Resonant photoemission study ofBa1−xKxBiO3single crystals

Abstract

Single crystals of superconducting ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_{3}$ and semiconducting ${\mathrm{Ba}}_{0.9}{\mathrm{K}}_{0.1}{\mathrm{BiO}}_{3}$ have been studied by resonant photoemission. In the photon energy range 14--25 eV, a valence-band feature with a binding energy around 2.6 eV becomes enhanced at the photon energies 16, 19, and 22.5 eV. In order to identify this possibly resonating feature, valence-band photoemission spectra were also measured at photon energies (90--120 eV) around the excitation threshold of the Ba $4d$ core level. A comparison between the results obtained at these different photon energy regions suggests that the valence-band enhancements at 16 and 19 eV can be assigned to resonating Ba states, while the comparatively weaker enhancement at 22.5 eV is consistent with both a Ba and/or an O photoemission resonance. The results also indicate Ba states close to the Fermi level.