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 γ in Sr 2 RuO 4 . According to the calculation, third order vertex correction terms, which are not direct contribution from spin fluctuation, are important, while the bare susceptibility χ 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 Sr 2 RuO 4 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.