Electronic States in Two-Dimensional Triangular Cobalt Oxides: Role of Electronic Correlation

We obtain the electronic states and structures of two-dimensional cobalt oxides, Na${}_x$CoO${}_2$ (x=0, 0.35, 0.5 and 0.75) by utilizing the full-potential linear muffin-tin orbitals (FP-LMTO) methods, from which some essential electronic interaction parameters are estimated: the bare on-site Coulomb interaction of cobalt U${}_{dd}$=7.5 eV renormalizes to 5 eV for x=0.35, the $pd$ hybridizations t${}_{pd\sigma }$ and t${}_{pd\pi }$ are -1.40 and 0.70 eV, respectively. The density of states at E${}_F$ decreases from 6-7 states/eV in the local density approximation (LDA) to about 1.0 states/eV in the LDA+U scheme. The role of the intercalation of water molecules and the microscopic mechanism of the superconductivity in Na${}_{0.35}$CoO${}_2$$\cdot$mH${}_2$O is discussed.