Ferromagnetic Fluctuation and Possible Triplet Superconductivity inNaxCoO2·yH2O: Fluctuation-Exchange Study of the Multiorbital Hubbard Model

Spin and charge fluctuations and superconductivity in ${\mathrm{Na}}_x{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$ 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 ${\mathrm{Na}}_x{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$, 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.