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-\(s + d_{x^{2} - y^{2}}\)”-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.