Hard-X-ray photoelectron spectroscopy of NaxCoO2.yH2O

We study the bulk electronic structure of NaxCoO2.yH2O using Hard X-ray (HX, hν = 5.95 KeV) synchrotron photoelectron spectroscopy (PES). The Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy Udd∼3-5.5 eV. Photon-dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band structure calculations. We discuss the importance of HX-PES for studying correlated transition metal oxides.