Electronic structure of Li_xCoO₂ studied by photoemission spectroscopy and unrestricted Hartree-Fock calculations

We report on an electronic structural study of Li_xCoO ₂ single crystals (x=0.99, 0.71, 0.66, and 0.46) which have hole-doped CoO₂ triangular lattices. The valence-band photoemission spectra show that the Fermi level is located near the top of the Co3d t _2g bands and that, by the reduction in x, the Co3d 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^′ components. The electronic structure parameters such as the charge-transfer energy Δ are obtained by the cluster-model analysis of the Co2p 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 Li_xCoO ₂.