Spin Fluctuation-Induced Superconductivity in \kappa-BEDT-TTF Compounds

TL;DR

This paper investigates spin fluctuation-induced superconductivity in ppa-BEDT-TTF organic compounds using a FLEX approximation on a Hubbard model, revealing gap development, resonance peaks, and T^3 relaxation behavior consistent with experiments.

Contribution

It extends previous work by analyzing the temperature dependence of the energy gap, dynamical susceptibilities, and relaxation rates, providing new insights into the superconducting mechanism in these materials.

Findings

Energy gap develops below T_c more rapidly than BCS prediction.

Sharp resonance peaks in dynamical susceptibilities resemble cuprate superconductors.

Nuclear spin-lattice relaxation rate 1/T_1 exhibits T^3 behavior below T_c.

Abstract

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^\prime), 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 behavior below T_c in accordance with experiment. Estimated values of 1/T_1 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 vs. \tau^\prime/\tau plane.