Hard-X-ray photoelectron spectroscopy of Na_xCoO_2·yH_2O

摘要

We study the bulk electronic structure of Na_xCoO_2·yH_2O using Hard X-ray (HX, hv = 5.95 KeV) synchrotron photoelectron spectroscopy (PES). The Co 2p core level spectra show well-separated Co~(3+) and Co~(4+) ions. Cluster calculations suggest low spin Co~(3+) and Co~(4+) character, and a moderate on-site Coulomb correlation energy U_(dd)～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.