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

A half-filled-band Hubbard model on an anisotropic triangular lattice (t in two bond directions and t' in the other) is studied using an optimization variational Monte Carlo method, to consider a Mott transition and superconductivity arising in kappa-(BEDT-TTF)(2)X. Adopting wave functions with doublon-holon binding factors, we reveal that a first-order Mott (conductor-to-nonmagnetic insulator) transition takes place at U = U-c, approximately of the bandwidth, for a wide t'/t range. This transition is not directly connected to magnetism. Robust d-wave superconductivity appears in a restricted parameter range: immediately below U-c and the moderate strength of frustration (0.4 less than or similar to t'/t less than or similar to 0.7), where short-range anti ferromagnetic correlation sufficiently develops but does not come to a long-range order. The relevance to experiments is also discussed.