interactions between two cavity fields and a superconducting qubit -
Abstract
- It has been shown that there are not only transverse but also longitudinal couplings between microwave fields and a superconducting qubit with broken inversion symmetry of the potential energy. Using multiphoton processes induced by longitudinal coupling fields and frequency matching conditions, we design a universal algorithm to produce arbitrary superpositions of two-mode photon states of microwave fields in two separated transmission line resonators, which are coupled to a superconducting qubit. Based on our algorithm, we analyze the generation of evenly-populated states and NOON states. Compared to other proposals with only single-photon process, we provide an efficient way to produce entangled microwave states when the interactions between superconducting qubits and microwave fields are in the ultrastrong regime.
Yan-Jun Zhao, Chang-Qing Wang, Xiaobo Zhu, Yu-xi Liu [pdf] DOI: 10.1038/srep23646 1506.06363v3 [pdf]
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Generating nonclassical photon states via longitudinal couplings between superconducting qubits and microwave fields -
Abstract
- Besides the conventional transverse couplings between superconducting qubits (SQs) and electromagnetic fields, there are additional longitudinal couplings when the inversion symmetry of the potential energies of the SQs is broken. We study nonclassical-state generation in a SQ which is driven by a classical field and coupled to a single-mode microwave field. We find that the classical field can induce transitions between two energy levels of the SQs, which either generate or annihilate, in a controllable way, different photon numbers of the cavity field. The effective Hamiltonians of these classical-field-assisted multiphoton processes of the single-mode cavity field are very similar to those for cold ions, confined to a coaxial RF-ion trap and driven by a classical field. We show that arbitrary superpositions of Fock states can be more efficiently generated using these controllable multiphoton transitions, in contrast to the single-photon resonant transition when there is only a SQ-field transverse coupling. The experimental feasibility for different SQs is also discussed.
Yan-Jun Zhao, Yu-Long Liu, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 91, 053820 (2015) [pdf] DOI: 10.1103/PhysRevA.91.053820
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Tunable photon blockade in a hybrid system consisting of an optomechanical device coupled to a two-level system -
Abstract
- We study photon blockade and anti-bunching in the cavity of an optomechanical system in which the mechanical resonator is coupled to a two-level system (TLS). In particular, we analyze the effects of the coupling strength (to the mechanical mode), transition frequency, and decay rate of TLS on the photon blockade. The statistical properties of the cavity field are affected by the TLS, because the TLS changes the energy-level structure of the optomechanical system via dressed states formed by the TLS and the mechanical resonator. We find that the photon blockade and tunneling can be significantly changed by the transition frequency of the TLS and the coupling strength between the TLS and the mechanical resonator. Therefore, our study provides a method to tune the photon blockade and tunneling using a controllable TLS.
Hui Wang, Xiu Gu, Yu-xi Liu, Adam Miranowicz, Franco Nori Journal reference: Phys. Rev. A 92, 033806 (2015) [pdf] DOI: 10.1103/PhysRevA.92.033806
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Coherent population transfer between uncoupled or weakly coupled states in ladder-type superconducting qutrits -
Abstract
- Stimulated Raman adiabatic passage (STIRAP) offers significant advantages for coherent population transfer between un- or weakly-coupled states and has the potential of realizing efficient quantum gate, qubit entanglement, and quantum information transfer. Here we report on the realization of STIRAP in a superconducting phase qutrit - a ladder-type system in which the ground state population is coherently transferred to the second-excited state via the dark state subspace. The result agrees well with the numerical simulation of the master equation, which further demonstrates that with the state-of-the-art superconducting qutrits the transfer efficiency readily exceeds $99\%$ while keeping the population in the first-excited state below $1\%$. We show that population transfer via STIRAP is significantly more robust against variations of the experimental parameters compared to that via the conventional resonant $\pi$ pulse method. Our work opens up a new venue for exploring STIRAP for quantum information processing using the superconducting artificial atoms.
H. K. Xu, W. Y. Liu, G. M. Xue, F. F. Su, H. Deng, Ye Tian, D. N. Zheng, Siyuan Han, Y. P. Zhong, H. Wang, Yu-Xi Liu, S. P. Zhao [pdf] DOI: 10.1038/ncomms11018 1508.01849v1 [pdf]
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The structure of f(R)-brane model -
Abstract
- Recently, a family of interesting analytical brane solutions were found in $f(R)$ gravity with $f(R)=R+\alpha R^2$ in Ref. [Phys. Lett. B 729, 127 (2014)]. In these solutions, inner brane structure can be turned on by tuning the value of the parameter $\alpha$. In this paper, we investigate how the parameter $\alpha$ affects the localization and the quasilocalization of the tensorial gravitons around these solutions. It is found that, in a range of $\alpha$, despite the brane has an inner structure, there is no graviton resonance. However, in some other regions of the parameter space, although the brane has no internal structure, the effective potential for the graviton KK modes has a singular structure, and there exists a series of graviton resonant modes. The contribution of the massive graviton KK modes to the Newton's law of gravity is discussed shortly.
Zeng-Guang Xu, Yuan Zhong, Hao Yu, Yu-Xiao Liu Journal reference: Eur.Phys.J. C75 (2015) 368 [pdf] DOI: 10.1140/epjc/s10052-015-3597-0
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Mechanical
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Abstract
- We propose to realize mechanical parity-time PT symmetry in two coupled optomechanical systems. To provide gain to one mechanical resonator and the same amount of damping to the other, the two optical cavities should be driven by blue- and red-detuned laser fields, respectively. After adiabatically eliminating the degrees of freedom of the cavity modes, we derive a formula to describe the PT symmetry of two coupled mechanical resonators. Mechanical PT-symmetric phase transition is demonstrated by the dynamical behavior of the mechanical resonators. Moreover, we study the effect of the quantum noises on the dynamical behavior of the mechanical resonators when the system is in the quantum regime.
Xun-Wei Xu, Yu-xi Liu, Chang-Pu Sun, Yong Li Journal reference: Phys. Rev. A 92, 013852 (2015) [pdf] DOI: 10.1103/PhysRevA.92.013852
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Simulation on buildup of electron cloud in a proton circular accelerator -
Abstract
- Electron cloud interaction with high energy positive beam are believed responsible for various undesirable effects such as vacuum degradation, collective beam instability and even beam loss in high power proton circular accelerator. An important uncertainty in predicting electron cloud instability lies in the detail processes on the generation and accumulation of the electron cloud. The simulation on the build-up of electron cloud is necessary to further studies on beam instability caused by electron cloud. China Spallation Neutron Source (CSNS) is the largest scientific project in building, whose accelerator complex includes two main parts: an H- linac and a rapid cycling synchrotron (RCS). The RCS accumulates the 80Mev proton beam and accelerates it to 1.6GeV with a repetition rate 25Hz. During the beam injection with lower energy, the emerging electron cloud may cause a serious instability and beam loss on the vacuum pipe. A simulation code has been developed to simulate the build-up, distribution and density of electron cloud in CSNS/RCS.
Yu-Dong Liu, Kai-Wei Li [pdf] DOI: 10.1088/1674-1137/39/10/107002 1412.2886v2 [pdf]
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Polarized lepton-nucleon elastic scattering and a search for a light scalar boson -
Abstract
- Lepton-nucleon elastic scattering, using the one-photon and one-scalar-boson exchange mechanisms considering all possible polarizations, is used to study searches for a new scalar boson and suggest new measurements of the nucleon form factors. A new light scalar boson, which feebly couples to leptons and nucleons, may account for the proton radius and muon $g-2$ puzzles. We show that the scalar boson produces relatively large effects in certain kinematic region when using sufficient control of lepton and nucleon spin polarization. We generalize current techniques to measure the ratio $G_E/G_M$ and present a new method to separately measure $G_M^2$ and $G_E^2$ using polarized incoming and outgoing muons.
Yu-Sheng Liu, Gerald A. Miller Journal reference: Phys. Rev. C 92, 035209 (2015) [pdf] DOI: 10.1103/PhysRevC.92.035209
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Entanglement distribution over quantum code-division multiple-access networks -
Abstract
- We present a method for quantum entanglement distribution over a so-called code-division-multiple-access network, in which two pairs of users share the same quantum channel to transmit information. The main idea of this method is to use different broad-band chaotic phase shifts, generated by electro-optic modulators (EOMs) and chaotic Colpitts circuits, to encode the information-bearing quantum signals coming from different users, and then recover the masked quantum signals at the receiver side by imposing opposite chaotic phase shifts. The chaotic phase shifts given to different pairs of users are almost uncorrelated due to the randomness of chaos and thus the quantum signals from different pair of users can be distinguished even when they are sent via the same quantum channel. It is shown that two maximally-entangled states can be generated between two pairs of users by our method mediated by bright coherent lights, which can be more easily implemented in experiments compared with single-photon lights. Our method is robust under the channel noises if only the decay rates of the information-bearing fields induced by the channel noises are not quite high. Our study opens up new perspectives for addressing and transmitting quantum information in future quantum networks.
Chang-long Zhu, Nan Yang, Yu-xi Liu, Franco Nori, Jing Zhang Journal reference: Phys. Rev. A 92, 042327 (2015) [pdf] DOI: 10.1103/PhysRevA.92.042327
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Black hole solution and strong gravitational lensing in Eddington-inspired Born–Infeld gravity -
Abstract
- A new theory of gravity called Eddington-inspired Born-Infeld (EiBI) gravity was recently proposed by Ba\~{n}ados and Ferreira. This theory leads to some exciting new features, such as free of cosmological singularities. In this paper, we first obtain a charged EiBI black hole solution with a nonvanishing cosmological constant when the electromagnetic field is included in. Then based on it, we study the strong gravitational lensing by the asymptotic flat charged EiBI black hole. The strong deflection limit coefficients and observables are shown to closely depend on the additional coupling parameter $\kappa$ in the EiBI gravity. It is found that, compared with the corresponding charged black hole in general relativity, the positive coupling parameter $\kappa$ will shrink the black hole horizon and photon sphere. Moreover, the coupling parameter will decrease the angular position and relative magnitudes of the relativistic images, while increase the angular separation, which may shine new light on testing such gravity theory in near future by the astronomical instruments.
Shao-Wen Wei, Ke Yang, Yu-Xiao Liu Journal reference: Eur. Phys. J. C (2015) 75:253 [pdf] DOI: 10.1140/epjc/s10052-015-3469-7
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Phase evolution of the two-dimensional Kondo lattice model near half-filling -
Abstract
- Within a mean-field approximation, the ground state and finite temperature phase diagrams of the two-dimensional Kondo lattice model have been carefully studied as functions of the Kondo coupling $J$ and the conduction electron concentration $n_{c}$. In addition to the conventional hybridization between local moments and itinerant electrons, a staggered hybridization is proposed to characterize the interplay between the antiferromagnetism and the Kondo screening effect. As a result, a heavy fermion antiferromagnetic phase is obtained and separated from the pure antiferromagnetic ordered phase by a first-order Lifshitz phase transition, while a continuous phase transition exists between the heavy fermion antiferromagnetic phase and the Kondo paramagnetic phase. We have developed a efficient theory to calculate these phase boundaries. As $n_{c}$ decreases from the half-filling, the region of the heavy fermion antiferromagnetic phase shrinks and finally disappears at a critical point $n_{c}^{*}=0.8228$, leaving a first-order critical line between the pure antiferromagnetic phase and the Kondo paramagnetic phase for $n_{c}<n_{c}^{* }$.="}$." at="At" half-filling="half-filling" limit,="limit," a="a" finite="finite" temperature="temperature" phase="phase" diagram="diagram" is="is" also="also" determined="determined" on="on" the="the" kondo="Kondo" coupling="coupling" and="and" temperature="temperature" ($j$-$t$)="($J$-$T$)" plane.="plane." notably,="Notably," as="as" the="the" temperature="temperature" is="is" increased,="increased," the="the" region="region" of="of" the="the" heavy="heavy" fermion="fermion" antiferromagnetic="antiferromagnetic" phase="phase" is="is" reduced="reduced" continuously,="continuously," and="and" finally="finally" converges="converges" to="to" a="a" single="single" point,="point," together="together" with="with" the="the" pure="pure" antiferromagnetic="antiferromagnetic" phase="phase" and="and" the="the" kondo="Kondo" paramagnetic="paramagnetic" phase.="phase." the="the" phase="phase" diagrams="diagrams" with="with" such="such" triple="triple" point="point" may="may" account="account" for="for" the="the" observed="observed" phase="phase" transitions="transitions" in="in" related="related" heavy="heavy" fermion="fermion" materials.
Huan Li, Yu Liu, Guangming Zhang, Lu Yu Journal reference: Journal of Physics: Condensed Matter 27, 425601 (2015) [pdf] DOI: 10.1088/0953-8984/27/42/425601
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On the black hole limit of rotating discs of charged dust -
Abstract
- Investigating the rigidly rotating disc of dust with constant specific charge, we find that it leads to an extreme Kerr-Newman black hole in the ultra-relativistic limit. A necessary and sufficient condition for a black hole limit is, that the electric potential in the co-rotating frame is constant on the disc. In that case certain other relations follow. These relations are reviewed with a highly accurate post-Newtonian expansion. Remarkably it is possible to survey the leading order behaviour close to the black hole limit with the post-Newtonian expansion. We find that the disc solution close to that limit can be approximated very well by a "hyper\-extreme" Kerr-Newman solution with the same gravitational mass, angular momentum and charge.
Martin Breithaupt, Yu-Chun Liu, Reinhard Meinel, Stefan Palenta Journal reference: Class.Quant.Grav.32:135022, 2015 [pdf] DOI: 10.1088/0264-9381/32/13/135022
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Tensor perturbations of Palatini
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Abstract
- We investigate the thick brane model in Palatini $f(\mathcal{R})$ gravity. The brane is generated by a real scalar field with a scalar potential. We solve the system analytically and obtain a series of thick brane solutions for the $f(\mathcal{R})=\mathcal{R}+\alpha \mathcal{R}^2$-brane model. It is shown that tensor perturbations of the metric are stable for $df({\mathcal{R}})/d{\mathcal{R}}>0$. For nonconstant curvature solutions, the graviton zero mode can be localized on the brane, which indicates that the four-dimensional gravity can be recovered on the brane. Mass spectrum of graviton KK modes and their corrections to the Newtonian potential are also discussed.
Bao-Min Gu, Bin Guo, Hao Yu, Yu-Xiao Liu Journal reference: Phys. Rev. D 92, 024011 (2015) [pdf] DOI: 10.1103/PhysRevD.92.024011
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Noise suppression of on-chip mechanical resonators by chaotic coherent feedback -
Abstract
- We propose a method to decouple the nanomechanical resonator in optomechanical systems from the environmental noise by introducing a chaotic coherent feedback loop. We find that the chaotic controller in the feedback loop can modulate the dynamics of the controlled optomechanical system and induce a broadband response of the mechanical mode. This broadband response of the mechanical mode will cut off the coupling between the mechanical mode and the environment and thus suppress the environmental noise of the mechanical modes. As an application, we use the protected optomechanical system to act as a quantum memory. It's shown that the noise-decoupled optomechanical quantum memory is efficient for storing information transferred from coherent or squeezed light.
Nan Yang, Jing Zhang, Hui Wang, Yu-xi Liu, Re-Bing Wu, Lian-qing Liu, Chun-Wen Li, Franco Nori Journal reference: Phys. Rev. A 92, 033812 (2015) [pdf] DOI: 10.1103/PhysRevA.92.033812
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Phase transition in finite density and temperature lattice QCD -
Abstract
- We investigate the behavior of the chiral condensate in lattice QCD at finite temperature and finite chemical potential. The study was done using two flavors of light quarks and with a series of $\beta$ and $ma$ at the lattice size $24\times12^{2}\times6$. The calculation was done in the Taylar expansion formalism. We are able to calculate the first and second order derivatives of $\langle\bar{\psi}\psi\rangle$ in both isoscalar and isovector channels. With the first derivatives being small, we find that the second derivatives are sizable close to the phase transition and the magnitude of $\bar{\psi}\psi$ decreases under the influence of finite chemical potential in both channels.
Rui Wang, Ying Chen, Ming Gong, Chuan Liu, Yu-Bin Liu, Zhao-Feng Liu, Jian-Ping Ma, Xiang-Fei Meng, Jian-Bo Zhang [pdf] DOI: 10.1088/1674-1137/39/6/063103 1504.02159v1 [pdf]
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Uncertainty Principle, Shannon–Nyquist Sampling and Beyond -
Abstract
- Donoho and Stark have shown that a precise deterministic recovery of missing information contained in a time interval shorter than the time-frequency uncertainty limit is possible. We analyze this signal recovery mechanism from a physics point of view and show that the well-known Shannon-Nyquist sampling theorem, which is fundamental in signal processing, also uses essentially the same mechanism. The uncertainty relation in the context of information theory, which is based on Fourier analysis, provides a criterion to distinguish Shannon-Nyquist sampling from compressed sensing. A new signal recovery formula, which is analogous to Donoho-Stark formula, is given using the idea of Shannon-Nyquist sampling; in this formulation, the smearing of information below the uncertainty limit as well as the recovery of information with specified bandwidth take place. We also discuss the recovery of states from the domain below the uncertainty limit of coordinate and momentum in quantum mechanics and show that in principle the state recovery works by assuming ideal measurement procedures. The recovery of the lost information in the sub-uncertainty domain means that the loss of information in such a small domain is not fatal, which is in accord with our common understanding of the uncertainty principle, although its precise recovery is something we are not used to in quantum mechanics. The uncertainty principle provides a universal sampling criterion covering both the classical Shannon-Nyquist sampling theorem and the quantum mechanical measurement.
Kazuo Fujikawa, Mo-Lin Ge, Yu-Long Liu, Qing Zhao [pdf] DOI: 10.7566/JPSJ.84.064801 1504.01467v1 [pdf]
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Hybrid model approach for strange and multistrange hadrons in
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Abstract
- Using the VISHNU hybrid model, we calculate the multiplicity, spectra, and elliptic flow of $\Lambda$, $\Xi$ and $\Omega$ in 2.76 A TeV Pb+Pb collisions. Comparisons between our calculations and the ALICE measurements show that the model generally describes the soft hadron data of these strange and multi-strange hadrons at several centrality bins. Mass ordering of elliptic flow among $\pi$, K, p, $\Lambda$, $\Xi$ and $\Omega$ has also been studied and discussed. With a nice description of the particle yields, we explore chemical and thermal freeze-out of various hadrons species at the LHC within the framework of the VISHNU hybrid model.
Xiangrong Zhu, Fanli Meng, Huichao Song, Yu-Xin Liu [pdf] DOI: 10.1103/PhysRevC.91.034904 1501.03286v2 [pdf]
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Clapeyron equations and fitting formula of the coexistence curve in the extended phase space of charged AdS black holes -
Abstract
- In this paper, we first review the equal area laws and Clapeyron equations in the extended phase space of the charged anti-de Sitter black holes. With different fixed parameters, the Maxwell's equal area law holds not only in the pressure-thermodynamic volume oscillatory line, but also in the charge-electric potential and temperature-entropy oscillatory lines. The conventional Clapeyron equation is generalized and two extra equations are found. Moreover, we show that the coexistence curve of the small and large charged black holes is charge independent in the reduced parameter space for any dimension of spacetime. The highly accurate fitting formula for the coexistence curve is also presented. Using this fitting formula of the coexistence curve, we find that the Clapeyron equations are highly consistent with the calculated values. The fitting formula is also very useful for further study on the thermodynamic property of the system varying along the coexistence curve.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 91, 044018 (2015) [pdf] DOI: 10.1103/PhysRevD.91.044018
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Brane worlds in gravity with auxiliary fields -
Abstract
- Recently, Pani, Sotiriou, and Vernieri explored a new theory of gravity by adding nondynamical fields, i.e., gravity with auxiliary fields [Phys. Rev. D 88, 121502(R) (2013)]. In this gravity theory, higher-order derivatives of matter fields generically appear in the field equations. In this paper we extend this theory to any dimensions and discuss the thick braneworld model in five dimensions. Domain wall solutions are obtained numerically. The stability of the brane system under the tensor perturbation is analyzed. We find that the system is stable under the tensor perturbation and the gravity zero mode is localized on the brane. Therefore, the four-dimensional Newtonian potential can be realized on the brane.
Bin Guo, Yu-Xiao Liu, Ke Yang Journal reference: Eur.Phys.J. C75 (2015) 2, 63 [pdf] DOI: 10.1140/epjc/s10052-015-3273-4
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Low-energy scattering of the
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Abstract
- In this paper, low-energy scattering of the $(D^{*}\bar{D}^{*})^\pm$ meson system is studied within L\"uscher's finite-size formalism using $N_{f}=2$ twisted mass gauge field configurations. With three different pion mass values, the $s$-wave threshold scattering parameters, namely the scattering length $a_0$ and the effective range $r_0$, are extracted in $J^P=1^+$ channel. Our results indicate that, in this particular channel, the interaction between the two vector charmed mesons is weakly repulsive in nature hence do not support the possibility of a shallow bound state for the two mesons, at least for the pion mass values being studied. This study provides some useful information on the nature of the newly discovered resonance-like structure $Z_c(4025)$ observed in various experiments.
Ying Chen, Ming Gong, Yu-Hong Lei, Ning Li, Jian Liang, Chuan Liu, Jin-Long Liu, Yong-Fu Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Zhan-Lin Wang, Jian-Bo Zhang Journal reference: Phys. Rev. D 92, 054507 (2015) [pdf] DOI: 10.1103/PhysRevD.92.054507
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Performance assessment of CsI(Tl) screens on various substrates for
X-ray imaging -
Abstract
- Thallium-doped cesium iodide (CsI(Tl)) screens are widely used in X-ray imaging devices because of the columnar structure of CsI(Tl) layer, but few reports focus on the optical role of the substrate in the screen system. In this paper, four substrates including fused silica (SiO2), silver-film coated SiO2, graphite (C) and fiber optic plate (FOP) are used to fabricate CsI(Tl) screens by thermal evaporation. Their imaging performance is evaluated by relative light output (RLO), modulation transfer function (MTF), normalized noise power spectrum (NNPS) and noise equivalent quanta (NEQ). The results reveal that although CsI(Tl) film on graphite plate yields images with the lowest light output, it presents relatively higher spatial resolution and better signal-to-noise characteristics. However, films on SiO2 plate obtain low MTF but high NNPS curves, whether or not coated with silver film. Furthermore, scintillation screens on FOP have bright images with low NNPS and high NEQ, but have the lowest MTF. By controlling the substrate optical features, CsI(Tl) films can be tailed to suit a given application.
Zhaodong Feng, Peng Jiang, Hongkai Zhang, Bozhen Zhao, Xiubo Qin, Cunfeng Wei, Yu Liu, Long Wei [pdf]
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Urban spatial-temporal activity structures: a New Approach to Inferring
the Intra-urban Functional Regions via Social Media Check-In Data -
Abstract
- Most existing literature focuses on the exterior temporal rhythm of human movement to infer the functional regions in a city, but they neglects the underlying interdependence between the functional regions and human activities which uncovers more detailed characteristics of regions. In this research, we proposed a novel model based on the low rank approximation (LRA) to detect the functional regions using the data from about 15 million check-in records during a yearlong period in Shanghai, China. We find a series of latent structures, called urban spatial-temporal activity structure (USTAS). While interpreting these structures, a series of outstanding underlying associations between the spatial and temporal activity patterns can be found. Moreover, we can not only reproduce the observed data with a lower dimensional representative but also simultaneously project both the spatial and temporal activity patterns in the same coordinate system. By utilizing the K-means clustering algorithm, five significant types of clusters which are directly annotated with a corresponding combination of temporal activities can be obtained. This provides a clear picture of how the groups of regions are associated with different activities at different time of day. Besides the commercial and transportation dominant area, we also detect two kinds of residential areas, the developed residential areas and the developing residential areas. We further verify the spatial distribution of these clusters in the view of urban form analysis. The results shows a high consistency with the government planning from the same periods, indicating our model is applicable for inferring the functional regions via social media check-in data, and can benefit a wide range of fields, such as urban planning, public services and location-based recommender systems and other purposes.
Ye Zhi, Yu Liu, Shaowen Wang, Min Deng, Jing Gao, Haifeng Li [pdf]
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Mott-Kondo insulator behavior in the iron oxychalcogenides -
Abstract
- We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.
B. Freelon, Yu Hao Liu, Jeng-Lung Chen, L. Craco, M. S. Laad, S. Leoni, Jiaqi Chen, Li Tao, Hangdong Wang, R. Flauca, Z. Yamani, Minghu Fang, Chinglin Chang, J. -H. Guo, Z. Hussain Journal reference: Phys. Rev. B 92, 155139 (2015) [pdf] DOI: 10.1103/PhysRevB.92.155139
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Dynamic creation of a topologically-ordered Hamiltonian using spin-pulse control in the Heisenberg model -
Abstract
- Hamiltonian engineering is an important approach for quantum information processing, when appropriate materials do not exist in nature or are unstable. So far there is no stable material for the Kitaev spin Hamiltonian with anisotropic interactions on a honeycomb lattice (A. Kitaev, Annals of Physics vol 321, 2 (2006)), which plays a crucial role in the realization of both Abelian and non-Abelian anyons. Here, we show how to dynamically realize the Kitaev spin Hamiltonian from the conventional Heisenberg spin Hamiltonian using a pulse-control technique. By repeating the same pulse sequence, the quantum state is dynamically preserved. The effects of the spin-orbit interaction and the hyperfine interaction are also investigated.
Tetsufumi Tanamoto, Keiji Ono, Yu-xi Liu, Franco Nori Journal reference: Scientific Rep. 5, 10076 (2015) [pdf] DOI: 10.1038/srep10076
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Phase-dependent optical response properties in an optomechanical system by coherently driving the mechanical resonator -
Abstract
- 2014
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Pairing Symmetry of Heavy Fermion Superconductivity in the Two-Dimensional Kondo—Heisenberg Lattice Model -
Abstract
- In the two-dimensional Kondo-Heisenberg lattice model away from half-filled, the local antiferromagnetic exchange coupling can provide the pairing mechanism of quasiparticles via the Kondo screening effect, leading to the heavy fermion superconductivity. We find that the pairing symmetry \textit{strongly} depends on the Fermi surface (FS) structure in the normal metallic state. When $J_{H}/J_{K}$ is very small, the FS is a small hole-like circle around the corner of the Brillouin zone, and the s-wave pairing symmetry has a lower ground state energy. For the intermediate coupling values of $J_{H}/J_{K}$, the extended s-wave pairing symmetry gives the favored ground state. However, when $J_{H}/J_{K}$ is larger than a critical value, the FS transforms into four small hole pockets crossing the boundary of the magnetic Brillouin zone, and the d-wave pairing symmetry becomes more favorable. In that regime, the resulting superconducting state is characterized by either nodal d-wave or nodeless d-wave state, depending on the conduction electron filling factor as well. A continuous phase transition exists between these two states. This result may be related to the phase transition of the nodal d-wave state to a fully gapped state, which is recently observed in Yb doped CeCoIn$_{5}$.
Yu Liu, Guang-Ming Zhang, Lu Yu Journal reference: Chin. Phys. Lett. 31(2014), 087102 [pdf] DOI: 10.1088/0256-307X/31/8/087102
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Emergent dynamical symmetry at the triple point of nuclear deformations -
Abstract
- Based on the boson realization of the Euclidean algebras, it is shown that the five-dimensional Euclidean dynamical symmetry may emerge at the triple point of the shape phase diagram of the interacting boson model, which thus offers a symmetry-based understanding of this isolated point. It is further shown that the low-lying dynamics in $^{108}$Pd, $^{134}$Ba, $^{64}$Zn, and $^{114}$Cd may be dominated by the Euclidean dynamical symmetry.
Yu Zhang, Feng Pan, Yu-xin Liu, Yan-an Luo, J. P. Draayer Journal reference: Physical Review C 90 (2014) 064318 [pdf] DOI: 10.1103/PhysRevC.90.064318
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Controllable microwave three-wave mixing via a single three-level
superconducting quantum circuit -
Abstract
- Three-wave mixing in second-order nonlinear optical processes cannot occur in atomic systems due to the electric-dipole selection rules. In contrast, we demonstrate that second-order nonlinear processes can occur in a superconducting quantum circuit (i.e., a superconducting artificial atom) when the inversion symmetry of the potential energy is broken by simply changing the applied magnetic flux. In particular, we show that difference- and sum-frequencies (and second harmonics) can be generated in the microwave regime in a controllable manner by using a single three-level superconducting flux quantum circuit (SFQC). For our proposed parameters, the frequency tunability of this circuit can be achieved in the range of about 17 GHz for the sum-frequency generation, and around 42 GHz (or 26 GHz) for the difference-frequency generation. Our proposal provides a simple method to generate second-order nonlinear processes within current experimental parameters of SFQCs.
Yu-xi Liu, Hui-Chen Sun, Z. H. Peng, Adam Miranowicz, J. S. Tsai, Franco Nori [pdf]
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Signal Flows in Non-Markovian Linear Quantum Feedback Networks -
Abstract
- Enabled by rapidly developing quantum technologies, it is possible to network quantum systems at a much larger scale in the near future. To deal with non-Markovian dynamics that is prevalent in solid-state devices, we propose a general transfer function based framework for modeling linear quantum networks, in which signal flow graphs are applied to characterize the network topology by flow of quantum signals. We define a noncommutative ring $\mathbb{D}$ and use its elements to construct Hamiltonians, transformations and transfer functions for both active and passive systems. The signal flow graph obtained for direct and indirect coherent quantum feedback systems clearly show the feedback loop via bidirectional signal flows. Importantly, the transfer function from input to output field is derived for non-Markovian quantum systems with colored inputs, from which the Markovian input-output relation can be easily obtained as a limiting case. Moreover, the transfer function possesses a symmetry structure that is analogous to the well-know scattering transformation in \sd picture. Finally, we show that these transfer functions can be integrated to build complex feedback networks via interconnections, serial products and feedback, which may include either direct or indirect coherent feedback loops, and transfer functions between quantum signal nodes can be calculated by the Riegle's matrix gain rule. The theory paves the way for modeling, analyzing and synthesizing non-Markovian linear quantum feedback networks in the frequency-domain.
Re-Bing Wu, Jing Zhang, Yu-xi Liu, Tzyh-Jong Tarn [pdf]
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Euclidean dynamical symmetry in nuclear shape phase transitions -
Abstract
- The Euclidean dynamical symmetry hidden in the critical region of nuclear shape phase transitions is revealed by a novel algebraic F(5) description. With a nonlinear projection, it is shown that the dynamics in the critical region of the spherical--axial deformed and the spherical--$\gamma$ soft shape phase transitions can indeed be manifested by this description, which thus provides a unified symmetry--based interpretation of the critical phenomena in the region.
Yu Zhang, Yu-Xin Liu, Feng Pan, Yang Sun, J. P. Draayer Journal reference: Physics Letter B 732 (2014) 55-58 [pdf] DOI: 10.1016/j.physletb.2014.03.017
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Warped brane worlds in critical gravity -
Abstract
- We investigate the brane models in arbitrary dimensional critical gravity presented in [Phys. Rev. Lett. 106, 181302 (2011)]. For the model of the thin branes with codimension one, the Gibbons-Hawking surface term and the junction conditions are derived, with which the analytical solutions for the flat, AdS, and dS branes are obtained at the critical point of the critical gravity. It is found that all these branes are embedded in an AdS$_{n}$ spacetime, but, in general, the effective cosmological constant $\Lambda$ of the AdS$_{n}$ spacetime is not equal to the naked one $\Lambda_0$ in the critical gravity, which can be positive, zero, and negative. Another interesting result is that the brane tension can also be positive, zero, or negative, depending on the symmetry of the thin brane and the values of the parameters of the theory, which is very different from the case in general relativity. It is shown that the mass hierarchy problem can be solved in the braneworld model in the higher-derivative critical gravity. We also study the thick brane model and find analytical and numerical solutions of the flat, AdS, and dS branes. It is find that some branes will have inner structure when some parameters of the theory are larger than their critical values, which may result in resonant KK modes for some bulk matter fields. The flat branes with positive energy density and AdS branes with negative energy density are embedded in an $n$-dimensional AdS spacetime, while the dS branes with positive energy density are embedded in an $n$-dimensional Minkowski one.
Yi Zhong, Feng-Wei Chen, Qun-Ying Xie, Yu-Xiao Liu Journal reference: Eur.Phys.J. C74 (2014) 12, 3185 [pdf] DOI: 10.1140/epjc/s10052-014-3185-8
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K-field kinks: stability, exact solutions and new features -
Abstract
- We study a class of noncanonical real scalar field models in $(1+1)$-dimensional flat space-time. We first derive the general criterion for the classical linear stability of an arbitrary static soliton solution of these models. Then we construct first-order formalisms for some typical models and derive the corresponding kink solutions. The linear structures of these solutions are also qualitatively analyzed and compared with the canonical kink solutions.
Yuan Zhong, Yu-Xiao Liu Journal reference: JHEP 10 (2014) 041 [pdf] DOI: 10.1007/JHEP10(2014)041
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Stability and (quasi)localization of gravitational fluctuations in an Eddington-inspired Born-Infeld brane system -
Abstract
- Stability and localization of the gravitational perturbations for a special brane system in Eddington-inspired Born-Infeld (EiBI) gravity were studied in [Phys. Rev. D 85, 124053 (2012)]. In this paper, we show that the gravitational perturbations for a general brane system are stable, the four-dimensional graviton (massless KK graviton) can be localized on the brane, and the mass spectrum of massive KK gravitons are gapless and continuous. Two models are constructed as examples. In the first model, which is a generalization of [Phys. Rev. D 85, 124053 (2012)], the brane has no inner structure and there is no gravitational resonance (quasi-localized KK gravitons). In the second one, the background scalar field is a double-kink when the parameter in the model approaches its critical value. Correspondingly, the brane has inner structure and some gravitational resonances appear.
Qi-Ming Fu, Li Zhao, Ke Yang, Bao-Min Gu, Yu-Xiao Liu Journal reference: Phys. Rev. D 90, 104007 (2014) [pdf] DOI: 10.1103/PhysRevD.90.104007
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State-dependent photon blockade via quantum-reservoir engineering -
Abstract
- An arbitrary initial state of an optical or microwave field in a lossy driven nonlinear cavity can be changed, in the steady-state limit, into a partially incoherent superposition of only the vacuum and the single-photon states. This effect is known as single-photon blockade, which is usually analyzed for a Kerr-type nonlinear cavity parametrically driven by a single-photon process assuming single-photon loss mechanisms. We study photon blockade engineering via a squeezed reservoir, i.e., a quantum reservoir, where only two-photon absorption is allowed. Namely, we analyze a lossy nonlinear cavity parametrically driven by a two-photon process and allowing two-photon loss mechanisms, as described by the master equation derived for a two-photon absorbing reservoir. The nonlinear cavity engineering can be realized by a linear cavity with a tunable two-level system via the Jaynes-Cummings interaction in the dispersive limit. We show that by tuning properly the frequencies of the driving field and the two-level system, the steady state of the cavity field can be the single-photon Fock state or a partially incoherent superposition of several Fock states with photon numbers, e.g., (0,2), (1,3), (0,1,2), or (0,2,4). We observe that an arbitrary initial coherent or incoherent superposition of Fock states with an even (odd) number of photons can be changed into a partially incoherent superposition of a few Fock states of the same photon-number parity. A general solution for an arbitrary initial state is a weighted mixture of the above two solutions with even and odd photon numbers, where the weights are given by the probabilities of measuring the even and odd numbers of photons of the initial cavity field, respectively. Thus, in contrast to the standard photon blockade, we prove that the steady state in the engineered photon blockade, can depend on its initial state.
Adam Miranowicz, Jiri Bajer, Malgorzata Paprzycka, Yu-xi Liu, Alexandre M. Zagoskin, Franco Nori Journal reference: Phys. Rev. A 90, 033831 (2014) [pdf] DOI: 10.1103/PhysRevA.90.033831
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Triple points and phase diagrams in the extended phase space of charged Gauss-Bonnet black holes in AdS space -
Abstract
- We study the triple points and phase diagrams in the extended phase space of the charged Gauss-Bonnet black holes in $d$-dimensional anti-de Sitter space, where the cosmological constant appears as a dynamical pressure of the system and its conjugate quantity is the thermodynamic volume of the black holes. Employing the equation of state $T=T(v, P)$, we demonstrate that the information of the phase transition and behavior of the Gibbs free energy are potential encoded in the $T-v$ ($T-r_{h}$) line with fixed pressure $P$. We get the phase diagrams for the charged Gauss-Bonnet black holes with different values of the charge $Q$ and dimension $d$. The result shows that the small/large black hole phase transitions appear for any $d$, which is reminiscent of the liquid/gas transition of a van der Waals type. Moreover, the interesting thermodynamic phenomena, i.e., the triple points and the small/intermediate/large black hole phase transitions are observed for $d=6$ and $Q\in(0.1705, 0.1946)$.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 90, 044057 (2014) [pdf] DOI: 10.1103/PhysRevD.90.044057
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Abstract
- We follow the Chumbes-Holf da Silva-Hott (CHH) mechanism to study the (qusi-)localization of $U(1)$ gauge field on the Bloch brane. The localization of $U(1)$ gauge field is discussed for four kinds of Bloch brane solutions: the original Bloch brane solution, the generalized Bloch brane solution, and the degenerate Bloch brane solutions I and II. With the CHH mechanism, we find that the mass spectrum of the gauge field Kaluza-Klein modes is continuous and only the zero mode of the gauge field is localized on the brane. For the massive modes, there exist resonant modes only on the degenerate Bloch branes.
Zhen-Hua Zhao, Yu-Xiao Liu, Yuan Zhong Journal reference: Phys. Rev. D 90, 045031 (2014) [pdf] DOI: 10.1103/PhysRevD.90.045031
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Modulation Classification via Gibbs Sampling Based on a Latent Dirichlet Bayesian Network -
Abstract
- A novel Bayesian modulation classification scheme is proposed for a single-antenna system over frequency-selective fading channels. The method is based on Gibbs sampling as applied to a latent Dirichlet Bayesian network (BN). The use of the proposed latent Dirichlet BN provides a systematic solution to the convergence problem encountered by the conventional Gibbs sampling approach for modulation classification. The method generalizes, and is shown to improve upon, the state of the art.
Yu Liu, Osvaldo Simeone, Alexander M. Haimovich, Wei Su [pdf] DOI: 10.1109/LSP.2014.2327193 1408.0765v2 [pdf]
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Large scale structure formation in Eddington-inspired Born-Infeld gravity -
Abstract
- We study the large scale structure formation in Eddington-inspired Born-Infeld (EiBI) gravity. It is found that the linear growth of scalar perturbations in EiBI gravity deviates from that in general relativity for modes with large wave numbers ($k$), but the deviation is largely suppressed with the expansion of the Universe. We investigate the integrated Sachs-Wolfe effect in EiBI gravity, and find that its effect on the angular power spectrum of the anisotropy of the cosmic microwave background (CMB) is almost the same as that in the Lambda-cold dark matter ($\Lambda$CDM) model. We further calculate the linear matter power spectrum in EiBI gravity and compare it with that in the $\Lambda$CDM model. Deviation is found on small scales ($k\gtrsim 0.1 h$ Mpc$^{-1}$), which can be tested in the future by observations from galaxy surveys.
Xiao-Long Du, Ke Yang, Xin-He Meng, Yu-Xiao Liu Journal reference: Phys. Rev. D 90, 044054 (2014) [pdf] DOI: 10.1103/PhysRevD.90.044054
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New localization mechanism of fermions on braneworlds -
Abstract
- It is known that by introducing the Yukawa coupling between the fermion and the background scalar field, a bulk spin-half fermion can be localized on general Randall-Sundrum braneworlds generated by a kinklike background scalar. However, this localization mechanism does not work anymore for Randall-Sundrum braneworlds generated by a scalar whose configuration is an even function of the extra dimension. In this paper, we present a new localization mechanism for spin-half fermions for such a class of braneworld models, in which extra dimension has the topology $S^1/Z_2$. By two examples, it is shown that the new localization mechanism produces interesting results. In the first model with the brane generated by two scalars, the zero mode of the left-handed fermion is localized on the brane and there is a mass gap between the fermion zero mode and excited KK modes. In the second model with the brane generated by a dilaton scalar, the zero mode of the left- or right-chiral fermion can be localized on the brane and there is no mass gap.
Yu-Xiao Liu, Zeng-Guang Xu, Feng-Wei Chen, Shao-Wen Wei Journal reference: Phys. Rev. D 89, 086001 (2014) [pdf] DOI: 10.1103/PhysRevD.89.086001
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Optomechanical analog of two-color electromagnetically induced transparency: Photon transmission through an optomechanical device with a two-level system -
Abstract
- Some optomechanical systems can be transparent to a probe field when a strong driving field is applied. These systems can provide an optomechanical analogue of electromagnetically-induced transparency (EIT). We study the transmission of a probe field through a hybrid optomechanical system consisting of a cavity and a mechanical resonator with a two-level system (qubit). The qubit might be an intrinsic defect inside the mechanical resonator, a superconducting artificial atom, or another two-level system. The mechanical resonator is coupled to the cavity field via radiation pressure and to the qubit via the Jaynes-Cummings interaction. We find that the dressed two-level system and mechanical phonon can form two sets of three-level systems. Thus, there are two transparency windows in the discussed system. We interpret this effect as an optomechanical analog of two-color EIT (or double-EIT). We demonstrate how to switch between one and two EIT windows by changing the transition frequency of the qubit. We show that the absorption and dispersion of the system are mainly affected by the qubit-phonon coupling strength and the transition frequency of the qubit.
Hui Wang, Xiu Gu, Yu-xi Liu, Adam Miranowicz, Franco Nori Journal reference: Phys. Rev. A 90, 023817 (2014) [pdf] DOI: 10.1103/PhysRevA.90.023817
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Nonlinear quantum input-output analysis using Volterra series -
Abstract
- Quantum input-output theory plays a very important role for analyzing the dynamics of quantum systems, especially large-scale quantum networks. As an extension of the input-output formalism of Gardiner and Collet, we develop a new approach based on the quantum version of the Volterra series which can be used to analyze nonlinear quantum input-output dynamics. By this approach, we can ignore the internal dynamics of the quantum input-output system and represent the system dynamics by a series of kernel functions. This approach has the great advantage of modelling weak-nonlinear quantum networks. In our approach, the number of parameters, represented by the kernel functions, used to describe the input-output response of a weak-nonlinear quantum network, increases linearly with the scale of the quantum network, not exponentially as usual. Additionally, our approach can be used to formulate the quantum network with both nonlinear and nonconservative components, e.g., quantum amplifiers, which cannot be modelled by the existing methods, such as the Hudson-Parthasarathy model and the quantum transfer function model. We apply our general method to several examples, including Kerr cavities, optomechanical transducers, and a particular coherent feedback system with a nonlinear component and a quantum amplifier in the feedback loop. This approach provides a powerful way to the modelling and control of nonlinear quantum networks.
Jing Zhang, Yu-xi Liu, Re-Bing Wu, Kurt Jacobs, Sahin Kaya Ozdemir, Lan Yang, Tzyh-Jong Tarn, Franco Nori [pdf]
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Delineating Intra-Urban Spatial Connectivity Patterns by
Travel-Activities: A Case Study of Beijing, China -
Abstract
- Travel activities have been widely applied to quantify spatial interactions between places, regions and nations. In this paper, we model the spatial connectivities between 652 Traffic Analysis Zones (TAZs) in Beijing by a taxi OD dataset. First, we unveil the gravitational structure of intra-urban spatial connectivities of Beijing. On overall, the inter-TAZ interactions are well governed by the Gravity Model $G_{ij} = {\lambda}p_{i}p_{j}/d_{ij}$, where $p_{i}$, $p_{j}$ are degrees of TAZ $i$, $j$ and $d_{ij}$ the distance between them, with a goodness-of-fit around 0.8. Second, the network based analysis well reveals the polycentric form of Beijing. Last, we detect the semantics of inter-TAZ connectivities based on their spatiotemporal patterns. We further find that inter-TAZ connections deviating from the Gravity Model can be well explained by link semantics.
Chaogui Kang, Yu Liu, Lun Wu [pdf]
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Coherent-feedback-induced photon blockade and optical bistability by an
optomechanical controller -
Abstract
- It is well-known that some nonlinear phenomena such as strong photon blockade are hard to be observed in optomechanical system with current experimental technology. Here, we present a coherent feedback control strategy in which a linear cavity is coherently controlled by an optomechanical controller in a feedback manner. The coherent feedback loop transfers and enhances quantum nonlinearity from the controller to the controlled cavity, which makes it possible to observe strong nonlinear effects in either linear cavity or optomechanical cavity. More interestingly, we find that the strong photon blockade under single-photon optomechanical weak coupling condition could be observed in the quantum regime. Additionally, the coherent feedback loop leads to two-photon and multiphoton tunnelings for the controlled linear cavity, which are also typical quantum nonlinear phenomenon. We hope that our work can give new perspectives in engineering nonlinear quantum phenomena.
Yu-Long Liu, Zhong-Peng Liu, Jing Zhang, Yu-xi Liu [pdf]
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Localization of q-form fields on
- In this paper, we investigate localization of a free massless $q-$form bulk field on thin and thick $AdS_{p+1}$ branes with codimension one. It is found that the zero mode of the $q-$form field with $q>(p+2)/2$ can be localized on the thin negative tension brane, which is different from the flat brane case given in [JHEP 10 (2012) 060]. For the thick $AdS_{p+1}$ branes, the $q-$form field with $q>(p+2)/2$ also has a localized zero mode under some conditions. Furthermore, we find that there are massive bound KK modes of the $q-$form field, which are localized on this type $p-$branes.
Chun-E Fu, Yu-Xiao Liu, Heng Guo, Feng-Wei Chen, Sheng-Li Zhang Journal reference: Physics Letters B 735 (2014) 7 [pdf] DOI: 10.1016/j.physletb.2014.06.010
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Pure geometric branes and mass hierarchy -
Abstract
- We consider a toy model with flat thin branes embedded in a 5-dimensional Weyl integrable manifold, where the geometric Weyl scalar provides the material that constitute the brane configurations. The brane configuration is similar to the Randall-Sundrum model. However, it is found that the massless graviton is localized on the brane with negative tension. So in order to solve the gauge hierarchy problem our world should be confined on the positive tension brane, and this is crucial to reproduce a correct Friedmann-like equation on the brane. The spacings of graviton mass spectrum are very tiny, but these massive gravitons are hidden in low energy experiments because they are weakly coupled with matter on the brane.
Ke Yang, Yuan Zhong, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Int.J.Mod.Phys. A29 (2014) 23, 1450120 [pdf] DOI: 10.1142/S0217751X14501206
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Transparency and amplification in a hybrid system of mechanical
resonator and circuit QED -
Abstract
- We theoretically study the transparency and amplification of a weak probe field applied to the cavity in hy- brid systems formed by a driven superconducting circuit QED system and a mechanical resonator, or a driven optomechanical system and a superconducting qubit. We find that both the mechanical resonator and the su- perconducting qubit can result in the transparency to a weak probe field in such hybrid systems when a strong driving field is applied to the cavity. We also find that the weak probe field can be amplified in some parameter regimes. We further study the statistical properties of the output field via the degrees of second-order coherence. We find that the nonclassicality of the output field strongly depends on the system parameters. Our studies show that one can control single-photon transmission in the optomechanical system via a tunable artificial atom or in the circuit QED system via a mechanical resonator.
Hui Wang, Hui-Chen Sun, Jing Zhang, Yu-xi Liu [pdf]
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Batalin-Vilkovisky structure over the Hochschild cohomology ring of a
group algebra -
Abstract
- We realize explicitly the well-known additive decomposition of the Hochschild cohomology ring of a group algebra in the elements level. As a result, we describe the cup product, the Batalin-Vilkovisky operator and the Lie bracket in the Hochschild cohomology ring of a group algebra.
Yu-Ming Liu, Guodong Zhou [pdf]
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Phonon amplification in two coupled cavities containing one mechanical resonator -
Abstract
- We study a general theory of phonon lasing [I. S. Grudinin et al., Phys. Rev. Lett. 104, 083901 (2010)] in coupled optomechancial systems. We derive the dynamical equation of the phonon lasing using supermodes formed by two cavity modes. A general threshold condition for phonon lasing is obtained. We also show the differences between phonon lasing and photon lasing, generated by photonic supermodes and two-level atomic systems, respectively. We find that the phonon lasing can be realized in certain parameter regime near the threshold. The phase diagram and second-order correlation function of the phonon lasing are also studied to show some interesting phenomena that cannot be observed in the common photon lasing with the two-level systems.
Hui Wang, Zhixin Wang, Jing Zhang, Sahin Kaya Ozdemir, Lan Yang, Yu-xi Liu Journal reference: Phys. Rev. A 90, 053814 (2014) [pdf] DOI: 10.1103/PhysRevA.90.053814
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Parton distribution amplitudes of light vector mesons -
Abstract
- A rainbow-ladder truncation of QCD's Dyson-Schwinger equations is used to calculate rho- and phi-meson valence-quark (twist-two parton) distribution amplitudes (PDAs) via a light-front projection of their Bethe-Salpeter wave functions, which possess S- and D-wave components of comparable size in the meson rest frame. All computed PDAs are broad concave functions, whose dilation with respect to the asymptotic distribution is an expression of dynamical chiral symmetry breaking. The PDAs can be used to define an ordering of valence-quark light-front spatial-extent within mesons: this size is smallest within the pion and increases through the perp-polarisation to the parallel-polarisation of the vector mesons; effects associated with the breaking of SU(3)-flavour symmetry are significantly smaller than those associated with altering the polarisation of vector mesons. Notably, the predicted pointwise behaviour of the rho-meson PDAs is in quantitative agreement with that inferred recently via an analysis of diffractive vector-meson photoproduction experiments.
Fei Gao, Lei Chang, Yu-Xin Liu, Craig D. Roberts, Sebastian M. Schmidt [pdf] DOI: 10.1103/PhysRevD.90.014011 1405.0289v1 [pdf]
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Low-energy scattering of the
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Abstract
- In this exploratory lattice study, low-energy scattering of the $(D\bar{D}^{*})^\pm$ meson system is analyzed using lattice QCD with $N_f=2$ twisted mass fermion configurations with three pion mass values. The calculation is performed within single-channel L\"uscher's finite-size formalism. The threshold scattering parameters, namely the scattering length $a_0$ and the effective range $r_0$, for the $s$-wave scattering in $J^P=1^+$ channel are extracted. For the cases in our study, the interaction between the two charmed mesons is weakly repulsive. Our lattice results therefore do not support the possibility of a shallow bound state for the two mesons for the pion mass values we studied. This calculation provides some useful information on the nature of the newly discovered resonance-like structure $Z_c(3900)$ by various experimental groups.
Ying Chen, Ming Gong, Yu-Hong Lei, Ning Li, Jian Liang, Chuan Liu, Hang Liu, Jin-Long Liu, Liuming Liu, Yong-Fu Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Zhan-Lin Wang, Yi-Bo Yang, Jian-Bo Zhang Journal reference: Phys. Rev. D 89, 094506 (2014) [pdf] DOI: 10.1103/PhysRevD.89.094506
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Brane structure and metastable graviton in five-dimensional model with
(non)canonical scalar field -
Abstract
- The appearance of inner brane structure is an interesting issue in domain wall {brane model}. Because such structure usually leads to quasilocalized modes of various kinds of bulk fields. In this paper, we construct a domain wall brane model by using a scalar field $\phi$, which couples to its kinetic term. The inner brane structure emerges as the scalar-kinetic coupling increases. With such brane structure, we show that it is possible to obtain gravity resonant modes in both tensor and scalar sectors. The number of the resonant modes depends on the vacuum expectation value of $\phi$ and the form of scalar-kinetic coupling. The correspondence between our model and the canonical one is also discussed. The noncanonical and canonical background scalar fields are connected by an integral equation, while the warp factor remains the same. Via this correspondence, the canonical and noncanonical models share the same linear perturbation spectrum. So the gravity resonances {obtained} in the noncanonical frame can also be obtained in the standard model. However, due to the inequivalence between the corresponding background scalar solutions, the localization condition for the left-chiral fermion zero mode can be largely different in different frames. Our estimate shows that the magnitude of the Yukawa coupling in the noncanonical frame might be hundreds times larger than the one in the canonical frame, if one demands the localization of the left-chiral fermion zero mode as well as the appearance of a few gravity resonance modes.
Yuan Zhong, Yu-Xiao Liu, Zhen-Hua Zhao [pdf]
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Cavity polariton in a quasilattice of qubits and its selective radiation -
Abstract
- In a circuit quantum eletrodynamic system, a chain of N qubits inhomogeneously coupled to a cavity field forms a mesoscopic quasi-lattice, which is characterized by its degree of deformation from a normal lattice. This deformation is a function of the relative spacing, that is the ratio of the qubit spacing to the cavity wavelength. A polariton mode arise in the quasi-lattice as the dressed mode of the lattice excitation by the cavity photon. We show that the transition probability of the polariton mode is either enhanced or decreased compared to that of a single qubit by the deformation, giving a selective spontaneous radiation spectrum. Further, unlike a microscopic lattice with large-N limit and nearly zero relative spacing, the polariton in the quasi-lattice has uneven decay rate over the relative spacing. We show that this unevenness coincides with the cooperative emission effect expected from the superradiance model, where alternative excitations in the qubits of the lattice result in maximum decay.
Hou Ian, Yu-xi Liu Journal reference: Phys. Rev. A 89, 043804 (2014) [pdf] DOI: 10.1103/PhysRevA.89.043804
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Particle Collisions in the Lower Dimensional Rotating Black Hole Space-Time with the Cosmological Constant -
Abstract
- In this paper, we study the effect of ultra-high energy collisions of two particles with different energies near the horizon of a 2+1 dimensional BTZ black hole (BSW effect). We find that the particle with the critical angular momentum could exist inside the outer horizon of BTZ black hole regardless of the particle energy. Therefore, for the non-extremal BTZ black hole, the BSW process is possible on the inner horizon with the fine tuning of parameters which are characterized by the motion of particle. While for the extremal BTZ black hole, the particle with the critical angular momentum could only exist on the degenerate horizon, and the BSW process could also happen there.
Jie Yang, Yun-Liang Li, Yang Li, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Adv.High Energy Phys. 2014 (2014) 204016 [pdf] DOI: 10.1155/2014/204016
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Glueballs in charmonia radiative decays -
Abstract
- Scalar \cite{scalar_paper} and tensor \cite{tensor_paper} glueballs created in $J/\psi$ radiative decays are studied in quenched lattice QCD. Using two anisotropic lattices to approach the continuum limit, we compute the relevant form factors responsible for the decay rates for $J/\psi\rightarrow\gamma G_{0^{++}}$ and $J/\psi\rightarrow\gamma G_{2^{++}}$. Comparing with the existing experimental data, it is argued that $f_0(1710)$ is a favorable candidate for scalar glueball. The decay rate for $J/\psi\rightarrow\gamma G_{2^{++}}$ is found to be quite substantial. A comprehensive search in the tensor channel on BESIII is therefore suggested.
Ying Chen, Long-Cheng Gui, Gang Li, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Yi-Bo Yang, Jian-Bo Zhang [pdf]
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From blockade to transparency: controllable photon transmission through
a circuit QED system -
Abstract
- A strong photon-photon nonlinear interaction is a necessary condition for photon blockade. Moreover, this nonlinearity can also result a bistable behavior in the cavity field. We analyze the relation between detecting field and photon blockade in a superconducting circuit QED system, and show that photon blockade cannot occur when the detecting field is in the bistable regime. This photon blockade is the microwave-photonics analog of the Coulomb blockade. We further demonstrate that the photon transmission through such system can be controlled (from photon blockade to transparency) by the detecting field. Numerical calculations show that our proposal is experimentally realizable with current technology.
Yu-xi Liu, Xun-Wei Xu, Adam Miranowicz, Franco Nori [pdf]
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Elliptic flow of Λ, Ξ and Ω in 2.76 A TeV Pb+Pb collisions -
Abstract
- The elliptic flow v_2 for strange and multi-strange bayons in 2.76 A TeV Pb+Pb collisions is investigated with VISHNU hybrid model that connects 2+1-d viscous hydrodynamics with a hadron cascade model. It is found that VISHNU nicely describes v_2(p_T) data for \Lambda, \Xi and \Omega at various centralities. Comparing with the ALICE data, it roughly reproduces the mass-ordering of v_2 among \pi, K, p, \Xi and \Omega within current statistics, but gives an inverse v_2 mass-ordering between p and \Lambda.
Huichao Song, Fanli Meng, Xianyin Xin, Yu-Xin Liu [pdf] DOI: 10.1088/1742-6596/509/1/012089 1310.3462v2 [pdf]
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Dirac dynamical resonance states around Schwarzschild black holes -
Abstract
- Recently, a novel kind of scalar wigs around Schwarzschild black holes---scalar dynamical resonance states were introduced in [Phys. Rev. D 84, 083008 (2011)] and [Phys. Rev. Lett. 109, 081102 (2012)]. In this paper, we investigate the existence and evolution of Dirac dynamical resonance states. First we look for stationary resonance states of a Dirac field around a Schwarzchild black hole by using the Schr\"{o}dinger-like equations reduced from the Dirac equation in Schwarzschild spacetime. Then Dirac pseudo-stationary configurations are constructed from the stationary resonance states. We use these configurations as initial data and investigate their numerical evolutions and energy decay. These dynamical solutions are the so-called "Dirac dynamical resonance states". It is found that the energy of the Dirac dynamical resonance states shows an exponential decay. The decay rate of energy is affected by the resonant frequency, the mass of Dirac field, the total angular momentum, and the spin-orbit interaction. In particular, for ultra-light Dirac field, the corresponding particles can stay around a Schwarzschild black hole for a very long time, even for cosmological time-scales.
Xiang-Nan Zhou, Xiao-Long Du, Ke Yang, Yu-Xiao Liu Journal reference: Phys. Rev. D 89, 043006 (2014) [pdf] DOI: 10.1103/PhysRevD.89.043006
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Zero mode in a strongly coupled quark gluon plasma -
Abstract
- In connection with massless two-flavour QCD, we analyse the chiral symmetry restoring phase transition using three distinct gluon-quark vertices and two different assumptions about the long-range part of the quark-quark interaction. In each case, we solve the gap equation, locate the transition temperature T_c, and use the maximum entropy method to extract the dressed-quark spectral function at T>T_c. Our best estimate for the chiral transition temperature is T_c=(147 +/- 8)MeV; and the deconfinement transition is coincident. For temperatures markedly above T_c, we find a spectral density that is consistent with those produced using a hard thermal loop expansion, exhibiting both a normal and plasmino mode. On a domain T\in[T_c,T_s], with T_s approximately 1.5T_c, however, with each of the six kernels we considered, the spectral function contains a significant additional feature. Namely, it displays a third peak, associated with a zero mode, which is essentially nonperturbative in origin and dominates the spectral function at T=T_c. We suggest that the existence of this mode is a signal for the formation of a strongly-coupled quark-gluon plasma and that this strongly-interacting state of matter is likely a distinctive feature of the QCD phase transition.
Fei Gao, Si-Xue Qin, Yu-Xin Liu, Craig D. Roberts, Sebastian M. Schmidt Journal reference: Phys.Rev.D 89,076009,2014 [pdf] DOI: 10.1103/PhysRevD.89.076009
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Nonperturbative procedure for stable
-
Abstract
- We propose a novel first-order formalism for a type of $K$-brane systems. An example solution is presented and studied. We illustrate how the noncanonical kinetic term can affect the properties of the model, such as the stability of the solutions, the localization of fermion and graviton. We argue that our solution is stable against linear perturbations. The tensor zero mode of graviton can be localized while the scalar zero mode cannot. The localization condition for fermion is also discussed.
Yuan Zhong, Yu-Xiao Liu, Zhen-Hua Zhao Journal reference: Phys. Rev. D 89, 104034 (2014) [pdf] DOI: 10.1103/PhysRevD.89.104034
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Coexistence of single- and multi-photon processes due to longitudinal couplings between superconducting flux qubits and external fields -
Abstract
- In contrast to natural atoms, the potential energies for superconducting flux qubit (SFQ) circuits can be artificially controlled. When the inversion symmetry of the potential energy is broken, we find that the multi-photon processes can coexist in the multi-level SFQ circuits. Moreover, there are not only transverse but also longitudinal couplings between the external magnetic fields and the SFQs when the inversion symmetry of potential energy is broken. The longitudinal coupling would induce some new phenomena in the SFQs. Here we will show how the longitudinal coupling can result in the coexistence of multi-photon processes in a two-level system formed by a SFQ circuit. We also show that the SFQs can become transparent to the transverse coupling fields when the longitudinal coupling fields satisfy the certain conditions. We further show that the quantum Zeno effect can also be induced by the longitudinal coupling in the SFQs. Finally we clarify why the longitudinal coupling can induce coexistence and disappearance of single- and two-photon processes for a driven SFQ, which is coupled to a single-mode quantized field.
Yu-xi Liu, Cheng-Xi Yang, Hui-Chen Sun, Xiang-bin Wang Journal reference: New Journal of Physics 16, 015031 (2014) [pdf] DOI: 10.1088/1367-2630/16/1/015031
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Electromagnetically induced transparency and Autler-Townes splitting in superconducting flux quantum circuits -
Abstract
- We study the microwave absorption of a driven three-level quantum system, which is realized by a superconducting flux quantum circuit (SFQC), with a magnetic driving field applied to the two upper levels. The interaction between the three-level system and its environment is studied within the Born-Markov approximation, and we take into account the effects of the driving field on the damping rates of the three-level system. We study the linear response of the driven three-level SFQC to a weak probe field. The linear magnetic susceptibility of the SFQC can be changed by both the driving field and the bias magnetic flux. When the bias magnetic flux is at the optimal point, the transition from the ground state to the second excited state is forbidden and the three-level SFQC has a ladder-type transition. Thus, the SFQC responds to the probe field like natural atoms with ladder-type transitions. However, when the bias magnetic flux deviates from the optimal point, the three-level SFQC has a cyclic transition, thus it responds to the probe field like a combination of natural atoms with ladder-type transitions and natural atoms with $\Lambda$-type transitions. In particular, we provide detailed discussions on the conditions for realizing electromagnetically induced transparency and Autler-Townes splitting in three-level SFQCs.
Hui-Chen Sun, Yu-xi Liu, J. Q. You, E. Il'ichev, Franco Nori Journal reference: Phys. Rev. A 89, 063822 (2014) [pdf] DOI: 10.1103/PhysRevA.89.063822
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Establishing a universal relation between gravitational waves and black hole lensing -
Abstract
- Black hole lensing and gravitational waves are, respectively, closely dependent of the property of the lens and radiation source. In this letter, a universal relation between them is established for a rotating black hole acting simultaneously as a lens and a gravitational wave source, in an asymptotically flat spacetime. The relation only relies on the lens geometry and observable, while is independent of the specific nature of the black hole. Therefore, the possible gravitational wave sources could be located with modern astronomical instrument from the side of the lensing without knowing the specific nature of the black hole lens. Moreover, the low bound of the frequency of the gravitational waves can also be well determined.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 89, 047502 (2014) [pdf] DOI: 10.1103/PhysRevD.89.047502
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Gilbert Damping in Noncollinear Ferromagnets -
Abstract
- The precession and damping of a collinear magnetization displaced from its equilibrium are described by the Landau-Lifshitz-Gilbert equation. For a noncollinear magnetization, it is not known how the damping should be described. We use first-principles scattering theory to investigate the damping in one-dimensional transverse domain walls (DWs) of the important ferromagnetic alloy Ni$_{80}$Fe$_{20}$ and interpret the results in terms of phenomenological models. The damping is found to depend not only on the magnetization texture but also on the specific dynamic modes of Bloch and N\'eel DWs. Even in the highly disordered Ni$_{80}$Fe$_{20}$ alloy, the damping is found to be remarkably nonlocal.
Zhe Yuan, Kjetil M. D. Hals, Yi Liu, Anton A. Starikov, Arne Brataas, Paul J. Kelly Journal reference: Physical Review Letters 113, 266603 (2014) [pdf] DOI: 10.1103/PhysRevLett.113.266603
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Pairing Symmetry of Heavy Fermion Superconductivity in the Two-Dimensional Kondo—Heisenberg Lattice Model -
Abstract
- 2013
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Engineering of nonclassical motional states in optomechanical systems -
Abstract
- We propose to synthesize arbitrary nonclassical motional states in optomechanical systems by using sideband excitations and photon blockade. We first demonstrate that the Hamiltonian of the optomechanical systems can be reduced, in the strong single-photon optomechanical coupling regime when the photon blockade occurs, to one describing the interaction between a driven two-level trapped ion and the vibrating modes, and then show a method to generate target states by using a series of classical pulses with desired frequencies, phases, and durations. We further analyze the effect of the photon leakage, due to small anharmonicity, on the fidelity of the expected motional state, and study environment induced decoherence. Moreover, we also discuss the experimental feasibility and provide operational parameters using the possible experimental data.
Xun-Wei Xu, Hui Wang, Jing Zhang, Yu-xi Liu Journal reference: Phys. Rev. A 88, 063819 (2013) [pdf] DOI: 10.1103/PhysRevA.88.063819
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Linear perturbations in Eddington-inspired Born-Infeld gravity -
Abstract
- We study the full linear perturbations of a homogeneous and isotropic spacetime in the Eddington-inspired Born-Infeld gravity. The stability of the perturbations are analyzed in the Eddington regime. We find that, for positive $\kappa$, the scalar modes are stable in the infinite wavelength limit ($k=0$) but unstable for $k\neq0$. The vector modes are stable and the tensor mode is unstable in the Eddington regime, independent of the wave vector $k$. However, these modes are unstable and hence cause the instabilities for negative $\kappa$.
Ke Yang, Xiao-Long Du, Yu-Xiao Liu Journal reference: Phys. Rev. D 88, 124037 (2013) [pdf] DOI: 10.1103/PhysRevD.88.124037
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Number-resolved master equation approach to quantum transport under the self-consistent Born approximation -
Abstract
- We construct a particle-number(n)-resolved master equation (ME) approach under the self-consistent Born approximation (SCBA) for quantum transport through mesoscopic systems. The formulation is essentially non-Markovian and incorporates the interlay of the multi-tunneling processes and many-body correlations. The proposed n-SCBA-ME goes completely beyond the scope of the Born-Markov master equation, being applicable to transport under small bias voltage, in non-Markovian regime and with strong Coulomb correlations. For steady state, it can recover not only the exact result of noninteracting transport under arbitrary voltages, but also the challenging nonequilibrium Kondo effect. Moreover, the n-SCBA-ME approach is efficient for the study of shot noise.We demonstrate the application by a couple of representative examples, including particularly the nonequilibrium Kondo system.
Yu Liu, Jinshuang Jin, Jun Li, Xin-Qi Li, YiJing Yan Journal reference: Science China Physics,Mechanics & Astronomy, 2013, 56(10): 1866-1873 [pdf] DOI: 10.1007/s11433-013-5238-7
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A dynamical mechanism for generating quark confinement -
Abstract
- We explore the dynamical mechanism for generating the infrared singular quark-gluon vertex and quark confinement based on the gauge invariance in covariant-gauge quantum chromodynamics(QCD). We first derive the gauge-invariance constraint relation for the infrared-limit behavior of the quark-gluon vertex, which shows the mechanism for generating the infrared behavior of the quark-gluon vertex. We hence unravel a novel mechanism for generating an infrared singular quark-gluon vertex and then a linear rising potential for confining massive quarks, where the infrared singularity in the form factors composing the quark-ghost scattering kernel plays a crucial role. The mechanism for linking chiral symmetry breaking with quark confinement is also shown.
Han-Xin He, Yu-Xin Liu [pdf]
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Interface tension and interface entropy in the
-
Abstract
- We study the QCD phases and their transitions in 2+1 flavour NJL model, with focus on the interface effects such as the interface tension, the interface entropy and critical bubble size in the coexistence region of the first order phase transitions. Our results show that, under the thin-wall approximation, interface contribution to total entropy density changes its discontinuity scale in the first order phase transition. However, the entropy density of dynamical chiral symmetry (DCS) phase is always greater than that of the dynamical chiral symmetry broken (DCSB) phase in both heating and hadronization processes. To address this entropy puzzle, the thin-wall approximation is evaluated in the present work. We find that the puzzle can be attributed to a drastic overestimate of the critical bubble size at low temperature in the hadronization process. With an improvement on the thin-wall approximation, the entropy puzzle is well solved with the total entropy density of the hadron-DCSB phase exceeding apparently that of the DCS-quark phase at low temperature.
Wei-yao Ke, Yu-xin Liu [pdf] DOI: 10.1103/PhysRevD.89.074041 1312.2295v1 [pdf]
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Observing the shadow of Einstein-Maxwell-Dilaton-Axion black hole -
Abstract
- In this paper, the shadows cast by Einstein-Maxwell-Dilaton-Axion black hole and naked singularity are studied. The shadow of a rotating black hole is found to be a dark zone covered by a deformed circle. For a fixed value of the spin $a$, the size of the shadow decreases with the dilaton parameter $b$. The distortion of the shadow monotonically increases with $b$ and takes its maximal when the black hole approaches to the extremal case. Due to the optical properties, the area of the black hole shadow is supposed to equal to the high-energy absorption cross section. Based on this assumption, the energy emission rate is investigated. For a naked singularity, the shadow has a dark arc and a dark spot or straight, and the corresponding observables are obtained. These results show that there is a significant effect of the spin $a$ and dilaton parameter $b$ on these shadows. Moreover, we examine the observables of the shadow cast by the supermassive black hole at the center of the Milky Way, which is very useful for us to probe the nature of the black hole through the astronomical observations in the near future.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: JCAP 11, 063 (2013) [pdf] DOI: 10.1088/1475-7516/2013/11/063
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A Qualitative Representation and Similarity Measurement Method in
Geographic Information Retrieval -
Abstract
- The modern geographic information retrieval technology is based on quantitative models and methods. The semantic information in web documents and queries cannot be effectively represented, leading to information lost or misunderstanding so that the results are either unreliable or inconsistent. A new qualitative approach is thus proposed for supporting geographic information retrieval based on qualitative representation, semantic matching, and qualitative reasoning. A qualitative representation model and the corresponding similarity measurement method are defined. Information in documents and user queries are represented using propositional logic, which considers the thematic and geographic semantics synthetically. Thematic information is represented as thematic propositions on the base of domain ontology. Similarly, spatial information is represented as geo-spatial propositions with the support of geographic knowledge base. Then the similarity is divided into thematic similarity and spatial similarity. The former is calculated by the weighted distance of proposition keywords in the domain ontology, and the latter similarity is further divided into conceptual similarity and spatial similarity. Represented by propositions and information units, the similarity measurement can take evidence theory and fuzzy logic to combine all sub similarities to get the final similarity between documents and queries. This novel retrieval method is mainly used to retrieve the qualitative geographic information to support the semantic matching and results ranking. It does not deal with geometric computation and is consistent with human commonsense cognition, and thus can improve the efficiency of geographic information retrieval technology.
Yong Gao, Lei Liu, Xing Lin, Yu Liu [pdf]
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Uncovering Patterns of Inter-Urban Trip and Spatial Interaction from Social Media Check-In Data -
Abstract
- The article revisits spatial interaction and distance decay from the perspective of human mobility patterns and spatially-embedded networks based on an empirical data set. We extract nationwide inter-urban movements in China from a check-in data set that covers half million individuals and 370 cities to analyze the underlying patterns of trips and spatial interactions. By fitting the gravity model, we find that the observed spatial interactions are governed by a power law distance decay effect. The obtained gravity model also well reproduces the exponential trip displacement distribution. However, due to the ecological fallacy issue, the movement of an individual may not obey the same distance decay effect. We also construct a spatial network where the edge weights denote the interaction strengths. The communities detected from the network are spatially connected and roughly consistent with province boundaries. We attribute this pattern to different distance decay parameters between intra-province and inter-province trips.
Yu Liu, Zhengwei Sui, Chaogui Kang, Yong Gao Journal reference: PLoS ONE 9(1): e86026 [pdf] DOI: 10.1371/journal.pone.0086026
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Brane worlds in critical gravity -
Abstract
- Recently, Lu and Pope proposed critical gravities in [Phys. Rev. Lett. 106, 181302 (2011)]. In this paper we construct analytic brane solutions in critical gravity with matter. The Gibbons-Hawking surface term and junction condition are investigated, and the thin and thick brane solutions are obtained. All these branes are embedded in five-dimensional anti-de Sitter spacetimes. Our solutions are stable against scalar perturbations, and the zero modes of scalar perturbations cannot be localized on the branes.
Feng-Wei Chen, Yu-Xiao Liu, Yuan Zhong, Yong-Qiang Wang, Shao-Feng Wu Journal reference: Phys. Rev. D 88, 104033 (2013) [pdf] DOI: 10.1103/PhysRevD.88.104033
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Equidistribution of polynomial sequences in function fields, with
applications -
Abstract
- We prove a function field analog of Weyl's classical theorem on equidistribution of polynomial sequences. Our result covers the case when the degree of the polynomial is greater than or equal to the characteristic of the field, which is a natural barrier when applying the Weyl differencing process to function fields. We also discuss applications to van der Corput and intersective sets in function fields.
Thai Hoang Le, Yu-Ru Liu [pdf]
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Feedback-induced nonlinearity and superconducting on-chip quantum optics -
Abstract
- Quantum coherent feedback has been proven to be an efficient way to tune the dynamics of quantum optical systems and, recently, those of solid-state quantum circuits. Here, inspired by the recent progress of quantum feedback experiments, especially those in mesoscopic circuits, we prove that superconducting circuit QED systems, shunted with a coherent feedback loop, can change the dynamics of a superconducting transmission line resonator, i.e., a linear quantum cavity, and lead to strong on-chip nonlinear optical phenomena. We find that bistability can occur under the semiclassical approximation, and photon anti-bunching can be shown in the quantum regime. Our study presents new perspectives for engineering nonlinear quantum dynamics on a chip.
Zhong-Peng Liu, Hui Wang, Jing Zhang, Yu-xi Liu, Re-Bing Wu, Franco Nori Journal reference: Phys. Rev. A 88, 063851 (2013) [pdf] DOI: 10.1103/PhysRevA.88.063851
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Quantum internet using code division multiple access -
Abstract
- A crucial open problem in large-scale quantum networks is how to efficiently transmit quantum data among many pairs of users via a common data-transmission medium. We propose a solution by developing a quantum code division multiple access (q-CDMA) approach in which quantum information is chaotically encoded to spread its spectral content, and then decoded via chaos synchronization to separate different sender-receiver pairs. In comparison to other existing approaches, such as frequency division multiple access (FDMA), the proposed q-CDMA can greatly increase the information rates per channel used, especially for very noisy quantum channels.
Jing Zhang, Yu-xi Liu, Sahin Kaya Ozdemir, Re-Bing Wu, Feifei Gao, Xiang-Bin Wang, Lan Yang, Franco Nori Journal reference: Sci. Rep. 3, 2211 (2013) [pdf] DOI: 10.1038/srep02211
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Further results on the nullity of signed graphs -
Abstract
- The nullity of a graph is the multiplicity of the eigenvalues zero in its spectrum. A signed graph is a graph with a sign attached to each of its edges. In this paper, we obtain the coefficient theorem of the characteristic polynomial of a signed graph, give two formulae on the nullity of signed graphs with cut-points. As applications of the above results, we investigate the nullity of the bicyclic signed graph $\Gamma(\infty(p,q,l))$, obtain the nullity set of unbalanced bicyclic signed graphs, and thus determine the nullity set of bicyclic signed graphs.
Yu Liu, Lhua You [pdf]
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Lattice Study of Radiative
-
Abstract
- The radiative decay of $J/\psi$ into a pure gauge tensor glueball is studied in the quenched lattice QCD formalism. With two anisotropic lattices, the mutlipole amplitudes E_1(0), M_2(0) and E_3(0) are obtained to be 0.114(12)(6)GeV, -0.011(5)(1)GeV, and 0.023(8)(1)GeV, respectively. The first error comes from the statistics, the Q^2 interpolation, and the continuum extrapolation, while the second is due to the uncertainty of the scale parameter r_0^{-1}=410(20) MeV. Thus the partial decay width $\Gamma(J/\psi\rightarrow \gamma G_{2^{++}})$ is estimated to be 1.01(22)(10) keV which corresponds to a large branch ratio 1.1(2)(1)x10^{-2}. The phenomenological implication of this result is also discussed.
Yi-Bo Yang, Long-Cheng Gui, Ying Chen, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Jian-Bo Zhang Journal reference: Phys. Rev. Lett. 111, 091601 (2013) [pdf] DOI: 10.1103/PhysRevLett.111.091601
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Linearization of thick
-
Abstract
- We study the linearization of a class of thick K-branes, namely, four-dimensional domain walls generated by a scalar field with particular nonstandard kinetic terms. The master equations for linear perturbations are derived from the point of view of both dynamical equations and quadratic action. The spectra of the canonical normal modes are studied using supersymmetric quantum mechanics. Our results indicate that the scalar perturbation is nonlocalizable in general. Conditions for stable $K$-branes are also found.
Yuan Zhong, Yu-Xiao Liu Journal reference: Phys. Rev. D 88, 024017 (2013) [pdf] DOI: 10.1103/PhysRevD.88.024017
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DIFFRACTION, REFRACTION, AND REFLECTION OF AN EXTREME-ULTRAVIOLET WAVE OBSERVED DURING ITS INTERACTIONS WITH REMOTE ACTIVE REGIONS -
Abstract
- We present observations of the diffraction, refraction, and reflection of a global extreme-ultraviolet (EUV) wave propagating in the solar corona. These intriguing phenomena are observed when the wave interacts with two remote active regions, and they together exhibit the wave property of this EUV wave. When the wave approached AR11465, it became weaker and finally disappeared in the active region, but a few minutes latter a new wavefront appeared behind the active region, and it was not concentric with the incoming wave. In addition, a reflected wave was also observed simultaneously on the wave incoming side. When the wave approached AR11459, it transmitted through the active region directly and without reflection. The formation of the new wavefront and the transmission could be explained with diffraction and refraction effects, respectively. We propose that the different behaviors observed during the interactions may caused by different speed gradients at the boundaries of the two active regions. For the origin of the EUV wave, we find that it formed ahead of a group of expanding loops a few minutes after the start of the loops' expansion, which represents the initiation of the associated coronal mass ejection (CME). Based on these results, we conclude that the EUV wave should be a nonlinear magnetosonic wave or shock driven by the associated CME, which propagated faster than the ambient fast-mode speed and gradually slowed down to an ordinary linear wave. Our observations support the hybrid model that includes both fast wave and slow non-wave components.
Yuandeng Shen, Yu Liu, Jiangtao Su, Hui Li, Ruijuan Zhao, Zhanjun Tian, Kiyoshi Ichimoto, Kazunari Shibata Journal reference: ApJ, 2013, 773, 33 [pdf] DOI: 10.1088/2041-8205/773/2/L33
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Hearts of twin cotorsion pairs on exact categories -
Abstract
- In the papers of Nakaoka, he introduced the notion of hearts of (twin) cotorsion pairs on triangulated categories and showed that they have structures of (semi-) abelian categories. We study in this article a twin cotorsion pair (S,T),(U,V) on an exact category B with enough projectives and injectives and introduce a notion of the heart. First we show that its heart is preabelian. Moreover we show the heart of a single cotorsion pair is abelian. These results are analog of Nakaoka's results in triangulated categories. We also consider special cases where the heart has nicer structure. By our results, the heart of a special twin cotorsion pair (S,T),(T,V), is integral and almost abelian. Finally we show that the Gabriel-Zisman localisation of the heart at the class of regular morphisms is abelian, and moreover it is equivalent to the category of finitely presented modules over a stable subcategory of B.
Yu Liu [pdf]
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Modulation Classification of MIMO-OFDM Signals by Independent Component
Analysis and Support Vector Machines -
Abstract
- A modulation classification (MC) scheme based on Independent Component Analysis (ICA) in conjunction with either maximum likelihood (ML) or Support Vector Machines (SVM) is proposed for MIMO-OFDM signals over frequency selective, time varying channels. The method is blind in the sense that it is assumed that the receiver has no information about the channel and transmitted signals other than that the spatial streams of signals are statistically independent. The processing consists of separation of the MIMO streams followed by modulation classification of the separated signals. While in general, blind separation of signals over frequency selective channels is a difficult problem, the non-frequency selective nature of the channel experienced by individual symbols in a MIMO-OFDM system enables the application of well-known ICA algorithms. Modulation classification is implemented by maximum likelihood and by an SVM-based modulation classification method relying on pre-selected modulation-dependent features. To improve performance in time varying channels, the invariance of the channel is exploited across the coherence bandwidth and the time coherence. The proposed method is shown to perform with high probability of correct classification over realistic ITU pedestrian and vehicular channels. An upper bound on the probability of correct classification is developed based on the Cramer Rao bound of channel estimation.
Yu Liu, Alexander M. Haimovich, Wei Su, Jason Dabin, Emmanuel Kanterakis [pdf]
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Localization of bulk matter fields, the hierarchy problem and corrections to Coulomb’s law on a pure de Sitter thick braneworld -
Abstract
- In this paper we investigate the localization and mass spectra of matter fields with spin 0, 1 and 1/2 on a geometric thick brane generated by pure 4D and 5D positive cosmological constants without bulk scalar fields. This model possesses a 4D cosmological constant that can be made as small as one desires without fine-tuning it with the bulk cosmological constant. The RS model is obtained as an analytic continuation of the flat brane limit of this braneworld configuration when the Hubble parameter disappears. Within this inflating braneworld model it is possible to formulate a mechanism for obtaining TeV mass scales from Planck ones by adding a positive thin brane, where the Standard Model fields are trapped, at a distance y_2 from the origin, where the Planck thick brane resides. The brane separation must be of the same order than the inverse thickness parameter of the model in order for the mechanism to generate the desired hierarchy. This result is obtained by imposing the recovery of both the correct 4D gravitational couplings and the actually observed accelerated expansion of the universe in our de Sitter braneworld. Regarding the localization of matter in the purely geometric thick braneworld, for spin 0 massless and massive scalar fields as well as for spin 1 vector fields, the potentials of the Kaluza--Klein (KK) modes in thecorresponding Schroedinger equations are modified Poeschl-Teller potentials, which lead to the localization of the scalar and vector zero modes on the brane as well as to mass gaps in the mass spectra. We also compute the corrections to Coulomb's law coming from massive KK vector modes. For spin 1/2 fermions, we introduce the bulk mass term MF(z)\bar{\Psi}\Psi in the action and show that localization of the massless left-chiral fermion zero mode is feasible for two mass functions MF(z) with a finite/infinite number of massive KK bound states.
Heng Guo, Alfredo Herrera-Aguilar, Yu-Xiao Liu, Dagoberto Malagon-Morejon, Refugio Rigel Mora-Luna Journal reference: Phys. Rev. D87 (2013) 095011 [pdf] DOI: 10.1103/PhysRevD.87.095011
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Detection of a charged two-level system by using the Kondo and the
Fano-Kondo effects in quantum dots -
Abstract
- The Kondo effect and the Fano-Kondo effect are important phenomena that have been observed in quantum dots (QDs). We theoretically investigate the transport properties of a coupled QD system in order to study the possibility of detecting a qubit state from the modulation of the conductance peak in the Kondo effect and the dip in the Fano-Kondo effect. We show that the peak and dip of the conductance are both shifted depending on the energy-level of the qubit. In particular, we find that we can estimate the optimal point and tunneling coupling between the |0> and |1> states of the qubit by measuring the shift of the positions of the conductance peak and dip, as functions of the applied gate voltage on the qubit and the distance between the qubit and the detector.
Tetsufumi Tanamoto, Yu-xi Liu, Xuedong Hu, Franco Nori [pdf]
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Photon-induced tunneling in optomechanical systems -
Abstract
- In contrast to recent studies [Rabl, Phys. Rev. Lett. 107, 063601 (2011); Nunnenkamp et al., Phys. Rev. Lett. 107, 063602 (2011)] on photon blockade that prevents subsequent photons from resonantly entering the cavity in optomechanical systems, we study the photon-induced tunneling that increases the probability of admitting subsequent photons in those systems. In particular, we analytically and numerically show how twoor three-photon tunneling can occur by avoiding single-photon blockade. Our study provides another way on photon control using a single mechanical resonator in optomechanical systems.
Xun-Wei Xu, Yuan-Jie Li, Yu-xi Liu Journal reference: Phys. Rev. A 87, 025803 (2013) [pdf] DOI: 10.1103/PhysRevA.87.025803
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Planetary Companions to Three Evolved Intermediate-Mass Stars: HD 2952, HD 120084, and
-
Abstract
- We report the detections of planetary companions orbiting around three evolved intermediate-mass stars from precise radial velocity measurements at Okayama Astrophysical Observatory. HD 2952 (K0III, 2.5 M_sun) and omega Ser (G8III, 2.2 M_sun) host a relatively low mass planet with minimum mass of m_2sin i=1.6 M_J and 1.7 M_J in nearly circular orbits with period of P=312 and 277 d, respectively. HD 120084 (G7 III, 2.4 M_sun) hosts an eccentric planet with m_2sin i=4.5 M_J in an orbit with P=2082 d and eccentricity of e=0.66. The planet has one of the largest eccentricities among those ever discovered around evolved intermediate-mass stars, almost all of which have eccentricity smaller than 0.4. We also show that radial velocity variations of stellar oscillations for G giants can be averaged out below a level of a few m/s at least in timescale of a week by high cadence observations, which enables us to detect a super-Earth and a Neptune-mass planet in short-period orbits even around such giant stars.
Bun'ei Sato, Masashi Omiya, Hiroki Harakawa, Yu-Juan Liu, Hideyuki Izumiura, Eiji Kambe, Yoichi Takeda, Michitoshi Yoshida, Yoichi Itoh, Hiroyasu Ando, Eiichiro Kokubo, Shigeru Ida [pdf] DOI: 10.1093/pasj/65.4.85 1304.4328v2 [pdf]
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Weak ferromagnetism with the Kondo screening effect in the Kondo lattice systems -
Abstract
- We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phase transition exists to separate the partially polarized ferromagnetic ordered phase from the paramagnetic heavy Fermi liquid phase. These results may be used to explain the weak ferromagnetism observed recently in the Kondo lattice materials.
Yu Liu, Guang-Ming Zhang, Lu Yu Journal reference: Physical Review B 87, 134409 (2013) [pdf] DOI: 10.1103/PhysRevB.87.134409
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New Superconductivity in Layered 1T-TaS2-xSex Single Crystals Fabricated
by Chemical Vapor Transport -
Abstract
- Layered transition-metal dichalcogenides 1T-TaS2-xSex (0<=x<=2) single crystals have been successfully fabricated by using a chemical vapor transport technique in which Ta locates in octahedral coordination with S and Se atoms. This is the first superconducting example by the substitution of S site, which violates an initial rule based on the fact that superconductivity merely emerges in 1T-TaS2 by applying the high pressure or substitution of Ta site. We demonstrate the appearance of a series of electronic states in 1T-TaS2-xSex with Se content. Namely, the Mott phase melts into a nearly commensurate charge-density-wave (NCCDW) phase, superconductivity in a wide x range develops within the NCCDW state, and finally commensurate charge-density-wave (CCDW) phase reproduces for heavy Se content. The present results reveal that superconductivity is only characterized by robust Ta 5d band, demonstrating the universal nature in 1T-TaS2 systems that superconductivity and NCCDW phase coexist in the real space.
Yu Liu, Ran Ang, Wenjian Lu, Wenhai Song, Lijun Li, Yuping Sun [pdf]
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Two-photon and three-photon blockades in driven nonlinear systems -
Abstract
- Photon blockade, in analogy to Coulomb's or phonon blockades, is a phenomenon when a single photon in a nonlinear cavity blocks the transmission of a second photon. This effect can occur in Kerr-type systems driven by a laser due to strong nonlinear photon-photon interactions. We predict the occurrence of higher-order photon blockades where the transmission of more than two photons is effectively blocked by single- and two-photon states. This photon blockade can be achieved by tuning the frequency of the laser driving field to be equal to the sum of the Kerr nonlinearity and the cavity resonance frequency. We refer to this phenomenon as two-photon blockade or two-photon state truncation via nonlinear scissors, and can also be interpreted as photon-induced tunneling. We also show that, for a driving-field frequency fulfilling another resonance condition and for higher strengths of the driving field, even a three-photon blockade can occur but less clearly than in the case of single- and two-photon blockades. We demonstrate how various photon blockades can be identified by analyzing photon-number correlations, coherence and entropic properties, Wigner functions, and spectra of squeezing. We show that two- and three-photon blockades can, in principle, be observed in various cavity and circuit quantum electrodynamical systems for which the standard single-photon blockade was observed without the need of using higher-order driving interactions or Kerr media exhibiting higher-order nonlinear susceptibility.
Adam Miranowicz, Malgorzata Paprzycka, Yu-xi Liu, Jiri Bajer, Franco Nori Journal reference: Phys. Rev. A 87, 023809 (2013) [pdf] DOI: 10.1103/PhysRevA.87.023809
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Phase gate of one qubit simultaneously controlling
-
Abstract
- We propose how to realize a three-step controlled-phase gate of one superconducting qubit simultaneously controlling n qubits selected from N qubits in a cavity (1<n<n). the="The" operation="operation" time="time" of="of" this="this" gate="gate" is="is" independent="independent" of="of" the="The" number="number" n="n" of="of" qubits="qubits" involved="involved" in="in" the="The" gate="gate" operation.="operation." this="this" phase="phase" gate="gate" controlling="controlling" at="at" once="once" n="n" qubits="qubits" is="is" insensitive="insensitive" to="to" the="The" initial="initial" state="state" of="of" the="The" cavity="cavity" mode="mode" and="and" can="can" be="be" used="used" to="to" produce="produce" an="an" analogous="analogous" cnot="CNOT" gate="gate" simultaneously="simultaneously" acting="acting" on="on" n="n" qubits.
Chui-Ping Yang, Yu-xi Liu, Franco Nori [pdf] DOI: 10.1103/PhysRevA.81.062323 0907.4201v3 [pdf]
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Entangled-state engineering of vibrational modes in a multimembrane optomechanical system -
Abstract
- We propose a method to generate entangled states of the vibrational modes of N membranes which are coupled to a cavity mode via the radiation pressure. Using sideband excitations, we show that arbitrary entangled states of vibrational modes of different membranes can be produced in principle by sequentially applying a series of classical pulses with desired frequencies, phases and durations. As examples, we show how to synthesize several typical entangled states, for example, Bell states, NOON states, GHZ states and W states. The environmental effect, information leakage, and experimental feasibility are briefly discussed. Our proposal can also be applied to other experimental setups of optomechanical systems, in which many mechanical resonators are coupled to a common sing-mode cavity field via the radiation pressure.
Xun-Wei Xu, Yan-Jun Zhao, Yu-xi Liu Journal reference: Phys. Rev. A 88, 022325 (2013) [pdf] DOI: 10.1103/PhysRevA.88.022325
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Nonequilibrium Shot Noise Spectrum Through a Quantum Dot in the Kondo Regime: A Master Equation Approach under Self-Consistent Born Approximation -
Abstract
- We construct a number($n$)-resolved master equation (ME) approach under self-consistent Born approximation (SCBA) for noise spectrum calculation. The formulation is essentially non-Markovian and incorporates properly the interlay of the multi-tunneling processes and many-body correlations. We apply this approach to the challenging nonequilibrium Kondo system and predict a profound nonequilibrium Kondo signature in the shot noise spectrum. The proposed $n$-SCBA-ME scheme goes completely beyond the scope of the Born-Markovian master equation approach, in the sense of being applicable to the shot noise of transport under small bias voltage, in non-Markovian regime, and with strong Coulomb correlations as favorably demonstrated in the nonequilibrium Kondo system.
Yu Liu, Jinshuang Jin, Jun Li, Xin-Qi Li, YiJing Yan Journal reference: Commun. Theor. Phys. 60, 503 (2013) [pdf] DOI: 10.1088/0253-6102/60/4/20
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Variant-Frequency Semantics for Green Futures -
Abstract
- This paper describes an operational semantics for futures, with the primary target on energy efficiency. The work in progress is built around an insight that different threads can coordinate by running at different "paces," so that the time for synchronization and the resulting wasteful energy consumption can be reduced. We exploit several inherent characteristics of futures to determine how the paces of involving threads can be coordinated. The semantics is inspired by recent advances in computer architectures, where the frequencies of CPU cores can be adjusted dynamically. The work is a first-step toward a direction where variant frequencies are directly modeled as an essential semantic feature in concurrent programming languages.
Yu David Liu Journal reference: EPTCS 109, 2013, pp. 1-6 [pdf] DOI: 10.4204/EPTCS.109.1
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Practical corollaries of transverse Ward–Green–Takahashi identities -
Abstract
- The gauge principle is fundamental in formulating the Standard Model. Fermion--gauge-boson couplings are the inescapable consequence and the primary determining factor for observable phenomena. Vertices describing such couplings are simple in perturbation theory and yet the existence of strong-interaction bound-states guarantees that many phenomena within the Model are nonperturbative. It is therefore crucial to understand how dynamics dresses the vertices and thereby fundamentally alters the appearance of fermion--gauge-boson interactions. We consider the coupling of a dressed-fermion to an Abelian gauge boson, and describe a unified treatment and solution of the familiar longitudinal Ward-Green-Takahashi identity and its less well known transverse counterparts. Novel consequences for the dressed-fermion--gauge-boson vertex are exposed.
Si-xue Qin, Lei Chang, Yu-xin Liu, Craig D. Roberts, Sebastian M. Schmidt Journal reference: Phys. Lett. B 722, 384 (2013) [pdf] DOI: 10.1016/j.physletb.2013.04.034
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Quotients of exact categories by cluster tilting subcategories as module categories -
Abstract
- We prove that some subquotient categories of exact categories are abelian. This generalizes a result by Koenig-Zhu in the case of (algebraic) triangulated categories. As a particular case, if an exact category B with enough projectives and injectives has a cluster tilting subcategory M, then B/M is abelian. More precisely, it is equivalent to the category of finitely presented modules over the stable category of M.
Laurent Demonet, Yu Liu Journal reference: J. Pure Appl. Algebra 217 (2013), no. 12, 2282-2297 [pdf] DOI: 10.1016/j.jpaa.2013.03.007
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Critical phenomena and thermodynamic geometry of charged Gauss-Bonnet AdS black holes -
Abstract
- In this paper, we study the phase structure and equilibrium state space geometry of charged topological Gauss-Bonnet black holes in $d$-dimensional anti-de Sitter spacetime. Several critical points are obtained in the canonical ensemble, and the critical phenomena and critical exponents near them are examined. We find that the phase structures and critical phenomena drastically depend on the cosmological constant $\Lambda$ and dimensionality $d$. The result also shows that there exists an analogy between the black hole and the van der Waals liquid gas system. Moreover, we explore the phase transition and possible property of the microstructure using the state space geometry. It is found that the Ruppeiner curvature diverges exactly at the points where the heat capacity at constant charge of the black hole diverges. This black hole is also found to be a multiple system, i.e., it is similar to the ideal gas of fermions in some range of the parameters, while to the ideal gas of bosons in another range.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D 87, 044014 (2013) [pdf] DOI: 10.1103/PhysRevD.87.044014
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Black hole spectroscopy from null geodesics -
Abstract
- The quasinormal mode frequencies can be understood from the massless particles trapped at the unstable circular null geodesics and slowly leaking out to infinity. Base on this viewpoint, in this paper, we construct the quantum entropy spectrum of the stationary black holes from the null geodesics using the new physical interpretation of the quasinormal mode frequencies proposed by Maggiore. Following this idea, we calculate the spacing of the entropy spectrum for various types of black holes with or without the charge and spin in any dimension $d$ of the spacetime. The result shows that the spacing closely depends on the charge, spin, and the dimension of the spacetime. Moreover, for a black hole far from the extremal case, the spacing is found to be larger than $2\pi\hbar$ for $d=4$, while smaller than $2\pi\hbar$ for $d\geq 5$, which is very different from the result of the previous work by using the usual quasinormal mode frequencies, where the spacing of the entropy spectrum is $2\pi\hbar$ and is independent of the black hole parameters and dimension $d$.
Shao-Wen Wei, Yu-Xiao Liu, Chun-E Fu [pdf]
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Baryon and meson screening masses -
Abstract
- In a strongly-coupled quark-gluon plasma, collective excitations of gluons and quarks should dominate over the excitation of individual quasi-free gluon and quark modes. To explore this possibility, we computed screening masses for ground-state light-quark mesons and baryons at leading-order in a symmetry-preserving truncation scheme for the Dyson-Schwinger equations using a confining formulation of a contact-interaction at nonzero temperature. Meson screening masses are obtained from Bethe-Salpeter equations; and baryon analogues from a novel construction of the Faddeev equation, which employs an improved quark-exchange approximation in the kernel. Our treatment implements a deconfinement transition that is coincident with chiral symmetry restoration in the chiral limit, when both transitions are second order. Despite deconfinement, in all T=0 bound-state channels, strong correlations persist above the critical temperature, T>T_c; and, in the spectrum defined by the associated screening masses, degeneracy between parity-partner correlations is apparent for T >1.3T_c. Notwithstanding these results, there are reasons (including Golberger-Treiman relations) to suppose that the inertial masses of light-quark bound-states, when they may be defined, vanish at the deconfinement temperature; and that this is a signal of bound-state dissolution. Where a sensible comparison is possible, our predictions are consistent with results from contemporary numerical simulations of lattice-regularised QCD.
Kun-lun Wang, Yu-xin Liu, Lei Chang, Craig D. Roberts, Sebastian M. Schmidt [pdf] DOI: 10.1103/PhysRevD.87.074038 1301.6762v1 [pdf]
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Scalar Glueball in Radiative
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Abstract
- The form factors in the radiative decay of $J/\psi$ to a scalar glueball are studied within quenched lattice QCD on anisotropic lattices. The continuum extrapolation is carried out by using two different lattice spacings. With the results of these form factors, the partial width of $J/\psi$ radiatively decaying into the pure gauge scalar glueball is predicted to be 0.35(8) keV, which corresponds to a branching ratio of 3.8(9)x10^{-3}. By comparing with the experiments, out results indicate that f_0(1710) has a larger overlap with the pure gauge glueball than other related scalar mesons.
Long-Cheng Gui, Ying Chen, Gang Li, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Yi-Bo Yang, Jian-Bo Zhang Journal reference: Phys. Rev. Lett. 110, 021601 (2013) [pdf] DOI: 10.1103/PhysRevLett.110.021601
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Lattice study on
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Abstract
- Properties of $2^{-+}$ charmonium $\eta_{c2}$ are investigated in quenched lattice QCD. The mass of $\eta_{c2}$ is determined to be 3.80(3) GeV, which is close to the mass of $D$-wave charmonium $\psi(3770)$ and in agreement with quark model predictions. The transition width of $\eta_{c2}\to \gamma J/\psi$ is also obtained with a value $\Gamma=3.8(9)$ keV. Since the possible $2^{-+}$ assignment to X(3872) has not been ruled out by experiments, our results help to clarify the nature of X(3872).
Yi-Bo Yang, Ying Chen, Long-Cheng Gui, Chuan Liu, Yu-Bin Liu, Zhaofeng Liu, Jian-Ping Ma, Jian-Bo Zhang Journal reference: Phys. ReV. D 87, 014501 (2013) [pdf] DOI: 10.1103/PhysRevD.87.014501
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Non-Markovian quantum input-output networks -
Abstract
- Quantum input-output response analysis is a useful method for modeling the dynamics of complex quantum networks, such as those for communication or quantum control via cascade connections. Non-Markovian effects have not yet been studied in such networks. Here we extend the Markovian input-output network formalism developed in optical systems to non-Markovian cascaded networks which can be used, e.g., to analyze the input-output response of mesoscopic quantum networks. We use this formalism to explore the behavior of superconducting qubit networks, where we examine the effect of finite cavity bandwidths. We also discuss its application to open- and closed-loop control networks, and show how these networks create effective Hamiltonians for the controlled system.
Jing Zhang, Yu-xi Liu, Re-Bing Wu, Kurt Jacobs, Franco Nori Journal reference: Phys. Rev. A 87, 032117 (2013) [pdf] DOI: 10.1103/PhysRevA.87.032117
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Geometric Curvatures of Plane Symmetry Black Hole -
Abstract
- In this paper, we study the properties and thermodynamic stability of the plane symmetry black hole from the viewpoint of geometry. Weinhold metric and Ruppeiner metric are obtained, respectively. The Weinhold curvature gives phase transition points, which correspond to the first-order phase transition only at N=1, where $N$ is a parameter in the plane symmetry black hole. While the Ruppeiner one shows first-order phase transition points for arbitrary $N\neq 1$. Both of which give no any information about the second-order phase transition. Considering the Legendre invariant proposed by Quevedo et. al., we obtain a unified geometry metric, which gives a correctly the behavior of the thermodynamic interactions and phase transitions. The geometry is also found to be curved and the scalar curvature goes to negative infinity at the Davies' phase transition points when the logarithmic correction is included.
Shao-Wen Wei, Yu-Xiao Liu, Chun-E Fu, Hai-Tao Li Journal reference: Adv.High Energy Phys. 2013 (2013) 734138 [pdf] DOI: 10.1155/2013/734138
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Engineering of nonclassical motional states in optomechanical systems -
Abstract
- 2012
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Gravity localization and mass hierarchy in scalar-tensor branes -
Abstract
- We consider a braneworld model in the scalar-tensor gravity. In order to solve the gauge hierarchy problem in this model, our world should be confined on the positive tension brane rather than on the negative one. This is crucial to reproduce a correct Friedmann-like equation on the brane. Interestingly, it is found that the spacing of mass spectrum in this scenario is very tiny, but the light gravitons cannot be observed individually in colliders because of their sufficiently weak interaction with matter fields on the visible brane.
Ke Yang, Yu-Xiao Liu, Yuan Zhong, Xiao-Long Du, Shao-Wen Wei Journal reference: Phys. Rev. D 86, 127502 (2012) [pdf] DOI: 10.1103/PhysRevD.86.127502
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Spectral analysis and identification of noises in quantum systems -
Abstract
- In quantum information processing, knowledge of the noise in the system is crucial for high-precision manipulation and tomography of coherent quantum operations. Existing strategies for identifying this noise require the use of additional quantum devices or control pulses. We present a noise-identification method directly based on the system's non-Markovian response of an ensemble measurement to the noise. The noise spectrum is identified by reversing the response relationship in the frequency domain. For illustration, the method is applied to superconducting charge qubits, but it is equally applicable to any type of qubits. We find that the identification strategy recovers the well-known Fermi's golden rule under the lowest-order perturbation approximation, which corresponds to the Markovian limit when the measurement time is much longer than the noise correlation time. Beyond such approximation, it is possible to further improve the precision at the so-called optimal point by incorporating the transient response data in the non-Markovian regime. This method is verified with experimental data from coherent oscillations in a superconducting charge qubit.
Re-Bing Wu, Tie-Fu Li, A. G. Kofman, Jing Zhang, Yu-xi Liu, Yu. A. Pashkin, Jaw-Shen Tsai, Franco Nori [pdf] DOI: 10.1103/PhysRevA.87.022324 1211.5186v2 [pdf]
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Strong field limit analysis of gravitational lensing in Kerr-Taub-NUT spacetime -
Abstract
- In this paper, we investigate the strong gravitational lensing by the stationary, axially-symmetric black hole in Kerr-Taub-NUT spacetime in the strong field limit. The deflection angle of light ray and other strong deflection limit coefficients are obtained numerically and they are found to be closely dependent on the NUT charge $n$ and spin $a$. The magnification and the positions of the relativistic images are computed. The caustics are studied and the results show that these caustics drift away from the optical axis, which is quite different from the Schwarzschild black hole case. Moreover, the intersections of the critical curves on the equatorial plane are obtained and it is shown that they increase with the NUT charge. These results show that there is a significant effect of the NUT charge on the strong gravitational lensing.
Shao-Wen Wei, Yu-Xiao Liu, Chun-E Fu, Ke Yang Journal reference: JCAP 1210 (2012) 053 [pdf] DOI: 10.1088/1475-7516/2012/10/053
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Emergence of cosmic space and the generalized holographic equipartition -
Abstract
- Recently, a novel idea about our expanding Universe was proposed by T. Padmanabhan [arXiv:1206.4916]. He suggested that the expansion of our Universe can be thought of as the emergence of space as cosmic time progresses. The emergence is governed by the basic relation that the increase rate of Hubble volume is linearly determined by the difference between the number of degrees of freedom on the horizon surface and the one in the bulk. In this paper, following this idea, we generalize the basic relation to derive the Friedmann equations of an $(n+1)$-dimensional Friedmann-Robertson-Walker universe corresponding to general relativity, Gauss-Bonnet gravity, and Lovelock gravity.
Ke Yang, Yu-Xiao Liu, Yong-Qiang Wang Journal reference: Phys. Rev. D 86, 104013 (2012) [pdf] DOI: 10.1103/PhysRevD.86.104013
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Thermodynamic geometry of black hole in the deformed Hořava-Lifshitz gravity -
Abstract
- We investigate the thermodynamic geometry and phase transition of Kehagias-Sfetsos black hole in the deformed Horava-Lifshitz gravity with coupling constant $\lambda=1$. The phase transition in black hole thermodynamics is thought to be associated with the divergence of the capacities. And the structures of these divergent points are studied. We also find that the thermodynamic curvature produced by the Ruppeiner metric is positive definite for all $r_+ > r_-$ and is divergence at $\eta_2=0$ corresponded to the divergent points of $C_{\Phi}$ and $C_T$. These results suggest that the microstructure of the black hole has an effective repulsive interaction, which is very similar to the ideal gas of fermions. These may shine some light on the microstructure of the black hole.
Shao-Wen Wei, Yu-Xiao Liu, Yong-Qiang Wang, Heng Guo Journal reference: Europhys.Lett. 99 (2012) 20004 [pdf] DOI: 10.1209/0295-5075/99/20004
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Localization of 5D Elko spinors on Minkowski branes -
Abstract
- Recently, a new spin-1/2 fermionic quantum field with mass dimension one in four dimensions--Elko field $\lambda$ was introduced as a candidate of dark matter. In this paper, we investigate the localization of 5D Elko {spinors} on Minkowski branes by presenting the equation of {the} Elko KK modes. For the 5D free massless Elko field, the zero mode can be localized on Randall-Sundrum thin brane but can not be localized on the majority of thick branes. There {do} not exist bound massive KK modes on all these branes. If the 5D mass term is introduced, there will exist bound Elko zero mode in Randall-Sundrum brane model. And when we introduce the Yukawa type coupling $\eta \phi^2 \mathop \lambda\limits^\neg\lambda$ with $\phi$ the background scalar field, the Elko zero mode can be localized on some special thick branes with a particular coupling constant $\eta$. Nevertheless, the massive KK modes still can not be localized on these branes. These results are very different from that of the conventional Dirac spinor {field} and the scalar field.
Yu-Xiao Liu, Xiang-Nan Zhou, Ke Yang, Feng-Wei Chen Journal reference: Phys. Rev. D 86 (2012) 064012 [pdf] DOI: 10.1103/PhysRevD.86.064012
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q-form fields on p-branes -
Abstract
- In this paper, we give one general method for localizing any form (q-form) field on p-branes with one extra dimension, and apply it to some typical p-brane models. It is found that, for the thin and thick Minkowski branes with an infinite extra dimension, the zero mode of the q-form fields with q<(p-1)/2 can be localized on the branes. For the thick Minkowski p-branes with one finite extra dimension, the localizable q-form fields are those with q<p/2, and="and" there="there" are="are" also="also" some="some" massive="massive" bound="bound" kaluza-klein="Kaluza-Klein" modes="modes" for="for" these="these" q-form="q-form" fields="fields" on="on" the="the" branes.="branes." for="for" the="the" same="same" q-form="q-form" field,="field," the="the" number="number" of="of" the="the" bound="bound" kaluza-klein="Kaluza-Klein" modes="modes" (but="(but" except="except" the="the" scalar="scalar" field="field" (q="0))" increases="increases" with="with" the="the" dimension="dimension" of="of" the="the" p-branes.="p-branes." moreover,="Moreover," on="on" the="the" same="same" p-brane,="p-brane," the="the" q-form="q-form" fields="fields" with="with" higher="higher" q="q" have="have" less="less" number="number" of="of" massive="massive" bound="bound" kk="KK" modes.="modes." while="While" for="for" a="a" family="family" of="of" pure="pure" geometrical="geometrical" thick="thick" p-branes="p-branes" with="with" a="a" compact="compact" extra="extra" dimension,="dimension," the="the" q-form="q-form" fields="fields" with="with" q<p/2="q<p/2" always="always" have="have" a="a" localized="localized" zero="zero" mode.="mode." for="for" a="a" special="special" pure="pure" geometrical="geometrical" thick="thick" p-brane,="p-brane," there="there" also="also" exist="exist" some="some" massive="massive" bound="bound" kk="KK" modes="modes" of="of" the="the" q-form="q-form" fields="fields" with="with" q<p/2,="q<p/2," whose="whose" number="number" increases="increases" with="with" the="the" dimension="dimension" of="of" the="the" p-brane.
Chun-E Fu, Yu-Xiao Liu, Ke Yang, Shao-Wen Wei Journal reference: JHEP 10 (2012) 060 [pdf] DOI: 10.1007/JHEP10(2012)060
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BLACK HOLES AND QUASIBLACK HOLES IN EINSTEIN-MAXWELL THEORY -
Abstract
- Continuous sequences of asymptotically flat solutions to the Einstein-Maxwell equations describing regular equilibrium configurations of ordinary matter can reach a black hole limit. For a distant observer, the spacetime becomes more and more indistinguishable from the metric of an extreme Kerr-Newman black hole outside the horizon when approaching the limit. From an internal perspective, a still regular but non-asymptotically flat spacetime with the extreme Kerr-Newman near-horizon geometry at spatial infinity forms at the limit. Interesting special cases are sequences of Papapetrou-Majumdar distributions of electrically counterpoised dust leading to extreme Reissner-Nordstrom black holes and sequences of rotating uncharged fluid bodies leading to extreme Kerr black holes.
Reinhard Meinel, Martin Breithaupt, Yu-Chun Liu Journal reference: Proceedings of the Thirteenth Marcel Grossmann Meeting on General Relativity, Singapore: World Scientific, 2015, p.1186 [pdf] DOI: 10.1142/9789814623995_0117
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Turbulent Flame Speeds of G-equation Models in Unsteady Cellular Flows -
Abstract
- We perform a computationl study of front speeds of G-equation models in time dependent cellular flows. The G-equations arise in premixed turbulent combustion, and are Hamilton-Jacobi type level set partial differential equations (PDEs). The curvature-strain G equations are also non-convex with degenerate diffusion. The computation is based on monotone finite difference discretization and weighted essentially nonoscillatory (WENO) methods. We found that the large time front speeds lock into the frequency of time periodic cellular flows in curvature-strain G-equations similar to what occurs in the basic inviscid G-equation. However, such frequency locking phenomenon disappears in viscous G-equation, and in the inviscid G-equation if time periodic oscillation of the cellular flow is replaced by time stochastic oscillation.
Yu-Yu Liu, Jack Xin, Yifeng Yu [pdf]
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Holographic SIS Josephson junction -
Abstract
- We construct a holographic model for the superconductor-insulator-superconductor (SIS) Josephson junction at zero temperature by considering a complex scalar field coupled with a Maxwell field in the four-dimensional anti-de Sitter soliton background. From the gravity side we reproduce the sine relation between the Josephson current and the phase difference across the junction. We also study the dependence of the maximal current on the dimension of the condensate operator and on the width of the junction, and obtain expected results.
Yong-Qiang Wang, Yu-Xiao Liu, Rong-Gen Cai, Shingo Takeuchi, Hai-Qing Zhang Journal reference: JHEP 09(2012)058 [pdf] DOI: 10.1007/JHEP09(2012)058
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Existence and stability of multiple solutions to the gap equation -
Abstract
- We argue by way of examples that, as a nonlinear integral equation, the gap equation can and does possess many physically distinct solutions for the dressed-quark propagator. The examples are drawn from a class that is successful in describing a broad range of hadron physics observables. We apply the homotopy continuation method to each of our four exemplars and thereby find all solutions that exist within the interesting domains of light current-quark masses and interaction strengths; and simultaneously provide an explanation of the nature and number of the solutions, many of which may be associated with dynamical chiral symmetry breaking. Introducing a stability criterion based on the scalar and pseudoscalar susceptibilities we demonstrate, however, that for any nonzero current-quark mass only the regular Nambu solution of the gap equation is stable against perturbations. This guarantees that the existence of multiple solutions to the gap equation cannot complicate the description of phenomena in hadron physics.
Kun-lun Wang, Si-xue Qin, Yu-xin Liu, Lei Chang, Craig D. Roberts, Sebastian M. Schmidt Journal reference: Phys. Rev. D 86, 114001 (2012) [pdf] DOI: 10.1103/PhysRevD.86.114001
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ON A CORONAL BLOWOUT JET: THE FIRST OBSERVATION OF A SIMULTANEOUSLY PRODUCED BUBBLE-LIKE CME AND A JET-LIKE CME IN A SOLAR EVENT -
Abstract
- The coronal blowout jet is a peculiar category among various jet phenomena, of which the sheared base arch, often carrying a small filament, experiences a miniature version of blowout eruption that produces large-scale coronal mass ejection (CME). In this paper, we report such a coronal blowout jet with high-resolution multi-wavelength and multi-angle observations taken from Solar Dynamics Observatory, Solar Terrestrial Relations Observatory, and Big Bear Solar Observatory. For the first time, we find that a simultaneous bubble-like and a jet-like CMEs were dynamically related to the blowout jet that showed cool and hot components next to each other. Our observational results indicate that: (1) the cool component was resulted from the eruption of the filament contained within the jet's base arch, and it further caused the bubble-like CME; (2) the jet-like CME was associated with the hot component, which was the outward moving heated plasmas generated by the reconnection of the base arch and its ambient open field lines. On the other hand, bifurcation of the jet's cool component was also observed, which was resulted from the uncoupling of the erupting filament's two legs that were highly twisted at the very beginning. Based on these results, we propose a model to interpret the coronal blowout jet, of which the external reconnection not only produces the jet-like CME but also leads to the rising of the filament. Subsequently, internal reconnection starts underneath the rising filament and thereby causes the bubble-like CME.
Yuandeng Shen, Yu Liu, Jiangtao Su, Yuanyong Deng Journal reference: 2012ApJ...745..164S [pdf] DOI: 10.1088/0004-637X/745/2/164
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SYMPATHETIC PARTIAL AND FULL FILAMENT ERUPTIONS OBSERVED IN ONE SOLAR BREAKOUT EVENT -
Abstract
- We report two sympathetic solar eruptions, including a partial and a full flux rope eruption in a quadrupolar magnetic region, where a large and a small filament resided above the middle and the east neutral lines respectively. The large filament first rose slowly at a speed of 8 km/s for 23 minutes and then it was accelerated to 102 km/s. Finally, this filament erupted successfully and caused a coronal mass ejection. During the slow rising phase, various evidence for breakout-like external reconnection has been identified at high and low temperature lines. The eruption of the small filament started around the end of the large filament's slow rising. This filament erupted partially and no associating coronal mass ejection could be detected. Based on a potential field extrapolation, we find that the topology of the three-dimensional coronal field above the source region is composed of three low-lying lobes and a large overlying flux system, and a null point located between the middle lobe and the overlying antiparallel flux system. We propose a possible mechanism within the framework of the magnetic breakout model to interpret the sympathetic filament eruptions, in which the magnetic implosion mechanism is thought to be a possible linkage between the sympathetic eruptions, and the external reconnection at the null point transfers field lines from the middle lobe to the lateral lobes and thereby leads to the full (partial) eruption of the observed large (small) filament. Other possible mechanisms are also discussed briefly. We conclude that the structural properties of coronal fields are important for producing sympathetic eruptions.
Yuandeng Shen, Yu Liu, Jiangtao Su Journal reference: 2012ApJ...750...12S [pdf] DOI: 10.1088/0004-637X/750/1/12
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OBSERVATIONAL STUDY OF THE QUASI-PERIODIC FAST-PROPAGATING MAGNETOSONIC WAVES AND THE ASSOCIATED FLARE ON 2011 MAY 30 -
Abstract
- On 2011 May 30, quasi-periodic fast propagating (QFP) magnetosonic waves accompanied by a C2.8 flare were directly imaged by the Atomospheric Imaging Assembly instrument on board the Solar Dynamics Observatory. The QFP waves successively emanated from the flare kernel, they propagated along a cluster of open coronal loops with a phase speed of 834 km/s during the flare's rising phase, and the multiple arc-shaped wave trains can be fitted with a series of concentric circles. We generate the k-omega diagram of the Fourier power and find a straight ridge that represents the dispersion relation of the waves. Along the ridge, we find a lot of prominent nodes which represent the available frequencies of the QFP waves. On the other hand, the frequencies of the flare are also obtained by analyzing the flare light curves using the wavelet technique. The results indicate that almost all the main frequencies of the flare are consistent with those of the QFP waves. This suggests that the flare and the QFP waves were possibly excited by a common physical origin. On the other hand, a few low frequencies revealed by the k-omega diagram can not be found in the accompanying flare. We propose that these low frequencies were possibly due to the leakage of the pressure-driven p-mode oscillations from the photosphere into the low corona, which should be a noticeable mechanism for driving the QFP waves observed in the corona.
Yuandeng Shen, Yu Liu Journal reference: 2012ApJ...753...53S [pdf] DOI: 10.1088/0004-637X/753/1/53
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SIMULTANEOUS OBSERVATIONS OF A LARGE-SCALE WAVE EVENT IN THE SOLAR ATMOSPHERE: FROM PHOTOSPHERE TO CORONA -
Abstract
- For the first time, we report a large-scale wave that was observed simultaneously in the photosphere, chromosphere, transition region and low corona layers of the solar atmosphere. Using the high temporal and high spatial resolution observations taken by the Solar Magnetic Activity Research Telescope at Hida Observatory and the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamic Observatory, we find that the wave evolved synchronously at different heights of the solar atmosphere, and it propagated at a speed of 605 km/s and showed a significant deceleration (-424 m/s2) in the extreme-ultraviolet (EUV) observations. During the initial stage, the wave speed in the EUV observations was 1000 km/s, similar to those measured from the AIA 1700 {\AA} (967 km/s) and 1600 {\AA} (893 km/s) observations. The wave was reflected by a remote region with open fields, and a slower wave-like feature at a speed of 220 km/s was also identified following the primary fast wave. In addition, a type-II radio burst was observed to be associated with the wave. We conclude that this wave should be a fast magnetosonic shock wave, which was firstly driven by the associated coronal mass ejection and then propagated freely in the corona. As the shock wave propagated, its legs swept the solar surface and thereby resulted in the wave signatures observed in the lower layers of the solar atmosphere. The slower wave-like structure following the primary wave was probably caused by the reconfiguration of the low coronal magnetic fields, as predicted in the field-line stretching model.
Yuandeng Shen, Yu Liu Journal reference: 2012ApJ...752L..23S [pdf] DOI: 10.1088/2041-8205/752/2/L23
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EVIDENCE FOR THE WAVE NATURE OF AN EXTREME ULTRAVIOLET WAVE OBSERVED BY THE ATMOSPHERIC IMAGING ASSEMBLY ON BOARD THE
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Abstract
- Extreme Ultraviolet (EUV) waves have been found for about 15 years. However, significant controversy remains over their physical natures and origins. In this paper, we report an EUV wave that was accompanied by an X1.9 flare and a partial halo coronal mass ejection. Using high temporal and spatial resolution observations taken by the {\em Solar Dynamics Observatory} and the Solar-TErrestrial RElations Observatory, we are able to investigate the detailed kinematics of the EUV wave. We find several arguments that support the fast-mode wave scenario: (1) The speed of the EUV wave (570 km/s) is higher than the sound speed of quiet-Sun corona. (2) Significant deceleration of the EUV wave (-130 m/s2) is found during its propagation. (3) The EUV wave resulted in the oscillations of a loop and a filament along its propagation path, and a reflected wave from the polar coronal hole is also detected. (4) Refraction or reflection effect is observed when the EUV wave was passing through two coronal bright points. (5) The dimming region behind the wavefront stopped to expand when the wavefront started to become diffuse. (6) The profiles of the wavefront exhibited a dispersive nature, and the magnetosonic Mach number of the EUV wave derived from the highest intensity jump is about 1.4. In addition, triangulation indicates that the EUV wave propagated within a height range of about 60-100 Mm above the photosphere. We propose that the EUV wave observed should be a nonlinear fast-mode magnetosonic wave that propagated freely in the corona after it was driven by the CME expanding flanks during the initial period.
Yuandeng Shen, Yu Liu Journal reference: 2012ApJ...754....7S [pdf] DOI: 10.1088/0004-637X/754/1/7
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Strong coupling of a spin qubit to a superconducting stripline cavity -
Abstract
- We study electron-spin-photon coupling in a single-spin double quantum dot embedded in a superconducting stripline cavity. With an external magnetic field, we show that either a spin-orbit interaction (for InAs) or an inhomogeneous magnetic field (for Si and GaAs) could produce a strong spin-photon coupling, with a coupling strength of the order of 1 MHz. With an isotopically purified Si double dot, which has a very long spin coherence time for the electron, it is possible to reach the strong-coupling limit between the spin and the cavity photon, as in cavity quantum electrodynamics. The coupling strength and relaxation rates are calculated based on parameters of existing devices, making this proposal experimentally feasible.
Xuedong Hu, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. B 86, 035314 (2012) [pdf] DOI: 10.1103/PhysRevB.86.035314
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Motion of charged particles and quasinormal modes around the magnetically and tidally deformed black hole -
Abstract
- Here we consider two phenomena in the vicinity of a black hole deformed by the tidal gravitational force of surrounding matter and by a strong magnetic field: equatorial motion of charged particles and the decay of a test scalar field. We were able to analyze both phenomena with analytical and simple numerical tools, which was unexpected given the low symmetry of the system. We show that both the tidal gravitational force and the magnetic field strongly enhance the release of the binding energy for the matter spiralling into the black hole. In the presence of the magnetic field, the left and right handed rotations of charged particles are not equivalent and for sufficiently large $|q| B$ there are stable anti-Larmor orbits very close to the event horizon, although Larmor orbits are only stable at some distance from the black hole. The larger the tidal force, the closer the innermost stable orbit to the black hole for both types of rotation. It was also shown that the real oscillation frequencies of the characteristic quasinormal modes are considerably suppressed by the tidal force.
R. A. Konoplya, Yu-Chun Liu Journal reference: Phys.Rev. D86 (2012) 084007 [pdf] DOI: 10.1103/PhysRevD.86.084007
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Non-minimal coupling branes -
Abstract
- We study a thick and Z_2 symmetric braneworld model with a non-minimally coupled bulk scalar. Several analytic solutions are found. There are two categories of the brane configuration: standard branes and deformed ones. The former is just the same with the solutions in general relativity, whereas the latter has negative effective energy densities in the location of the branes. The question of stability under metric tensor perturbation is also investigated, and there is no tachyon. We show that the gravity zero mode (namely the 4D massless graviton) is the only one state that can be localized on the branes. So Newtonian potential can be recovered in 4D space-time.
Yu-Xiao Liu, Feng-Wei Chen, Heng-Guo, Xiang-Nan Zhou Journal reference: JHEP 1205 (2012) 108 [pdf] DOI: 10.1007/JHEP05(2012)108
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Abstract
- The possible heavy fermion superconductivity is carefully reexamined in the two-dimensional Kondo lattice model with an antiferromagnetic Heisenberg superexchange between local magnetic moments. In order to establish an effective mean field theory in the limit of the paramagnetic heavy Fermi liquid and near the half-filling case, we find that the spinon singlet pairing from the local antiferromagnetic short-range correlations can reduce the ground state energy substantially. In the presence of the Kondo screening effect, the Cooper pairs between the conduction electrons is induced. Depending on the ratio of the Heisenberg and the Kondo exchange couplings, the resulting superconducting state is characterized by either a d-wave nodal or d-wave nodeless state, and a continuous phase transition exists between these two states. These results are related to some quasi-two dimensional heary fermion superconductors.
Yu Liu, Huan Li, Guang-Ming Zhang, L. Yu Journal reference: Physical Review B 86, 024526 (2012) [pdf] DOI: 10.1103/PhysRevB.86.024526
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Quantum Coherent Nonlinear Feedback With Applications to Quantum Optics on Chip -
Abstract
- In the control of classical mechanical systems, the feedback has been successfully applied to the production of the desired nonlinear dynamics. However, how much this can be done is still an open problem in quantum mechanical systems. This paper proposes a scheme of generating strong nonlinear quantum effects via the recently developed coherent feedback techniques, which can be shown to outperform the measurement-based quantum feedback scheme that can only generate pseudo-nonlinear quantum effects. Such advancement is demonstrated by two application examples in quantum optics on chip. In the first example, we show that the nonlinear Kerr effect can be generated and amplified to be comparable with the linear effect in a transmission line resonator (TLR). In the second example, we show that by tuning the gains of the quantum amplifiers in a TLR coherent feedback network, non-Gaussian "light" (microwave field) can be generated and manipulated via the nonlinear effects which exhibits fully quantum sub-Poisson photoncount statistics and photon antibunching phenomenon. The scheme opens promising applications in demonstrating strong nonlinear quantum optics on chip, which is extremely weak and inflexible in traditional quantum optical devices.
Jing Zhang, Re-Bing Wu, Yu-xi Liu, Chun-Wen Li, Tzyh-Jong Tarn Journal reference: IEEE Trans. Automat. Contr. 57(8), 1997 (2012) [pdf] DOI: 10.1109/TAC.2012.2195871
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Thick branes with a nonminimally coupled bulk-scalar field -
Abstract
- In this paper, we investigate thick branes with a nonminimally coupled background scalar field, whose solution is a single-kink or a double-kink. The effects of the nonminimal coupling constant $\xi$ on the structure of the thick branes and the localization of gravity, fermions, scalars and vectors are discussed. It is shown that each brane will split into two sub-branes as increasing the nonminimal coupling constant $\xi$. By investigating the tensor perturbation equations of gravity and the general covariant Dirac equation of fermions, we find that both the gravity zero mode and left-chiral fermion zero mode are localized at the center of the single-kink branes and localized between the two sub-branes generated by the double-kink, which indicates that the constant $\xi$ does not effect the localization of these zero modes. However, the zero mode of scalars is localized on each sub-brane (for both single-kink and double-kink branes) when $\xi$ is larger than its critical value $\xi_0$. The effects of the nonminimal coupling constant $\xi$ on the resonances of gravity and fermions with finite lifetime on the branes are also discussed.
Heng Guo, Yu-Xiao Liu, Zhen-Hua Zhao, Feng-Wei Chen Journal reference: Phys. Rev. D 85, 124033 (2012) [pdf] DOI: 10.1103/PhysRevD.85.124033
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Domain wall brane in Eddington-inspired Born-Infeld gravity -
Abstract
- Recently, inspired by Eddington's theory, an alternative gravity called Eddington-inspired Born-Infeld gravity was proposed by Ba$\tilde{\text{n}}$ados and Ferreira. It is equivalent to Einstein's general relativity in vacuum, but deviates from it when matter is included. Interestingly, it seems that the cosmological singularities are prevented in this theory. Based on the new theory, we investigate a thick brane model with a scalar field presenting in the five-dimensional background. A domain wall solution is obtained, and further, we find that at low energy the four-dimensional Einstein gravity is recovered on the brane. Moreover, the stability of gravitational perturbations is ensured in this model.
Yu-Xiao Liu, Ke Yang, Heng Guo, Yuan Zhong Journal reference: Phys. Rev. D 85, 124053 (2012) [pdf] DOI: 10.1103/PhysRevD.85.124053
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Stacks associated to abelian tensor categories -
Abstract
- For an abelian tensor category a stack is constructed. As an application we show that our construction can be used to recover a quasi-compact separated scheme from the category of its quasi-coherent sheaves. In another application, we show how the "dual stack" of the classifying stack $BG$ of a finite group $G$ can be obtained by altering the tensor product on the category $\rep{G}$ of $G$-representations. Using glueing techniques we show that the dual pair of a $G$-gerbe, in the sense of [TT10], can be constructed by glueing local dual stacks.
Yu-Han Liu, Hsian-Hua Tseng [pdf]
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Triangulated quotient categories -
Abstract
- A notion of mutation of subcategories in a right triangulated category is defined in this paper. When (Z,Z) is a D-mutation pair in a right triangulated category C, the quotient category Z/D carries naturally a right triangulated structure. More-over, if the right triangulated category satisfies some reasonable conditions, then the right triangulated quotient category Z/D becomes a triangulated category. When C is triangulated, our result unifies the constructions of the quotient triangulated categories by Iyama-Yoshino and by J{\o}rgensen respectively.
Yu Liu, Bin Zhu [pdf]
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Deformed brane with finite extra dimension -
Abstract
- We construct a deformed brane solution generated by a double-kink scalar field and a dilaton scalar field. In this brane scenario the extra dimension is finite, which is due to the introduction of the dilaton field with special form. The finity of the extra dimension will result in the localization of the zero mode for the vector fields. While the localization of the Kalb-Ramond fields {depends} on the coupling to the dilaton. For the fermion fields, {with different values of the dilaton-fermion coupling constant}, there are three types of the effective potential for the fermion KK modes. Moreover, we investigate the effect of the deformation of the brane on the localization, and find that the number of the resonances will increase with the distances of the two sub-branes.
Yu-Xiao Liu, Chun-E Fu, Heng Guo, Hai-Tao Li Journal reference: Phys.Rev. D 85 (2012) 084023 [pdf] DOI: 10.1103/PhysRevD.85.084023
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Gravity localization and effective Newtonian potential for Bent thick branes -
Abstract
- In this letter, we first investigate the gravity localization and mass spectrum of gravity KK modes on de Sitter and Anti-de Sitter thick branes. Then, the effective Newtonian gravitational potentials for these bent branes are discussed by the two typical examples. The corrections of the Newtonian potential turns out to be $\Delta U(r)\sim 1/r^{2}$ at small $r$ for both cases. These corrections are very different from that of the Randall-Sundrum brane model $\Delta U(r)\sim 1/r^{3}$.
Heng Guo, Yu-Xiao Liu, Shao-Wen Wei, Chun-E Fu Journal reference: Europhys. Letters 97 (2012) 60003 [pdf] DOI: 10.1209/0295-5075/97/60003
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Equatorial and quasiequatorial gravitational lensingby a Kerr black hole pierced by a cosmic string -
Abstract
- In the present paper, we study numerically the equatorial lensing and quasiequatorial lensing by Kerr black hole pierced by a cosmic string in the strong deflection limit. We calculate the strong deflection limit coefficients and the deflection angle, which are found to depend closely on the cosmic string parameter $\beta$ and dimensionless spin $a_{*}$. The magnification and positions of relativistic images are also computed in the strong deflection limit and a two-dimensional lens equation is derived. The most important and outstanding effect is that the caustics drift away from the optical axis and shift in the clockwise direction with respect to the Kerr black hole. For fixed $a_{*}$ of the black hole, the caustics drift farther away from the optical axis for a large value of $\beta$. And for fixed $\beta$, they drift farther for high $a_{*}$. We also obtain the intersections of the critical curves with the equatorial plane, which decrease with $a_{*}$ and $\beta$. In particular, we obtain a quantity $\bar{\mu}_{k+1}/\bar{\mu}_{k}$, which is independent of the black hole spin and mass. Thus, through measuring it, one is allowed to determine the value of $\beta$ from astronomical observations.
Shao-Wen Wei, Yu-Xiao Liu Journal reference: Phys. Rev. D85 (2012) 064044 [pdf] DOI: 10.1103/PhysRevD.85.064044
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A numerical study of turbulent flame speeds of curvature and strain G-equations in cellular flows -
Abstract
- We study front speeds of curvature and strain G-equations arising in turbulent combustion. These G-equations are Hamilton-Jacobi type level set partial differential equations (PDEs) with non-coercive Hamiltonians and degenerate nonlinear second order diffusion. The Hamiltonian of strain G-equation is also non-convex. Numerical computation is performed based on monotone discretization and weighted essentially nonoscillatory (WENO) approximation of transformed G-equations on a fixed periodic domain. The advection field in the computation is a two dimensional Hamiltonian flow consisting of a periodic array of counter-rotating vortices, or cellular flows. Depending on whether the evolution is predominantly in the hyperbolic or parabolic regimes, suitable explicit and semi-implicit time stepping methods are chosen. The turbulent flame speeds are computed as the linear growth rates of large time solutions. A new nonlinear parabolic PDE is proposed for the reinitialization of level set functions to prevent piling up of multiple bundles of level sets on the periodic domain. We found that the turbulent flame speed $s_T$ of the curvature G-equation is enhanced as the intensity $A$ of cellular flows increases, at a rate between those of the inviscid and viscous G-equations. The $s_T$ of the strain G-equation increases in small $A$, decreases in larger $A$, then drops down to zero at a large enough but finite value $A_{*}$. The flame front ceases to propagate at this critical intensity $A_*$, and is quenched by the cellular flow.
Yu-Yu Liu, Jack Xin, Yifeng Yu [pdf] DOI: 10.1016/j.physd.2012.09.008 1202.6152v1 [pdf]
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Fast generation of multiparticle entangled state for flux qubits in a circle array of transmission line resonators with tunable coupling -
Abstract
- We study a one-step approach to the fast generation of Greenberger-Horne-Zeilinger (GHZ) states in a circuit QED system with superconducting flux qubits. The GHZ state can be generated in about 10 ns, which is much shorter than the coherence time of flux qubits and comparable with the time of single-qubit operation. In our proposal, a time-dependent microwave field is applied to a superconducting transmission line resonator (TLR) and displaces the resonator in a controlled manner, thus inducing indirect qubit-qubit coupling without residual entanglement between the qubits and the resonator. The design of a tunably coupled TLR circle array provides us with the potential for extending this one-step scheme to the case of many qubits coupled via several TLRs.
Z. H. Peng, Yu-xi Liu, Y. Nakamura, J. S. Tsai Journal reference: Phys. Rev. B 85, 024537(2012) [pdf] DOI: 10.1103/PhysRevB.85.024537
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Collective Perspective on Advances in Dyson—Schwinger Equation QCD -
Abstract
- We survey contemporary studies of hadrons and strongly interacting quarks using QCD's Dyson-Schwinger equations, addressing: aspects of confinement and dynamical chiral symmetry breaking; the hadron spectrum; hadron elastic and transition form factors, from small- to large-Q^2; parton distribution functions; the physics of hadrons containing one or more heavy quarks; and properties of the quark gluon plasma.
Adnan Bashir, Lei Chang, Ian C. Cloet, Bruno El-Bennich, Yu-xin Liu, Craig D. Roberts, Peter C. Tandy Journal reference: Commun. Theor. Phys. 58 (2012) 79-134 [pdf] DOI: 10.1088/0253-6102/58/1/16
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Excitation spectrum for an inhomogeneously dipole-field-coupled superconducting qubit chain -
Abstract
- When a chain of superconducting qubits couples to a coplanar resonator in a cavity, each of its N qubits (equally-spaced with distance l) experiences a different dipole-field coupling strength due to the waveform of the cavity field. We find that this inhomogeneous coupling leads to a pair of l-dependent ladder operators for the angular momentum of the spin chain. Varying the qubit spacing l changes the transition amplitudes between the angular momentum levels. We derive an exact diagonalization of the general N-qubit Hamiltonian and, through the N=4 case, demonstrate how the l-dependent operators lead to a denser one-excitation spectrum and a probability redistribution of the eigenstates. Moreover, it will be shown that the variation of l between its two limiting values coincides with the crossover between Frenkel- and Wannier-type excitons in the superconducting spin chain.
Hou Ian, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 85, 053833 (2012) [pdf] DOI: 10.1103/PhysRevA.85.053833
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Multiple Lifshitz transitions driven by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model -
Abstract
- With a mean field approach, the heavy Fermi liquid in the two-dimensional Kondo lattice model is carefully considered in the presence of short-range antiferromagnetic correlations. As the ratio of the local Heisenberg superexchange coupling to the Kondo coupling increases, the Fermi surface structure changes dramatically. From the analysis of the ground state energy density, multiple Lifshitz type phase transitions occur at zero temperature.
Yu Liu, Guang-Ming Zhang Journal reference: Journal of Physics: Conference Series 391, 012171 (2012) [pdf] DOI: 10.1088/1742-6596/391/1/012171
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Investigation of rainbow-ladder truncation for excited and exotic mesons -
Abstract
- Ground-state, radially-excited and exotic scalar-, vector- and flavoured-pseudoscalar-mesons are studied in rainbow-ladder truncation using an interaction kernel that is consonant with modern DSE- and lattice-QCD results. The inability of this truncation to provide realistic predictions for the masses of excited- and exotic-states is confirmed and explained. On the other hand, its application does provide information that is potentially useful in proceeding beyond this leading-order truncation, e.g.: assisting with development of projection techniques that ease the computation of excited state properties; placing qualitative constraints on the long-range behaviour of the interaction kernel; and highlighting and illustrating some features of hadron observables that do not depend on details of the dynamics.
Si-xue Qin, Lei Chang, Yu-xin Liu, Craig D. Roberts, David J. Wilson Journal reference: Phys. Rev. C 85, 035202 (2012) [pdf] DOI: 10.1103/PhysRevC.85.035202
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Gravity localization and mass hierarchy in scalar-tensor branes -
Abstract
- 2011
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Particle acceleration in Kerr–(anti-)de Sitter black hole backgrounds -
Abstract
- Recently, Ban\~{a}dos, Silk and West (BSW) found that the center-of-mass energy of two colliding test particles in the neighborhood of an extreme Kerr black hole could be arbitrarily high when one particle has the critical angular momentum $L_\text{C}$. In their paper, they considered the black holes living in a Minkowski space-time with a zero cosmological constant. In this work, we study this process of particles in the backgrounds of the Kerr black holes living in a space-time with a nonzero cosmological constant. We find that for kerr black holes living in a space-time with a negative cosmological constant (Kerr-anti-de Sitter black holes), this process of particles could happen only if the Kerr-anti-de Sitter black hole is extreme and an additional fine tuning is satisfied; while for kerr black holes living in a space-time with a positive cosmological constant (Kerr-de Sitter black holes), this process of particles could happen no matter the Kerr-de Sitter black hole is extreme or not. We also study the particle collision outside the outer horizon of the Kerr-(anti-) de Sitter black hole in some cases. We find that there is a possibility that we could tune one colliding particle's angular momentum to gain the large center-of-mass energy without increase the particle's energy.
Yang Li, Jie Yang, Yun-Liang Li, Shao-Wen Wei, Yu-Xiao Liu Journal reference: Class. Quantum Grav. 28 225006 (2011) [pdf] DOI: 10.1088/0264-9381/28/22/225006
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Dyson-Schwinger equations with a parametrized metric -
Abstract
- We construct and solve the Dyson-Schwinger equation (DSE) of quark propagator with a parameterized metric, which connects the Euclidean metric with the Minkowskian one. We show, in some models, the Minkowskian vacuum is different from the Euclidean vacuum. The usual analytic continuation of Green function does not make sense in these cases. While with the algorithm we proposed and the quark-gluon vertex ansatz which preserves the Ward-Takahashi identity, the vacuum keeps being unchanged in the evolution of the metric. In this case, analytic continuation becomes meaningful and can be fully carried out.
Wei Yuan, Si-xue Qin, Huan Chen, Yu-xin Liu Journal reference: Phys.Rev.D81:114022,2010 [pdf] DOI: 10.1103/PhysRevD.81.114022
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Holographic p-wave Josephson junction -
Abstract
- In this work we generalized holographic model for s-wave DC Josephson junction constructed in arXiv:1101.3326[hep-th] to a holographic description for p-wave Josephson junction. By solving numerically the coupled equations of motion of Yang-Mills theory for a non-Abelian SU(2) gauge fields in (3+1)-dimensional AdS spacetimes, we shown that DC current of the p-wave Josephson junction is proportional to the sine of the phase difference across the junction like the s-wave case.
Yong-Qiang Wang, Yu-Xiao Liu, Zhen-Hua Zhao [pdf]
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Prime spectra of derived quiver representations -
Abstract
- We compute Balmer's prime spectrum for the derived category of quiver representations for a finite ordered quiver and show that it does not recover the quiver. We then associate an algebra to every k-linear triangulated tensor category and show that the path algebra can be recovered this way.
Yu-Han Liu, Susan J. Sierra [pdf]
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Interaction model for the gap equation -
Abstract
- We explain a form for the rainbow-ladder kernel whose momentum-dependence is consonant with modern DSE- and lattice-QCD results, and assess its capability as a tool in hadron physics. In every respect tested, this form produces results for observables that are at least equal to the best otherwise obtained in a comparable approach. Moreover, it enables the natural extraction of a monotonic running-coupling and -gluon-mass.
Si-xue Qin, Lei Chang, Yu-xin Liu, Craig D. Roberts, David J. Wilson Journal reference: Phys.Rev.C84:042202,2011 [pdf] DOI: 10.1103/PhysRevC.84.042202
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A Note on Friedmann Equation of FRW Universe in Deformed Hořava—Lifshitz Gravity from Entropic Force -
Abstract
- With entropic interpretation of gravity proposed by Verlinde, we obtain the Friedmann equation of the Friedmann-Robertson-Walker universe for the deformed Ho\v{r}ava-Lifshitz gravity. It is shown that, when the parameter of Ho\v{r}ava-Lifshitz gravity $\omega\rightarrow \infty$, the modified Friedmann equation will go back to the one in Einstein gravity. This results may imply that the entropic interpretation of gravity is effective for the deformed Ho\v{r}ava-Lifshitz gravity.
Shao-Wen Wei, Yu-Xiao Liu, Yong-Qiang Wang Journal reference: Commun.Theor.Phys.56:455-458,2011 [pdf] DOI: 10.1088/0253-6102/56/3/11
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Relationship between high-energy absorption cross section and strong gravitational lensing for black hole -
Abstract
- In this paper, we obtain a relation between the high-energy absorption cross section and the strong gravitational lensing for a static and spherically symmetric black hole. It provides us a possible way to measure the high-energy absorption cross section for a black hole from strong gravitational lensing through astronomical observation. More importantly, it allows us to compute the total energy emission rate for high-energy particles emitted from the black hole acting as a gravitational lens. It could tell us the range of the frequency, among which the black hole emits the most of its energy and the gravitational waves are most likely to be observed. We also apply it to the Janis-Newman-Winicour solution. The results suggest that we can test the cosmic censorship hypothesis through the observation of gravitational lensing by the weakly naked singularities acting as gravitational lenses.
Shao-Wen Wei, Yu-Xiao Liu, Heng Guo Journal reference: Phys. Rev. D 84, 041501(R) (2011) [pdf] DOI: 10.1103/PhysRevD.84.041501
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Localization of gravity and bulk matters on a thick anti–de Sitter brane -
Abstract
- In this paper, we investigate the localization and the mass spectra of gravity and various bulk matter fields on a thick anti-de Sitter (AdS) brane, by presenting the mass-independent potentials of the Kaluza-Klein (KK) modes in the corresponding Schr\"{o}dinger equations. For gravity, the potential of the KK modes tends to infinity at the boundaries of the extra dimension, which leads to an infinite number of the bound KK modes. Although the gravity zero mode cannot be localized on the AdS brane, the massive modes are trapped on the brane. The scalar perturbations of the thick AdS brane have been analyzed, and the brane is stable under the scalar perturbations. For spin-0 scalar fields and spin-1 vector fields, the potentials of the KK modes also tend to infinity at the boundaries of the extra dimension, and the characteristic of the localization is the same as the case of gravity. For spin-1/2 fermions, by introducing the usual Yukawa coupling $\eta\bar{\Psi}\phi\Psi$ with the positive coupling constant $\eta$, the four-dimensional massless left-chiral fermion and massive Dirac fermions are obtained on the AdS thick brane.
Yu-Xiao Liu, Heng Guo, Chun-E Fu, Hai-Tao Li Journal reference: Phys.Rev.D84:044033,2011 [pdf] DOI: 10.1103/PhysRevD.84.044033
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Derived equivalence and birationality -
Abstract
- A different proof to a known criterion of derived equivalence implying birationality is given. Derived equivalent smooth projective curves over an algebraically closed field are proved to be isomorphic. A different proof of derived equivalence implying birationality for varieties of general type (originally due to Kawamata) is given.
Yu-Han Liu [pdf]
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General stationary charged black holes as charged particle accelerators -
Abstract
- We study the possibility of getting infinite energy in the center of mass frame of colliding charged particles in a general stationary charged black hole. For black holes with two-fold degenerate horizon, it is found that arbitrary high center-of-mass energy can be attained, provided that one of the particle has critical angular momentum or critical charge, and the remained parameters of particles and black holes satisfy certain restriction. For black holes with multiple-fold degenerate event horizons, the restriction is released. For non-degenerate black holes, the ultra-high center-of-mass is possible to be reached by invoking the multiple scattering mechanism. We obtain a condition for the existence of innermost stable circular orbit with critical angular momentum or charge on any-fold degenerate horizons, which is essential to get ultra-high center-of-mass energy without fine-tuning problem. We also discuss the proper time spending by the particle to reach the horizon and the duality between frame dragging effect and electromagnetic interaction. Some of these general results are applied to braneworld small black holes.
Yi Zhu, Shao-Feng Wu, Yu-Xiao Liu, Ying Jiang Journal reference: Phys.Rev.D84:043006,2011 [pdf] DOI: 10.1103/PhysRevD.84.043006
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Bulk matter fields on two-field thick branes -
Abstract
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In this paper we obtain a new solution of a brane made up of a scalar field
coupled to a dilaton. There is a unique parameter $b$ in the solution, which
decides the distribution of the energy density and will effect the localization
of bulk matter fields. For free vector fields, we find that the zero mode can
be localized on the brane. And for vector fields coupled with the dilaton via
$\text{e}^{\tau\pi}F_{MN}F^{MN}$, the condition for localizing the zero mode is
$\tau\geq-\sqrt{b/3}$ with $0
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In this paper we obtain a new solution of a brane made up of a scalar field
coupled to a dilaton. There is a unique parameter $b$ in the solution, which
decides the distribution of the energy density and will effect the localization
of bulk matter fields. For free vector fields, we find that the zero mode can
be localized on the brane. And for vector fields coupled with the dilaton via
$\text{e}^{\tau\pi}F_{MN}F^{MN}$, the condition for localizing the zero mode is
$\tau\geq-\sqrt{b/3}$ with $0
Chun-E Fu, Yu-Xiao Liu, Heng Guo Journal reference: Phys.Rev.D84:044036,2011 [pdf] DOI: 10.1103/PhysRevD.84.044036
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Functors on triangulated tensor categories -
Abstract
- We define and study the functorial spectrum for every triangulated tensor category. A reconstruction result for topologically noetherian schemes similar to (and based on) a theorem by Balmer is proved. An alternative proof of the reconstruction theorem by Bondal-Orlov for smooth projective varieties with ample (anti-)canonical bundles is given.
Yu-Han Liu [pdf]
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Domain wall brane in squared curvature gravity -
Abstract
- We suggest a thick braneworld model in the squared curvature gravity theory. Despite the appearance of higher order derivatives, the localization of gravity and various bulk matter fields is shown to be possible. The existence of the normalizable gravitational zero mode indicates that our four-dimensional gravity is reproduced. In order to localize the chiral fermions on the brane, two types of coupling between the fermions and the brane forming scalar is introduced. The first coupling leads us to a Schr\"odinger equation with a volcano potential, and the other a P\"oschl-Teller potential. In both cases, the zero mode exists only for the left-hand fermions. Several massive KK states of the fermions can be trapped on the brane, either as resonant states or as bound states.
Yu-Xiao Liu, Yuan Zhong, Zhen-Hua Zhao, Hai-Tao Li Journal reference: JHEP 1106:135,2011 [pdf] DOI: 10.1007/JHEP06(2011)135
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Hardy spaces associated with Schrodinger operators on the Heisenberg
group -
Abstract
- Let $L= -\Delta_{\mathbb{H}^n}+V$ be a Schr\"odinger operator on the Heisenberg group $\mathbb{H}^n$, where $\Delta_{\mathbb{H}^n}$ is the sub-Laplacian and the nonnegative potential $V$ belongs to the reverse H\"older class $B_{\frac{Q}{2}}$ and $Q$ is the homogeneous dimension of $\mathbb{H}^n$. The Riesz transforms associated with the Schr\"odinger operator $L$ are bounded from $L^1(\mathbb{H}^n)$ to $L^{1,\infty}(\mathbb{H}^n)$. The $L^1$ integrability of the Riesz transforms associated with $L$ characterizes a certain Hardy type space denoted by $H^1_L(\mathbb{H}^n)$ which is larger than the usual Hardy space $H^1(\mathbb{H}^n)$. We define $H^1_L(\mathbb{H}^n)$ in terms of the maximal function with respect to the semigroup $\big \{e^{-s L}:\; s>0 \big\}$, and give the atomic decomposition of $H^1_L(\mathbb{H}^n)$. As an application of the atomic decomposition theorem, we prove that $H^1_L(\mathbb{H}^n)$ can be characterized by the Riesz transforms associated with $L$. All results hold for stratified groups as well.
Chin-Cheng Lin, Heping Liu, Yu Liu [pdf]
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Effects of temperature on thick branes and the fermion (quasi-)localization -
Abstract
- Following Campos's work [Phys. Rev. Lett. 88, 141602 (2002)], we investigate the effects of temperature on flat, de Sitter (dS), and anti-de Following Campos's work [Phys. Rev. Lett. \textbf{88}, 141602 (2002)], we investigate the effects of temperature on flat, de Sitter (dS), and anti-de Sitter (AdS) thick branes in five-dimensional (5D) warped spacetime, and on the fermion (quasi-)localization. First, in the case of flat brane, when the critical temperature reaches, the solution of the background scalar field and the warp factor is not unique. So the thickness of the flat thick brane is uncertain at the critical value of the temperature parameter, which is found to be lower than the one in flat 5D spacetime. The mass spectra of the fermion Kaluza-Klein (KK) modes are continuous, and there is a series of fermion resonances. The number and lifetime of the resonances are finite and increase with the temperature parameter, but the mass of the resonances decreases with the temperature parameter. Second, in the case of dS brane, we do not find such a critical value of the temperature parameter. The mass spectra of the fermion KK modes are also continuous, and there is a series of fermion resonances. The effects of temperature on resonance number, lifetime, and mass are the same with the case of flat brane. Last, in the case of AdS brane, {the critical value of the temperature parameter can less or greater than the one in the flat 5D spacetime.} The spectra of fermion KK modes are discrete, and the mass of fermion KK modes does not decrease monotonically with increasing temperature parameter.
Zhen-Hua Zhao, Yu-Xiao Liu, Yong-Qiang Wang, Hai-Tao Li Journal reference: JHEP 1106 (2011) 045 [pdf] DOI: 10.1007/JHEP06(2011)045
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KINEMATICS AND FINE STRUCTURE OF AN UNWINDING POLAR JET OBSERVED BY THE
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Abstract
- We present an observational study of the kinematics and fine structure of an unwinding polar jet, with high temporal and spatial observations taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) and the Solar Magnetic Activity Research Telescope (SMART). During the rising period, the shape of the jet resembled a cylinder with helical structures on the surface, while the mass of the jet was mainly distributed on the cylinder's shell. In the radial direction, the jet expanded successively at its western side. The radial expansion presented three distinct phases: the gradually expanding phase, the fast expanding phase, and the steady phase. Each phase lasted for about 12 minutes. The angular speed of the unwinding jet and the twist transferred into the outer corona during the eruption are estimated to be 11.1 \times 10{-3} rad/s (period = 564 s) and 1.17 to 2.55 turns (or 2.34 to 5.1{\pi}) respectively. On the other hand, by calculating the azimuthal component of the magnetic field in the jet and comparing the free energy stored in the non-potential magnetic field with the jet's total energy, we find that the non-potential magnetic field in the jet is enough to supply the energy for the ejection. These new observational results strongly support the scenario that the jets are driven by the magnetic twist, which is stored in the twisted closed field of a bipole, and released through magnetic reconnection between the bipole and its ambient open field.
Yuandeng Shen, Yu Liu, Jiangtao Su, Ahmed Ibrahim Journal reference: 2011 ApJ 735 L43 [pdf] DOI: 10.1088/2041-8205/735/2/L43
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Large-deviation analysis for counting statistics in mesoscopic transport -
Abstract
- We present an efficient approach, based on a number-conditioned master equation, for large-deviation analysis in mesoscopic transports. Beyond the conventional full-counting-statistics study, the large-deviation approach encodes complete information of both the typical trajectories and the rare ones, in terms of revealing a continuous change of the dynamical phase in trajectory space. The approach is illustrated with two examples: (i) transport through a single quantum dot, where we reveal the inhomogeneous distribution of trajectories in general case and find a particular scale invariance point in trajectory statistics; and (ii) transport through a double dots, where we find a dynamical phase transition between two distinct phases induced by the Coulomb correlation and quantum interference.
Jun Li, Yu Liu, Jing Ping, Shu-Shen Li, Xin-Qi Li, YiJing Yan Journal reference: Phys. Rev. B 84, 115319 (2011) [pdf] DOI: 10.1103/PhysRevB.84.115319
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A time series of filament eruptions observed by three eyes from space: from failed to successful eruptions -
Abstract
- We present stereoscopic observations of six sequent eruptions of a filament in the active region NOAA 11045 on 2010 Feb 8, with the advantage of the STEREO twin viewpoints in combination with the earth viewpoint from SOHO instruments and ground-based telescopes. The last one of the six eruptions is with a coronal mass ejection, while the others are not. The flare in this successful one is more intensive than in the others. Moreover, the filament material velocity of the successful one is also the largest among them. Interestingly, all the filament velocities are found proportional to their flare powers. We calculate magnetic field intensity at low altitude, the decay indexes of the external field above the filament, and the asymmetry properties of the overlying fields before and after the failed eruptions and find little difference between them, indicating the same coronal confinement for the failed and the successful eruptions. The results suggest that, besides the confinement of coronal magnetic field, the energy released in low corona should be another crucial element for production of a failed or successful filament eruption. That is, only a coronal mass ejection can be launched away if the energy released exceeds some critical value, given the same coronal conditions.
Yuandeng Shen, Yu Liu, Rui Liu Journal reference: 2011, RAA, 11, 594-606 [pdf] DOI: 10.1088/1674-4527/11/5/009
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Sudden vanishing and reappearance of nonclassical effects: General occurrence of finite-time decays and periodic vanishings of nonclassicality and entanglement witnesses -
Abstract
- Analyses of phenomena exhibiting finite-time decay of quantum entanglement have recently attracted considerable attention. Such decay is often referred to as sudden vanishing (or sudden death) of entanglement, which can be followed by its sudden reappearance (or sudden rebirth). We analyze various finite-time decays (for dissipative systems) and analogous periodic vanishings (for unitary systems) of nonclassical correlations as described by violations of classical inequalities and the corresponding nonclassicality witnesses (or quantumness witnesses), which are not necessarily entanglement witnesses. We show that these sudden vanishings are universal phenomena and can be observed: (i) not only for two- or multi-mode but also for single-mode nonclassical fields, (ii) not solely for dissipative systems, and (iii) at evolution times which are usually different from those of sudden vanishings and reappearances of quantum entanglement.
Monika Bartkowiak, Adam Miranowicz, Xiaoguang Wang, Yu-xi Liu, Wieslaw Leonski, Franco Nori Journal reference: Phys. Rev. A 83, 053814 (2011) [pdf] DOI: 10.1103/PhysRevA.83.053814
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d + id holographic superconductors -
Abstract
- A holographic model of $d+id$ superconductors based on the action proposed by Benini, Herzog, and Yarom [arXiv:1006.0731] is studied. This model has a charged spin two field in an AdS black hole spacetime. Working in the probe limit, the normalizable solution of the spin two field in the bulk gives rise to a $d+id$ superconducting order parameter at the boundary of the AdS. We calculate the fermion spectral function in this\ superconducting background and confirm the existence of fermi arcs for non-vanishing Majorana couplings. By changing the relative strength $\gamma $ of the $d$ and $id$ condensations, the position and the size of the fermi arcs are changed. When $\gamma =1$, the spectrum becomes isotropic and the spectral function is s-wave like. By changing the fermion mass, the fermi momentum is changed. We also calculate the conductivity for these holographic $d+id$ superconductors where time reversal symmetry has been broken spontaneously. A non-vanishing Hall conductivity is obtained even without an external magnetic field.
Jiunn-Wei Chen, Yu-Sheng Liu, Debaprasad Maity Journal reference: JHEP 1105:032,2011 [pdf] DOI: 10.1007/JHEP05(2011)032
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Tensor perturbations of
- We analyze the tensor perturbations of flat thick domain wall branes in $f(R)$ gravity. Our results indicate that under the transverse and traceless gauge, the metric perturbations decouple from the perturbation of the scalar field. Besides, the perturbed equation reduces to the familiar Klein-Gordon equation for massless spin-2 particles only when the bulk curvature is a constant or when $f(R)=R$. As an application of our results, we consider the possibility of localizing gravity on some flat thick branes. The stability of these brane solutions is also shortly discussed.
Yuan Zhong, Yu-Xiao Liu, Ke Yang Journal reference: Phys.Lett.B699:398-402,2011 [pdf] DOI: 10.1016/j.physletb.2011.04.037
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Circular photogalvanic effect induced by near-infrared radiation in InAs quantum wires patterned quasi-two-dimensional electron system -
Abstract
- In this work we investigated the InAs/InAlAs quantum wires (QWRs) superlattice by optically exciting the structure with near-infrared radiation. By varying the helicity of the radiation at room temperature we observed the circular photogalvanic effect related to the $C_{2v}$ symmetry of the structure, which could be attributed to the formation of a quasi two-dimensional system underlying in the vicinity of the QWRs pattern. The ratio of Rashba and Dresselhaus terms shows an evolution of the spin-orbit interaction in quasi two-dimensional structure with the QWR layer deposition thickness.
Chongyun Jiang, Yonghai Chen, Hui Ma, Jinling Yu, Yu Liu [pdf] DOI: 10.1063/1.3596467 1104.4407v1 [pdf]
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Holographic Josephson Junction in 3+1 dimensions -
Abstract
- In arXiv:1101.3326[hep-th], a (2+1)-dimensional holographic Josephson junction was constructed, and it was shown that the DC Josephson current is proportional to the sine of the phase difference across the junction. In this paper, we extend this study to a holographic description for the (3+1)-dimensional holographic DC Josephson junction. By solving numerically the coupled differential equations, we also obtain the familiar characteristics of Josephson junctions.
Yong-Qiang Wang, Yu-Xiao Liu, Zhen-Hua Zhao [pdf]
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Radiative transitions in charmonium from
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Abstract
- We present a study for charmonium radiative transitions: $J/\psi\rightarrow\eta_c\gamma$, $\chi_{c0}\rightarrow J/\Psi\gamma$ and $h_c\rightarrow\eta_c\gamma$ using $N_f=2$ twisted mass lattice QCD gauge configurations. The single-quark vector form factors for $\eta_c$ and $\chi_{c0}$ are also determined. The simulation is performed at a lattice spacing of $a= 0.06666$ fm and the lattice size is $32^3\times 64$. After extrapolation of lattice data at nonzero $Q^2$ to 0, we compare our results with previous quenched lattice results and the available experimental values.
Ying Chen, De-Chuan Du, Bao-Zhong Guo, Ning Li, Chuan Liu, Hang Liu, Yu-Bin Liu, Jian-Ping Ma, Xiang-Fei Meng, Zhi-Yuan Niu, Jian-Bo Zhang Journal reference: Phys.Rev.D84:034503,2011 [pdf] DOI: 10.1103/PhysRevD.84.034503
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Fermion resonances on a thick brane with a piecewise warp factor -
Abstract
- In this paper, we mainly investigate the problems of resonances of massive KK fermions on a single scalar constructed thick brane with a piecewise warp factor matching smoothly. The distance between two boundaries and the other parameters are determined by one free parameter through three junction conditions. For the generalized Yukawa coupling $\eta\bar{\Psi}\phi^{k}\Psi$ with odd $k=1,3,5,...$, the mass eigenvalue $m$, width $\Gamma$, lifetime $\tau$, and maximal probability $P_{max}$ of fermion resonances are obtained. Our numerical calculations show that the brane without internal structure also favors the appearance of resonant states for both left- and right-handed fermions. The scalar-fermion coupling and the thickness of the brane influence the resonant behaviors of the massive KK fermions.
Hai-Tao Li, Yu-Xiao Liu, Zhen-Hua Zhao, Heng Guo Journal reference: Phys.Rev.D83:045006,2011 [pdf] DOI: 10.1103/PhysRevD.83.045006
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Area spectrum of Schwarzschild black hole inspired by noncommutative
geometry -
Abstract
- It is known that, in the noncommutative Schwarzschild black hole spacetime, the point-like object is replaced by the smeared object, whose mass density is described by a Gaussian distribution of minimal width $\sqrt{\theta}$ with $\theta$ the noncommutative parameter. The elimination of the point-like structures makes it quite different from the conventional Schwarzschild black hole. In this paper, we mainly investigate the area spectrum and entropy spectrum for the noncommutative Schwarzschild black hole with $0\leq \theta\leq (\frac{M}{1.90412})^{2}$. By the use of the new physical interpretation of the quasinormal modes of black holes presented by Maggiore, we obtain the quantized area spectrum and entropy spectrum with the modified Hod's and Kunstatter's methods, respectively. The results show that: (1) The area spectrum and entropy spectrum are discrete. (2) The spectrum spacings are dependent on the parameter $\frac{M}{\sqrt{\theta}}$. (3) The spacing of the area spectrum of the noncommutative Schwarzschild black hole is smaller than that of the conventional one. So does the spacing of the entropy spectrum. (4) The spectra from the two methods are consistent with each other.
Shao-Wen Wei, Yu-Xiao Liu, Zhen-Hua Zhao, Chun-E Fu [pdf]
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Atomic-scale Field-effect Transistor as a Thermoelectric Power Generator
and Self-powered Device -
Abstract
- Using first-principles approaches, we have investigated the thermoelectric properties and the energy conversion efficiency of the paired metal-Br-Al junction. Owing to the narrow states in the vicinity of the chemical potential, the nanojunction has large Seebeck coefficients such that it can be considered an efficient thermoelectric power generator. We also consider the nanojunction in a three-terminal geometry, where the current, voltage, power, and efficiency can be efficiently modulated by the gate voltages. Such current-voltage characteristics could be useful in the design of nano-scale electronic devices such, as a transistor or switch. Notably, the nanojunction as a transistor with a fixed finite temperature difference between electrodes can power itself using the Seebeck effect.
Yu-Shen Liu, Hsuan-Te Yao, Yu-Chang Chen [pdf]
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Experimental investigation of longitudinal space–time correlations of the velocity field in turbulent Rayleigh–Bénard convection -
Abstract
- We report an experimental investigation of the longitudinal space-time cross-correlation function of the velocity field, $C(r,\tau)$, in a cylindrical turbulent Rayleigh-B\'{e}nard convection cell using the particle image velocimetry (PIV) technique. We show that while the Taylor's frozen-flow hypothesis does not hold in turbulent thermal convection, the recent elliptic model advanced for turbulent shear flows [He & Zhang, \emph{Phys. Rev. E} \textbf{73}, 055303(R) (2006)] is valid for the present velocity field for all over the cell, i.e., the isocorrelation contours of the measured $C(r,\tau)$ have a shape of elliptical curves and hence $C(r,\tau)$ can be related to $C(r_E,0)$ via $r_E^2=(r-\beta\tau)^2+\gamma^2\tau^2$ with $\beta$ and $\gamma$ being two characteristic velocities. We further show that the fitted $\beta$ is proportional to the mean velocity of the flow, but the values of $\gamma$ are larger than the theoretical predictions. Specifically, we focus on two representative regions in the cell: the region near the cell sidewall and the cell's central region. It is found that $\beta$ and $\gamma$ are approximately the same near the sidewall, while $\beta\simeq0$ at cell center.
Quan Zhou, Chun-Mei Li, Zhi-Ming Lu, Yu-Lu Liu [pdf] DOI: 10.1017/jfm.2011.249 1012.0153v2 [pdf]
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Suppressing non-Markovian noises by coupling the qubit to a chaotic
device -
Abstract
- To suppress decoherence of solid-state qubits which are coupled to the non-Markovian noises, we propose a strategy to couple the qubit with a chaotic device, of which the broad power distribution in the high-frequency domain can be used to freeze the noises just like the dynamical decoupling control (DDC) method. Compared with the DDC, high-frequency components can be generated by the chaotic device even driven by a low-frequency field and we do not need to optimize the control fields to generate complex control pulses. As an application to superconducting circuits, we find that various noises in a wide frequency domain, including low-frequency $1/f$, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubit to a Duffing oscillator, and the decoherence rate of the qubit is efficiently decreased for about 100 times in magnitude.
Jing Zhang, Yu-xi Liu, Wei-Min Zhang, Re-Bing Wu, Tzyh-Jong Tarn [pdf]
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Deterministic chaos can act as a decoherence suppressor -
Abstract
- We propose a strategy to suppress decoherence of a solid-state qubit coupled to non-Markovian noises by attaching the qubit to a chaotic setup with the broad power distribution in particular in the high-frequency domain. Different from the existing decoherence control methods such as the usual dynamics decoupling control, high-frequency components of our control are generated by the chaotic setup driven by a low-frequency field, and the generation of complex optimized control pulses is not necessary. We apply the scheme to superconducting quantum circuits and find that various noises in a wide frequency domain, including low-frequency $1/f$, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubits to a Duffing oscillator as the chaotic setup. Significantly, the decoherence time of the qubit is prolonged approximately $100$ times in magnitude.
Jing Zhang, Yu-xi Liu, Wei-Min Zhang, Lian-Ao Wu, Re-Bing Wu, Tzyh-Jong Tarn Journal reference: Phys. Rev. B 84, 214304 (2011) [pdf] DOI: 10.1103/PhysRevB.84.214304
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Counting statistics in nanoscale junctions -
Abstract
- We present first-principles calculations for moments of the current up to the third order in atomic-scale junctions. The quantum correlations of the current are calculated using the current operator in terms of the wave functions obtained self-consistently within the static density-functional theory. We investigate the relationships of the conductance, the second, and the third moment of the current for carbon atom chains of various lengths bridging two metal electrodes in the linear and nonlinear regimes. The conductance, the second-, and the third-order Fano factors exhibit odd-even oscillation with the number of carbon atoms due to the full and half filled {\pi}* orbital near the Fermi levels. The third-order Fano factor and the conductance are positively correlated.
Yu-Shen Liu, Yu-Chang Chen Journal reference: Phys. Rev. B 83, 035401 (2011) [pdf] DOI: 10.1103/PhysRevB.83.035401
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Effect of Thermoelectric Cooling in Nanoscale Junctions -
Abstract
- We propose a thermoelectric cooling device based on an atomic-sized junction. Using first-principles approaches, we investigate the working conditions and the coefficient of performance (COP) of an atomic-scale electronic refrigerator where the effects of phonon's thermal current and local heating are included. It is observed that the functioning of the thermoelectric nano-refrigerator is restricted to a narrow range of driving voltages. Compared with the bulk thermoelectric system with the overwhelmingly irreversible Joule heating, the 4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric refrigerator with the same $ZT$ due to suppressed local heating via the quasi-ballistic electron transport and small driving voltages. Quantum nature due to the size minimization offered by atomic-level control of properties facilitates electron cooling beyond the expectation of the conventional thermoelectric device theory.
Yu-Shen Liu, Bailey C. Hsu, Yu-Chang Chen Journal reference: J. Phys. Chem. C 115, 6111 (2011) [pdf] DOI: 10.1021/jp110920q
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Seebeck coefficients in nanoscale junctions: Effects of electron-vibration scattering and local heating -
Abstract
- We report first-principles calculations of inelastic Seebeck coefficients in an aluminum monatomic junction. We compare the elastic and inelastic Seebeck coefficients with and without local heating. In the low temperature regime, the signature of normal modes in the profiles of the inelastic Seebeck effects is salient. The inelastic Seebeck effects are enhanced by the normal modes, and further magnified by local heating. In the high temperature regime, the inelastic Seebeck effects are weakly suppressed due to the quasi-ballistic transport.
Bailey C. Hsu, Yu-Shen Liu, Sheng Hsien Lin, Yu-Chang Chen Journal reference: Phys. Rev. B 83, 041404(R) (2011) [pdf] DOI: 10.1103/PhysRevB.83.041404
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Multistability of electromagnetically induced transparency in atom-assisted optomechanical cavities -
Abstract
- We study how an oscillating mirror affects the electromagnetically induced transparency (EIT) of an atomic ensemble, which is confined in a gas cell placed inside a micro-cavity with an oscillating mirror in one end. The oscillating mirror is modeled as a quantum mechanical harmonic oscillator. The cavity field acts as a probe light of the EIT system and also produces a light pressure on the oscillating mirror. The back-action from the mirror to the cavity field results in several (from one to five) steady-states for this atom-assisted optomechanical cavity, producing a complex structure in its EIT. We calculate the susceptibility with respect to the few (from one to three) stable solutions found here for the equilibrium positions of the oscillating mirror. We find that the EIT of the atomic ensemble can be significantly changed by the oscillating mirror, and also that the various steady states of the mirror have different effects on the EIT.
Yue Chang, T. Shi, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 83, 063826 (2011) [pdf] DOI: 10.1103/PhysRevA.83.063826
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Particle acceleration in Kerr–(anti-)de Sitter black hole backgrounds -
Abstract
- 2010
-
Particle collisions on stringy black hole background -
Abstract
- The collision of two particles in the background of a Sen black hole is studied. With the equations of motion of the particles, the center-of-mass energy is investigated when the collision takes place at the horizon of a Sen black hole. For an extremal Sen black hole, we find that the center-of-mass energy will be arbitrarily high with two conditions: (1) spin $a\neq 0$ and (2) one of the colliding particles has the critical angular momentum $l_{\text{c}}=2$. For a nonextremal Sen black hole, we show that, in order to obtain an unlimited center-of-mass energy, one of the colliding particles should have the critical angular momentum $l'_{\text{c}}=2 r_{+}/a$ ($r_{+}$ is the radius of the outer horizon for a nonextremal black hole). However, a particle with the angular momentum $l=l'_{\text{c}}$ could not approach the black hole from outside of the horizon through free fall, which implies that the collision with arbitrarily high center-of-mass energy could not take place. Thus, there is an upper bound of the center-of-mass energy for the nonextremal black hole. We also obtain the maximal center-of-mass energy for a near-extremal black hole and the result implies that the Planck-scale energy is hard to be approached. Furthermore, we also consider the back-reaction effects. The result shows that, neglecting the gravitational radiation, it has a weak effect on the center-of-mass energy. However, we argue that the maximum allowed center-of-mass energy will be greatly reduced to below the Planck-scale when the gravitational radiation is included.
Shao-Wen Wei, Yu-Xiao Liu, Hai-Tao Li, Feng-Wei Chen Journal reference: JHEP 1012:066,2010 [pdf] DOI: 10.1007/JHEP12(2010)066
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Bulk matter fields on a GRS-inspired braneworld -
Abstract
- In this paper we investigate the localization and mass spectra of bulk matter fields on a Gergory-Rubakov-Sibiryakov-inspired braneworld. In this braneworld model, there are one thick brane located at the origin of the extra dimension and two thin branes at two sides. For spin 1/2 fermions coupled with the background scalar $\phi$ via $\eta\bar{\Psi}\phi^p\Psi$ with $p$ a positive odd integer, the zero mode of left-hand fermions can be localized on the thick brane for finite distance of the two thin branes, and there exist some massive bound modes and resonance modes. The resonances correspond to the quasi-localized massive fermions. For free massless spin 0 scalars, the zero mode can not be localized on the thick brane when the two thin branes are located finitely. While for a massive scalar $\Phi$ coupled with itself and the background scalar field $\phi$, in order to get a localized zero mode on the thick brane, a fine-tuning relation should be introduced. Some massive bound modes and resonances also will appear. For spin 1 vectors, there is no bound KK mode because the effective potential felt by vectors vanishes outside the two thin branes. We also investigate the physics when the distance of the two thin branes tends to infinity.
Yu-Xiao Liu, Chun-E Fu, Heng Guo, Shao-Wen Wei, Zhen-Hua Zhao Journal reference: JCAP 1012 (2010) 031 [pdf] DOI: 10.1088/1475-7516/2010/12/031
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Measurement-based quantum phase estimation algorithm for finding eigenvalues of non-unitary matrices -
Abstract
- We propose a quantum algorithm for finding eigenvalues of non-unitary matrices. We show how to construct, through interactions in a quantum system and projective measurements, a non-Hermitian or non-unitary matrix and obtain its eigenvalues and eigenvectors. This proposal combines ideas of frequent measurement, measured quantum Fourier transform, and quantum state tomography. It provides a generalization of the conventional phase estimation algorithm, which is limited to Hermitian or unitary matrices.
Hefeng Wang, Lian-Ao Wu, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A, 82, 062303 (2010) [pdf] DOI: 10.1103/PhysRevA.82.062303
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Quark-mass dependence of two-nucleon observables -
Abstract
- We study the implications of lattice QCD determinations of the S-wave nucleon-nucleon scattering lengths at unphysical light quark masses. It is found that with the help of nuclear effective field theory (NEFT), not only the quark mass dependence of the effective range parameters, but also the leading quark mass dependence of all the low energy deuteron matrix elements can be obtained. The quark mass dependence of deuteron charge radius, magnetic moment, polarizability and the deuteron photodisintegration cross section are shown based on the NPLQCD lattice calculation of the scattering lengths at 354 MeV pion mass and the NEFT power counting scheme of Beane, Kaplan and Vuorinen. Further improvement can be obtained by performing the lattice calculation at smaller quark masses. Our result can be used to constrain the time variation of isoscalar combination of u and d quark mass m_q, to help the anthropic principle study to find the m_q range which allows the existence of life, and to provide a weak test of the multiverse conjecture.
Jiunn-Wei Chen, Tze-Kei Lee, C. -P. Liu, Yu-Sheng Liu [pdf] DOI: 10.1103/PhysRevC.86.054001 1012.0453v1 [pdf]
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Charged spinning black holes as particle accelerators -
Abstract
- It has recently been pointed out that the spinning Kerr black hole with maximal spin could act as a particle collider with arbitrarily high center-of-mass energy. In this paper, we will extend the result to the charged spinning black hole, the Kerr-Newman black hole. The center-of-mass energy of collision for two uncharged particles falling freely from rest at infinity depends not only on the spin $a$ but also on the charge $Q$ of the black hole. We find that an unlimited center-of-mass energy can be approached with the conditions: (1) the collision takes place at the horizon of an extremal black hole; (2) one of the colliding particles has critical angular momentum; (3) the spin $a$ of the extremal black hole satisfies $\frac{1}{\sqrt{3}}\leq \frac{a}{M}\leq 1$, where $M$ is the mass of the Kerr-Newman black hole. The third condition implies that to obtain an arbitrarily high energy, the extremal Kerr-Newman black hole must have a large value of spin, which is a significant difference between the Kerr and Kerr-Newman black holes. Furthermore, we also show that, for a near-extremal black hole, there always exists a finite upper bound for center-of-mass energy, which decreases with the increase of the charge $Q$.
Shao-Wen Wei, Yu-Xiao Liu, Heng Guo, Chun-E Fu Journal reference: Phys.Rev.D82:103005,2010 [pdf] DOI: 10.1103/PhysRevD.82.103005
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Phase Diagram and Critical End Point for Strongly Interacting Quarks -
Abstract
- We introduce a method based on the chiral susceptibility, which enables one to draw a phase diagram in the chemical-potential/temperature plane for strongly-interacting quarks whose interactions are described by any reasonable gap equation, even if the diagrammatic content of the quark-gluon vertex is unknown. We locate a critical endpoint (CEP) at (\mu^E,T^E) ~ (1.0,0.9)T_c, where T_c is the critical temperature for chiral symmetry restoration at \mu=0; and find that a domain of phase coexistence opens at the CEP whose area increases as a confinement length-scale grows.
Si-xue Qin, Lei Chang, Huan Chen, Yu-xin Liu, Craig D. Roberts Journal reference: Phys.Rev.Lett.106:172301,2011 [pdf] DOI: 10.1103/PhysRevLett.106.172301
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Phase gate of one qubit simultaneously controlling $n$ qubits in a
cavity -
Abstract
- We propose how to realize a three-step controlled-phase gate of one qubit simultaneously controlling $n$ qubits in a cavity or coupled to a resonator. The $n$ two-qubit controlled-phase gates, forming this multiqubit phase gate, can be performed simultaneously. The operation time of this phase gate is independent of the number $n$ of qubits. This phase gate controlling at once $n$ qubits is insensitive to the initial state of the cavity mode and can be used to produce an analogous CNOT gate simultaneously acting on $n$ qubits. We present two alternative approaches to implement this gate. One approach is based on tuning the qubit frequency while the other method tunes the resonator frequency. Using superconducting qubits coupled to a resonator as an example, we show how to implement the proposed gate with one superconducting qubit simultaneously controlling $n$ qubits selected from $N$ qubits coupled to a resonator ($1<n<n$). we="We" also="also" give="give" a="a" discussion="discussion" on="on" realizing="realizing" the="the" proposed="proposed" gate="gate" with="with" atoms,="atoms," by="by" using="using" one="one" cavity="cavity" initially="initially" in="in" an="an" arbitrary="arbitrary" state.
Chui-Ping Yang, Yu-xi Liu, Franco Nori [pdf]
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Effects of the variation of mass on fermion localization and resonances on thick branes -
Abstract
- A few years ago, Campos investigated the critical phenomena of thick branes in warped spacetimes [Phys. Rev. Lett. {\bf 88} (2002) 141602]. Inspired by his work, we consider a toy model of thick branes generated by a real scalar field with the potential $V(\phi)=a\phi^2-b\phi^4+c\phi^6$, and investigate the variation of the mass parameter $a$ on the branes as well as the localization and resonances of fermions. An interesting result is found: there is a critical value for the mass parameter $a$, and when the critical value of $a$ is reached the solution of the background scalar field is not unique and has the shape of a double kink. This happens in both cases with and without gravity. It is also shown that the numbers of the bound Kaluza-Klein modes of fermions on the gravity-free brane and the resonant states of fermions on the brane with gravity increase with the value of $a$.
Zhen-Hua Zhao, Yu-Xiao Liu, Hai-Tao Li, Yong-Qiang Wang Journal reference: Phys.Rev.D82:084030,2010 [pdf] DOI: 10.1103/PhysRevD.82.084030
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de Sitter thick brane solution in Weyl geometry -
Abstract
- In this paper, we consider a de Sitter thick brane model in a pure geometric Weyl integrable five-dimensional space-time, which is a generalization of Riemann geometry and is invariant under a so-called Weyl rescaling. We find a solution of this model via performing a conformal transformation to map the Weylian structure into a familiar Riemannian one with a conformal metric. The metric perturbations of the model are discussed. For gravitational perturbation, we get the effective modified P$\ddot{\text{o}}$schl-Teller potential in corresponding Schr$\ddot{\text{o}}$dinger equation for Kaluza-Klein (KK) modes of the graviton. There is only one bound state, which is a normalizable massless zero mode and represents a stable 4-dimensional graviton. Furthermore, there exists a mass gap between the massless mode and continuous KK modes. We also find that the model is stable under the scalar perturbation in the metric. The correction to the Newtonian potential on the brane is proportional to $e^{-3 r \beta/2}/r^2$, where $\beta$ is the de Sitter parameter of the brane. This is very different from the correction caused by a volcano-like effective potential.
Yu-Xiao Liu, Ke Yang, Yuan Zhong Journal reference: JHEP 1010:069,2010 [pdf] DOI: 10.1007/JHEP10(2010)069
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Quark spectral density and strongly-coupled quark-gluon plasma -
Abstract
- The maximum entropy method is used to compute the dressed-quark spectral density from the self-consistent numerical solution of a rainbow truncation of QCD's gap equation at temperatures above that for which chiral symmetry is restored. In addition to the normal and plasmino modes, the spectral function also exhibits an essentially nonperturbative zero mode for temperatures extending to 1.4-1.8-times the critical temperature, T_c. In the neighbourhood of T_c, this long-wavelength mode contains the bulk of the spectral strength and so long as this mode persists, the system may fairly be described as a strongly-coupled state of matter.
Si-xue Qin, Lei Chang, Yu-xin Liu, Craig D. Roberts Journal reference: Phys.Rev.D84:014017,2011 [pdf] DOI: 10.1103/PhysRevD.84.014017
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Dressed-Quark Anomalous Magnetic Moments -
Abstract
- Perturbation theory predicts that a massless fermion cannot possess a measurable magnetic moment. We explain, however, that the nonperturbative phenomenon of dynamical chiral symmetry breaking generates a momentum-dependent anomalous chromomagnetic moment for dressed light-quarks, which is large at infrared momenta; and demonstrate that consequently these same quarks also possess an anomalous electromagnetic moment with similar magnitude and opposite sign.
Lei Chang, Yu-Xin Liu, Craig D. Roberts Journal reference: Phys.Rev.Lett.106:072001,2011 [pdf] DOI: 10.1103/PhysRevLett.106.072001
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Temperature dependence of the effective bag constant and the radius of a nucleon in the global color symmetry model of QCD -
Abstract
- We study the temperature dependence of the effective bag constant, the mass, and the radius of a nucleon in the formalism of the simple global color symmetry model in the Dyson-Schwinger equation approach of QCD with a Gaussian-type effective gluon propagator. We obtain that, as the temperature is lower than a critical value, the effective bag constant and the mass decrease and the radius increases with the temperature increasing. As the critical temperature is reached, the effective bag constant and the mass vanish and the radius tends to infinity. At the same time, the chiral quark condensate disappears. These phenomena indicate that the deconfinement and the chiral symmetry restoration phase transitions can take place at high temperature. The dependence of the critical temperature on the interaction strength parameter in the effective gluon propagator of the approach is given.
Yuan Mo, Si-xue Qin, Yu-xin Liu Journal reference: Phys.Rev.C82:025206,2010 [pdf] DOI: 10.1103/PhysRevC.82.025206
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Vacuum-induced Berry phases in single-mode Jaynes-Cummings models -
Abstract
- Motivated by the work [Phys. Rev. Lett. 89, 220404 (2002)] for detecting the vacuum-induced Berry phases with two-mode Jaynes-Cummings models (JCMs), we show here that, for a parameter-dependent single-mode JCM, certain atom-field states also acquire the photon-number-dependent Berry phases after the parameter slowly changed and eventually returned to its initial value. This geometric effect related to the field quantization still exists, even the filed is kept in its vacuum state. Specifically, a feasible Ramsey interference experiment with cavity quantum electrodynamics (QED) system is designed to detect the vacuum-induced Berry phase.
Yu Liu, L. F. Wei, W. Z. Jia, J. Q. Liang [pdf] DOI: 10.1103/PhysRevA.82.045801 1008.3777v2 [pdf]
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Hidden conformal symmetry of the Kerr-Newman black hole -
Abstract
- We investigate the hidden conformal symmetry of the 4-dimensional non-extremal Kerr-Newman (KN) black hole with the idea of the near-region Kerr/CFT correspondence proposed by Castro, Maloney and Strominger in arXiv:1004.0996[hep-th]. The near-region KN black hole is dual to a 2D CFT with left and right temperatures $T_L=(2 M^2-Q^2)/(4\pi J)$ and $T_R=\sqrt{M^4-J^2-M^2Q^2}/(2\pi J)$. Furthermore, we reproduce the microscopic entropy of the KN black hole via the Cardy formula, which is in agreement with the macroscopic Bekenstein-Hawking entropy and precisely match the absorption cross section of a near-region scalar field in the KN black hole with the finite-temperature absorption cross section for a 2D CFT.
Yong-Qiang Wang, Yu-Xiao Liu Journal reference: JHEP 1008:087,2010 [pdf] DOI: 10.1007/JHEP08(2010)087
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Asymptotics for Turbulent Flame Speeds of the Viscous G-Equation Enhanced by Cellular and Shear Flows -
Abstract
- G-equations are well-known front propagation models in turbulent combustion and describe the front motion law in the form of local normal velocity equal to a constant (laminar speed) plus the normal projection of fluid velocity. In level set formulation, G-equations are Hamilton-Jacobi equations with convex ($L^1$ type) but non-coercive Hamiltonians. Viscous G-equations arise from either numerical approximations or regularizations by small diffusion. The nonlinear eigenvalue $\bar H$ from the cell problem of the viscous G-equation can be viewed as an approximation of the inviscid turbulent flame speed $s_T$. An important problem in turbulent combustion theory is to study properties of $s_T$, in particular how $s_T$ depends on the flow amplitude $A$. In this paper, we will study the behavior of $\bar H=\bar H(A,d)$ as $A\to +\infty$ at any fixed diffusion constant $d > 0$. For the cellular flow, we show that $$ \bar H(A,d)\leq O(\sqrt {\mathrm {log}A}) \quad \text{for all $d>0$}. $$ Compared with the inviscid G-equation ($d=0$), the diffusion dramatically slows down the front propagation. For the shear flow, the limit \nit $\lim_{A\to +\infty}{\bar H(A,d)\over A} = \lambda (d) >0$ where $\lambda (d)$ is strictly decreasing in $d$, and has zero derivative at $d=0$. The linear growth law is also valid for $s_T$ of the curvature dependent G-equation in shear flows.
Yu-Yu Liu, Jack Xin, Yifeng Yu [pdf] DOI: 10.1007/s00205-011-0418-y 1007.3332v2 [pdf]
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Testing nonclassicality in multimode fields: A unified derivation of classical inequalities -
Abstract
- We consider a way to generate operational inequalities to test nonclassicality (or quantumness) of multimode (or multiparty) bosonic fields that unifies the derivation of many known inequalities and allows to propose new ones. The nonclassicality criteria are based on Vogel's criterion corresponding to analyzing the positivity of multimode P functions or, equivalently, the positivity of matrices of expectation values of, e.g., creation and annihilation operators. We analyze not only monomials, but also polynomial functions of such moments, which can sometimes enable simpler derivations of physically relevant inequalities. As an example, we derive various classical inequalities which can be violated only by nonclassical fields. In particular, we show how the criteria introduced here easily reduce to the well-known inequalities describing: (a) multimode quadrature squeezing and its generalizations including sum, difference and principal squeezing, (b) two-mode one-time photon-number correlations including sub-Poisson photon-number correlations and effects corresponding to violations of the Cauchy-Schwarz and Muirhead inequalities, (c) two-time single-mode photon-number correlations including photon antibunching and hyperbunching, and (d) two- and three-mode quantum entanglement. Other simple inequalities for testing nonclassicality are also proposed. We have found some general relations between the nonclassicality and entanglement criteria, in particular, those resulting from the Cauchy-Schwarz inequality. It is shown that some known entanglement inequalities can be derived as nonclassicality inequalities within our formalism, while some other known entanglement inequalities can be seen as sums of more than one inequality derived from the nonclassicality criterion. This approach enables a deeper analysis of the entanglement for a given nonclassicality.
Adam Miranowicz, Monika Bartkowiak, Xiaoguang Wang, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 82, 013824 (2010) [pdf] DOI: 10.1103/PhysRevA.82.013824
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Tunable electromagnetically induced transparency and absorption with dressed superconducting qubits -
Abstract
- Electromagnetically induced transparency and absorption (EIT and EIA) are usually demonstrated by three-level atomic or atom-like systems. In contrast to the usual case, we theoretically study the EIT and EIA in an equivalent three-level system, which is constructed by dressing a superconducting two-level system (qubit) dressed by a single-mode cavity field. In this equivalent system, we find that both the EIT and the EIA can be tuned by controlling the level-spacing of the superconducting qubit and hence controlling the dressed system. This tunability is due to the dressed relaxation and dephasing rates which vary parametrically with the level-spacing of the original qubit and thus affect the transition properties of the dressed qubit and the susceptibility. These dressed relaxation and dephasing rates characterize the reaction of the dressed qubit to an incident probe field. We also use recent experimental data on superconducting qubits (charge, phase, and flux qubits) to demonstrate our approach and show the possibility of experimentally realizing this proposal.
Hou Ian, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 81, 063823 (2010) [pdf] DOI: 10.1103/PhysRevA.81.063823
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A note on the temperature and energy of four-dimensional black holes from an entropic force -
Abstract
- We investigate the temperature and energy on holographic screens for 4-dimensional black holes with the entropic force idea proposed by Verlinde. We find that the "Unruh-Verlinde temperature" is equal to the Hawking temperature on the horizon and can be considered as a generalized Hawking temperature on the holographic screen outside the horizons. The energy on the holographic screen is not the black hole mass $M$ but the reduced mass $M_0$, which is related to the black hole parameters. With the replacement of the black hole mass $M$ by the reduced mass $M_0$, the entropic force can be written as $F=\frac{GmM_0}{r^2}$, which could be tested by experiments.
Yu-Xiao Liu, Yong-Qiang Wang, Shao-Wen Wei Journal reference: Class.Quant.Grav.27:185002,2010 [pdf] DOI: 10.1088/0264-9381/27/18/185002
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Fermion localization on asymmetric two-field thick branes -
Abstract
- In this paper we investigate the localization of fermions on asymmetric thick branes generated by two scalars $\phi$ and $\chi$. In order to trap fermions on the asymmetric branes with kink-like warp factors, the couplings with the background scalars $\eta\bar{\Psi}F(\chi,\phi)\Psi$ are introduced, where $F(\chi,\phi)$ is a function of $\phi$ and $\chi$. We find that the coupling $\eta\bar{\Psi}\chi\phi\Psi$ do not support the localization of 4-dimensional fermions on the branes. While, for the case $\eta\bar{\Psi}\chi\Psi+\eta'\bar{\Psi}\phi\Psi$, which is the kink-fermion coupling corresponding to one-scalar-generated brane scenarios, the zero mode of left-handed fermions could be trapped on the branes under some conditions.
Zhen-Hua Zhao, Yu-Xiao Liu, Hai-Tao Li Journal reference: Class.Quant.Grav.27:185001,2010 [pdf] DOI: 10.1088/0264-9381/27/18/185001
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Qubit-induced phonon blockade as a signature of quantum behavior in nanomechanical resonators -
Abstract
- The observation of quantized nanomechanical oscillations by detecting femtometer-scale displacements is a significant challenge for experimentalists. We propose that phonon blockade can serve as a signature of quantum behavior in nanomechanical resonators. In analogy to photon blockade and Coulomb blockade for electrons, the main idea for phonon blockade is that the second phonon cannot be excited when there is one phonon in the nonlinear oscillator. To realize phonon blockade, a superconducting quantum two-level system is coupled to the nanomechanical resonator and is used to induce the phonon self-interaction. Using Monte Carlo simulations, the dynamics of the induced nonlinear oscillator is studied via the Cahill-Glauber $s$-parametrized quasiprobability distributions. We show how the oscillation of the resonator can occur in the quantum regime and demonstrate how the phonon blockade can be observed with currently accessible experimental parameters.
Yu-xi Liu, Adam Miranowicz, Y. B. Gao, J. Bajer, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 82, 032101 (2010) [pdf] DOI: 10.1103/PhysRevA.82.032101
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Truly random number generation via entropy amplification -
Abstract
- We present a simple setup to implement truly random number generator based on the measurement of the laser phase noise. From the entropy point of view, we estimate the number of truly random bits that can be extracted from the sampled Byte. With a simple method of adopting the $m$-least-significant-bit, we amplify the entropy of the original bit sequence and realize a truly random bit generation rate of $300$ Mbps.
Yu Liu, Mingyi Zhu, Hong Guo [pdf]
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Scalar-kinetic branes -
Abstract
- This work tries to find out thick brane solutions in braneworld scenarios described by a real scalar field in the presence of a scalar-kinetic term $F(X,\phi)=X\phi^m$ with a single extra dimension, where $X=\frac12\nabla_M\phi\nabla^M\phi$ stands for the standard kinetic term and $m=0,1,2\cdots$. We mainly consider bent branes, namely de Sitter and Anti-de Sitter four-dimensional slices. The solutions of a flat brane are obtained when taking the four-dimensional cosmological constant $\Lambda_4\rightarrow 0$. When the parameter $m=0$, these solutions turn to those of the standard scenario. The localization and spectrum of graviton on these branes are also analyzed.
Yu-Xiao Liu, Yuan Zhong, Ke Yang Journal reference: Europhys.Lett.90:51001,2010 [pdf] DOI: 10.1209/0295-5075/90/51001
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Entropy/area spectra of the charged black hole from quasinormal modes -
Abstract
- With the new physical interpretation of quasinormal modes proposed by Maggiore, the quantum area spectra of black holes have been investigated recently. Adopting the modified Hod's treatment, results show that the area spectra for black holes are equally spaced and the spacings are in a unified form, $\triangle A=8\pi \hbar$, in Einstein gravity. On the other hand, following Kunstatter's method, the studies show that the area spectrum for a nonrotating black hole with no charge is equidistant. And for a rotating (or charged) black hole, it is also equidistant and independent of the angular momentum $J$ (or charge $q$) when the black hole is far from the extremal case. In this paper, we mainly deal with the area spectrum of the stringy charged Garfinkle-Horowitz-Strominger black hole, originating from effective action that emerges in the low-energy string theory. We find that both methods give the same results-that the area spectrum is equally spaced and does not depend on the charge $q$. Our study may provide new insights into understanding the area spectrum and entropy spectrum for stringy black holes.
Shao-Wen Wei, Yu-Xiao Liu, Ke Yang, Yuan Zhong Journal reference: Phys.Rev.D81:104042,2010 [pdf] DOI: 10.1103/PhysRevD.81.104042
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Transition from weak to strong measurements by nonlinear quantum feedback control -
Abstract
- We find that feedback control may induce "pseudo" nonlinear dynamics in a damped harmonic oscillator, whose centroid trajectory in the phase space behaves like a classical nonlinear system. Thus, similar to nonlinear amplifiers (e.g., rf-driven Josephson junctions), feedback control on the harmonic oscillator can induce nonlinear bifurcation, which can be used to amplify small signals and further to measure quantum states of qubits. Using the circuit QED systems as an example, we show how to apply our method to measure superconducting charge qubits.
Jing Zhang, Yu-xi Liu, Re-Bing Wu, Chun-Wen Li, Tzyh-Jong Tarn Journal reference: Phys. Rev. A 82, 022101 (2010) [pdf] DOI: 10.1103/PhysRevA.82.022101
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Entropy puzzle and the quark combination model -
Abstract
- We use two available methods, the Duhem-Gibbs relation and the entropy formula in terms of particle phase space distributions, to calculate the entropy in a quark combination model. The entropy of the system extracted from the Duhem-Gibbs relation is found to increase in hadronization if the average temperature of the hadronic phase is lower than that of the quark phase. The increase of the entropy can also be confirmed from the entropy formula if the volume of the hadronic phase is larger than 2.5-3.0 times that of the quark phase. So whether the entropy increases or decreases during combination depends on the temperature before and after combination and on how much expansion the system undergoes during combination. The current study provides an example to shed light on the entropy issue in the quark combination model.
Jun Song, Zuo-tang Liang, Yu-xin Liu, Feng-lan Shao, Qun Wang Journal reference: Phys.Rev.C81:057901,2010 [pdf] DOI: 10.1103/PhysRevC.81.057901
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Controlling the transport of single photons by tuning the frequency of either one or two cavities in an array of coupled cavities -
Abstract
- We theoretically study how to control transport, bound states, and resonant states of a single photon in a one-dimensional coupled-cavity array. We find that the transport of a single photon in the cavity array can be controlled by tuning the frequency of either one or two cavities. If one of the cavities in the array has a tunable frequency, and its frequency is tuned to be larger (or smaller) than those of other cavities, then there is a photon bound state above (or below) the energy band of the coupled cavity array. However, if two cavities in the array have tunable frequencies, then there exist both bound states and resonant states. When the frequencies of the two cavities are chosen to be much larger than those of other cavities and the hopping couplings between any two nearest-neighbor cavities are weak, a single photon with a resonant wave vector can be trapped in the region between the two frequency-tunable cavities. In this case, a quantum supercavity can be formed by these two frequency-tunable cavities. We also study how to apply this photon transport control to an array of coupled superconducting transmission line resonators.
Jie-Qiao Liao, Z. R. Gong, Lan Zhou, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 81, 042304 (2010) [pdf] DOI: 10.1103/PhysRevA.81.042304
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CHIRAL MAGNETIC EFFECT AND QCD PHASE TRANSITIONS WITH EFFECTIVE MODELS -
Abstract
- We study the influence of the chiral phase transition on the chiral magnetic effect. The chiral electric current density along the magnetic field, the electric charge difference between on each side of the reaction plane, and the azimuthal charged-particle correlations as functions of the temperature during the QCD phase transitions are calculated. It is found that with the decrease of the temperature, the chiral electric current density, the electric charge difference, and the azimuthal charged-particle correlations all get a sudden suppression at the critical temperature of the chiral phase transition, because the large quark constituent mass in the chiral symmetry broken phase quite suppresses the axial anomaly and the chiral magnetic effect. We suggest that the azimuthal charged-particle correlations (including the correlators divided by the total multiplicity of produced charged particles which are used in current experiments and another kind of correlators not divided by the total multiplicity) can be employed to identify the occurrence of the QCD phase transitions in RHIC energy scan experiments.
Wei-jie Fu, Yu-xin Liu, Yue-liang Wu [pdf] DOI: 10.1142/S0217751X11054541 1003.4169v1 [pdf]
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Localization of matters on Anti-de Sitter thick branes -
Abstract
- By presenting the mass-independent potentials of the Kaluza-Klein (KK) modes in the corresponding Schr\"{o}dinger equations, we investigate the localization and mass spectra of various bulk matter fields on an AdS thick brane. For a spin 0 scalar $\Phi$ coupled with itself and the domain-wall-forming field $\phi$ via a coupling potential $V=(\lambda \phi^{2}-u^{2})\Phi^{2}+\tau\Phi^{4}$, the localization and spectrum are decided by a critical coupling constant $\lambda_{0}$. When $\lambda>\lambda_0$, the potential of the scalar KK modes in the corresponding Schr\"{o}dinger equation tends to infinite when far away from the brane, which results in that there exist infinite discrete scalar bound KK states, and the massless modes could be trapped on the AdS brane by fine-tuning of parameters. When $\lambda<\lambda_0$, the potential of the scalar KK modes tends to negative infinite when far away from the brane, hence there does not exist any scalar bound KK state. For a spin 1 vector, the situation is same like the scalar with a coupling constant $\lambda>\lambda_0$, but the zero mode can not be localized on the brane. For a spin 1/2 fermion, we introduce the usual Yukawa coupling $\eta\bar{\Psi}\phi\Psi$, and find that the localization of the fermion is decided by a critical coupling constant $\eta_0$. For $\eta > \eta_0$, the four-dimensional massless left chiral fermion and massive Dirac fermions consisted of the pairs of coupled left-hand and right-hand KK modes could be localized on the AdS brane, and the massive Dirac fermions have a set of discrete mass spectrum. While for the case $0<\eta < \eta_{0}$, no four-dimensional Dirac fermion can be localized on the AdS brane.
Yu-Xiao Liu, Heng Guo, Chun-E Fu, Ji-Rong Ren Journal reference: JHEP 1002:080,2010 [pdf] DOI: 10.1007/JHEP02(2010)080
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Fluctuations and correlations of conserved charges in QCD at finite temperature with effective models -
Abstract
- We study fluctuations of conserved charges including baryon number, electric charge, and strangeness as well as the correlations among these conserved charges in the 2+1 flavor Polyakov--Nambu--Jona-Lasinio model at finite temperature. The calculated results are compared with those obtained from recent lattice calculations performed with an improved staggered fermion action at two values of the lattice cutoff with almost physical up and down quark masses and a physical value for the strange quark mass. We find that our calculated results are well consistent with those obtained in lattice calculations except for some quantitative differences for fluctuations related with strange quarks. Our calculations indicate that there is a pronounced cusp in the ratio of the quartic to quadratic fluctuations of baryon number, i.e. $\chi_{4}^{B}/\chi_{2}^{B}$, at the critical temperature during the phase transition, which confirms that $\chi_{4}^{B}/\chi_{2}^{B}$ is a useful probe of the deconfinement and chiral phase transition.
Wei-jie Fu, Yu-xin Liu, Yue-Liang Wu Journal reference: Phys.Rev.D81:014028,2010 [pdf] DOI: 10.1103/PhysRevD.81.014028
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Sudden vanishing of spin squeezing under decoherence -
Abstract
- In order to witness multipartite correlations beyond pairwise entanglement, spin-squeezing parameters are analytically calculated for a spin ensemble in a collective initial state under three different decoherence channels. It is shown that, in analogy to pairwise entanglement, the spin squeezing described by different parameters can suddenly become zero at different vanishing times. This finding shows the general occurrence of sudden vanishing phenomena of quantum correlations in many-body systems, which here is referred to as spin-squeezing sudden death (SSSD). It is shown that the SSSD usually occurs due to decoherence and that SSSD never occurs for some initial states in the amplitude-damping channel. We also analytically obtain the vanishing times of spin squeezing.
Xiaoguang Wang, Adam Miranowicz, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 81, 022106 (2010) [pdf] DOI: 10.1103/PhysRevA.81.022106
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Chiral Magnetic Effect and Chiral Phase Transition -
Abstract
- We study the influence of the chiral phase transition on the chiral magnetic effect. The azimuthal charge-particle correlations as functions of the temperature are calculated. It is found that there is a pronounced cusp in the correlations as the temperature reaches its critical value for the QCD phase transition. It is predicted that there will be a drastic suppression of the charge-particle correlations as the collision energy in RHIC decreases to below a critical value. We show then the azimuthal charge-particle correlations can be the signal to identify the occurrence of the QCD phase transitions in RHIC energy scan experiments.
Wei-jie Fu, Yu-xin Liu, Yue-liang Wu [pdf] DOI: 10.1088/0253-6102/55/1/23 1002.0418v1 [pdf]
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Glueballs at finite temperature in
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Abstract
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Thermal properties of glueballs in SU(3) Yang-Mills theory are investigated
in a large temperature range from $0.3T_c$ to $1.9T_c$ on anisotropic lattices.
The glueball operators are optimized for the projection of the ground states by
the variational method with a smearing scheme. Their thermal correlators are
calculated in all 20 symmetry channels. It is found in all channels that the
pole masses $M_G$ of glueballs remain almost constant when the temperature is
approaching the critical temperature $T_c$ from below, and start to reduce
gradually with the temperature going above $T_c$. The correlators in the
$0^{++}$, $0^{-+}$, and $2^{++}$ channels are also analyzed based on the
Breit-Wigner $\emph{Ansatz}$ by assuming a thermal width $\Gamma$ to the pole
mass $\omega_0$ of each thermal glueball ground state. While the values of
$\omega_0$ are insensitive to $T$ in the whole temperature range, the thermal
widths $\Gamma$ exhibit distinct behaviors at temperatures below and above
$T_c$. The widths are very small (approximately few percent of $\omega_0$ or
even smaller) when $T
-
Thermal properties of glueballs in SU(3) Yang-Mills theory are investigated
in a large temperature range from $0.3T_c$ to $1.9T_c$ on anisotropic lattices.
The glueball operators are optimized for the projection of the ground states by
the variational method with a smearing scheme. Their thermal correlators are
calculated in all 20 symmetry channels. It is found in all channels that the
pole masses $M_G$ of glueballs remain almost constant when the temperature is
approaching the critical temperature $T_c$ from below, and start to reduce
gradually with the temperature going above $T_c$. The correlators in the
$0^{++}$, $0^{-+}$, and $2^{++}$ channels are also analyzed based on the
Breit-Wigner $\emph{Ansatz}$ by assuming a thermal width $\Gamma$ to the pole
mass $\omega_0$ of each thermal glueball ground state. While the values of
$\omega_0$ are insensitive to $T$ in the whole temperature range, the thermal
widths $\Gamma$ exhibit distinct behaviors at temperatures below and above
$T_c$. The widths are very small (approximately few percent of $\omega_0$ or
even smaller) when $T
Xiang-Fei Meng, Gang Li, Ying Chen, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Jian-Bo Zhang Journal reference: Phys.Rev.D80:114502,2009 [pdf] DOI: 10.1103/PhysRevD.80.114502
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Truly random number generation based on measurement of phase noise of a laser -
Abstract
- We present a simple approach to realize truly random number generation based on measurement of the phase noise of a single mode vertical cavity surface emitting laser (VCSEL). The true randomness of the quantum phase noise originates from the spontaneous emission of photons and the random bit generation rate is ultimately limited only by the laser linewidth. With the final bit generation rate of 20 Mbit/s, the physically guaranteed truly random bit sequence passes the three standard random tests. Moreover, for the first time, a {\it continuously} generated random bit sequence up to 14 Gbit is verified by two additional criteria for its true randomness.
Hong Guo, Wenzhuo Tang, Yu Liu, Wei Wei [pdf] DOI: 10.1103/PhysRevE.81.051137 0908.2893v3 [pdf]
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Particle collisions on stringy black hole background -
Abstract
- 2009
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Quantum Zeno switch for single-photon coherent transport -
Abstract
- Using a dynamical quantum Zeno effect, we propose a general approach to control the coupling between a two-level system (TLS) and its surroundings, by modulating the energy level spacing of the TLS with a high frequency signal. We show that the TLS--surroundings interaction can be turned on or off when the ratio between the amplitude and the frequency of the modulating field is adjusted to be a zero of a Bessel function. The quantum Zeno effect of the TLS can also be observed by the vanishing of the photon reflection at these zeros. Based on these results, we propose a quantum switch to control the transport of a single photon in a 1D waveguide. Our analytical results agree well with numerical results using Floquet theory.
Lan Zhou, S. Yang, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 80, 062109 (2009) [pdf] DOI: 10.1103/PhysRevA.80.062109
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Area Spectrum of the Large AdS Black Hole from Quasinormal Modes -
Abstract
- Using the new physical interpretation of quasinormal modes proposed by Maggiore, we calculate the area and entropy spectra for the 3-dimensioal and 5-dimensional large AdS black holes. The spectra are obtained by imposing the Bohr-Sommerfeld quantization condition to the adiabatic invariant quantity. With this semiclassical method, we find that the spacings of the area and entropy spectra are equidistant and independent of the AdS radius of the black hole for both the cases. However, the spacings of the spectra are not the same for different dimension of space-time. The equidistant area spectra will be broken when the black hole has other parameters (i.e., charge and angular momentum) or in a non-Einstein's gravity theory.
Shao-Wen Wei, Yu-Xiao Liu [pdf]
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Vacuum pseudoscalar susceptibility -
Abstract
- We derive a novel model-independent result for the pion susceptibility in QCD via the isovector-pseudoscalar vacuum polarisation. In the neighbourhood of the chiral-limit, the pion susceptibility can be expressed as a sum of two independent terms. The first expresses the pion-pole contribution. The second is identical to the vacuum chiral susceptibility, which describes the response of QCD's ground-state to a fluctuation in the current-quark mass. In this result one finds a straightforward explanation of a mismatch between extant estimates of the pion susceptibility.
Lei Chang, Yu-Xin Liu, Craig D. Roberts, Yuan-Mei Shi, Wei-Min Sun, Hong-Shi Zong Journal reference: Phys.Rev.C81:032201,2010 [pdf] DOI: 10.1103/PhysRevC.81.032201
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Cold quarks in medium: An equation of state -
Abstract
- We derive a compact, semi-algebraic expression for the cold quark matter equation of state (EoS) in a covariant model that exhibits coincident deconfinement and chiral symmetry restoring transitions in-medium. Along the way we obtain algebraic expressions for: the number- and scalar-density distributions in both the confining Nambu and deconfined Wigner phases; and the vacuum-pressure difference between these phases, which defines a bag constant. The confining interaction materially alters the distribution functions from those of a Fermi gas and consequently has a significant impact on the model's thermodynamic properties, which is apparent in the EoS.
Thomas Klahn, Craig D. Roberts, Lei Chang, Huan Chen, Yu-Xin Liu Journal reference: Phys.Rev.C82:035801,2010 [pdf] DOI: 10.1103/PhysRevC.82.035801
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Anomaly analysis of Hawking radiation from Kaluza–Klein black hole with squashed horizon -
Abstract
- Considering gravitational and gauge anomalies at the horizon, a new method that to derive Hawking radiations from black holes has been developed by Wilczek et al. In this paper, we apply this method to non-rotating and rotating Kaluza-Klein black holes with squashed horizon, respectively. For the rotating case, we found that, after the dimensional reduction, an effective U(1) gauge field is generated by an angular isometry. The results show that the gauge current and energy-momentum tensor fluxes are exactly equivalent to Hawking radiation from the event horizon.
Shao-Wen Wei, Ran Li, Yu-Xiao Liu, Ji-Rong Ren Journal reference: Eur. Phys. J. C65:281, 2010; Eur.Phys.J.C65:281-287,2010 [pdf] DOI: 10.1140/epjc/s10052-009-1203-z
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Plasmon-phonon strongly coupled mode in epitaxial graphene -
Abstract
- We report the dispersion measurements, using angle-resolved reflection electron-energy-loss-spectroscopy (AREELS), on two-dimensional (2D) plasmons in single and multilayer graphene which couple strongly to surface optical phonon (FK phonon) modes of silicon carbide substrate. The coupled modes show discrete dispersion behaviors in the single and bilayer graphene. With increasing graphene layers on SiC(0001), a transition from plasmon-like dispersion to phonon-like dispersion is observed. For plasmon-like modes, the dispersion is strongly damped by electron-hole pair excitations at entering single-particle continuum, while phonon-like mode is undamped. In the region free of coupling, the graphene 2D plasmon exhibits acoustic behavior with linear dispersion with slope and damping determined by the Fermi surface topology.
Yu Liu, Roy F. Willis [pdf] DOI: 10.1103/PhysRevB.81.081406 0910.2735v1 [pdf]
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Exponential quadratic operators and evolution of bosonic systems coupled to a heat bath -
Abstract
- Using exponential quadratic operators, we present a general framework for studying the exact dynamics of system-bath interaction in which the Hamiltonian is described by the quadratic form of bosonic operators. To demonstrate the versatility of the approach, we study how the environment affects the squeezing of quadrature components of the system. We further propose that the squeezing can be enhanced when parity kicks are applied to the system.
Xiao-Tong Ni, Yu-Xi Liu, L C Kwek, Xiang-Bin Wang [pdf] DOI: 10.1103/PhysRevA.81.062125 0910.1001v1 [pdf]
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Effective Hamiltonian approach to the Kerr nonlinearity in an optomechanical system -
Abstract
- Using the Born-Oppenheimer approximation, we derive an effective Hamiltonian for an optomechanical system that leads to a nonlinear Kerr effect in the system's vacuum. The oscillating mirror at one edge of the optomechanical system induces a squeezing effect in the intensity spectrum of the cavity field. A near-resonant laser field is applied at the other edge to drive the cavity field, in order to enhance the Kerr effect. We also propose a quantum-nondemolition-measurement setup to monitor a system with two cavities separated by a common oscillating mirror, based on our effective Hamiltonian approach.
Z. R. Gong, H. Ian, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 80, 065801 (2009) [pdf] DOI: 10.1103/PhysRevA.80.065801
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Fermion resonances on multi-field thick branes -
Abstract
- Motivated by the recent work on the fermion resonances on scalar-constructed thick branes (arXiv:0901.3543 and arXiv:0904.1785), we extend the idea to multi-scalar generated thick branes and complete previous work. The fermion localization and resonances on the three-field and two-field thick branes are investigated. With the Numerov method, our numerical results show that the resonance states also exist in the brane besides the single-field thick branes and the two-field thick branes in the former cases. This interesting phenomenon is related to the internal structure of the brane and the coupling of fermions and scalars. We find that the Kaluza-Klein chiral decomposition of massive fermion resonances is the parity-chiral decomposition. For the couplings $\eta\bar{\Psi}\phi^{k}\chi\rho\Psi$ and $\eta\bar{\Psi}\phi^{k}\chi\rho\Psi$ for three-field and two-field models with odd positive $k$, respectively, the number of the resonant states decreases with $k$. This result is opposite to the one obtained in the single-field de Sitter thick brane (arXiv:0904.1785).
Yu-Xiao Liu, Hai-Tao Li, Zhen-Hua Zhao, Jing-Xin Li, Ji-Rong Ren Journal reference: JHEP0910:091,2009 [pdf] DOI: 10.1088/1126-6708/2009/10/091
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Localization and mass spectra of fermions on symmetric and asymmetric thick branes -
Abstract
- A three-parameter (positive odd integer $s$, thickness factor $\lambda$, and asymmetry factor $a$) family of asymmetric thick brane solutions in five dimensions were constructed from a two-parameter ($s$ and $\lambda$) family of symmetric ones in [R. Guerrero, R.O. Rodriguez, and R. Torrealba, Phys. Rev. D \textbf{72}, 124012 (2005).]. The values $s=1$ and $s\geq3$ correspond to single branes and double branes, respectively. These branes have very rich inner structure. In this paper, by presenting the mass-independent potentials of Kaluza--Klein (KK) modes in the corresponding Schr\"{o}dinger equations, we investigate the localization and mass spectra of fermions on the symmetric and asymmetric thick branes in an AdS background. In order to analyze the effect of gravity-fermion interaction (i.e., the effect of the inner structure of the branes) and scalar-fermion interaction to the spectrum of fermion KK modes, we consider three kinds of typical kink-fermion couplings. The spectra of left chiral fermions for these couplings are consisted of a bound zero mode and a series of gapless continuous massive KK modes, some discrete bound KK modes including zero mode (exist mass gaps) and a series of continuous massive KK modes, infinite discrete bound KK modes, respectively. The structures of the spectra are investigated in detail.
Yu-Xiao Liu, Chun-E Fu, Li Zhao, Yi-Shi Duan Journal reference: Phys.Rev.D80:065020,2009 [pdf] DOI: 10.1103/PhysRevD.80.065020
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Fermion localization and resonances on a de Sitter thick brane -
Abstract
- In arXiv:0901.3543, the simplest Yukawa coupling $\eta\bar{\Psi}\phi\chi\Psi$ was considered for a two-scalar-generated Bloch brane model. Fermionic resonances for both chiralities were obtained, and their appearance is related to branes with internal structure. Inspired on this result, we investigate the localization and resonance spectrum of fermions on a one-scalar-generated $dS$ thick brane with a class of scalar-fermion couplings $\eta\bar{\Psi}\phi^k\Psi$ with positive odd integer $k$. A set of massive fermionic resonances for both chiralities are obtained when provided large couple constant $\eta$. We find that the masses and life-times of left and right chiral resonances are almost the same, which demonstrates that it is possible to compose massive Dirac fermions from the left and right chiral resonances. The resonance with lower mass has longer life-time. For a same set of parameters, the number of resonances increases with $k$ and the life-time of the lower level resonance for larger $k$ is much longer than the one for smaller $k$.
Yu-Xiao Liu, Jie Yang, Zhen-Hua Zhao, Chun-E Fu, Yi-Shi Duan Journal reference: Phys.Rev.D80:065019,2009 [pdf] DOI: 10.1103/PhysRevD.80.065019
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Slow Light of an Amplitude-Modulated Gaussian Pulse in Cesium Vapor -
Abstract
- Slow light of an amplitude modulated Gaussian (AMG) pulse in cesium vapor is demonstrated and studied, as an appropriate amplitude modulation to a single pulse can expand its spectrum and thus increase the utilization efficiency of the bandwidth of a slow light system. In a single-$\Lambda$ type electromagnetically induced transparency (EIT) system, the slowed AMG pulse experiences severe distortion, mainly owing to the frequency dependent transmission of medium. Additionally, due to its spectral distribution, the frequency dependent dispersion of the medium causes simultaneous slow and fast light of different spectral components and thus a certain dispersive distortion of the AMG pulse. Further, a post-processing method is proposed to recover the slowed (distorted) pulse, which indicates that by introducing a linear optical system with a desired gain spectrum we can recover the pulse in an "all-optical" way. Finally, we discuss the limitations during this compensation procedure in detail. Although it is demonstrated in the cesium vapor using EIT, this method should be applicable to a wide range of slow light systems.
Wenzhuo Tang, Bin Luo, Yu Liu, Hong Guo [pdf] DOI: 10.1109/JQE.2009.2034122 0904.1779v3 [pdf]
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Perfect function transfer and interference effects in interacting boson lattices -
Abstract
- We show how to perfectly transfer, without state initialization and remote collaboration, arbitrary functions in interacting boson lattices. We describe a possible implementation of state transfer through bosonic atoms trapped in optical lattices or polaritons in on-chip coupled cavities. Significantly, a family of Hamiltonians, both linear and nonlinear, is found which are related to the Bose-Hubbard model and that enable the perfect transfer of arbitrary functions. It is shown that the state transfer between two sites in two-dimensional lattices can result in quantum interference due to the different numbers of intermediate sites in different paths. The signature factor in nuclear physics can be useful to characterize this quantum interference.
Lian-Ao Wu, Adam Miranowicz, XiangBin Wang, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 80, 012332 (2009) [pdf] DOI: 10.1103/PhysRevA.80.012332
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Quantum switch for single-photon transport in a coupled superconducting transmission-line-resonator array -
Abstract
- We propose and study an approach to realize quantum switch for single-photon transport in a coupled superconducting transmission line resonator (TLR) array with one controllable hopping interaction. We find that the single-photon with arbitrary wavevector can transport in a controllable way in this system. We also study how to realize controllable hopping interaction between two TLRs via a superconducting quantum interference device (SQUID). When the frequency of the SQUID is largely detuned from those of the two TLRs, the variables of the SQUID can be adiabatically eliminated and thus a controllable interaction between two TLRs can be obtained.
Jie-Qiao Liao, Jin-Feng Huang, Yu-xi Liu, Le-Man Kuang, C. P. Sun Journal reference: Phys. Rev. A 80, 014301 (2009) [pdf] DOI: 10.1103/PhysRevA.80.014301
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Explicit field realizations of
-
Abstract
- The fact that certain non-linear $W_{2,s}$ algebras can be linearized by the inclusion of a spin-1 current can provide a simple way to realize $W_{2,s}$ algebras from linear $W_{1,2,s}$ algebras. In this paper, we first construct the explicit field realizations of linear $W_{1,2,s}$ algebras with double-scalar and double-spinor, respectively. Then, after a change of basis, the realizations of $W_{2,s}$ algebras are presented. The results show that all these realizations are Romans-type realizations.
Shao-Wen Wei, Yu-Xiao Liu, Li-Jie Zhang, Ji-Rong Ren Journal reference: Phys.Rev.D79:126011,2009 [pdf] DOI: 10.1103/PhysRevD.79.126011
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Seebeck coefficient of thermoelectric molecular junctions: First-principles calculations -
Abstract
- A first-principles approach is presented for the thermoelectricity in molecular junctions formed by a single molecule contact. The study investigates the Seebeck coefficient considering the source-drain electrodes with distinct temperatures and chemical potentials in a three-terminal geometry junction. We compare the Seebeck coefficient in the amino-substituted and unsubstituted butanethiol junction and observe interesting thermoelectric properties in the amino-substituted junction. Due to the novel states around the Fermi levels introduced by the amino-substitution, the Seebeck coefficient could be easily modulated by using gate voltages and biases. When the temperature in one of the electrodes is fixed, the Seebeck coefficient varies significantly with the temperature in the other electrode, and such dependence could be modulated by varying the gate voltages. As the biases increase, richer features in the Seebeck coefficient are observed, which are closely related to the transmission functions in the vicinity of the left and right Fermi levels.
Yu-Shen Liu, Yu-Chang Chen Journal reference: Phys. Rev B 79, 193101 (2009) [pdf] DOI: 10.1103/PhysRevB.79.193101
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Low-energy
-
Abstract
- Low-energy scattering of $D^*$ and $D_1$ meson are studied using quenched lattice QCD with improved lattice actions on anisotropic lattices. The calculation is performed within L\"uscher's finite-size formalism which establishes the relation between the scattering phase in the infinite volume and the exact energy level in the finite volume. The threshold scattering parameters, namely the scattering length $a_0$ and the effective range $r_0$, for the s-wave scattering in $J^P=0^-$ channel are extracted. After the chiral and continuum extrapolations, we obtain: $a_0=2.52(47)$fm and $r_0=0.7(1)$fm where the errors are purely statistical. Based on these results, we discuss the possibility of a shallow bound state for the two charmed mesons within the non-relativistic potential scattering model. It is argued that, albeit the interaction between the two charmed mesons being attractive, it is unlikely that they can form a shallow bound state in this channel. This calculation provides some useful information on the nature of the newly discovered resonance-like structure $Z^+(4430)$ by the Belle Collaboration.
Guo-Zhan Meng, Ming Gong, Ying Chen, Song He, Gang Li, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Xiang-Fei Meng, Zhi-Yuan Niu, Yan Shen, Jian-Bo Zhang, Yuan-Jiang Zhang Journal reference: Phys.Rev.D80:034503,2009 [pdf] DOI: 10.1103/PhysRevD.80.034503
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Anomaly analysis of Hawking radiation from 2+1 dimensional spinning
black hole -
Abstract
- Considering gravitational and gauge anomalies at the horizon, a new successful method that to derive Hawking radiations from black holes has been developed recently by Wilczek et al.. By using the dimensional reduction technique, we apply this method to a non-vacuum solution, the 2+1 dimensional spinning black hole. The Hawking temperature and angular velocity on the horizon are obtained. The results may partially imply that this method is independent of the gravity theory, the dimension of spacetime and the topological structure of the event horizon.
Shao-Wen Wei, Ran Li, Yu-Xiao Liu, Ji-Rong Ren [pdf]
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Physical states and BRST operators for higher-spin W strings -
Abstract
- In this paper, we mainly investigate the $W_{2,s}^{M}\otimes W_{2,s}^{L}$ system, in which the matter and the Liouville subsystems generate $W_{2,s}^{M}$ and $W_{2,s}^L$ algebras respectively. We first give a brief discussion of the physical states for corresponding $W$ stings. The lower states are given by freezing the spin-2 and spin-$s$ currents. Then, introducing two pairs of ghost-like fields, we give the realizations of $W_{1,2,s}$ algebras. Based on these linear realizations, BRST operators for $W_{2,s}$ algebras are obtained. Finally, we construct new BRST charges of Liouville system for $W_{2,s}^{L}$ strings at the specific values of central charges $c$: $c=-{22/5}$ for $W_{2,3}^{L}$ algebra, $c=-24$ for $W_{2,4}^{L}$ algebra and $c=-2,-{286/3}$ for $W_{2,6}^{L}$ algebra, at which the corresponding $W_{2,s}^L$ algebras are singular.
Yu-Xiao Liu, Shao-Wen Wei, Li-Jie Zhang, Ji-Rong Ren Journal reference: Eur.Phys.J.C60:675-684,2009 [pdf] DOI: 10.1140/epjc/s10052-009-0958-6
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Mesonic excitations and pi--pi scattering lengths at finite temperature
in the two-flavor Polyakov--Nambu--Jona-Lasinio model -
Abstract
- The mesonic excitations and s-wave pi--pi scattering lengths at finite temperature are studied in the two-flavor Polyakov--Nambu--Jona-Lasinio (PNJL) model. The masses of pi-meson and sigma-meson, pion-decay constant, the pion-quark coupling strength, and the scattering lengths $a_{0}$ and $a_{2}$ at finite temperature are calculated in the PNJL model with two forms of Polyakov-loop effective potential. The obtained results are almost independent of the choice of the effective potentials. The calculated results in the PNJL model are also compared with those in the conventional Nambu--Jona-Lasinio model and indicate that the effect of color confinement screens the effect of temperature below the critical one in the PNJL model. Furthermore, the Goldberger-Treiman relation and the Gell-Mann--Oakes--Renner relation are extended to the case at finite temperature in the PNJL model.
Wei-jie Fu, Yu-xin Liu Journal reference: Phys.Rev.D79:074011,2009 [pdf] DOI: 10.1103/=
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Do Thermoelectric Materials in Nanojunctions Display Material Property
or Junction Property? -
Abstract
- The miniaturization of thermoelectric nanojunctions raises a fundamental question: do the thermoelectric quantities of the bridging materials in nanojunctions remain to display material properties or show junction properties? In order to answer this question, we investigate the Seebeck coefficient $S$ and the thermoelectric figure of merit $ZT$ especially in relation to the length characteristics of the junctions from the first-principles approaches. For $S$, the metallic atomic chains reveal strong length characteristics related to strong hybridization in the electronic structures between the atoms and electrodes, while the insulating molecular wires display strong material properties due to the cancelation of exponential scalings in the DOSs. For $ZT$, the atomic wires remain to show strong junction properties. However, the length chrematistics of the insulation molecular wires depend on a characteristic temperature $T_{0}= \sqrt{\beta/\gamma(l)}$ around 10K. When $T \ll T_{0}$, where the electron transport dominates the thermal current, the molecular junctions remain to show material properties. When $T \gg T_{0}$, where the phonon transport dominates the thermal current, the molecular junctions display junction properties.
Yu-Chang Chen, Yu-Shen Liu [pdf]
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Quantization of black hole entropy from quasinormal modes -
Abstract
- In Phys. Rev. D 78 (2008) 104018 [arXiv:0807.1481], the conclusion that "entropy eigenvalues of GB black hole are discrete and equally spaced, but the area spacing is not equidistant" was firstly presented by Kothawala, Padmanabhan and Sarkar. In this paper, using the new physical interpretation of quasinormal modes proposed by Maggiore, we calculate the quantum spectra of entropy for various types of non-rotating black holes with no charge. The spectrum is obtained by imposing Bohr-Sommerfeld quantization condition to the adiabatic invariant quantity. We conjecture that the spacing of entropy spectrum is equidistant and is independent of the dimension of spacetime. However, the spacing of area spectrum depends on gravity theory. In Einstein's gravity, it is equally spaced, otherwise it is non-equidistant. This conjecture agrees with the result of Kothawala, Padmanabhan and Sarkar.
Shao-Wen Wei, Ran Li, Yu-Xiao Liu, Ji-Rong Ren Journal reference: JHEP0903:076,2009 [pdf] DOI: 10.1088/1126-6708/2009/03/076
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Universal existence of exact quantum state transmissions in interacting media -
Abstract
- We consider an exact state transmission, where a density matrix in one information processor A at time $t=0$ is exactly equal to that in another processor B at a later time. We demonstrate that there always exists a complete set of orthogonal states, which can be employed to perform the exact state transmission. Our result is very general in the sense that it holds for arbitrary media between the two processors and for any time interval. We illustrate our results in terms of models of spin, fermionic and bosonic chains. This complete set can be used as bases to study the perfect state transfer, which is associated with degenerated subspaces of this set of states. Interestingly, this formalism leads to a proposal of perfect state transfer via adiabatic passage, which does not depend on the specific form of the driving Hamiltonian.
Lian-Ao Wu, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 80, 042315 (2009) [pdf] DOI: 10.1103/PhysRevA.80.042315
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Thermoelectric Efficiency in Nanojunctions: A Comparison between Atomic Junctions and Molecular Junctions -
Abstract
- Using first-principles approaches, this study investigated the efficiency of energy conversion in nanojunctions, described by the thermoelectric figure of merit $ZT$. We obtained the qualitative and quantitative descriptions for the dependence of $ZT$ on temperatures and lengths. A characteristic temperature: $T_{0}= \sqrt{\beta/\gamma(l)}$ was observed. When $T\ll T_{0}$, $ZT\propto T^{2}$. When $T\gg T_{0}$, $ZT$ tends to a saturation value. The dependence of $ZT$ on the wire length for the metallic atomic chains is opposite to that for the insulating molecules: for aluminum atomic (conducting) wires, the saturation value of $ZT$ increases as the length increases; while for alkanethiol (insulating) chains, the saturation value of $ZT$ decreases as the length increases. $ZT$ can also be enhanced by choosing low-elasticity bridging materials or creating poor thermal contacts in nanojunctions. The results of this study may be of interest to research attempting to increase the efficiency of energy conversion in nano thermoelectric devices.
Yu-Shen Liu, Yi-Ren Chen, Yu-Chang Chen Journal reference: ACS Nano, 2009, 3 (11), pp 3497--3504 [pdf] DOI: 10.1021/nn900986r
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Cooling and squeezing the fluctuations of a nanomechanical beam by indirect quantum feedback control -
Abstract
- We study cooling and squeezing the fluctuations of a nanomechanical beam using quantum feedback control. In our model, the nanomechanical beam is coupled to a transmission line resonator via a superconducting quantum interference device (SQUID). The leakage of the electromagnetic field from the transmission line resonator is measured using homodyne detection. This measured signal is then used to design a quantum-feedback-control signal to drive the electromagnetic field in the transmission line resonator. Although the control is imposed on the transmission line resonator, this quantum-feedback-control signal indirectly affects the thermal motion of the nanomechanical beam via the inductive beam-resonator coupling, making it possible to cool and squeeze the fluctuations of the beam, allowing it to approach the standard quantum limit.
Jing Zhang, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 79, 052102 (2009) [pdf] DOI: 10.1103/PhysRevA.79.052102
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Probing Nano-Mechanical QED Effects -
Abstract
- We propose and study an "intrinsic probing" approach, without introducing any external detector, to mimic cavity QED effects in a qubit-nanomechanical resonator system. This metallic nanomechanical resonator can act as an intrinsic detector when a weak driving current passes through it. The nanomechanical resonator acts as both the cavity and the detector. A cavity QED-like effect is demonstrated by the correlation spectrum of the electromotive force between the two ends of the nanomechanical resonator. Using the quantum regression theorem and perturbation theory, we analytically calculate the correlation spectrum. In the weak driving limit, we study the effect on the vacuum Rabi splitting of both the strength of the driving as well as the frequency-detuning between the charge qubit and the nanomechanical resonator. Numerical calculations confirm the validity of our intrinsic probing approach.
Y. B. Gao, S. Yang, Yu-xi Liu, C. P. Sun, Franco Nori [pdf]
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Bulk matters on symmetric and asymmetric de Sitter thick branes -
Abstract
- An asymmetric thick domain wall solution with de Sitter ($dS$) expansion in five dimensions can be constructed from a symmetric one by using a same scalar (kink) with different potentials. In this paper, by presenting the mass-independent potentials of Kaluza--Klein (KK) modes in the corresponding Schr\"{o}dinger equations, we investigate the localization and mass spectra of various bulk matter fields on the symmetric and asymmetric $dS$ thick branes. It is shown that the spectrum of scalar KK modes on the symmetric $dS$ brane contains only one bound mode (the massless mode). However, for the asymmetric $dS$ brane with a large asymmetric factor $a$, there are two bound scalar KK modes: a zero mode and a massive mode. For spin 1 vectors, the spectra of KK modes on both $dS$ branes consist of a bound massless mode and a set of continuous ones, i.e., the asymmetric factor does not change the number of the bound vector KK modes. For spin 1/2 fermions with the scalar-fermion coupling $\eta\bar\Psi \sin(\phi/\phi_0)\cos^{-\delta}(\phi/\phi_0)\Psi$, there exist some discrete bound KK modes and a series of continuous ones. The asymmetric factor $a$ reduces the number of the bound fermion KK modes.
Yu-Xiao Liu, Zhen-Hua Zhao, Shao-Wen Wei, Yi-Shi Duan Journal reference: JCAP 02 (2009) 003 [pdf] DOI: 10.1088/1475-7516/2009/02/003
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Slow light of an amplitude modulated Gaussian pulse in
electromagnetically induced transparency medium -
Abstract
- The slow light effects of an amplitude modulated Gaussian (AMG) pulse in a cesium atomic vapor are presented. In a single-$\Lambda$ type electromagnetically induced transparency (EIT) medium, more severe distortion is observed for an AMG pulse than a Gaussian one. Using Fourier spectrum analysis, we find that the distortion, as well as the loss, is dominantly caused by linear absorption than dispersion. Accordingly, a compensation method is proposed to reshape the slow light pulse based on the transmission spectrum. In addition, we find a novel way to obtain simultaneous slow and fast light.
Wenzhuo Tang, Bin Luo, Yu Liu, Hong Guo [pdf]
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Generating stationary entangled states in superconducting qubits -
Abstract
- When a two-qubit system is initially maximally-entangled, two independent decoherence channels, one per qubit, would greatly reduce the entanglement of the two-qubit system when it reaches its stationary state. We propose a method on how to minimize such a loss of entanglement in open quantum systems. We find that the quantum entanglement of general two-qubit systems with controllable parameters can be protected by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity $F_{\rm max}$ between the stationary entangled state, $\rho_{\infty}$, and the maximally-entangled state, $\rho_m$, can be about $2/3\approx\max\{{\rm tr}(\rho_{\infty}\rho_m)\}=F_{\rm max}$, corresponding to a maximum stationary concurrence, $C_{\rm max}$, of about $1/3\approx C(\rho_{\infty})=C_{\rm max}$. This is significant because the quantum entanglement of the two-qubit system can be protected, even for a long time. We apply our proposal to several types of two-qubit superconducting circuits, and show how the entanglement of these two-qubit circuits can be optimized by varying experimentally-controllable parameters.
Jing Zhang, Yu-xi Liu, Chun-Wen Li, Tzyh-Jong Tarn, Franco Nori Journal reference: Phys. Rev. A 79, 052308 (2009) [pdf] DOI: 10.1103/PhysRevA.79.052308
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Efficient Quantum Circuits for One-Way Quantum Computing -
Abstract
- While Ising-type interactions are ideal for implementing controlled phase flip gates in one-way quantum computing, natural interactions between solid-state qubits are most often described by either the XY or the Heisenberg models. We show an efficient way of generating cluster states directly using either the iSWAP gate for the XY model, or the $\sqrt{\rm SWAP}$ gate for the Heisenberg model. Our approach thus makes one-way quantum computing more feasible for solid-state devices.
Tetsufumi Tanamoto, Yu-xi Liu, Xuedong Hu, Franco Nori Journal reference: Phys. Rev. Lett. 102, 100501 (2009) [pdf] DOI: 10.1103/PhysRevLett.102.100501
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Quantum Zeno switch for single-photon coherent transport -
Abstract
- 2008
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Quantum supercavity with atomic mirrors -
Abstract
- We study single-photon transport in an array of coupled microcavities where two two-level atomic systems are embedded in two separate cavities of the array. We find that a single-photon can be totally reflected by a single two-level system. However, two separate two-level systems can also create, between them, single-photon quasi-bound states. Therefore, a single two-level system in the cavity array can act as a mirror while a different type of cavity can be formed by using two two-level systems, acting as tunable "mirrors", inside two separate cavities in the array. In analogy with superlattices in solid state, we call this new "cavity inside a coupled-cavity array" a super-cavity. This supercavity is the quantum analog of Fabry-Perot interferometers. Moreover, we show that the physical properties of this quantum super-cavity can be adjusted by changing the frequencies of these two-level systems.
Lan Zhou, H. Dong, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 78, 063827 (2008) [pdf] DOI: 10.1103/PhysRevA.78.063827
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Chiral susceptibility and the scalar Ward identity -
Abstract
- The chiral susceptibility is given by the scalar vacuum polarisation at zero total momentum. This follows directly from the expression for the vacuum quark condensate so long as a nonperturbative symmetry preserving truncation scheme is employed. For QCD in-vacuum the susceptibility can rigorously be defined via a Pauli-Villars regularisation procedure. Owing to the scalar Ward identity, irrespective of the form or Ansatz for the kernel of the gap equation, the consistent scalar vertex at zero total momentum can automatically be obtained and hence the consistent susceptibility. This enables calculation of the chiral susceptibility for markedly different vertex Ansaetze. For the two cases considered, the results were consistent and the minor quantitative differences easily understood. The susceptibility can be used to demarcate the domain of coupling strength within a theory upon which chiral symmetry is dynamically broken. Degenerate massless scalar and pseudoscalar bound-states appear at the critical coupling for dynamical chiral symmetry breaking.
Lei Chang, Yu-xin Liu, Craig D. Roberts, Yuan-mei Shi, Wei-min Sun, Hong-shi Zong Journal reference: Phys.Rev.C79:035209,2009 [pdf] DOI: 10.1103/PhysRevC.79.035209
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Property of the low-lying states at the critical point of the phase
transition in U(4) vibron model -
Abstract
- We study the properties of the low-lying states at the critical point of the phase transition from U(3) to O(4) symmetry in the U(4) vibron model in detail. By analyzing the general characteristics and comparing the calculated results of the energy spectra and the E1, E2 transition rates in E(3) symmetry, in $r^4$ potential model and the finite boson number case in boson space, we find that the results in the $r^4$ potential demonstrates the characteristic of the classical limit at the critical point well and the E(3) symmetry over-predict the energy levels and under-predict the E1 and E2 transition rates of the states at the critical point. However, the E(3) symmetry may describe part of the properties of the system with boson number around 10 to 20. We also confirm that the 12C+12C system is an empirical evidence of the state at the critical point of the phase transition in the U(4) model when concerning the energies of the low-lying resonant states.
Ze-bo Li, Xia-ping Tang, Yu Zhang, Xing-chen Yang, Zhao Wang, Yu-xin Liu [pdf]
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Distinguishing Newly Born Strange Stars from Neutron Stars with
-
Abstract
- The gravity-mode (g-mode) eigenfrequencies of newly born strange quark stars (SQSs) and neutron stars (NSs) are studied. It is found that the eigenfrequencies in SQSs are much lower than those in NSs by almost one order of magnitude, since the components of a SQS are all extremely relativistic particles while nucleons in a NS are non-relativistic. We therefore propose that newly born SQSs can be distinguished from the NSs by detecting the eigenfrequencies of the g-mode pulsations of supernovae cores through gravitational radiation by LIGO-class detectors.
Wei-jie Fu, Hai-qing Wei, Yu-xin Liu Journal reference: Phys.Rev.Lett.101:181102,2008 [pdf] DOI: 10.1103/PhysRevLett.101.181102
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Revisiting the vector and axial-vector vacuum susceptibilities -
Abstract
- We re-investigate the vector and axial-vector vacuum susceptibilities by taking advantage of the vector and axial-vector Ward-Takahashi identities. We show analytically that, in the chiral limit, the vector vacuum susceptibility is zero and the axial-vector vacuum susceptibility equals three fourths of the square of the pion decay constant. Besides, our analysis reproduces the Weinberg sum rule.
Lei Chang, Yu-xin Liu, Wei-min Sun, Hong-shi Zong Journal reference: Phys.Lett.B669:327-330,2008 [pdf] DOI: 10.1016/j.physletb.2008.10.010
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Controllable Scattering of a Single Photon inside a One-Dimensional Resonator Waveguide -
Abstract
- We analyze coherent transport of photons, which propagate in a one-dimensional coupled-resonator waveguide (CRW) and are scattered by a controllable two-level system located inside the CRW. Our approach, which uses discrete coordinates, unifies "low" and "high" energy effective theories for single photon scattering. We show that the controllable two-level system can behave as a quantum switch for the coherent transport of photons. This study may inspire new electro-optical single-photon quantum devices. We also suggest an experimental setup based on superconducting transmission line resonators and qubits
Lan Zhou, Z. R. Gong, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. Lett 101, 100501 (2008) [pdf] DOI: 10.1103/PhysRevLett.101.100501
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Magnetic Vortex Line Configuration of Faddeev-Niemi Knot -
Abstract
- This paper has been withdrawn by the author due to the inaccurate result.
Yi-Shi Duan, Li-Da Zhang, Yu-Xiao Liu [pdf]
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The modified London equation, Abrikosov-like vortices and knot solitons in two-gap superconductors -
Abstract
- We derive the exact modified London equation for the two-gap superconductor, compare it with its single-gap counterpart. We show that the vortices in the two-gap superconductor are soft (or continuous) core vortices. In particular, we discuss the topological structure of the finite energy vortices (Abrikosov-like vortices), and find that they can be viewed as the incarnation of the baby skyrmion stretched in the third direction. Besides, we point out that the knot soliton in the two-gap superconductor is the twisted Abrikosov-like vortex with its two periodic ends connected smoothly. The relation between the magnetic monopoles and the Abrikosov-like vortices is also discussed briefly.
Li-Da Zhang, Yi-Shi Duan, Yu-Xiao Liu Journal reference: J. Phys. A: Math. Theor. 41 (2008) 375201 [pdf] DOI: 10.1088/1751-8113/41/37/375201
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Magnetic monopoles in ferromagnetic spin-triplet superconductors -
Abstract
- Using the $\phi$-mapping method, we argue that ferromagnetic spin-triplet superconductors allow formation of unstable magnetic monopoles. In particular, we show that the limit points and the bifurcation points of the $\phi$-mapping will serve as the interaction points of these magnetic monopoles.
Li-Da Zhang, Yi-Shi Duan, Yu-Xiao Liu Journal reference: J. Phys.: Condens. Matter 20 (2008) 235219 [pdf] DOI: 10.1088/0953-8984/20/23/235219
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SELF-DUAL VORTICES IN THE ABELIAN CHERN–SIMONS MODEL WITH TWO COMPLEX SCALAR FIELDS -
Abstract
- Making use of $\phi$-mapping topological current method, we discuss the self-dual vortices in the Abelian Chern-Simons model with two complex scalar fields. For each scalar field, an exact nontrivial equation with a topological term which is missing in many references is derived analytically. The general angular momentum is obtained. The magnetic flux which relates the two scalar fields is calculated. Furthermore, we investigate the vortex evolution processes, and find that because of the present of the vortex molecule, these evolution processes is more complicated than the vortex evolution processes in the corresponding single scalar field model.
Yi-Shi Duan, Li-Da Zhang, Yu-Xiao Liu Journal reference: Mod.Phys.Lett.A23:2189-2198,2008 [pdf] DOI: 10.1142/S0217732308026273
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A NEW DESCRIPTION OF COSMIC STRINGS IN BRANE WORLD SCENARIO -
Abstract
- In the light of $\phi$-mapping topological current theory, the structure of cosmic strings are obtained from the Abelian Higgs model, which is an effective description to the brane world cosmic string system. In this topological description of the cosmic string, combining the result of decomposition of U(1) gauge potential, we analytically reach the familiar conclusions that in the brane world scenario the magnetic flux of the cosmic string is quantized and the RR charge of it is screened.
Yi-Shi Duan, Li-Da Zhang, Yu-Xiao Liu Journal reference: Mod.Phys.Lett.A23:2023-2030,2008 [pdf] DOI: 10.1142/S021773230802611X
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Fermions on thick branes in the background of sine-Gordon kinks -
Abstract
- A class of thick branes in the background of sine-Gordon kinks with a scalar potential $V(\phi)=p(1+\cos\frac{2\phi}{q})$ was constructed by R. Koley and S. Kar [Classical Quantum Gravity \textbf{22}, 753 (2005)]. In this paper, in the background of the warped geometry, we investigate the issue of localization of spin half fermions on these branes in the presence of two types of scalar-fermion couplings: $\eta\bar{\Psi}\phi\Psi$ and $\eta\bar{\Psi}\sin\phi \Psi$. By presenting the mass-independent potentials in the corresponding Schr\"{o}dinger equations, we obtain the lowest Kaluza--Klein (KK) modes and a continuous gapless spectrum of KK states with $m^2>0$ for both types of couplings. For the Yukawa coupling $\eta\bar{\Psi}\phi\Psi$, the effective potential of the right chiral fermions for positive $q$ and $\eta$ is always positive, hence only the effective potential of the left chiral fermions could trap the corresponding zero mode. This is a well-known conclusion which had been discussed extensively in the literature. However, for the coupling $\eta\bar{\Psi}\sin\phi \Psi$, the effective potential of the right chiral fermions for positive $q$ and $\eta$ is no longer always positive. Although the value of the potential at the location of the brane is still positive, it has a series of wells and barriers on each side, which ensures that the right chiral fermion zero mode could be trapped. Thus we may draw the remarkable conclusion: for positive $\eta$ and $q$, the potentials of both the left and right chiral fermions could trap the corresponding zero modes under certain restrictions.
Yu-Xiao Liu, Li-Da Zhang, Li-Jie Zhang, Yi-Shi Duan Journal reference: Phys.Rev.D78:065025,2008 [pdf] DOI: 10.1103/PhysRevD.78.065025
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Localization and mass spectrum of matters on Weyl thick branes -
Abstract
- In this paper, we study localization and mass spectrum of various matter fields on a family of thick brane configurations in a pure geometric Weyl integrable 5-dimensional space time, a non-Riemannian modification of 5-dimensional Kaluza--Klein (KK) theory. We present the shape of the mass-independent potential of the corresponding Schr\"{o}dinger problem and obtain the KK modes and mass spectrum, where a special coupling of spinors and scalars is considered for fermions. It is shown that, for a class of brane configurations, there exists a continuum gapless spectrum of KK modes with any $m^2>0$ for scalars, vectors and ones of left chiral and right chiral fermions. All of the corresponding massless modes are found to be normalizable on the branes. However, for a special of brane configuration, the corresponding effective Schr\"{o}dinger equations have modified P\"{o}schl-Teller potentials. These potentials suggest that there exist mass gap and a series of continuous spectrum starting at positive $m^2$. There are one bound state for spin one vectors, which is just the normalizable vector zero mode, and two bound KK modes for scalars. The total number of bound states for spin half fermions is determined by the coupling constant $\eta$. In the case of no coupling ($\eta=0$), there are no any localized fermion KK modes including zero modes for both left and right chiral fermions. For positive (negative) coupling constant, the number of bound states of right chiral fermions is one less (more) than that of left chiral fermions. In both cases ($\eta>0$ and $\eta<0$), only one of the zero modes for left chiral fermions and right chiral fermions is bound and normalizable.
Yu-Xiao Liu, Li-Da Zhang, Shao-Wen Wei, Yi-Shi Duan Journal reference: JHEP 0808:041,2008 [pdf] DOI: 10.1088/1126-6708/2008/08/041
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LOCALIZATION OF FERMIONIC FIELDS ON BRANEWORLDS WITH BULK TACHYON MATTER -
Abstract
- Recently, Pal and Skar in [arXiv:hep-th/0701266] proposed a mechanism to arise the warped braneworld models from bulk tachyon matter, which are endowed with a thin brane and a thick brane. In this framework, we investigate localization of fermionic fields on these branes. As in the 1/2 spin case, the field can be localized on both the thin and thick branes with inclusion of scalar background. In the 3/2 spin extension, the general supergravity action coupled to chiral supermultiplets is considered to produce the localization on both the branes as a result.
Xin-Hui Zhang, Yu-Xiao Liu, Yi-Shi Duan Journal reference: Mod.Phys.Lett.A23:2093-2101,2008 [pdf] DOI: 10.1142/S0217732308026133
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Efficient purification protocols using
-
Abstract
- We show an efficient purification protocol in solid-state qubits by replacing the usual bilateral CNOT gate by the bilateral iSWAP gate. We also show that this replacement can be applied to breeding and hashing protocols, which are useful for quantum state purification. These replacements reduce the number of fragile and cumbersome two-qubit operations, making more feasible quantum-information-processing with solid-state qubits. As examples, we also present quantitative analyses for the required time to perform state purification using either superconducting or semiconducting qubits.
Tetsufumi Tanamoto, Koji Maruyama, Yu-xi Liu, Xuedong Hu, Franco Nori Journal reference: Phys. Rev. A 78, 062313 (2008) [pdf] DOI: 10.1103/PhysRevA.78.062313
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Chemical potential and the gap equation -
Abstract
- In general the kernel of QCD's gap equation possesses a domain of analyticity upon which the equation's solution at nonzero chemical potential is simply obtained from the in-vacuum result through analytic continuation. On this domain the single-quark number- and scalar-density distribution functions are mu-independent. This is illustrated via two models for the gap equation's kernel. The models are alike in concentrating support in the infrared. They differ in the form of the vertex but qualitatively the results are largely insensitive to the Ansatz. In vacuum both models realise chiral symmetry in the Nambu-Goldstone mode and in the chiral limit, with increasing chemical potential, exhibit a first-order chiral symmetry restoring transition at mu~M(0), where M(p^2) is the dressed-quark mass function. There is evidence to suggest that any associated deconfinement transition is coincident and also of first-order.
Huan Chen, Wei Yuan, Lei Chang, Yu-Xin Liu, Thomas Klahn, Craig D. Roberts Journal reference: Phys.Rev.D78:116015,2008 [pdf] DOI: 10.1103/PhysRevD.78.116015
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Spinor field realizations of the half-integer strings -
Abstract
- The grading Becchi-Rouet-Stora-Tyutin (BRST) method gives a way to construct the integer $W_{2,s}$ strings, where the BRST charge is written as $Q_B=Q_0+Q_1$. Using this method, we reconstruct the nilpotent BRST charges $Q_{0}$ for the integer $W_{2,s}$ strings and the half-integer $W_{2,s}$ strings. Then we construct the exact grading BRST charge with spinor fields and give the new realizations of the half-integer $W_{2,s}$ strings for the cases of $s=3/2$, 5/2, and 7/2.
Shao-Wen Wei, Yu-Xiao Liu, Li-Jie Zhang, Ji-Rong Ren Journal reference: Nucl.Phys.B809:426-438,2009 [pdf] DOI: 10.1016/j.nuclphysb.2008.08.027
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FERMIONS IN GRAVITY AND GAUGE BACKGROUNDS ON A BRANE WORLD -
Abstract
- We solve the fermionic zero modes in gravity and gauge backgrounds on a brane involving a warped geometry, and study the localization of spin 1/2 fermionic field on the brane world. The result is that there exist massless spin 1/2 fermions which can be localized on the bulk with the exponentially decreasing warp factor if including U(1) gauge background. Two special cases of gauge backgrounds on the extra dimensional manifold are discussed.
Li Zhao, Yu-Xiao Liu, Yi-Shi Duan Journal reference: Mod.Phys.Lett.A23:1129-1139,2008 [pdf] DOI: 10.1142/S0217732308025796
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Observational Test of Coronal Magnetic Field Models. I. Comparison with Potential Field Model -
Abstract
- Recent advances have made it possible to obtain two-dimensional line-of-sight magnetic field maps of the solar corona from spectropolarimetric observations of the Fe XIII 1075 nm forbidden coronal emission line. Together with the linear polarization measurements that map the azimuthal direction of the coronal magnetic field, these coronal vector magnetograms now allow for direct observational testing of theoretical coronal magnetic field models. This paper presents a study testing the validity of potential-field coronal magnetic field models. We constructed a theoretical coronal magnetic field model of active region AR 10582 observed by the SOLARC coronagraph in 2004 by a global potential field extrapolation of the synoptic map of Carrington Rotation 2014. Synthesized linear and circular polarization maps from thin layers of the coronal magnetic field model above the active region along the line of sight are compared with the observed maps. We found that reasonable agreement occurs from layers located just above the sunspot of AR 10582, near the plane of the sky. This result provides the first observational evidence that potential field extrapolation can yield a reasonable approximation of the magnetic field configuration of the solar corona for simple and stable active regions.
Yu Liu, Haosheng Lin [pdf] DOI: 10.1086/588645 0710.3223v2 [pdf]
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A general scheme for multiparty controlled quantum teleportation of an
arbitrary N-particle state -
Abstract
- There is much interest in the multiparty quantum communications where quantum teleportation using high dimensional entangled quantum channel is one of the promising tools. In this paper, we propose a more general scheme for M-party controlled teleportation of an arbitrary N-particle quantum state using N-1 identical Einstein-Podolsky-Rosen pairs and one (M+2)-particle Greenberger-Horne-Zeilinger state together as quantum channel. Based on which a 2-party controlled teleportation of an arbitrary 3-particle state is tested with our scheme as an example.
Ye Jun, Yue Li, Yu Liu, Sha Hua [pdf]
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Cavity optomechanical coupling assisted by an atomic gas -
Abstract
- We theoretically study a cavity filled with atoms, which provides the optical-mechanical interaction between the modified cavity photonic field and a movable mirror at one end. We show that the cavity field ``dresses'' these atoms, producing two types of polaritons, effectively enhancing the radiation pressure of the cavity field upon the end mirror, as well as establishing an additional squeezing mode of the end mirror. This squeezing produces an adiabatic entanglement, which is absent in usual vacuum cavities, between the oscillating mirror and the rest of the system. We analyze the entanglement and quantify it using the Loschmidt echo and fidelity.
H. Ian, Z. R. Gong, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 78, 013824 (2008) [pdf] DOI: 10.1103/PhysRevA.78.013824
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Localization of matters on pure geometrical thick branes -
Abstract
- In the literatures, several types of thick smooth brane configurations in a pure geometric Weyl integrable 5-dimensional space time have been presented. The Weyl geometry is a non-Riemannian modification of 5-dimensional Kaluza--Klein (KK) theory. All these thick brane solutions preserve 4-dimensional Poincar\'e invariance, and some of them break $Z_2$--symmetry along the extra dimension. In this paper, we study localization of various matter fields on these pure geometrical thick branes, which also localize the graviton. We present the shape of the potential of the corresponding Schr$\mathrm{\ddot{o}}$dinger problem and obtain the lowest KK mode. It is showed that, for both spin 0 scalars and spin 1 vectors, there exists a continuum gapless spectrum of KK states with $m^2>0$. But only the massless mode of scalars is found to be normalizable on the brane. However, for the massless left or right chiral fermion localization, there must be some kind of Yukawa coupling. For a special coupling, there exist a series of discrete massive KK modes with $m^2 >0$. It is also showed that for a given coupling constant only one of the massless chiral modes is localized on the branes.
Yu-Xiao Liu, Xin-Hui Zhang, Li-Da Zhang, Yi-Shi Duan Journal reference: JHEP 0802:067,2008 [pdf] DOI: 10.1088/1126-6708/2008/02/067
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Simultaneous Cooling of an Artificial Atom and Its Neighboring Quantum System -
Abstract
- We propose an approach for cooling both an artificial atom (e.g., a flux qubit) and its neighboring quantum system, the latter modeled by either a quantum two-level system or a quantum resonator. The flux qubit is cooled by manipulating its states, following an inverse process of state population inversion, and then the qubit is switched on to resonantly interact with the neighboring quantum system. By repeating these steps, the two subsystems can be simultaneously cooled. Our results show that this cooling is robust and effective, irrespective of the chosen quantum systems connected to the qubit.
J. Q. You, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. Lett. 100, 047001 (2008) [pdf] DOI: 10.1103/PhysRevLett.100.047001
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Testing power-law cosmology with galaxy clusters -
Abstract
- Power-law cosmologies, in which the cosmological scale factor evolves as a power law in the age, $a \propto t^{\alpha}$ with $\alpha \ga 1$, regardless of the matter content or cosmological epoch, is comfortably concordant with a host of cosmological observations.} {In this article, we use recent measurements of the X-ray gas mass fractions in clusters of galaxies to constrain the $\alpha$ parameter with curvature $k = \pm1, 0$. We find that the best fit happens for an open scenario with the power index $\alpha = 1.14 \pm 0.05$, though the flat and closed model can not be rule out at very high confidence level.} {Our results are in agreement with other recent analyses and show that the X-ray gas mass fraction measurements in clusters of galaxies provide a complementary test to the power law cosmology.
Zong-Hong Zhu, Ming Hu, J. S. Alcaniz, Yu-Xing Liu Journal reference: Astro.Astrophys. 483 (2008) 15-18 [pdf] DOI: 10.1051/0004-6361:20077797
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Chiral susceptibility and chiral phase transition in Nambu–Jona-Lasinio model -
Abstract
- We study the solutions of the gap equation, the thermodynamic potential and the chiral susceptibility in and beyond the chiral limit at finite chemical potential in the Nambu--Jona-Lasinio (NJL) model. We give an explicit relation between the chiral susceptibility and the thermodynamic potential in the NJL model. We find that the chiral susceptibility is a quantity being able to represent the furcation of the solutions of the gap equation and the concavo-convexity of the thermodynamic potential in NJL model. It indicates that the chiral susceptibility can identify the stable state and the possibility of the chiral phase transition in NJL model.
Yue Zhao, Lei Chang, Wei Yuan, Yu-xin Liu Journal reference: European Physical Journal C 56: 483-492 (2008) [pdf] DOI: 10.1140/epjc/s10052-008-0673-8
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Quantum supercavity with atomic mirrors -
Abstract
- 2007
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Control of photon propagation via electromagnetically induced transparency in lossless media -
Abstract
- We study the influence of a lossless material medium on the coherent storage and quantum state transfer of a quantized probe light in an ensemble of $\Lambda $-type atoms. The medium is modeled as uniformly distributed two-level atoms with same energy level spacing, coupling to a probe light. This coupled system can be simplified to a collection of two-mode polaritons which couple to one transition of the $\Lambda$-type atoms. We show that, when the other transition of $\Lambda$-type atoms is controlled by a classical light, the electromagnetically induced transparency can also occur for the polaritons. In this case the coherent storage and quantum transfer for photon states are achievable through the novel dark states with respect to the polaritons. By calculating the corresponding dispersion relation, we find the ensemble of the three-level atoms with $\Lambda$-type transitions may serve as quantum memory for it slows or even stops the light propagation through the mechanism of electromagnetically induced transparency. the corresponding dispersion relation, we find the ensemble of the three-level atoms with $\Lambda$-type transitions may serve as quantum memory for it slows or even stops the light propagation through the mechanism of electromagnetically induced transparency.
Liang He, Yu-xi Liu, S. Yi, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 75, 063818 (2007) [pdf] DOI: 10.1103/PhysRevA.75.063818
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SELF-DUAL VORTICES IN THE FRACTIONAL QUANTUM HALL SYSTEM -
Abstract
- Based on the $\phi$-mapping theory, we obtain an exact Bogomol'nyi self-dual equation with a topological term, which is ignored in traditional self-dual equation, in the fractional quantum Hall system. It is revealed that there exist self-dual vortices in the system. We investigate the inner topological structure of the self-dual vortices and show that the topological charges of the vortices are quantized by Hopf indices and Brouwer degrees. Furthermore, we study the branch processes in detail. The vortices are found generating or annihilating at the limit points and encountering, splitting or merging at the bifurcation points of the vector field $\vec\phi$.
Xin-Hui Zhang, Yi-Shi Duan, Yu-Xiao Liu, Li Zhao Journal reference: Int.J.Mod.Phys.B00:1-11,2009 [pdf] DOI: 10.1142/S0217979209052480
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Rigorous description of exchange-correlation energy of many-electron
systems -
Abstract
- With the eigenfunctional theory, we study a general interacting electron system, and give a rigorous expression of its ground state energy which is composed of two parts, one part is contributed by the non-interacting electrons, and another one is represented by the correlation functions that are controlled by the electron correlation. Moreover, according to the rigorous expression of the ground state energy, an effective method beyond the local density approximation of the density functional theory is proposed.
Yu-Liang Liu [pdf]
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DETECTING EXTRA DIMENSION BY HELIUM-LIKE IONS -
Abstract
- Considering that gravitational force might deviate from Newton's inverse-square law and become much stronger in small scale, we present a method to detect the possible existence of extra dimensions in the ADD model. By making use of an effective variational wave function, we obtain the nonrelativistic ground energy of a helium atom and its isoelectronic sequence. Based on these results, we calculate gravity correction of the ADD model. Our calculation may provide a rough estimation about the magnitude of the corresponding frequencies which could be measured in later experiments.
Yu-Xiao Liu, Xin-Hui Zhang, Yi-Shi Duan Journal reference: Mod.Phys.Lett.A23:1853-1860,2008 [pdf] DOI: 10.1142/S0217732308026029
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Superconducting qubits can be coupled and addressed as trapped ions -
Abstract
- Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting qubits (SCQs) which is very similar to the successful one now being used for trapped ions. The SCQs are coupled to the "vibrational" mode provided by a superconducting LC circuit or its equivalent (e.g., a SQUID). Both single-qubit rotations and qubit-LC-circuit couplings/decouplings can be controlled by the frequencies of the time-dependent magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the LC circuit, can be selectively performed, and the information transfer can be realized in a controllable way.
Yu-xi Liu, L. F. Wei, J. R. Johansson, J. S. Tsai, Franco Nori Journal reference: Phys. Rev. B 76, 144518 (2007) [pdf] DOI: 10.1103/PhysRevB.76.144518
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SPIE Proceedings -
Abstract
- We present a quantum secure direct communication(QSDC) scheme as an extension for a proposed supervised secure entanglement sharing protocol. Starting with a quick review on the supervised entanglement sharing protocol -- the "Wuhan" protocol [Y. Li and Y. Liu, arXiv:0709.1449v2], we primarily focus on its further extend using for a QSDC task, in which the communication attendant Alice encodes the secret message directly onto a sequence of 2-level particles which then can be faithfully teleported to Bob using the shared maximal entanglement states obtained by the previous "Wuhan" protocol. We also evaluate the security of the QSDC scheme, where an individual self-attack performed by Alice and Bob -- the out of control attack(OCA) is introduced and the robustness of our scheme on the OCA is documented.
Yue Li, Yu Liu [pdf] DOI: 10.1117/12.755810 0711.2827v1 [pdf]
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Supervised secure entanglement sharing for faithful quantum
teleportation via tripartite W states -
Abstract
- We present a supervised secure entanglement sharing protocol via tripartite W states for faithful quantum teleportation. By guaranteeing a secure entanglement distribution in the charge of a third believed supervisor, quantum information of an unknown state of a 2-level particle can be faithfully teleported from the sender to the remote receiver via the Bell states distilled from the tripartite W states. We emphasize that reliable teleportation after our protocol between two communication parties depends on the agreement of the supervisor to cooperate via taking the W states as both the quantum channel and eavesdropping detector. The security against typical individual eavesdropping attacks is proved and its experimental feasibility is briefly illustrated.
Yue Li, Yu Liu [pdf]
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Double Electromagnetically Induced Transparency in a Tripod-type Atom
System -
Abstract
- The electromagnetically induced transparency (EIT) phenomenon in a four level atomic system with tripod configuration is studied. The results show that this configuration is equivalent to the combination of two single three-level $\Lambda$ configurations, which, under certain conditions, results in the so-called double-EIT (DEIT) phenomenon. The properties of the double transparency windows for DEIT are discussed in detail and the possible experimental scheme is proposed.
Xiao Li, Yu Liu, Hong Guo [pdf]
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Dyson–Schwinger equation and quantum phase transitions in massless QCD -
Abstract
- We study the stability of the highest symmetric solution (Wigner-solution) of Dyson-Schwinger equations in chiral limit and at zero temperature. Our results confirm that if the chemical potential is not very large, the QCD vacuum is in the chiral symmetry breaking phase and the quantum phase-transition of the chiral symmetry restoration is in first order. Meanwhile it seems that there is not competition between chiral symmetry breaking phase and color superconductivity phase since the color superconductivity phase appears only if the chemical potential is very large. Moreover, we propose that chiral symmetry breaking arises from the positive feedback with respect to the mass perturbation.
Wei Yuan, Huan Chen, Yu-xin Liu Journal reference: Phys.Lett.B637:69-74,2006 [pdf] DOI: 10.1016/j.physletb.2006.03.076
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Rotation-driven prolate-to-oblate shape phase transition in 190W: A projected shell model study -
Abstract
- A shape phase transition is demonstrated to occur in 190W by applying the Projected Shell Model, which goes beyond the usual mean-field approximation. Rotation alignment of neutrons in the high-j, i_{13/2} orbital drives the yrast sequence of the system, changing suddenly from prolate to oblate shape at angular momentum 10$\hbar$. We propose observables to test the picture.
Yang Sun, Philip M. Walker, Fu-Rong Xu, Yu-Xin Liu Journal reference: Phys.Lett.B659:165-169,2008 [pdf] DOI: 10.1016/j.physletb.2007.10.067
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Cooling a micromechanical beam by coupling it to a transmission line -
Abstract
- We study a method to cool down the vibration mode of a micro-mechanical beam using a capacitively-coupled superconducting transmission line. The Coulomb force between the transmission line and the beam is determined by the driving microwave on the transmission line and the displacement of the beam. When the frequency of the driving microwave is smaller than that of the transmission line resonator, the Coulomb force can oppose the velocity of the beam. Thus, the beam can be cooled. This mechanism, which may enable to prepare the beam in its quantum ground state of vibration, is feasible under current experimental conditions.
Fei Xue, Y. D. Wang, Yu-xi Liu, Franco Nori Journal reference: PHYSICAL REVIEW B 76, 205302 (2007) [pdf] DOI: 10.1103/PhysRevB.76.205302
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Abstract
- We extend the Polyakov-loop improved Nambu--Jona-Lasinio (PNJL) model to 2+1 flavor case to study the chiral and deconfinement transitions of strongly interacting matter at finite temperature and nonzero chemical potential. The Polyakov-loop, the chiral susceptibility of light quarks (u and d) and the strange quark number susceptibility as functions of temperature at zero chemical potential are determined and compared with the recent results of Lattice QCD simulations. We find that there is always an inflection point in the curve of strange quark number susceptibility accompanying the appearance of the deconfinement phase, which is consistent with the result of Lattice QCD simulations. Predictions for the case at nonzero chemical potential and finite temperature are made as well. We give the phase diagram in terms of the chemical potential and temperature and find that the critical endpoint (CEP) moves down to low temperature and finally disappears with the decrease of the strength of the 't Hooft flavor-mixing interaction.
Wei-jie Fu, Zhao Zhang, Yu-xin Liu Journal reference: Phys.Rev.D77:014006,2008 [pdf] DOI: 10.1103/PhysRevD.77.014006
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Fermionic Zero Modes in Gauge and Gravity Backgrounds on a Sphere -
Abstract
- In this letter we study fermionic zero modes in gauge and gravity backgrounds taking a two dimensional compact manifold $S^2$ as extra dimensions. The result is that there exist massless Dirac fermions which have normalizable zero modes under quite general assumptions about these backgrounds on the bulk. Several special cases of gauge background on the sphere are discussed and some simple fermionic zero modes are obtained.
Yu-Xiao Liu, Li-Jie Zhang, Yi-Shi Duan Journal reference: Commun.Theor.Phys.49:1577-1579,2008 [pdf] DOI: 10.1088/0253-6102/49/6/47
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Fermionic zero modes in self-dual vortex background on a torus -
Abstract
- We study fermionic zero modes in the self-dual vortex background on an extra two-dimensional Riemann surface in 5+1 dimensions. Using the generalized Abelian Higgs model, we obtain the inner topological structure of the self-dual vortex and establish the exact self-duality equation with topological term. Then we analyze the Dirac operator on an extra torus and the effective Lagrangian of four-dimensional fermions with the self-dual vortex background. Solving the Dirac equation, the fermionic zero modes on a torus with the self-dual vortex background in two simple cases are obtained.
Yu-Xiao Liu, Yong-Qiang Wang, Yi-Shi Duan [pdf]
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Energy-level shifts of a stationary hydrogen atom in a static external gravitational field with Schwarzschild geometry -
Abstract
- The first order perturbations of the energy levels of a stationary hydrogen atom in static external gravitational field, with Schwarzschild metric, are investigated. The energy shifts are calculated for the relativistic 1S, 2S, 2P, 3S, 3P, 3D, 4S, 4P, 4D and 4F levels. The results show that the energy-level shifts of the states with total angular momentum quantum number 1/2 are all zero, and the ratio of absolute energy shifts with total angular momentum quantum number 5/2 is 1:4:5. This feature can be used to help us to distinguish the gravitational effect from other effect.
Zhen-Hua Zhao, Yu-Xiao Liu, Xi-Guo Li Journal reference: Phys.Rev.D76:064016,2007 [pdf] DOI: 10.1103/PhysRevD.76.064016
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Teleportation of qubit states through dissipative channels: Conditions for surpassing the no-cloning limit -
Abstract
- We investigate quantum teleportation through dissipative channels and calculate teleportation fidelity as a function of damping rates. It is found that the average fidelity of teleportation and the range of states to be teleported depend on the type and rate of the damping in the channel. Using the fully entangled fraction, we derive two bounds on the damping rates of the channels: one is to beat the classical limit and the second is to guarantee the non-existence of any other copy with better fidelity. Effect of the initially distributed maximally entangled state on the process is presented; and the concurrence and the fully entangled fraction of the shared states are discussed. We intend to show that prior information on the dissipative channel and the range of qubit states to be teleported is helpful for the evaluation of the success of teleportation, where success is defined as surpassing the fidelity limit imposed by the fidelity of 1-to-2 optimal cloning machine for the specific range of qubits.
Sahin Kaya Ozdemir, Karol Bartkiewicz, Yu-xi Liu, Adam Miranowicz Journal reference: Phys. Rev. A 76, 042325 (2007) [pdf] DOI: 10.1103/PhysRevA.76.042325
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The total energy-momentum of the universe in teleparallel gravity -
Abstract
- We investigate the conservation law of energy-momentum in teleparallel gravity by using general Noether theorem. The energy-momentum current has also superpotential and is therefore identically conserved. The total energy-momentum, which includes the contributions of both matter and gravitational fields, is given by the integral of scalar densities over a three-dimensional spacelike hypersurface. As an example, the universe in teleparallel gravity is investigated. It is shown that the total energy-momentum vanishes independently of both the curvature parameter and the three dimensionless coupling constants of teleparallel gravity.
Yu-Xiao Liu, Zhen-Hua Zhao, Jie Yang, Yi-Shi Duan [pdf]
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Flavor symmetry breaking and meson masses -
Abstract
- The axial-vector Ward-Takahashi identity is used to derive mass formulae for neutral pseudoscalar mesons. Flavour symmetry breaking entails non-ideal flavour content for these states. Adding that the \eta^\prime is not a Goldstone mode, exact chiral-limit relations are developed from the identity. They connect the dressed-quark propagator to the topological susceptibility. It is confirmed that in the chiral limit the \eta^\prime mass is proportional to the matrix element which connects this state to the vacuum via the topological susceptibility. The implications of the mass formulae are illustrated using an elementary dynamical model, which includes an Ansatz for that part of the Bethe-Salpeter kernel related to the non-Abelian anomaly. In addition to the current-quark masses, the model involves two parameters, one of which is a mass-scale. It is employed in an analysis of pseudoscalar- and vector-meson bound-states. While the effects of SU(N_f=2) and SU(N_f=3) flavour symmetry breaking are emphasised, the five-flavour spectra are described. Despite its simplicity, the model is elucidative and phenomenologically efficacious; e.g., it predicts \eta-\eta^\prime mixing angles of ~ (-15 degrees) and \pi^0-\eta angles of ~ 1 degree.
Mandar S. Bhagwat, Lei Chang, Yu-Xin Liu, Craig D. Roberts, Peter C. Tandy Journal reference: Phys.Rev.C76:045203,2007 [pdf] DOI: 10.1103/PhysRevC.76.045203
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ENERGY–MOMENTUM FOR RANDALL–SUNDRUM MODELS -
Abstract
- We investigate the conservation law of energy-momentum for Randall-Sundrum models by the general displacement transform. The energy-momentum current has a superpotential and are therefore identically conserved. It is shown that for Randall-Sundrum solution, the momentum vanishes and most of the bulk energy is localized near the Planck brane. The energy density is $\epsilon = \epsilon_0 e^{-3k \mid y \mid}$.
Yu-Xiao Liu, Li-Jie Zhang, Yong-Qiang Wang, Yi-Shi Duan Journal reference: Mod.Phys.Lett.A23:769-779,2008 [pdf] DOI: 10.1142/S0217732308024110
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Two-mode squeezed states and entangled states of two mechanical resonators -
Abstract
- We study a device consisting of a dc-SQUID with two sections of its loop acting as two mechanical resonators. An analog of the parametric down-conversion process in quantum optics can be realized with this device. We show that a two-mode squeezed state can be generated for two overdamped mechanical resonators, where the damping constants of the two mechanical resonators are larger than the coupling strengths between the dc-SQUID and the two mechanical resonators. Thus we show that entangled states of these two mechanical resonators can be generated.
Fei Xue, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. B 76, 064305 (2007) [pdf] DOI: 10.1103/PhysRevB.76.064305
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Hadron scattering in an asymmetric box* -
Abstract
- We propose to study hadron-hadron scattering using lattice QCD in an asymmetric box which allows one to access more non-degenerate low-momentum modes for a given volume. The conventional L\"{u}scher's formula applicable in a symmetric box is modified accordingly. To illustrate the feasibility of this approach, pion-pion elastic scattering phase shifts in the I=2, J=0 channel are calculated within quenched approximation using improved gauge and Wilson fermion actions on anisotropic lattices in an asymmetric box. After the chiral and continuum extrapolation, we find that our quenched results for the scattering phase shifts in this channel are consistent with the experimental data when the three-momentum of the pion is below 300MeV. Agreement is also found when compared with previous theoretical results from lattice and other means. Moreover, with the usage of asymmetric volume, we are able to compute the scattering phases in the low-momentum range (pion three momentum less than about 350MeV in the center of mass frame) for over a dozen values of the pion three-momenta, much more than using the conventional symmetric box with comparable volume.
Xin Li, Ying Chen, Guo-Zhan Meng, Xu Feng, Ming Gong, Song He, Gang Li, Chuan Liu, Yu-Bin Liu, Jian-Ping Ma, Xiang-Fei Meng, Yan Shen, Jian-Bo Zhang Journal reference: JHEP 0706:053,2007 [pdf] DOI: 10.1088/1126-6708/2007/06/053
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Localization of Gravitino on Braneworlds in Six Dimensions -
Abstract
- This paper has been withdrawn by the author due to the unsuitable content.
Yu-Xiao Liu, Zhen-Hua Zhao, Shao-Feng Wu, Yi-Shi Duan [pdf]
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Fermions in self-dual vortex background on a string-like defect -
Abstract
- By using the self-dual vortex background on extra two-dimensional Riemann surfaces in 5+1 dimensions, the localization mechanism of bulk fermions on a string-like defect with the exponentially decreasing warp-factor is obtained. We give the conditions under which localized spin 1/2 and 3/2 fermions can be obtained.
Yu-Xiao Liu, Li Zhao, Xin-Hui Zhang, Yi-Shi Duan Journal reference: Nucl.Phys.B785:234-245,2007 [pdf] DOI: 10.1016/j.nuclphysb.2007.05.018
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Gravitational Corrections to Energy-Levels of a Hydrogen Atom -
Abstract
- The first order perturbations of the energy levels of a hydrogen atom in central internal gravitational field are investigated. The internal gravitational field is produced by the mass of the atomic nucleus. The energy shifts are calculated for the relativistic 1S, 2S, 2P, 3S, 3P, 3D, 4S and 4P levels with Schwarzschild metric. The calculated results show that the gravitational corrections are sensitive to the total angular momentum quantum number.
Zhen-Hua Zhao, Yu-Xiao Liu, Xi-Guo Li Journal reference: Commun.Theor.Phys.47:658-662,2007 [pdf] DOI: 10.1088/0253-6102/47/4/018
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Coupling of pion condensate, chiral condensate, and Polyakov loop in an extended Nambu–Jona-Lasinio model -
Abstract
- The Nambu Jona-Lasinio model with a Polyakov loop is extended to finite isospin chemical potential case, which is characterized by simultaneous coupling of pion condensate, chiral condensate and Polyakov loop. The pion condensate, chiral condensate and the Polyakov loop as functions of temperature and isospin chemical potential are investigated by minimizing the thermodynamic potential of the system. The resulting $(T,\mu_I)$ phase diagram is studied with emphasis on the critical point and Polyakov loop dynamics. The tricritical point for the pion superfluidity phase transition is confirmed and the phase transition for isospin symmetry restoration in high isospin chemical potential region perfectly coincides with the crossover phase transition for Polyakov loop. These results are in agreement with the Lattice QCD data.
Zhao Zhang, Yu-Xin Liu Journal reference: Phys.Rev.C75:064910,2007 [pdf] DOI: 10.1103/PhysRevC.75.064910
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Localization of fermions on a string-like defect -
Abstract
- We study localization of bulk fermions on a string-like defect with the exponentially decreasing warp factor in six dimensions with inclusion of U(1) gauge background from the viewpoint of field theory, and give the conditions under which localized spin 1/2 and 3/2 fermions can be obtained.
Yu-Xiao Liu, Li Zhao, Yi-Shi Duan Journal reference: JHEP 0704:097,2007 [pdf] DOI: 10.1088/1126-6708/2007/04/097
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Switchable coupling between charge and flux qubits -
Abstract
- We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective inter-qubit coupling controlled by the external magnetic flux. This switchable inter-qubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.
Xiao-Ling He, J. Q. You, Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Phys. Rev. B 76, 024517 (2007) [pdf] DOI: 10.1103/PhysRevB.76.024517
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Fermion absorption cross section and topology of spherically symmetric black holes -
Abstract
- In 1997, Liberati and Pollifrone in Phys. Rev. D56 (1997) 6458 (hep-th/9708014) achieved a new formulation of the Bekenstein-Hawking formula, where the entropy and the Euler characteristic are related by $S=\chi A/8$. In this work we present a relation between the low-energy absorption cross section for minimally coupled fermions and the Euler characteristic of (3+1)-dimensional spherically symmetric black holes, i.e. $\sigma =\chi g_h^{-1}A$. Based on the relation, using the Gauss--Bonnet--Chern theorem and the $\phi$-mapping method, an absorption cross section density is introduced to describe the topology of the absorption cross section. It is shown that the absorption cross section and its density are determined by the singularities of the timelike Killing vector field of the spacetime and these singularities carry the topological numbers, Hopf indices and Brouwer degrees, naturally.
Yu-Xiao Liu, Li Zhao, Zhen-Bin Cao, Yi-Shi Duan Journal reference: Phys.Lett.B650:286-292,2007 [pdf] DOI: 10.1016/j.physletb.2007.04.001
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Quantum thermodynamic cycles and quantum heat engines -
Abstract
- In order to describe quantum heat engines, here we systematically study isothermal and isochoric processes for quantum thermodynamic cycles. Based on these results the quantum versions of both the Carnot heat engine and the Otto heat engine are defined without ambiguities. We also study the properties of quantum Carnot and Otto heat engines in comparison with their classical counterparts. Relations and mappings between these two quantum heat engines are also investigated by considering their respective quantum thermodynamic processes. In addition, we discuss the role of Maxwell's demon in quantum thermodynamic cycles. We find that there is no violation of the second law, even in the existence of such a demon, when the demon is included correctly as part of the working substance of the heat engine.
H. T. Quan, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. E 76. 031105 (2007) [pdf] DOI: 10.1103/PhysRevE.76.031105
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Topological Excitation in Skyrme Theory -
Abstract
- Based on the $\phi$-mapping topological current theory and the decomposition of gauge potential theory, we investigate knotted vortex lines and monopoles in Skyrme theory and simply discuss the branch processes (splitting, merging and intersection) during the evolution of the monopoles.
Yi-Shi Duan, Xin-Hui Zhang, Yu-Xiao Liu Journal reference: Commun.Theor.Phys. 47 (2007) 85-88 [pdf] DOI: 10.1088/0253-6102/47/1/017
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Variable-frequency-controlled coupling in charge qubit circuits: Effects of microwave field on qubit-state readout -
Abstract
- To implement quantum information processing, microwave fields are often used to manipulate superconuducting qubits. We study how the coupling between superconducting charge qubits can be controlled by variable-frequency magnetic fields. We also study the effects of the microwave fields on the readout of the charge-qubit states. The measurement of the charge-qubit states can be used to demonstrate the statistical properties of photons.
Xiao-Ling He, Yu-xi Liu, J. Q. You, Franco Nori Journal reference: Phys. Rev. A 76, 022317 (2007) [pdf] DOI: 10.1103/PhysRevA.76.022317
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Two-flavor QCD phases and condensates at finite isospin chemical potential -
Abstract
- We study the phase structure and condensates of two-flavor QCD at finite isospin chemical potential in the framework of a confining, Dyson-Schwinger equation model. We find that the pion superfluidity phase is favored at high enough isospin chemical potential. A new gauge invariant mixed quark-gluon condensate induced by isospin chemical potential is proposed based on Operator Product Expansion. We investigate the sign and magnitude of this new condensate and show that it's an important condensate in QCD sum rules at finite isospin density.
Zhao Zhang, Yu-xin Liu Journal reference: Phys.Rev.C75:035201,2007 [pdf] DOI: 10.1103/PhysRevC.75.035201
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Quantum Entanglement and Teleportation of Quantum-Dot States in Microcavities -
Abstract
- Generation and control of quantum entanglement are studied in an equivalent-neighbor system of spatially-separated semiconductor quantum dots coupled by a single-mode cavity field. Generation of genuinely multipartite entanglement of qubit states realized by conduction-band electron-spin states in quantum dots is discussed. A protocol for quantum teleportation of electron-spin states via cavity decay is briefly described.
A. Miranowicz, S. K. Ozdemir, Yu-xi Liu, G. Chimczak, M. Koashi, N. Imoto Journal reference: e-J. Surf. Sci. Nanotech. 5, 51-59 (2007). [pdf] DOI: 10.1380/ejssnt.2007.51
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Persistent single-photon production by tunable on-chip micromaser with a superconducting quantum circuit -
Abstract
- We propose a tunable on-chip micromaser using a superconducting quantum circuit (SQC). By taking advantage of externally controllable state transitions, a state population inversion can be achieved and preserved for the two working levels of the SQC and, when needed, the SQC can generate a single photon. We can regularly repeat these processes in each cycle when the previously generated photon in the cavity is decaying, so that a periodic sequence of single photons can be produced persistently. This provides a controllable way for implementing a persistent single-photon source on a microelectronic chip.
J. Q. You, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. B 75, 104516 (2007). [pdf] DOI: 10.1103/PhysRevB.75.104516
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ANGULAR MOMENTUM CONSERVATION LAW FOR RANDALL–SUNDRUM MODELS -
Abstract
- In Randall-Sundrum models, by the use of general Noether theorem, the covariant angular momentum conservation law is obtained with the respect to the local Lorentz transformations. The angular momentum current has also superpotential and is therefore identically conserved. The space-like components $J_{ij}$ of the angular momentum for Randall-Sundrum models are zero. But the component $J_{04}$ is infinite.
Yu-Xiao Liu, Yi-Shi Duan, Li-Jie Zhang Journal reference: Mod.Phys.Lett.A22:2855-2864,2007 [pdf] DOI: 10.1142/S0217732307023365
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Control of photon propagation via electromagnetically induced transparency in lossless media -
Abstract
- 2006
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Knotted solitons in a charged two-condensate Bose system -
Abstract
- By making use of the decomposition of U(1) gauge potential theory and the \phi mapping method, we propose that a charged two-condensate Bose system possesses vortex lines and two classes of knotted solitons. The topological charges of the vortex lines are characterized by the Hopf indices and the Brower degrees of \phi-mapping, and the knotted solitons are described by the nontrivial Hopf invariant and the BF action, respectively.
Yi-Shi Duan, Xin-Hui Zhang, Yu-Xiao Liu, Li Zhao Journal reference: PHYSICAL REVIEW B 74, 144508 (2006) [pdf] DOI: 10.1103/PhysRevB.74.144508
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New Spinor Field Realizations of the Non-Critical
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Abstract
- We investigate the new spinor field realizations of the $W_{3}$ algebra, making use of the fact that the $W_{3}$ algebra can be linearized by the addition of a spin-1 current. We then use these new realizations to build the nilpotent Becchi-Rouet-Stora--Tyutin (BRST) charges of the spinor non-critical $W_{3}$ string.
Li-Jie Zhang, Yu-Xiao Liu, Ji-Rong Ren Journal reference: Chin. Phys. Lett. 23 (2006) 797-799 [pdf] DOI: 10.1088/0256-307X/23/4/010
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Switchable resonant coupling of flux qubits -
Abstract
- We propose a coupling scheme, where two or more flux qubits with different eigenfrequencies share Josephson junctions with a coupler loop devoid of its own quantum dynamics. Switchable two-qubit coupling is realized by tuning the frequency of the AC magnetic flux through the coupler to a combination frequency of two of the qubits. The coupling allows any or all of the qubits to be simultaneously at the degeneracy point and can change sign.
M. Grajcar, Yu-xi Liu, Franco Nori, A. M. Zagoskin Journal reference: Phys. Rev. B 74, 172505 (2006) [pdf] DOI: 10.1103/PhysRevB.74.172505
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Dynamical chiral symmetry breaking and a critical mass -
Abstract
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On a bounded, measurable domain of non-negative current-quark mass, realistic
models of QCD's gap equation can simultaneously admit two inequivalent
dynamical chiral symmetry breaking (DCSB) solutions and a solution that is
unambiguously connected with the realisation of chiral symmetry in the Wigner
mode. The Wigner solution and one of the DCSB solutions are destabilised by a
current-quark mass and both disappear when that mass exceeds a critical value.
This critical value also bounds the domain on which the surviving DCSB solution
possesses a chiral expansion. This value can therefore be viewed as an upper
bound on the domain within which a perturbative expansion in the current-quark
mass around the chiral limit is uniformly valid for physical quantities. For a
pseudoscalar meson constituted of equal mass current-quarks, it corresponds to
a mass m_{0^-}~0.45GeV. In our discussion we employ properties of the two DCSB
solutions of the gap equation that enable a valid definition of
in the presence of a nonzero current-mass. The behaviour of this condensate indicates that the essentially dynamical component of chiral symmetry breaking decreases with increasing current-quark mass.
-
On a bounded, measurable domain of non-negative current-quark mass, realistic
models of QCD's gap equation can simultaneously admit two inequivalent
dynamical chiral symmetry breaking (DCSB) solutions and a solution that is
unambiguously connected with the realisation of chiral symmetry in the Wigner
mode. The Wigner solution and one of the DCSB solutions are destabilised by a
current-quark mass and both disappear when that mass exceeds a critical value.
This critical value also bounds the domain on which the surviving DCSB solution
possesses a chiral expansion. This value can therefore be viewed as an upper
bound on the domain within which a perturbative expansion in the current-quark
mass around the chiral limit is uniformly valid for physical quantities. For a
pseudoscalar meson constituted of equal mass current-quarks, it corresponds to
a mass m_{0^-}~0.45GeV. In our discussion we employ properties of the two DCSB
solutions of the gap equation that enable a valid definition of
Lei Chang, Yu-Xin Liu, Mandar S. Bhagwat, Craig D. Roberts, Stewart V. Wright Journal reference: Phys.Rev.C75:015201,2007 [pdf] DOI: 10.1103/PhysRevC.75.015201
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Collective Shape-Phases of Interacting Fermion Systems -
Abstract
- A microscopic theory is presented for identifying shape-phase structures and transitions in interacting fermion systems. The method provides a microscopic description for collective shape-phases, and reveals detailed dependence of such shape-phases on microscopic interaction strengths. The theory is generally applicable to fermion systems such as nuclei, quarks, and in particular trapped cold atoms, where shape-phases may be observed and investigated in a controlled manner.
Yu-xin Liu, Zhan-feng Hou, Yu Zhang, Haiqing Wei [pdf]
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Chemical potential dependence of chiral quark condensate in Dyson–Schwinger equation approach of QCD -
Abstract
- We propose a chemical potential dependent effective gluon propagator and study the chiral quark condensate in strongly interacting matter in the framework of Dyson-Schwinger equation approach. The obtained results manifest that, as the effect of the chemical potential on the effective gluon propagator is taken into account, the chiral quark condensate decreases gradually with the increasing of the chemical potential if it is less than the critical value, and the condensate vanishes suddenly at the critical chemical potential. The inclusion of the chemical potential in the effective gluon propagator enhances the decreasing rate and decreases the critical chemical potential. It indicates that the chiral symmetry can be restored completely at a critical chemical potential and restored partially as the chemical potential is less than the critical value. If the effective gluon propagator is independent of the chemical potential, the chiral symmetry can only be restored suddenly but no gradual restoration.
Lei Chang, Huan Chen, Bin Wang, Wei Yuan, Yu-xin Liu Journal reference: Phys.Lett.B644:315-321,2007 [pdf] DOI: 10.1016/j.physletb.2006.11.073
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Probing Tiny Motions of Nanomechanical Resonators: Classical or Quantum Mechanical? -
Abstract
- We propose a spectroscopic approach to probe tiny vibrations of a nanomechanical resonator (NAMR), which may reveal classical or quantum behavior depending on the decoherence-inducing environment. Our proposal is based on the detection of the voltage-fluctuation spectrum in a superconducting transmission line resonator (TLR), which is {\it indirectly} coupled to the NAMR via a controllable Josephson qubit acting as a quantum transducer. The classical (quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark shifts of the qubit levels, which can be measured by the voltage fluctuations in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical or not, could be probed by detecting the voltage-fluctuation spectrum of the TLR.
L. F. Wei, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. Lett. 97, 237201 (2006) [pdf] DOI: 10.1103/PhysRevLett.97.237201
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Classical Nature of the Evolution of Dark Energy Density -
Abstract
- By ignoring the local density fluctuations, we construct an uniform Higgs-field's (inflaton's) quantum theory with varying effective Planck constant ($\hbar_{v}(t) \propto R(t)^{-3}$) for the evolution of the dark energy density during the epoch after inflation. With presumable sufficient inflation in the very early period (time-scale is $t_{inf}$), so that $\hbar_{v}\to 0$, the state of universe decomposes into some decoherent components, which could be the essential meaning of phase transition, and each of them could be well described by classical mechanics for an inharmonic oscillator in the corresponding potential-well with a viscous force. We find that the cosmological constant at present is $\Lambda_{now}\approx 2.05\times 10^{-3}$ eV, which is almost independent of the choice of potential for inflaton, and agrees excellently with the recent observations. In addition, we find that, during the cosmic epoch after inflation, the dark energy is almost conserved as well as the matter's energy, therefore the "why now" problem can be avoided.
Wei Yuan, Yu-xin Liu [pdf]
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REMARK ON THE CONSISTENCY OF THE LADDER APPROXIMATION AND THE RAINBOW APPROXIMATION OF DYSON–SCHWINGER EQUATIONS OF QCD -
Abstract
- We study the consistency of the ladder approximation and the rainbow approximation of the Dyson-Schwinger equation of QCD. By considering the non-Abelian property of QCD, we show that the QED-type Ward-Takahashi identity is not required for the rainbow-ladder approximation of QCD. It indicates that there does not exists any internal inconsistency in the usual rainbow-ladder approximation of QCD. In addition, we propose an modified ladder approximation which guarantees the Slavnov-Taylor identity for the quark-gluon vertex omitting the ghost effect in the approximation.
Lei Chang, Yu-xin Liu Journal reference: Int.J.Mod.Phys.A23:1711-1717,2008 [pdf] DOI: 10.1142/S0217751X08039645
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Scalable superconducting qubit circuits using dressed states -
Abstract
- We study a coupling/decoupling method between a superconducting qubit and a data bus that uses a controllable time-dependent electromagnetic field (TDEF). As in recent experiments, the data bus can be either an LC circuit or a cavity field. When the qubit and the data bus are initially fabricated, their detuning should be made far larger than their coupling constant, so these can be treated as two independent subsystems. However, if a TDEF is applied to the qubit, then a "dressed qubit" (i.e., qubit plus the electromagnetic field) can be formed. By choosing appropriate parameters for the TDEF, the dressed qubit can be coupled to the data bus and, thus, the qubit and the data bus can exchange information with the assistance of the TDEF. This mechanism allows the scalability of the circuit to many qubits. With the help of the TDEF, any two qubits can be selectively coupled to (and decoupled from) a common data bus. Therefore, quantum information can be transferred from one qubit to another.
Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. A 74, 052321 (2006) [pdf] DOI: 10.1103/PhysRevA.74.052321
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Maxwell’s Demon Assisted Thermodynamic Cycle in Superconducting Quantum Circuits -
Abstract
- We propose a quantum analog of the internal combustion engine used in most cars. Specifically, we study how to implement the Otto-type quantum heat engine (QHE) with the assistance of a Maxwell's demon. Three steps are required: thermalization, quantum measurement, and quantum feedback controlled by the Maxwell demon. We derive the positive-work condition of this composite QHE. Our QHE can be constructed using superconducting quantum circuits. We explicitly demonstrate the essential role of the demon in this macroscopic QHE.
H. T. Quan, Y. D. Wang, Yu-xi Liu, C. P. Sun, Franco Nori Journal reference: Phys. Rev. Lett. 97, 180402 (2006) [pdf] DOI: 10.1103/PhysRevLett.97.180402
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Producing Cluster States in Charge Qubits and Flux Qubits -
Abstract
- We propose a method to efficiently generate cluster states in charge qubits, both semiconducting and superconducting, as well as flux qubits. We show that highly-entangled cluster states can be realized by a `one-touch' entanglement operation by tuning gate bias voltages for charge qubits. We also investigate the robustness of these cluster states for non-uniform qubits, which are unavoidable in solid-state systems. We find that quantum computation based on cluster states is a promising approach for solid-state qubits.
Tetsufumi Tanamoto, Yu-xi Liu, Shinobu Fujita, Xuedong Hu, Franco Nori Journal reference: Phys. Rev. Lett. 97, 230501(2006) [pdf] DOI: 10.1103/PhysRevLett.97.230501
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Generation and Control of Greenberger-Horne-Zeilinger Entanglement in Superconducting Circuits -
Abstract
- Going beyond the entanglement of microscopic objects (such as photons, spins, and ions), here we propose an efficient approach to produce and control the quantum entanglement of three macroscopic coupled superconducting qubits. By conditionally rotating, one by one, selected Josephson charge qubits, we show that their Greenberger-Horne-Zeilinger (GHZ) entangled states can be deterministically generated. The existence of GHZ correlations between these qubits could be experimentally demonstrated by effective single-qubit operations followed by high-fidelity single-shot readouts. The possibility of using the prepared GHZ correlations to test the macroscopic conflict between the noncommutativity of quantum mechanics and the commutativity of classical physics is also discussed.
L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Physical Review Letters, 96, 246803 (2006) [pdf] DOI: 10.1103/PhysRevLett.96.246803
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Correlated photons and collective excitations of a cyclic atomic ensemble -
Abstract
- We systematically study the interaction between two quantized optical fields and a cyclic atomic ensemble driven by a classic optical field. This so-called atomic cyclic ensemble consists of three-level atoms with Delta-type transitions due to the symmetry breaking, which can also be implemented in the superconducting quantum circuit by Yu-xi Liu et al. [Phys. Rev. Lett. 95, 087001 (2005)]. We explore the dynamic mechanisms to creating the quantum entanglements among photon states, and between photons and atomic collective excitations by the coherent manipulation of the atom-photon system. It is shown that the quantum information can be completely transferred from one quantized optical mode to another, and the quantum information carried by the two quantized optical fields can be stored in the collective modes of this atomic ensemble by adiabatically controlling the classic field Rabi frequencies.
Yong Li, Li Zheng, Yu-xi Liu, C. P. Sun Journal reference: Phys. Rev. A 73, 043805 (2006) [pdf] DOI: 10.1103/PhysRevA.73.043805
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Macroscopic Einstein-Podolsky-Rosen pairs in superconducting circuits -
Abstract
- We propose an efficient approach to prepare Einstein-Podolsky-Rosen (EPR) pairs in currently existing Josephson nanocircuits with capacitive couplings. In these fixed coupling circuits, two-qubit logic gates could be easily implemented while, strictly speaking, single-qubit gates cannot be easily realized. For a known two-qubit state, conditional single-qubit operation could still be designed to evolve only the selected qubit and keep the other qubit unchanged; the rotation of the selected qubit depends on the state of the other one. These conditional single-qubit operations allow to deterministically generate the well-known Einstein-Podolsky-Rosen pairs, represented by EPR-Bell (or Bell) states. Quantum-state tomography is further proposed to experimentally confirm the generation of these states. The decays of the prepared EPR pairs are analyzed using numerical simulations. Possible application of the generated EPR pairs to test Bell's Inequality is also discussed.
L. F. Wei, Yu-xi Liu, Markus J. Storcz, Franco Nori Journal reference: Phys. Rev. A 73, 052307 (2006) [pdf] DOI: 10.1103/PhysRevA.73.052307
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Controllable Coupling between Flux Qubits -
Abstract
- We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively-coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplitudes of the bias magnetic fluxes through the qubit loops, and also offers a remarkable way to implement any logic gate as well as tomographically measure flux qubit states.
Yu-xi Liu, L. F. Wei, J. S. Tsai, Franco Nori Journal reference: Phys. Rev. Lett. 96, 067003 (2006) [pdf] DOI: 10.1103/PhysRevLett.96.067003
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FERMIONIC ZERO MODES IN GAUGE AND GRAVITY BACKGROUNDS ON T
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Abstract
- In this note we study fermionic zero modes in gauge and gravity backgrounds taking a two dimensional compact manifold $T^2$ as extra dimensions. The result is that there exist massless Dirac fermions which have normalizable zero modes under quite general assumptions about these backgrounds on the bulk. Several special cases of gauge background on the torus are discussed and some simple fermionic zero modes are obtained.
Yi-Shi Duan, Yu-Xiao Liu, Yong-Qiang Wang Journal reference: Mod.Phys.Lett.A21:2019-2025,2006 [pdf] DOI: 10.1142/S0217732306020068
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Spinor Field Realizations of Non-critical
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Abstract
- In this paper, we investigate the spinor field realizations of the $W_{2,4}$ algebra, making use of the fact that the $W_{2,4}$ algebra can be linearized through the addition of a spin-1 current. And then the nilpotent BRST charges of the spinor non-critical $W_{2,4}$ string were built with these realizations.
Li-Jie Zhang, Yu-Xiao Liu Journal reference: Commun.Theor.Phys.46:675-678,2006 [pdf] DOI: 10.1088/0253-6102/46/4/023
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Soliton with a pion field in the global color symmetry model -
Abstract
- We calculate the property of the global color symmetry model soliton with the pion field being included explicitly. The calculated results indicate that the pion field provides a strong attraction so that the eigen-energy of a quark and the mass of a soliton reduce drastically, in contrast to those with only the sigma field.
Bin Wang, Hui-chao Song, Lei Chang, Huan Chen, Yu-xin Liu Journal reference: Phys.Rev.C73:015206,2006 [pdf] DOI: 10.1103/PhysRevC.73.015206
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Theoretical study on rotational bands and shape coexistence of
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Abstract
- By taking the particle triaxial-rotor model with variable moment of inertia, we investigate the energy spectra, the deformations and the single particle configurations of the nuclei $^{183,185,187}$Tl systemically. The calculated energy spectra agree with experimental data quite well. The obtained results indicate that the aligned bands observed in $^{183,185,187}$Tl originate from the $[530]{{1/2}}^{-}$, $[532]{{3/2}}^{-}$, $[660]{{1/2}}^{+}$ proton configuration coupled to a prolate deformed core, respectively. Whereas, the negative parity bands built upon the ${{9/2}}^{-}$ isomeric states in $^{183,185,187}$Tl are formed by a proton with the $[505]{{9/2}}^{-}$ configuration coupled to a core with triaxial oblate deformation, and the positive parity band on the ${{13/2}}^{+}$ isomeric state in $^{187}$Tl is generated by a proton with configuration $[606]{{13/2}}^{+}$ coupled to a triaxial oblate core.
Guo-Jie Chen, Yu-xin Liu, Hui-chao Song, Hui Cao Journal reference: Phys.Rev.C73:034304,2006 [pdf] DOI: 10.1103/PhysRevC.73.034304
-
Phase transition of finite size quark droplets with isospin chemical potential in the Nambu–Jona-Lasinio model -
Abstract
- Making use of the NJL model and the multiple reflection expansion pproximation, we study the phase transition of the finite size droplet with u and d quarks. We find that the dynamical masses of u, d quarks are different, and the chiral symmetry can be restored at different critical radii for u, d quark. It rovides a clue to understand the effective nucleon mass splitting in nuclear matter. Meanwhile, it shows that the maximal isospin chemical potential at zero temperature is much smaller than the mass of pion in free space.
Guo-yun Shao, Lei Chang, Yu-xin Liu, Xue-lei Wang Journal reference: Phys.Rev. D73 (2006) 076003 [pdf] DOI: 10.1103/PhysRevD.73.076003
-
Approach to the Rotation Driven Vibrational to Axially Rotational Shape
Phase Transition Along the Yrast Line of a Nucleus -
Abstract
- By analyzing the potential energy surface, the shape phase diagram and the energy spectrum of the nucleus in U(5) symmetry in the IBM, we propose that the U(5) symmetry with parameters $(A+B) < 0$ may be a model to describe the rotation driven vibrational to axially rotational phase transition along the yrast line. With such a model, we have described successfully the observed rotation driven shape phase transition along the yrast line of individual nucleus and proposed some other empirical evidences.
Yu-xin Liu, Liang-zhu Mu, Haiqing Wei [pdf]
-
Macroscopic Quantum Criticality in a Circuit QED -
Abstract
- Cavity quantum electrodynamic (QED) is studied for two strongly-coupled charge qubits interacting with a single-mode quantized field, which is provided by a on-chip transmission line resonator. We analyze the dressed state structure of this superconducting circuit QED system and the selection rules of electromagnetic-induced transitions between any two of these dressed states. Its macroscopic quantum criticality, in the form of ground state level crossing, is also analyzed, resulting from competition between the Ising-type inter-qubit coupling and the controllable on-site potentials.
Y. D. Wang, H. T. Quan, Yu-xi Liu, C. P. Sun, Franco Nori [pdf]
-
Quantum transducers: Integrating transmission lines and nanomechanical resonators via charge qubits -
Abstract
- We propose a mechanism to interface a transmission line resonator (TLR) with a nano-mechanical resonator (NAMR) by commonly coupling them to a charge qubit, a Cooper pair box with a controllable gate voltage. Integrated in this quantum transducer or simple quantum network, the charge qubit plays the role of a controllable quantum node coherently exchanging quantum information between the TLR and NAMR. With such an interface, a maser-like process is predicted to create a quasi-classical state of the NAMR by controlling a single-mode classical current in the TLR. Alternatively, a "Cooper pair" coherent output through the transmission line can be driven by a single-mode classical oscillation of the NAMR.
C. P. Sun, L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 73, 022318 (2006) [pdf] DOI: 10.1103/PhysRevA.73.022318
-
Knotted solitons in a charged two-condensate Bose system -
Abstract
- 2005
-
Coherent photons and macroscopic cat states generated by a cyclic
artificial atom with broken symmetry -
Abstract
- We propose a novel method to generate non-classical states of a single-mode microwave field, and to produce macroscopic cat states by virtue of a three-level system with $\Delta$-shaped (or cyclic) transitions. This exotic system can be implemented by a superconducting quantum circuit with a broken symmetry in its effective potential. Using the cyclic population transfer, controllable single-mode photon states can be created in the third transition when two classical fields are applied to induce the other two transitions. This is because, for large detuning, two classical fields are equivalent to an effective external force, which derives the quantized single mode. Our approach is valid not only for superconducting quantum circuits but also for any three-level quantum system with $\Delta$-shaped transitions
C. P. Sun, Yu-xi Liu, L. F. Wei, Franco Nori [pdf]
-
Pion-nucleon sigma term in the global color model of QCD -
Abstract
- We study the pion-nucleon sigma term in vacuum and in nuclear matter in the framework of global color model of QCD. With the effective gluon propagator being taken as the $\delta$-function in momentum space of Munczek-Nomirovsky model, we estimate that the sigma term at chiral limit in the vacuum is 9/2 times the current quark mass and it decreases with the nuclear matter density. With the presently obtained in-medium pion-nucleon sigma term, we study the in-medium chiral quark condensate and obtain a reasonable variation behavior against the nuclear matter density.
Lei Chang, Yu-xin Liu, Hua Guo Journal reference: Phys.Rev. D72 (2005) 094023 [pdf] DOI: 10.1103/PhysRevD.72.094023
-
Measuring the quality factor of a microwave cavity using superconducting qubit devices -
Abstract
- We propose a method to create superpositions of two macroscopic quantum states of a single-mode microwave cavity field interacting with a superconducting charge qubit. The decoherence of such superpositions can be determined by measuring either the Wigner function of the cavity field or the charge qubit states. Then the quality factor Q of the cavity can be inferred from the decoherence of the superposed states. The proposed method is experimentally realizable within current technology even when the $Q$ value is relatively low, and the interaction between the qubit and the cavity field is weak.
Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Phys. Rev. A 72, 033818 (2005) [pdf] DOI: 10.1103/PhysRevA.72.033818
-
Optical Selection Rules and Phase-Dependent Adiabatic State Control in a Superconducting Quantum Circuit -
Abstract
- We analyze the optical selection rules of the microwave-assisted transitions in a flux qubit superconducting quantum circuit (SQC). We show that the parities of the states relevant to the superconducting phase in the SQC are well-defined when the external magnetic flux $\Phi_{e}=\Phi_{0}/2$, then the selection rules are same as the ones for the electric-dipole transitions in usual atoms. When $\Phi_{e}\neq \Phi_{0}/2$, the symmetry of the potential of the artificial "atom'' is broken, a so-called $\Delta$-type "cyclic" three-level atom is formed, where one- and two-photon processes can coexist. We study how the population of these three states can be selectively transferred by adiabatically controlling the electromagnetic field pulses. Different from $\Lambda$-type atoms, the adiabatic population transfer in our three-level $\Delta$-atom can be controlled not only by the amplitudes but also by the phases of the pulses.
Yu-xi Liu, J. Q. You, L. F. Wei, C. P. Sun, Franco Nori Journal reference: Phys. Rev. Lett. 95, 087001 (2005) [pdf] DOI: 10.1103/PhysRevLett.95.087001
-
Spinor field realizations of non-critical strings -
Abstract
- In this paper, we construct the nilpotent Becchi-Rouet-Stora-Tyutin($BRST$) charges of spinor non-critical $W_{2,s}$ strings. The cases of $s=3,4$ are discussed in detail, and spinor realization for $s=4$ is given explicitly. The $BRST$ charges are graded.
Yi-Shi Duan, Yu-Xiao Liu, Li-Jie Zhang Journal reference: Nucl.Phys.B699:174-182,2004 [pdf] DOI: 10.1016/j.nuclphysb.2004.08.046
-
FERMIONIC ZERO MODES IN SELF-DUAL VORTEX BACKGROUND -
Abstract
- We study fermionic zero modes in the background of self-dual vortex on a two-dimensional non-compact extra space in 5+1 dimensions. In the Abelian Higgs model, we present an unified description of the topological and non-topological self-dual vortex on the extra two dimensions. Based on it, we study localization of bulk fermions on a brane with inclusion of Yang-Mills and gravity backgrounds in six dimensions. Through two simple cases, it is shown that the vortex background contributes a phase shift to the fermionic zero mode, this phase is actually origin from the Aharonov-Bohm effect.
Yong-Qiang Wang, Tie-Yan Si, Yu-Xiao Liu, Yi-Shi Duan Journal reference: Mod.Phys.Lett. A20 (2005) 3045-3053 [pdf] DOI: 10.1142/S0217732305018037
-
A new form of self-duality equations with topological term -
Abstract
- Based on the U(1) gauge potential decomposition theory and $\phi$-mapping theory, the topological inner structure of the self-duality (Bogomol'nyi-type) equations are studied. The special form of the gauge potential decomposition is obtained directly from the first of the self-duality equations. Using this decomposition, the topological inner structure of the Chern-Simons-Higgs (CSH) vortex is discussed. Furthermore, we obtain a rigorous self-dual equation with topological term for the first time, in which the topological term has been ignored by other physicists.
Yong-Qiang Wang, Yu-Xiao Liu, Yi-Shi Duan [pdf]
-
Testing Bell’s inequality in a constantly coupled Josephson circuit by effective single-qubit operations -
Abstract
- In superconducting circuits with interbit untunable (e.g., capacitive) couplings, ideal local quantum operations cannot be exactly performed on individual Josephson qubits. Here we propose an effective dynamical decoupling approach to overcome the "fixed-interaction" difficulty for effectively implementing elemental logical gates for quantum computation. The proposed single-qubit operations and local measurements should allow testing Bell's inequality with a pair of capacitively-coupled Josephson qubits. This provides a powerful approach, besides spectral-analysis [Nature \textbf{421}, 823 (2003); Science \textbf{300}, 1548 (2003)], to verify the existence of macroscopic quantum entanglement between two fixed-coupling Josephson qubits.
L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. B 72, 104516 (2005) [pdf] DOI: 10.1103/PhysRevB.72.104516
-
Ghost field realizations of the spinor
-
Abstract
- It has been shown that certain W algebras can be linearized by the inclusion of a spin-1 current. This Provides a way of obtaining new realizations of the W algebras. In this paper, we investigate the new ghost field realizations of the W(2,s)(s=3,4) algebras, making use of the fact that these two algebras can be linearized. We then construct the nilpotent BRST charges of the spinor non-critical W(2,s) strings with these new realizations.
Yu-Xiao Liu, Li-Jie Zhang, Ji-Rong Ren Journal reference: JHEP 0501:005,2005 [pdf] DOI: 10.1088/1126-6708/2005/01/005
-
Tomographic measurements on superconducting qubit states -
Abstract
- We propose an approach to reconstruct any superconducting charge qubit state by using quantum state tomography. This procedure requires a series of measurements on a large enough number of identically prepared copies of the quantum system. The experimental feasibility of this procedure is explained and the time scales for different quantum operations are estimated according to experimentally accessible parameters. Based on the state tomography, we also investigate the possibility of the process tomography.
Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Phys. Rev. B 72, 014547 (2005) [pdf] DOI: 10.1103/PhysRevB.72.014547
-
Shape Phase Transition of the Axially Symmetric States Between the U(5)
and SU(3) Symmetries -
Abstract
- The shape phase structure and its transition of the nucleus in the transitional region between the U(5) and SU(3) symmetries is restudied within the framework of coherent-state theory with angular momentum projection in IBM-1. The certain angular momentum (or rotation-driven) effect on the nuclear shape is discussed. A coexistence of prolate and oblate shapes is found for the ground states of the transitional nuclei. A phase diagram in terms of the deformation parameter and angular momentum is given.
Liang-zhu Mu, Yu-xin Liu [pdf]
-
Preparation of macroscopic quantum superposition states of a cavity field via coupling to a superconducting charge qubit -
Abstract
- We propose how to generate macroscopic quantum superposition states using a microwave cavity containing a superconducting charge qubit. Based on the measurement of charge states, we show that the superpositions of two macroscopically distinguishable coherent states of a single-mode cavity field can be generated by a controllable interaction between a cavity field and a charge qubit. After such superpositions of the cavity field are created, the interaction can be switched off by the classical magnetic field, and there is no information transfer between the cavity field and the charge qubit. We also discuss the generation of the superpositions of two squeezed coherent states.
Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Phys. Rev. A 71, 063820 (2005) [pdf] DOI: 10.1103/PhysRevA.71.063820
-
New method for numerically solving the chemical potential dependence of the dressed quark propagator -
Abstract
- Based on the rainbow approximation of Dyson-Schwinger equation and the assumption that the inverse dressed quark propagator at finite chemical potential is analytic in the neighborhood of $\mu=0$, a new method for obtaining the dressed quark propagator at finite chemical potential $\mu$ from the one at zero chemical potential is developed. Using this method the dressed quark propagator at finite chemical potential can be obtained directly from the one at zero chemical potential without the necessity of numerically solving the corresponding coupled integral equations by iteration methods. A comparison with previous results is given.
Feng-yao Hou, Lei Chang, Wei-min Sun, Hong-shi Zong, Yu-xin Liu Journal reference: Phys.Rev. C72 (2005) 034901 [pdf] DOI: 10.1103/PhysRevC.72.034901
-
Study the effects of metallic ions on the combination of DNA and
histones with molecular combing technique -
Abstract
- The effects of monovalent (Na+, K+) and divalent (Mg2+, Ca2+, Mn2+) ions on the interaction between DNA and histone are studied using the molecular combing technique. Lamda-DNA molecules and DNA-histone complexes incubated with metal cations (Na+, K+, Mg2+, Ca2+, Mn2+) are stretched on hydrophobic surfaces, and directly observed by fluorescence microscopy. The results indicate that when these cations are added into the DNA solution, the fluorescence intensities of the stained DNA are reduced differently. The monovalent cations (Na+, K+) inhibit binding of histone to DNA. The divalent cations (Mg2+, Ca2+, Mn2+) enhance significantly the binding of histone to DNA and the binding of the DNA-histone complex to the hydrophobic surface. Mn2+ also induces condensation and aggregation of the DNA-histone complex.
Yu-Ying Liu, Peng-Ye Wang, Shuo-Xing Dou, Ping Xie, Wei-Chi Wang, Hua-Wei Yin [pdf]
-
Observation of Single-Photon Switching -
Abstract
- We report an experimental demonstration of single-photon switching in laser-cooled $^{87}$Rb atoms. A resonant probe pulse with an energy per unit area of one photon per $\lambda^2/2\pi$ propagates through the optically thick atoms. Its energy transmittance is greater than 63% or loss is less than $e^{-1}$ due to the effect of electromagnetically induced transparency. In the presence of a switching pulse with an energy per unit area of 1.4 photons per $\lambda^2/2\pi$, the energy transmittance of the same probe pulse becomes less than 37% or $e^{-1}$. This substantial reduction of the probe transmittance caused by single switching photons has potential applications in single-photon-level nonlinear optics and the manipulation of quantum information.
Yong-Fan Chen, Zen-Hsiang Tsai, Yu-Chen Liu, Ite A. Yu [pdf]
-
Quantum computation with Josephson qubits using a current-biased information bus -
Abstract
- We propose an effective scheme for manipulating quantum information stored in a superconducting nanocircuit. The Josephson qubits are coupled via their separate interactions with an information bus, a large current-biased Josephson junction treated as an oscillator with adjustable frequency. The bus is sequentially coupled to only one qubit at a time. Distant Josephson qubits without any direct interaction can be indirectly coupled with each other by independently interacting with the bus sequentially, via exciting/de-exciting vibrational quanta in the bus. This is a superconducting analog of the successful ion trap experiments on quantum computing. Our approach differs from previous schemes that simultaneously coupled two qubits to the bus, as opposed to their sequential coupling considered here. The significant quantum logic gates can be realized by using these tunable and selective couplings. The decoherence properties of the proposed quantum system are analyzed within the Bloch-Redfield formalism. Numerical estimations of certain important experimental parameters are provided.
L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. B 71, 134506 (2005) [pdf] DOI: 10.1103/PhysRevB.71.134506
-
Coherent photons and macroscopic cat states generated by a cyclic
artificial atom with broken symmetry -
Abstract
- 2004
-
Corrections to the Nonrelativistic Ground Energy of a Helium Atom -
Abstract
- Considering the nuclear motion, the authors give out the nonrelativistic ground energy of a helium atom by using a simple but effective variational wave function with a flexible parameter $k$. Based on this result, the relativistic and radiative corrections to the nonrelativistic Hamiltonian are discussed. The high precision value of the helium ground energy is evaluated to be -2.90338 a.u., and the relative error is 0.00034%.
Yi-Shi Duan, Yu-Xiao Liu, Li-Jie Zhang Journal reference: Chin. Phys. Lett. 21 (2004) 1714-1716 [pdf] DOI: 10.1088/0256-307X/21/9/010
-
Spinor Field Realizations of $W_{2,6}$ String and $W_{6}$ String -
Abstract
- In this paper the spinor field BRST charges of the W2,6 string and W6 string are constructed, where the BRST charges are graded.
Shu-Cheng Zhao, Li-Jie Zhang, Yu-Xiao Liu [pdf]
-
Engineering quantum pure states of a trapped cold ion beyond the Lamb-Dicke limit -
Abstract
- Based on the conditional quantum dynamics of laser-ion interaction, we propose an efficient theoretical scheme to deterministically generate quantum pure states of a single trapped cold ion without performing the Lamb-Dicke approximation. An arbitrary quantum state can be created by using a series of classical laser beams with selected frequencies, initial phases and durations. As special examples, we further show how to create or approximate several typical macroscopic quantum states, such as the phase state and (even/odd) coherent states. Unlike previous schemes operated in the Lamb-Dicke regime, the present one does well for arbitrary strength coupling between the internal and external degrees of freedom of the ion. The experimental realizability of this approach is also discussed.
L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Phys. Rev. A 70, 063801 (2004) [pdf] DOI: 10.1103/PhysRevA.70.063801
-
Kraus representation of a damped harmonic oscillator and its application -
Abstract
- By definition, the Kraus representation of a harmonic oscillator suffering from the environment effect, modeled as the amplitude damping or the phase damping, is directly given by a simple operator algebra solution. As examples and applications, we first give a Kraus representation of a single qubit whose computational basis states are defined as bosonic vacuum and single particle number states. We further discuss the environment effect on qubits whose computational basis states are defined as the bosonic odd and even coherent states. The environment effects on entangled qubits defined by two different kinds of computational basis are compared with the use of fidelity.
Yu-xi Liu, Sahin Kaya Ozdemir, Adam Miranowicz, Nobuyuki Imoto Journal reference: Phys. Rev. A 70, 042308 (2004) [pdf] DOI: 10.1103/PhysRevA.70.042308
-
Quantum tomography for solid-state qubits -
Abstract
- We propose a method for the tomographic reconstruction of qubit states for a general class of solid state systems in which the Hamiltonians are represented by spin operators, e.g., with Heisenberg-, $XXZ$-, or XY- type exchange interactions. We analyze the implementation of the projective operator measurements, or spin measurements, on qubit states. All the qubit states for the spin Hamiltonians can be reconstructed by using experimental data.
Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Europhysics Letters 67, 874-880 (2004) [pdf] DOI: 10.1209/epl/i2004-10154-1
-
Coupling Josephson qubits via a current-biased information bus -
Abstract
- Josephson qubits without direct interaction can be effectively coupled by sequentially connecting them to an information bus: a current-biased large Josephson junction treated as an oscillator with adjustable frequency. The coupling between any qubit and the bus can be controlled by modulating the magnetic flux applied to that qubit. This tunable and selective coupling provides two-qubit entangled states for implementing elementary quantum logic operations, and for experimentally testing Bell's inequality.
L. F. Wei, Yu-xi Liu, Franco Nori Journal reference: Europhysics Letters 67, 1004 (2004) [pdf] DOI: 10.1209/epl/i2004-10149-x
-
Generation of nonclassical photon states using a superconducting qubit in a microcavity -
Abstract
- Based on the interaction between the radiation field and a superconductor, we propose a way to engineer quantum states using a SQUID charge qubit inside a microcavity. This device can act as a deterministic single photon source as well as generate any Fock states and an arbitrary superposition of Fock states for the cavity field. The controllable interaction between the cavity field and the qubit can be realized by the tunable gate voltage and classical magnetic field applied to the SQUID.
Yu-xi Liu, L. F. Wei, Franco Nori Journal reference: Europhysics Letters 67, 941-947 (2004) [pdf] DOI: 10.1209/epl/i2004-10144-3
-
MULTI-QUARK STATES IN THE INHERENT NODAL STRUCTURE ANALYSIS -
Abstract
- The strangeness $s=+1$ pentaquark states as $qqqq\bar{q}$ clusters are investigated in this letter. Starting from the inherent geometric symmetry, we analyzed the inherent nodal structure of the system. As the nodeless states, the low-lying states are picked out. Then the S-wave state $(J^P, T)= ({{1/2}}^{-}, 0)$ and P-wave state $(J^P, T)= ({{1/2}}^{+}, 0)$ may be the candidates of low-lying pentaquark states. By comparing the accessibility of the two states and referring the presently obtained K-N interaction potential, we propose that the quantum numbers of the observed pentaquark state $\Theta^{+}$ may be $(J^P, T)=({{1/2}}^{+}, 0)$ and L=1.
Yu-xin Liu, Jing-sheng Li, Cheng-guang Bao [pdf] DOI: 10.1142/S0217751X0502375X hep-ph/0401197v3 [pdf]
-
Exciton entanglement in two coupled semiconductor microcrystallites -
Abstract
- Entanglement of the excitonic states in the system of two coupled semiconductor microcrystallites, whose sizes are much larger than the Bohr radius of exciton in bulk semiconductor but smaller than the relevant optical wavelength, is quantified in terms of the entropy of entanglement. It is observed that the nonlinear interaction between excitons increases the maximum values of the entropy of the entanglement more than that of the linear coupling model. Therefore, a system of two coupled microcrystallites can be used as a good source of entanglement with fixed exciton number. The relationship between the entropy of the entanglement and the population imbalance of two microcrystallites is numerically shown and the uppermost envelope function for them is estimated by applying the Jaynes principle.
Yu-xi Liu, Sahin K. Ozdemir, Adam Miranowicz, Masato Koashi, Nobuyuki Imoto Journal reference: J. Phys. A 37, 4423-4436 (2004) [pdf] DOI: 10.1088/0305-4470/37/15/010
-
Ionic effect on combing of single DNA molecules and observation of their force-induced melting by fluorescence microscopy -
Abstract
- Molecular combing is a powerful and simple method for aligning DNA molecules onto a surface. Using this technique combined with fluorescence microscopy, we observed that the length of lambda-DNA molecules was extended to about 1.6 times their contour length (unextended length, 16.2 micrometers) by the combing method on hydrophobic polymethylmetacrylate (PMMA) coated surfaces. The effects of sodium and magnesium ions and pH of the DNA solution were investigated. Interestingly, we observed force-induced melting of single DNA molecules.
Yu-Ying Liu, Peng-Ye Wang, Shuo-Xing Dou, Wei-Chi Wang, Ping Xie, Hua-Wei Yin, Xing-Dong Zhang Journal reference: JOURNAL OF CHEMICAL PHYSICS, 121, 4302 (2004) [pdf] DOI: 10.1063/1.1777220
-
Corrections to the Nonrelativistic Ground Energy of a Helium Atom -
Abstract
- 2003
-
Approach to Vibrational to Axially Rotational Shape Phase Transition
Along the Yrast Line -
Abstract
- With the energy surface of the nucleus in U(5) symmetry being analyzed in the framework of thermodynamics, the vibration and rotation phase diagram in terms of the angular momentum and deformation parameter is given. Together with examining the energy spectrum, we propose a theoretical approach to describe the vibrational to axially rotational phase transition along the yrast line. By analyzing the available experimental data we show that the vibrational to rotational shape phase transition along the yrast line takes place in many nuclei.
Yu-xin Liu, Liang-zhu Mu [pdf]
-
Quark condensates in nuclear matter in the global color symmetry model of QCD -
Abstract
- With the global color symmetry model being extended to finite chemical potential, we study the density dependence of the local and nonlocal scalar quark condensates in nuclear matter. The calculated results indicate that the quark condensates increase smoothly with the increasing of nuclear matter density before the critical value (about 12$\rho_0$) is reached. It also manifests that the chiral symmetry is restored suddenly as the density of nuclear matter reaches its critical value. Meanwhile, the nonlocal quark condensate in nuclear matter changes nonmonotonously against the space-time distance among the quarks.
Yu-xin Liu, Dong-feng Gao, Hua Guo Journal reference: Phys.Rev. C68 (2003) 035204 [pdf] DOI: 10.1103/PhysRevC.68.035204
-
Reevaluation of the density dependence of nucleon radius and mass in the global color symmetry model of QCD -
Abstract
- With the global color symmetry model (GCM) at finite chemical potential, the density dependence of the bag constant, the total energy and the radius of a nucleon in nuclear matter is investigated. A relation between the nuclear matter density and the chemical potential with the action of QCD being taken into account is obtained. A maximal nuclear matter density for the existence of the bag with three quarks confined within is given. The calculated results indicate that, before the maximal density is reached, the bag constant and the total energy of a nucleon decrease, and the radius of a nucleon increases slowly, with the increasing of the nuclear matter density. As the maximal nuclear matter density is reached, the mass of the nucleon vanishes and the radius becomes infinite suddenly. It manifests that a phase transition from nucleons to quarks takes place.
Yu-xin Liu, Dong-feng Gao, Jian-hang Zhou, Hua Guo Journal reference: Nucl.Phys. A725 (2003) 127-138 [pdf] DOI: 10.1016/S0375-9474(03)01574-4
-
Superradiance of low-density Frenkel excitons in a crystal slab of three-level atoms: The quantum interference effect -
Abstract
- We systematically study the fluorescence of low density Frenkel excitons in a crystal slab containing $N_T$ V-type three-level atoms. Based on symmetric quasi-spin realization of SU(3) in large $N$ limit, the two-mode exciton operators are invoked to depict various collective excitations of the collection of these V-type atoms starting from their ground state. By making use of the rotating wave approximation, the light intensity of radiation for the single lattice layer is investigated in detail. As a quantum coherence effect, the quantum beat phenomenon is discussed in detail for different initial excitonic states. We also test the above results analytically without the consideration of the rotating wave approximation and the self-interaction of radiance field is also included.
G. R. Jin, P. Zhang, Yu-xi Liu, C. P. Sun Journal reference: Phys. Rev. B 68, 134301 (2003) [pdf] DOI: 10.1103/PhysRevB.68.134301
-
Low-lying
-
Abstract
- The dibaryon states as six-quark clusters of exotic QCD states are investigated in this paper. With the inherent nodal surface structure analysis, the wave functions of the six-quark clusters (in another word, the dibaryons) are classified. The contribution of the hidden color channels are discussed. The quantum numbers of the low-lying dibaryon states are obtained. The States $[\Omega\Omega]_{(0,0^{+})}$, $[\Omega\Omega]_{(0,2^{-})}$, $[\Xi^{*}\Omega]_{(1/2,0^{+})}$, $[\Sigma^{*}\Sigma^{*}]_{(0,4^{-})}$ and the hidden color channel states with the same quantum numbers are proposed to be the candidates of dibaryons, which may be observed in experiments.
Yu-xin Liu, Jing-sheng Li, Cheng-guang Bao Journal reference: Phys. Rev. C 67 (2003), 055207 [pdf] DOI: 10.1103/PhysRevC.67.055207
-
Size-dependent decoherence of excitonic states in semiconductor microcrystallites -
Abstract
- The size-dependent decoherence of the exciton states resulting from the spontaneous emission is investigated in a semiconductor spherical microcrystallite under condition $a_{B}\ll R_{0}\leq\lambda$. In general, the larger size of the microcrystallite corresponds to the shorter coherence time. If the initial state is a superposition of two different excitonic coherent states, the coherence time depends on both the overlap of two excitonic coherent states and the size of the microcrystallite. When the system with fixed size is initially in the even or odd coherent states, the larger average number of the excitons corresponds to the faster decoherence. When the average number of the excitons is given, the bigger size of the microcrystallite corresponds to the faster decoherence. The decoherence of the exciton states for the materials GaAs and CdS is numerically studied by our theoretical analysis.
Yu-xi Liu, A. Miranowicz, S. K. Ozdemir, Masato Koashi, Nobuyuki Imoto Journal reference: Phys. Rev. A 67, 034303 (2003) [pdf] DOI: 10.1103/PhysRevA.67.034303
-
Energy level statistics of the U(5) and O(6) symmetries in the interacting boson model -
Abstract
- We study the energy level statistics of the states in U(5) and O(6) dynamical symmetries of the interacting boson model and the high spin states with backbending in U(5) symmetry. In the calculations, the degeneracy resulting from the additional quantum number is eliminated manually. The calculated results indicate that the finite boson number $N$ effect is prominent. When $N$ has a value close to a realistic one, increasing the interaction strength of subgroup O(5) makes the statistics vary from Poisson-type to GOE-type and further recover to Poisson-type. However, in the case of $N \to \infty$, they all tend to be Poisson-type. The fluctuation property of the energy levels with backbending in high spin states in U(5) symmetry involves a signal of shape phase transition between spherical vibration and axial rotation.
Jing Shu, Ying Ran, Tao Ji, Yu-Xin Liu Journal reference: Phys.Rev. C67 (2003) 044304 [pdf] DOI: 10.1103/PhysRevC.67.044304
-
Low-lying States of $^{6}$He and $^{6}$Be in Nodal Surface Structure
Analysis -
Abstract
- The low-lying states of light nuclei $^6$He and $^6$Be are studied. Using the inherent nodal surface(INS) analysis approach, we deduce the quantum numbers and the spatial symmetries of the low-lying states with positive parity and negative parity of the two nuclei. The energy spectrum obtained agrees well with the experimental data.
Yu-xin Liu, Jing-sheng Li, Cheng-guang Bao [pdf]
-
SU(5) symmetry of spdfg interacting boson model -
Abstract
- The extended interacting boson model with s-, p-, d-, f- and g-bosons being included (spdfg IBM) are investigated. The algebraic structure including the generators, the Casimir operators of the groups at the SU(5) dynamical symmetry and the branching rules of the irreducible representation reductions along the group chain are obtained. The typical energy spectrum of the Symmetry is given.
Jing-sheng Li, Yu-xin Liu, Peng Gao Journal reference: Sci.ChinaG48:204-213,2003 [pdf] DOI: 10.1360/03yg9028
-
Approach to Vibrational to Axially Rotational Shape Phase Transition
Along the Yrast Line -
Abstract
- 2002
-
Realization of symmetric sharing of entanglement in semiconductor microcrystallites coupled by a cavity field -
Abstract
- The entanglement of excitonic states in a system of $N$ spatially separated semiconductor microcrystallites is investigated. The interaction among the different microcrystallites is mediated by a single-mode cavity field. It is found that the symmetric sharing of the entanglement (measured by the concurrence) between any pair of the excitonic state with $N$ qubits defined by the number states (vacuum and a single-exciton states) or the coherent states (odd and even coherent states) can be prepared by the cavity field for this system.
Yu-xi Liu, Adam Miranowicz, Masato Koashi, Nobuyuki Imoto Journal reference: Phys. Rev. A 66, 062309 (2002) [pdf] DOI: 10.1103/PhysRevA.66.062309
-
Generation of maximum spin entanglement induced by a cavity field in quantum-dot systems -
Abstract
- Equivalent-neighbor interactions of the conduction-band electron spins of quantum dots in the model of Imamoglu et al. [Phys. Rev. Lett. 83, 4204 (1999)] are analyzed. Analytical solution and its Schmidt decomposition are found and applied to evaluate how much the initially excited dots can be entangled to the remaining dots if all of them are initially disentangled. It is demonstrated that the perfect maximally entangled states (MES) can only be generated in the systems of up to 6 dots with a single dot initially excited. It is also shown that highly entangled states, approximating the MES with a good accuracy, can still be generated in systems of odd number of dots with almost half of them being excited. A sudden decrease of entanglement is observed by increasing the total number of dots in a system with a fixed number of excitations.
A. Miranowicz, S. K. Ozdemir, Yu-xi Liu, M. Koashi, N. Imoto, Y. Hirayama Journal reference: Phys. Rev. A 65, 062321 (2002) [pdf] DOI: 10.1103/PhysRevA.65.062321
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Statistical Properties of E(5) and X(5) Symmetries -
Abstract
- We study the energy level statistics of the states in E(5) and X(5) transitional dynamical symmetries. The calculated results indicate that the statistics of E(5) symmetry is regular and follows Poisson statistics, while the statistics of X(5) symmetry involves two maxima in the nearest neighbor level spacing distribution $P(s)$ and the $\Delta_{3}$ statistics follows the GOE statistics. It provides an evidence that the X(5) symmetry is at the critical point exhibiting competing degree of freedom.
Jing Shu, Hong-Bo Jia, Yu-Xin Liu [pdf]
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Dynamics of entanglement for coherent excitonic states in a system of two coupled quantum dots and cavity QED -
Abstract
- The dynamics of the entanglement for coherent excitonic states in the system of two coupled large semiconductor quantum dots ($R/a_{B}\gg 1$) mediated by a single-mode cavity field is investigated. Maximally entangled coherent excitonic states can be generated by cavity field initially prepared in odd coherent state. The entanglement of the excitonic coherent states between two dots reaches maximum when no photon is detected in the cavity. The effects of the zero-temperature environment on the entanglement of excitonic coherent state are also studied using the concurrence for two subsystems of the excitons
Yu-xi Liu, S. K. Ozdemir, Masato Koashi, Nobuyuki Imoto Journal reference: PRA65, 042326(2002) [pdf] DOI: 10.1103/PhysRevA.65.042326
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A Nonlinear Realized Approach of SU(2) Chiral Symmetry Spontaneous
Breaking and Properties of Nuclear Matter -
Abstract
- A nonlinear realization of SU(2) chiral symmetry spontaneous breaking approach is developed in the composite operator formalism. A Lagrangian including quark, gluon and Goldstone boson degrees of freedom of the chiral quark model is obtained from the QCD Lagrangian. A way to link the chiral symmetry spontaneous breaking formalism at hadron level and that at quark level is predicted. too. The application to nuclear matter shows that the approach is quite successful in describing the properties of nuclear matter and the quark condensate in it.
Xiao-Fu Lu, Bao-Xi Sun, Yu-Xin Liu, Hua Guo, En-Guang Zhao [pdf]
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Semiconductor-cavity QED in high-
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Abstract
- The non-resonant interaction between the high-density excitons in a quantum well and a single mode cavity field is investigated. An analytical expression for the physical spectrum of the excitons is obtained. The spectral properties of the excitons, which are initially prepared in the number states or the superposed states of the two different number states by the resonant femtosecond pulse pumping experiment, are studied. Numerical study of the physical spectrum is carried out and a discussion of the detuning effect is presented.
Yu-xi Liu, N. Imoto, S. K. Ozdemir, Guang-ri Jin, C. P. Sun Journal reference: Phys. Rev. A65, 023805(2002) [pdf] DOI: 10.1103/PhysRevA.65.023805
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Realization of symmetric sharing of entanglement in semiconductor microcrystallites coupled by a cavity field -
Abstract
- 2001
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Dynamics of optically driven exciton and quantum decoherence -
Abstract
- By using the normal ordering method, we study the state evolution of an optically driven excitons in a quantum well immersed in a leaky cavity, which was introduced by Yu-xi Liu et.al. [Phys. Rev. A {\bf 63}, 033816 (2001)]. The influence of the external laser field on the quantum decoherence of a mesoscopically superposed states of the excitons is investigated. Our result shows that, thought the characteristic time of decoherence does not depend on the external field, the phase of the decoherence factor can be well controlled by adjusting the external parameters.
G. R. Jin, Yu-xi Liu, D. L. Zhou, X. X. Yi, C. P. Sun [pdf]
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Density dependence of nucleon bag constant, radius and mass in an effective field theory model of QCD -
Abstract
- With the global color symmetry model (GCM) being extended to finite chemical potential, the density dependence of the bag constant, the total energy and the radius of a nucleon, as well as the quark condensate in nuclear matter are investigated. A maximal nuclear matter density for the existence of the bag with three quarks confined within is obtained. The calculated results indicate that, before the maximal density is reached, the bag constant, the total energy of a nucleon and the quark condensate decrease gradually, and the radius of a nucleon increases, with the increasing of the nuclear matter density. Nevertheless no sudden change emerges. As the maximal nuclear matter density is reached, a phase transition from nucleons to quarks takes place and the chiral symmetry is restored.
Yu-xin Liu, Dong-feng Gao, Hua Guo Journal reference: Nucl.Phys. A695 (2001) 353-364 [pdf] DOI: 10.1016/S0375-9474(01)01120-4
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Unified theory of quantum many-particle systems -
Abstract
- Using eigen-functional bosonization method, we study quantum many-particle systems, and show that the quantum many-particle problems end in to solve the differential equation of the phase fields which represent the particle correlation strength. Thus, the physical properties of these systems are completely determined by the differential equation of the phase fields. We mainly focus on the study of D-dimensional electron gas with/without transverse gauge fields, two-dimensional electron gas under an external magnetic field, D-dimensional boson systems, a D-dimensional Heisenberg model and a one-band Hubbard model on a square lattice, and give their exact (accurate for Heisenberg model) functional expressions of the ground state energy and action, and the eigen-functional wave functions of the fermions/bosons. With them, we can calculate a variety of correlation functions of the systems, such as single particle Green's functions and their ground state wave functions. In present theoretical framework, we can unifiably represent the Landau Fermi liquid, non-Fermi liquid ($D\geq 2$) and Tomonaga-Luttinger liquid.
Yu-Liang Liu [pdf]
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Exact expression of the ground state energy of quantum many-particle
systems as a functional of the particle density -
Abstract
- By introducing a phase field and solving the eigen-functional equation of particles, we obtain the exact expressions of the ground state energy as a functional of the particle density for interacting electron/boson systems, and a two-dimensional electron gas under an external magnetic field, respectively. With the eigen-functionals of the particles, we can construct the ground state wave-function of the systems. Moreover, with the expressions of the ground state energy, we can exactly determine the ground state energy and the ground state particle density of the systems by taking $% \delta E_g[\rho ]/\delta \rho (x)=0$.
Yu-Liang Liu [pdf]
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Normal Ordering Solution to Quantum Dissipation and Its Induced Decoherence -
Abstract
- We implement the normal ordering technique to study the quantum dissipation of a single mode harmonic oscillator system. The dynamic evolution of the system is investigated for a reasonable initial state by solving the Schr\"{o}dinger equation directly through the normal ordering technique. The decoherence process of the system for the cases $T=0K$ and $T\neq0K$ is investigated as an application.
G. R. Jin, D. L. Zhou, Yu-xi Liu, X. X. Yi, C. P. Sun [pdf] DOI: 10.1088/0253-6102/36/3/331 quant-ph/0101078v2 [pdf]
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Dynamics of optically driven exciton and quantum decoherence -
Abstract
- 2000
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Influence of gauge fluctuations on fermion pairing order parameter -
Abstract
- Using a prototype model, we study the influence of gauge fluctuations on fermion pairing order parameter which has the gauge symmetry, and demonstrate that the gauge fluctuations can destroy the long range order of the fermion pairing order parameter, and make it only have short range correlation. If this parameter is a superconducting order parameter, we show that the Meissner effect of the system keeps intact, and the system is in the superconducting state even though the long range order of the superconducting order parameter is destroied by the gauge fluctuations. Our calculations support that the pseudo-gap region of the high Tc cuprate superconductivity is a spin pseudo-gap region rather than an electron pre-paired region.
Yu-Liang Liu [pdf]
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Unified thoery of strongly correlated electron systems -
Abstract
- In framework of eigen-functional bosonization method, we introduce an imaginary phase field to uniquely represent electron correlation, and demonstrate that the Landau Fermi liquid theory and the Tomonaga-Luttinger liquid theory can be unified. It is very clear in this framework that the Tomonaga-Luttinger liquid behavior of one-dimensional interacting electron gases originates from their Fermi structure, and the non-Landau-Fermi liquid behavior of 2D interacting electron gases is induced by the long-range electron interaction, while 3D interacting electron gases generally show the Landau Fermi liquid behavior.
Yu-Liang Liu [pdf]
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Eigen-functional bosonization and Eikonal-type equations in
one-dimensional strongly correlated electron system -
Abstract
- With the eigen-functional bosonization method, we study one-dimensional strongly correlated electron systems with large momentum ($2k_{F}$ and/or $4k_{F}$) transfer term(s), and demonstrate that this kind of problems ends in to solve the Eikonal-type equations, and these equations are universal, and independent of whether or not the system is integrable. In contrast to usual perturbation theory, this method is valid not only for weak electron interaction, but also for strong electron interaction. Comparing with exact solution of some integrable models, it can give correct results in one-loop approximation. This method can also be used to study electron-phonon interaction systems, and two coupled spin chain or quantum wire systems.
Yu-Liang Liu [pdf]
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Quantum decoherence of excitons in a leaky cavity with quasimode -
Abstract
- For the excitons in the quantum well placed within a leaky cavity, the quantum decoherence of a mesoscopically superposed states is investigated based on the factorization theory for quantum dissipation. It is found that the coherence of the exciton superposition states will decrease in an oscillating form when the cavity field interacting with the exciton is of the form of quasimode. The effect of the thermal cavity fields on the quantum decoherence of the superposition states of the exciton is studied and it is observed that the higher the temperature of the environment is, the shorter the decoherence characteristic time is.
Yu-xi Liu, C. P. Sun, S. X. Yu [pdf] DOI: 10.1103/PhysRevA.63.033816 quant-ph/0006059v1 [pdf]
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Semiconductor-cavity QED in high-
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Abstract
- The high density Frenkel exciton which interacts with a single mode microcavity field is dealed with in the framework of the q-deformed boson. It is shown that the q-defomation of bosonic commutation relations is satisfied naturally by the exciton operators when the low density limit is deviated. An analytical expression of the physical spectrum for the exciton is given by using of the dressed states of the cavity field and the exciton. We also give the numerical study and compare the theoretical results with the experimental results
Yu-Xi Liu, C. P Sun, S. X. Yu, D. L. Zhou [pdf] DOI: 10.1103/PhysRevA.63.023802 quant-ph/0001067v1 [pdf]
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Symmetries and Mei Conserved Quantities for Nonholonomic Systems with Servoconstraints -
Abstract
- We study the hybrid exciton-polaritons in a bad microcavity containing the organic and inorganic quantum wells. The corresponding polariton states are given. The analytical solution and the numerical result of the stationary spectrum for the cavity field are finished
Yu-Xi Liu, Chang-pu Sun [pdf] DOI: 10.1088/0253-6102/51/1/01 quant-ph/0001037v1 [pdf]
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Antibunching effect of the radiation field in a microcavity with a
mirror undergoing heavily damping oscillation -
Abstract
- The interaction between the radiation field in a microcavity with a mirror undergoing damping oscillation is investigated. Under the heavily damping cases, the mirror variables are adiabatically eliminated. The the stationary conditions of the system are discussed. The small fluctuation approximation around steady values is applied to analysis the antibunching effect of the cavity field. The antibunching condition is given under two limit cases.
Yu-xi Liu, Chang-pu Sun [pdf]
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Influence of gauge fluctuations on fermion pairing order parameter -
Abstract
- 1999
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Electronic transport through a quantum wire coupled to a quantum dot -
Abstract
- In this paper we re-examine the problem of electronic transports through a system consisting of a quantum dot which has well-defined discrete energy levels connected to an infinite quantum wire, using the bosonization method and phase shift representation, we show that all previously known results can be obtained through our method in a very simple way. Furthermore, the evolution of the system from ultraviolet to infrared critical fixed points appears naturally our method.
Yu-Liang Liu, T. K. Ng [pdf] DOI: 10.1103/PhysRevB.61.2911 cond-mat/9912197v1 [pdf]
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Connection between the closeness of classical orbits and the factorization of the radial Schrödinger equation -
Abstract
- It was shown that the Runge-Lenz vector for a hydrogen atom is equivalent to the raising and lowering operators derived from the factorization of radial Schr\"{o}dinger equation. Similar situation exists for an isotropic harmonic oscillator. It seems that there may exist intimate relation between the closeness of classical orbits and the factorization of radial Schr\"{o}dinger equation. Some discussion was made about the factorization of a 1D Schr\"{o}dinger equation.
Yu-feng Liu, Wu-jun Huo, Jinyan Zeng Journal reference: Phys.Rev.A58:862,1998 [pdf] DOI: 10.1103/PhysRevA.58.862
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Universal behavior of quantum impurity scattering in a Tomonaga-Luttinger liquid -
Abstract
- Using bosonization and path integral methods, we study general low temperature behavior of non-magnetic and magnetic impurity scattering in Tomonaga-Luttinger liquid, and calculate electron Green function for a general backward scattering potential. We demonstrate that electron density of state near the impurity site is suppressed by the backward scattering, but it mainly remains invariant as far away from the impurity, and at zero temperature the electrons are completely reflected on the impurity site, the system breaks into two subsystems but right- and left-moving electron fields have a twisted boundary condition. We also show that a testing charge can only be partially screened by conduction electrons, and in strong interaction region the impurity susceptibility has a 1/T-type low temperature behavior.
Yu-Liang Liu [pdf] DOI: 10.1103/PhysRevB.59.7937 cond-mat/9901032v1 [pdf]
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Electronic transport through a quantum wire coupled to a quantum dot -
Abstract
- 1997
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Low Temperature Behavior of the Kondo Effect in Tomonaga-Luttinger
Liquid -
Abstract
- Using the bosonization method, we study the low temperature behavior of the Kondo effect in the Tomonaga-Luttinger liquid and clearly show that the power law temperature dependence of the impurity susceptibility is completely determined by the repulsive electron-electron interaction existing in the total spin channel and is independent of the electron-electron interaction existing in the charge channels.
Yu-Liang Liu [pdf]
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Tunneling conductance and Coulomb-blockade peak splitting of two quantum dots connected by a quantum point contact -
Abstract
- By using bosonization method and unitary transformation, we give a general relation between the dimensionless tunneling conductance and the fractional Coulomb blockade conductance peak splitting which is valid both for weak and strong transmission between two quantum dots, and show that the tunneling conductance has a linear temperature dependence in the low energy and low temperature limit.
Yu-Liang Liu [pdf] DOI: 10.1103/PhysRevB.56.6732 cond-mat/9706056v1 [pdf]
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Conductance of a finite quantum wire connected to reservoirs -
Abstract
- We study a finite quantum wire connected to external leads, and show that the conductance of the system significantly depends upon the length of the quantum wire and the position of the impurity in it. For a very long quantum wire and the impurity far away from its two ends, the conductance has the same behavior as that for an infinity quantum wire above some very little energy scale. However, for a very short quantum wire, the conductance is independent of the electron-electron interactions in it and closing to $e^{2}/(2\pi\hbar)$ in a higher temperature range. While, in a lower temperature range, the conductance shows the same property as that for an infinity quantum wire.
Yu-Liang Liu [pdf] DOI: 10.1016/S0375-9601(97)00432-5 cond-mat/9706018v1 [pdf]
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Low-Temperature Behavior of a Magnetic Impurity in a Heisenberg Chain -
Abstract
- Using the bosonization technique, we have studied a spin-1/2 magnetic impurity in Heisenberg chain, and shown that the impurity specific heat and spin susceptibility have an anomalous temperature dependence.
Yu-Liang Liu [pdf] DOI: 10.1103/PhysRevLett.79.293 cond-mat/9706017v1 [pdf]
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Low Temperature Behavior of the Kondo Effect in Tomonaga-Luttinger
Liquid -
Abstract
- 1996
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An exact expression of the collective excitation energy gap of
fractional quantum Hall effect -
Abstract
- We have exactly solved the eigenequation of a two-dimensional Dirac fermion moving on the surface of a sphere under the influence of a radial magnetic field B, and obtained an exact expression of the collective excitation energy gap for the filling factors $\nu=p/(2mp\pm 1)$, m and p are non-zero integers, which is very well agreement with the computing results.
Yu-Liang Liu [pdf]
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Finite-size scaling for low-energy excitations in integer Heisenberg spin chains -
Abstract
- In this paper we study the finite size scaling for low energy excitations of $S=1$ and $S=2$ Heisenberg chains, using the density matrix renormalization group technique. A crossover from $1/L$ behavior (with $L$ as the chain length) for medium chain length to $1/L^2$ scaling for long chain length is found for excitations in the continuum band as the length of the open chain increases. Topological spin $S=1/2$ excitations are shown to give rise to the two lowest energy states for both open and periodic $S=1$ chains. In periodic chains these two excitations are ``confined'' next to each other, while for open chains they are two free edge 1/2 spins. The finite size scaling of the two lowest energy excitations of open $S=2$ chains is determined by coupling the two free edge $S=1$ spins. The gap and correlation length for $S=2$ open Heisenberg chains are shown to be 0.082 (in units of the exchange $J$) and 47, respectively.
Shaojin Qin, Yu-Liang Liu, Lu Yu [pdf] DOI: 10.1103/PhysRevB.55.2721 cond-mat/9610100v1 [pdf]
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Non-Fermi-liquid behavior in an extended Anderson model -
Abstract
- An extended Anderson model, including screening channels (non-hybridizing, but interacting with the local orbit), is studied within the Anderson-Yuval approach, originally devised for the single-channel Kondo problem. By comparing the perturbation expansions of this model and a generalized resonant level model, the spin-spin correlation functions are calculated which show non-Fermi liquid exponent depending on the strength of the scattering potential. The relevance of this result to experiments in some heavy fermion systems is briefly discussed.
Yu-Liang Liu, Zhao-Bin Su, Lu Yu [pdf] DOI: 10.1103/PhysRevB.54.15185 cond-mat/9608128v1 [pdf]
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Normal state property of the t-J model -
Abstract
- Using the spin-hole coherent state representation and taking a long range antiferromagnetic N\`{e}el order as a background of the localized spin degree part, we have studied the normal state behavior of the t-J model, and shown that a strongly short-range antiferromagnetic correlation of the localized spin degree part is responsible for the anomalous non-Korringa-like relaxation behavior of the planar copper spin, the Korringa-like behavior of the planar oxygen spin may derive from the charge degree part describing a Zhang-Rice spin-singlet; The charge degree part feels a strongly staggered magnetic field induced by this short-range antiferromagnetic correlation as a doping hole hopping, this staggered magnetic field enforces the charge degrees to have different responses to external magnetic and electric fields and to show two relaxation rate behaviors corresponding to the planar resistivity and Hall angle, respectively. We have found that the temperature dependence of magnetoresistance is $T^{-n}$, $n\simeq 3$, near the optimal doping, $n\simeq 4$, in the underdoping region, violating Kohler's rule, the transport relaxation rate is of the order of $2k_{B}T$, all that are consistent with the normal state of the cuprate superconductors.
Yu-Liang Liu [pdf]
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An exact expression of the collective excitation energy gap of
fractional quantum Hall effect -
Abstract
- 1995
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The Magnetic Behavior of the t-J Model -
Abstract
- Using the spin-hole coherent state representation, and taking the long range antiferromagnetic N\'{e}el order as the background of the spin degree part, we have studied the magnetic behavior of the t-J model in the usual slave boson and slave fermion treatment of the single occupation constraint, and shown that we can qualitatively explain the anomalous magnetic and transport properties of the normal state of the cuprate superconducting materials by the t-J model.
Yu-Liang Liu [pdf]
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Universal properties of the normal state of cuprate superconducting materials -
Abstract
- We have proposed a model Hamiltonian, which describes a simple physical picture that the holes with single occupation constraint introduced by doping move in the antiferromagnetic background of the copper spins, to describe the normal state of the cuprate superconducting materials, and used the renormalization group method to calculate its anomalous magnetic and transport properties. The anomalous magnetic behavior of the normal state is controlled by both the copper spin and the spin part of the doping hole residing on the O sites. The physical resistivity is determined by both the quasiparticle-spin-fluctuation and the quasiparticle-gauge-fluctuation scatterings and the Hall coefficient is determined by the parity-odd gauge interaction deriving from the nature of the hard-core boson which describes the charge part of the doping holes.
Yu-Liang Liu [pdf] DOI: 10.1016/0375-9601(95)00211-K cond-mat/9510015v1 [pdf]
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The Magnetic Behavior of the t-J Model -
Abstract
- 1994
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Quantum fluctuations of
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Abstract
- The dynamic Jahn-Teller splitting of the six equivalent $D_{5d}$ polarons due to quantum fluctuations is studied in the framework of the Bogoliubov-de Gennes formalism. The tunneling induced level splittings are determined to be $^2 T_{1u} \bigoplus ^2 T_{2u}$ and $^1 A_g \bigoplus ^1 H_g$ for $C_{60}^{1-}$ and $C_{60}^{2-}$, respectively, which should give rise to observable effects in experiments.
Chui-Lin Wang, Wen-Zheng Wang, Yu-Liang Liu, Zhao-Bin Su, Lu Yu [pdf] DOI: 10.1103/PhysRevB.50.5676 cond-mat/9406099v1 [pdf]
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Quantum fluctuations of
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Abstract