A mechanical of spin-triplet superconductivity in Hubbard model on triangular lattice: application to UNi2Al3

We discuss the possibility of spin-triplet superconductivity in a two-dimensional Hubbard model on a triangular lattice within the third-order perturbation theory. When we vary the symmetry in the dispersion of the bare energy band from D2 to D6, spin-singlet superconductivity in the D2-symmetric system is suppressed and we obtain spin-triplet superconductivity in near the D6-symmetric system. In this case, it is found that the vertex terms, which are not included in the interaction mediated by the spin fluctuation, are essential for realizing the spin-triplet pairing. We point out the possibility that obtained results correspond to the difference between the superconductivity of UNi2Al3 and that of UPd2Al3. (author)