Abstract
Within third-order perturbation theory, we discuss the possibility of spin-tripletand spin-singlet superconductivities in a two-dimensional Hubbard modelon a triangular lattice for some model parameters corresponding to theheavy-fermion superconductors UM2Al3 (M = Pd,Ni)and organic superconductors under uniaxial strain. In the model of theheavy-fermion superconductors UM2Al3 (M = Pd,Ni),when we vary the symmetry in the dispersion of the bare energy band from D2 toD6,spin-singlet superconductivity in the D2-symmetricsystem is suppressed and we obtain spin-triplet superconductivity in the nearly D6-symmetricsystem. We point out the possibility that the results obtainedcorrespond to the difference between the superconductivity of UPd2Al3 and that ofUNi2Al3.In the model of organic superconductors under uniaxial strain, when we increasethe diagonal hopping integral to above 1.2, we obtain the tendency for spin-tripletsuperconductivity to emerge. In all cases mentioned above, it is found that thevertex correction terms, which are not included in the interaction mediated bythe spin fluctuation, are essential for realizing the spin-triplet pairing.
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