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We preform a multi-orbital analysis on the novel superconductivity inNa_{x}CoO_{2} \cdot yH_{2}O. We construct a three-orbital model whichreproduces the band structure expected from the LDA calculation. The effectiveinteraction leading to the pairing is estimated by means of the perturbationtheory. It is shown that the spin triplet superconductivity is stabilized inthe wide parameter region. This is basically owing to the ferromagneticcharacter of spin fluctuation. The p-wave and f-wave superconductivity arenearly degenerate. The former is realized when the Hund's rule coupling islarge, and vice versa. In a part of the parameter space, the d-wavesuperconductivity is also stabilized. We point out that the orbital degeneracyplays an essential role for these results through the wave function ofquasi-particles. The nearly degeneracy of p-wave and f-wave superconductivityis explained by analysing the orbital character of each Fermi surface. Wediscuss the validity of some reduced models. While the single band Hubbardmodel reproducing the Fermi surface is qualitatively inappropriate, we find aneffective two-orbital model appropriate for studying the superconductivity. Weinvestigate the vertex corrections higher than the third order on the basis ofthe two-orbital model. It is shown that the vertex correction induces thescreening effect but does not affect on the qualitative results.Comment: To appear in J. Phys. Soc. Jpn. 74 (2005) No.

Multi-orbital Analysis on the Superconductivity in NaxCoO2·yH2O

Multi-orbital Analysis on the Superconductivity in Na_xCoO₂·yH₂O