Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W45: Superconductivity in NonFermi LiquidsInvited Live Streamed

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Sponsoring Units: DCMP Chair: Philip Phillips, University of Illinois at UrbanaChampai Room: McCormick Place W375D 
Thursday, March 17, 2022 3:00PM  3:36PM 
W45.00001: Superconductivity out of a NonFermi Liquid Invited Speaker: Andrey V Chubukov In this talk I will discuss the interplay between nonFermi liquid behavior and pairing near a quantumcritical point (QCP) in a metal. These tendencies are intertwined in the sense that both originate from the same interaction mediated by gapless fluctuations of a critical order parameter. The two tendencies compete because fermionic incoherence destroys the Cooper logarithm, while the pairing eliminates scattering at low energies and restores fermionic coherence. I will discuss this physics for a class of models with an effective dynamical interaction V (W) ~1/W^{γ} (the γmodel). This model describes, in particular, the pairing at a 2D Isingnematic critical point in (γ=1/3), a 2D antiferromagnetic critical point (γ=1/2) and the pairing by an Einstein phonon with vanishing dressed Debye frequency (γ=2). I will argue that the pairing wins, unless the pairing component of the interaction is artificially reduced, but because of fermionic incoherence in the normal state, the pairing problem is fundamentally different from BCS/Eliashberg one. I will show that a system develops pseudogap behaviour in the temperature range between the onset of the pairing at Tp and the onset of phase coherence at the actual superconducting Tc. The ratio Tc/Tp decreases with γ and vanishes at γ =2. I will present two complementary arguments of why this happens. One is the softening of longitudinal gap fluctuations, which become gapless at γ =2. Another is the emergence of a 1D array of dynamical vortices, whose number diverges at γ =2. I will argue that a fundamentally novel superconducting ground state emerges at γ >2, with a macroscopically degenerate bound state at the onset of the continuum. Overall, I will argue that γ=2 is a critical point of a dynamical topological transition and discuss topological distinction between superconductivity at γ<2 and γ>2. I will present and discuss a generic phase diagram. 
Thursday, March 17, 2022 3:36PM  4:12PM 
W45.00002: Cooper pairing without quasiparticles Invited Speaker: Ilya Esterlis Superconductivity is the ultimate fate of a Fermi liquid at low temperatures. More broadly, superconductivity occurs in many systems where sharp Fermi liquid quasiparticles are absent and the nature of pairing in such incoherent systems thus raises an important problem. I will present work on a natural generalization of the SachdevYeKitaev model to systems of interacting electrons and bosons, which demonstrates that both critical and fully incoherent electrons still undergo a superconducting transition. I will describe various properties of the resulting state, such as the possibility of a strongly coupled fluid of Cooper pairs below Tc. I will also describe pair breaking effects due to timereversal symmetry breaking, leading to a theory that may be understood as a nonFermi liquid counterpart of the AbrikosovGor'kov pair breaking theory by paramagnetic impurities in superconductors. 
Thursday, March 17, 2022 4:12PM  4:48PM 
W45.00003: Exact theory for superconductivity in a doped Mott insulator Invited Speaker: Edwin Huang Because the cuprate superconductors are doped Mott insulators, it would be advantageous to solve even a toy model that exhibits both Mottness and superconductivity. In this talk, I consider the HatsugaiKohmoto model, an exactly solvable system that is a prototypical Mott insulator above a critical interaction strength at half filling. Upon doping or reducing the interaction strength, our exact calculations show that the system becomes a nonFermi liquid metal with a superconducting instability. In the presence of a weak pairing interaction, the instability produces a thermal transition to a superconducting phase, which is distinct from the BCS state, as evidenced by a gaptotransition temperature ratio exceeding the universal BCS limit. The elementary excitations of this superconductor are not Bogoliubov quasiparticles but rather superpositions of doublons and holons, composite excitations signaling that the superconducting ground state of the doped Mott insulator inherits the nonFermi liquid character of the normal state. An unexpected feature of this model is that it exhibits a superconductivityinduced transfer of spectral weight from high to low energies as seen in the cuprates as well as a suppression of the superfluid density relative to that in BCS theory. 
Thursday, March 17, 2022 4:48PM  5:24PM 
W45.00004: Solvable strongcoupling models for pairing and charge4e superconductivity in nonFermi liquids Invited Speaker: Yuxuan Wang We present SachdevYeKitaevlike models that captures the interplay between fermionic incoherence and strong attractive interaction in the pairing instability of nonFermi liquids. The first class of models feature random alltoall interactions among fermions and bosons. We show that this model exhibits ``selftuned quantum criticality" for a range of fermion filling independent on the bare boson mass. Unlike BCS instability, the pairing phase of electrons occur at T=0 only for some range of parameters, which is separated by the nonFermi liquid phase via an infiniteorder KosterlizThouless critical point. In the second class of models, we consider a solvable SachdevYeKitaevlike model for charge4e superconductivity. We determine spectral properties, the superfluid density, and the pairing instability of a charge4e superconductor, which applies even in the strongcoupling regime. 
Thursday, March 17, 2022 5:24PM  6:00PM 
W45.00005: Strange metal transport and violation of the conjectured Planckian scattering rate limit in electrondoped cuprates Invited Speaker: Richard L Greene I will discuss recent transport studies of the electrondoped cuprate system La2xCexCuO4 that exhibits strange metal transport in the normal state down to 35 mK. The strange metal behavior is manifested as a linearinT resistivity from 35 mK to 20K and a ~T^2 resistivity from 50K to 400K over a range of doping above and below a Fermi surface reconstruction at x = 0.14 [12]. Other strange metal behavior includes an anomalous low temperature linearinH magnetoresistance and lnT thermopower [35]. I also show that the magnitude of the ~T^2 resistivity far exceeds the conjectured Planckian bound on inelastic scattering [6]. 
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