Superconductivity in Charge Ordered Organic Conductor –α-(ET)₂I₃ Salt–

We theoretically examine superconductivity in quasi-two-dimensional organic conductor, α-(ET) 2 I 3 salt, which has been found under uniaxial pressure and at temperature below the charge ordering. The interplay of many bands and several repulsive interactions gives rise to novel role of the charge ordering (CO), which leads to the coexistence with the superconducting (SC) state. The hole and electron pockets at Fermi surface, which exist in normal state and at ambient pressure, vanish in the charge ordered insulating state due to the formation of the charge gap. Under pressures, the charge gap disappears and the narrow gap semiconductor (NGS) with CO exhibits the density of state with a sharp peak just below the Fermi energy. Based on the mean field of CO state, charge and spin susceptibilities, and the paring interaction are evaluated by using the random phase approximation. The onset for the SC state is calculated in terms of a linearized Eliashberg equation, where the pairing interaction consists of both charge and spin fluctuations. The NGS results in the exotic SC state, which exhibits an extended s -state with large anisotropy of the gap. It is shown that the SC state is the inter-site pairing and is governed by the inter-site spin fluctuation.