Momentum-Resolved Electronic Structure of the High-T_c Superconductor Parent Compound BaBiO₃

We investigate the band structure of BaBiO₃, an insulating parent compound of doped high-T_c superconductors, using in situ angle-resolved photoemission spectroscopy on thin films. The data compare favorably overall with density functional theory calculations within the local density approximation, demonstrating that electron correlations are weak. The bands exhibit Brillouin zone folding consistent with known BiO₆ breathing distortions. Though the distortions are often thought to coincide with Bi³⁺/Bi⁵⁺ charge ordering, core level spectra show that bismuth is monovalent. We further demonstrate that the bands closest to the Fermi level are primarily oxygen derived, while the bismuth 6s states mostly contribute to dispersive bands at deeper binding energy. The results support a model of Bi-O charge transfer in which hole pairs are localized on combinations of the O 2p orbitals.