Perturbation theory on the transition temperature and electronic properties of organic superconductor

We study the superconducting transition temperature and the electronic properties of the metallic phase of K-type (BEDT-TTF)(2)X which shows unconventional properties ill experiments, on the basis of the third order perturbation theory for a simple effective Hubbard model of a nearly triangular lattice. Appropriate transition temperatures and d(x2)-(y2) Symmetry of the gap function are obtained in good agreement with experimental results. T 58 also calculate the transition temperature by the fluctuation-exchange approximation (FLEX) in order to compare the two approaches; FLEX gives higher transition temperatures rather than the perturbation approach. However, it is also found that the vertex corrections, which are ignored in FLEX, have a crucial effect on T, for strongly frustrated systems. The density of states and the normal self-energy calculated in this perturbation scheme show the nature of the conventional Fermi liquid near the Mott-insulator. Thus, our perturbation approach is applicable to the conventional metallic phase of this compound! while it cannot explain the (pseudo-)spin gap phenomenon which signals the non-Fermi liquid.