Alternatives for Fermi level location in cubic bismuthates deduced from ACAR spectra

On the basis of a simple tight-binding model for the band structure of superconducting perovskites we address some open questions related to the Fermi level location in Ba1−xKxBiO3invoked by the ACAR study of this material. The latter reports a roughly cubic Fermi surface centered around the Γ point of the Brillouin zone. Two basically different alternatives explaining the observed shape and location are discussed. They reflect, respectively, the two possible effects of potassium doping of BaBiO3: the smaller positive valency of K1+(compared to Ba2+) creates (1) a hole in the upper metallic Bi6sband, or (2) a hole in the narrow O2pσbands, as in the case of hole doping of layered cuprates. The corresponding ACAR spectra are calculated and compared to the experiment. The importance of ACAR spectra of halogen-containing layered cuprates is analyzed as well. A comparison of such spectra with those related to cubic materials should help elucidate the nature, in general, of conductivity in perovskites.