Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A57: Superconductivity: Conventional TheoriesRecordings Available

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Sponsoring Units: DCMP Chair: Herbert Fotso, University at Albany SUNY Room: Hyatt Regency Hotel Clark 
Monday, March 14, 2022 8:00AM  8:12AM 
A57.00001: Intertwined superconductivity, chargedensity wave, and supersolid order in topologically trivial flatbands Debanjan Chowdhury, Erez Berg, Johannes S Hofmann Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a nonperturbative problem, where BCS theory does not apply. What sets the superconducting phase stiffness, and relatedly the transition temperature, in the flatband limit is a question of both fundamental and practical interest. We will demonstrate using analytical and numerically exact quantum MonteCarlo computations that the phase stiffness is set by the strength of attractive interactions even for models with topologically trivial flatbands, as long as certain criteria are met. By including further neighbor interactions, we can drive the system into a panoply of ordered or phaseseparated states. 
Monday, March 14, 2022 8:12AM  8:24AM 
A57.00002: Selfconsistent approach to local pairing in multiorbital superconductors Victor DrouinTouchette, Piers Coleman The wide morphology of Fermi surfaces of the Ironbased superconductors, coupled with a universal relation between gap and Tc, leads us to consider a local pairing mechanism. It was shown within a meanfield theory that the interplay of large Hund’s coupling and spinorbit coupling within the t2g orbitals of a single Iron atom leads to the local resonance of triplet states [1]. In this talk, we show that the spin symmetric, orbitally antisymmetric preformed pairs can escape their local environment and lead to fully gapped superconductivity in the bulk. We map this problem to a multiorbital Anderson impurity with anisotropic XXZlike Hund’s coupling and use largeN and Schwinger boson techniques [2] to study the ground state and thermodynamic properties. We show that local Cooper pairing can be dynamically generated in this setting, and that selfconsistency between the electron bath and the impurity’s degrees of freedom leads to a fully gapped triplet condensate, thus taking the tRVB theory beyond the meanfield level. 
Monday, March 14, 2022 8:24AM  8:36AM 
A57.00003: Anisotropic superconductivity calculations with the fullbandwidth MigdalEliashberg formalism using the EPW code Samad Hajinazar, Hari Paudyal, Elena R Margine Recent developments in the superconductivity module of the EPW code include solving the fullbandwidth (FBW) anisotropic MigdalEliashberg equations. This approach enables the description of phononmediated superconductors beyond the constant density of states approximation. Here we overview the implemented formalism, a brief howto for invoking the new features, and benchmarking tests' results demonstrating the computational scaling and performance of the FBW method. 
Monday, March 14, 2022 8:36AM  8:48AM 
A57.00004: Twodimensional spectroscopy of superconductors Yihua Qiang, Thais V Trevisan, Victor L Quito, Peter P Orth Recent progress in the generation of coherent terahertz pulses allows probing several manybody phenomena, providing unique fingerprints of exotic excitations. In higherdimensional spectroscopy, terahertz pulses delayed in time are irradiated to the sample and the corresponding nonlinear response functions are measured. In this talk, we present theoretical results on the twodimensional spectroscopy of clean and dirty superconductors. We show how higherorder optical responses of disordered superconductors can quantify the effects of impurities and provide fingerprints about the scattering and lifetime of Bogoliubons. For clean superconductors, we study the multiband case and address the possibility of resolving the orbital content and the gap symmetry. 
Monday, March 14, 2022 8:48AM  9:00AM 
A57.00005: Emergent inhomogeneity in dirty superconductors: microscopic theory Daniil S Antonenko, Pavel M Ostrovsky, D.Sc., Mikhail A Skvortsov, D.Sc. Superconducting state is known to become inhomogeneous as a material gets closer to the metaltoinsulator transition. The origin of inhomogeneity is naturally attributed to the increase of disorder, but a particular mechanism behind the effect is still unknown. We report on the microscopic theory of the emergent inhomogeneity in moderately disordered superconductors, which takes into account electron scattering on a few impurities located at a distance of the mean free path or closer. The contribution of such processes can parametrically exceed diffusive contribution considered previously. Emergent inhomogeneity is manifested in spatial fluctuations of the superconducting critical temperature and the quasiparticle gap, which can be probed by scanning tunneling measurements. Our findings suggest that with increasing disorder strength the superconducting state first becomes strongly inhomogeneous, and only then the metaltoinsulator transition occurs. 
Monday, March 14, 2022 9:00AM  9:12AM 
A57.00006: Simulating the static magnetic response of thin film superconducting devices Logan BishopVan Horn, Kathryn Moler Quantitative understanding of the spatial distribution of magnetic fields and screening currents in twodimensional (2D) superconductors and superconducting devices composed of thin films is critical to interpreting the results of magnetic measurements of such systems. A convenient numerical method for solving the static 2D London equation, which describes the linear magnetic response of 2D superconductors, was introduced by Brandt and Clem [Phys. Rev. B 69, 184509 (2004), Phys. Rev. B 72, 024529 (2005)]. Here, we outline the model and present an efficient, opensource Python implementation of Brandt and Clem's matrix inversion method, which solves for the magnetic field and current distributions in devices composed of thin inhomogenous superconducting films of arbitrary geometry in the presence of trapped flux, vortices, and inhomogeneous applied fields. As a demonstration, we apply the model to scanning superconducting quantum interference device (SQUID) microscopy. Beyond magnetic microscopy, this tool can be used to model screening effects and calculate self and mutualinductance in superconducting devices, and simulate the magnetic response of inhomogeneous 2D superconductors. 
Monday, March 14, 2022 9:12AM  9:24AM 
A57.00007: Firstprinciples calculation of the superconducting properties of Niobium Mehdi Zarea, James A Sauls We report results for the superconducting transition temperature and anisotropic energy gap for pure Niobium based on Eliashberg's equations and electron and phonon band structures computed from Density Functional Theory. The Fermi surface and the Fermi velocity are also calculated. The phonon bands are in excellent agreement with inelastic neutron scattering data. The phonon density of states and electronphonon coupling define the electronphonon spectral function, α^{2}F(p,p';ω), and the electronphonon pairing interaction, which is the basis for computing the superconducting properties. The electronphonon spectral function is overall agreement with tunneling spectroscopy data. The strongcoupling gap at T=0 is modestly enhanced, Δ=1.55 meV, compared to weakcoupling BCS value. The superconducting gap exhibits strong anisotropy on the Fermi surface. This leads to violation of Anderson's theorem for nonmagnetic impurity scattering in conventional isotropic superconductors. We analyze the distribution of gap anisotropy and compute the suppression of the superconducting transition temperature using a selfconsistent Tmatrix theory for quasiparticleimpurity scattering to describe Niobium doped with nonmagnetic impurities. 
Monday, March 14, 2022 9:24AM  9:36AM 
A57.00008: Multiscale modeling of quantum chip devices using ARTEMIS Revathi Jambunathan, Andrew J Nonaka, Prabhat Kumar, Zhi Yao Quantum devices with superconducting qubits are a promising technology in the realm of nextgeneration computing and information processing. However, as these systems grow in complexity, it has become critical to quantify losses or "crosstalk" errors between qubits and electromagnetic signals in the microwave resonators. These undesirable effects can occur at the cmscale due to electromagnetic waveinteractions from the material interfaces in the circuit, or at the micrometer or nanometer lengthscale caused by electromagnetic interference with the qubits. Designing efficient devices minimizing these interferences requires fullphysical modeling of the circuit. We have developed a multiscale electromagnetic code, ARTEMIS, for modeling the interaction of electromagnetic signals and the qubit components of the quantum circuit. ARTEMIS is a performanceportable software framework that solves Maxwell's equations using the finitedifference timedomain approach for realistic circuitry with heterogeneous materials. We will present our numerical approach towards coupling the Maxwell solver for electromagnetic signals and the classical London equation solver to study the nonlinear effects of the superconducting qubits and quantify the performance of realistic quantum circuit components. These simulations will aid in the design optimization of circuit configurations. 
Monday, March 14, 2022 9:36AM  9:48AM 
A57.00009: Cooper instability in uniform electron gas: Irrelevance of KohnLuttinger mechanism Tao Wang, Xiansheng Cai, Kun Chen, Boris Svistunov, Nikolay Prokofiev We study the Cooper instability in jellium model in the controlled regime of 
Monday, March 14, 2022 9:48AM  10:00AM 
A57.00010: Functionalintegral approach to Gaussian fluctuations in Eliashberg theory Mason Protter, Rufus Boyack, Frank Marsiglio The Eliashberg theory of superconductivity is based on a dynamical electronphonon interaction as opposed to a static interaction present in BCS theory. The standard derivation of Eliashberg theory is based on an equation of motion approach, which incorporates certain assumptions such as Migdal's approximation for the pairing vertex. In this paper we provide a functionalintegralbased derivation of Eliashberg theory and we also consider its Gaussianfluctuation extension. The functional approach enables a selfconsistent method of computing the meanfield equations, which arise as saddlepoint conditions, and here we observe that the conventional Eliashberg selfenergy and pairing function both appear as HubbardStratonovich auxiliary fields. An important consequence of this fact is that it provides a systematic derivation of the Cooper and densitychannel interactions in the Gaussianfluctuation response. We also investigate the fluctuation contribution to the diamagnetic susceptibility near the critical temperature. 
Monday, March 14, 2022 10:00AM  10:12AM 
A57.00011: Nature of Unconventional Pairing in the Kagome Superconductors AV_{3}Sb_{5} (A=K,Rb,Cs) Xianxin Wu, Tilman Schwemmer, Tobias Müller, Armando Consiglio, Giorgio Sangiovanni, Domenico Di Sante, Yasir Iqbal, Werner R Hanke, Andreas P Schnyder, Michael Denner, Mark H Fischer, Titus Neupert, Ronny Thomale The recent discovery of AV3Sb5 (A=K,Rb,Cs) has uncovered an intriguing arena for exotic Fermi surface instabilities in a kagome metal. Among them, superconductivity is found in the vicinity of multiple van Hove singularities, exhibiting indications of unconventional pairing. We show that the sublattice interference mechanism is central to understanding the formation of superconductivity in a kagome metal. Starting from an appropriately chosen minimal tightbinding model with multiple van Hove singularities close to the Fermi level for AV3Sb5, we provide a random phase approximation analysis of superconducting instabilities. Nonlocal Coulomb repulsion, the sublattice profile of the van Hove bands, and the interaction strength turn out to be the crucial parameters to determine the preferred pairing symmetry. Implications for potentially topological surface states are discussed, along with a proposal for additional measurements to pin down the nature of superconductivity in AV3Sb5. 
Monday, March 14, 2022 10:12AM  10:24AM 
A57.00012: Generic HT phase diagram of a staggeredRashba superconductor Mark H Fischer, Manfred W Sigrist Superconductivity in a crystal lacking inversion exhibits complex (Rashba) spinorbitcoupling effects, resulting in mixedparity pairing and an unusual magnetic response. In our work, we study the generic phase diagram of a layered, globally centrosymmetric superconductor with alternating Rashba spinorbit coupling in the stacking of layers within a generalized GinzburgLandau model. The superconducting order parameter (locally) mixes even and an oddparity pairing components, which exchange their roles as dominant pairing channel upon increasing the magnetic field. As a consequence, the upper critical field exhibits an unusual kink feature. Furthermore, the associated phase transition within the mixed phase is of first order and, due to the local mixing of even and oddparity components, is stable against splitting into two secondorder transitions. The resulting generic phase diagram qualitatively agrees with the recently found HT phase diagram of the heavyfermion superconductor CeRh_{2}As_{2} and emphasizes the role of local inversionsymmetry breaking in this compound. 
Monday, March 14, 2022 10:24AM  10:36AM 
A57.00013: Theory of transport in nanoscale superconducting devices: charge imbalance and thermoelectric effects Kevin M Seja, Tomas Lofwander, Louhane Jacob We present computational strategies for steadystate transport simulations of nanoscale superconductors coupled to normalmetal reservoirs. The EilenbergerLarkinOvchinnikov nonequilibrium quasiclassical transport equations are solved selfconsistently with selfenergies to ensure chargecurrent conservation. Within the framework, different order parameter symmetries are studied for arbitrary impurity concentrations ranging from clean to dirty limits. In addition to the traditional quasi1D treatment, we introduce a finiteelement method and study real twodimensional geometries. 
Monday, March 14, 2022 10:36AM  10:48AM 
A57.00014: DMRG study of superconductivity of Hubbard model with electronphonon interactions HaoXin Wang, Hong Yao, Yifan Jiang Recently several experimental and theoretical studies suggest that electronphonon coupling (EPC) could be an essential ingredient in understanding high temperature superconductors. In this work, we investigate the effect of EFC on the Hubbard model by using largescale density matrix renormalization group. For the $U=8$ Hubbard model with EPC constant $\lambda$ ~ 0.3, we find significant suppression of charge density wave. The influence of phonon on superconductivity and the effect of retardation will also be discussed. 
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