Origin of the Superconductivity in \beta'-(BEDT-TTF)_2ICl_2 under High Pressure and \beta-(BEDT-TTF)_2X at Ambient Pressure

We present a theoretical study on the superconductivity in \beta’-(BEDT-TTF)2ICl_2 at Tc=14.2K under high hydrostatic pressure found by Taniguchi et al., which is the highest record among organic superconductors. Its electronic structure is well expressed by the anisotropic triangular lattice Hubbard model at half filling. In the present work, we study this effective model by using the fluctuation-exchange (FLEX) approximation. In the obtained phase diagram, the superconductivity with d{x^2-y^2} like symmetry is realized next to the antiferromagnetic (AF) insulating phase, as a result of the 1D-2D dimensional crossover driven by the pressure. The obtained maximum Tc is 16-18K. In addition, the superconductivity in \beta-(BEDT-TTF)_2X is also understood in the same framework.