Theoretical analysis on the multi-orbital model for the superconductivity in Na_xCoO₂

We analyze the multi-orbital model which describes the electronic structure of recently discovered superconductor NaxCoO2·yH2O. The instability to the unconventional superconductivity is investigated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region, basically owing to the ferromagnetic spin fluctuation. In a small part of the parameter region, the d-wave superconductivity is also stabilized. We point out that the orbital degree of freedom plays an essential role for the superconductivity.