Origin of superconductivity inβ′−(BEDT−TTF)2ICl2under high pressure and inβ−(BEDT−TTF)2Xat atmospheric pressure

We present a theoretical study on the superconductivity in ${\beta }^{\prime }-(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{ICl}}_2$ at $T_{\mathrm{c}}=14.2\mathrm{}\mathrm{K}$ under high hydrostatic pressure, which was recently found by Taniguchi et al. and is the highest recorded for organic superconductors. Its electronic structure is well expressed by the anisotropic triangular lattice Hubbard model at half filling. In the present paper, we study this effective model by using the fluctuation-exchange approximation. In the obtained phase diagram, the superconductivity with $d_{x^2\mathrm{-}{y}^2}$ like symmetry is realized next to the antiferromagnetic insulating phase, as a result of the one-dimensional–two-dimensional crossover driven by the pressure. The obtained maximum $T_{\mathrm{c}}$ is 16–18 K. In addition, the superconductivity in $\beta -(\mathrm{BEDT}-\mathrm{TTF}{)}_{2}X$ is also understood in the same framework.