Nonmagnetic insulating states near the Mott transitions on lattices with geometrical frustration and implications for κ-(ET)2Cu2(CN)3

We study phase diagrams of the Hubbard model on anisotropic triangular lattices, which also represents a model for kappa-type BEDT-TTF compounds. In contrast with mean-field predictions, path-integral renormalization group calculations show a universal presence of a nonmagnetic insulator sandwiched by an antiferromagnetic insulator and paramagnetic metals. The nonmagnetic phase does not show simple translational symmetry breakings such as flux phases, implying a genuine Mott insulator. We discuss possible relevance to the nonmagnetic insulating phase found in kappa-(ET)(2)Cu-2(CN)(3).