Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R18: Electron-Phonon Coupling and High-Temperature SuperconductivityInvited
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Sponsoring Units: GSNP Chair: John Sous, Columbia Univ Room: 205 |
Thursday, March 5, 2020 8:00AM - 8:36AM |
R18.00001: Reconsidering the electron-phonon problem and bounds on Tc Invited Speaker: Steven Kivelson We exploit the fact that the Holstein model of the electron-phonon problem can be treated without approximation using fermion-minus-sign-free determinent quantum Monte Carlo methods to establish results that can be compared quantitatively and unambiguously with approximate methods based on Migdal-Eliashberg (ME) theory. In the relevant limit in which the phonon frequencies are small compared to the Fermi energy (strong retardation), we find that ME theory is extremely accurate up to moderate values of the dimensionless electron-phonon coupling λ, and then breaks down relatively suddenly beyond a characteristic value, λ*~1, beyond which polaron physics is significant. One consequence of this is that – in contrast with earlier beliefs based on ME theory – the superconducting Tc(λ) has its maximum value at λ ≈λ*. This implies that there is an upper bound on Tc from the electron phonon mechanism Tc ≤ A wmax, where wmax is the maximum phonon energy and we estimate that A ≈ 1/10. |
Thursday, March 5, 2020 8:36AM - 9:12AM |
R18.00002: Light bipolarons from strong Peierls electron-phonon coupling Invited Speaker: Mona Berciu It is widely accepted that phonon-mediated high-temperature superconductivity is impossible at ambient pressure, because of the very large effective masses of polarons and bipolarons at strong electron-phonon coupling. We challenge this by showing that strongly bound yet very light bipolarons appear for strong Peierls coupling. These bipolarons exhibit many unconventional properties; e.g., at strong coupling there are two low-energy bipolaron bands that are stable against strong Coulomb repulsion. Using numerical simulations and analytical arguments, we show that these properties result from the specific form of the phonon-mediated interaction, which is of “pair hopping” instead of regular density-density type. This unusual effective interaction is bound to have nontrivial consequences for the superconducting state expected to arise at finite carrier concentrations and should favor a large critical temperature. |
Thursday, March 5, 2020 9:12AM - 9:48AM |
R18.00003: Superconductivity in Ultra-Low Density Dirac Materials Driven by a Ferroelectric Quantum Critical Point Invited Speaker: Jonathan Ruhman According to standard lore, low-density materials such as semimetals should not become superconducting. They are characterized by a small Fermi energy, which is comparable to the Debye frequency and moreover, their density of states is extremely small. Nonetheless, superconductivity is ubiquitous in topological materials, including YPtBi, Bi2Se3, Bi, Cd3As2, PbTe and SnTe, which raise the question regarding the microscopic picture of pairing in these systems. In this talk, I will discuss the option of pairing mediated by ferroelectric fluctuations close to a quantum critical point in a Dirac semimetal. I will show that such fluctuations lead to pairing, even when the Fermi energy is smaller than any bosonic frequency in the problem and possibly, even at charge neutrality. |
Thursday, March 5, 2020 9:48AM - 10:24AM |
R18.00004: New Perspectives from Spectroscopy on the Bismuth Oxide Superconductors Invited Speaker: Nicholas Plumb Despite being known for decades, the perovskite bismuth oxide superconductors (max Tc > 30 K) were never experimentally probed to nearly the extent of more famous high-Tc materials, such as cuprates and iron-based superconductors. This is a pity, because their phenomenology and underlying physics connect with a wide array of contemporary interests: not only unconventional/high-Tc superconductivity, but also metal-insulator and insulator-superconductor transitions, (bi)polarons, CDWs/charge-order, disordered systems, and so on. Recently we have studied Ba1-xKxBiO3 films using angle-resolved photoemission and resonant inelastic x-ray scattering. These experiments establish the unusual "bond disproportionated" nature of the parent compound and track its evolution into the superconducting doping region. The under- to optimally-doped region of the phase diagram is particularly fascinating. There spectra show a highly dispersive conduction band forming a well-defined Fermi surface, despite a total absence of sharp peaks that would typically accompany weakly interacting quasiparticles. We observe, moreover, two types of pseudogap-like spectral behaviors: The first extends over a broad energy scale and persists above room temperature; the other is set in a narrow region around EF and opens in a well-defined temperature range above Tc. I will discuss how these psuedogaps appear to be linked - namely that they represent the precipitation of ordered bipolaronic insulating regions out of a disordered polaronic liquid. |
Thursday, March 5, 2020 10:24AM - 11:00AM |
R18.00005: Electron-Phonon Coupling and Dilute Superconductivity in SrTiO3 Invited Speaker: Harold Hwang The nature of superconductivity in the dilute semiconductor SrTiO3 has remained an open question for over half a century. Thin film heterostructures provide new opportunities to examine SrTiO3 superconductivity using a newly developed method for engineering band alignments at oxide interfaces to access the electronic structure of Nb-doped SrTiO3 using high-resolution tunneling spectroscopy. While we observe strong coupling to the highest-energy longitudinal optic phonon consistent with λ ~ 1, the superconducting gap is found to be in the weak-coupling limit of BCS theory, i.e. λBCS~ 0.1. This discrepancy arises in the context of an unusual anti-adiabatic condition for superconductivity in SrTiO3 (Fermi energy < Debye energy), which we find precisely bounds the superconducting dome. |
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