Charge ordered metal and pressure-induced superconductivity in the two-dimensional organic conductor β^′′-(DODHT) ₂PF₆

The superconductivity in a quasi-two-dimensional organic conductor, β^′′-(DODHT)₂PF₆ salt, which is expected to appear after the melting of a charge ordering (CO) state, has been examined using an extended Hubbard model with anisotropy for both the transfer energies and the nearest-neighbor repulsive interactions between the DODHT molecules. The fluctuation is treated by the random phase approximation based on mean-field calculation of the CO. When pressure is applied, an insulating state with CO (COI) changes into a metallic state with weakened CO (COM) at an intermediate pressure, and a normal state without CO emerges at higher pressures. For the COM state at intermediate pressure, the COI state and the normal state also exist as metastable states, and their free energies are nearly the same within a narrow energy range of 10^-3eV. Thus these three states may coexist with each other by forming a phase separation at finite temperature. We find that the spin fluctuation around such a mean-field COM state gives rise to a superconducting state with a full gap. Further, we suggest that d-wave superconductivity mediated by the charge fluctuation occurs in the normal state with a quasi-one-dimensional Fermi surface when the nearest-neighbor interaction becomes as large as the on-site interaction.