Local Inversion-Symmetry Breaking in a Bismuthate High-Tc Superconductor*

The doped perovskite BaBiO₃ was the first high-T_c oxide to be discovered, with the highest attainable T_c values close to 35 K. Yet pivotal questions regarding the nature of both the metallic and superconducting states remain unresolved. Although it has generally been 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. To shed new light on these questions, we have performed an extensive diffuse x-ray scattering study of the local structure of Ba_1-xK_xBiO₃ across its insulator-metal boundary, together with 3D-ΔPDF analysis and Monte Carlo modeling [1]. 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, with far-reaching implications for the electronic physics. 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. 

[1] S. Griffitt et al., Nat. Commun. 14, 845 (2023)