Superconductivity and a Mott transition in a Hubbard model on an anisotropic triangular lattice

With a mind to a Mott transition and superconductivity (SC) arising in κ-(BEDT-TTF)2X, we study a half-filled-band Hubbard model on an anisotropic triangular lattice (t in two bond directions and t′ in the other). Using an optimization variational Monte Carlo method, we find that with doublon–holon binding factors in the trial functions, a first-order Mott transition takes place at U = Uc approximately of the band width. For a moderate frustration (t′/t = 0.4), the d-wave superconducting correlation Pd is sizably enhanced just under Uc/t, whereas for a strong frustration (t′/t = 0.8), Pd is hardly enhanced for any value of U/t. Since the behavior of Pd is closely connected to that of the spin structure factor S(π, π), the SC in this model must be induced by the antiferromagnetic correlation.