Spin fluctuation-induced superconductivity in κ-BEDT-TTF compounds

Spin fluctuation-induced superconductivity in quasi-two dimensional organic compounds, kappa-BEDT-TTF salts, is investigated within a fluctuation exchange (FLEX) approximation using a half-filled Hubbard model with a right-angled isosceles triangular lattice (transfer matrices -tau, -tau'), extending a previous work above T-c. An energy gap of A(2) or (x(2) - y(2))-type develops with decreasing temperature below T-c more rapidly than in the BCS model. The calculated dynamical susceptibilities enough below T-c show sharp resonance peaks like those in certain cuprates superconductors. The calculated nuclear spin-lattice relaxation rate 1/T-1 shows a T-3 behaviour below T-c in accordance with experiment. Estimated values of 1/T1 are roughly consistent with experimental results. A prediction is made for the doping concentration dependence of T-c and the antiferromagnetic and superconductive instability points are calculated in the U/tau against tau'/tau plane.