Excitation spectra ofLaCoO3

Abstract

We study the excitation spectra of ${\mathrm{LaCoO}}_{3}$ using a multiorbital tight-binding model. Making the self-energy correction due to local three-body scattering to the Hartree-Fock (HF) approximation, we calculate the spectral density for the low-spin phase. It is shown that the band gap is strongly reduced from the HF value, and that the intensity is transferred from the lower part to the upper part of the valence band and to the satellite in the spectral density projected onto the Co 3d states. The results are in good agreement with the spectra of photoemission experiments. We also calculate the spectral densities for the intermediate-spin phase and the high-spin phase in relation to the nonmagnetic-to-paramagnetic transition. The peak intensities around the top of the valence band are smaller than that for the low-spin phase. In ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CoO}}_{3}$, the energies for the intermediate-spin and high-spin phases are found to become lower than for the low-spin phase with increasing x, indicating the instability of the low-spin phase. This is consistent with the susceptibility data and the photoemission spectra.