Momentum-Resolved Electronic Structure of the High-$T_{c}$   Superconductor Parent Compound BaBiO$_{3}$
  We investigate the band structure of BaBiO$_{3}$, an insulating parent compound of doped high-$T_{c}$ superconductors, using \emph{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 BiO$_{6}$ breathing distortions. Though the distortions are often thought to coincide with Bi$^{3+}$/Bi$^{5+}$ 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. 
N. C. Plumb, D. J. Gawryluk, Y. Wang, Z. Risti\'c, J. Park, B. Q. Lv,   Z. Wang, C. E. Matt, N. Xu, T. Shang, K. Conder, J. ...
cond-mat.supr-con