Spin-triplet superconductivity induced by charge fluctuations in extended Hubbard model

The pairing symmetry in the electron mechanism for superconductivity is explored when charge fluctuations coexist with spin fluctuations. The extended Hubbard model is adopted to obtain, with the fluctuation exchange approximation, a phase diagram against the on-site Coulomb repulsion U and the off-site repulsion V for a square lattice with a second-neighbor hopping t′. We have observed the following. (i) For large U(> 9), a triplet superconductivity with sin(k_x + k_y) symmetry can appear just below the charge density wave phase. The pairing is degenerate with sin(k_x - k_y), so a chiral (k_x + k_y) + i sin(k_x - k_y) that breaks the time-reversal symmetry should result, which is a candidate for the gap function on the γ band of Sr₂RuO₄ and is consistent with a recent measurement of the specific heat. (ii) By systematically deforming the Fermi surface with varied t′, the region most favored by the triplet is identified to be the region where the Fermi surface traverses the van Hove singularity with the charge susceptibility strongly enhanced.