A model for the doping dependence of the superconducting transition temperature in cobaltates

Some properties of hydrated cobalt oxides NaxCoO2⋅yH2O in the superconductor phase are studied using an extended Hubbard model on a triangular lattice. We derive the superconductive ordering temperatures for singlet and triplet symmetries by means of the Green’s functions of the system, calculated within a combined mean field and Hubbard I type approximation. It is found that both symmetries are possible. The coexistence of d+id′ and p+ip′ pairing, in a certain doping region, is observed, but the s-extended pairing seems to be the more stable. The critical temperatures for superconductivity are calculated for both symmetries and compared to experimental results.