Electronic correlations in Lix̳ CoO₂

Abstract

The purpose of this thesis is further the understanding of the electronic properties of LixCoO2 using density functional theory (DFT) and the dynamical mean-field theory (DMFT). Three main problems are addressed. First, the influence of hybridization among the eg and oxygen orbitals is studied using DFT and a modified Hubbard model which is solved within DMFT. It has long been known that doping holes into the t2g bands are accompanied by a rehybridization which causes electron density to be added to the eg states and hole density to the oxygen states. This so-called rehybridization mechanism has been demonstrated to be a competition between the hybridization, which prefers to occupy the eg orbitals, and the Co on-site coulomb repulsion, which prefers to have the eg orbitals empty to avoid the strong coulomb interaction. It is also shown that eg-oxygen hybridization effectively screens the low energy t2g excitations, which has implications for the low energy Hamiltonian corresponding to hydrated Nal/3CoO2. Second, the hereto anomalous first-order metal-insulator transition in LixCoO2 (0.75 < x < 0.95) is identified as a Mott transition of impurity states. DFT supercell calculations indicate that for dilute Li vacancy concentrations (ie x > 0.95), the vacancy potential binds its hole and forms an impurity state which leads to a Mott insulator. We argue that the first-order transition is due to a decomposition of the impurity band, and is perhaps the only known example of a first-order Mott transition in a doped semiconductor. Third, LiCoO2 possesses a high energy photoemission satellite which hereto could not be predicted by any first-principles method. LDA+DMFT solved within multi-band iterated perturbation theory successfully predicts the satellite.