Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A09: Superconductivity: Theory General |
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Sponsoring Units: DMP DCMP Chair: Pavel Volkov Room: BCEC 151A |
Monday, March 4, 2019 8:00AM - 8:12AM |
A09.00001: Multiple solutions for superconductivity at T=0 in a quantum-critical metal Andrey Chubukov, Artem G Abanov We discuss the interplay between non-Fermi liquid behaviour and superconductivity at a quantum-critical point (QCP) in a metal. It is widely thought that the tendency towards superconductivity and towards non-Fermi liquid behaviour compete with each other, and if the pairing interaction is reduced below a certain threshold, the system displays a naked non-Fermi liquid QC behaviour. We show that at T=0 there is indeed a threshold below which the system remains in the normal state. We argue that above the threshold the system develops a discrete, but infinite set of solutions for the pairing gap. In particular, for ant value of the pairing interaction above the threshold there exists a solution of the linearized gap equation. This is very different from a conventional superconductivity, when the solution of the linearized gap equation exists only a critical strength of the pairing interaction. We present some exact results the set of QC models with frequency-dependent effective interaction and discuss the consequences of the existence of multiple solutions for superconductivity at a QCP. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A09.00002: The spectial role of the first Matsubara frequency for superconductivity near a quantum critical point: the non linear gap equation below Tc and spectral properties in real frequency axis. Yi-Ming Wu, Artem G Abanov, Yuxuan Wang, Andrey Chubukov We consider a model of itinerant fermions near a quantum critical point, where the effective interaction is mediated by order parameter fluctuations. This interaction makes fermions ncoherent and, at the same time, gives rise to attraction in at least one pairing channel. These two tendencies compete with each other. We argue that the situation is more tricky because the self-energy without thermal fluctuation piece, |
Monday, March 4, 2019 8:24AM - 8:36AM |
A09.00003: Strange metal crossover in the doped holographic superconductor Gaston Giordano, Nicolas Grandi, Adrian Lugo, Rodrigo Soto Garrido In this talk, we study the "normal" phase of a holographic superconductor. The specific model used was originally introduced by Kiritsis and Li (JHEP01(2016)147) to incorporate the different orders phases similar to the ones observed in High Temperature Superconductors. These phases include the normal phase, the SC phase, the antiferromagnetic phase and a striped phase as a function of both, temperature and doping. Within this model we study the fermionic spectral functions in the normal phase at zero and finite temperatures. Using analytic and numerical methods, we found a crossover in the normal phase from a strange metal with short lived excitations at small doping, into a Fermi liquid with well defined quasiparticles at large doping. The critical doping at which excitations becomes long lived increases with temperature. The emerging phase diagram is qualitatively similar to that of High Temperature Superconductors. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A09.00004: Non-Fermi liquid scattering against emergent Bose liquid: manifestations in the kink and other exotic quasiparticle behaviors in the normal-state cuprate superconductors Shengtao Jiang, Long Zou, Wei Ku The normal state of cuprate superconductors exhibits many exotic behaviors qualitatively different from the Fermi liquid, the foundation of condensed matter physics. Here we demonstrate that non-Fermi liquid behaviors emerge naturally from scattering against an emergent Bose liquid[1]. Particularly, we find a finite zero-energy scattering rate at low-temperature limit that grows linearly with respect to temperature, against clean fermions' generic non-dissipative characteristics. Surprisingly, three other seemingly unrelated experimental observations are also produced, including the well-studied "kink" in the quasi-particle dispersion, as well as the puzzling correspondences between the normal and superconducting state. Our findings provide a general route for fermionic systems to generate non-Fermi liquid behavior, and suggest that by room temperature large number of the doped holes in the cuprates have already formed an emergent Bose liquid of tightly bound pairs, whose low-temperature condensation gives unconventional superconductivity. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A09.00005: Renormalization group study of superconductivity in Nickelates Michael Klett, Tilman Schwemmer, Xianxin Wu, David Riegler, Ronny Thomale Since the discovery of high Tc superconductivity in the cuprates by Bednorz and Mueller, materials bearing similarities to copper oxide compounds have emerged as a contemporary research topic of high interest. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A09.00006: Pairing Mechanism in Hunds Metal Superconductors and the Universality of the Superconducting Gap to Critical Temperature Ratio Tsung-Han Lee, Andrey Chubukov, Hu Miao, Gabriel Kotliar We propose a simple spin-fluctuation Eliashberg model that contains the physical ingredients of a Hund's metal, the local spin-fluctuations with super-linear correlators, (Ω0/|Ω|)γ with γ>1, interacting with electrons. Our model yields a universal superconducting gap to critical temperature ratio 2△max/Tc≈7.2 that coincides with the experimental observation in a series of iron-based superconductors. We also discuss the implications of our model in the other classes of superconductors, e.g., cuprates and heavy fermion superconductors. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A09.00007: A Simple Model for Quantum Tunneling of Cooper Pairs Edgar Patino, Daniel Lozano, Denis Chevallier We propose a simple phenomenological model for quantum tunneling of Cooper pairs based on |
Monday, March 4, 2019 9:24AM - 9:36AM |
A09.00008: Gap anisotropy in nematic quantum critical superconductors Avraham Klein, Yi-Ming Wu, Andrey Chubukov When pairing is driven by nematic quantum fluctuations, it results in a unique form of superconductivity. Nematic fluctuations, while anisotropic, couple to the entire Fermi surface, in contrast to, e.g. spin fluctuations that typically couple to so-called ‘hotspots’. The result is an entire series of possible anisotropic pairing states, with closely spaced condensation temperatures, all with s-wave symmetry. The existence of these states drives a strong temperature evolution of the anisotropy of the superconducting gap. This evolution is a direct consequence of critical fluctuations, and provides a clear signature of quantum criticality inside the superconducting state. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A09.00009: Collective modes in 2 dimensional fermionic superfluid near Pomeranchuk instability WeiHan Hsiao In this work we investigate order parameter collective modes in 2 dimensional fermionic superfluids in finite angular momentum pairing channels. We discover subgap bosonic excitations with finite mass analogous to order parameter collective modes in 3 dimensional superfluid 3He. We further investigate the effect of fermion vacuum on these modes and it is shown that some of these modes in the spectrum soften as the underlying Fermi liquid approaches Pomeranchuk instability, or the nematic critical point when considering a p-wave chiral superfluid. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A09.00010: Fragile superconductivity in the presence of weakly disordered charge density waves Yue YU, Steven Kivelson When superconducting (SC) and charge-density wave (CDW) orders compete, novel low temperature behaviors can result. From an analysis of the Landau-Ginzburg-Wilson theory of competing orders, we demonstrate the generic occurrence of a “fragile” SC phase at low temperatures and high fields in the presence of weak disorder. Here, the SC order is largely concentrated in the vicinity of dilute dislocations in the CDW order, leading to transition temperatures and critical currents that are parametrically smaller than those characterizing the zero field SC phase. This may provide the outline of an explanation of the recently discovered “resilient” superconducting phase at high fields in underdoped YBa2Cu3O6+δ. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A09.00011: Study of the superconducting Tc dome in the phase diagram of the two-dimensional Holstein model Yan Wang, Phillip Dee, Ken Nakatsukasa, Steven Johnston Tc dome, i.e., the nonmonotonic superconducting Tc dependence as a function of parameters such as doping or pressure, is ubiquitous in the phase diagrams of unconventional superconductors, where the pairing interaction has a magnetic and electronic origin. By contrast, Tc dome is rarely seen in conventional electron-phonon superconductors. We report the Tc dome in the phase diagram of the two-dimensional Holstein model, where electrons and phonons interact through a local coupling. By solving the model using strong coupling Migdal-Eliashberg formalism and self-consistently renormalizing both the electrons and phonons, a Tc dome as function of filling n arises in the phase diagram, alongside a charge-density-wave phase around the half-filling. The dome is tied to the competition of three renormalized quantities as the filling increases: the increasing coupling constant λ, the decreasing ωlog, and the nonmonotonically varying density of states at the Fermi-level. Interestingly, we also find the effective electron-electron interaction becomes nonlocal in r-space and even repulsive at neighboring lattice sites. Our discovery could shed light on the high Tc of the strongly electron-phonon coupled system such as the dense hydrogen-rich compounds and the monolayer FeSe on SrTiO3. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A09.00012: Polarons, bipolarons and few-polaron states in the Peierls model of electron-phonon coupling John Sous, Alberto Nocera, Mona Inesa Berciu We consider the one-dimensional Peierls model of electron-phonon coupling describing the modulation of the electron hopping by phonons. We review recent results on single polarons and bipolarons, and extend our discussion to few-polaron states in the regime of competing phononic and electronic excitations. Single polarons experience a transition from weak coupling to strong coupling, while the Peierls coupling stabilitizes strongly bound yet very ligh bipolarons. Few polarons, however, experience higher order interactions that result in the formation of multipolaron bound states. In the anti-adiabatic limit of large phonon frequencies, we find that bipolarons are stable against phase separation. We discuss the relevance of these ideas to superconductivity in higher dimensions. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A09.00013: Emergence of a Macroscopic Phase in an Isolated Electron-Phonon Superconductor Wataru Kohno, Takafumi Kita Within the number-conserving formalism introduced by Ambegaokar, we constructed a variational wave function of electron-phonon superconductors. The many-body correlations originated from electron-phonon coupling are incorporated in our variational wave function. Moreover, this wave function gives rise to a superposition over the number of condensed particles and the Cooper-pair wave function with a macroscopic phase can be defined naturally. Therefore, we conclude dynamical exchanges of phonons induce a well-defined macroscopic phase with emergence of superconductivity in an electron-phonon superconductor. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A09.00014: Generalization of Anderson's Theorem for Disordered Superconductors John Dodaro, Steven Kivelson, Catherine Kallin We show that at the level of BCS mean-field theory, the superconducting Tc is always increased in the presence of disorder, regardless of order parameter symmetry, disorder strength, and spatial dimension. This result reflects the physics of rare events - formally analogous to the problem of Lifshitz tails in disordered semiconductors - and arises from considerations of spatially inhomogeneous solutions of the gap equation. We discuss the relevance of these effects for short coherence length superconductors and compare them with an exact treatment of disorder within dirty d-wave BCS theory to describe superfluid stiffness results on the overdoped cuprates. |
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