Ferromagnetic fluctuation and possible triplet superconductivity due to the inter-orbital Hund's-rule coupling in Na_0.35CoO_2*1.3H_2O: FLEX study of the multi-orbital Hubbard model

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Abstract
Spin and charge fluctuations and superconductivity in a recently discovered superconductor Na_0.35CoO_2*1.3H_2O are studied based on a multi-orbital Hubbard model. Tight-binding parameters are determined to reproduce the LDA band dispersions with the Fermi surfaces, which consist of a large cylindrical one around $\Gamma$-point and six hole pockets near the K-points. By applying the fluctuation exchange (FLEX) 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_0.35CoO_2*1.3H_2O, Co t_{2g} orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor mediated by inter-orbital-interaction-induced FM fluctuation.

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