Superconductivity mechanism of molecular conductors through control of the dimerization and frustration

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We study experimentally and theoretically the shift of the superconducting transition temperature (T_C) under uniaxial compression in β-type organic superconductors possessing unidirectional molecular stacks. Contrary to the BDA-TTP salts, BEDT-TTF salts shows T_C maximum in both compressions parallel and perpendicular to the molecular stack. T_C calculations are carried out on both the dimerized and nondimerized Hubbard models. We have found that the competition between the spin frustration and the effect induced by the fluctuation causes the nonmonotonic shift of T_C and that the behavior of T_C in BEDT-TTF salts with a stronger dimerization is well reproduced by the dimer model, while that in weakly dimerized BDA-TTP salts is rather well reproduced by the nondimerized model.