Electronic structure ofLixCoO2studied by photoemission spectroscopy and unrestricted Hartree-Fock calculations

Abstract

We report on an electronic structural study of ${\text{Li}}_{x}{\text{CoO}}_{2}$ single crystals ($x=0.99$, 0.71, 0.66, and 0.46) which have hole-doped ${\text{CoO}}_{2}$ triangular lattices. The valence-band photoemission spectra show that the Fermi level is located near the top of the $\text{Co}\text{ }3d$ ${t}_{2g}$ bands and that, by the reduction in $x$, the $\text{Co}\text{ }3d$ ${t}_{2g}$ peak is shifted to the lower binding-energy side. This energy shift is consistent with the chemical-potential shift by the hole doping to the ${t}_{2g}$ bands. The fine structures near the Fermi level indicate the splitting of the ${t}_{2g}$ bands into the ${a}_{1g}$ and ${e}_{g}^{\ensuremath{'}}$ components. The electronic structure parameters such as the charge-transfer energy $\ensuremath{\Delta}$ are obtained by the cluster-model analysis of the $\text{Co}\text{ }2p$ core-level spectra. The unrestricted Hartree-Fock calculation using the obtained parameter values predicts that the doped holes are accommodated by the ${a}_{1g}$ band up to the doping level $x$ of 0.46 which is consistent with the observation in the valence-band spectra. However, the valence-band spectra cannot be reproduced by the unrestricted Hartree-Fock calculation indicating that the correlation effect from the electron-electron and electron-phonon interactions is substantial in ${\text{Li}}_{x}{\text{CoO}}_{2}$.