摘要
Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of Na x CoO 2 have predominantly a 1g symmetry with significant O 2p character. In contrast to the prediction of band theory, doping-dependent O 1s x-ray absorption shows a large transfer of spectral weight, providing spectral evidence for strong electron correlations of the layered cobaltates. We also found that Na x CoO 2 exhibits a charge-transfer electronic character rather than a Mott-Hubbard character. © 2005 The American Physical Society.