Ferromagnetic fluctuation and possible triplet superconductivity in NaxCoO2•yH2O:: Fluctuation-exchange study of the multiorbital Hubbard model -: art. no. 147005

Spin and charge fluctuations and superconductivity in Na(x)CoO(2)center dot yH(2)O are studied based on a multiorbital Hubbard model. Tight-binding parameters are determined to reproduce the results of band calculations. By applying the fluctuation-exchange approximation, we show that the Hund's-rule coupling between the Co t(2g) orbitals causes ferromagnetic (FM) spin fluctuation. Triplet f(y(y)(2)-3x(2))-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in Na(x)CoO(2)center dot yH(2)O, the Co t(2g) orbitals and interorbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor in which the orbital degrees of freedom play substantial roles.