Possibility of BCS-BEC crossover in is-type organic superconductors

The realization of BCS-BEC crossover in superconductors, which smoothly connects Bardeen-CooperSchrieffer (BCS) theory with Bose-Einstein condensation (BEC) in fermion systems, is an intriguing recent topic in strongly correlated electron systems. The organic superconductor kappa-(BEDT-TTF)4Hg2.89Br8 (kappa-HgBr) under pressure is one of the leading candidates, owing to its unique metallic spin-liquid nature and tunable electron correlation. We theoretically investigate the extended Hubbard model for kappa-HgBr and discuss the possibility of the BCS-BEC crossover by systematically calculating superconducting correlation function, coherence length, superfluid weight, and chemical potential. Our findings show that the BCS-BEC crossover can be observed when competing phases, such as Mott insulators and charge and/or spin orders, are suppressed by appropriate hole doping. kappa-HgBr is just the case because both the Mott insulating phase and magnetic orders are absent due to its nonstoichiometric Hg composition and geometrical frustration. We further propose that other kappa-type organic superconductors could serve as potential candidates of the BCS-BEC crossover if their band fillings and degree of geometrical frustration are systematically tuned.