Local inversion-symmetry breaking in a bismuthate high-$T_c$ superconductor

TL;DR

This study reveals nanoscale inversion-symmetry breaking in a high-temperature superconductor, challenging previous assumptions about disproportionation and suggesting new insights into its pairing mechanism and electronic properties.

Contribution

The paper demonstrates the absence of disproportionation and uncovers local inversion-symmetry breaking in BaBiO$_3$, providing new understanding of its structural and electronic behavior.

Findings

No evidence of disproportionation in Ba$_{1-x}$K$_x$BiO$_3$

Nanoscale inversion-symmetry breaking observed

Implications for pairing mechanism and electronic physics

Abstract

The doped perovskite BaBiO$_3$ exhibits a maximum superconducting transition temperature ($T_c$) of 34 K and was the first high-$T_c$ oxide to be discovered, yet pivotal questions regarding the nature of both the metallic and superconducting states remain unresolved. Although it is generally thought that superconductivity in the bismuthates is of the conventional s-wave type, the pairing mechanism is still debated, with strong electron-phonon coupling and bismuth valence or bond disproportionation possibly playing a role. Here we use diffuse x-ray scattering and Monte Carlo modeling to study the local structure of Ba$_{1-x}$K$_x$BiO$_3$ across its insulator-metal boundary. We find no evidence for either long- or short-range disproportionation, which resolves a major conundrum, as disproportionation and the related polaronic effects are likely not relevant for the metallic and superconducting states. Instead, we uncover nanoscale structural correlations that break inversion symmetry, which has far-reaching implications for the electronic physics, including the pairing mechanism. This unexpected finding furthermore establishes that the bismuthates belong to the broader classes of materials with hidden spin-orbit coupling and a tendency towards inversion-breaking displacements.