Doping effects on the electronic and structural properties ofCoO2: AnLSDA+Ustudy

A systematic local spin density approximation $\left(\mathrm{LSDA}\right)+\mathrm{U}$ study of doping effects on the electronic and structural properties of single layer ${\mathrm{CoO}}_2$ is presented. Undoped ${\mathrm{CoO}}_2$ is a charge transfer insulator within $\mathrm{LSDA}+\mathrm{U}$ and a metal with a high density of states (DOS) at the Fermi level within LSDA. ${{(\mathrm{CoO}}_2)}^{1.0-}$, on the other hand, is a band insulator with a gap of $2.2\mathrm{eV}$. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated $\mathrm{Co}$ $d$ electrons, the calculated electronic structure of ${{(\mathrm{CoO}}_2)}^{x-}$ depends sensitively on the doping level $x$. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of the $E_g$ phonon at doping $x\sim 0.25$ seems to indicate a strong electron-phonon coupling in this system. Possible intermediate spin states of $\mathrm{Co}$ ions, $\mathrm{Na}$ ordering, as well as magnetic and charge orderings in this system are also discussed.