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In order to shed light on the electronic structure of Na_xCoO_2, andmotivated by recent Co L-edge X-ray absorption spectra (XAS) experiments withpolarized light, we calculate the electronic spectrum of a CoO_6 clusterincluding all interactions between 3d orbitals. We obtain the ground state fortwo electronic occupations in the cluster that correspond nominally to all O inthe O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited statesobtained by promotion of a Co 2p electron to a 3d electron, and thecorresponding matrix elements are calculated. A fit of the observedexperimental spectra is good and points out a large Co-O covalency and cubiccrystal field effects, that result in low spin Co 3d configurations. Ourresults indicate that the effective hopping between different Co atoms plays amajor role in determining the symmetry of the ground state in the lattice.Remaining quantitative discrepancies with the XAS experiments are expected tocome from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure

physical review. b, condensed matter and materials physics

Polarization dependence of x-ray absorption spectra of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>Na</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>CoO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>: Electronic structure from cluster calculations

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