Local inversion-symmetry breaking in a bismuthate high-T superconductor.

The doped perovskite BaBiO exhibits a maximum superconducting transition temperature (T) of 34 K and was the first high-T 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 BaKBiO across its insulator-metal boundary.