Role of Hybridization inNaxCoO2and the Effect of Hydration

Abstract

Density functional theory is used to understand the electronic properties of ${\mathrm{Na}}_{1/3}{\mathrm{CoO}}_2$ and $\mathrm{N}{\mathrm{a}}_{\mathrm{1}\mathrm{/}\mathrm{3}}\mathrm{Co}{\mathrm{O}}_{\mathrm{2}}({\mathrm{H}}_{\mathrm{2}}\mathrm{O}{)}_{4/3}$. Comparing the charge density of $\mathrm{Co}{\mathrm{O}}_{\mathrm{2}}$ and the Na doped phases indicates that doping does not simply add electrons to the $t_{2g}$ states. In fact, the electron added in the $t_{2g}$ state is dressed by hole density in the $e_g$ state and electron density in the oxygen states via rehybridization. In order to fully understand this phenomenon, a simple extension of the Hubbard Hamiltonian is proposed and solved using the dynamical mean-field theory. This model confirms that the rehybridization is driven by a competition between the on-site Coulomb interaction and the hybridization, and results in an effective screening of the low-energy excitations. Finally, we show that hydration causes the electronic structure to become more two dimensional.