Superconductivity in Organic Compounds with Pseudo-Triangular Lattice

We study spin fluctuation (SF) mediated superconductivity (SC) in a half-filled square lattice Hubbard model with the transfer matrices - t between nearest neighbor sites and - t ' between a half of next nearest neighbor sites neighboring along only one of the directions, considering application of this model to organic κ-(BEDT-TTF) 2 X compounds. Varying the t '/ t value from 0 to 1, one can interpolate between a square and an equilateral triangular lattice, the latter giving frustration to antiferromagnetically (AF) coupled spin systems. Within the fluctuation exchange (FLEX) approximation, we calculate χ( q ,ω), T c and the SC order parameter for various model parameter values and find that both AF and SC are suppressed as one approaches the frustration geometry or |( t '/ t )-1| →0. The SC phase, however, extends beyond the AF phase boundary fairly close to t '/ t = 1 for realistic U / t values. The order parameter is of x 2 - y 2 -type for t '/ t 1.