Polarization dependence of x-ray absorption spectra in Na_xCoO_2

TL;DR

This study uses theoretical calculations to analyze the polarization dependence of x-ray absorption spectra in Na_xCoO_2, revealing insights into its electronic structure, covalency, and the influence of interatomic hopping.

Contribution

It provides a detailed theoretical analysis of polarized XAS spectra in Na_xCoO_2, highlighting the importance of covalency and hopping effects on electronic structure.

Findings

Good fit with experimental spectra indicating strong Co-O covalency

Low spin Co 3d configurations due to crystal field effects

Effective hopping influences ground state symmetry

Abstract

In order to shed light on the electronic structure of Na_xCoO_2, and motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a 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 O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states obtained by promotion of a Co 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 Co-O covalency and cubic crystal field effects, that result in low spin Co 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.