Superconductivity in Organic Compounds with Pseudo-Triangular Lattice (Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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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 к-(BEDT-TTF)_2X 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 x(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 and of xy-type for t'/t > 1.
社団法人日本物理学会の論文
2004-04-15