Spin Fluctuation-Induced Superconductivity in Organic Compounds

Spin fluctuation-induced superconductivity in two-dimensional organic compounds such as κ-(ET)_2-X is investigated by using a simplified dimer Hubbard model with right-angled isosceles triangular lattice (transfer matrices -τ, -τ^). The dynamical susceptiblity and the self-energy are calculated self-consistently within the fluctuation exchange approximation and the value for T_c as obtained by solving the linearized Eliashberg-type equations is in good agreement with experiment. The pairing symmetry is of dx^2-y^2 type. The calculated (U/τ)-dependence of T_c compares qualitatively well with the observed pressure dependence of T_c. Varying the value for τ^/τ from 0 to 1 we interpolate between the square lattice and the regular triangular lattice and find firstly that values of T_c for κ-(ET)_2-X and cuprates scale well and secondly that T_c tends to decrease with increasing τ^/τ and no superconductivity is found for τ^/τ=1, the regular triangular lattice.