Phase competition and superconductivity in κ-(BEDT-TTF)₂X: Importance of intermolecular Coulomb interactions

We theoretically study the competition among different electronic phases in molecular conductors κ-(BEDT-TTF)₂X. The ground-state properties of a 3/4-filled extended Hubbard model with the κ-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-s + d_{x2 - y2}"- wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.