Magnetic and Metal-Insulator Transitions through Bandwidth Control in Two-Dimensional Hubbard Models with Nearest and Next-Nearest Neighbor Transfers

Numerical studies on Mott transitions caused by the control of the ratio between bandwidth and electron-electron interaction ($U$) are reported. Physical properties near the transitions in the groundstate of two-dimensional half-filled models with the nearest and the next-nearest neighbor transfers ($-t$ and $t'$, respectively) are studiedby using the recently proposed path-integral renormalizationgroup(PIRG) algorithm. The nature of the bandwidth-control transitions shows sharp contrast with that of the filling-control transitions: For $t'/t=0.2$, a metal-insulator (MI) transition occurs at $U/t=3.25\pm 0.05$ while a magnetic transition from the paramagnetic to the antiferromagnetic (AF) phase occurs at $U/t=3.45\pm 0.05$. Both transitions do not contradict the {\it first-order} transitions; for $t'/t=0.5$, a MI and an AF transitions take place at $U/t =4.75\pm 0.25$ and $7.25\pm 0.25$, respectively. At $t'/t=0.5$, short-ranged incommensurate spin correlations are observed between the MI and AF transition points. In both cases of $t'/t=0.2$ and 0.5, a {\it paramagnetic insulator phase} is stabilized between the MI and AF transitions. The region of the spin-liquid insulator becomes wider with increasing $t'/t$. This result shows consistency with the speculations in the strongcoupling limit, namely the frustrated Heisenberg model indicating an existence of a spin-liquid insulator phase.