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

Abstract

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, hasbeen examined using an extended Hubbard model with anisotropy for both the transferenergies and the nearest-neighbor repulsive interactions between the DODHT molecules.The fluctuation is treated by the random phase approximation based on mean-fieldcalculation 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 anormal state without CO emerges at higher pressures. For the COM state atintermediate pressure, the COI state and the normal state also exist as metastablestates, and their free energies are nearly the same within a narrow energy range of10−3 eV. Thus these three states may coexist with each other by forming a phase separation atfinite temperature. We find that the spin fluctuation around such a mean-field COM stategives rise to a superconducting state with a full gap. Further, we suggest that d-wavesuperconductivity mediated by the charge fluctuation occurs in the normal state with aquasi-one-dimensional Fermi surface when the nearest-neighbor interaction becomes aslarge as the on-site interaction.