Temperature-dependent photoemission and x-ray absorption studies of the metal-insulator transition inBi1−xLaxNiO3

Abstract

Perovskite-type $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for $0.05\ensuremath{\le}x\ensuremath{\le}0.1$ a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Ni}{\mathrm{O}}_{3}$ to investigate how the electronic structure changes across the MIT. From the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}2p$ XAS spectra of $x=0.05$, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for $x=0.05$ at a low temperature, whereas for $x=0.1$ and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ band and that the ``insulating'' phase below the MIT is indeed a mixture of insulating and metallic regions.