Photoemission and x-ray absorption study of superconducting and semiconducting Ba1-xKxBiO3 single crystals.

Abstract

Semiconducting ${\mathrm{Ba}}_{0.9}$${\mathrm{K}}_{0.1}$Bi${\mathrm{O}}_{3}$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_{3}$ single crystals cleaved in situ have been studied by core level and valence band photoelectron spectroscopy and O $K$ edge x-ray absorption spectroscopy. It was found that the general shape of the valence band spectrum agrees with the shape predicted by band structure calculations, but the intensity near the Fermi level was lower in the experimental spectrum as compared to the calculated. The O $K$ edge spectra showed that the metallic phase is not related to the presence of doping inducted O $2p$ holes. This property of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ shows that the semiconductor-metal transition of this system is of a different nature than that of the hole doped cuprate high-${T}_{c}$ superconductors. The core level photoemission spectra of the cations showed a small asymmetry for ${\mathrm{Ba}}_{0.9}$${\mathrm{K}}_{0.1}$Bi${\mathrm{O}}_{3}$. Corresponding spectra for ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_{3}$ showed a larger asymmetry resulting in a resolved high binding energy shoulder in the Bi $4f$ spectrum. The origin of this feature is discussed.