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

Electronic structures and states of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ $(x=0,$ 0.35, 0.5, and 0.75) with the same lattice parameters as superconducting ${\mathrm{Na}}_x{\mathrm{CoO}}_2\cdot m{\mathrm{H}}_2\mathrm{O}$ 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 $a_{1g}$ character. The effective on-site Coulomb interaction $U_{\mathrm{dd}}=4\mathrm{}\mathrm{eV}$ for $x=0;$ the $\mathrm{pd}$ hybridizations $t_{\mathrm{pd}}^{\sigma }$ and $t_{\mathrm{pd}}^{\pi }$ are $-1.40$ and 0.70 eV, respectively; the density of states at $E_F$ for $x=0.35\mathrm{}$ is about 1.0 states/eV in the local-density approximation $\left(\mathrm{LDA}\right)+U$ scheme, much smaller than 6–7 states/eV in the LDA scheme. The role of the electronic correlation on the superconductivity in ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_2\cdot m{\mathrm{H}}_2\mathrm{O}$ is discussed.