Electronic structure and states in two-dimensional triangular cobalt oxides: The role of electronic correlation

Electronic structures and states of NaₓCoO₂ $(x=0,$ 0.35, 0.5, and 0.75) with the same lattice parameters as superconducting NaₓCoO₂·mH₂O have been studied by utilizing the full-potential linear muffin-tin orbital methods, from which some electronic interaction parameters are obtained. Most of the carriers in doped compounds have a1g character. The effective on-site Coulomb interaction Udd=4eV for $x=0;$ the pd hybridizations tpd^σ and tpd^π are -1.40 and 0.70 eV, respectively; the density of states at EF for $x=0.35$ is about 1.0 states/eV in the local-density approximation (LDA)+U scheme, much smaller than 6--7 states/eV in the LDA scheme. The role of the electronic correlation on the superconductivity in Na0.35CoO₂·mH₂O is discussed.