Two-Gap Superconductivity and Decisive Role of Rare-Earth d Electrons in Infinite-Layer Nickelates

Van Hove Charge Density Waves on Hexagonal Lattices and Kagome Superconductors
October 25, 2022

Monday, October 31, 2022 from 2:30-3:30 p.m.
Location: 325 Physics South Hall & Zoom
Speaker: Zhenglu Li, University of California at Berkeley; Lawrence Berkeley National Laboratory

Abstract: The discovery of superconductivity in infinite-layer nickelates, with transition temperature (Tc) up to 23 K, provides an exciting new avenue to study correlated electrons and emergent phases. Superconductivity in the nickelates has been mostly perceived to be unconventional and originated from the Ni d-electrons due to its analog to cuprate superconductors. The conventional mechanism for superconductivity – phonon-mediated pairing – was presumably ruled out because density functional theory (DFT) calculations reported a very weak electron-phonon coupling in the nickelates. In this talk, by including electron self-energy effects on the electronic structure and electron-phonon coupling (with ab initio GW calculations), I will show that infinite-layer Nd0.8Sr0.2NiO2 is a dominantly two-gap phonon-mediated superconductor. We find electron correlations alter the character of its multi-band Fermi surface and also strongly enhance the electron-phonon coupling, leading to a large Tc in agreement with experiment. The computed electron-phonon coupling constant λ is enhanced by an unprecedented factor of 5.5 as compared to DFT. Solutions of the anisotropic Eliashberg equations yield two dominant s-wave gaps – a large gap on states of rare-earth Nd d-electron and interstitial orbital characters but a small gap on those of transition-metal Ni d-electron character. The superconducting quasiparticle density of states prominently reflects the two-gap nature and explains well tunneling experiments. Our results demonstrate that the phonon mechanism accounts for superconductivity in the nickelates, revealing an unforeseen two-gap s-wave nature as well as providing new insights to demystify the similarities and distinctions between the nickelate and cuprate superconductors.

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