Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C48: Superconductivity Theory: Mainly Ruthenates and NickelatesLive
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Sponsoring Units: DCMP Chair: Herbert Fotso, University at Albany |
Monday, March 15, 2021 3:00PM - 3:12PM Live |
C48.00001: FLEX modeling of the strain dependence of the superconducting state of Sr2RuO4 John Deisz We apply the fluctuation exchange approximation (FLEX) to model the evolution of the superconducting state of Sr2RuO4 under strain. Using first-principles-based parameters for band-structure, spin-orbit coupling and electron-electron interactions, FLEX produces a dx2-y2 state on the γ-band in the unstrained limit, with minor p-wave and α- and β-band contributions induced by spin-orbit coupling. As is observed experimentally, FLEX results show a steep drop-off in Tc when strain pushes a γ-band van Hove singularity through and below the Fermi level. However, the model produces a weaker dependence of Tc at low strain levels than is reported experimentally. For temperatures below Tc we have yet to observe a secondary transition that incorporates a component that is nearly-degenerate to the dominant pairing state, though lower-temperature simulations are ongoing. We also report results for the spin susceptibility below Tc. |
Monday, March 15, 2021 3:12PM - 3:24PM Live |
C48.00002: Multi-Orbital B1g Superconducting Order Parameter with Complex Components Induced by Spin-Orbit Coupling in Strontium Ruthenate Olivier Gingras, Michel Cote, A.-M. S. Tremblay Strontium ruthenate is an archetypal unconventional superconductor yet experiments give seemingly different pictures for the symmetry of its superconducting order parameter. Starting from a realistic electronic structure, we solve the frequency dependent Eliashberg equations with a spin-fluctuation pairing vertex. Varying interaction strength and Hund's coupling, we found a phase diagram with three candidate order parameters when spin-orbit coupling is neglected: 1) a spin-singlet d-wave, 2) a momentum-independent inter-orbital spin-triplet and 3) a spin-triplet odd-frequency s-wave [1]. Including spin-orbit coupling, we find that 1) and 2) are both parts of a same multi-orbital order parameter which transforms like the one-component B1g irreducible representation. We show that this order parameter has to be complex and find that it includes odd-frequency contributions that make it appear to break time-reversal symmetry. Comparison with experiments will be presented. |
Monday, March 15, 2021 3:24PM - 3:36PM Live |
C48.00003: Shadowed Triplet Pairings in Hund's Metals with Spin-Orbit Coupling Jonathan Clepkens, Austin Lindquist, Hae-Young Kee Hund’s coupling in multi-orbital systems allows for the possibility of even-parity orbital-antisymmetric spin-triplet pairing which can be stabilized by spin-orbit coupling (SOC). This pairing appears at the Fermi surface (FS) as a pseudospin-singlet, with the momentum-dependence determined by the SOC and the underlying triplet character remaining in the form of interband pairing away from the FS. Here, we examine the role of momentum-dependent SOC (k-SOC) in generating non-trivial pairing symmetries at the FS, and the hidden triplet nature associated with this inter-orbital pairing, which we dub a “shadowed triplet”. Applying this concept to Sr2RuO4, we first derive several forms of k-SOC with d-wave form-factors from a microscopic model and subsequently show that for a reasonable range of SOC parameters, a pairing state with s + idxy symmetry at the FS can be stabilized. This pairing state is distinct from pure spin-singlet and -triplet pairings due to its unique character of pseudospin-singlet pairing on the FS with underlying pseudospin-triplet nature away from the FS. We discuss experimental probes to differentiate the shadowed triplet pairing from conventional pseudospin-triplet and -singlet pairings. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C48.00004: Stabilizing Even-Parity Chiral Superconductivity in Sr2RuO4 Han Gyeol Suh, Henri Menke, Philip Brydon, Carsten Timm, Aline Ramires, Daniel Agterberg The singlet-like response observed in spin-susceptibility measurements has reopened the question of the pairing state of Sr2RuO4. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus c66 at the superconducting transition temperature, the available experiments point towards an even-parity chiral superconductor with kz(kx±iky)-like Eg symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of Sr2RuO4. In this talk, I will discuss how the orbital degree of freedom can encode the two-component nature of the Eg order parameter, allowing for an s-wave orbital-antisymmetric spin-triplet Eg state that can be stabilized by on-site Hund's coupling. Also, I will discuss nodal Bogoliubov Fermi surfaces in this state. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C48.00005: Time-reversal-odd-bilinears: Key to the anomalous Hall effect in chiral superconductors Mathew Denys, Philip Brydon Motivated by the observation of the polar Kerr effect in e.g. UPt3 and Sr2RuO4, we consider the origin of the anomalous Hall conductivity in a clean chiral superconductor. Using a generic model of a two-band system, we demonstrate that the existence of the anomalous Hall conductivity is guaranteed if one can construct a "time-reversal-odd-bilinear" from the pairing potential. In particular, this confirms the importance of interband pairing in generating the anomalous Hall conductivity. We illustrate this result with a toy model of a chiral p-wave superconductor. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C48.00006: Effects of longer-range interaction on superconductivity in Sr2RuO4 Xin Wang, Zhiqiang Wang, Catherine Kallin The symmetry of the superconducting order parameter (OP) in Sr2RuO4 remains a puzzle. None of the currently proposed OPs can account for all important experiments, highlighting the importance of exploring different alternatives. Recently, dx2-y2 + igxy(x2-y2) and dx2-y2 + i(px ± py) have been proposed as OP candidates. The gxy(x2-y2) -wave component has not been found to be stable in on-site multi-orbital Hubbard interaction. In this work, we investigate the effect of longer-range interactions on different Cooper pairing channels in different regimes of the interaction parameter space. Our study combines a weak-coupling renormalization group and random phase approximation calculations. From the OPs found in the dominant pairing channels, we further study experimental implications, such as spontaneous edge currents due to possible spontaneous time-reversal symmetry breaking when two OPs are degenerate or are close to degeneracy. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C48.00007: Superconducting Instability From Spin Fluctuations in Sr2RuO4 Astrid Rømer, Peter Hirschfeld, Brian Møller Andersen During the last couple of years, research on the enigmatic and canonical unconventional superconductor Sr2RuO4 has undergone a renaissance. In particular, new NMR and ultrasound experiments [1,2,3] place considerable constraints on the superconducting order parameter. At the same time, Sr2RuO4 is in many respects simple enough that we should be able to understand why and how electrons pair. Superconductivity condenses around 1K from a well-characterized Fermi liquid, and samples are extremely clean. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C48.00008: A microscopic BdG analysis of time-reversal symmetry breaking at domain walls in a model of Sr2RuO4 Andrew Yuan, Steven Allan Kivelson, Erez Berg It has been shown1 that many seemingly contradictory experimental findings concerning the superconducting state in Sr2RuO4 can be accounted for as resulting from an accidental near degeneracy between d(x2-y2}and g(xy(x2-y2)) superconducting states. We perform a BdG analysis of the effect of spatially varying strain on such a state, and its impact on the transition to a state with spontaneously broken time-reversal symmetry (TRS). In the presence of finite xy-strain, the superconducting state consists of 2 possible symmetry-related TRS preserving states: d+g and d-g. However, at domain walls between two such regions, TRS is broken, resulting in a d+ig state. We find rich microscopic structure associated with such domain walls, including equilibrium orbital currents (and hence induced magnetic fields) that extend to surprisingly long distances from the domain wall. This observation suggests a resolution of the apparent inconsistency between the observation of spontaneous bulk magnetic fields by μSr with the lack of a strong feature in the specific heat marking the transition to a TRS breaking state. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C48.00009: Exchange Interactions and Competing S=0 and S=1 States in Doped NdNiO2 Xiangang Wan, Vsevolod Ivanov, Giacomo R Resta, Ivan Vasilevich Leonov, Sergey Savrasov Using density functional based LDA+U methods and linear–response theory, we study the magnetic exchange interactions of the superconductor Nd1−xSrxNiO2, finding that hole doping significantly enchances the inter-layer exchange coupling compared to the nearest-neighbor interactions. This can be understood as a consequence of the large overlap of the Ni-3d3z2-r2 orbital in the z-direction, in contrast to its small hybridization within the NiO2 plane which results in a flat band near the Fermi level. Additionally, while previous works emphasize the importance of the Ni-3dx2-y2 and Nd-5d orbitals, we demonstrate that the Nd-5d orbitals have no effect on magnetic exchange interactions, instead proposing a Ni-3dx2-y2/Ni-3d3z2-r2 two–band model for this material. This model is solved using DMFT, revealing an underlying Mott insulating state, which upon doping produces competing S=0 and S=1 two–hole states at low energies, depending on the precise values of the intra–atomic Hunds coupling. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C48.00010: Possible incipient-band-induced unconventional superconductivity in nickelates with large crystal field splitting Kazuhiko Kuroki, Naoya Kitamine, Masayuki Ochi We theoretically design nickelate superconductors with d8+δ electron configuration [1]. We exploit the mixed anion strategy to enhance the crystal field splitting, so that only the dx2-y2 band intersects the Fermi level, while all the other four 3d bands are incipient. The idea is inspired by a recent study [2] which shows that a two-orbital model with large level offset exhibits high Tc s±wave superconductivity when one of the bands is incipient. Through orbital basis transformation, the large level offset in the two-orbital model is shown to correspond to a large interlayer hopping in a bilayer system, which is known to give rise to high Tc s±wave superconductivity. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C48.00011: Enhanced Superconductivity in Quasi-periodic Crystals Shi-Zeng Lin, Zhijie Fan, Gia-Wei Chern We study superconductivity in a family of one dimensional incommensurate system with s-wave pairing interaction. The incommensurate potential can alter the spatial characteristics of electrons in the normal state, leading to either extended, critical, or localized wave functions. We find that superconductivity is significantly enhanced when the electronic wave function exhibits a critical multifractal structure. This criticality also manifests itself in the power-law dependence of superconducting temperature on the pairing strength. As a consequence, an extended superconducting domain is expected to exist around the localization-delocalization transition, which can be induced by either tuning the amplitude of the incommensurate potential, or by varying the chemical potential across a mobility edge. Our results thus suggest a novel approach to enhance superconducting transition temperature through engineering of incommensurate potential. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C48.00012: Proposals to detect Bogoliubov Fermi surfaces Hanbit Oh, Eun-Gook Moon An exotic superconducting state with Fermi surfaces of Bogoliubov quasiparticles, Bogoliubov Fermi-surface (BG-FS), has recently been proposed in a centrosymmetric j=3/2 system with a Z2 topological invariant. Several candidate materials of a BG-FS are proposed in Sr2RuO4 and doped FeSe, but its existence has not been confirmed yet. In this work, we provide theoretical proposals to pin down a BG-FS. First, we use the inversion instability of a BG-FS and suggest that second-harmonic-generation (SHG) experiments with a strain gradient to identify enhanced fluctuations of an inversion order parameter. [1] Second, a nontrivial current response of a Bogoliubov Fermi surface superconductivity is used. The importance of dissipation channels from a Fermi surface of Bogoliubov quasi-particles is emphasized. Specifically, we derive the expressions for the optical conductivity and compare them with that of conventional Bardeen-Cooper-Schrieffer superconductivity. Possible applications of our theory to iron-based superconductors and heavy fermion systems including FeSe will also be discussed. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C48.00013: Magnetic interaction in UTe2 Tatsuya Shishidou, Daniel Agterberg, Michael Weinert UTe2 has drawn considerable interest for its unconventional superconducting phase(s). Evidence suggests spin-triplet superconductivity, with magnetic fluctuations responsible for the pairing. A symmetry-based argument of how anisotropic magnetic interactions prefer particular symmetries of the pairing gap function [1] is provided. In this work, based on density-functional theory calculations and model calculations, we discuss possible forms of magnetic interactions in UTe2. |
Monday, March 15, 2021 5:36PM - 5:48PM Not Participating |
C48.00014: Influence of uniaxial pressure and spin-orbit coupling on spin fluctuations in superconducting strontium ruthenate Nikita Allaglo, Olivier Gingras, A.-M. S. Tremblay Strontium ruthenate has one of the best characterized normal-state Fermi-liquid. Since experiments and theory give contradictory results, predicting its superconducting order parameter from first principles has thus become an important challenge. Studies of superconductivity based on a correlated electronic structure and spin-fluctuation theory have predicted three possible order parameters but neglected spin-orbit coupling [1], although it was shown that this coupling might be important [2]. We evaluate ab initio the effect of spin-orbit coupling on the spin fluctuations that act as pairing glue and discuss its impact on superconductivity. Moreover, we investigate the effect of uniaxial pressure on these fluctuations because experiments have revealed intriguing behaviors under such conditions [3]. |
Monday, March 15, 2021 5:48PM - 6:00PM Live |
C48.00015: Theory of (s + id) pairing in mixed-valent correlated metals Emilian Nica, Onur Erten Motivated by the recent discovery of superconductivity in square-planar nickelates as well as by longstanding puzzling experiments in heavy-fermion superconductors, we study Cooper pairing between correlated d-electrons coupled to a band of weakly-correlated electrons. We perform self-consistent large N calculations on an effective t - J model for the d-electrons with additional hybridization. Unlike previous studies of mixed-valent systems, we focus on parameter regimes where both hybridized bands are relevant to determining the pairing |
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