Doping-induced metal-insulator transition in two-dimensional Hubbard t-U and extended Hubbard t-U-W models

We show numerically that the nature of the doping-induced metal-insulator transition in the two-dimensional Hubbard model with hopping matrix element t and Coulomb repulsion U is radically altered by the inclusion of a term W that depends upon a square of a single-particle nearest-neighbor hopping. This result is reached by computing the localization length xi(l), in the insulating state. At W/t = 0.05 and U/t = 4, we find results consistent with xi(l)similar to\mu - mu(c)\(-1/2) where mu(c) is the critical chemical potential. In contrast, xi(l)similar to\mu - mu(c)\(-1/4) for the Hubbard model at U/t = 4, At half-filling, we calculate the density of states N(omega). The large value of N(omega) in the vicinity of w = mu(c) present at W = 0 is suppressed with growing values of W. At finite doping, the d-wave pair-field correlations are enhanced with growing values of W. The numerical results imply that at finite values of W doping the antiferromagnetic Mott insulator leads to a d(x2-y2) superconductor.