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 non-repeaters, 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 non-repeaters 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 non-repeaters 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 power-law 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 non-evolving population in Euclidean space. We find $\alpha$ is steeper for high-DM events and shallower for low-DM 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]
      The CHIME/FRB Collaboration, :, Mandana Amiri, Bridget C. Andersen, Kevin Bandura, Sabrina Berger, Mohit Bhardwaj, Michelle M. Boyce, P. J. Boyle, Charanjot Brar, Daniela Breitman, Tomas Cassanelli, Pragya Chawla, Tianyue Chen, J. -F. Cliche, Amanda Cook, Davor Cubranic, Alice P. Curtin, Meiling Deng, Matt Dobbs, Fengqiu, Dong, Gwendolyn Eadie, Mateus Fandino, Emmanuel Fonseca, B. M. Gaensler, Utkarsh Giri, Deborah C. Good, Mark Halpern, Alex S. Hill, Gary Hinshaw, Alexander Josephy, Jane F. Kaczmarek, Zarif Kader, Joseph W. Kania, Victoria M. Kaspi, T. L. Landecker, Dustin Lang, Calvin Leung, Dongzi Li, Hsiu-Hsien Lin, Kiyoshi W. Masui, Ryan Mckinven, Juan Mena-Parra, Marcus Merryfield, Bradley W. Meyers, Daniele Michilli, Nikola Milutinovic, Arash Mirhosseini, Moritz Münchmeyer, Arun Naidu, Laura Newburgh, Cherry Ng, Chitrang Patel, Ue-Li Pen, Emily Petroff, Tristan Pinsonneault-Marotte, Ziggy Pleunis, Masoud Rafiei-Ravandi, Mubdi Rahman, Scott M. Ransom, Andre Renard, Pranav Sanghavi, Paul Scholz, J. Richard Shaw, Kaitlyn Shin, Seth R. Siegel, Andrew E. Sikora, Saurabh Singh, Kendrick M. Smith, Ingrid Stairs, Chia Min Tan, S. P. Tendulkar, Keith Vanderlinde, Haochen Wang, Dallas Wulf, A. V. Zwaniga

    • Sub-second periodicity in a fast radio burst - Abstract
      • The origin of fast radio bursts (FRBs), millisecond-duration flashes of radio waves that are visible at distances of billions of light-years, remains an open astrophysical question. Here we report the detection of the multi-component 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 multi-component 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 neutron-star origin of these events. Moreover, our detections favour emission arising from the neutron-star magnetosphere, as opposed to emission regions located further away from the star, as predicted by some models. Possible explanations for the observed periodicity include super-giant pulses from a neutron star that are possibly related to a magnetar outburst and interacting neutron stars in a binary system.
    • 2107.08463v1 [pdf]
      The CHIME/FRB Collaboration, Bridget C. Andersen, Kevin Bandura, Mohit Bhardwaj, P. J. Boyle, Charanjot Brar, Daniela Breitman, Tomas Cassanelli, Shami Chatterjee, Pragya Chawla, Jean-François Cliche, Davor Cubranic, Alice P. Curtin, Meiling Deng, Matt Dobbs, Fengqiu Adam Dong, Emmanuel Fonseca, B. M. Gaensler, Utkarsh Giri, Deborah C. Good, Alex S. Hill, Alexander Josephy, J. F. Kaczmarek, Zarif Kader, Joseph Kania, Victoria M. Kaspi, Calvin Leung, D. Z. Li, Hsiu-Hsien Lin, Kiyoshi W. Masui, Ryan Mckinven, Juan Mena-Parra, Marcus Merryfield, B. W. Meyers, D. Michilli, Arun Naidu, Laura Newburgh, C. Ng, Anna Ordog, Chitrang Patel, Aaron B. Pearlman, Ue-Li Pen, Emily Petroff, Ziggy Pleunis, Masoud Rafiei-Ravandi, Mubdi Rahman, Scott Ransom, Andre Renard, Pranav Sanghavi, Paul Scholz, J. Richard Shaw, Kaitlyn Shin, Seth R. Siegel, Saurabh Singh, Kendrick Smith, Ingrid Stairs, Chia Min Tan, Shriharsh P. Tendulkar, Keith Vanderlinde, D. V. Wiebe, Dallas Wulf, Andrew Zwaniga

    • Revealing the competition between charge-density 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 charge-density 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 $T_{\rm CDW}(\varepsilon)=94.5-4.72\varepsilon-0.60\varepsilon^2$. 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 first-principles 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 strain-induced 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

    • Theory of the charge-density 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 charge-density wave (CDW) phase is presently unsettled, which complicates the interpretation of the superconducting ground state. In this paper, we use group-theory and density-functional 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 in-plane momentum but different out-of-plane momenta, whose frequencies depend strongly on the electronic temperature. This is indicative of an electronically-driven CDW, stabilized by features of the in-plane electronic dispersion. Motivated by the DFT analysis, we construct a Landau free-energy expansion for coupled CDW order parameters with wave-vectors 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 nearly-degenerate. 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 unit-cell 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 tri-hexagonal and staggered Star-of-David - 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

    • 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 January-November. 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 X-ray counterparts in Swift/BAT and Fermi/GBM data, and for two of the FRB 20200120E bursts, we rule out coincident SGR 1806$-$20-like X-ray bursts. Due to the proximity of FRB 20200120E, future follow-up for prompt multi-wavelength counterparts and sub-arcsecond 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. Rafiei-Ravandi, P. Scholz, K. Shin, K. M. Smith, I. H. Stairs, A. V. Zwaniga
      DOI: 10.3847/2041-8213/abeaa6
      2103.01295v2 [pdf]

    • LOFAR Detection of 110–188 MHz Emission and Frequency-dependent Activity from FRB 20180916B - Abstract
      • FRB 20180916B is a well-studied 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 16.3-day periodicity in its activity. Here we report on the detection of 18 bursts using LOFAR at 110-188 MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are seen down to the lowest-observed frequency of 110 MHz, suggesting that their spectra extend even lower. These observations provide an order-of-magnitude stronger constraint on the optical depth due to free-free 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 300-1700 MHz. Compared with higher frequencies, the larger burst widths (~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due to scattering. We find ~2-3 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 (200-450 MHz) and CHIME/FRB (400-800 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 high-mass 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
      DOI: 10.3847/2041-8213/abec72
      2012.08372v3 [pdf]

    • Correlation-Induced 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 two-dimensional 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, sign-problem-free quantum Monte Carlo simulations to solve an effective interacting lattice model for TBG at charge neutrality. Besides the usual cluster Hubbard-like repulsion, this model also contains an assisted hopping interaction that emerges due to the non-trivial topological properties of TBG. Such a non-local 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 intervalley-coherent insulator, and a valence bond solid. The charge-neutrality correlated insulating phases discovered here provide the sought-after 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, Han-Qing 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 longer-range Coulomb interactions. We perform a material-specific microscopic theoretical study of pairing by spin- and charge-fluctuations in Sr$_2$RuO$_4$, including the effects of spin-orbit coupling, and both local and longer-range Coulomb repulsion. The latter has important consequences for Sr$_2$RuO$_4$ due to the near-degeneracy of symmetry-distinct pairing states in this material. We find that both the $g$- and $d_{x^2-y^2}$-wave channels remain noncompetitive compared to leading nodal $s'$, $d_{xy}$, and helical ($p$) solutions. This suggests nodal time-reversal symmetry broken $s'+id_{xy}$ or $s'+ip$ phases, promoted by longer-range 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

    • Orbital-dependent self-energy 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 multi-band superconductors. In particular, by comparing standard RPA-based spin-fluctuation mediated gap structures to those obtained within the FLEX formalism for an iron-based superconductor, we obtain directly the feedback effects from electron-electron interactions on the momentum-space gap structure. We show how self-energy effects can lead to an orbital inversion of the orbital-resolved 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 self-energy feedback on the pairing gap to be generally relevant for superconductivity in strongly correlated multi-orbital 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 iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1-Fe 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 nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of inter-orbital $d_{xz}-d_{xy}$ and $d_{yz}-d_{xy}$ character, with spontaneously broken amplitudes for these two components. This inter-orbital nematic order naturally leads to distinct hybridization gaps at the $X$ and $Y$ points of the 1-Fe Brillouin zone, and may thereby produce highly anisotropic Fermi surfaces with only a single electron pocket at one of these momentum-space locations. The associated superconducting gap structure obtained with the generated low-energy electronic band structure from spin-fluctuation 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 self-energy effects, and explore the role of orbital-dependent 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 Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45 - Abstract
      • By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based 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 tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (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 tip-gating scenario within the single-impurity 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/s41467-020-20529-x

  • 2019
    • Quantum Phase Transition of Correlated Iron-Based 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 high-temperature 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$_{1-x}$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 strong-coupling 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 Born-limit scattering effect in a s$\pm$-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.
    • Jia-Xin Yin, Songtian S. Zhang, Guangyang Dai, Yuanyuan Zhao, Andreas Kreisel, Gennevieve Macam, Xianxin Wu, Hu Miao, Zhi-Quan 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, Feng-Chuan 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 high-rate 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 burst-averaged 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 multi-wavelength follow-up 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]
      The CHIME/FRB Collaboration, :, B. C. Andersen, K. Bandura, M. Bhardwaj, P. Boubel, M. M. Boyce, P. J. Boyle, C. Brar, T. Cassanelli, P. Chawla, D. Cubranic, M. Deng, M. Dobbs, M. Fandino, E. Fonseca, B. M. Gaensler, A. J. Gilbert, U. Giri, D. C. Good, M. Halpern, A. S. Hill, G. Hinshaw, C. Höfer, A. Josephy, V. M. Kaspi, R. Kothes, T. L. Landecker, D. A. Lang, D. Z. Li, H. -H. Lin, K. W. Masui, J. Mena-Parra, M. Merryfield, R. Mckinven, D. Michilli, N. Milutinovic, A. Naidu, L. B. Newburgh, C. Ng, C. Patel, U. Pen, T. Pinsonneault-Marotte, Z. Pleunis, M. Rafiei-Ravandi, M. Rahman, S. M. Ransom, A. Renard, P. Scholz, S. R. Siegel, S. Singh, K. M. Smith, I. H. Stairs, S. P. Tendulkar, I. Tretyakov, K. Vanderlinde, P. Yadav, A. V. Zwaniga

    • Effects of spin-orbit coupling on spin-fluctuation induced pairing in iron-based superconductors - Abstract
      • We perform a theoretical study of the leading pairing instabilities and the associated superconducting gap functions within the spin-fluctuation mediated pairing scenario in the presence of spin-orbit coupling (SOC). Focussing on iron-based superconductors (FeSCs), our model Hamiltonian consists of a realistic density functional theory (DFT)-derived ten-band hopping term, spin-orbit coupling, and electron-electron interactions included via the multi-orbital Hubbard-Hund 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 DFT-obtained bands for iron-based 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 SOC-characteristic gap oscillations along the various Fermi surfaces. By contrast in the strongly hole-doped case featuring only hole-pockets around the $\Gamma$-point, the leading solution is $d$-wave pseudo-spin singlet, but with a notable SOC-driven tendency towards helical pseudo-spin 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 pseudo-spin 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

    • CHIME/FRB Detection of the Original Repeating Fast Radio Burst Source FRB 121102 - Abstract
      • We report the detection of a single burst from the first-discovered repeating Fast Radio Burst source, FRB 121102, with CHIME/FRB, which operates in the frequency band 400-800 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$3 Jy ms and shows complex time and frequency morphology. The 34 ms width of the burst is the largest seen for this object at any frequency. We find evidence of sub-burst structure that drifts downward in frequency at a rate of -3.9$\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 0.1-10 per day in the 400-800 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. Mena-Parra, M. Merryfield, D. Michilli, N. Milutinovic, A. Naidu, U. Pen, M. Rafiei-Ravandi, 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
      DOI: 10.3847/2041-8213/ab2c00
      1906.11305v1 [pdf]

    • Competing Electronic Phases near the Onset of Superconductivity in Hole-doped SrFe - Abstract
      • An intriguingly complex phase diagram of Na-doped SrFe2As2 is uncovered using high-resolution thermal-expansion, magnetization and heat-capacity 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 double-Q phase consisting of a mixture of symmetry-distinct commensurate magnetic orders with a peculiar temperature-dependent 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 spin-orbit coupling on the neutron spin resonance in iron-based superconductors - Abstract
      • The so-called 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 iron-based superconductors, the neutron resonance has been extensively studied experimentally, and a peculiar spin-space anisotropy has been identified by polarized inelastic neutron scattering experiments. Here we perform a theoretical study of the energy- and spin-resolved magnetic susceptibility in the superconducting state with $ s_{+-} $-wave order parameter, relevant to iron-pnictide and iron-chalcogenide superconductors. Our model is based on a realistic bandstructure including spin-orbit coupling with electronic Hubbard-Hund interactions included at the RPA level. Spin-orbit coupling is taken into account both in the generation of spin-fluctuation mediated pairing, as well as the numerical computation of the spin susceptibility in the superconducting state. We find that spin-orbit 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

    • Disorder-induced electronic nematicity - Abstract
      • We expose the theoretical mechanisms underlying disorder-induced nematicity in systems exhibiting strong fluctuations or ordering in the nematic channel. Our analysis consists of a symmetry-based Ginzburg-Landau approach and associated microscopic calculations. We show that a single featureless point-like impurity induces nematicity locally, already above the critical nematic transition temperature. The persistence of fourfold rotational symmetry constrains the resulting disorder-induced 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 disorder-induced nematic order in iron-based 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 four-fold 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 = 6-11 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies 3-5 meV, indicating a gapped four-fold (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 5-6 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/s41563-019-0369-5

    • 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 spin-fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band-structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the $\gamma$ band to the van Hove points. A surprising near-degeneracy of the nodal $s^\prime$- and $d_{x^2-y^2}$-wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken $s^\prime+id_{x^2-y^2}$ 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 low-mass 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 large-scale 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 Segura-Cox, Philip C. Myers, Tyler L. Bourke, John J. Tobin, Łukasz Tychoniec
      DOI: 10.3847/1538-4357/ab05c7
      1902.05956v1 [pdf]

    • Theoretical study of impurity-induced 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 short-range magnetism, and explain the driving mechanism for the local order by resonant eg-orbital states. In addition, we investigate the importance of orbital-selective self-energy 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 iron-based 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

  • 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 interaction-driven 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 low-energy momentum and energy dependent response measured by inelastic neutron scattering experiments. We find a correlation-induced 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 Lifshitz-point instability to finite-momentum superconductivity - Abstract
      • We derive the effective Ginzburg-Landau theory for finite momentum (FFLO/PDW) superconductivity without spin population imbalance from a model with local attraction and repulsive pair-hopping. 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 pair-hopping the phase diagram contains a line of Lifshitz points where vanishing superfluid stiffness induces a continuous change to a long wavelength Fulde-Ferrell (FF) state. For larger pair-hopping there is a bicritical region where the pair-momentum changes discontinuously. Here the FF type state is near degenerate with the Larkin-Ovchinnikov (LO) or Pair-Density-wave (PDW) type state. At the intersection of these two regimes there is a "Super-Lifshitz" point with extra soft fluctuations. The instability to finite momentum superconductivity occurs for arbitrarily weak pair-hopping for sufficiently large attraction suggesting that even a small repulsive pair-hopping 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 (sign-preserving gaps) and unconventional (sign-changing gaps) superconductors (SC). In the first scenario, relevant to multi-band 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 off-Fermi-level bands, lead to significant enhancements of Tc in the condensate formed by the near-Fermi-level bands. The second scenario focuses on the dense impurity limit where random disorder-generated 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 Iron-Based 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 paramagnetic-magnetic 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 five-orbital models, suitable for BaFe$_2$As$_2$ and LaFeAsO, we find that the incommensurate magnetic phases discussed here are accessible in iron-based superconductors. Our investigation unveils a set of potential candidates for the unidentified $C_2$-symmetric magnetic phase that was recently observed in Ba$_{1-x}$Na$_x$Fe$_{2}$As$_{2}$. Among the phases stabilized we find a spin-whirl crystal, which is a textured magnetic $C_4$-symmetric phase. The possible experimental observation of textured magnetic orders in iron-based 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 self-consistent real space treatment of pairing in the disordered one-band 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 pair-breaking 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 disorder-dependent Tc-suppression rate expected from Abrikosov-Gor'kov theory, or even in disorder-generated 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 spin-orbit coupling: Frustration between - Abstract
      • We investigate the impact of spin anisotropic interactions, promoted by spin-orbit coupling, on the magnetic phase diagram of the iron-based 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 mean-field and renormalization-group 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 renormalization-group flow, which is absent in the spin isotropic case, and is associated with a near-degeneracy 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 Spin-Orbit 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 wave-vectors, they break distinct symmetries of the lattice. Importantly, the highest superconducting transition temperature takes place close to this proliferation of near-degenerate magnetic states. In this paper, we employ a renormalization group calculation to show that such a behavior naturally arises due to the effects of spin-orbit 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 orbital-selective quasiparticles in the Hund’s metal state of FeSe - Abstract
      • Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high temperature superconductivity (HTS). By contrast, the parent phase of iron-based HTS is never a correlated insulator. But this distinction may be deceptive because Fe has five active d-orbitals 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 inter-orbital 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" orbital-selective="orbital-selective" cooper="Cooper" pairing="pairing" studies.="studies." thus,="Thus," orbital-selective="orbital-selective" strong="strong" correlations="correlations" dominate="dominate" the="the" parent="parent" state="state" of="of" iron-based="iron-based" 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/s41563-018-0151-0

    • 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 jerk-search-enabled detection of Ter5am (PSR~J1748$-$2446am), a new highly-accelerated pulsar in a compact, eccentric, and relativistic orbit, with a likely pulsar mass of 1.649$^{+0.037}_{-0.11}$\,\msun.
    • Bridget C. Andersen, Scott M. Ransom
      DOI: 10.3847/2041-8213/aad59f
      1807.07900v1 [pdf]

    • Spin Waves in Detwinned - Abstract
      • Understanding magnetic interactions in the parent compounds of high-temperature 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 magnon-phonon interaction in multiferroic - Abstract
      • Inelastic neutron scattering has been used to study the magneto-elastic 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 magnon-phonon character. An external magnetic field along the $c$-axis is observed to cause a linear field-induced 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 magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced 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

    • Spin-Orbit Coupling and Magnetic Anisotropy in Iron-Based Superconductors - Abstract
      • We determine theoretically the effect of spin-orbit coupling on the magnetic excitation spectrum of itinerant multi-orbital systems, with specific application to iron-based superconductors. Our microscopic model includes a realistic ten-band kinetic Hamiltonian, atomic spin-orbit coupling, and multi-orbital Hubbard interactions. Our results highlight the remarkable variability of the resulting magnetic anisotropy despite constant spin-orbit coupling. At the same time, the magnetic anisotropy exhibits robust universal behavior upon changes in the bandstructure corresponding to different materials of iron-based 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 spin-orbit 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

  • 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 iron-pnictides. Recent experimental findings in the tetragonal magnetic phase attest to the existence of the so-called charge and spin ordered density wave over the spin-vortex 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 $d_{x^2-y^2}$ 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 materials-specific 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$_{2-x}$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 iron-based 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, iron-based 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 low-temperature scanning tunnelling microscopy and spectroscopy we can characterize the spin resonance in real space. We show that inelastic tunnelling leads to the characteristic dip-hump 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 non-local 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 orbital-selective Cooper pairing in FeSe - Abstract
      • The superconductor FeSe is of intense interest thanks to its unusual non-magnetic 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 orbital-selective 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 iron-based superconductors - Abstract
      • We discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based 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 spin-fluctuation 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 iron-based 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 F-doped 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 F-doped LaFe$_{1-x}$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 five-band model show that correlation-enhanced 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 F-doped 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 sign-changing gaps in multiband superconductors - Abstract
      • Recently, a test for a sign-changing 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 sign-changing 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 pressure-induced magnetic state in the iron-chalcogenide 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 nearest-neighbor 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 self-consistent way and determine the generic topology of the temperature-pressure 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 multi-Q 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 multi-Q magnetic ground states, hosting unusual states such as chiral density-waves and quantum Hall insulators. Distinguishing single-Q and multi-Q 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 single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors -- one single-Q and two double-Q phases. Our results open a promising avenue to investigate complex magnetic configurations in itinerant systems via standard scanning tunneling spectroscopy, without requiring spin-resolved capability.
    • Maria N. Gastiasoro, Ilya Eremin, Rafael M. Fernandes, Brian M. Andersen
      Journal reference: Nature Communications 8, 14317 (2017) [pdf]
      DOI: 10.1038/ncomms14317

  • 2016
    • Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs - Abstract
      • Since the discovery of iron-based 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, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles 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 Fe-based 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 Li-registered 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 transition-metal 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 Fe-based 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 optimally-doped SmFeAsO0.88-F0.12 iron-based 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 muon-spin 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 correlation-enhanced RKKY-couplings 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 two-dimensional 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 `self-organized' 1D topologically non-trivial superconducting phase within the set of subgap Yu-Shiba-Rusinov (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 short-range ferromagnetic and long-range antiferromagnetic electron-induced 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 spin-orbit 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 One-Band Hubbard Model - Abstract
      • We study spin-fluctuation-mediated superconductivity in the one-band 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 next-nearest 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/s10948-016-3758-x

    • Collective magnetic excitations of - Abstract
      • We study the collective magnetic excitations of the recently discovered $C_{4}$ symmetric spin-density wave states of iron-based superconductors with particular emphasis on their orbital character based on an itinerant multiorbital approach. This is important since the $C_{4}$ symmetric spin-density wave states exist only at moderate interaction strengths where damping effects from a coupling to the continuum of particle-hole 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 iron-pnictide 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 iron-site defects on superconductivity in LiFeAs - Abstract
      • In conventional s-wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s+- order parameter they can occur for both magnetic and non-magnetic 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 iron-site 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 multi-orbital 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 Abrikosov-Gor'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 high-Tc superconductivity at the sub-1% level, and when non-magnetic disorder, counter-intuitively, hardly affects the unconventional superconducting state while concomitantly inducing an inhomogeneous full-volume magnetic phase. Recent experimental studies of Fe-based 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 weak-coupling limit for coexisting spin-density-wave order and spin-fluctuation-mediated superconductivity. Both longitudinal and transverse spin fluctuations contribute significantly to the effective interaction potential, which creates Cooper pairs of the quasi-particles of the antiferromagnetic metallic state. We find a dominant $d_{x^2-y^2}$-wave solution in both electron- and hole-doped cases. In the quasi-spin 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 $d_{x^2-y^2}$. The sub-leading 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 spin-density-wave band, is considered for a range of electron dopings in the regime of well-developed magnetic order. We demonstrate that these interband pairing gaps, which are non-zero in the magnetic state, must have the same parity under inversion as the normal intraband gaps. The self-consistent solution to the full system of five coupled gap equations give intraband and interband pairing gaps of $d_{x^2-y^2}$ structure and similar gap magnitude. In conclusion, the $d_{x^2-y^2}$ gap dominates for both hole and electron doping inside the spin-density-wave 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 Pair-Density-Wave 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 spin-wave spectrum including the neutron resonance. In contrast to the standard case of d-wave superconductivity, we show that the pair density wave phase exhibits neither a spin-gap 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

    • Self-organized topological superconductivity in a Yu-Shiba-Rusinov 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 spin-spin interaction mediated through the subgap band of Yu-Shiba-Rusinov states. Moreover, the simple spin-spin 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 electron-driven 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 Fe-based 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 iron-based superconductors by use of a realistic five-band model relevant to these systems, and superconductivity stabilized by spin-fluctuation 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 low-bias 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

    • Spin-driven nematic instability of the multiorbital Hubbard model: Application to iron-based superconductors - Abstract
      • Nematic order resulting from the partial melting of density-waves has been proposed as the mechanism to explain nematicity in iron-based superconductors. An outstanding question, however, is whether the microscopic electronic model for these systems -- the multi-orbital Hubbard model -- displays such an ordered state as its leading instability. In contrast to usual electronic instabilities, such as magnetic and charge order, this fluctuation-driven phenomenon cannot be captured by the standard RPA method. Here, by including fluctuations beyond RPA in the multi-orbital Hubbard model, we derive its nematic susceptibility and contrast it with its ferro-orbital order susceptibility, showing that its leading instability is the spin-driven nematic phase. Our results also demonstrate the primary role played by the $d_{xy}$ orbital in driving the nematic transition, and reveal that high-energy 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

  • 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 Fe-based superconductor FeSe, based on a tight-binding model developed to account for the temperature-dependent 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 1-Fe 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 spin-orbit coupling and Hund's rule coupling in iron pnictides - Abstract
      • In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform 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 spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped 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

    • Impurity-induced antiferromagnetic order in Pauli-limited nodal superconductors: Application to heavy-fermion - Abstract
      • We investigate the properties of the coexistence phase of itinerant antiferromagnetism and nodal $d$-wave superconductivity (Q-phase) 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 Q-phase at lower magnetic fields. Our most crucial finding is that, even at zero applied field, dilute magnetic impurities cooperate via RKKY-like exchange interactions to generate a long-range ordered coexistence state identical to the Q-phase. 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 long-range magnetic correlations in optical lattices - Abstract
      • In Pedersen et al. (2011) we proposed a method to utilize a temporally dependent superlattice potential to mediate spin-selective 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 (error-free) 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 (single-Q) order forms a dome, but competing non-collinear and non-uniform double-Q 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 double-Q order. Superconductivity is shown to compete more strongly with double-Q magnetic phases, which can lead to re-entrance of the C2 (single-Q) order in agreement with recent thermal expansion measurements on K-doped Ba-122 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 one-band Hubbard model in the paramagnetic weak-coupling limit: A numerical RPA study - Abstract
      • We study the spin-fluctuation-mediated superconducting pairing gap in a weak-coupling 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 p-wave at very low electron fillings to d_{x^2-y^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 sub-dominant triplet solution over the singlet d-wave 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 quasi-particle spectrum, and show how spin-fluctuation-mediated pairing leads to significant deviations from the first harmonics both in the singlet d_{x^2-y^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 Bogoliubov-de 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 out-of-plane 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 low-energy properties consistent with recent photoemission and quantum oscillations measurements on FeSe, assuming mean-field like s and/or d-wave 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 spin-relaxation 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 Fe-Based Superconductors - Abstract
      • The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe$_{1-x}$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 two-band 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

    • Field-induced interplanar magnetic correlations in the high-temperature superconductor - Abstract
      • We present neutron scattering studies of the inter-planar correlations in the high-temperature 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 out-of-plane 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 quench-cooling 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 sub-dominant 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. Toft-Petersen, 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 spin-density wave phase of iron pnictides - Abstract
      • We present a general study of the magnetic excitations within a weak-coupling five-orbital model relevant to itinerant iron pnictides. As a function of enhanced electronic correlations, the spin excitations in the symmetry broken spin-density wave phase evolve from broad low-energy 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 long-range 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 spin-density 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 La-based cuprates - Abstract
      • Doping a parent antiferromagnetic Mott insulator in cuprates leads to short-range electronic correlations and eventually to high-Tc 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 high-Tc 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 high-Tc superconductors is subject of current research. Here we demonstrate that ferroelectricity and the associated magnetoelectric coupling are in fact common in La-214 cuprates namely, La$_{2-x}$Sr$_x$CuO$_4$, La$_2$Li$_x$Cu$_{1-x}$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 Dzyaloshinskii-Moriya 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 field-induced magnetism in electron-doped - 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 five-band 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 long-range 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 Non-magnetic Impurities in Fe-based Superconductors - Abstract
      • We study impurity-induced magnetic order within a five-band Hubbard model relevant to the normal paramagnetic phase of iron-based superconductors. The existence of the local magnetic order is explained in terms of an impurity-enhancement 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 impurity-induced 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/s10948-014-2908-2

    • 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 $d_{x^2-y^2}$-wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodeless odd-parity 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: 10.1088/1367-2630/17/2/023022

  • 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 Fe-based 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 Iron-Pnictide Superconductors from the Interaction between Conduction Electrons and Magnetic Impurities - Abstract
      • Recent experimental studies have revealed several unexpected properties of Mn-doped BaFe2As2. These include extension of the stripe-like 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 real-space five-band 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 Ruderman-Kittel-Kasuya-Yosida (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 spin-density wave phase of Fe-based superconductors - Abstract
      • We investigate the emergent impurity-induced states arising from point-like scatterers in the spin-density wave phase of iron-based superconductors within a microscopic five-band model. Independent of the details of the band-structure 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 a-axis, 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 dopant-induced 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

  • 2013
    • Impurity Bound States and Disorder-Induced Orbital and Magnetic Order in the s ± State of Fe-Based Superconductors - Abstract
      • We study the presence of impurity bound states within a five-band Hubbard model relevant to iron-based superconductors. In agreement with earlier studies, we find that in the absence of Coulomb correlations there exists a range of repulsive impurity potentials where in-gap 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 low-energy bound states into the gap.
    • Maria N. Gastiasoro, Brian M. Andersen
      Journal reference: J. Supercond. Novel Magn. 26, 2651 (2013) [pdf]
      DOI: 10.1007/s10948-013-2153-0

    • Impurity states and cooperative magnetic order in Fe-based superconductors - Abstract
      • We study impurity bound states and impurity-induced order in the superconducting state of LiFeAs within a realistic five-band 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 sub-gap bound states, whereas stronger impurities like Cu do. We also obtain the bound state spectrum for magnetic impurities, such as Mn, and show how spin-resolved 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

    • Impurity-induced subgap bound states in alkali-doped 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_{2-y}Se_2. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest neighbor d-wave and next-nearest neighbor s-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that sub-gap bound states exist only for d-wave 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 impurity-induced magnetism in the case of d-wave superconductivity. For magnetic impurities, both s-wave and d-wave superconducting states support sub-gap bound states. The above results can be explained by a simple analytic model that provides a semi-quantitative 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

    • Low-energy bound states at interfaces between superconducting and block antiferromagnetic regions in K - Abstract
      • The high-Tc alkali doped iron selenide superconductors KxFe{2-y}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 low-energy states near the interfaces between block antiferromagnets and superconductors. It is found that abundant low-energy in-gap bound states exist near such interfaces irrespective of whether the superconductor has d- or s-wave 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{2-y}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 low-energy spin fluctuations and anisotropy gap in La - Abstract
      • We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature dependence of the low-energy 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 in-elastic 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 low-energy 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 d-Wave Superconductors - Abstract
      • The spin-fluctuation based pairing mechanism has proven successful in explaining the pairing symmetries due to Fermi surface nesting of both cuprates and iron-based materials. In this work, we study signatures of a spin-fluctuation 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 Coulomb-driven 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/s10948-012-2058-3

    • Bi-Quadratic 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 Ginzburg-Landau free energy approach that the magnetoelectric effect can be explained by the presence of bi-quadratic 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 Dzyaloshinskii-Moriya interaction in generating this magnetoelectric response. This work is particularly relevant for such relaxor systems where the material-dependent 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/s10948-012-2059-2

  • 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 d-wave superconductor in the very same compounds. Specifically, we show how short-range fluctuations of the competing order essentially restore the nodal d-wave 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 quasi-particles and, depending of the strength of the competing order, also in small induced nodal gaps as found in recent experiments on underdoped La{2-x}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 spin-fluctuation-mediated pairing interaction by impurities in - Abstract
      • We present a self-consistent real space formulation of spin-fluctuation mediated d-wave 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

    • Low-temperature 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 spin-charge coupling in this lightly doped "parent" high temperature copper-oxide superconductor. The ferroelectric state is proposed to develop from polar nanoregions, in which spatial inversion symmetry is locally broken due to non-stoichiometric 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 intra-unit 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 non-magnetic impurity in a coexistence phase with superconductivity. We find a distinct C4 symmetry breaking near the disorder which is also detectable in the resulting quasi-particle 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 spin-dependent tunneling to probe magnetic correlations in optical lattices - Abstract
      • We suggest a simple experimental method for probing antiferromagnetic spin correlations of two-component 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 bi-quadratic magnetoelectric coupling can explain the experimentally observed polarization dependence on magnetic field. This coupling leads to several novel low-temperature 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 heavy-fermion 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 d-wave 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 disorder-induced stripe phase, impurities become spin nematogens with a C_2 symmetric impurity resonance state, and the disorder-averaged 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 High-T c Superconductors - Abstract
      • We study the spin-density wave (stripe) instability in lattices with mixed low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal symmetry. Within an explicit mean-field 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 real-space 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/s10948-012-1623-0

  • 2011
    • Two routes to magnetic order by disorder in underdoped cuprates - Abstract
      • We study disorder-induced magnetism within the Gutzwiller approximation applied to the t-J 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 quasi-long range ordered state in the underdoped regime. We identify two distinct disorder-induced 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 in-between 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 low-energy behavior largely independent of both disorder and magnetization. However, the magnetic regions are characterized by larger (reduced) superconducting gap (coherence peaks) and a sub-gap 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 IV-characteristics 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 spin-induced Yu-Shiba-Russinov bound states in the strongly asymmetric limit. The interplay between these two mechanisms leads to qualitatively different IV-characteristics in the cross-over regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and re-positioned conductance peaks in sub-gap 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 long-range 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 time-dependent superlattice potential to generate spin-dependent 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 d-wave 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 low-energy spin excitations in high-T materials - Abstract
      • We review a theoretical scenario for the origin of the spin-glass phase of underdoped cuprate materials. In particular it is shown how disorder in a correlated d-wave superconductor generates a magnetic phase by inducing local droplets of antiferromagnetic order which eventually merge and form a quasi-long 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 spin-density 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

  • 2010
    • Nonequilibrium transport via spin-induced subgap states in superconductor/quantum dot/normal metal cotunnel junctions - Abstract
      • We study low-temperature 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 T-matrix, from which we obtain the nonlinear cotunneling conductance. For even occupied dots the system can be conceived as an effective S/N-cotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd occupied dot, however, leads to the formation of sub-gap resonances inside the superconducting gap which gives rise to a characteristic peak-dip 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

    • Disorder-Induced Freezing of Dynamical Spin Fluctuations in Underdoped Cuprate Superconductors - Abstract
      • We study the dynamical spin susceptibility of a correlated d-wave superconductor (dSC) in the presence of disorder, using an unrestricted Hartree-Fock approach. This model provides a concrete realization of the notion that disorder slows down spin fluctuations, which eventually "freeze out". The evolution of disorder-induced 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 disorder-generated 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 phase-decoupled one-dimensional 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 particle-hole 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 inter-stripe spacing. We consider pairing which has either local d-wave or s-wave 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 pair-density 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 high-temperature superconductors - Abstract
      • The interface properties of high-temperature 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 high-Tc 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 tight-binding Hamiltonian for the d-wave 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

    • d-Wave superconductivity as a catalyst for antiferromagnetism in underdoped cuprates - Abstract
      • The underdoped high-Tc 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 d-wave pairing symmetry and a remarkable magnetic response to nonmagnetic perturbations, whereby droplets of spin-density 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 mean-field theory of d-wave 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 correlation-induced 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 d-wave symmetry. Furthermore the formation of anti-phase 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: 10.1088/1367-2630/12/5/053043

  • 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 g-mode 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 g-mode 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 data-analysis 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. Turck-Chièze
      DOI: 10.1007/s00159-009-0027-z
      0910.0848v2 [pdf]

    • Antiferromagnetic noise correlations in optical lattices - Abstract
      • We analyze how noise correlations probed by time-of-flight (TOF) experiments reveal antiferromagnetic (AF) correlations of fermionic atoms in two-dimensional (2D) and three-dimensional (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 spin-resolved 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

    • Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots - Abstract
      • We study low-temperature transport through carbon nanotube quantum dots in the Coulomb blockade regime coupled to niobium-based superconducting leads. We observe pronounced conductance peaks at finite source-drain 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 tunneling-renormalizations of the two subbands in the nanotube. Finally, we discuss the origin of an especially pronounced sub-gap structure observed in every fourth Coulomb diamond.
    • K. Grove-Rasmussen, 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 real-space diagonalization of a Hubbard model treated in an inhomogeneous unrestricted Hartree-Fock 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 disorder-induced 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

    • Magnetic-field-induced soft mode in spin-gapped high- - Abstract
      • We present an explanation of the dynamical in-gap 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 spin-only 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 many-body phenomena of anti-bunching, 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 short-range coexisting order present in the d-wave superconducting phase, and that the most likely origin of this order is disorder-induced incommensurate antiferromagnetism. We show explicitly how the peaks are extinguished in the related situation with coexisting long-range 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

  • 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 hole-density but are crystallographically different outside the CuO2 plane. A key challenge, therefore, has been to identify a predominant out-of-plane 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 inter-atomic distances within individual crystalline unit cells, affect the superconducting energy-gap maximum $\Delta$ of Bi2Sr2CaCu2O8. In this material, quasi-periodic variations of unit cell geometry occur in the form of a bulk crystalline 'supermodulation'. Within each supermodulation period, we find a $\sim 9\pm1%$ co-sinusoidal variation in local $\Delta$ that is anti-correlated 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 non-random out-of-plane 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 large-gap 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 large-gap 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 low-temperature 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 d-wave nodal quasiparticles. Here, we show that the breakdown of universality may be understood as the consequence of disorder-induced magnetic states in the presence of increasing antiferromagnetic correlations in the underdoped state, even as these same correlations protect the nodal low-energy 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 d-wave superconductors (d). Such d/AF/d junctions exhibit a rich dependence of the Josephson current on the interlayer parameters, including the possibility of 0-pi 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 long-standing puzzle of the critical current versus grain boundary angle in high-T_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

  • 2007
    • Magnetic and superfluid phases of confined fermions in two-dimensional optical lattices - Abstract
      • We examine antiferromagnetic and d-wave superfluid phases of cold fermionic atoms with repulsive interactions in a two-dimensional 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 anti-bunching dips and pairing peaks in the density-density 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

    • Disorder-Induced Static Antiferromagnetism in Cuprate Superconductors - Abstract
      • Using model calculations of a disordered d-wave superconductor with on-site 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 low-energy 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 Bi-2212 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 x-ray 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

    • Kondo-Enhanced 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 sub-gap-peaks 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 Kondo-correlations and Andreev tunneling in dots with an odd number of spins, and substantiate this interpretation by a poor man's scaling analysis.
    • T. Sand-Jespersen, J. Paaske, B. M. Andersen, K. Grove-Rasmussen, 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 d-wave 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 impurity-induced 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 semi-quantitative 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 impurity-induced 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 high-T_c materials. The stripes are modeled as coupled spin-1/2 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 high-energy peaks rotated by 45 degrees compared to the incommensurate low-energy 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 d-wave superconductors by impurity-induced local moments - Abstract
      • The low-temperature thermal conductivity \kappa_0/T of d-wave 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, 460-462, 744 (2007) [pdf]
      DOI: 10.1016/j.physc.2007.03.364

    • Signatures of modulated pair interaction in cuprate superconductors - Abstract
      • Recent low-temperature scanning tunnelling spectroscopy experiments on the surface of BSCCO-2212 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, 460-462, 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$_{7-δ}$ - 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 $d_{x^2-y^2}$-wave superconductors in the Meissner regime in the presence of nonmagnetic impurities within the self-consistent T-matrix approximation.
    • Brian M. Andersen, James C. Booth, P. J. Hirschfeld

  • 2006
    • Recursion method for the quasiparticle structure of a single vortex with induced magnetic order - Abstract
      • We use a real-space recursion method to calculate the local density of states (LDOS) within a model that contains both d-wave 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 low-energy 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 field-induced order, and predict field-induced 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 d-wave 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 semi-quantitative 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 Charge-Density-Wave 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 ferromagnet-insulator-ferromagnet 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 two-sublattice 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/ s-wave SC interfaces, whereas they lie near the edge of the continuous spectrum for AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave 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 d-density 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 superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a remarkable atomic scale dependence on the interlayer thickness. At low temperatures the junction is either a 0- or pi-junction 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 pi-0 transition can take place as a function of temperature. This originates from the interplay of spin-split 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 k-points 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 Cu-O 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 out-of-plane disorder in BSCCO is used. Weak extended potential scatterers, which are assumed to represent cation disorder, suppress large-momentum features and broaden the low-energy "q$_7$''-peaks, whereas scattering at order parameter variations, possibly caused by a dopant-modulated 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 Short-Ranged Pairing Potential Impurities - Abstract
      • We study Andreev states near atomic scale modulations in the pairing potential in both $s$- and d-wave 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

  • 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 Bogoliubov-de Gennes equations with a tight-binding model on a square lattice, we study both the self-consistent order parameter fields proximate to interfaces between antiferromagnets (AF) and s-wave (sSC) or d-wave (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 low-energy 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

    • Dopant-Modulated Pair Interaction in Cuprate Superconductors - Abstract
      • Comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic structure inhomogeneity observed in BSCCO-2212, 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$_{x^2-y^2}$-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy 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

  • 2003
    • Quantum interference between multiple impurities in anisotropic superconductors - Abstract
      • We study the quantum interference between impurities in d-wave superconductors within a potential scattering formalism that easily applies to multiple impurities. The evolution of the low-energy local density of states for both magnetic and nonmagnetic short-ranged 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 d-wave superconducting (DSC) and the d-density 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 two-impurity 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 non-magnetic perturbations in a d-wave superconductor. In partucular, we discuss recent elastic neutron scattering and scanning tunneling experiments on High-T_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 high-T_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 T-matrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.
    • Brian M. Andersen, Per Hedegard

  • 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 d-wave superconductors. The well-studied zero energy mode at the {110} interface between an insulator and a d-wave 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

  • 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