Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K57: Superconductivity: Unconventional Theories IIRecordings Available
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Sponsoring Units: DCMP Chair: Ireneusz Weymann, Adam Mickiewicz University Room: Hyatt Regency Hotel -Clark |
Tuesday, March 15, 2022 3:00PM - 3:12PM |
K57.00001: Mechanism of superconductivity in the Hubbard model at intermediate interaction strength Xinyang Dong, Emanuel C Gull The mechanism of electronic superconductivity in systems with intertwined orders has long been debated. |
Tuesday, March 15, 2022 3:12PM - 3:24PM |
K57.00002: The ??-model: density of states of low-energy fermionic excitations Shang-Shun Zhang, Yi-Ming Wu, Artem G Abanov, Andrey V Chubukov We study the competition between non-Fermi liquid and superconductivity near a quantum critical point of a metal, where critical bosonic fluctuations mediate the interaction between electrons. When the Fermi energy is much larger than the typical bosonic energy, the low-energy properties of the system are effectively described by effective Eliashberg theory with singular bosonic propagator 1/|??|?? (the ??-model). This talk is based on our series of studies on this model and particularly focuses on the density of states (DOS) of the low-energy fermionic excitations. The normal state is a non-Fermi liquid state due to the singular self-energy at low frequencies and the ground state is a superconductor. At zero temperature, the DOS vanishes below the pairing gap and forms a continuum above. For small ??, the behavior of the DOS is qualitatively the same as the a BCS superconductor. As ?? increases, more spectral weight transfers to the lower-edge of the continuum. We discuss in detail how the DOS evolves with ??. At ??=2, the continuum collapses into a discrete set of ??-peaks, corresponding to the formation of polaronic bound states. For 2<??<3, the DOS again has a form of a gapped continuum, but contains a level with macroscopic degeneracy at the edge of the continuum. |
Tuesday, March 15, 2022 3:24PM - 3:36PM |
K57.00003: Odd frequency pairing in quantum critical metals. Yi-Ming Wu, Andrey V Chubukov, Shang-Shun Zhang, Artem G Abanov We consider superconductivity pairing at a quantum critical point(QCP). Close to such a QCP, the fermion interaction becomes dynamic and gives rise to non-Fermi liquid behavior as well as pairing instability, which can be described by a general model where the pairing interaction scales as V(Ωn)∼1/|Ωn|γ. Based on this model, we investigate the case when 0<γ<1 and find there exists an infinite number of odd frequency solutions [Δ(-ωm)=-Δ(ωm)] to the superconductivity gap equations, with each of them having a unique gap structure Δn(ωm) and onset pairing temperature Tp,n (n=0,1,2,...). We study the one with the largest pairing temperature Tp,0, and find in zero temperature limit, the gap function Δ(ωm) scales as |ωm|asgn(ωm) with a<1 at small frequencies. On real axis, this scaling implies that the quasiparticle density of states N(ω) tends to 0 at ω=0. This differs from previous studies on odd frequency pairing in normal metal/superconductor or ferromagnet/superconductor heterostructures where the odd frequency pairing is associated with a peak in N(ω) at ω=0. We also investigate the temperature evolution of N(ω) for odd frequency pairing, and find it resembles a conventional BCS superconductor, i.e. as temperature increases, the gap in N(ω) gradually closes till it reaches the onset temperature. For completeness, we also consider the case when γ>1. By analyzing the linearized gap equation, we find there is no odd-frequency superconducting order at finite temperature if γ>1. |
Tuesday, March 15, 2022 3:36PM - 3:48PM |
K57.00004: Superconductivity from energy fluctuations in quantum critical polar metals Pavel A Volkov, Premala Chandra, Piers Coleman We demonstrate a new mechanism of s-wave superconductivity, that does not require retardation to overcome the Coulomb repulsion. In quantum critical polar metals the latter is heavily screened while the critical transverse optic phonons decouple from the electron charge. We show that the residual interactions between quasiparticles are carried by energy fluctuations of the polar medium and are attractive leading to pairing. This mechanism naturally leads to a dome-like dependence of the superconducting Tc on carrier density. Our estimates show that this mechanism may explain the occurence of superconductivity at low densities in doped SrTiO3. We provide predictions for the enhancement of superconductivity near polar quantum criticality in two and three dimensional materials that can be used to test our theory. |
Tuesday, March 15, 2022 3:48PM - 4:00PM |
K57.00005: Transient Higgs modes in time-reversal-symmetry-breaking BCS superconductors Tianrui Xu, Kazuaki Takasan, Joel E Moore We study the transient dynamics of superconductors initially prepared at time-reversal-symmetry(T)-breaking states and assume the post-pump system evolves with a BCS Hamiltonian that preserves T. Such scenarios could occur in pump-probe experiments on superconductors, where the initial pump fields, such as circularly polarized laser lights, break T of the superconductors. Two types of T-breaking initial states are discussed: Zeeman splitting (h) of the fermion dispersions and complex superconducting order parameters ($\Delta_x+i\Delta_y$). We found that h can induce a transient Higgs mode, while $\Delta_y$ determines how the frequency of the Higgs mode deviates from $\Delta_\infty$. Finally, we discuss how such modes could be observed in time- and angle-resolved photoemission spectroscopy. |
Tuesday, March 15, 2022 4:00PM - 4:12PM |
K57.00006: Theory of orbital do-vector in a two-band spin-singlet superconductor: application to nematic superconductivity Meng Zeng, Dong-Hui Xu, Zi-Ming Wang, Lun-Hui Hu, Fu-Chun Zhang We study even-parity spin-singlet orbital-triplet pairing states in a two-band superconductor. An orbital do(k)-vector is introduced to characterize such pairings. Naively, one might think the double degeneracy of orbitals would be lifted by orbital hybridizations due to the crystal field splitting or electron- electron interactions, then spin-singlet orbital-dependent pairings may be severely suppressed. However, we demonstrate that these pairings are not excluded in real materials and a corresponding orbital do-vector could be stabilized along certain axis in orbital subspace. Even more remarkably, the interplay between the many-body interaction induced nematic order and the superconducting order leads to the establishment of a nematic orbital do-vector, which gives rise to the coexistence of nematicity and superconductivity. The generalization to a single-band superconductor with two valleys (e.g. honeycomb lattice with two sublattices) is also discussed. The nematic superconductivity in both FeSe and magic-angle twisted bilayer graphene might be interpreted within our theory framework. Moreover, the complex orbital do-vector spontaneously breaks time-reversal symmetry (TRS), which may induce the TRS-breaking orbital-polarization, analogous to the spin magnetism. |
Tuesday, March 15, 2022 4:12PM - 4:24PM |
K57.00007: Pair density waves in the strong-coupling limit of the Holstein-Hubbard model Zhaoyu Han, Kevin S Huang, Steven A Kivelson, Hong Yao We study an effective t-J-V model that is equivalent to the strong coupling limit of the Holstein-Hubbard model. In the dilute electron limit, on a triangular lattice with negative hopping elements, we find parameter regimes that can give rise to pair-density-wave (PDW), a novel superconducting state with an oscillating pairing order parameter. For different degrees of retardation, we identify two possible mechanisms. When the retardation is large, quantum hopping is suppressed, and the electrons tend to form hard-core singlet dimers. The Bose condensation of the dimers to their band minimum at K points serves as a strong-coupling mechanism of PDW formation. To study finite retardation cases, we performed a density-matrix renormalization group study on long, multi-leg cylinders. We find strong quasi-long-range PDW order with a divergent PDW susceptibility, as well as the spontaneous breaking of time-reversal and inversion symmetries. We conclude that the state is valley-polarized and the PDW arises from the intra-pocket pairing so that the pairing center is located at a non-zero momentum. The valley polarization physics that is well characterized by a mean-field theory can potentially be generalized to higher dimensions. This provides a weak coupling mechanism of PDW formation. |
Tuesday, March 15, 2022 4:24PM - 4:36PM Withdrawn |
K57.00008: Unconventional Superconductivity arising from Multipolar Kondo Interactions Adarsh S Patri, Yong-Baek Kim The nature of unconventional superconductivity is intimately linked to the microscopic nature of the pairing interactions. In this talk, motivated by cubic heavy fermion compounds with embedded multipolar moments, we theoretically investigate superconducting instabilities instigated by multipolar Kondo interactions. Employing multipolar fluctuations (mediated by RKKY interaction) coupled to conduction electrons via two-channel Kondo and novel multipolar Kondo interactions, we discuss the nature of the realized unconventional superconducting states. This work lays the foundation for discovery and classification of superconducting states in rare-earth metallic compounds with multipolar local moments. |
Tuesday, March 15, 2022 4:36PM - 4:48PM |
K57.00009: Pairing Instabilities of the Yukawa-SYK Models with Controlled Fermion Incoherence Wonjune Choi, Omid Tavakol, Yong Baek Kim One of the major obstacles in understanding unconventional superconductivity is the lack of a controlled theoretical framework to treat the competing effects of non-Fermi liquids and superconductivity born out of strong electronic correlations. As a solvable example of superconductors in the strong coupling limit, we study the pairing instabilities of the Yukawa-Sachdev-Ye-Kitaev (Yukawa-SYK) model, where spinful fermions are coupled to bosons by random all-to-all Yukawa-type interactions. In contrast to the previously studied models, the random Yukawa couplings are sampled from a collection of Gaussian ensembles whose variances follow a continuous distribution rather than a fixed constant. The continuous distribution of the variances offers a new tool that can be used to control the fermion incoherence or the "non-Fermi liquidness". This allows a systematic investigation of different normal states ranging from the Fermi liquid to non-Fermi liquids. With this novel control knob for the fermion incoherence, we demonstrate how non-Fermi liquids may show stronger tendency to form Cooper pairs in such Yukawa-SYK models. |
Tuesday, March 15, 2022 4:48PM - 5:00PM |
K57.00010: Consequences of density-wave order in a staggered Rashba superconductor Anastasiia Skurativska, Manfred W Sigrist, Mark H Fischer Superconductors with local inversion-symmetry breaking can exhibit properties usually associated with non-centrosymmetric systems, such as local mixing of even and odd superconducting order parameters or unusual magnetic response. An example of a system with such local non-centrosymmetricity is a stack of layers with alternating Rashba spin-orbit coupling due to mirror-symmetry breaking with respect to the individual layers. Motivated by recent experiments on the Ce-based superconductor CeRh2As2, which were interpreted as showing possible quadrupole-density-wave order, we investigate the effect of density-wave order on the physics related to local inversion-symmetry breaking. In particular, we study how the partial gapping out of the Fermi surface changes the effect of local inversion-symmetry breaking. |
Tuesday, March 15, 2022 5:00PM - 5:12PM |
K57.00011: Interaction of a Néel-type skyrmion with a superconducting vortex Elizaveta Andriyakhina, Igor Burmistrov Superconductor-ferromagnet heterostructures hosting vortices and skyrmions are a new area of the interplay between superconductivity and magnetism. We study the interaction of a Néel-type skyrmion and a Pearl vortex in thin heterostructures due to stray fields. Surprisingly, we find that it can be energetically favorable for the Pearl vortex to be situated at some nonzero distance from the center of the Néel-type skyrmion. The presence of a vortex-antivortex pair is found to result in the increase of the skyrmion radius. Our theory predicts that a spontaneous generation of a vortex-antivortex pair is possible under some conditions in the presence of a Néel-type skyrmion. |
Tuesday, March 15, 2022 5:12PM - 5:24PM |
K57.00012: Bond Bipolarons: Sign-free Monte Carlo Approach Chao Zhang, Nikolay V Prokofiev, Boris Svistunov Polarons originating from phonon displacement modulated hopping have relatively light masses and, thus, are of significant current interest as candidates for bipolaron mechanism of high-temperature superconductivity [Phys. Rev. Lett. {\bf 121}, 247001 (2018)]. We observe that the bond model, when the dominant coupling comes from atomic vibrations on lattice bonds, can be solved by efficient sign-free Monte Carlo methods based on the path-integral formulation of the particle sector in combination with either the (real-space) diagrammatic or Fock-path-integral representation of the phonon sector. We introduce the corresponding algorithms and provide illustrative results for bipolarons in two dimensions. The results suggest that the route towards high-temperature superconductivity (if any) in the multiparametric space of the model lies between the Scylla of large size of moderately light bipolarons and Charybdis of large mass of compact bipolarons. As a result, on-site repulsion is helping $s$-wave superconductivity in sharp contrast with existing expectations. |
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