Publications by Brian Møller Andersen
 2021

The First CHIME/FRB Fast Radio Burst Catalog 
Abstract
 We present a catalog of 536 fast radio bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project between 400 and 800 MHz from 2018 July 25 to 2019 July 1, including 62 bursts from 18 previously reported repeating sources. The catalog represents the first large sample, including bursts from repeaters and nonrepeaters, observed in a single survey with uniform selection effects. This facilitates comparative and absolute studies of the FRB population. We show that repeaters and apparent nonrepeaters have sky locations and dispersion measures (DMs) that are consistent with being drawn from the same distribution. However, bursts from repeating sources differ from apparent nonrepeaters in intrinsic temporal width and spectral bandwidth. Through injection of simulated events into our detection pipeline, we perform an absolute calibration of selection effects to account for systematic biases. We find evidence for a population of FRBs  comprising a large fraction of the overall population  with a scattering time at 600 MHz in excess of 10 ms, of which only a small fraction are observed by CHIME/FRB. We infer a powerlaw index for the cumulative fluence distribution of $\alpha=1.40\pm0.11(\textrm{stat.})^{+0.06}_{0.09}(\textrm{sys.})$, consistent with the $3/2$ expectation for a nonevolving population in Euclidean space. We find $\alpha$ is steeper for highDM events and shallower for lowDM events, which is what would be expected when DM is correlated with distance. We infer a sky rate of $[820\pm60(\textrm{stat.})^{+220}_{200}({\textrm{sys.}})]/\textrm{sky}/\textrm{day}$ above a fluence of 5 Jy ms at 600 MHz, with scattering time at $600$ MHz under 10 ms, and DM above 100 pc cm$^{3}$.
 2106.04352v2 [pdf]

Subsecond periodicity in a fast radio burst 
Abstract
 The origin of fast radio bursts (FRBs), millisecondduration flashes of radio waves that are visible at distances of billions of lightyears, remains an open astrophysical question. Here we report the detection of the multicomponent FRB 20191221A with the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB), and the identification of a periodic separation of 216.8(1) ms between its components with a significance of 6.5 sigmas. The long (~ 3 s) duration and nine or more components forming the pulse profile make this source an outlier in the FRB population. We also report two additional FRBs, 20210206A and 20210213A, whose multicomponent pulse profiles show some indication of periodic separations of 2.8(1) and 10.7(1) ms, respectively, suggesting the possible existence of a group of FRBs with complex and periodic pulse profiles. Such short periodicities provide strong evidence for a neutronstar origin of these events. Moreover, our detections favour emission arising from the neutronstar magnetosphere, as opposed to emission regions located further away from the star, as predicted by some models. Possible explanations for the observed periodicity include supergiant pulses from a neutron star that are possibly related to a magnetar outburst and interacting neutron stars in a binary system.
 2107.08463v1 [pdf]

Revealing the competition between chargedensity wave and
superconductivity in CsV$_3$Sb$_5$ through uniaxial strain 
Abstract
 In this paper we report the impact of uniaxial strain $\varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor CsV$_3$Sb$_5$. At ambient conditions, CsV$_3$Sb$_5$ shows a chargedensity wave (CDW) transition at $T_{\rm CDW}=94.5$ K and superconducts below $T_C = 3.34$ K. In our study, when the uniaxial strain $\varepsilon$ is varied from $0.90\%$ to $0.90\%$, $T_C$ monotonically increases by $\sim 33\%$ from 3.0 K to 4.0 K, giving rise to the empirical relation $T_C (\varepsilon)=3.4+0.56\varepsilon+0.12\varepsilon^2$. On the other hand, for $\varepsilon$ changing from $0.76\%$ to $1.26\%$, $T_{\rm CDW}$ decreases monotonically by $\sim 10\%$ from 97.5 K to 87.5 K with $T_{\rm CDW}(\varepsilon)=94.54.72\varepsilon0.60\varepsilon^2$. The opposite response of $T_C$ and $T_{\rm CDW}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our firstprinciples calculations and phenomenological analysis, we conclude that the enhancement in $T_C$ with decreasing $c$ is caused primarily by the suppression of $T_{\rm CDW}$, rather than straininduced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{\rm CDW}$ with respect to the changes in the $c$axis arises from the impact of the latter on the trilinear coupling between the $M_1^+$ and $L_2^$ phonon modes associated with the CDW. Overall, our work reveals that the $c$axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of CsV$_3$Sb$_5$.
 2107.04545v2 [pdf]
Tiema Qian, Morten H. Christensen, Chaowei Hu, Amartyajyoti Saha, Brian M. Andersen, Rafael M. Fernandes, Turan Birol, Ni Ni [pdf]

Theory of the chargedensity wave in $A$V$_3$Sb$_5$ kagome metals 
Abstract
 The family of metallic kagome compounds $A$V$_3$Sb$_5$ ($A$=K, Rb, Cs) was recently discovered to exhibit both superconductivity and charge order. The nature of the chargedensity wave (CDW) phase is presently unsettled, which complicates the interpretation of the superconducting ground state. In this paper, we use grouptheory and densityfunctional theory (DFT) to derive and solve a phenomenological Landau model for this CDW state. The DFT results reveal three unstable phonon modes with the same inplane momentum but different outofplane momenta, whose frequencies depend strongly on the electronic temperature. This is indicative of an electronicallydriven CDW, stabilized by features of the inplane electronic dispersion. Motivated by the DFT analysis, we construct a Landau freeenergy expansion for coupled CDW order parameters with wavevectors at the $M$ and $L$ points of the hexagonal Brillouin zone. We find an unusual trilinear term coupling these different order parameters, which can promote the simultaneous condensation of both CDWs even if the two modes are not nearlydegenerate. We classify the different types of coupled multi$\bf{Q}$ CDW orders, focusing on those that break the sixfold rotational symmetry and lead to a unitcell doubling along all three crystallographic directions, as suggested by experiments. We determine a region in parameter space, characterized by large nonlinear Landau coefficients, where these phases  dubbed staggered trihexagonal and staggered StarofDavid  are the leading instabilities of the system. Finally, we discuss the implications of our results for the kagome metals.
 2107.04546v2 [pdf]
Morten H. Christensen, Turan Birol, Brian M. Andersen, Rafael M. Fernandes [pdf]

A Nearby Repeating Fast Radio Burst in the Direction of M81 
Abstract
 We report on the discovery of FRB 20200120E, a repeating fast radio burst (FRB) with low dispersion measure (DM), detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB project. The source DM of 87.82 pc cm$^{3}$ is the lowest recorded from an FRB to date, yet is significantly higher than the maximum expected from the Milky Way interstellar medium in this direction (~ 50 pc cm$^{3}$). We have detected three bursts and one candidate burst from the source over the period 2020 JanuaryNovember. The baseband voltage data for the event on 2020 January 20 enabled a sky localization of the source to within $\simeq$ 14 sq. arcmin (90% confidence). The FRB localization is close to M81, a spiral galaxy at a distance of 3.6 Mpc. The FRB appears on the outskirts of M81 (projected offset $\sim$ 20 kpc) but well inside its extended HI and thick disks. We empirically estimate the probability of chance coincidence with M81 to be $< 10^{2}$. However, we cannot reject a Milky Way halo origin for the FRB. Within the FRB localization region, we find several interesting cataloged M81 sources and a radio point source detected in the Very Large Array Sky Survey (VLASS). We searched for prompt Xray counterparts in Swift/BAT and Fermi/GBM data, and for two of the FRB 20200120E bursts, we rule out coincident SGR 1806$$20like Xray bursts. Due to the proximity of FRB 20200120E, future followup for prompt multiwavelength counterparts and subarcsecond localization could be constraining of proposed FRB models.
M. Bhardwaj, B. M. Gaensler, V. M. Kaspi, T. L. Landecker, R. Mckinven, D. Michilli, Z. Pleunis, S. P. Tendulkar, B. C. Andersen, P. J. Boyle, T. Cassanelli, P. Chawla, A. Cook, M. Dobbs, E. Fonseca, J. Kaczmarek, C. Leung, K. Masui, M. Münchmeyer, C. Ng, M. RafieiRavandi, P. Scholz, K. Shin, K. M. Smith, I. H. Stairs, A. V. Zwaniga [pdf] DOI: 10.3847/20418213/abeaa6 2103.01295v2 [pdf]

LOFAR Detection of 110–188 MHz Emission and Frequencydependent Activity from FRB 20180916B 
Abstract
 FRB 20180916B is a wellstudied repeating fast radio burst source. Its proximity (~150 Mpc), along with detailed studies of the bursts, have revealed many clues about its nature  including a 16.3day periodicity in its activity. Here we report on the detection of 18 bursts using LOFAR at 110188 MHz, by far the lowestfrequency detections of any FRB to date. Some bursts are seen down to the lowestobserved frequency of 110 MHz, suggesting that their spectra extend even lower. These observations provide an orderofmagnitude stronger constraint on the optical depth due to freefree absorption in the source's local environment. The absence of circular polarization and nearly flat polarization angle curves are consistent with burst properties seen at 3001700 MHz. Compared with higher frequencies, the larger burst widths (~40160 ms at 150 MHz) and lower linear polarization fractions are likely due to scattering. We find ~23 rad/m^2 variations in the Faraday rotation measure that may be correlated with the activity cycle of the source. We compare the LOFAR burst arrival times to those of 38 previously published and 22 newly detected bursts from the uGMRT (200450 MHz) and CHIME/FRB (400800 MHz). Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected by LOFAR. We find that the burst activity is systematically delayed towards lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results in the context of a model in which FRB 20180916B is an interacting binary system featuring a neutron star and highmass stellar companion.
Z. Pleunis, D. Michilli, C. G. Bassa, J. W. T. Hessels, A. Naidu, B. C. Andersen, P. Chawla, E. Fonseca, A. Gopinath, V. M. Kaspi, V. I. Kondratiev, D. Z. Li, M. Bhardwaj, P. J. Boyle, C. Brar, T. Cassanelli, Y. Gupta, A. Josephy, R. Karuppusamy, A. Keimpema, F. Kirsten, C. Leung, B. Marcote, K. Masui, R. Mckinven, B. W. Meyers, C. Ng, K. Nimmo, Z. Paragi, M. Rahman, P. Scholz, K. Shin, K. M. Smith, I. H. Stairs, S. P. Tendulkar [pdf] DOI: 10.3847/20418213/abec72 2012.08372v3 [pdf]

CorrelationInduced Insulating Topological Phases at Charge Neutrality in Twisted Bilayer Graphene 
Abstract
 Twisted bilayer graphene (TBG) provides a unique framework to elucidate the interplay between strong correlations and topological phenomena in twodimensional systems. The existence of multiple electronic degrees of freedom  charge, spin, and valley  gives rise to a plethora of possible ordered states and instabilities. Identifying which of them are realized in the regime of strong correlations is fundamental to shed light on the nature of the superconducting and correlated insulating states observed in the TBG experiments. Here, we use unbiased, signproblemfree quantum Monte Carlo simulations to solve an effective interacting lattice model for TBG at charge neutrality. Besides the usual cluster Hubbardlike repulsion, this model also contains an assisted hopping interaction that emerges due to the nontrivial topological properties of TBG. Such a nonlocal interaction fundamentally alters the phase diagram at charge neutrality, gapping the Dirac cones even for infinitesimally small interaction. As the interaction strength increases, a sequence of different correlated insulating phases emerge, including a quantum valley Hall state with topological edge states, an intervalleycoherent insulator, and a valence bond solid. The chargeneutrality correlated insulating phases discovered here provide the soughtafter reference states needed for a comprehensive understanding of the insulating states at integer fillings and the proximate superconducting states of TBG.
Yuan Da Liao, Jian Kang, Clara N. Breiø, Xiao Yan Xu, HanQing Wu, Brian M. Andersen, Rafael M. Fernandes, Zi Yang Meng Journal reference: Phys. Rev. X 11, 011014 (2021) [pdf] DOI: 10.1103/PhysRevX.11.011014

Superconducting state of

Abstract
 The symmetry of the superconducting condensate in Sr$_2$RuO$_4$ remains controversial after nuclear magnetic resonance (NMR) experiments recently overturned the dominant chiral $p$wave paradigm. Several theoretical proposals have been put forward to account for existing experiments, including a $d+ig$wave admixture, conjectured to be stabilized by longerrange Coulomb interactions. We perform a materialspecific microscopic theoretical study of pairing by spin and chargefluctuations in Sr$_2$RuO$_4$, including the effects of spinorbit coupling, and both local and longerrange Coulomb repulsion. The latter has important consequences for Sr$_2$RuO$_4$ due to the neardegeneracy of symmetrydistinct pairing states in this material. We find that both the $g$ and $d_{x^2y^2}$wave channels remain noncompetitive compared to leading nodal $s'$, $d_{xy}$, and helical ($p$) solutions. This suggests nodal timereversal symmetry broken $s'+id_{xy}$ or $s'+ip$ phases, promoted by longerrange Coulomb repulsion, as the most favorable candidates for Sr$_2$RuO$_4$. We analyse the properties of these states, and show that the $s'+id_{xy}$ solution agrees with the bulk of available experimental data, including recent discoveries from NMR, muon spin relaxation ($\mu$SR), and ultrasound measurements.
Astrid T. Rømer, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 104, 064507 (2021) [pdf] DOI: 10.1103/PhysRevB.104.064507

Orbitaldependent selfenergy effects and consequences for the superconducting gap structure in multiorbital correlated electron systems 
Abstract
 We perform a theoretical study of the effects of electronic correlations on the superconducting gap structure of multiband superconductors. In particular, by comparing standard RPAbased spinfluctuation mediated gap structures to those obtained within the FLEX formalism for an ironbased superconductor, we obtain directly the feedback effects from electronelectron interactions on the momentumspace gap structure. We show how selfenergy effects can lead to an orbital inversion of the orbitalresolved spin susceptibility, and thereby invert the hierarchy of the most important orbitals channels for superconducting pairing. This effect has important consequences for the detailed gap variations on the Fermi surface. We expect such selfenergy feedback on the pairing gap to be generally relevant for superconductivity in strongly correlated multiorbital systems.
Kristofer Björnson, Andreas Kreisel, Astrid T. Rømer, Brian M. Andersen Journal reference: Phys. Rev. B 103, 024508 (2021) [pdf] DOI: 10.1103/PhysRevB.103.024508

Interorbital nematicity and the origin of a single electron Fermi pocket in FeSe 
Abstract
 The electronic structure of the enigmatic ironbased superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. Here, we perform a theoretical study of the preferred nematic order originating from nearestneighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of interorbital $d_{xz}d_{xy}$ and $d_{yz}d_{xy}$ character, with spontaneously broken amplitudes for these two components. This interorbital nematic order naturally leads to distinct hybridization gaps at the $X$ and $Y$ points of the 1Fe Brillouin zone, and may thereby produce highly anisotropic Fermi surfaces with only a single electron pocket at one of these momentumspace locations. The associated superconducting gap structure obtained with the generated lowenergy electronic band structure from spinfluctuation mediated pairing agrees well with that measured experimentally. Finally, from a comparison of the computed spin susceptibility to available neutron scattering data, we discuss the necessity of additional selfenergy effects, and explore the role of orbitaldependent quasiparticle weights as a minimal means to include them.
Daniel Steffensen, Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 103, 054505 (2021) [pdf] DOI: 10.1103/PhysRevB.103.054505

Spatially dispersing YuShibaRusinov states in the unconventional superconductor FeTe0.55Se0.45 
Abstract
 By using scanning tunneling microscopy (STM) we find and characterize dispersive, energysymmetric ingap states in the ironbased superconductor $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tipsample distance. We find that the impurity state is of the YuShibaRusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$, which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tipgating scenario within the singleimpurity Anderson model and find good agreement to the experimental data.
Damianos Chatzopoulos, Doohee Cho, Koen M. Bastiaans, Gorm O. Steffensen, Damian Bouwmeester, Alireza Akbari, Genda Gu, Jens Paaske, Brian M. Andersen, Milan P. Allan Journal reference: Nature Communications 12, 298 (2021) [pdf] DOI: 10.1038/s4146702020529x

The First CHIME/FRB Fast Radio Burst Catalog 
Abstract
 2020

Topological Superconductivity Induced by Magnetic Textures 
Abstract
 We present an indepth classification of the topological phases and Majorana fermion (MF) excitations that arise from the bulk interplay between unconventional multiband spinsinglet superconductivity and various magnetic textures. We focus on magnetic texture crystals with a periodicallyrepeating primitive cell of the helix, whirl, and skyrmion types. Our analysis is relevant for a wide range of layered materials and hybrid devices, and accounts for both strong and weak, as well as crystalline topological phases. We identify a multitude of accessible topological phases which harbor flat, uni or bidirectional, (quasi)helical, or chiral MF edge modes. This rich variety of MFs originates from the interplay between topological phases with gapped and nodal bulk energy spectra, with the resulting types of spectra and MFs controlled by the size of the pairing and magnetic gaps.
 2012.09691v1 [pdf]
Daniel Steffensen, Morten H. Christensen, Brian M. Andersen, Panagiotis Kotetes [pdf]

First discovery of new pulsars and RRATs with CHIME/FRB 
Abstract
 We report the discovery of seven new Galactic pulsars with the Canadian Hydrogen Intensity Mapping Experiment's Fast Radio Burst backend (CHIME/FRB). These sources were first identified via single pulses in CHIME/FRB, then followed up with CHIME/Pulsar. Four sources appear to be rotating radio transients (RRATs), pulsarlike sources with occasional single pulse emission with an underlying periodicity. Of those four sources, three have detected periods ranging from 220 ms to 2.726 s. Three sources have more persistent but still intermittent emission and are likely intermittent or nulling pulsars. We have determined phasecoherent timing solutions for the latter three. These seven sources are the first discovery of previously unknown Galactic sources with CHIME/FRB and highlight the potential of fast radio burst detection instruments to search for intermittent Galactic radio sources.
 2012.02320v1 [pdf]

Role of anion in the pairing interaction of ironbased superconductivity 
Abstract
 Hightemperature ironbased superconductivity develops in a structure with unusual latticeorbital geometry, based on a planar layer of Fe atoms with 3d orbitals and tetrahedrally coordinated by anions. Here we elucidate the electronic role of anions in the ironbased superconductors utilizing stateoftheart scanning tunneling microscopy. By measuring the local electronic structure, we find that As anion in Ba0.4K0.6Fe2As2 has a striking impact on the electron pairing. The superconducting electronic feature can be switched off/on by removing/restoring As atoms on Fe layer at the atomic scale. Our analysis shows that this remarkable atomic switch effect is related to the geometrical cooperation between anion mediated hopping and unconventional pairing interaction. Our results uncover that the local Feanion coupling is fundamental for the pairing interaction of ironbased superconductivity, and promise the potential of bottomup engineering of electron pairing.
 2011.07701v1 [pdf]

Singular magnetic anisotropy in the nematic phase of FeSe 
Abstract
 FeSe is arguably the simplest, yet the most enigmatic, ironbased superconductor. Its nematic but nonmagnetic ground state is unprecedented in this class of materials and stands out as a current puzzle. Here, our NMR measurements in the nematic state of mechanically detwinned FeSe reveal that both the Knight shift and the spinlattice relaxation rate 1/T_1 possess an inplane anisotropy opposite to that of the iron pnictides LaFeAsO and BaFe2As2. Using a microscopic electron model that includes spinorbit coupling, our calculations show that an opposite quasiparticle weight ratio between the d_xz and d_yz orbitals leads to an opposite anisotropy of the orbital magnetic susceptibility, which explains our Knight shift results. We attribute this property to a different nature of nematic order in the two compounds, predominantly bondtype in FeSe and onsite ferroorbital in pnictides. The T_1 anisotropy is found to be inconsistent with existing neutron scattering data in FeSe, showing that the spin fluctuation spectrum reveals surprises at low energy, possibly from fluctuations that do not break C_4 symmetry. Therefore, our results reveal that important information is hidden in these anisotropies and they place stringent constraints on the lowenergy spin correlations as well as on the nature of nematicity in FeSe.
R. Zhou, D. D. Scherer, H. Mayaffre, P. Toulemonde, M. Ma, Y. Li, B. M. Andersen, M. H. Julien Journal reference: npj Quantum Mater. 5, 93 (2020) [pdf] DOI: 10.1038/s41535020002951

On the Remarkable Superconductivity of FeSe and Its Close Cousins 
Abstract
 Emergent electronic phenomena in ironbased superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origin and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spinhelical surface states and vortexinduced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe$_{1x}$Se$_x$, intercalated FeSe materials, and monolayer FeSe and FeTe$_{1x}$Se$_x$ on SrTiO$_3$. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSerelated materials, focusing again on interesting implications for superconductivity.
Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen Journal reference: Symmetry 12, 1402 (2020) [pdf] DOI: 10.3390/sym12091402

Majorana Zero Modes in Magnetic Texture Vortices 
Abstract
 We propose an alternative route to engineer Majorana zero modes (MZMs), which relies on inducing shift or spin vortex defects in magnetic textures which microscopically coexist or are in proximity to a superconductor. The present idea applies to a variety of superconducting materials and hybrid structures, irrespectively of their spinsinglet, triplet, or mixed type of pairing, as long as their bulk energy spectrum contains robust point nodes. Our mechanism provides a new framework to understand the recent observations of pairs of MZMs in superconductor  magnetic adatom systems. Moreover, it can inspire the experimental development of new platforms, consisting of nanowires in proximity to conventional superconductors with strong Rashba spinorbit coupling.
 2008.10626v1 [pdf]
Daniel Steffensen, Brian M. Andersen, Panagiotis Kotetes [pdf]

Spinorbit quantum impurity in a topological magnet 
Abstract
 Quantum states induced by singleatomic impurities are at the frontier of physics and material science. While such states have been reported in hightemperature superconductors and dilute magnetic semiconductors, they are unexplored in topological magnets which can feature spinorbit tunability. Here we use spinpolarized scanning tunneling microscopy/spectroscopy (STM/S) to study the engineered quantum impurity in a topological magnet Co3Sn2S2. We find that each substituted In impurity introduces a striking localized bound state. Our systematic magnetizationpolarized probe reveals that this bound state is spindown polarized, in lock with a negative orbital magnetization. Moreover, the magnetic bound states of neighboring impurities interact to form quantized orbitals, exhibiting an intriguing spinorbit splitting, analogous to the splitting of the topological fermion line. Our work collectively demonstrates the strong spinorbit effect of the singleatomic impurity at the quantum level, suggesting that a nonmagnetic impurity can introduce spinorbit coupled magnetic resonance in topological magnets.
JiaXin Yin, Nana Shumiya, Yuxiao Jiang, Huibin Zhou, Gennevieve Macam, Hano Omar Mohammad Sura, Songtian S. Zhang, Zijia Cheng, Zurab Guguchia, Yangmu Li, Qi Wang, Maksim Litskevich, Ilya Belopolski, Xian Yang, Tyler A. Cochran, Guoqing Chang, Qi Zhang, ZhiQuan Huang, FengChuan Chuang, Hsin Lin, Hechang Lei, Brian M. Andersen, Ziqiang Wang, Shuang Jia, M. Zahid Hasan Journal reference: Nature Communications 11, 4415 (2020) [pdf] DOI: 10.1038/s41467020181116

Nonlocal correlations in iron pnictides and chalcogenides 
Abstract
 Deviations of lowenergy electronic structure of ironbased superconductors from density functional theory predictions have been parametrized in terms of band and orbitaldependent mass renormalizations and energy shifts. The former have typically been described in terms of a local selfenergy within the framework of dynamical mean field theory, while the latter appears to require nonlocal effects due to interband scattering. By calculating the renormalized bandstructure in both random phase approximation (RPA) and the twoparticle selfconsistent approximation (TPSC), we show that correlations in pnictide systems like LaFeAsO and LiFeAs can be described rather well by a nonlocal selfenergy. In particular, Fermi pocket shrinkage as seen in experiment occurs due to repulsive interband finiteenergy scattering. For the canonical iron chalcogenide system FeSe in its bulk tetragonal phase, the situation is however more complex since even including momentumdependent band renormalizations cannot explain experimental findings. We propose that the longrange Coulomb interaction may play an important role in bandstructure renormalization in FeSe. We further compare our evaluations of nonlocal quasiparticle scattering lifetime within RPA and TPSC with experimental data for LiFeAs.
Shinibali Bhattacharyya, Kristofer Björnson, Karim Zantout, Daniel Steffensen, Laura Fanfarillo, Andreas Kreisel, Roser Valentí, Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. B 102, 035109 (2020) [pdf] DOI: 10.1103/PhysRevB.102.035109

A Distant Fast Radio Burst Associated with Its Host Galaxy by the Very Large Array 
Abstract
 We present the discovery and subarcsecond localization of a new Fast Radio Burst with the Karl G. Jansky Very Large Array and realfast search system. The FRB was discovered on 2019 June 14 with a dispersion measure of 959 pc/cm3. This is the highest DM of any localized FRB and its measured burst fluence of 0.6 Jy ms is less than nearly all other FRBs. The source is not detected to repeat in 15 hours of VLA observing and 153 hours of CHIME/FRB observing. We describe a suite of statistical and data quality tests we used to verify the significance of the event and its localization precision. Followup optical/infrared photometry with Keck and Gemini associate the FRB to a pair of galaxies with $\rm{r}\sim23$ mag. The falsealarm rate for radio transients of this significance that are associated with a host galaxy is roughly $3\times10^{4}\ \rm{hr}^{1}$. The two putative host galaxies have similar photometric redshifts of $z_{\rm{phot}}\sim0.6$, but different colors and stellar masses. Comparing the host distance to that implied by the dispersion measure suggests a modest (~ 50 pc/cm3) electron column density associated with the FRB environment or host galaxy/galaxies.
C. J. Law, B. J. Butler, J. X. Prochaska, B. Zackay, S. BurkeSpolaor, A. Mannings, N. Tejos, A. Josephy, B. Andersen, P. Chawla, K. E. Heintz, K. Aggarwal, G. C. Bower, P. B. Demorest, C. D. Kilpatrick, T. J. W. Lazio, J. Linford, R. Mckinven, S. Tendulkar, S. Simha [pdf] DOI: 10.3847/15384357/aba4ac 2007.02155v1 [pdf]

Periodic activity from a fast radio burst source 
Abstract
 Fast radio bursts (FRBs) are bright, millisecondduration radio transients originating from extragalactic distances. Their origin is unknown. Some FRB sources emit repeat bursts, ruling out cataclysmic origins for those events. Despite searches for periodicity in repeat burst arrival times on time scales from milliseconds to many days, these bursts have hitherto been observed to appear sporadically, and though clustered, without a regular pattern. Here we report the detection of a $16.35\pm0.15$ day periodicity (or possibly a higherfrequency alias of that periodicity) from a repeating FRB 180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September 16th, 2018 through February 4th, 2020, we find that all bursts arrive in a 5day phase window, and 50% of the bursts arrive in a 0.6day phase window. Our results suggest a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption, and disfavour models invoking purely sporadic processes.

A bright millisecondduration radio burst from a Galactic magnetar 
Abstract
 Magnetars are highly magnetized young neutron stars that occasionally produce enormous bursts and flares of Xrays and gammarays. Of the approximately thirty magnetars currently known in our Galaxy and Magellanic Clouds, five have exhibited transient radio pulsations. Fast radio bursts (FRBs) are millisecondduration bursts of radio waves arriving from cosmological distances. Some have been seen to repeat. A leading model for repeating FRBs is that they are extragalactic magnetars, powered by their intense magnetic fields. However, a challenge to this model has been that FRBs must have radio luminosities many orders of magnitude larger than those seen from known Galactic magnetars. Here we report the detection of an extremely intense radio burst from the Galactic magnetar SGR 1935+2154 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project. The fluence of this twocomponent bright radio burst and the estimated distance to SGR 1935+2154 together imply a 400800 MHz burst energy of $\sim 3 \times 10^{34}$ erg, which is three orders of magnitude brighter than those of any radioemitting magnetar detected thus far. Such a burst coming from a nearby galaxy would be indistinguishable from a typical FRB. This event thus bridges a large fraction of the radio energy gap between the population of Galactic magnetars and FRBs, strongly supporting the notion that magnetars are the origin of at least some FRBs.

Detection of Repeating FRB 180916.J0158+65 Down to Frequencies of 300 MHz 
Abstract
 We report on the detection of seven bursts from the periodically active, repeating fast radio burst (FRB) source FRB 180916.J0158+65 in the 300400MHz frequency range with the Green Bank Telescope (GBT). Emission in multiple bursts is visible down to the bottom of the GBT band, suggesting that the cutoff frequency (if it exists) for FRB emission is lower than 300 MHz. Observations were conducted during predicted periods of activity of the source, and had simultaneous coverage with the Low Frequency Array (LOFAR) and the FRB backend on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. We find that one of the GBTdetected bursts has potentially associated emission in the CHIME band (400800 MHz) but we detect no bursts in the LOFAR band (110190 MHz), placing a limit of $\alpha > 1.0$ on the spectral index of broadband emission from the source. We also find that emission from the source is severely bandlimited with burst bandwidths as low as $\sim$40 MHz. In addition, we place the strictest constraint on observable scattering of the source, $<$ 1.7 ms, at 350 MHz, suggesting that the circumburst environment does not have strong scattering properties. Additionally, knowing that the circumburst environment is optically thin to freefree absorption at 300 MHz, we find evidence against the association of a hypercompact HII region or a young supernova remnant (age $<$ 50 yr) with the source.
P. Chawla, B. C. Andersen, M. Bhardwaj, E. Fonseca, A. Josephy, V. M. Kaspi, D. Michilli, Z. Pleunis, K. M. Bandura, C. G. Bassa, P. J. Boyle, C. Brar, T. Cassanelli, D. Cubranic, M. Dobbs, F. Q. Dong, B. M. Gaensler, D. C. Good, J. W. T. Hessels, T. L. Landecker, C. Leung, D. Z. Li, H. . H. Lin, K. Masui, R. Mckinven, J. MenaParra, M. Merryfield, B. W. Meyers, A. Naidu, C. Ng, C. Patel, M. RafieiRavandi, M. Rahman, P. Sanghavi, P. Scholz, K. Shin, K. M. Smith, I. H. Stairs, S. P. Tendulkar, K. Vanderlinde [pdf] DOI: 10.3847/20418213/ab96bf 2004.02862v2 [pdf]

Theory of straininduced magnetic order and splitting of

Abstract
 The internal structure of the superconducting state in Sr$_2$RuO$_4$ remains elusive at present, and exhibits evidence for timereversal symmetry breaking. Recent muon spin relaxation measurements under uniaxial strain have revealed an increasing splitting between the superconducting critical temperature $T_c$ and the onset of timereversal symmetry breaking $T_{\rm TRSB}$ with applied strain [Grinenko et al., arXiv:2001.08152]. In addition, static magnetic order is induced by the uniaxial strain beyond $\sim$1 GPa, indicating that unstrained Sr$_2$RuO$_4$ is close to a magnetic quantum critical point. Here, we perform a theoretical study of the magnetic susceptibility and the associated pairing structure as a function of uniaxial strain. It is found that the recent muon relaxation data can be qualitatively explained from the perspective of spinfluctuation mediated pairing and the associated straindependence of accidentally degenerate pair states in unstrained Sr$_2$RuO$_4$. In addition, while unstrained Sr$_2$RuO$_4$ features mainly $(2\pi/3,2\pi/3)$ magnetic fluctuations, uniaxial strain promotes $(\pi,\pm\pi/2)$ magnetism.
Astrid T. Rømer, Andreas Kreisel, Marvin A. Müller, P. J. Hirschfeld, Ilya M. Eremin, Brian M. Andersen Journal reference: Phys. Rev. B 102, 054506 (2020) [pdf] DOI: 10.1103/PhysRevB.102.054506

Visualization of Local Magnetic Moments Emerging from Impurities in Hund’s Metal States of FeSe 
Abstract
 Understanding the origin of the magnetism of high temperature superconductors is crucial for establishing their unconventional pairing mechanism. Recently, theory predicts that FeSe is close to a magnetic quantum critical point, and thus weak perturbations such as impurities could induce local magnetic moments. To elucidate such quantum instability, we have employed scanning tunneling microscopy and spectroscopy. In particular, we have grown FeSe film on superconducting Pb(111) using molecular beam epitaxy and investigated magnetic excitation caused by impurities in the proximityinduced superconducting gap of FeSe. Our study provides a deep insight into the origin of the magnetic ordering of FeSe by showing the way local magnetic moments develop in response to impurities near the magnetic quantum critical point.
Sang Yong Song, J. H. J. Martiny, A. Kreisel, B. M. Andersen, Jungpil Seo Journal reference: Phys. Rev. Lett. 124, 117001 (2020) [pdf] DOI: 10.1103/PhysRevLett.124.117001

Spinorbit quantum impurity in a topological kagome magnet 
Abstract
 Quantum states induced by singleatomicimpurities are the current frontier of material and information science. Recently the spinorbit coupled correlated kagome magnets are emerging as a new class of topological quantum materials, although the effect of singleatomic impurities remains unexplored. Here we use stateoftheart scanning tunneling microscopy/spectroscopy (STM/S) to study the atomic indium impurity in a topological kagome magnet Co3Sn2S2, which is designed to support the spinorbit quantum state. We find each impurity features a strongly localized bound state. Our systematic magnetizationpolarized tunneling probe reveals its spindown polarized nature with an unusual moment of 5uB, indicative of additional orbital magnetization. As the separation between two impurities progressively shrinks, their respective bound states interact and form quantized molecular orbital states. The molecular orbital of three neighboring impurities further exhibits an intriguing splitting owing to the combination of geometry, magnetism, and spinorbit coupling, analogous to the splitting of the topological Weyl fermion line12,19. Our work demonstrates the quantumlevel interplay between magnetism and spinorbit coupling at an individual atomic impurity, which provides insights into the emergent impurity behavior in a topological kagome magnet and the potential of spinorbit quantum impurities for information science.
 2002.11783v1 [pdf]

Nine New Repeating Fast Radio Burst Sources from CHIME/FRB 
Abstract
 We report on the discovery and analysis of bursts from nine new repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 195 to 1380 pc cm$^{3}$. We detect two bursts from three of the new sources, three bursts from four of the new sources, four bursts from one new source, and five bursts from one new source. We determine sky coordinates of all sources with uncertainties of $\sim$10$^\prime$. We detect Faraday rotation measures for two sources, with values $20(1)$ and $499.8(7)$ rad m$^{2}$, that are substantially lower than the RM derived from bursts emitted by FRB 121102. We find that the DM distribution of our events, combined with the nine other repeaters discovered by CHIME/FRB, is indistinguishable from that of thus far nonrepeating CHIME/FRB events. However, as previously reported, the burst widths appear statistically significantly larger than the thus far nonrepeating CHIME/FRB events, further supporting the notion of inherently different emission mechanisms and/or local environments. These results are consistent with previous work, though are now derived from 18 repeating sources discovered by CHIME/FRB during its first year of operation. We identify candidate galaxies that may contain FRB 190303.J1353+48 (DM = 222.4 pc cm$^{3}$).
E. Fonseca, B. C. Andersen, M. Bhardwaj, P. Chawla, D. C. Good, A. Josephy, V. M. Kaspi, K. W. Masui, R. Mckinven, D. Michilli, Z. Pleunis, K. Shin, S. P. Tendulkar, K. M. Bandura, P. J. Boyle, C. Brar, T. Cassanelli, D. Cubranic, M. Dobbs, F. Q. Dong, B. M. Gaensler, G. Hinshaw, T. L. Landecker, C. Leung, D. Z. Li, H. H. Lin, J. MenaParra, M. Merryfield, A. Naidu, C. Ng, C. Patel, U. Pen, M. RafieiRavandi, M. Rahman, S. M. Ransom, P. Scholz, K. M. Smith, I. H. Stairs, K. Vanderlinde, P. Yadav, A. V. Zwaniga [pdf] DOI: 10.3847/20418213/ab7208 2001.03595v2 [pdf]

Fluctuationdriven superconductivity in Sr2RuO4 from weak repulsive interactions 
Abstract
 We provide results for the leading superconducting instabilities for a model pertaining to Sr$_2$RuO$_4$ obtained within spinfluctuation mediated superconductivity in the very weakcoupling limit. The theory incorporates spinorbit coupling (SOC) effects both in the band structure and in the pairing kernel in the form of associated magnetic anisotropies. The leading superconducting phase is found to be $d_{x^2y^2}$ and a nodal $s$wave state. However, the oddparity helical solution can become leading either for small SOC and Hund's coupling $J$ in the weak $U$limit, or in the opposite limit with large SOC and $J$ at larger values of the Hubbard$U$. The oddparity chiral solution is never found to be leading. Finally we discuss the form of the resulting superconducting spectral gaps in the different explored parameter regimes.
Astrid T. Rømer, Brian M. Andersen Journal reference: Modern Physics Letters B, Vol. 34, No. 19n20, 2040052 (2020) [pdf] DOI: 10.1142/S0217984920400527

A repeating fast radio burst source localized to a nearby spiral galaxy 
Abstract
 Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes. Their physical origin remains unknown, but dozens of possible models have been postulated. Some FRB sources exhibit repeat bursts. Though over a hundred FRB sources have been discovered to date, only four have been localised and associated with a host galaxy, with just one of the four known to repeat. The properties of the host galaxies, and the local environments of FRBs, provide important clues about their physical origins. However, the first known repeating FRB has been localised to a lowmetallicity, irregular dwarf galaxy, and the apparently nonrepeating sources to highermetallicity, massive elliptical or starforming galaxies, suggesting that perhaps the repeating and apparently nonrepeating sources could have distinct physical origins. Here we report the precise localisation of a second repeating FRB source, FRB 180916.J0158+65, to a starforming region in a nearby (redshift $z = 0.0337 \pm 0.0002$) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure further distinguish the local environment of FRB 180916.J0158+65 from that of the one previously localised repeating FRB source, FRB 121102. This demonstrates that repeating FRBs have a wide range of luminosities, and originate from diverse host galaxies and local environments.

Size and Shape Constraints of (486958) Arrokoth from Stellar Occultations 
Abstract
 We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twentyfive mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by SOFIA with one very short chord. Twentyfour deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twentytwo systems were deployed for the fourth event on 2018 Aug 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 $\pm$ 0.0005 hours. The occultation data also provided highprecision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies.

Variation of shear moduli across superconducting phase transitions 
Abstract
 We study how shear moduli of a correlated metal change across superconducting phase transitions. Using a microscopic theory we explain why for most classes of superconductors this change is small. The Febased and the A15 systems are notable exceptions where the change is boosted by five orders of magnitude. We show that this boost is a consequence of enhanced nematic correlation. The theory explains the unusual temperature dependence of the orthorhombic shear and the backbending of the nematic transition line in the superconducting phase of the Febased systems.
Dimitri Labat, Panagiotis Kotetes, Brian M. Andersen, Indranil Paul Journal reference: Phys. Rev. B 101, 144502 (2020) [pdf] DOI: 10.1103/PhysRevB.101.144502

Pairing in the twodimensional Hubbard model from weak to strong coupling 
Abstract
 The Hubbard model is the simplest model that is believed to exhibit superconductivity arising from purely repulsive interactions, and has been extensively applied to explore a variety of unconventional superconducting systems. Here we study the evolution of the leading superconducting instabilities of the singleorbital Hubbard model on a twodimensional square lattice as a function of onsite Coulomb repulsion $U$ and band filling by calculating the irreducible particleparticle scattering vertex obtained from dynamical cluster approximation (DCA) calculations, and compare the results to both perturbative KohnLuttinger (KL) theory as well as the widely used random phase approximation (RPA) spinfluctuation pairing scheme. Near halffilling we find remarkable agreement of the hierarchy of the leading pairing states between these three methods, implying adiabatic continuity between weak and strongcoupling pairing solutions of the Hubbard model. The $d_{x^2y^2}$wave instability is robust to increasing $U$ near halffilling as expected. Away from half filling, the predictions of KL and RPA at small $U$ for transitions to other pair states agree with DCA at intermediate $U$ as well as recent diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute limit, where it yields a $d_{xy}$ ground state instead of a $p$wave state established by diagrammatic Monte Carlo and loworder perturbative methods, as well as our DCA calculations. We discuss the origins of this discrepancy, highlighting the crucial role of the vertex corrections neglected in the RPA approach. Overall, comparison of the various methods over the entire phase diagram strongly suggests a smooth crossover of the superconducting interaction generated by local Hubbard interactions between weak and strong coupling.
Astrid T. Rømer, Thomas A. Maier, Andreas Kreisel, Ilya Eremin, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. Research 2, 013108 (2020) [pdf] DOI: 10.1103/PhysRevResearch.2.013108

Topological Superconductivity Induced by Magnetic Textures 
Abstract
 2019

Quantum Phase Transition of Correlated IronBased Superconductivity in

Abstract
 The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the hightemperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy (STM) to image the electronic impact of Co atoms on the ground state of the LiFe$_{1x}$Co$_x$As system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strongcoupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Bornlimit scattering effect in a s$\pm$wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.
JiaXin Yin, Songtian S. Zhang, Guangyang Dai, Yuanyuan Zhao, Andreas Kreisel, Gennevieve Macam, Xianxin Wu, Hu Miao, ZhiQuan Huang, Johannes H. J. Martiny, Brian M. Andersen, Nana Shumiya, Daniel Multer, Maksim Litskevich, Zijia Cheng, Xian Yang, Tyler A. Cochran, Guoqing Chang, Ilya Belopolski, Lingyi Xing, Xiancheng Wang, Yi Gao, FengChuan Chuang, Hsin Lin, Ziqiang Wang, Changqing Jin, Yunkyu Bang, M. Zahid Hasan Journal reference: Phys. Rev. Lett. 123, 217004 (2019) [pdf] DOI: 10.1103/PhysRevLett.123.217004

CHIME/FRB Detection of Eight New Repeating Fast Radio Burst Sources 
Abstract
 We report on the discovery of eight repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 103.5 to 1281 pc cm$^{3}$. They display varying degrees of activity: six sources were detected twice, another three times, and one ten times. These eight repeating FRBs likely represent the bright and/or highrate end of a distribution of infrequently repeating sources. For all sources, we determine sky coordinates with uncertainties of $\sim$10$^\prime$. FRB 180916.J0158+65 has a burstaveraged DM = $349.2 \pm 0.3$ pc cm$^{3}$ and a low DM excess over the modelled Galactic maximum (as low as $\sim$20 pc cm$^{3}$); this source also has a Faraday rotation measure (RM) of $114.6 \pm 0.6$ rad m$^{2}$, much lower than the RM measured for FRB 121102. FRB 181030.J1054+73 has the lowest DM for a repeater, $103.5 \pm 0.3$ pc cm$^{3}$, with a DM excess of $\sim$ 70 pc cm$^{3}$. Both sources are interesting targets for multiwavelength followup due to their apparent proximity. The DM distribution of our repeater sample is statistically indistinguishable from that of the first 12 CHIME/FRB sources that have not repeated. We find, with 4$\sigma$ significance, that repeater bursts are generally wider than those of CHIME/FRB bursts that have not repeated, suggesting different emission mechanisms. Our repeater events show complex morphologies that are reminiscent of the first two discovered repeating FRBs. The repetitive behavior of these sources will enable interferometric localizations and subsequent host galaxy identifications.
 1908.03507v3 [pdf]

Effects of spinorbit coupling on spinfluctuation induced pairing in
ironbased superconductors 
Abstract
 We perform a theoretical study of the leading pairing instabilities and the associated superconducting gap functions within the spinfluctuation mediated pairing scenario in the presence of spinorbit coupling (SOC). Focussing on ironbased superconductors (FeSCs), our model Hamiltonian consists of a realistic density functional theory (DFT)derived tenband hopping term, spinorbit coupling, and electronelectron interactions included via the multiorbital HubbardHund Hamiltonian. We perform an extensive parameter sweep and investigate different doping regimes including cases with only hole or only electron Fermi pockets. In addition, we explore two different bandstructures: a rather generic band derived for LaFeAsO but known to represent standard DFTobtained bands for ironbased superconductors, and a band specifically tailored for FeSe which exhibits a notably different Fermi surface compared to the generic case. It is found that for the generic FeSCs band, even rather large SOC has negligible effect on the resulting gap structure; the $s_{+}$ (pseudo)spin singlet pairing remains strongly favored and SOC does not lead to any SOCcharacteristic gap oscillations along the various Fermi surfaces. By contrast in the strongly holedoped case featuring only holepockets around the $\Gamma$point, the leading solution is $d$wave pseudospin singlet, but with a notable SOCdriven tendency towards helical pseudospin triplet pairing, which may even become the leading instability. In the heavily electron doped situation, featuring only electron pockets centered around the $M$point, the leading superconducting instabilities are pseudospin singlets with SOC favoring the $s$wave case as compared to $d$wave pairing, which is the favored gap symmetry for vanishing SOC.
 1909.01313v1 [pdf]
Daniel D. Scherer, Brian M. Andersen [pdf]

CHIME/FRB Detection of the Original Repeating Fast Radio Burst Source FRB 121102 
Abstract
 We report the detection of a single burst from the firstdiscovered repeating Fast Radio Burst source, FRB 121102, with CHIME/FRB, which operates in the frequency band 400800 MHz. The detected burst occurred on 2018 November 19 and its emission extends down to at least 600 MHz, the lowest frequency detection of this source yet. The burst, detected with a significance of 23.7$\sigma$, has fluence 12$\pm$3 Jy ms and shows complex time and frequency morphology. The 34 ms width of the burst is the largest seen for this object at any frequency. We find evidence of subburst structure that drifts downward in frequency at a rate of 3.9$\pm$0.2 MHz ms$^{1}$. Our best fit tentatively suggests a dispersion measure of 563.6$\pm$0.5 pc cm$^{3}$, which is ${\approx}$1% higher than previously measured values. We set an upper limit on the scattering time at 500 MHz of 9.6 ms, which is consistent with expectations from the extrapolation from higher frequency data. We have exposure to the position of FRB 121102 for a total of 11.3 hrs within the FWHM of the synthesized beams at 600 MHz from 2018 July 25 to 2019 February 25. We estimate on the basis of this single event an average burst rate for FRB 121102 of 0.110 per day in the 400800 MHz band for a median fluence threshold of 7 Jy ms in the stated time interval.
A. Josephy, P. Chawla, E. Fonseca, C. Ng, C. Patel, Z. Pleunis, P. Scholz, B. C. Andersen, K. Bandura, M. Bhardwaj, M. M. Boyce, P. J. Boyle, C. Brar, D. Cubranic, M. Dobbs, B. M. Gaensler, A. Gill, U. Giri, D. C. Good, M. Halpern, G. Hinshaw, V. M. Kaspi, T. L. Landecker, D. A. Lang, H. H. Lin, K. W. Masui, R. Mckinven, J. MenaParra, M. Merryfield, D. Michilli, N. Milutinovic, A. Naidu, U. Pen, M. RafieiRavandi, M. Rahman, S. M. Ransom, A. Renard, S. R. Siegel, K. M. Smith, I. H. Stairs, S. P. Tendulkar, K. Vanderlinde, P. Yadav, A. V. Zwaniga [pdf] DOI: 10.3847/20418213/ab2c00 1906.11305v1 [pdf]

Competing Electronic Phases near the Onset of Superconductivity in Holedoped SrFe

Abstract
 An intriguingly complex phase diagram of Nadoped SrFe2As2 is uncovered using highresolution thermalexpansion, magnetization and heatcapacity measurements. The detailed temperature dependence of the orthorhombic distortion and the anisotropy of the uniform magnetic susceptibility provide evidence for nine distinct electronic phases near the transition region between stripe antiferromagnetism and unconventional superconductivity. In particular, we report the finding of a new magnetic phase which competes surprisingly strongly with superconductivity. From theoretical studies we propose that this phase is a doubleQ phase consisting of a mixture of symmetrydistinct commensurate magnetic orders with a peculiar temperaturedependent magnetic moment reorientation.
L. Wang, M. He, D. D. Scherer, F. Hardy, P. Schweiss, T. Wolf, M. Merz, B. M. Andersen, C. Meingast Journal reference: J. Phys. Soc. Jpn. 88, 104710 (2019) [pdf] DOI: 10.7566/JPSJ.88.104710

Effects of spinorbit coupling on the neutron spin resonance in
ironbased superconductors 
Abstract
 The socalled neutron spin resonance consists of a prominent enhancement of the magnetic response at a particular energy and momentum transfer upon entering the superconducting state of unconventional superconductors. In the case of ironbased superconductors, the neutron resonance has been extensively studied experimentally, and a peculiar spinspace anisotropy has been identified by polarized inelastic neutron scattering experiments. Here we perform a theoretical study of the energy and spinresolved magnetic susceptibility in the superconducting state with $ s_{+} $wave order parameter, relevant to ironpnictide and ironchalcogenide superconductors. Our model is based on a realistic bandstructure including spinorbit coupling with electronic HubbardHund interactions included at the RPA level. Spinorbit coupling is taken into account both in the generation of spinfluctuation mediated pairing, as well as the numerical computation of the spin susceptibility in the superconducting state. We find that spinorbit coupling and superconductivity in conjunction can reproduce the salient experimentally observed features of the magnetic anisotropy of the neutron resonance. This includes the possibility of a double resonance, the tendency for a $c$axis polarized resonance, and the existence of enhanced magnetic anisotropy upon entering the superconducting phase.
 1906.08566v1 [pdf]
Daniel D. Scherer, Brian M. Andersen [pdf]

Disorderinduced electronic nematicity 
Abstract
 We expose the theoretical mechanisms underlying disorderinduced nematicity in systems exhibiting strong fluctuations or ordering in the nematic channel. Our analysis consists of a symmetrybased GinzburgLandau approach and associated microscopic calculations. We show that a single featureless pointlike impurity induces nematicity locally, already above the critical nematic transition temperature. The persistence of fourfold rotational symmetry constrains the resulting disorderinduced nematicity to be inhomogeneous and spatially average to zero. Going beyond the single impurity case, we discuss the effects of finite disorder concentrations on the appearance of nematicity. We identify the conditions that allow disorder to enhance the nematic transition temperature, and we provide a concrete example. The presented theoretical results can explain a large series of recent experimental discoveries of disorderinduced nematic order in ironbased superconductors.
Daniel Steffensen, Panagiotis Kotetes, Indranil Paul, Brian M. Andersen Journal reference: Phys. Rev. B 100, 064521 (2019) [pdf] DOI: 10.1103/PhysRevB.100.064521

Anisotropic spin fluctuations in detwinned FeSe 
Abstract
 Superconductivity in FeSe emerges from a nematic phase that breaks fourfold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations, or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors QAF = (1, 0) at low energies E = 611 meV in the normal state. This twofold (C2) anisotropy is reduced at lower energies 35 meV, indicating a gapped fourfold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.7 meV) at QAF with incommensurate scattering around 56 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.
Tong Chen, Youzhe Chen, Andreas Kreisel, Xingye Lu, Astrid Schneidewind, Yiming Qiu, J. T. Park, Toby G. Perring, J Ross Stewart, Huibo Cao, Rui Zhang, Yu Li, Yan Rong, Yuan Wei, Brian M. Andersen, P. J. Hirschfeld, Collin Broholm, Pengcheng Dai Journal reference: Nature materials 18.7 (2019): 709 [pdf] DOI: 10.1038/s4156301903695

Knight Shift and Leading Superconducting Instability from Spin Fluctuations in

Abstract
 Recent nuclear magnetic resonance studies [A. Pustogow {\it et al.}, arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario proposed for Sr$_2$RuO$_4$. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spinfluctuation mediated pairing for this compound. We map out the phase diagram as a function of spinorbit coupling, interaction parameters, and bandstructure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that evenparity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spinorbit coupling favors a nearnodal oddparity triplet superconducting state, which is either helical or chiral depending on the proximity of the $\gamma$ band to the van Hove points. A surprising neardegeneracy of the nodal $s^\prime$ and $d_{x^2y^2}$wave solutions leads to the possibility of a nearnodal timereversal symmetry broken $s^\prime+id_{x^2y^2}$ pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.
A. T. Rømer, D. D. Scherer, I. M. Eremin, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. Lett. 123, 247001 (2019) [pdf] DOI: 10.1103/PhysRevLett.123.247001

The Mass Evolution of Protostellar Disks and Envelopes in the Perseus Molecular Cloud 
Abstract
 In the standard picture for lowmass star formation, a dense molecular cloud undergoes gravitational collapse to form a protostellar system consisting of a new central star, a circumstellar disk, and a surrounding envelope of remaining material. The mass distribution of the system evolves as matter accretes from the largescale envelope through the disk and onto the protostar. While this general picture is supported by simulations and indirect observational measurements, the specific timescales related to disk growth and envelope dissipation remain poorly constrained. In this paper we conduct a rigorous test of a method introduced by J{\o}rgensen et al. (2009) to obtain mass measurements of disks and envelopes around embedded protostars with observations that do not resolve the disk (resolution of $\sim$1000\,AU). Using unresolved data from the recent Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey, we derive disk and envelope mass estimates for $59$ protostellar systems in the Perseus molecular cloud. We compare our results to independent disk mass measurements from the VLA Nascent Disk and Multiplicity (VANDAM) survey and find a strong linear correlation, suggesting that accurate disk masses can be measured from unresolved observations. Then, leveraging the size of the MASSES sample, we find no significant trend in protostellar mass distribution as a function of age, as approximated from bolometric temperatures. These results may indicate that the disk mass of a protostar is set near the onset of the Class 0 protostellar stage and remains roughly constant throughout the Class I protostellar stage.
Bridget C. Andersen, Ian W. Stephens, Michael M. Dunham, Riwaj Pokhrel, Jes K. Jørgensen, Søren Frimann, Dominique SeguraCox, Philip C. Myers, Tyler L. Bourke, John J. Tobin, Łukasz Tychoniec [pdf] DOI: 10.3847/15384357/ab05c7 1902.05956v1 [pdf]

Theoretical study of impurityinduced magnetism in FeSe 
Abstract
 Experimental evidence suggests that FeSe is close to a magnetic instability, and recent scanning tunneling microscopy (STM) measurements on FeSe multilayer films have revealed stripe order locally pinned near defect sites. Motivated by these findings, we perform a theoretical study of locally induced magnetic order near nonmagnetic impurities in a model relevant for FeSe. We find that relatively weak repulsive impurities indeed are capable of generating shortrange magnetism, and explain the driving mechanism for the local order by resonant egorbital states. In addition, we investigate the importance of orbitalselective selfenergy effects relevant for Hund's metals, and show how the structure of the induced magnetization cloud gets modified by orbital selectivity. Finally, we make concrete connection to STM measurements of ironbased superconductors by symmetry arguments of the induced magnetic order, and the basic properties of the Fe Wannier functions relevant for tunneling spectroscopy.
Johannes H. J. Martiny, Andreas Kreisel, Brian M. Andersen Journal reference: Phys. Rev. B 99, 014509 (2019) [pdf] DOI: 10.1103/PhysRevB.99.014509

Quantum Phase Transition of Correlated IronBased Superconductivity in

Abstract
 2018

Itinerant approach to magnetic neutron scattering of FeSe: Effect of orbital selectivity 
Abstract
 Recent STM experiments and theoretical considerations have highlighted the role of interactiondriven orbital selectivity in FeSe, and its role in generating the extremely anisotropic superconducting gap structure in this material. We study the magnetic excitation spectrum resulting from the coherent quasiparticles within the same renormalized random phase approximation approach used to explain the STM experiments, and show that it agrees well with the lowenergy momentum and energy dependent response measured by inelastic neutron scattering experiments. We find a correlationinduced suppression of $(\pi,\pi)$ scattering due to a small quasiparticle weight of states of $d_{xy}$ character. We compare predictions for twinned and untwinned crystals, and predict in particular a strongly $(\pi,0)$dominated response at low energies in untwinned systems, in contrast to previous itinerant theories.
Andreas Kreisel, Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. B 98, 214518 (2018) [pdf] DOI: 10.1103/PhysRevB.98.214518

Suppression of superfluid stiffness near a Lifshitzpoint instability to finitemomentum superconductivity 
Abstract
 We derive the effective GinzburgLandau theory for finite momentum (FFLO/PDW) superconductivity without spin population imbalance from a model with local attraction and repulsive pairhopping. We find that the GL free energy must include up to sixth order derivatives of the order parameter, providing a unified description of the interdependency of zero and finite momentum superconductivity. For weak pairhopping the phase diagram contains a line of Lifshitz points where vanishing superfluid stiffness induces a continuous change to a long wavelength FuldeFerrell (FF) state. For larger pairhopping there is a bicritical region where the pairmomentum changes discontinuously. Here the FF type state is near degenerate with the LarkinOvchinnikov (LO) or PairDensitywave (PDW) type state. At the intersection of these two regimes there is a "SuperLifshitz" point with extra soft fluctuations. The instability to finite momentum superconductivity occurs for arbitrarily weak pairhopping for sufficiently large attraction suggesting that even a small repulsive pairhopping may be significant in a microscopic model of strongly correlated superconductivity. Several generic features of the model may have bearing on the cuprate superconductors, including the suppression of superfluid stiffness in proximity to a Lifshitz point as well as the existence of subleading FFLO order (or vice versa) in the bicritical regime.
Jonatan Wårdh, Mats Granath, Brian M. Andersen Journal reference: Phys. Rev. B 98, 224501 (2018) [pdf] DOI: 10.1103/PhysRevB.98.224501

Enhancing superconductivity by disorder 
Abstract
 We study two mechanisms for enhancing the superconducting transition temperature Tc by nonmagnetic disorder in both conventional (signpreserving gaps) and unconventional (signchanging gaps) superconductors (SC). In the first scenario, relevant to multiband systems in the dilute impurity limit of both conventional and unconventional SC, we demonstrate how favorable density of states enhancements driven by resonant states in offFermilevel bands, lead to significant enhancements of Tc in the condensate formed by the nearFermilevel bands. The second scenario focuses on the dense impurity limit where random disordergenerated local density of states modulations cause a boosted Tc for conventional SC with short coherence lengths. We analyze the basic physics of both mechanisms within simplified models, and discuss the relevance to existing materials.
Maria N. Gastiasoro, Brian M. Andersen Journal reference: Phys. Rev. B 98, 184510 (2018) [pdf] DOI: 10.1103/PhysRevB.98.184510

Unravelling Incommensurate Magnetism and Its Emergence in IronBased Superconductors 
Abstract
 We focus on a broad class of tetragonal itinerant systems sharing a tendency towards the spontaneous formation of incommensurate magnetism with ordering wavevectors $\mathbf{Q}_{1,2}=(\pi\delta,0)/(0,\pi\delta)$ or $\mathbf{Q}_{1,2}=(\pi,\delta)/(\delta,\pi)$. Employing a Landau approach, we obtain the generic magnetic phase diagram and identify the leading instabilities near the paramagneticmagnetic transition. Nine distinct magnetic phases exist that either preserve or violate the assumed $C_4$symmetry of the paramagnetic phase. These are single and double$\mathbf{Q}$ phases consisting of magnetic stripes, helices and whirls, either in an individual or coexisting manner. These nine phases can be experimentally distinguished by polarized neutron scattering, or, for example, by combining measurements of the induced charge order and magnetoelectric coupling. Within two representative fiveorbital models, suitable for BaFe$_2$As$_2$ and LaFeAsO, we find that the incommensurate magnetic phases discussed here are accessible in ironbased superconductors. Our investigation unveils a set of potential candidates for the unidentified $C_2$symmetric magnetic phase that was recently observed in Ba$_{1x}$Na$_x$Fe$_{2}$As$_{2}$. Among the phases stabilized we find a spinwhirl crystal, which is a textured magnetic $C_4$symmetric phase. The possible experimental observation of textured magnetic orders in ironbased superconductor, opens new directions for realizing intrinsic topological superconductors.
Morten H. Christensen, Brian M. Andersen, Panagiotis Kotetes Journal reference: Phys. Rev. X 8, 041022 (2018) [pdf] DOI: 10.1103/PhysRevX.8.041022

Raising the Critical Temperature by Disorder in Unconventional Superconductors Mediated by Spin Fluctuations 
Abstract
 We propose a mechanism whereby disorder can enhance the transition temperature Tc of an unconventional superconductor with pairing driven by exchange of spin fluctuations. The theory is based on a selfconsistent real space treatment of pairing in the disordered oneband Hubbard model. It has been demonstrated before that impurities can enhance pairing by softening the spin fluctuations locally; here, we consider the competing effect of pairbreaking by the screened Coulomb potential also present. We show that, depending on the impurity potential strength and proximity to magnetic order, this mechanism results in a weakening of the disorderdependent Tcsuppression rate expected from AbrikosovGor'kov theory, or even in disordergenerated Tc enhancements.
Astrid T. Roemer, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. Lett. 121, 027002 (2018) [pdf] DOI: 10.1103/PhysRevLett.121.027002

Magnetic phase diagram of the iron pnictides in the presence of spinorbit coupling: Frustration between

Abstract
 We investigate the impact of spin anisotropic interactions, promoted by spinorbit coupling, on the magnetic phase diagram of the ironbased superconductors. Three distinct magnetic phases with Bragg peaks at $(\pi,0)$ and $(0,\pi)$ are possible in these systems: one $C_2$ (i.e. orthorhombic) symmetric stripe magnetic phase and two $C_4$ (i.e. tetragonal) symmetric magnetic phases. While the spin anisotropic interactions allow the magnetic moments to point in any direction in the $C_2$ phase, they restrict the possible moment orientations in the $C_4$ phases. As a result, an interesting scenario arises in which the spin anisotropic interactions favor a $C_2$ phase, but the other spin isotropic interactions favor a $C_4$ phase. We study this frustration via both meanfield and renormalizationgroup approaches. We find that, to lift this frustration, a rich magnetic landscape emerges well below the magnetic transition temperature, with novel $C_2$, $C_4$, and mixed $C_2$$C_4$ phases. Near the putative magnetic quantum critical point, spin anisotropies promote a stable Gaussian fixed point in the renormalizationgroup flow, which is absent in the spin isotropic case, and is associated with a neardegeneracy between $C_2$ and $C_4$ phases. We argue that this frustration is the reason why most $C_4$ phases in the iron pnictides only appear inside the $C_2$ phase, and discuss additional manifestations of this frustration in the phase diagrams of these materials.
Morten H. Christensen, Peter P. Orth, Brian M. Andersen, Rafael M. Fernandes Journal reference: Phys. Rev. B 98, 014523 (2018) [pdf] DOI: 10.1103/PhysRevB.98.014523

Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between SpinOrbit Coupling and Quantum Fluctuations 
Abstract
 Recent experiments in iron pnictide superconductors reveal that, as the putative magnetic quantum critical point is approached, different types of magnetic order coexist over a narrow region of the phase diagram. Although these magnetic configurations share the same wavevectors, they break distinct symmetries of the lattice. Importantly, the highest superconducting transition temperature takes place close to this proliferation of neardegenerate magnetic states. In this paper, we employ a renormalization group calculation to show that such a behavior naturally arises due to the effects of spinorbit coupling on the quantum magnetic fluctuations. Formally, the enhanced magnetic degeneracy near the quantum critical point is manifested as a stable Gaussian fixed point with a large basin of attraction. Implications of our findings to the superconductivity of the iron pnictides are also discussed.
Morten H. Christensen, Peter P. Orth, Brian M. Andersen, Rafael M. Fernandes Journal reference: Phys. Rev. Lett. 121, 057001 (2018) [pdf] DOI: 10.1103/PhysRevLett.121.057001

Imaging orbitalselective quasiparticles in the Hund’s metal state of FeSe 
Abstract
 Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copperbased high temperature superconductivity (HTS). By contrast, the parent phase of ironbased HTS is never a correlated insulator. But this distinction may be deceptive because Fe has five active dorbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's Metal state, in which alignment of the Fe spins suppresses interorbital fluctuations producing orbitally selective strong correlations. The spectral weights $Z_m$ of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle $Z_m$ appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that $Z_{xy}<z_{xz}<<z_{yz}$, consistent="consistent" with="with" earlier="earlier" orbitalselective="orbitalselective" cooper="Cooper" pairing="pairing" studies.="studies." thus,="Thus," orbitalselective="orbitalselective" strong="strong" correlations="correlations" dominate="dominate" the="the" parent="parent" state="state" of="of" ironbased="ironbased" hts="HTS" in="in" fese.
Andrey Kostin, Peter O. Sprau, Andreas Kreisel, Yi Xue Chong, Anna E. Böhmer, Paul C. Canfield, Peter J. Hirschfeld, Brian M. Andersen, J. C. Séamus Davis Journal reference: Nature Materials 17, 869 (2018) [pdf] DOI: 10.1038/s4156301801510

A Fourier Domain “Jerk” Search for Binary Pulsars 
Abstract
 While binary pulsar systems are fantastic laboratories for a wide array of astrophysics, they are particularly difficult to detect. The orbital motion of the pulsar changes its apparent spin frequency over the course of an observation, essentially "smearing" the response of the time series in the Fourier domain. We review the Fourier domain acceleration search (FDAS), which uses a matched filtering algorithm to correct for this smearing by assuming constant acceleration for a small enough portion of the orbit. We discuss the theory and implementation of a Fourier domain "jerk" search, developed as part of the \textsc{PRESTO} software package, which extends the FDAS to account for a linearly changing acceleration, or constant orbital jerk, of the pulsar. We test the performance of our algorithm on archival Green Bank Telescope observations of the globular cluster Terzan~5, and show that while the jerk search has a significantly longer runtime, it improves search sensitivity to binaries when the observation duration is $5$ to $15\%$ of the orbital period. Finally, we present the jerksearchenabled detection of Ter5am (PSR~J1748$$2446am), a new highlyaccelerated pulsar in a compact, eccentric, and relativistic orbit, with a likely pulsar mass of 1.649$^{+0.037}_{0.11}$\,\msun.
Bridget C. Andersen, Scott M. Ransom [pdf] DOI: 10.3847/20418213/aad59f 1807.07900v1 [pdf]

Spin Waves in Detwinned

Abstract
 Understanding magnetic interactions in the parent compounds of hightemperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as $A$Fe$_2$As$_2$ ($A=$ Ba, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone (BZ), measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of $\sim$100\% detwinned BaFe$_2$As$_2$. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model with important contributions from electronic correlations.
Xingye Lu, Daniel D. Scherer, David W. Tam, Wenliang Zhang, Rui Zhang, Huiqian Luo, Leland W. Harriger, H. C. Walker, D. T. Adroja, Brian M. Andersen, Pengcheng Dai Journal reference: Phys. Rev. Lett. 121, 067002 (2018) [pdf] DOI: 10.1103/PhysRevLett.121.067002

Magnetic ground state and magnonphonon interaction in multiferroic

Abstract
 Inelastic neutron scattering has been used to study the magnetoelastic excitations in the multiferroic manganite hexagonal YMnO$_3$. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the $(a,b)$plane. Neutron polarization analysis reveals that this mode has mixed magnonphonon character. An external magnetic field along the $c$axis is observed to cause a linear fieldinduced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magnetoelastic coupling via the singleion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnonphonon hybridized modes, and quantifies the magnetoelastic coupling. The combined information, including the fieldinduced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.
S. L. Holm, A. Kreisel, T. K. Schäffer, A. Bakke, M. Bertelsen, U. B. Hansen, M. Retuerto, J. Larsen, D. Prabhakaran, P. P. Deen, Z. Yamani, J. O. Birk, U. Stuhr, Ch. Niedermayer, A. L. Fennell, B. M. Andersen, K. Lefmann Journal reference: Phys. Rev. B 97, 134304 (2018) [pdf] DOI: 10.1103/PhysRevB.97.134304

SpinOrbit Coupling and Magnetic Anisotropy in IronBased Superconductors 
Abstract
 We determine theoretically the effect of spinorbit coupling on the magnetic excitation spectrum of itinerant multiorbital systems, with specific application to ironbased superconductors. Our microscopic model includes a realistic tenband kinetic Hamiltonian, atomic spinorbit coupling, and multiorbital Hubbard interactions. Our results highlight the remarkable variability of the resulting magnetic anisotropy despite constant spinorbit coupling. At the same time, the magnetic anisotropy exhibits robust universal behavior upon changes in the bandstructure corresponding to different materials of ironbased superconductors. A natural explanation of the observed universality emerges when considering optimal nesting as a resonance phenomenon. Our theory is also of relevance to other itinerant system with spinorbit coupling and nesting tendencies in the bandstructure.
Daniel D. Scherer, Brian M. Andersen Journal reference: Phys. Rev. Lett. 121, 037205 (2018) [pdf] DOI: 10.1103/PhysRevLett.121.037205

Itinerant approach to magnetic neutron scattering of FeSe: Effect of orbital selectivity 
Abstract
 2017

Role of multiorbital effects in the magnetic phase diagram of iron pnictides 
Abstract
 We elucidate the pivotal role of the bandstructure's orbital content in deciding the type of commensurate magnetic order stabilized within the itinerant scenario of ironpnictides. Recent experimental findings in the tetragonal magnetic phase attest to the existence of the socalled charge and spin ordered density wave over the spinvortex crystal phase, the latter of which tends to be favored in simplified band models of itinerant magnetism. Here we show that employing a multiorbital itinerant Landau approach based on realistic bandstructures can account for the experimentally observed magnetic phase, and thus shed light on the importance of the orbital content in deciding the magnetic order. In addition, we remark that the presence of a hole pocket centered at the Brillouin zone's ${\rm M}$point favors a magnetic stripe rather than a tetragonal magnetic phase. For inferring the symmetry properties of the different magnetic phases, we formulate our theory in terms of magnetic order parameters transforming according to irreducible representations of the ensuing D$_{\rm 4h}$ point group. The latter method not only provides transparent understanding of the symmetry breaking schemes but also reveals that the leading instabilities always belong to the $\{A_{1g},B_{1g}\}$ subset of irreducible representations, independent of their C$_2$ or C$_4$ nature.
Morten H. Christensen, Daniel D. Scherer, Panagiotis Kotetes, Brian M. Andersen Journal reference: Phys. Rev. B 96, 014523 (2017) [pdf] DOI: 10.1103/PhysRevB.96.014523

Universality of scanning tunneling microscopy in cuprate superconductors 
Abstract
 We consider the problem of local tunneling into cuprate superconductors, combining model based calculations for the superconducting order parameter with wavefunction information obtained from first principles electronic structure. For some time it has been proposed that scanning tunneling microscopy (STM) spectra do not reflect the properties of the superconducting layer in the CuO$_2$ plane directly beneath the STM tip, but rather a weighted sum of spatially proximate states determined by the details of the tunneling process. These "filter" ideas have been countered with the argument that similar conductance patterns have been seen around impurities and charge ordered states in systems with atomically quite different barrier layers. Here we use a recently developed Wannier function based method to calculate topographies, spectra, conductance maps and normalized conductance maps close to impurities. We find that it is the local planar Cu $d_{x^2y^2}$ Wannier function, qualitatively similar for many systems, that controls the form of the tunneling spectrum and the spatial patterns near perturbations. We explain how, despite the fact that STM observables depend on the materialsspecific details of the tunneling process and setup parameters, there is an overall universality in the qualitative features of conductance spectra. In particular, we discuss why STM results on Bi$_2$Sr$_2$CaCu$_2$O$_8$ and Ca$_{2x}$Na$_x$CuO$_2$Cl$_2$ are essentially identical.
Peayush Choubey, Andreas Kreisel, T. Berlijn, Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. B 96, 174523 (2017) [pdf] DOI: 10.1103/PhysRevB.96.174523

Imaging the real space structure of the spin fluctuations in an ironbased superconductor 
Abstract
 Spin fluctuations are a leading candidate for the pairing mechanism in high temperature superconductors, supported by the common appearance of a distinct resonance in the spin susceptibility across the cuprates, ironbased superconductors and many heavy fermion materials. The information we have about the spin resonance comes almost exclusively from neutron scattering. Here we demonstrate that by using lowtemperature scanning tunnelling microscopy and spectroscopy we can characterize the spin resonance in real space. We show that inelastic tunnelling leads to the characteristic diphump feature seen in tunnelling spectra in high temperature superconductors and that this feature arises from excitations of the spin fluctuations. Spatial mapping of this feature near defects allows us to probe nonlocal properties of the spin susceptibility and to image its real space structure.
Shun Chi, Ramakrishna Aluru, Stephanie Grothe, A. Kreisel, Udai Raj Singh, Brian M. Andersen, W. N. Hardy, Ruixing Liang, D. A. Bonn, S. A. Burke, Peter Wahl Journal reference: Nat. Commun. 8, 15996 (2017) [pdf] DOI: 10.1038/ncomms15996

Discovery of orbitalselective Cooper pairing in FeSe 
Abstract
 The superconductor FeSe is of intense interest thanks to its unusual nonmagnetic nematic state and potential for high temperature superconductivity. But its Cooper pairing mechanism has not been determined. Here we use Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the bands surrounding the $\Gamma = (0,0)$ and $X=(\pi / a_{Fe}, 0)$ points of FeSe, and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless, and exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbitalselective Cooper pairing which, in FeSe, is based preferentially on electrons from the $d_{yz}$ orbitals of the iron atoms.
Peter O. Sprau, Andrey Kostin, Andreas Kreisel, Anna E. Böhmer, Valentin Taufour, Paul C. Canfield, Shantanu Mukherjee, Peter J. Hirschfeld, Brian M. Andersen, J. C. Séamus Davis Journal reference: Science 357, 75 (2017) [pdf] DOI: 10.1126/science.aal1575

Orbital selective pairing and gap structures of ironbased superconductors 
Abstract
 We discuss the influence on spinfluctuation pairing theory of orbital selective strong correlation effects in Febased superconductors, particularly Fe chalcogenide systems. We propose that a key ingredient for an improved itinerant pairing theory is orbital selectivity, i.e., incorporating the reduced coherence of quasiparticles occupying specific orbital states. This modifies the usual spinfluctuation via suppression of pair scattering processes involving those less coherent states and results in orbital selective Cooper pairing of electrons in the remaining states. We show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated ironbased superconductors.
Andreas Kreisel, Brian M. Andersen, Peter O. Sprau, Andrey Kostin, J. C. Séamus Davis, P. J. Hirschfeld Journal reference: Phys. Rev. B 95, 174504 (2017) [pdf] DOI: 10.1103/PhysRevB.95.174504

Fast recovery of the stripe magnetic order by Mn/Fe substitution in Fdoped LaFeAsO superconductors 
Abstract
 $^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in Fdoped LaFe$_{1x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic fiveband model show that correlationenhanced RKKY exchange interactions between nearby Mn ions stabilize the observed magnetic order, dominated by $Q_1=(\pi,0)$ and $Q_2=(0,\pi)$ ordering vectors. These results give compelling evidence that Fdoped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.
M. Moroni, P. Carretta, G. Allodi, R. De Renzi, M. N. Gastiasoro, B. M. Andersen, P. Materne, H. H. Klauss, Y. Kobayashi, M. Sato, S. Sanna Journal reference: Phys. Rev. B 95, 180501 (2017) [pdf] DOI: 10.1103/PhysRevB.95.180501

Robustness of a quasiparticle interference test for signchanging gaps in multiband superconductors 
Abstract
 Recently, a test for a signchanging gap function in a candidate multiband unconventional superconductor involving quasiparticle interference data was proposed. The test was based on the antisymmetric, Fourier transformed conductance maps integrated over a range of momenta $\bf q$ corresponding to interband processes, which was argued to display a particular resonant form, provided the gaps changed sign between the Fermi surface sheets connected by $\bf q$. The calculation was performed for a single impurity, however, raising the question of how robust this measure is as a test of signchanging pairing in a realistic system with many impurities. Here we reproduce the results of the previous work within a model with two distinct Fermi surface sheets, and show explicitly that the previous result, while exact for a single nonmagnetic scatterer and also in the limit of a dense set of random impurities, can be difficult to implement for a few dilute impurities. In this case, however, appropriate isolation of a single impurity is sufficient to recover the expected result, allowing a robust statement about the gap signs to be made.
Johannes H. J. Martiny, Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 95, 184507 (2017) [pdf] DOI: 10.1103/PhysRevB.95.184507

Interplay of nematic and magnetic orders in FeSe under pressure 
Abstract
 We offer an explanation for the recently observed pressureinduced magnetic state in the ironchalcogenide FeSe based on \textit{ab initio} estimates for the pressure evolution of the most important Coulomb interaction parameters. We find that an increase of pressure leads to an overall decrease mostly in the nearestneighbor Coulomb repulsion, which in turn leads to a reduction of the nematic order and the generation of magnetic stripe order. We treat the concomitant effects of band renormalization and the induced interplay of nematic and magnetic order in a selfconsistent way and determine the generic topology of the temperaturepressure phase diagram, and find qualitative agreement with the experimentally determined phase diagram.
Daniel D. Scherer, Anthony Jacko, Christoph Friedrich, Ersoy Sasioglu, Stefan Blugel, Roser Valenti, Brian M. Andersen Journal reference: Phys. Rev. B 95, 094504 (2017) [pdf] DOI: 10.1103/PhysRevB.95.094504

Scanning tunnelling spectroscopy as a probe of multiQ magnetic states of itinerant magnets 
Abstract
 The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multiQ magnetic ground states, hosting unusual states such as chiral densitywaves and quantum Hall insulators. Distinguishing singleQ and multiQ magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between singleQ and multiQ phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to ironbased superconductors  one singleQ and two doubleQ phases. Our results open a promising avenue to investigate complex magnetic configurations in itinerant systems via standard scanning tunneling spectroscopy, without requiring spinresolved capability.
Maria N. Gastiasoro, Ilya Eremin, Rafael M. Fernandes, Brian M. Andersen Journal reference: Nature Communications 8, 14317 (2017) [pdf] DOI: 10.1038/ncomms14317

Role of multiorbital effects in the magnetic phase diagram of iron pnictides 
Abstract
 2016

Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs 
Abstract
 Since the discovery of ironbased superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, materialspecific comparisons to experimental results. The spinfluctuation theory of electronic pairing, based on firstprinciples electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. Here we present such a comparison between theory and experiment on the Febased superconductor LiFeAs. Results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As and Liregistered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transitionmetal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Febased systems.
A. Kreisel, R. Nelson, T. Berlijn, W. Ku, Ramakrishna Aluru, Shun Chi, Haibiao Zhou, Udai Raj Singh, Peter Wahl, Ruixing Liang, Walter N. Hardy, D. A. Bonn, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 94, 224518 (2016) [pdf] DOI: 10.1103/PhysRevB.94.224518

Role of magnetic dopants in the phase diagram of Sm 1111 pnictides: The case of Mn 
Abstract
 The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimallydoped SmFeAsO0.88F0.12 ironbased superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muonspin spectroscopy (muSR), both in zero and in transverse fields, respectively, to probe the magnetic and the superconducting order. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors [M. Gastiasoro et al., ArXiv 1606.09495], which includes correlationenhanced RKKYcouplings between the impurity moments.
G. Lamura, T. Shiroka, S. Bordignon, S. Sanna, M. Moroni, R. De Renzi, P. Carretta, P. K. Biswas, F. Caglieris, M. Putti, S. Wurmehl, S. J. Singh, J. Shimoyama, M. N. Gastiasoro, B. M. Andersen Journal reference: Phys. Rev. B 94, 214517 (2016) [pdf] DOI: 10.1103/PhysRevB.94.214517

Spiral magnetic order and topological superconductivity in a chain of magnetic adatoms on a twodimensional superconductor 
Abstract
 We study the magnetic and electronic phases of a 1D magnetic adatom chain on a 2D superconductor. In particular, we confirm the existence of a `selforganized' 1D topologically nontrivial superconducting phase within the set of subgap YuShibaRusinov (YSR) states formed along the magnetic chain. This phase is stabilized by incommensurate spiral correlations within the magnetic chain that arise from the competition between shortrange ferromagnetic and longrange antiferromagnetic electroninduced exchange interactions, similar to a recent study for a 3D superconductor [M. Schecter et al. Phys. Rev. B 93, 140503(R) 2016]. The exchange interaction along diagonal directions are also considered and found to display behavior similar to a 1D substrate when close to half filling. We show that the topological phase diagram is robust against local superconducting order parameter suppression and weak substrate spinorbit coupling. Lastly, we study the effect of a direct ferromagnetic exchange coupling between the adatoms, and find the region of spiral order in the phase diagram to be significantly enlarged in a wide range of the direct exchange coupling.
M. H. Christensen, M. Schecter, K. Flensberg, B. M. Andersen, J. Paaske Journal reference: Phys. Rev. B 94, 144509 (2016) [pdf] DOI: 10.1103/PhysRevB.94.144509

Superconducting Phase Diagram of the Paramagnetic OneBand Hubbard Model 
Abstract
 We study spinfluctuationmediated superconductivity in the oneband Hubbard model. Higher order effective interactions in $U$ give rise to a superconducting instability which is very sensitive to changes in the Fermi surface topology arising as a function of doping and changes in the band structure. We show the superconducting phase diagram as a function of doping and nextnearest neighbor hopping in the limit of very small Coulomb interaction strength and discuss peculiarities arising at the phase boundaries separating different superconducting domains.
Andreas Kreisel, Astrid T. Roemer, Peter J. Hirschfeld, Brian M. Andersen Journal reference: J Supercond Nov Magn (2016) [pdf] DOI: 10.1007/s109480163758x

Collective magnetic excitations of

Abstract
 We study the collective magnetic excitations of the recently discovered $C_{4}$ symmetric spindensity wave states of ironbased superconductors with particular emphasis on their orbital character based on an itinerant multiorbital approach. This is important since the $C_{4}$ symmetric spindensity wave states exist only at moderate interaction strengths where damping effects from a coupling to the continuum of particlehole excitations strongly modifies the shape of the excitation spectra compared to predictions based on a local moment picture. We uncover a distinct orbital polarization inherent to magnetic excitations in $C_{4}$ symmetric states, which provide a route to identify the different commensurate magnetic states appearing in the continuously updated phase diagram of the ironpnictide family.
Daniel D. Scherer, Ilya Eremin, Brian M. Andersen Journal reference: Phys. Rev. B 94, 180405 (2016) [pdf] DOI: 10.1103/PhysRevB.94.180405

Impact of ironsite defects on superconductivity in LiFeAs 
Abstract
 In conventional swave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s+ order parameter they can occur for both magnetic and nonmagnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. Here, we present a combined experimental and theoretical study of native and engineered ironsite defects in LiFeAs. Detailed comparison of tunneling spectra measured on impurities with spin fluctuation theory reveals a continuous evolution from negligible impurity bound state features for weaker scattering potential to clearly detectable states for somewhat stronger scattering potentials. All bound states for these intermediate strength potentials are pinned at or close to the gap edge of the smaller gap, a phenomenon that we explain and ascribe to multiorbital physics.
Shun Chi, Ramakrishna Aluru, Udai Raj Singh, Ruixing Liang, Walter N. Hardy, D. A. Bonn, A. Kreisel, Brian M. Andersen, R. Nelson, T. Berlijn, W. Ku, P. J. Hirschfeld, Peter Wahl Journal reference: Phys. Rev. B 94, 134515 (2016) [pdf] DOI: 10.1103/PhysRevB.94.134515

Unconventional Disorder Effects in Correlated Superconductors 
Abstract
 The understanding of disorder has profoundly influenced the development of condensed matter physics, explaining such fundamental effects as, for example, the transition from ballistic to diffusive propagation, and the presence of quantized steps in the quantum Hall effect. For superconductors, the response to disorder reveals crucial information about the internal gap symmetries of the condensate, and thereby the pairing mechanism itself. The destruction of superconductivity by disorder is traditionally described by AbrikosovGor'kov (AG) theory, which however ignores spatial modulations and ceases to be valid when impurities interfere, and interactions become important. Here we study the effects of disorder on unconventional superconductors in the presence of correlations, and explore a completely different disorder paradigm dominated by strong deviations from standard AG theory due to generation of local bound states and cooperative impurity behavior driven by Coulomb interactions. Specifically we explain under which circumstances magnetic disorder acts as a strong poison destroying highTc superconductivity at the sub1% level, and when nonmagnetic disorder, counterintuitively, hardly affects the unconventional superconducting state while concomitantly inducing an inhomogeneous fullvolume magnetic phase. Recent experimental studies of Febased superconductors have discovered that such unusual disorder behavior seem to be indeed present in those systems.
Maria N. Gastiasoro, Fabio Bernardini, Brian M. Andersen Journal reference: Phys. Rev. Lett. 117, 257002 (2016) [pdf] DOI: 10.1103/PhysRevLett.117.257002

Superconducting phase diagram of itinerant antiferromagnets 
Abstract
 We study the phase diagram of the Hubbard model in the weakcoupling limit for coexisting spindensitywave order and spinfluctuationmediated superconductivity. Both longitudinal and transverse spin fluctuations contribute significantly to the effective interaction potential, which creates Cooper pairs of the quasiparticles of the antiferromagnetic metallic state. We find a dominant $d_{x^2y^2}$wave solution in both electron and holedoped cases. In the quasispin triplet channel, the longitudinal fluctuations give rise to an effective attraction supporting a $p$wave gap, but are overcome by repulsive contributions from the transverse fluctuations which disfavor $p$wave pairing compared to $d_{x^2y^2}$. The subleading pair instability is found to be in the $g$wave channel, but complex admixtures of $d$ and $g$ are not energetically favored since their nodal structures coincide. Inclusion of interband pairing, in which each fermion in the Cooper pair belongs to a different spindensitywave band, is considered for a range of electron dopings in the regime of welldeveloped magnetic order. We demonstrate that these interband pairing gaps, which are nonzero in the magnetic state, must have the same parity under inversion as the normal intraband gaps. The selfconsistent solution to the full system of five coupled gap equations give intraband and interband pairing gaps of $d_{x^2y^2}$ structure and similar gap magnitude. In conclusion, the $d_{x^2y^2}$ gap dominates for both hole and electron doping inside the spindensitywave phase.
A. T. Roemer, I. Eremin, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. B 93, 174519 (2016) [pdf] DOI: 10.1103/PhysRevB.93.174519

Magnetic Fluctuations in PairDensityWave Superconductors 
Abstract
 Pair density wave superconductivity constitutes a novel electronic condensate proposed to be realized in certain unconventional superconductors. Establishing its potential existence is important for our fundamental understanding of superconductivity in correlated materials. Here we compute the dynamical magnetic susceptibility in the presence of a pair density wave ordered state, and study its fingerprints on the spinwave spectrum including the neutron resonance. In contrast to the standard case of dwave superconductivity, we show that the pair density wave phase exhibits neither a spingap nor a magnetic resonance peak, in agreement with a recent neutron scattering experiment on underdoped La$_{1.905}$Ba$_{0.095}$CuO$_4$ [Z. Xu et al., Phys. Rev. Lett. 113, 177002 (2014)].
M. H. Christensen, H. Jacobsen, T. M. Maier, B. M. Andersen Journal reference: Phys. Rev. Lett. 116, 167001 (2016) [pdf] DOI: 10.1103/PhysRevLett.116.167001

Selforganized topological superconductivity in a YuShibaRusinov chain 
Abstract
 We study a chain of magnetic moments exchange coupled to a conventional three dimensional superconductor. In the normal state the chain orders into a collinear configuration, while in the superconducting phase we find that ferromagnetism is unstable to the formation of a magnetic spiral state. Beyond weak exchange coupling the spiral wavevector greatly exceeds the inverse superconducting coherence length as a result of the strong spinspin interaction mediated through the subgap band of YuShibaRusinov states. Moreover, the simple spinspin exchange description breaks down as the subgap band crosses the Fermi energy, wherein the spiral phase becomes stabilized by the spontaneous opening of a $p$wave superconducting gap within the band. This leads to the possibility of electrondriven topological superconductivity with Majorana boundary modes using magnetic atoms on superconducting surfaces.
M. Schecter, K. Flensberg, M. H. Christensen, B. M. Andersen, J. Paaske Journal reference: Phys. Rev. B 93, 140503 (2016) [pdf] DOI: 10.1103/PhysRevB.93.140503

Electronic vortex structure of Febased superconductors: Application to LiFeAs 
Abstract
 Detailed tunneling spectroscopy of vortex core states can provide important insight to the momentum structure of the superconducting order parameter. We present a theoretical study of vortex bound states in ironbased superconductors by use of a realistic fiveband model relevant to these systems, and superconductivity stabilized by spinfluctuation generated pairing vertices yielding an $s\pm$ gap structure. The computed local density of states agrees remarkably well with both the bias dependence of the local conductance and the spatial structure of the lowbias conductance as obtained by scanning tunneling microscopy measurements on LiFeAs [T. Hanaguri et al., Phys. Rev. B 85, 214505 (2012)].
B. Mencia Uranga, Maria N. Gastiasoro, Brian M. Andersen Journal reference: Phys. Rev. B 93, 224503 (2016) [pdf] DOI: 10.1103/PhysRevB.93.224503

Spindriven nematic instability of the multiorbital Hubbard model: Application to ironbased superconductors 
Abstract
 Nematic order resulting from the partial melting of densitywaves has been proposed as the mechanism to explain nematicity in ironbased superconductors. An outstanding question, however, is whether the microscopic electronic model for these systems  the multiorbital Hubbard model  displays such an ordered state as its leading instability. In contrast to usual electronic instabilities, such as magnetic and charge order, this fluctuationdriven phenomenon cannot be captured by the standard RPA method. Here, by including fluctuations beyond RPA in the multiorbital Hubbard model, we derive its nematic susceptibility and contrast it with its ferroorbital order susceptibility, showing that its leading instability is the spindriven nematic phase. Our results also demonstrate the primary role played by the $d_{xy}$ orbital in driving the nematic transition, and reveal that highenergy magnetic fluctuations are essential to stabilize nematic order in the absence of magnetic order.
M. H. Christensen, Jian Kang, B. M. Andersen, R. M. Fernandes Journal reference: Phys. Rev. B 93, 085136 (2016) [pdf] DOI: 10.1103/PhysRevB.93.085136

Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs 
Abstract
 2015

Spin excitations in a model of FeSe with orbital ordering 
Abstract
 We present a theoretical study of the dynamical spin susceptibility for the intriguing Febased superconductor FeSe, based on a tightbinding model developed to account for the temperaturedependent band structure in this system. The model allows for orbital ordering in the $d_{xz}/d_{yz}$ channel below the structural transition and presents a strongly $C_4$ symmetry broken Fermi surface at low temperatures which accounts for the nematic properties of this material. The calculated spin excitations are peaked at wave vector $(\pi,0)$ in the 1Fe Brillouin zone, with a broad maximum at energies of order a few meV. In this range, the occurrence of superconductivity sharpens this peak in energy, creating a $(\pi,0)$ "neutron resonance" as seen in recent experiments. With the exception of the quite low energy scale of these fluctuations, these results are roughly similar to standard behavior in Fe pnictide systems. At higher energies, however, intensity increases and shifts to wave vectors along the $(\pi,0)$  $(\pi,\pi)$ line. We compare with existing inelastic neutron experiments and NMR data, and give predictions for further studies.
A. Kreisel, Shantanu Mukherjee, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 92, 224515 (2015) [pdf] DOI: 10.1103/PhysRevB.92.224515

Spin reorientation driven by the interplay between spinorbit coupling and Hund's rule coupling in iron pnictides 
Abstract
 In most magneticallyordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in holedoped iron pnictides have observed a reorientation of the magnetic moments from inplane to outofplane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal nonuniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spinorbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in holedoped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron and holedoped compounds, with only the latter displaying tetragonal magnetic states.
M. H. Christensen, Jian Kang, B. M. Andersen, I. Eremin, R. M. Fernandes Journal reference: Phys. Rev. B 92, 214509 (2015) [pdf] DOI: 10.1103/PhysRevB.92.214509

Impurityinduced antiferromagnetic order in Paulilimited nodal superconductors: Application to heavyfermion

Abstract
 We investigate the properties of the coexistence phase of itinerant antiferromagnetism and nodal $d$wave superconductivity (Qphase) discovered in heavy fermion CeCoIn5 under applied magnetic field. We solve the minimal model that includes $d$wave superconductivity and underlying magnetic correlations in real space to elucidate the structure of the $Q$phase in the presence of an externally applied magnetic field. We further focus on the role of magnetic impurities, and show that they nucleate the Qphase at lower magnetic fields. Our most crucial finding is that, even at zero applied field, dilute magnetic impurities cooperate via RKKYlike exchange interactions to generate a longrange ordered coexistence state identical to the Qphase. This result is in agreement with recent neutron scattering measurements [S. Raymond et al., J. Phys. Soc. Jpn. {\bf 83}, 013707 (2014)].
Johannes. H. J. Martiny, Maria N. Gastiasoro, I. Vekhter, Brian M. Andersen Journal reference: Phys. Rev. B 92, 224510 (2015) [pdf] DOI: 10.1103/PhysRevB.92.224510

Using superlattice potentials to probe longrange magnetic correlations in optical lattices 
Abstract
 In Pedersen et al. (2011) we proposed a method to utilize a temporally dependent superlattice potential to mediate spinselective transport, and thereby probe long and short range magnetic correlations in optical lattices. Specifically this can be used for detecting antiferromagnetic ordering in repulsive fermionic optical lattice systems, but more generally it can serve as a means of directly probing correlations among the atoms by measuring the mean value of an observable, the number of double occupied sites. Here, we provide a detailed investigation of the physical processes which limit the effectiveness of this "conveyer belt method". Furthermore we propose a simple ways to improve the procedure, resulting in an essentially perfect (errorfree) probing of the magnetic correlations. These results shows that suitably constructed superlattices constitute a promising way of manipulating atoms of different spin species as well as probing their interactions.
Kim G. L. Pedersen, Brian M. Andersen, Georg M. Bruun, Anders S. Sørensen Journal reference: Phys. Rev. A 92, 063633 (2015) [pdf] DOI: 10.1103/PhysRevA.92.063633

Competing magnetic double

Abstract
 We perform a microscopic theoretical study of the generic properties of competing magnetic phases in iron pnictides. As a function of electron filling and temperature, the magnetic stripe (singleQ) order forms a dome, but competing noncollinear and nonuniform doubleQ phases exist at the foot of the dome in agreement with recent experiments. We compute and compare the electronic properties of the different magnetic phases, investigate the role of competing superconductivity, and show how disorder may stabilize doubleQ order. Superconductivity is shown to compete more strongly with doubleQ magnetic phases, which can lead to reentrance of the C2 (singleQ) order in agreement with recent thermal expansion measurements on Kdoped Ba122 crystals.
Maria N. Gastiasoro, Brian M. Andersen Journal reference: Phys. Rev. B 92, 140506 (2015) [pdf] DOI: 10.1103/PhysRevB.92.140506

Pairing symmetry of the oneband Hubbard model in the paramagnetic weakcoupling limit: A numerical RPA study 
Abstract
 We study the spinfluctuationmediated superconducting pairing gap in a weakcoupling approach to the Hubbard model for a two dimensional square lattice in the paramagnetic state. Performing a comprehensive theoretical study of the phase diagram as a function of filling, we find that the superconducting gap exhibits transitions from pwave at very low electron fillings to d_{x^2y^2}wave symmetry close to half filling in agreement with previous reports. At intermediate filling levels, different gap symmetries appear as a consequence of the changes in the Fermi surface topology and the associated structure of the spin susceptibility. In particular, the vicinity of a van Hove singularity in the electronic structure close to the Fermi level has important consequences for the gap structure in favoring the otherwise subdominant triplet solution over the singlet dwave solution. By solving the full gap equation, we find that the energetically favorable triplet solutions are chiral and break time reversal symmetry. Finally, we also calculate the detailed angular gap structure of the quasiparticle spectrum, and show how spinfluctuationmediated pairing leads to significant deviations from the first harmonics both in the singlet d_{x^2y^2} gap as well as the chiral triplet gap solution.
A. T. Romer, A. Kreisel, I. Eremin, M. A. Malakhov, T. A. Maier, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. B 92, 104505 (2015) [pdf] DOI: 10.1103/PhysRevB.92.104505

Interpretation of Scanning Tunneling Quasiparticle Interference and Impurity States in Cuprates 
Abstract
 We apply a recently developed method combining first principles based Wannier functions with solutions to the Bogoliubovde Gennes equations to the problem of interpreting STM data in cuprate superconductors. We show that the observed images of Zn on the surface of Bi$_2$Sr$_2$CaCu$_2$O$_8$ can only be understood by accounting for the tails of the Cu Wannier functions, which include significant weight on apical O sites in neighboring unit cells. This calculation thus puts earlier crude "filter" theories on a microscopic foundation and solves a long standing puzzle. We then study quasiparticle interference phenomena induced by outofplane weak potential scatterers, and show how patterns long observed in cuprates can be understood in terms of the interference of Wannier functions above the surface. Our results show excellent agreement with experiment and enable a better understanding of novel phenomena in the cuprates via STM imaging.
A. Kreisel, Peayush Choubey, T. Berlijn, B. M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 114, 217002 (2015) [pdf] DOI: 10.1103/PhysRevLett.114.217002

Model of Electronic Structure and Superconductivity in Orbitally Ordered FeSe 
Abstract
 We provide a band structure with lowenergy properties consistent with recent photoemission and quantum oscillations measurements on FeSe, assuming meanfield like s and/or dwave orbital ordering at the structural transition. We show how the resulting model provides a consistent explanation of the temperature dependence of the measured Knight shift and the spinrelaxation rate. Furthermore, the superconducting gap structure obtained from spin fluctuation theory exhibits nodes on the electron pockets, consistent with the 'V'shaped density of states obtained by tunneling spectroscopy on this material, and the temperature dependence of the London penetration depth. Our studies prove that the recent experimental observations of the electronic properties of FeSe are consistent with orbital order, but leave open the microscopic origin of the unusual band structure of this material.
Shantanu Mukherjee, A. Kreisel, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. Lett. 115, 026402 (2015) [pdf] DOI: 10.1103/PhysRevLett.115.026402

Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of FeBased Superconductors 
Abstract
 The unusual temperature dependence of the resistivity and its inplane anisotropy observed in the Febased superconducting materials, particularly Ba(Fe$_{1x}$Co$_x$)$_2$As$_2$, has been a longstanding puzzle. Here we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple twoband model of a dirty spin density wave metal. The sharp drop in resistivity below the N\'eel temperature $T_N$ in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.
Y. Wang, Maria N. Gastiasoro, Brian M. Andersen, M. Tomić, Harald O. Jeschke, Roser Valentí, Indranil Paul, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 114, 097003 (2015) [pdf] DOI: 10.1103/PhysRevLett.114.097003

Fieldinduced interplanar magnetic correlations in the hightemperature superconductor

Abstract
 We present neutron scattering studies of the interplanar correlations in the hightemperature superconductor La1.88Sr0.12CuO4 (T_c=27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the outofplane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L~0 is probed. Interestingly, the results of quenchcooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the dominating and subdominant twin displaying peaks at even or odd L, respectively.
A. T. Roemer, P. Jensen, H. Jacobsen, L. Udby, B. M. Andersen, M. Bertelsen, S. L. Holm, N. B. Christensen, R. ToftPetersen, M. Skoulatos, M. Laver, A. Schneidewind, P. Link, M. Oda, M. Ido, N. Momono, K. Lefmann Journal reference: Phys. Rev. B 91, 174507 (2015) [pdf] DOI: 10.1103/PhysRevB.91.174507

Spin excitations in the nematic phase and the metallic stripe spindensity wave phase of iron pnictides 
Abstract
 We present a general study of the magnetic excitations within a weakcoupling fiveorbital model relevant to itinerant iron pnictides. As a function of enhanced electronic correlations, the spin excitations in the symmetry broken spindensity wave phase evolve from broad lowenergy modes in the limit of weak interactions to sharply dispersing spin wave prevailing to higher energies at larger interaction strengths. We show how the resulting spin response at high energies depends qualitatively on the magnitude of the interactions. We also calculate the magnetic excitations in the nematic phase by including an orbital splitting, and find a pronounced C_2 symmetric excitation spectrum right above the transition to longrange magnetic order. Finally, we discuss the C_2 versus C_4 symmetry of the spin excitations as a function of energy for both the nematic and the spindensity wave phase.
M. Kovacic, M. H. Christensen, M. N. Gastiasoro, B. M. Andersen Journal reference: Phys. Rev. B 91, 064424 (2015) [pdf] DOI: 10.1103/PhysRevB.91.064424

Ferroelectricity in underdoped Labased cuprates 
Abstract
 Doping a parent antiferromagnetic Mott insulator in cuprates leads to shortrange electronic correlations and eventually to highTc superconductivity. However, the nature of charge correlations in the lightly doped cuprates remains unclear. Understanding the intermediate electronic phase in the phase diagram (between the parent insulator and the highTc superconductor) is expected to elucidate the complexity both inside and outside the superconducting dome, and in particular in the underdoped region. One such phase is ferroelectricity whose origin and relation to the properties of highTc superconductors is subject of current research. Here we demonstrate that ferroelectricity and the associated magnetoelectric coupling are in fact common in La214 cuprates namely, La$_{2x}$Sr$_x$CuO$_4$, La$_2$Li$_x$Cu$_{1x}$O$_4$ and La$_2$CuO$_{4+x}$. It is proposed that ferroelectricity results from local CuO$_6$ octahedral distortions, associated with the dopant atoms and/or clustering of the doped charge carriers, which break spatial inversion symmetry at the local scale whereas magnetoelectric coupling can be tuned through DzyaloshinskiiMoriya interaction.
Z. Viskadourakis, S. S. Sunku, S. Mukherjee, B. M. Andersen, T. Ito, T. Sasagawa, C. Panagopoulos Journal reference: Sci. Rep. 5, 15268 (2015) [pdf] DOI: 10.1038/srep15268

Competing superconducting and magnetic order parameters and fieldinduced magnetism in electrondoped

Abstract
 We have studied the magnetic and superconducting properties of Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data by use of a fiveband Hamiltonian relevant to iron pnictides. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a longrange ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments.
J. Larsen, B. Mencia Uranga, G. Stieber, S. L. Holm, C. Bernhard, T. Wolf, K. Lefmann, B. M. Andersen, C. Niedermayer Journal reference: Phys. Rev. B 91, 024504 (2015) [pdf] DOI: 10.1103/PhysRevB.91.024504

Local Magnetization Nucleated by Nonmagnetic Impurities in Febased Superconductors 
Abstract
 We study impurityinduced magnetic order within a fiveband Hubbard model relevant to the normal paramagnetic phase of ironbased superconductors. The existence of the local magnetic order is explained in terms of an impurityenhancement of states near the Fermi level, and we map out the resulting phase diagram of the existence of magnetization as a function of impurity strength and Coulomb correlations. In particular, the presence of impurityinduced magnetism in only a certain range of potential scattering strengths can be understood from the specific behavior of the impurity resonant state.
Maria N. Gastiasoro, Brian M. Andersen Journal reference: J. Supercond. Novel Magn. 28, 1321 (2015) [pdf] DOI: 10.1007/s1094801429082

Doping asymmetry of superconductivity coexisting with antiferromagnetism in spin fluctuation theory 
Abstract
 We generalize the theory of Cooper pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets. We show that Cooper pairing arises from transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts primarily within one type of pocket. We find a nodeless $d_{x^2y^2}$wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodeless oddparity spin singlet $p$wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram, as well as for the "nodal gap" observed recently for strongly underdoped cuprates.
W. Rowe, I. Eremin, A. Rømer, B. M. Andersen, P. J. Hirschfeld Journal reference: New J. Phys. 17, 023022 (2015) [pdf] DOI: 10.1088/13672630/17/2/023022

Spin excitations in a model of FeSe with orbital ordering 
Abstract
 2014

Emergent Defect States as a Source of Resistivity Anisotropy in the Nematic Phase of Iron Pnictides 
Abstract
 We consider the role of potential scatterers in the nematic phase of Febased superconductors above the transition temperature to the (pi,0) magnetic state but below the orthorhombic structural transition. The anisotropic spin fluctuations in this region can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials which scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. We calculate the scattering potentials, relaxation rates, and conductivity in this region, and show that such emergent defect states are essential for the transport anisotropy observed in experiments.
Maria N. Gastiasoro, I. Paul, Y. Wang, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. Lett. 113, 127001 (2014) [pdf] DOI: 10.1103/PhysRevLett.113.127001

Enhancement of Magnetic Stripe Order in IronPnictide Superconductors from the Interaction between Conduction Electrons and Magnetic Impurities 
Abstract
 Recent experimental studies have revealed several unexpected properties of Mndoped BaFe2As2. These include extension of the stripelike magnetic (pi,0) phase to high temperatures above a critical Mn concentration only, the presence of diffusive and weakly temperature dependent magnetic (pi,pi) checkerboard scattering, and an apparent absent structural distortion from tetragonal to orthorhombic. Here, we study the effects of magnetic impurities both below and above the N\'eel transition temperature within a realspace fiveband model appropriate to the iron pnictides. We show how these experimental findings can be explained by a cooperative behavior of the magnetic impurities and the conduction electrons mediating the RudermanKittelKasuyaYosida (RKKY) interactions between them.
Maria N. Gastiasoro, Brian M. Andersen Journal reference: Phys. Rev. Lett. 113, 067002 (2014) [pdf] DOI: 10.1103/PhysRevLett.113.067002

Origin of electronic dimers in the spindensity wave phase of Febased superconductors 
Abstract
 We investigate the emergent impurityinduced states arising from pointlike scatterers in the spindensity wave phase of ironbased superconductors within a microscopic fiveband model. Independent of the details of the bandstructure and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic aaxis, and have typical lengths of order 10 lattice constants in excellent agreement with recent scanning tunnelling experiments. These electronic dimers provide a natural explanation of the dopantinduced transport anisotropy found e.g. in the 122 iron pnictides.
Maria N. Gastiasoro, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 89, 100502(R) (2014) [pdf] DOI: 10.1103/PhysRevB.89.100502

Emergent Defect States as a Source of Resistivity Anisotropy in the Nematic Phase of Iron Pnictides 
Abstract
 2013

Impurity Bound States and DisorderInduced Orbital and Magnetic Order in the s ± State of FeBased Superconductors 
Abstract
 We study the presence of impurity bound states within a fiveband Hubbard model relevant to ironbased superconductors. In agreement with earlier studies, we find that in the absence of Coulomb correlations there exists a range of repulsive impurity potentials where ingap states are generated. In the presence of weak correlations, these states are generally pushed to the edges of the gap, whereas for larger correlations the onsite impurity potential induces a local magnetic region which reintroduces the lowenergy bound states into the gap.
Maria N. Gastiasoro, Brian M. Andersen Journal reference: J. Supercond. Novel Magn. 26, 2651 (2013) [pdf] DOI: 10.1007/s1094801321530

Impurity states and cooperative magnetic order in Febased superconductors 
Abstract
 We study impurity bound states and impurityinduced order in the superconducting state of LiFeAs within a realistic fiveband model based on the band structure and impurity potentials obtained from density functional theory (DFT). In agreement with recent experiments, we find that Co impurities are too weak produce subgap bound states, whereas stronger impurities like Cu do. We also obtain the bound state spectrum for magnetic impurities, such as Mn, and show how spinresolved tunnelling may determine the nature of the various defect sites in iron pnictides, a prerequisite for using impurity bound states as a probe of the ground state pairing symmetry. Lastly we show how impurities pin both orbital and magnetic order, providing an explanation for a growing set of experimental evidence for unusual magnetic phases in doped iron pnictides.
Maria N. Gastiasoro, P. J. Hirschfeld, Brian M. Andersen Journal reference: Phys. Rev. B 88, 220509(R) (2013) [pdf] DOI: 10.1103/PhysRevB.88.220509

Impurityinduced subgap bound states in alkalidoped iron chalcogenide superconductors 
Abstract
 Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali doped iron chalcogenide superconductors K_xFe_{2y}Se_2. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest neighbor dwave and nextnearest neighbor swave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that subgap bound states exist only for dwave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such disorder Coulomb interactions can lead to local impurityinduced magnetism in the case of dwave superconductivity. For magnetic impurities, both swave and dwave superconducting states support subgap bound states. The above results can be explained by a simple analytic model that provides a semiquantitative understanding of the variation of the impurity bound states energies as a function of impurity potential and chemical doping level.
Shantanu Mukherjee, Maria N. Gastiasoro, Brian M. Andersen Journal reference: Phys. Rev. B 88, 134508 (2013) [pdf] DOI: 10.1103/PhysRevB.88.134508

Lowenergy bound states at interfaces between superconducting and block antiferromagnetic regions in K

Abstract
 The highTc alkali doped iron selenide superconductors KxFe{2y}Se2 have been recently shown to be intrinsically phase separated into Fe vacancy ordered block antiferromagnetic regions and superconducting regions at low temperatures. In this work, we use a microscopic five orbital Hubbard model to obtain the electronic lowenergy states near the interfaces between block antiferromagnets and superconductors. It is found that abundant lowenergy ingap bound states exist near such interfaces irrespective of whether the superconductor has d or swave pairing symmetry. By contrast, it is shown how nonmagnetic scattering planes can provide a natural means to distinguish between these two leading pairing instabilities of the KxFe{2y}Se2 materials.
S. Mukherjee, M. N. Gastiasoro, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. B 88, 014519 (2013) [pdf] DOI: 10.1103/PhysRevB.88.014519

Glassy lowenergy spin fluctuations and anisotropy gap in La

Abstract
 We present highresolution tripleaxis neutron scattering studies of the hightemperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature dependence of the lowenergy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature TN ~ Tc inferred from elastic neutron scattering and the freezing temperature Tf ~ 11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe ordered ground state of La1.875Ba0.125CuO4. The difference between TN and Tf implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly inelastic scattering  at least in the temperature range between Tf and Tc  which is not resolved in the present experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4 with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p=0.12, where local probes indicate a sharp maximum in Tf(p), we arrive at the view that the lowenergy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p=0.12 and superconducting spin gaps dominating elsewhere.
A. T. Rømer, J. Chang, N. B. Christensen, B. M. Andersen, K. Lefmann, L. Mähler, J. Gavilano, R. Gilardi, Ch. Niedermayer, H. M. Rønnow, A. Schneidewind, P. Link, M. Oda, M. Ido, N. Momono, J. Mesot Journal reference: Phys. Rev. B 87, 144513 (2013) [pdf] DOI: 10.1103/PhysRevB.87.144513

Modulations of the Local Pairing Interaction Near Magnetic Impurities in dWave Superconductors 
Abstract
 The spinfluctuation based pairing mechanism has proven successful in explaining the pairing symmetries due to Fermi surface nesting of both cuprates and ironbased materials. In this work, we study signatures of a spinfluctuation mediated pairing at the local scale. Specifically, we focus on magnetic impurities and calculate both the local antiferromagnetism and the resulting modulated pairing interaction. The latter gives rise to distinct local enhancements of the superconducting gap in the immediate vicinity of the impurities. Our results show that Coulombdriven pairing naturally leads to unusual superconducting gap modulations near disorder potentials.
A. T. Roemer, S. Graser, P. J. Hirschfeld, B. M. Andersen Journal reference: J. Supercond. Novel Magn. 26, 1729 (2013) [pdf] DOI: 10.1007/s1094801220583

BiQuadratic Magnetoelectric Coupling in Underdoped La2CuO4+x 
Abstract
 The recent discovery of relaxor ferroelectricity and magnetoelectric effect in lightly doped cuprate material La_2CuO_{4+x} has provided a number of questions concerning its theoretical description. It has been argued using a GinzburgLandau free energy approach that the magnetoelectric effect can be explained by the presence of biquadratic interaction terms in the free energy. Here, by using the same free energy functional, we study the variety of behavior which can emerge in the electric polarization under an external magnetic field. Subsequently, we discuss the role of DzyaloshinskiiMoriya interaction in generating this magnetoelectric response. This work is particularly relevant for such relaxor systems where the materialdependent parameters would be affected by changes in e.g. chemical doping or cooling rate.
S. Mukherjee, B. M. Andersen, Z. Viskadourakis, I. Radulov, C. Panagopoulos Journal reference: J. Supercond. Novel Magn. 26, 1649 (2013) [pdf] DOI: 10.1007/s1094801220592

Impurity Bound States and DisorderInduced Orbital and Magnetic Order in the s ± State of FeBased Superconductors 
Abstract
 2012

Robust Nodal

Abstract
 We resolve an existing controversy between, on the one hand, convincing evidence for the existence of competing order in underdoped cuprates, and, on the other hand, spectroscopic data consistent with a seemingly homogeneous dwave superconductor in the very same compounds. Specifically, we show how shortrange fluctuations of the competing order essentially restore the nodal dwave spectrum from the qualitatively distinct folded dispersion resulting from homogeneous coexisting phases. The signatures of the fluctuating competing order can be found mainly in a splitting of the antinodal quasiparticles and, depending of the strength of the competing order, also in small induced nodal gaps as found in recent experiments on underdoped La{2x}SrxCuO4.
W. A. Atkinson, J. David Bazak, B. M. Andersen Journal reference: Phys. Rev. Lett. 109, 267004 (2012) [pdf] DOI: 10.1103/PhysRevLett.109.267004

Local modulations of the spinfluctuationmediated pairing interaction by impurities in

Abstract
 We present a selfconsistent real space formulation of spinfluctuation mediated dwave pairing. By calculating all relevant inhomogeneous spin and charge susceptibilities in real space within the random phase approximation (RPA), we obtain the effective pairing interaction and study its spatial dependence near both local potential and hopping impurities. A remarkably large enhancement of the pairing interaction may be obtained near the impurity site. We discuss the relevance of our result to inhomogeneities observed by scanning tunneling spectroscopy on the surface of cuprate superconductors.
A. T. Roemer, S. Graser, T. S. Nunner, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. B 86, 054507 (2012) [pdf] DOI: 10.1103/PhysRevB.86.054507

Lowtemperature ferroelectric phase and magnetoelectric coupling in underdoped La

Abstract
 We report the discovery of a ferroelectric ground state below 4.5 K in highly underdoped La_2CuO_(4+x) accompanied by slow charge dynamics which develop below T~40 K. An anisotropic magnetoelectric response has also been observed, indicating considerable spincharge coupling in this lightly doped "parent" high temperature copperoxide superconductor. The ferroelectric state is proposed to develop from polar nanoregions, in which spatial inversion symmetry is locally broken due to nonstoichiometric carrier doping.
Z. Viskadourakis, I. Radulov, A. P. Petrović, S. Mukherjee, B. M. Andersen, G. Jelbert, N. S. Headings, S. M. Hayden, K. Kiefer, S. Landsgesell, D. N. Argyriou, C. Panagopoulos Journal reference: Phys. Rev. B 85, 214502 (2012) [pdf] DOI: 10.1103/PhysRevB.85.214502

Signatures of orbital loop currents in the spatially resolved local density of states 
Abstract
 Polarized neutron scattering measurements have suggested that intraunit cell antiferromagnetism may be associated with the pseudogap phase. Assuming that loop current order is responsible for the observed magnetism, we calculate some signatures of such circulating currents in the local density of states around a single nonmagnetic impurity in a coexistence phase with superconductivity. We find a distinct C4 symmetry breaking near the disorder which is also detectable in the resulting quasiparticle interference patterns.
W. H. P. Nielsen, W. A. Atkinson, B. M. Andersen Journal reference: Phys. Rev. B 86, 054510 (2012) [pdf] DOI: 10.1103/PhysRevB.86.054510

Inducing spindependent tunneling to probe magnetic correlations in optical lattices 
Abstract
 We suggest a simple experimental method for probing antiferromagnetic spin correlations of twocomponent Fermi gases in optical lattices. The method relies on a spin selective Raman transition to excite atoms of one spin species to their first excited vibrational mode where the tunneling is large. The resulting difference in the tunneling dynamics of the two spin species can then be exploited, to reveal the spin correlations by measuring the number of doubly occupied lattice sites at a later time. We perform quantum Monte Carlo simulations of the spin system and solve the optical lattice dynamics numerically to show how the timed probe can be used to identify antiferromagnetic spin correlations in optical lattices.
K. G. L. Pedersen, B. M. Andersen, O. F. Syljuasen, G. M. Bruun, A. S. Sorensen Journal reference: Phys. Rev. A 85, 053642 (2012) [pdf] DOI: 10.1103/PhysRevA.85.053642

Theory of the magnetoeletric effect in a lightly doped high

Abstract
 In a recent study Viskadourakis et al. discovered that extremely underdoped La_2CuO_(4+x) is a relaxor ferroelectric and a magnetoelectric material at low temperatures. It is further observed that the magnetoelectric response is anisotropic for different directions of electric polarization and applied magnetic field. By constructing an appropriate Landau theory, we show that a biquadratic magnetoelectric coupling can explain the experimentally observed polarization dependence on magnetic field. This coupling leads to several novel lowtemperature effects including a feedback enhancement of the magnetization below the ferroelectric transition, and a predicted magnetocapacitive effect.
S. Mukherjee, B. M. Andersen, Z. Viskadourakis, I. Radulov, C. Panagopoulos Journal reference: Phys. Rev. B 85, 140405(R) (2012) [pdf] DOI: 10.1103/PhysRevB.85.140405

Textured superconductivity in the presence of a coexisting order: Ce115s and other heavyfermion compounds 
Abstract
 Recent experimental and theoretical studies have highlighted the possible role of a electronic nematic liquid in underdoped cuprate superconductors. We calculate, within a model of dwave superconductor with Hubbard correlations, the spin susceptibility in the case of a small explicitly broken rotational symmetry of the underlying lattice. We then exhibit how the induced spin response asymmetry is strongly enhanced by correlations as one approaches the instability to stripe order. In the disorderinduced stripe phase, impurities become spin nematogens with a C_2 symmetric impurity resonance state, and the disorderaveraged spin susceptibility remains only C_2 symmetric at low energies, similar to recent data from neutron scattering experiments on underdoped YBCO.
Brian M. Andersen, Siegfried Graser, P. J. Hirschfeld Journal reference: Europhys. Lett. 97, 47002 (2012) [pdf] DOI: 10.1016/j.physc.2012.04.030

Pinning of Stripes by Local Structural Distortions in Cuprate HighT c Superconductors 
Abstract
 We study the spindensity wave (stripe) instability in lattices with mixed lowtemperature orthorhombic (LTO) and lowtemperature tetragonal (LTT) crystal symmetry. Within an explicit meanfield model it is shown how local LTT regions act as pinning centers for static stripe formation. We calculate the modulations in the local density of states near these local stripe regions and find that mainly the coherence peaks and the van Hove singularity (VHS) are spatially modulated. Lastly, we use the realspace approach to simulate recent tunneling data in the overdoped regime where the VHS has been detected by utilizing local normal state regions.
U. Tricoli, B. M. Andersen Journal reference: J. Supercond. Novel Magn. 25, 1329 (2012) [pdf] DOI: 10.1007/s1094801216230

Robust Nodal

Abstract
 2011

Two routes to magnetic order by disorder in underdoped cuprates 
Abstract
 We study disorderinduced magnetism within the Gutzwiller approximation applied to the tJ model relevant for cuprate superconductors. In particular, we show how disorder generates magnetic phases by inducing local droplets of antiferromagnetic order which eventually merge, and form a quasilong range ordered state in the underdoped regime. We identify two distinct disorderinduced magnetic phases of this type depending on the strength of the scatterers. For weak potential scatterers used to model dopant disorder, charge reorganization may push local regions inbetween the impurities across the magnetic phase boundary, whereas for strong scatterers used to model substitutional ions, a local static magnetic moment is formed around each impurity. We calculate the density of states and find a remarkably universal lowenergy behavior largely independent of both disorder and magnetization. However, the magnetic regions are characterized by larger (reduced) superconducting gap (coherence peaks) and a subgap kink in the density of states.
R. B. Christensen, P. J. Hirschfeld, B. M. Andersen Journal reference: Phys. Rev. B 84, 184511 (2011) [pdf] DOI: 10.1103/PhysRevB.84.184511

Nonequilibrium Transport through a Spinful Quantum Dot with Superconducting Leads 
Abstract
 We study the nonlinear cotunneling current through a spinful quantum dot contacted by two superconducting leads. Applying a general nonequilibrium Green function formalism to an effective Kondo model, we study the rich variation in the IVcharacteristics with varying asymmetry in the tunnel coupling to source and drain electrodes. The current is found to be carried respectively by multiple Andreev reflections in the symmetric limit, and by spininduced YuShibaRussinov bound states in the strongly asymmetric limit. The interplay between these two mechanisms leads to qualitatively different IVcharacteristics in the crossover regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and repositioned conductance peaks in subgap cotunneling spectroscopy.
B. M. Andersen, K. Flensberg, V. Koerting, J. Paaske Journal reference: Phys. Rev. Lett. 107, 256802 (2011) [pdf] DOI: 10.1103/PhysRevLett.107.256802

Measuring spin correlations in optical lattices using superlattice potentials 
Abstract
 We suggest two experimental methods for probing both short and longrange spin correlations of atoms in optical lattices using superlattice potentials. The first method involves an adiabatic doubling of the periodicity of the underlying lattice to probe neighboring singlet (triplet) correlations for fermions (bosons) by the occupation of the new vibrational ground state. The second method utilizes a timedependent superlattice potential to generate spindependent transport by any number of prescribed lattice sites, and probes correlations by the resulting number of doubly occupied sites. For experimentally relevant parameters, we demonstrate how both methods yield large signatures of antiferromagnetic (AF) correlations of strongly repulsive fermionic atoms in a single shot of the experiment. Lastly, we show how this method may also be applied to probe dwave pairing, a possible ground state candidate for the doped repulsive Hubbard model.
K. G. L. Pedersen, B. M. Andersen, G. M. Bruun, O. F. Syljuasen, A. S. Sorensen Journal reference: Phys. Rev. A 84, 041603(R) (2011) [pdf] DOI: 10.1103/PhysRevA.84.041603

Effects of impurities and vortices on the lowenergy spin excitations in highT materials 
Abstract
 We review a theoretical scenario for the origin of the spinglass phase of underdoped cuprate materials. In particular it is shown how disorder in a correlated dwave superconductor generates a magnetic phase by inducing local droplets of antiferromagnetic order which eventually merge and form a quasilong range ordered state. When correlations are sufficiently strong, disorder is unimportant for the generation of static magnetism but plays an additional role of pinning disordered stripe configurations. We calculate the spin excitations in a disordered spindensity wave phase, and show how disorder and/or applied magnetic fields lead to a slowing down of the dynamical spin fluctuations in agreement with neutron scattering and muon spin rotation (muSR) experiments.
Brian M. Andersen, Markus Schmid, Siegfried Graser, P. J. Hirschfeld, Arno P. Kampf Journal reference: J. Phys. Chem. Solids 72, 358 (2011) [pdf] DOI: 10.1016/j.jpcs.2010.10.025

Two routes to magnetic order by disorder in underdoped cuprates 
Abstract
 2010

Nonequilibrium transport via spininduced subgap states in superconductor/quantum dot/normal metal cotunnel junctions 
Abstract
 We study lowtemperature transport through a Coulomb blockaded quantum dot (QD) contacted by a normal (N), and a superconducting (S) electrode. Within an effective cotunneling model the conduction electron self energy is calculated to leading order in the cotunneling amplitudes and subsequently resummed to obtain the nonequilibrium Tmatrix, from which we obtain the nonlinear cotunneling conductance. For even occupied dots the system can be conceived as an effective S/Ncotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd occupied dot, however, leads to the formation of subgap resonances inside the superconducting gap which gives rise to a characteristic peakdip structure in the differential conductance, as observed in recent experiments.
V. Koerting, B. M. Andersen, K. Flensberg, J. Paaske Journal reference: Phys. Rev. B 82, 245108 (2010) [pdf] DOI: 10.1103/PhysRevB.82.245108

DisorderInduced Freezing of Dynamical Spin Fluctuations in Underdoped Cuprate Superconductors 
Abstract
 We study the dynamical spin susceptibility of a correlated dwave superconductor (dSC) in the presence of disorder, using an unrestricted HartreeFock approach. This model provides a concrete realization of the notion that disorder slows down spin fluctuations, which eventually "freeze out". The evolution of disorderinduced spectral weight transfer agrees qualitatively with experimental observations on underdoped cuprate superconductors. For sufficiently large disorder concentrations, static spin density wave (SDW) order is created when droplets of magnetism nucleated by impurities overlap. We also study the disordered stripe state coexisting with a dSC and compare its magnetic fluctuation spectrum to that of the disordergenerated SDW phase.
Brian M. Andersen, Siegfried Graser, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 105, 147002 (2010) [pdf] DOI: 10.1103/PhysRevLett.105.147002

Modeling a striped pseudogap state 
Abstract
 We study the electronic structure within a system of phasedecoupled onedimensional superconductors coexisting with stripe spin and charge density wave order. This system has a nodal Fermi surface (Fermi arc) in the form of a hole pocket and an antinodal pseudogap. The spectral function in the antinodes is approximately particlehole symmetric contrary to the gapped regions just outside the pocket. We find that states at the Fermi energy are extended whereas states near the pseudogap energy have localization lengths as short as the interstripe spacing. We consider pairing which has either local dwave or swave symmetry and find similar results in both cases, consistent with the pseudogap being an effect of local pair correlations. We suggest that this state is a stripe ordered caricature of the pseudogap phase in underdoped cuprates with coexisting spin, charge, and pairdensity wave correlations. Lastly, we also model a superconducting state which 1) evolves smoothly from the pseudogap state, 2) has a signature subgap peak in the density of states, and 3) has the coherent pair density concentrated to the nodal region.
M. Granath, B. M. Andersen Journal reference: Phys. Rev. B 81, 024501 (2010) [pdf] DOI: 10.1103/PhysRevB.81.024501

How grain boundaries limit supercurrents in hightemperature superconductors 
Abstract
 The interface properties of hightemperature cuprate superconductors have been of interest for many years, and play an essential role in Josephson junctions, superconducting cables, and microwave electronics. In particular, the maximum critical current achievable in highTc wires and tapes is well known to be limited by the presence of grain boundaries, regions of mismatch between crystallites with misoriented crystalline axes. In studies of single, artificially fabricated grain boundaries the striking observation has been made that the critical current Jc of a grain boundary junction depends exponentially on the misorientation angle. Until now microscopic understanding of this apparently universal behavior has been lacking. We present here the results of a microscopic evaluation based on a construction of fully 3D YBCO grain boundaries by molecular dynamics. With these structures, we calculate an effective tightbinding Hamiltonian for the dwave superconductor with a grain boundary. The critical current is then shown to follow an exponential suppression with grain boundary angle. We identify the buildup of charge inhomogeneities as the dominant mechanism for the suppression of the supercurrent.
S. Graser, P. J. Hirschfeld, T. Kopp, R. Gutser, B. M. Andersen, J. Mannhart Journal reference: Nature Physics 6, 609 (2010) [pdf] DOI: 10.1038/nphys1687

dWave superconductivity as a catalyst for antiferromagnetism in underdoped cuprates 
Abstract
 The underdoped highTc materials are characterized by a competition between Cooper pairing and antiferromagnetic (AF) order. Important differences between the superconducting (SC) state of these materials and conventional superconductors include the dwave pairing symmetry and a remarkable magnetic response to nonmagnetic perturbations, whereby droplets of spindensity wave (SDW) order can form around impurities and the cores of vortices. In a simple picture, whenever SC is suppressed locally, SDW order is nucleated. Within a meanfield theory of dwave SC in an applied magnetic field including disorder and Hubbard correlations, we show in fact that the creation of SDW order is not simply due to suppression of the SC order parameter, but rather due to a correlationinduced splitting of the electronic bound state created by the perturbation. Since the bound state exists because of the sign change of the order parameter along quasiparticle trajectories, the induced SDW order is a direct consequence of the dwave symmetry. Furthermore the formation of antiphase domain walls is important for obtaining the correct temperature dependence of the induced magnetism as measured by neutron diffraction.
Markus Schmid, Brian M. Andersen, Arno P. Kampf, P. J. Hirschfeld Journal reference: New J. Phys. 12, 053043 (2010) [pdf] DOI: 10.1088/13672630/12/5/053043

Nonequilibrium transport via spininduced subgap states in superconductor/quantum dot/normal metal cotunnel junctions 
Abstract
 2009

The quest for the solar g modes 
Abstract
 Solar gravity modes (or g modes)  oscillations of the solar interior for which buoyancy acts as the restoring force  have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well observed acoustic modes (or p modes). The high amplitude of the gmode eigenfunctions in the core and the evanesence of the modes in the convection zone make the modes particularly sensitive to the physical and dynamical conditions in the core. Owing to the existence of the convection zone, the g modes have very low amplitudes at photospheric levels, which makes the modes extremely hard to detect. In this paper, we review the current state of play regarding attempts to detect g modes. We review the theory of g modes, including theoretical estimation of the gmode frequencies, amplitudes and damping rates. Then we go on to discuss the techniques that have been used to try to detect g modes. We review results in the literature, and finish by looking to the future, and the potential advances that can be made  from both data and dataanalysis perspectives  to give unambiguous detections of individual g modes. The review ends by concluding that, at the time of writing, there is indeed a consensus amongst the authors that there is currently no undisputed detection of solar g modes.
T. Appourchaux, K. Belkacem, A. M. Broomhall, W. J. Chaplin, D. O. Gough, G. Houdek, J. Provost, F. Baudin, P. Boumier, Y. Elsworth, R. A. García, B. Andersen, W. Finsterle, C. Fröhlich, A. Gabriel, G. Grec, A. Jiménez, A. Kosovichev, T. Sekii, T. Toutain, S. TurckChièze [pdf] DOI: 10.1007/s001590090027z 0910.0848v2 [pdf]

Antiferromagnetic noise correlations in optical lattices 
Abstract
 We analyze how noise correlations probed by timeofflight (TOF) experiments reveal antiferromagnetic (AF) correlations of fermionic atoms in twodimensional (2D) and threedimensional (3D) optical lattices. Combining analytical and quantum Monte Carlo (QMC) calculations using experimentally realistic parameters, we show that AF correlations can be detected for temperatures above and below the critical temperature for AF ordering. It is demonstrated that spinresolved noise correlations yield important information about the spin ordering. Finally, we show how to extract the spin correlation length and the related critical exponent of the AF transition from the noise.
G. M. Bruun, O. F. Syljuasen, K. G. L. Pedersen, B. M. Andersen, E. Demler, A. S. Sorensen Journal reference: Phys. Rev. A 80, 033622 (2009) [pdf] DOI: 10.1103/PhysRevA.80.033622

Superconductivityenhanced bias spectroscopy in carbon nanotube quantum dots 
Abstract
 We study lowtemperature transport through carbon nanotube quantum dots in the Coulomb blockade regime coupled to niobiumbased superconducting leads. We observe pronounced conductance peaks at finite sourcedrain bias, which we ascribe to elastic and inelastic cotunneling processes enhanced by the coherence peaks in the density of states of the superconducting leads. The inelastic cotunneling lines display a marked dependence on the applied gate voltage which we relate to different tunnelingrenormalizations of the two subbands in the nanotube. Finally, we discuss the origin of an especially pronounced subgap structure observed in every fourth Coulomb diamond.
K. GroveRasmussen, H. I. Jørgensen, B. M. Andersen, J. Paaske, T. S. Jespersen, J. Nygård, K. Flensberg, P. E. Lindelof Journal reference: Phys. Rev. B 79, 134518 (2009) [pdf] DOI: 10.1103/PhysRevB.79.134518

Theory of resistivity upturns in metallic cuprates 
Abstract
 We propose that the experimentally observed resistivity upturn of cuprates at low temperatures may be explained by properly accounting for the effects of disorder in a strongly correlated metallic host. Within a calculation of the DC conductivity using realspace diagonalization of a Hubbard model treated in an inhomogeneous unrestricted HartreeFock approximation, we find that correlations induce magnetic droplets around impurities, and give rise to additional magnetic scattering which causes the resistivity upturn. A pseudogap in the density of states is shown to enhance both the disorderinduced magnetic state and the resistivity upturns.
W. Chen, Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. B 80, 134518 (2009) [pdf] DOI: 10.1103/PhysRevB.80.134518

Magneticfieldinduced soft mode in spingapped high

Abstract
 We present an explanation of the dynamical ingap spin mode in LSCO induced by an applied magnetic field H as recently observed by J. Chang et al. Our model consists of a phenomenological spinonly Hamiltonian, and the softening of the spin mode is caused by vortex pinning of dynamical stripe fluctuations which we model by a local ordering of the exchange interactions. The spin gap vanishes experimentally around H=7T which in our scenario corresponds to the field required for overlapping vortex regions.
Brian M. Andersen, Olav F. Syljuasen, Per Hedegard Journal reference: Phys. Rev. B 80, 052509 (2009) [pdf] DOI: 10.1103/PhysRevB.80.052509

Probing Spatial Spin Correlations of Ultracold Gases by Quantum Noise Spectroscopy 
Abstract
 Spin noise spectroscopy with a single laser beam is demonstrated theoretically to provide a direct probe of the spatial correlations of cold fermionic gases. We show how the generic manybody phenomena of antibunching, pairing, antiferromagnetic, and algebraic spin liquid correlations can be revealed by measuring the spin noise as a function of laser width, temperature, and frequency.
G. M. Bruun, Brian M. Andersen, Eugene Demler, Anders S. Sørensen Journal reference: Phys. Rev. Lett. 102, 030401 (2009) [pdf] DOI: 10.1103/PhysRevLett.102.030401

Extinction of quasiparticle interference in underdoped cuprates with coexisting order 
Abstract
 Recent scanning tunnelling spectroscopy measurements [Y. Koksaka et al., Nature 454, 1072 (2008)] have shown that dispersing quasiparticle interference peaks in Fourier transformed conductance maps disappear as the bias voltage exceeds a certain threshold corresponding to the coincidence of the contour of constant quasiparticle energy with the antiferromagnetic zone boundary. Here we argue that this is caused by quasistatic shortrange coexisting order present in the dwave superconducting phase, and that the most likely origin of this order is disorderinduced incommensurate antiferromagnetism. We show explicitly how the peaks are extinguished in the related situation with coexisting longrange antiferromagnetic order, and discuss the connection with the realistic disordered case. Since it is the localized quasiparticle interference peaks rather than the underlying antinodal states themselves which are destroyed at a critical bias, our proposal resolves a conflict between scanning tunneling spectroscopy and photoemission regarding the nature of these states.
Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. B 79, 144515 (2009) [pdf] DOI: 10.1103/PhysRevB.79.144515

The quest for the solar g modes 
Abstract
 2008

Imaging the impact on cuprate superconductivity of varying the interatomic distances within individual crystal unit cells 
Abstract
 Many theoretical models of high temperature superconductivity focus only on the doping dependence of the CuO2 plane electronic structure. But such models are manifestly insufficient to explain the strong variations in superconducting critical temperature Tc among cuprates which have identical holedensity but are crystallographically different outside the CuO2 plane. A key challenge, therefore, has been to identify a predominant outofplane influence controlling the superconductivity  with much attention focusing on the distance $d_A$ between the apical oxygen and the planar copper atom. Here we report direct determination of how variations of interatomic distances within individual crystalline unit cells, affect the superconducting energygap maximum $\Delta$ of Bi2Sr2CaCu2O8. In this material, quasiperiodic variations of unit cell geometry occur in the form of a bulk crystalline 'supermodulation'. Within each supermodulation period, we find a $\sim 9\pm1%$ cosinusoidal variation in local $\Delta$ that is anticorrelated with the associated $d_A$ variations. Furthermore, we show that phenomenological consistency would exist between these effects and the random $\Delta$ variations found near dopant atoms if the primary effect of the interstitial dopant atom is to displace the apical oxygen so as to diminish $d_A$ or tilt the CuO5 pyramid. Thus we reveal a strong nonrandom outofplane effect on cuprate superconductivity at atomic scale.
J. A. Slezak, Jinho Lee, M. Wang, K. McElroy, K. Fujita, B. M. Andersen, P. J. Hirschfeld, H. Eisaki, S. Uchida, J. C. Davis Journal reference: Proc. Natl. Acad. Sci. USA 105, 3203 (2008) [pdf] DOI: 10.1073/pnas.0706795105

Extinction of impurity resonances in largegap regions of inhomogeneous

Abstract
 Impurity resonances observed by scanning tunneling spectroscopy in the superconducting state have been used to deduce properties of the underlying pure state. Here we study a longstanding puzzle associated with these measurements, the apparent extinction of these resonances for Ni and Zn impurities in largegap regions of the inhomogeneous BSCCO superconductor. We calculate the effect of order parameter and hopping suppression near the impurity site, and find that these two effects are sufficient to explain the missing resonances in the case of Ni. There are several possible scenarios for the extinction of the Zn resonances, which we discuss in turn; in addition, we propose measurements which could distinguish among them.
Brian M. Andersen, S. Graser, P. J. Hirschfeld Journal reference: Phys. Rev. B, 78 134502 (2008) [pdf] DOI: 10.1103/PhysRevB.78.134502

Breakdown of Universal Transport in Correlated

Abstract
 The prediction and observation of lowtemperature universal thermal conductivity in cuprates has served as a keystone of theoretical approaches to the superconducting state, but recent measurements on underdoped samples show strong violations of this apparently fundamental property of dwave nodal quasiparticles. Here, we show that the breakdown of universality may be understood as the consequence of disorderinduced magnetic states in the presence of increasing antiferromagnetic correlations in the underdoped state, even as these same correlations protect the nodal lowenergy density of states in agreement with recent scanning tunneling experiments.
Brian M. Andersen, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 100, 257003 (2008) [pdf] DOI: 10.1103/PhysRevLett.100.257003

Josephson effects in

Abstract
 We calculate the dc supercurrent through a Josephson tunnel junction consisting of an antiferromagnetic (AF) interlayer sandwiched between two dwave superconductors (d). Such d/AF/d junctions exhibit a rich dependence of the Josephson current on the interlayer parameters, including the possibility of 0pi transitions with varying temperature or interlayer thickness. Furthermore, we study d/I/d junctions when the dSC leads include subdominant magnetic correlations. In this case induced magnetism near the interface can strongly diminish the critical current for 110 oriented junctions whereas no suppression is obtained for the 100 orientation. This may help resolve a longstanding puzzle of the critical current versus grain boundary angle in highT_c superconductors.
Brian M. Andersen, Yu. S. Barash, S. Graser, P. J. Hirschfeld Journal reference: Phys. Rev. B 77, 054501 (2008) [pdf] DOI: 10.1103/PhysRevB.77.054501

Imaging the impact on cuprate superconductivity of varying the interatomic distances within individual crystal unit cells 
Abstract
 2007

Magnetic and superfluid phases of confined fermions in twodimensional optical lattices 
Abstract
 We examine antiferromagnetic and dwave superfluid phases of cold fermionic atoms with repulsive interactions in a twodimensional optical lattice combined with a harmonic trapping potential. For experimentally realistic parameters, the trapping potential leads to the coexistence of magnetic and superfluid ordered phases with the normal phase. We study the intriguing shell structures arising from the competition between the magnetic and superfluid order as a function of the filling fraction. In certain cases antiferromagnetism induce superfluidity by charge redistributions. We furthermore demonstrate how these shell structures can be detected as distinct antibunching dips and pairing peaks in the densitydensity correlation function probed in expansion experiments.
Brian M. Andersen, G. M. Bruun Journal reference: Phys. Rev. A 76, 041602(R) (2007) [pdf] DOI: 10.1103/PhysRevA.76.041602

DisorderInduced Static Antiferromagnetism in Cuprate Superconductors 
Abstract
 Using model calculations of a disordered dwave superconductor with onsite Hubbard repulsion, we show how dopant disorder can stabilize novel states with antiferromagnetic order. We find that the critical strength of correlations or impurity potential necessary to create an ordered magnetic state in the presence of finite disorder is reduced compared to that required to create a single isolated magnetic droplet. This may explain why in cuprates like LSCO lowenergy probes have identified a static magnetic component which persists well into the superconducting state, whereas in cleaner systems like YBCO it is absent or minimal. Finally we address the case of nominally clean LSCO samples at optimal doping, where such ordered magnetic moments are absent, but where they can be induced by small concentrations of strong scatterers.
Brian M. Andersen, P. J. Hirschfeld, Arno P. Kampf, Markus Schmid Journal reference: Phys. Rev. Lett. 99, 147002 (2007) [pdf] DOI: 10.1103/PhysRevLett.99.147002

Superconducting gap variations induced by structural supermodulation in

Abstract
 We discuss the possibility that the strain field introduced by the structural supermodulation in Bi2212 and certain other cuprate materials may modulate the superconducting pairing interaction. We calculate the amplitude of this effect, visible in scanning tunneling spectroscopy experiments, and thereby relate a change in the local superconducting gap with the change in the local dopant displacements induced by the supermodulation. In principle, since this modulation is periodic, sufficiently accurate xray measurements or ab initio calculations should enable one to determine which atomic displacements enhance pairing and therefore T_c.
Brian M. Andersen, P. J. Hirschfeld, James A. Slezak Journal reference: Phys. Rev. B 76, 020507 (2007) [pdf] DOI: 10.1103/PhysRevB.76.020507

KondoEnhanced Andreev Tunneling in InAs Nanowire Quantum Dots 
Abstract
 We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with subgappeaks at $V_{sd}=\Delta/e$ markedly stronger(weaker) than the quasiparticle tunneling peaks at $V_{sd}=2\Delta/e$ for odd(even) occupation. We attribute the enhanced $\Delta$peak to an interplay between Kondocorrelations and Andreev tunneling in dots with an odd number of spins, and substantiate this interpretation by a poor man's scaling analysis.
T. SandJespersen, J. Paaske, B. M. Andersen, K. GroveRasmussen, H. I. Jørgensen, M. Aagesen, C. Sørensen, P. E. Lindelof, K. Flensberg, J. Nygård Journal reference: Phys. Rev. Lett. 99, 126603 (2007) [pdf] DOI: 10.1103/PhysRevLett.99.126603

Antiferromagnetic correlations and impurity broadening of NMR linewidths in cuprate superconductors 
Abstract
 We study a model of a dwave superconductor with strong potential scatterers in the presence of antiferromagnetic correlations and apply it to experimental nuclear magnetic resonance (NMR) results on Zn impurities in the superconducting state of YBCO. We then focus on the contribution of impurityinduced paramagnetic moments, with Hubbard correlations in the host system accounted for in Hartree approximation. We show that local magnetism around individual impurities broadens the line, but quasiparticle interference between impurity states plays an important role in smearing out impurity satellite peaks. The model, together with estimates of vortex lattice effects, provides a semiquantitative description of the impurity concentration dependence of the NMR line shape in the superconducting state, and gives a qualitative description of the temperature dependence of the line asymmetry. We argue that impurityinduced paramagnetism and resonant local density of states effects are both necessary to explain existing experiments.
J. W. Harter, B. M. Andersen, J. Bobroff, M. Gabay, P. J. Hirschfeld Journal reference: Phys. Rev. B 75, 054520 (2007) [pdf] DOI: 10.1103/PhysRevB.75.054520

Detailed magnetic excitation spectra of ordered and disordered spin ladders 
Abstract
 We calculate the magnetic excitation spectrum in the stripe phase of highT_c materials. The stripes are modeled as coupled spin1/2 ladders and the spin dynamics is extracted using Quantum Monte Carlo (QMC) simulations, which can capture the strong quantum fluctuations near quantum critical points of coupled spin ladders. We find a characteristic hourglass magnetic excitation spectrum with highenergy peaks rotated by 45 degrees compared to the incommensurate lowenergy peaks in good agreement with the experimental data. The excitations are investigated quantitatively as a function of interladder coupling, ladder width, and domain formation with stripe disorder.
Brian M. Andersen, Olav F. Syljuasen Journal reference: Phys. Rev. B 75, 012506 (2007) [pdf] DOI: 10.1103/PhysRevB.75.012506

Renormalization of thermal conductivity of disordered dwave superconductors by impurityinduced local moments 
Abstract
 The lowtemperature thermal conductivity \kappa_0/T of dwave superconductors is generally thought to attain a "universal" value independent of disorder at sufficiently low temperatures, providing an important measure of the magnitude of the gap slope near its nodes. We discuss situations in which this inference can break down because of competing order, and quasiparticle localization. Specifically, we study an inhomogeneous BCS mean field model with electronic correlations included via a Hartree approximation for the Hubbard interaction, and show that the suppression of \kappa_0/T by localization effects can be strongly enhanced by magnetic moment formation around potential scatterers.
Brian M. Andersen, P. J. Hirschfeld Journal reference: Physica C: Superconductivity, 460462, 744 (2007) [pdf] DOI: 10.1016/j.physc.2007.03.364

Signatures of modulated pair interaction in cuprate superconductors 
Abstract
 Recent lowtemperature scanning tunnelling spectroscopy experiments on the surface of BSCCO2212 have revealed a strong positive correlation between the position of localized resonances at 960 meV identified with interstitial oxygen dopants and the size of the local spectral gap. We review efforts to understand these correlations within a model where the dopants modulate the pair interaction on an atomic scale. We provide further evidence for this model by comparing the correlations between the dopants and the local density of states with experimental results.
Tamara S. Nunner, P. J. Hirschfeld, Brian M. Andersen, Ashot Melikyan, K. McElroy Journal reference: Physica C: Superconductivity, 460462, 446 (2007) [pdf] DOI: 10.1016/j.physc.2007.03.116

Disorder effects on the intrinsic nonlinear current density in
YBa$_2$Cu$_3$O$_{7δ}$ 
Abstract
 We present harmonic generation measurements of the intrinsic nonlinear current density $j_2$ of YBa$_2$Cu$_3$O$_{7\delta}$ (YBCO) films at temperatures close the $T_c$. Experiments on a range of different quality samples allow us to extract the dependence of $j_2$ on the penetration depth of the superconductor. In order to model these results, we calculate the intrinsic nonlinear current response of $d_{x^2y^2}$wave superconductors in the Meissner regime in the presence of nonmagnetic impurities within the selfconsistent Tmatrix approximation.
Brian M. Andersen, James C. Booth, P. J. Hirschfeld [pdf]

Magnetic and superfluid phases of confined fermions in twodimensional optical lattices 
Abstract
 2006

Recursion method for the quasiparticle structure of a single vortex with induced magnetic order 
Abstract
 We use a realspace recursion method to calculate the local density of states (LDOS) within a model that contains both dwave superconducting and antiferromagnetic order. We focus on the LDOS in the superconducting phase near single vortices with either normal or antiferromagnetic cores. Furthermore, we study the lowenergy quasiparticle structure when magnetic vortices operate as pinning centers for surrounding unidirectional spin density waves (stripes). We calculate the Fourier transformed LDOS and show how the energy dependence of relevant Fourier components can be used to determine the nature of the magnetic fieldinduced order, and predict fieldinduced LDOS features that can be tested by future scanning tunneling microscopy (STM) experiments.
Linda Udby, Brian M. Andersen, Per Hedegard Journal reference: Phys. Rev. B 73, 224510 (2006) [pdf] DOI: 10.1103/PhysRevB.73.224510

Thermodynamic transitions in inhomogeneous

Abstract
 We study the spectral and thermodynamic properties of inhomogeneous dwave superconductors within a model where the inhomogeneity originates from atomic scale pair disorder. This assumption has been shown to be consistent with the small charge and large gap modulations observed by scanning tunnelling spectroscopy (STS) on BSCCO. Here we calculate the specific heat within the same model, and show that it gives a semiquantitative description of the transition width in this material. This model therefore provides a consistent picture of both surface sensitive spectroscopy and bulk thermodynamic properties.
Brian M. Andersen, Ashot Melikyan, Tamara S. Nunner, P. J. Hirschfeld Journal reference: Phys. Rev. B 74, 060501(R) (2006) [pdf] DOI: 10.1103/PhysRevB.74.060501

Superconducting Junctions with Ferromagnetic, Antiferromagnetic or ChargeDensityWave Interlayers 
Abstract
 Spectra and spin structures of Andreev interface states and the Josephson current are investigated theoretically in junctions between clean superconductors (SC) with ordered interlayers. The Josephson current through the ferromagnetinsulatorferromagnet interlayer can exhibit a nonmonotonic dependence on the misorientation angle. The characteristic behavior takes place if the pi state is the equilibrium state of the junction in the particular case of parallel magnetizations. We find a novel channel of quasiparticle reflection (Q reflection) from the simplest twosublattice antiferromagnet (AF) on a bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev states arise at the AF/SC interface. When the Q reflection dominates the specular one, Andreev bound states have almost zero energy on AF/ swave SC interfaces, whereas they lie near the edge of the continuous spectrum for AF/dwave SC boundaries. For an swave SC/AF/swave SC junction, the bound states are found to split and carry the supercurrent. Our analytical results are based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets. Similar effects can take place on interfaces of superconductors with charge density wave materials (CDW), including the possible ddensity wave state (DDW) of the cuprates.
Yuri Barash, I. V. Bobkova, Brian M. Andersen, T. Kopp, P. J. Hirschfeld Journal reference: AIP Conference Proceedings, Vol. 850, 889 (2006). [pdf] DOI: 10.1063/1.2354989


Abstract
 We show that the dc Josephson current through superconductorantiferromagnetsuperconductor (S/AF/S) junctions manifests a remarkable atomic scale dependence on the interlayer thickness. At low temperatures the junction is either a 0 or pijunction depending on whether the AF interlayer consists of an even or odd number of atomic layers. This is associated with different symmetries of the AF interlayers in the two cases. In the junction with odd AF interlayers an additional pi0 transition can take place as a function of temperature. This originates from the interplay of spinsplit Andreev bound states. Experimental implications of these theoretical findings are discussed.
Brian M. Andersen, I. V. Bobkova, P. J. Hirschfeld, Yu. S. Barash Journal reference: Phys. Rev. Lett. 96, 117005 (2006) [pdf] DOI: 10.1103/PhysRevLett.96.117005

Fourier transform spectroscopy of

Abstract
 The local density of states power spectrum of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO) has been interpreted in terms of quasiparticle interference peaks corresponding to an "octet'' of scattering wave vectors connecting kpoints where the density of states is maximal. Until now, theoretical treatments have not been able to reproduce the experimentally observed weights and widths of these "octet'' peaks; in particular, the predominance of the dispersing "q$_1$'' peak parallel to the CuO bond directions has remained a mystery. In addition, such theories predict "background'' features which are not observed experimentally. Here, we show that most of the discrepancies can be resolved when a realistic model for the outofplane disorder in BSCCO is used. Weak extended potential scatterers, which are assumed to represent cation disorder, suppress largemomentum features and broaden the lowenergy "q$_7$''peaks, whereas scattering at order parameter variations, possibly caused by a dopantmodulated pair interaction around interstitial oxygens, strongly enhances the dispersing "q$_1$''peaks.
Tamara S. Nunner, Wei Chen, Brian M. Andersen, Ashot Melikyan, P. J. Hirschfeld Journal reference: Phys. Rev. B 73, 104511 (2006) [pdf] DOI: 10.1103/PhysRevB.73.104511

Andreev States near ShortRanged Pairing Potential Impurities 
Abstract
 We study Andreev states near atomic scale modulations in the pairing potential in both $s$ and dwave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.
Brian M. Andersen, Ashot Melikyan, Tamara S. Nunner, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 96, 097004 (2006) [pdf] DOI: 10.1103/PhysRevLett.96.097004

Recursion method for the quasiparticle structure of a single vortex with induced magnetic order 
Abstract
 2005

Bound states at the interface between antiferromagnets and superconductors 
Abstract
 We present a detailed theoretical investigation of interfaces and junctions involving itinerant antiferromagnets. By solving the Bogoliubovde Gennes equations with a tightbinding model on a square lattice, we study both the selfconsistent order parameter fields proximate to interfaces between antiferromagnets (AF) and swave (sSC) or dwave (dSC) superconductors, the dispersion of quasiparticle subgap states at interfaces and interlayers, and the local density of states (LDOS) as a function of distance from the interface. In addition, we present the quasiclassical approach to interfaces and junctions involving itinerant antiferromagnets developed in an earlier paper. Analytical results are in excellent agreement with what we obtain numerically. Strong effects of pair breaking in the presence of lowenergy interface Andreev states are found in particular for AF/sSC interfaces when interface potentials are not too high. Potential barriers induce additional extrema in the dispersive quasiparticle spectra with corresponding peaks in the LDOS. Discrete quasiparticle dispersive levels in AF  normal metal (N)  AF systems are found to strongly depend on the misorientation angle of the magnetizations in the two antiferromagnets.
B. M. Andersen, I. V. Bobkova, P. J. Hirschfeld, Yu. S. Barash Journal reference: Phys. Rev B 72, 184510 (2005) [pdf] DOI: 10.1103/PhysRevB.72.184510

DopantModulated Pair Interaction in Cuprate Superconductors 
Abstract
 Comparison of recent experimental STM data with singleimpurity and manyimpurity Bogoliubovde Gennes calculations strongly suggests that random outofplane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic structure inhomogeneity observed in BSCCO2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.
Tamara S. Nunner, Brian M. Andersen, Ashot Melikyan, P. J. Hirschfeld Journal reference: Phys. Rev. Lett. 95, 177003 (2005) [pdf] DOI: 10.1103/PhysRevLett.95.177003

Spin Dynamics in the Stripe Phase of the Cuprate Superconductors 
Abstract
 Within a model that supports stripe spin and charge order coexisting with a d$_{x^2y^2}$wave superconducting phase, we study the selfconsistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particlehole continuum can strongly damp parts of the lowenergy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the lowenergy collective magnetic modes of lanthanum based cuprate superconductors.
Brian M. Andersen, Per Hedegard Journal reference: Phys. Rev. Lett. 95, 037002 (2005) [pdf] DOI: 10.1103/PhysRevLett.95.037002

Bound states at the interface between antiferromagnets and superconductors 
Abstract
 2003

Quantum interference between multiple impurities in anisotropic superconductors 
Abstract
 We study the quantum interference between impurities in dwave superconductors within a potential scattering formalism that easily applies to multiple impurities. The evolution of the lowenergy local density of states for both magnetic and nonmagnetic shortranged scatterers are studied as a function of the spatial configuration of the impurities. Further we discuss the influence of subdominant bulk superconducting order parameters on the interference pattern from multiple impurities.
Brian M. Andersen, Per Hedegard Journal reference: Physical Review B, 67, 172505 (2003) [pdf] DOI: 10.1103/PhysRevB.67.172505

Two nonmagnetic impurities in the

Abstract
 The quantum interference between two nonmagnetic impurities is studied numerically in both the dwave superconducting (DSC) and the ddensity wave (DDW) state. In all calculations we include the tunnelling through excited states from the CuO$_2$ planes to the BiO layer probed by the STM tip. Compared to the single impurity case, a systematic study of the modulations in the twoimpurity local density of states can distinguish between the DSC or DDW states. This is important if the origin of the pseudogap phase is caused by preformed pairs or DDW order. Furthermore, in the DSC state the study of the LDOS around two nonmagnetic impurities provide further tests for the potential scattering model versus more strongly correlated models.
Brian M. Andersen Journal reference: Physical Review B 68, 094518 (2003) [pdf] DOI: 10.1103/PhysRevB.68.094518

Checkerboard local density of states in striped domains pinned by vortices 
Abstract
 Within a Green's function formalism we calculate the electronic structure around static extended magnetic and nonmagnetic perturbations in a dwave superconductor. In partucular, we discuss recent elastic neutron scattering and scanning tunneling experiments on HighT_c cuprates exposed to an applied magnetic field. A physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. The low energy electronic structure in such a region reveals a checkerboard interference pattern consistent with recent scanning tunneling experiments.
Brian M Andersen, Per Hedegard, Henrik Bruus Journal reference: Phys. Rev. B 67, 134528 (2003) [pdf] DOI: 10.1103/PhysRevB.67.134528

Electronic checkerboard pattern in striped racetrack domains: a
consistent picture of recent neutron and STM experiments 
Abstract
 We discuss recent elastic neutron scattering and scanning tunneling experiments on highT_c cuprates exposed to an applied magnetic field. In particular we show that a physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. Further, we calculate the electronic structure in such a region using a Tmatrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.
Brian M. Andersen, Per Hedegard [pdf]

Quantum interference between multiple impurities in anisotropic superconductors 
Abstract
 2002

Andreev bound states at the interface of antiferromagnets and

Abstract
 We set up a simple transfer matrix formalism to study the existence of bound states at interfaces and in junctions between antiferromagnets and dwave superconductors. The wellstudied zero energy mode at the {110} interface between an insulator and a dwave superconductor is spin split when the insulator is an antiferromagnet. This has as a consequence that any competing interface induced superconducting order parameter that breaks the time reversal symmetry needs to exceed a critical value before a charge current is induced along the interface.
Brian M. Andersen, P. Hedegaard Journal reference: Phys. Rev. B 66, 104516 (2002) [pdf] DOI: 10.1103/PhysRevB.66.104515

Andreev bound states at the interface of antiferromagnets and

Abstract
 2000

SO(5) theory of insulating vortex cores in high

Abstract
 We study the fermionic states of the antiferromagnetically ordered vortex cores predicted to exist in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Our model calculation gives a natural explanation of the recent STM measurements on BSCCO, which in surprising contrast to YBCO revealed completely insulating vortex cores.
Brian Møller Andersen, Henrik Bruus, Per Hedegård Journal reference: Phys. Rev. B 61, 6298 (2000) [pdf] DOI: 10.1103/PhysRevB.61.6298

SO(5) theory of insulating vortex cores in high

Abstract