Dominant antiferromagnetism in the Hubbard model on anisotropic triangular lattices

With kappa-(BEDT-TTF)(2)X in mind, we study a half-filled-band Hubbard model on an anisotropic triangular lattice, using an optimization variational Monte Carlo method. As a trial function, we use a state having antiferromagnetic (AF) and d-wave singlet gaps simultaneously, and allow for a band renormalization effect in it. It is found that, at a fairy small value of U/t, a first- order conductor-to-AF insulator transition takes place even for remarkably large strength of frustration. In the optimized insulating states, the staggered magnetization is always robust, even for fully frustrated case. Thereby, a regime of robust superconductor as well as of a nonmagnetic insulator vanishes in this wave function. (c) 2006 Elsevier B.V. All rights reserved.