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 V48: Superconductivity Theory: Multiband systems and unconventional orderLive
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Sponsoring Units: DCMP Chair: Anton Vorontsov, Montana State University, Bozeman |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V48.00001: Multiorbital singlet pairing and d + d superconductivity Emilian Nica, Qimiao Si Recent experiments in multiband Fe-based and heavy-fermion superconductors have challenged the long-held dichotomy between simple s- and d-wave spin-singlet pairing states. Here, we advance several time-reversal-invariant irreducible pairings that go beyond the standard singlet functions through a matrix structure in the band/orbital space, and elucidate their naturalness in multiband systems. We consider the sτ3 multiorbital superconducting state for Fe-chalcogenide superconductors. This state, corresponding to a d + d intra- and inter-band pairing, is shown to contrast with the more familiar d + id state in a way analogous to how the B- triplet pairing phase of 3He superfluid differs from its A- phase counterpart. In addition, we construct an analogue of the sτ3 pairing for the heavy-fermion superconductor CeCu2Si2, using degrees-of-freedom that incorporate spin-orbit coupling. Our results lead to the proposition that d-wave superconductors in correlated multiband systems will generically have a fully-gapped Fermi surface when they are examined at sufficiently low energies. |
Thursday, March 18, 2021 3:12PM - 3:24PM Live |
V48.00002: Robustness of unconventional s-wave superconducting states against disorder David Cavanagh, Philip Brydon Unconventional superconductors are infamously unstable against the presence of nonmagnetic disorder, while the critical temperature of a conventional superconductor is insensitive to such disorder, as encapsulated in Anderson’s theorem. Generalization of Anderson’s theorem to superconductors with multiple bands has proven difficult. We investigate the robustness against disorder of superconductivity in systems with two bands. In multi-band systems, unconventional superconducting states are possible with momentum-independent (s-wave) pairing functions. We have developed a general framework, based on the self-consistent Born approximation, to understand the stability of orbitally non-trivial superconductors against disorder, and applied this framework to several multi-band superconductors. Unconventional s-wave states are found to be significantly more robust against disorder than analogous single-band states. Additionally, superconductors with momentum-dependent gaps inherit some robustness against disorder from the s-wave states with the same symmetry, in a significant deviation from the behavior exhibited in single-band superconductors. |
Thursday, March 18, 2021 3:24PM - 3:36PM Live |
V48.00003: Attraction from repulsion: unconventional superconductivity in flat bands due to far-out pairs Zhiyu Dong, Leonid Levitov Many superconducting systems of current interest feature abnormally high Tc values coexisting with high rs values and other anomalies which point to a Coulomb-repulsion-dominated superconducting pairing mechanism. It is appealing to consider a route in which the Coulomb repulsion generates attraction ``directly", rather than acting through low-energy intermediaries used in previous proposals. If achievable, such a mechanism would fully benefit from the strength of the pristine Coulomb interaction. One mechanism considered previously involves repulsion-induced pairing between several Fermi surfaces, however, the resulting pairing interaction is weakened by Thomas-Fermi screening. This talk will describe a new repulsion-dominated pairing mechanism that utilizes a nimbus of far-out pairs in the empty band positioned above the conduction band. The resulting pairing interaction is unscreened, and thus provides an enhanced Coulomb-repulsion-driven superconductivity. We further illustrate the sizable effect resulting from such mechanism by applying it to the magic-angle twisted bilayer graphene (MATBG) where the upper bands in our model capture the role of the non-flat minibands, which is usually overlooked in the literature. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V48.00004: Bogoliubov-Fermi Surfaces in Noncentrosymmetric Multicomponent Superconductors Julia Link, Igor Herbut We show that when the time reversal symmetry is broken in a multicomponent superconducting condensate without inversion symmetry the resulting Bogoliubov quasiparticles generically exhibit mini-Bogoliubov-Fermi (BF) surfaces, for small superconducting order parameter [1]. The absence of inversion symmetry makes the BF surfaces stable with respect to weak perturbations. With sufficient increase of the order parameter, however, the Bogoliubov-Fermi surface may disappear through a Lifshitz transition, and the spectrum this way become fully gapped. Our demonstration is based on the computation of the effective Hamiltonian for the bands near the normal Fermi surface by the integration over high-energy states. Exceptions to the rule, and experimental consequences are briefly discussed. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V48.00005: Cooper instability in the Jellium model: Implicit renormalization approach Xiansheng Cai, Tao Wang, Kun Chen, Boris Svistunov, Nikolai Prokof'ev We study superconducting properties of the jellium model by the implicit renormalization approach, which enables us to reliably determine the exponentially low transition temperatures in s-, p-, and d-channels. Existence of superconducting states at r_s>2.0 within the RPA approximation indicates that dynamic screening may lead to an effective attractive Cooper-channel coupling, in disagreement with the notion of repulsive pseudopotential. Going beyond RPA, we consider an effective KO interaction, as well as the effects of higher-order diagrams accounting for renormalization of the electron self-energy and the vertex function with the projection renormalization method. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V48.00006: Unconventional superconductivity mediated by the "Higgs" amplitude mode in itinerant ferromagnets Roy Forestano, Joshuah Heath, Kevin Shawn Bedell Over 20 years ago, Blagoev et. al. [Phys. Rev. Lett. 82, 133 (1999)] predicted an s-wave pairing instability in a ferromagnetic Fermi liquid (FFL) as a consequence of spin fluctuations. Shortly after, it was discovered that, when magnetic interactions in the ferromagnetic superconductor UGe2 dominate, quasiparticles with parallel spin form pairs in odd-parity orbitals; i.e., a form of spin-triplet p-wave superconductivity emerges, in contrast to Blagoev et. al.'s prediction. In this work, we return to this issue by introducing the effects of a gapped amplitude (or "Higgs") mode on the vertex corrections and subsequent form of Cooper pairing. As the Higgs mode only propagates in the presence of a finite spin current, such an amplitude mode results in strong momentum-dependence in the many-body vertex. This results in the emergence of an unconventional form of superconductivity mediated by unconventional low-energy modes in a weak itinerant ferromagnet. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V48.00007: Vertex bosonization in the Hubbard model: a new perspective Pietro Bonetti, Demetrio Vilardi, Walter Metzner The two-particle vertex is an essential ingredient in the study of strongly correlated electrons as it provides useful information on the collective excitations of the system. A suitable representation of the vertex as a sum of bosonic propagators not only reduces its complexity, but also helps to identify those collective fluctuations which play a decisive role in the phase transitions to ordered states. We analyze a functional renormalization group (fRG) flow in which the effective interaction diverges before reaching the final scale, signaling the onset of spontaneous symmetry breaking. While keeping the full frequency dependencies, we continue the flow by explicitly introducing a bosonic field and deriving a reduced set of flow equations for the calculation of the order parameter in a mean-field-like fashion. This method satisfies fundamental constraints, such as the Ward identities, and it can be easily combined with the most recent fRG truncations. We also extend the present formulation to the newly developed DMF2RG, which combines the dynamical mean-field theory (DMFT) and the fRG. Our method represents a convenient starting point for the inclusion of bosonic fluctuations and for the access of the symmetry broken phases also with strong coupling methods. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V48.00008: Spontaneous Time-Reversal Symmetry Breaking in the Normal Phase of Unconventional Superconductors Meng Zeng, Lunhui Hu, Hong-Ye Hu, Yizhuang You, Congjun Wu Time-reversal symmetry (TRS) plays a crucial role in the study of unconventional superconductivity in strongly correlated systems. When two superconducting order parameters with different pairing symmetries compete, TRS can be spontaneously broken due to a second order Josephson coupling between the competing order parameters. In this work, we show that TRS breaking transition can occur due to superconducting phase fluctuations before the onset of superconductivity. To illustrate this phenomenon, we employ the Ginzburg-Landau theory, and use an effective two-component XY-model to perform a renormalization group (RG) analysis to study superconducting phase fluctuations. In the TRS breaking normal state, neither of the pairing orders develops phase coherence, but their relative phase is pinned at $\pm \pi/2$. Monte Carlo simulations are also used to obtain the phase diagram for the coupled XY model at various coupling strengths, which agrees well with the RG calculation. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V48.00009: Electron irradiation effects on superconductivity in PdTe2: An application of a generalized Anderson theorem Peter Orth, Erik I Timmons, Serafim Teknowijoyo, Marcin Konczykowski, Olivier Cavani, Makariy Tanatar, Sunil Ghimire, Kyuil Cho, Yongbin Lee, Liqin Ke, Na Hyun Jo, Sergey Budko, Paul C Canfield, Mathias Scheurer, Ruslan Prozorov We present a general theoretical framework for analyzing irradiation studies in spin-orbit-coupled multiorbital systems, which is based on a generalized Anderson theorem for multiband superconductors. We apply this framework to interpret results from a theory-experiment collaboration about the impact of nonmagnetic disorder on the normal and superconducting properties of the type-II Dirac semimetal PdTe2. Experimental measurements of longitudinal and Hall resistivity, thermal conductivity and London penetration depth using the tunnel-diode resonator technique are performed for various irradiation doses. They yield that the superconducting transition temperature is suppressed at a rate about 16 times lower than described by the Abrikosov-Gor’kov law. This imposes quantitative constraints on the gap anisotropies for each of the possible pairing candidate states. We conclude that the most likely pairing candidate is an unconventional A+−1g state, since other candidates require additional assumptions about the orbital structure of the disorder potential to be consistent with our experimental results. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V48.00010: Renormalization-Group Approach to Odd-Frequency Superconductivity Henoc Ejigu, Shan-Wen Tsai In the BCS theory, Cooper pairs are formed between electrons of opposite momenta near the Fermi surface and condense into the superconducting state. The Eliashberg theory takes into account retardation effects. In these theories, the pairing between electrons occurs at equal times. Here we investigate the off-time pairing, or odd-frequency superconductivity, first introduced by Berezinskii, using a renormalization-group approach. By considering a two-dimensional rotationally invariant Fermi surface at zero temperature, and assuming that the paired electrons have the same spin and opposite momenta near the Fermi surface, we probe the susceptibility of the system to forming Cooper pairs using the method of renormalization-group. We present a self-contained derivation of the flow equations for the vertex function and electron self-energy. We show numerical results for the vertex function in both weak-coupling and strong-coupling cases. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V48.00011: Odd frequency pairing of superconductivity in quantum critical metals Yi-Ming Wu, Andrey Chubukov Odd frequency pairing superconductivity is an unconventional pairing state where the order parameter (the gap function) △(ω) is an odd function of frequency (△(-ω)=-△(ω)). We analyze a possibility of odd frequency pairing for a model of fermions near a metallic quantum-critical point. In this model, the effective dynamical 4-fermion interaction, mediated by a critical boson, V(Ωm)∝1/|Ωm|γ, with γ>0, gives rise to both pairing and non-Fermi liquid behavior in the normal state. We assume that the interaction in the particle-particle channel is smaller by 1/N than that in the particle-hole channel, and adopt the Eliashberg formalism to analyze the interplay between the pairing and non-Fermi liquid. We show that the gap equaton allows solutions that are either even or odd in frequency. We show that odd frequency pairing develops as long as N is smaller than some critical value Ncr, which is a function of γ. For a generic γ<1, the onset temperature and the gap amplitude at T=0 for the odd frequency solution are much smaller than for the corresponding even frequency pairing, but the two become comparable at γ→1. In this limit, even frequency and odd frequency solutions may coexist. |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V48.00012: Thermodynamics and coherence effects in the Hatsugai-Kohmoto superconductor Luke Yeo, Edwin Huang, Philip Phillips Soluble Hamiltonian models with unconventional superconductivity are few and far between. The Hatsugai-Kohmoto superconductor is one such model. In a previous work, we have shown that it exhibits a number of striking phenoma, including (1) a gap-to-Tc ratio that diverges in the weak pair coupling limit; (2) a shift in spectral weight across the superconducting transition, (in a certain regime) on the order of the colour change seen in BSCCO; and (3) novel elementary excitations that descend from its non-Fermi liquid normal state. We further clarify the nature of this superconductor from the perspective of its thermodynamics and some foundational experimental observables. Specifically, we consider its thermodynamic entropy, specific heat, and free energy, as well as the system's ultrasonic attenuation and nuclear spin relaxation rates. |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V48.00013: Domes of Tc in single and multiband dilute superconductors with extended attractive interactions Nazim Boudjada, Finn Lasse Buessen, Arun Paramekanti One of the common features shared among many families of superconductors is the appearance of a superconducting dome as a function of doping. Although this is often associated with quantum critical points, the connection is not clear for dilute superconductors such as strontium titanate. In an effort to explain experimental signatures such as the weak dependence of Tc on the carrier density, the emergence of multiple domes, and the very dilute regime over which superconductivity is observed in both 2D and 3D, we propose a toy model of electrons coupled via a spatially extended interaction. Using a combination of mean field theory and functional renormalization group to account for competing orders, we show that for a characteristic interaction range L, there exists a peak in Tc around kFL∼1. Our analysis holds in 2D and 3D systems and predicts the existence of multiple domes in the vicinity of Lifshitz transitions for multiband materials. |
Thursday, March 18, 2021 5:36PM - 5:48PM Live |
V48.00014: Superconductivity from energy fluctuations in dilute quantum critical polar metals Premala Chandra, Pavel Volkov, Piers Coleman Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here we show that in dilute metals near polar quantum critical points the Coulomb repulsion can be surpassed by the attraction mediated by zero-point energy fluctuations of the critical polar mode. Our estimates show that this mechanism can explain the critical temperatures observed in doped SrTiO3, and we predict enhanced effects of polar quantum criticality on superconductivity in dilute two-dimensional metals. |
Thursday, March 18, 2021 5:48PM - 6:00PM On Demand |
V48.00015: Odd–Parity Spin–Triplet Superconductivity in Centrosymmetric Antiferromagnetic Metals Seung Hun Lee, Hongchul Choi, Bohm-Jung Yang We propose a route to achieve odd-parity spin-triplet superconductivity in metallic collinear antiferromagnets with inversion symmetry. Owing to the existence of hidden antiunitary symmetry, which we call the effective time-reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin-degenerate, and spin-singlet pairing is favored. However, by introducing a perturbation that breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin-polarized Fermi surfaces constrain the electrons to form spin-triplet Cooper pairs with odd-parity. Interestingly, all the odd-parity superconducting ground states we obtained host nontrivial band topologies. We propose that double perovskites with collinear antiferromagnetic or ferrimagnetic ordering, such as SrLaVMoO6, are promising candidate systems where our theoretical ideas can be applied to. |
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