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

Abstract

Spin fluctuation-induced superconductivity in quasi-two dimensional organic compounds, -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 -, -´), extending a previous work above T_c. An energy gap of A₂ or (x²-y²)-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/T₁ shows a T³ 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/ against ´/ plane.