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 kappa-(BEDT-TTF)(2)X, 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 = U-c approximately of the band width. For a moderate frustration (t'/t = 0.4), the d-wave superconducting correlation P-d is sizably enhanced just under U-c/t, whereas for a strong frustration (t'/t = 0-8), P-d is hardly enhanced for any value of U/t. Since the behavior of P-d is closely connected to that of the spin structure factor S(pi, pi), the SC in this model must be induced by the antiferromagnetic correlation. (c) 2006 Elsevier B.V. All rights reserved.