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 <1, 1> directions, considering application of this model to organic kappa-(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 chi(q, omega), 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 and of xy-type for t'/t > 1.