Phase diagram of $\beta'$-BEDT-TTF salts

We present theoretical studies on the phase diagram of layered organic charge transfer salts, $\beta'$-(BEDT-TTF)$_2$ICl$_2$ and $\beta'$-(BEDT-TTF)$_2$AuCl$_2$. The former shows the highest superconducting transition temperature ($T_{\rm c} $14.2~K under a high hydrostatic pressure) among the organic superconductors. We study an effective model using the fluctuation-exchange (FLEX) approximation based on the results of the first-principles calculations in DFT/GGA under applied pressures. In the obtained phase diagram of $\beta'$-(BEDT-TTF) $_2$ICl$_2$, the superconductivity with $d_{xy}$-like symmetry is realized next to the antiferromagnetic phase. The calculated $T_{\rm c}$ quantitatively coincides well with the experimental one.