Unveiling the superconducting mechanism of Ba$_{0.51}$K$_{0.49}$BiO$_3$

TL;DR

This study uses advanced spectroscopy and theoretical calculations to reveal that long-range Coulomb interactions significantly influence the bandwidth and electron-phonon coupling in Ba$_{0.51}$K$_{0.49}$BiO$_3$, clarifying its superconducting mechanism.

Contribution

It provides the first direct evidence of bandwidth expansion due to Fock exchange in bismuthate superconductors, confirming their BCS nature and highlighting the role of Coulomb interactions.

Findings

Bandwidth expanded by 34% due to Coulomb interactions

Isotropic superconducting gap with 2Δ₀/kBTc ≈ 3.51

Strong electron-phonon coupling with λ ≈ 1.3

Abstract

Bismuthates were the first family of oxide high-temperature superconductors, exhibiting superconducting transition temperatures (Tc) up to 32K, but the superconducting mechanism remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba$_{0.51}$K$_{0.49}$BiO$_3$ reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions --- the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics. Furthermore, we observe an isotropic superconducting gap with 2\Delta$_0$/k$_B$ T$_c$ = 3.51 $\pm$ 0.05, and strong electron-phonon interactions with a coupling constant \lambda$\sim$ 1.3 $\pm$ 0.2. These findings solve a long-standing mystery --- Ba$_{0.51}$K$_{0.49}$BiO$_3$ is an extraordinary Bardeen-Cooper-Schrieffer (BCS) superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high Tc. Such effects will also be critical for finding new superconductors.