Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F57: Superconductivity: Unconventional Theories IRecordings Available
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Sponsoring Units: DCMP Chair: Benjamin Fregoso, Kent State University Room: Hyatt Regency Hotel -Clark |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F57.00001: Frequency-dependent Inter-pseudospin Solutions of Superconducting Strontium Ruthenate Olivier Gingras, Michel Cote, A.-M. S Tremblay Recent experimental improvements have shifted the discussion on superconducting Sr2RuO4 away from the chiral p-wave solution, yet experiments give seemingly different pictures that are challenging to reconcile. In this multi-orbital system with important spin-orbit coupling, pairing mediated by electronic correlations leads to superconducting order parameters with rich structures. Taking advantage of pseudospin and inversion symmetries, we investigate the possible superconducting instabilities mediated by these spin and charge fluctuations. We find that spin-orbit coupling ubiquitously mixes even and odd contributions in orbital, spin and frequency spaces. We find two different leading inter-pseudospin symmetries: a B1g+ and an A2g- that have intra-orbital components respectively even and odd in Matsubara frequencies. An accidental degeneracy between those could help unify experimental evidence. |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F57.00002: Fractional fermionic Mott insulator and high-temperature superconductor in the doublon-conserving Hubbard model Rohit Hegde The Hubbard model's doublon-conserving (DC) series expansion most directly mimics the true Hamiltonian dynamic (which does not conserve doublon number) when interaction dominates over bandwidth and the series can be truncated at low order in t/u. This work explores low-temperature electronic and superconducting orders driven by a renormalized particle-hole symmetric DC Hubbard model at intermediate coupling crucially including correlated hopping, accompanied by strong, exotic three-body fluctuations at finite temperature. Fluctuations couple not only to spins and charges, but to their hoppings and currents, and include terms promoting chirality. Hartree-Fock calculations reveal a spontaneous metric fostering exoticism including unstable gauged ferromagnetism with undamped Higgs mode at half-filling, antiferromagnetism, a strongly insulating Wigner crystal of holons at fractional filling, and gapless striped states. BdG theory incorporates Hirsch's s-wave superconductor, but reveals a tendency to inhomogeneity and stripey-intertwining with charge and spin order. The harmony of Hubbard U, correlated hopping, and superexchange strongly prefers d-wave pairing over s-wave. Including the DC current in a linear response calculation reveals distinctly non-BCS electrodynamics, including a vanishing of superfluid density approaching half-filling. The pivotal role of t' is explored alongside psuedogap candidate-states. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F57.00003: Orbital entanglement mechanism of unconventional supercondcutivity Sergei Urazhdin, Alexander Mitrofanov The widely held notion that the superconducting holes in cuprates reside in Cu dx2-y2 orbitals is incorrect, because two-hole correlations responsible for superconductivity are not described by the molecular orbital approximation. We utilize Hubbard model to show that insted, the nearest-neighbor d2+ and d2- Cu orbitals become entangled due to orbitally-selective virtual hopping onto the p+, p- orbitals of oxygens shared by Cu neighbors. Without doping, these correlations lead to the Mott state or orbital antiferromagnetism. Doping suppresses these states, and the ground state becomes orbitally entangled singlet-like with a residual gauge symmetry. We provide thermodynamic arguments that the gauge symmetry becomes broken at sufficiently low temperature due to the collective correlation effects, resulting in the onset of superconductivity. We expect that the proposed orbital entanglement mechanism explains superconductivity in other unconventional superconductors, including SrTiO3, iron pnictides and twisted multilayer graphene, and elucidates their direct connection with ferromagnetism, which can be described as condensation of orbitally-entangled spin triplets. Our analysis also provids specific guidance in the search for better high-temperature superconductors. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F57.00004: The Effect of Repulsion on Superconductivity at Low Density Dan Phan, Andrey V Chubukov We examine the effect of repulsion on superconductivity in a three-dimensional system with a Bardeen-Pines-like interaction in the low-density limit, where the chemical potential is much smaller than the phonon frequency. We parameterize the strength of the repulsion by a dimensionless parameter f. We find that the superconducting transition temperature Tc approaches a nonzero value in the limit of zero density as long as f is below a certain threshold f*. We show that Tc goes to zero as a power of f*- f, in contrast to the high density limit, where Tc goes to zero exponentially quickly as f approaches f*. At the same time as Tc goes to zero, the nodes of the gap function along the Matsubara axis approach zero also as a power of f*- f. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F57.00005: On the possibility of mixed helical p-wave pairings in Sr$_2$RuO$_4$ Zhiqiang Wang, Wen Huang The exact nature of the unconventional superconductivity in Sr$_2$RuO$_4$ remains a mystery. At the phenomenological level, no superconducting order parameter proposed thus far seems able to coherently account for all essential experimental signatures. Among the latter is the prominent polar Kerr effect, which implies a nonzero ac anomalous Hall conductivity. Assuming the Kerr effect is intrinsic, it can be accounted for by a bulk chiral Cooper pairing with nonzero orbital angular momentum, such as $p+ip$ or $d+id$, which, however, has difficulties in being reconciled with other experimental results. Given the situation, we propose alternative possibilities with complex mixtures of distinct helical p-wave order parameters, namely, $A_{1u}+iA_{2u}$ and $B_{1u}+iB_{2u}$ in the group theory nomenclature. We show that these states can explain the Kerr data and exhibit salient features that may explain several other key observations in this material, including the absence of spontaneous edge current, a substantial Knight shift drop, and possibly signatures in uniaxial strain and ultrasound measurements. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F57.00006: Strain-induced time reversal breaking and half quantum vortices near a putative superconducting tetra-critical point in Sr2RuO4 Andrew C Yuan, Steven A Kivelson, Erez Berg It has been shown that many seemingly contradictory experimental findings concerning the superconducting state in Sr2RuO4 can be accounted for as resulting from the existence of an assumed tetra-critical point at near ambient pressure at which $d_{x^2-y^2}$ and $g_{xy(x^2-y^2)}$ superconducting states are degenerate. We perform both a Landau-Ginzburg and a microscopic mean-field analysis of the effect of spatially varying strain on such a state. In the presence of finite $xy$ shear strain, the superconducting state consists of two possible symmetry-related time-reversal symmetry (TRS) preserving states: $d \pm g$. However, at domain walls between two such regions, TRS can be broken, resulting in a $d+ig$ state. More generally, we find that various natural patterns of spatially varying strain induce a rich variety of superconducting textures, including half-quantum fluxoids. These results may resolve some of the apparent inconsistencies between the theoretical proposal and various experimental observations, including the suggestive evidence of half-quantum vortices. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F57.00007: Effects of longer-range interaction on superconductivity in Sr2RuO4 Xin Wang, Zhiqiang Wang, Catherine Kallin The superconducting symmetry of Sr2RuO4 (SRO) remains a puzzle. Currently, none of the proposed order parameters (OP) can account for all the key experiments, highlighting the importance of exploring different alternatives. Recently, among others, dx2-y2 + igxy(x2-y2) and s+idxy pairings have been proposed as OP candidates. However, their stability remains unclear. In this work, with effects of the sizable spin-orbit coupling accounted for, we theoretically study the superconducting instabilities in SRO in the presence of both local and longer-range interactions. We show that inclusion of a second nearest neighbor repulsion or orbital-anisotropy of the longer-range interaction can have a significant impact on the stability of the g-wave pairing. Different pairing states, including dx2-y2 + ig, s+idxy, mixed helical, and dx2-y2 + ip, are found to be stabilized in different regimes of the interaction parameter space. We further investigate their properties such as Knight shift and spontaneous edge current. Lastly, we make a connection to the orbital-basis classification of superconducting states that has been actively discussed recently. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F57.00008: Superconducting gaps for Sr2RuO4 and their manifestations on observables Marc-Antoine Gauthier, Olivier Gingras, A.-M. S Tremblay The superconducting gap of the Sr2RuO4 is still unknown [1]. The most recent striking inconsistency is observed under uniaxial pressure. While high-sensitivity specific heat measurements detect only one transition temperature [2], muon spin relaxation experiments suggest two [3]. Because the minimal effective model of Sr2RuO4 involves multiple orbitals coupled by spin-orbit interaction, the possible gap functions are extremely intricate and possess odd-frequency contributions [4]. Starting from a model Hamiltonian, we compute the specific heat, the spectral weight and the band diagram expected for various superconducting order parameters. We inspect the effect of inter-orbital pairing on those quantities and we generalize the formalism to account for frequency dependent gap functions. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F57.00009: Spin triplet superconductivity driven by finite momentum spin fluctuations Andreas Kreisel, Peter J Hirschfeld, Yundi Quan A small number of superconductors are believed to exhibit intrinsic spin triplet pairing, |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F57.00010: Thermodynamic properties of superconductors in the extended BCS-Bose crossover theory Luis Abraham García Hernández, Marcela Dolores Grether González, Manuel de Llano Superconductivity in a generalized Bose-Einstein condensation theory (GBEC) is addressed. This theory contains three coupled transcendental equations for all temperatures: two gap-like equations and a particle number density equation that guarantees charge conservation. Here we explore the special case of an extended BCS-Bose crossover picture where two-hole Cooper pairs are explicitly included. We solved these three associated coupled equations yielding two pure Bose-Einstein-condensation phases of two-electron Cooper pairs and/or of two-hole Cooper pairs plus a mixed phase with varying proportions of both kinds of pairs. We found that the mere inclusion of two-hole Cooper pairs can lead to critical temperatures enhanced by several orders of magnitude compared with BCS theory. We also found a weak-intermediate and strong-coupling regime. Within the BCS-Bose crossover theory, we calculated the entropy and from that we analysed the rest of the well-known thermodynamic quantities that can be compared with experimental data. Results fit better for elemental superconductors, suggesting that two-hole Cooper pairs might be indispensable to describe this kind of materials. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F57.00011: Superconducting Instability From Spin Fluctuations in Sr2RuO4 Astrid T Rømer, Peter J Hirschfeld, Brian M Andersen, Andreas Kreisel, Thomas A Maier The symmetry of the superconducting condensate in Sr2RuO4 remains controversial after recent experiments overturned the dominant chiral p-wave paradigm: NMR leaves almost no room for a triplet component and ultrasound experiments put very stringent constraints on the superconducting order parameter which is deemed to comprise two components[1,2,3]. At the same time, specific heat finds no evidence for a split transition under uniaxial strain [4] in sharp contrast to the findings of muon spin rotation[5]. We explore which superconducting order parameters are favorable from the perspective of spin-fluctuation mediated pairing, taking into account the three Ru orbitals, a sizable spin-orbit coupling, longer range Coulomb interaction as well as three-dimensional effects. A full self-consistent solution of the gap sheds light on the propensity to time-reversal breaking superconducting order based on the near-degeneracy of symmetry-distinct pairing states and we will discuss effects due to strain and impurities. [1] A. Pustogow et al., Nature 574, 72 (2019) [2] S. Ghosh et al., Nat. Phys. 17, 199 (2021) [3] S. Benhabib et al., Nat. Phys. 17, 194 (2021) [4] Y.-S. Li et al, PNAS 118, 10 (2021) [5] V. Grinenko et al, Nat. Phys. 17, 748 (2021) |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F57.00012: Emergence of multiple Higgs modes in a superconductor due to spontaneous breakdown of a Z2 symmetry Shunji Tsuchiya Higgs modes and Nambu-Goldstone (NG) modes are ubiquitous collective excitations that arise in systems with spontaneous symmetry breaking. Although Higgs modes are believed to emerge with NG modes when continuous symmetries are spontaneously broken, Higgs modes do not necessarily appear in systems exhibiting spontaneous breakdown of continuous symmetries. We study the Higgs mode in a Bardeen-Cooper-Schrieffer (BCS) superconductor. Motivated by the observation that U(1) symmetry of the BCS Hamiltonian is not essential for the Higgs mode, we study the Ising-like Hamiltonian in the pseudospin representation. We show that the Higgs mode emerges as the lowest excited state of the Ising-like Hamiltonian due to spontaneous breakdown of Z2 symmetry under the time-reversal operation T in the pseudospin space. We further predict the existence of multiple Higgs modes that have quantized energy 2(n+1)∆0 (0 ≤ n ≤ NkF ), where ∆0 is the superconducting gap, n is an integer, and NkF is the number of states on the Fermi surface. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F57.00013: Study of vortices-PDW mixed state predicted by the t-J model via renormalized mean-field theory YiHsuan Liu, Gia-Wei Chern, Ting-Kuo Lee
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Tuesday, March 15, 2022 10:36AM - 10:48AM Withdrawn |
F57.00014: Quantum Phase Fluctuations Dominate Unconventional Superconductivity Anthony C Hegg, Wei Ku, Ruoshi Jiang, Yucel Yildirim Unconventional superconductivity has remained one of the most important mysteries in condensed matter physics since its discovery several decades ago. In contrast to conventional gapped s-wave BCS theory, early experiments indicated T-linear superfluid density at low temperature and more recent experiments with ever-higher accuracy frequently find superlinear dependence at low temperature, casting doubt on conventional theory categorically. We examine many well-known prototypical unconventional superconductors from several families and discover systematic T3 depletion of the 3D superfluid density at low temperature. To explain this novel behavior, we study the many-body current-current response of bosons in a lattice and find generic T3 depletion of the superfluid density. In contrast to the universality found in the vicinity of phase transitions, such stable low-temperature behavior is highly sensitive to the underlying quantum many-body theory and can be used to distinguish between possible theories. The success of our bosonic model at low temperature implies that phase-fluctuation dominated superfluidity underlies the quantum nature of the superconductivity in many of these materials. |
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