Electron-spectroscopy study of the semiconductor-metal transition in La1-xSrxCoO3.

Abstract

A study of the semiconductor-metal transition in the series ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CoO}}_{3}$, x=0.0--0.4, using electron-spectroscopy techniques is presented. The results show that the ground state of ${\mathrm{LaCoO}}_{3}$ has strongly mixed character. From the experiments we estimate the on-site Coulomb correlation energies ${\mathit{U}}_{\mathit{d}\mathit{d}}$ and ${\mathit{U}}_{\mathit{p}\mathit{p}}$ to be 3.4 and 6.7 eV, respectively, while the (${\mathit{pd}}_{\mathrm{\ensuremath{\sigma}}}$) interaction strength is 2.2 eV. A comparison of the experiment with the results of a model many-body cluster calculation for the Co 2p spectra indicates a low-spin state of Co in ${\mathrm{LaCoO}}_{3}$. The band gap of ${\mathrm{LaCoO}}_{3}$ is estimated to be about 0.6 eV from ultraviolet photoemission spectroscopy and bremsstrahlung isochromat spectroscopy measurements. The semiconductor-metal transition results from overlap of doped hole states with the valence band for x\ensuremath{\ge}0.2.