Perturbation theory of spin-triplet superconductivity for Sr2RuO4

We discuss the possibility of spin-triplet superconductivity within the third order perturbation theory with respect to on-site Coulomb repulsion U, Critical temperature T-c, for spin-triplet pairing state is calculated in a single-band two-dimensional Hubbard model and relatively high T-c, is obtained for moderately large U. The present situation considered here is particularly intended for the main branch gamma in Sr2RuO4?, According to the calculation, third order vertex correction terms, which are not direct contribution from spin fluctuation, are important, while the bare susceptibility chi (0)(q) need not always have a prominent peak at q = 0 for the spin-triplet pairing state. The picture that strong ferromagnetic spin fluctuations mainly induce the spin-triplet superconductivity in Sr2RuO4 may not be appropriate, and such momentum dependence of renormalized effective interaction between quasi-particles as is not sufficiently taken into account in spin fluctuation mediated interaction is essential for realizing the spin-triplet pairing.