Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session M37: Recent Developments in the Theory of SuperconductivityInvited

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Sponsoring Units: DCMP Chair: HungYu Yang, University of California, Los Angeles Room: Room 233 
Wednesday, March 8, 2023 8:00AM  8:36AM 
M37.00001: Quantum Geometry and superconductivity in flat bands Invited Speaker: Paivi E Torma We have found that superconductivity and superfluidity have a connection to quantum geometry [1,2]. Namely, the superfluid weight in a multiband system has a previously unnoticed component which we call the geometric contribution. It is proportional to the minimal quantum metric of the band. Quantum metric is connected to the Berry curvature, and this allows to relate superconductivity with the topological properties of the band. Using this theory, we have shown that superconductivity is possible also in a flat band where individual electrons would not move. Recently, we and other groups have shown [3,4] that these results may be essential in explaining the observation of superconductivity in bilayer graphene and may eventually help realize superconductors at elevated temperatures. In addition to the promise of high critical temperatures and strong correlation effects, also the quantum transport in flat band shows unique behavior [5]. In the normal state above the superconducting critical temperature, a new type of insulator can be found [6]. We have also explored the effect of quantum geometry on BoseEinstein condensation and shown that the quantum distance and quantum metric determine the stability of the condensate, and quantum fluctuations dominate over meanfield effects [7]. Lightmatter interactions become strongly enhanced by quantum geometry in flat bands [8]. 
Wednesday, March 8, 2023 8:36AM  9:12AM 
M37.00002: Abinitio theory of phonondriven superconductivity Invited Speaker: E.K.U. Gross A prominent challenge of condensedmatter theory is to reliably predict materialspecific properties of superconductors, such as the critical temperature. The superconducting version of density functional theory (SCDFT) meets this challenge for phonondriven superconductors. The central quantity of SCDFT is a universal exchangecorrelation functional which depends both on the electronic density and on the superconducting order parameter [1]. With the best available approximations for this functional, SCDFT is a genuine abinitio theory with no adjustable parameters, allowing the prediction of the critical temperatures of phonondriven superconductors within a few percent of experiment [2]. Examples of intermetallic compounds as well as hydrogenrich materials under pressure will be presented. Apart from Tc, other quantities, such as the order parameter in real space, and the excitation gap are directly accessible in SCDFT. Highly complex materials such as NbSe2, featuring a competition between superconductivity and a charge density wave at low temperature, can be fully understood [3] with this approach. Furthermore, for a variety of materials, we analyze how the order parameter in real space is related to the chemical bonding structure [4]. Regions in the unit cell that provide an attractive coupling can be beautifully distinguished from those that contribute via Coulomb renormalization and those that are not coupled at all. By controlling the local bonding structure, SCDFT offers a route to locally tailor the order parameter in superconducting nanostructures [5]. 
Wednesday, March 8, 2023 9:12AM  9:48AM 
M37.00003: Superconductivity without quasiparticles: Quantum critical Eliashberg theory and its holographic dual Invited Speaker: Joerg Schmalian Superconductivity is abundant near quantumcritical points, where fluctuations suppress the formation of Fermi liquid quasiparticles and the BardeenCooperSchrieffer theory no longer applies. Two very distinct approaches have been developed to address this issue: quantumcritical Eliashberg theory and holographic superconductivity. The former includes a strongly retarded pairing interaction of illdefined fermions, the latter is rooted in the duality of quantum field theory and gravity theory. We demonstrate that both are different perspectives of the same theory. We derive holographic superconductivity in form of a gravity theory with emergent spacetime from a quantum manybody Hamiltonian  the Yukawa SYK model and finitedimensional generalizations thereof  where the Eliashberg formalism is exact. Exploiting the power of holography, we then determine the phase diagram, dynamic pairing susceptibility , and electromagnetic properties of the model. Finally, the theory can be used to study the crossover from non Fermi liquid to Fermi liquid superconductivity. 
Wednesday, March 8, 2023 9:48AM  10:24AM 
M37.00004: Quantum Critical Metals: From Loss of Quasiparticles to HighTc Superconductivity Invited Speaker: Qimiao Si Strange metals develop near quantum critical points in a variety of correlated systems. Some of the key issues include how the quantum critical state loses quasiparticles, how it drives superconductivity, and to what extent the strangemetal physics in different classes of correlated systems is connected with each other. 
Wednesday, March 8, 2023 10:24AM  11:00AM 
M37.00005: Triplet pairing mechanisms from Hund'sKondo models  applications to heavy fermion superconductors Invited Speaker: Tamaghna Hazra The family of heavy fermion materials, with active felectrons, hosts a large variety of candidatetriplet superconductors, with upper critical fields often exceeding the Pauli limit by an order of magnitude. Some, like UTe_{2} remain superconducting in fields over 60T indicating tightly bound pairs with coherence lengths shorter than 2nm. Notably, almost every triplet heavy fermion superconductor shares a common structural motif – two or more fshell atoms in the primitive unit cell related to each other by inversion, with only two exceptions UAu_{2} and YbRh_{2}Si_{2}. I will present a triplet pairing mechanism driven by Hund’s and Kondo coupling and enabled by this structural motif. In essence, Hund's coupling leads to preformed triplet pairs between the electrons trapped inside local moments. Kondo hybridization imbues these triplet correlations onto the conduction electron cloud that screens the moment. As these heavy electron pairs delocalize on a lattice of such moments, they remember the triplet entanglement of their parent configuration. In heavy fermion superconductors as diverse as UPt_{3}, UBe_{13}, UTe_{2}, CeRh_{2}As_{2}, UGe_{2}, U(Co,Rh)Ge, CeSb_{2} as well as PrTi_{2}Al_{20} and analogues, the moments are situated away from the inversion center, so that their preformed triplet pairs can be odd under inversion. As they delocalize, these heavy fermion pairs then have finite overlap with oddparity triplet pairs on the same Fermi surface, leading to a triplet pairing instability. This pairing mechanism is demonstrated by a twochannel Kondo model, in which Hund’s coupling modifies the Kondo interaction into a triplet superexchange between local moment and conduction spins. This unifies the triplet superconductivity and the local moment physics in a coherent framework, and we discuss experimental consequences and existing support for this pairing mechanism. The nearuniversal correlation with the structural motif suggests a common origin of heavy fermion triplet superconductivity in Hund’scoupled local moments. 
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