Polarization dependence of x-ray absorption spectra ofNaxCoO2: Electronic structure from cluster calculations

Abstract

In order to shed light on the electronic structure of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, and motivated by recent $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}L$-edge x-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a ${\mathrm{CoO}}_{6}$ cluster including all interactions between $3d$ orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the ${\mathrm{O}}^{\ensuremath{-}2}$ oxidation state, and ${\mathrm{Co}}^{+3}$ or ${\mathrm{Co}}^{+4}$. Then, all excited states obtained by promotion of a $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ electron to a $3d$ electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large $\mathrm{Co}\text{\ensuremath{-}}\mathrm{O}$ covalency and cubic crystal field effects, that result in low spin $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}3d$ configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.