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 be tween bandwidth and electron-electron interaction (U) are reported. By usin g the recently proposed path-integral renormalization group (PIRG) algorith m, physical properties near the transitions, in the ground state of two-dim ensional half-filled models with the nearest and the next-nearest neighbor transfers (-t and t', respectively) are studied as a prototype of geometric ally frustrated system. The nature of the bandwidth-control transitions sho ws sharp contrast with that of the filling-control transitions: First, the metal-insulator and magnetic transitions axe separated each other and the m etal-insulator (MI) transition occurs at smaller U, although the both trans ition interactions U increase vith increasing t'. Both transitions do not c ontradict the first-order transitions for smaller t'/t while the MI transit ions become continuous type accompanied by emergence of unusual metallic ph ase near the transition for large t'/t. A nonmagnetic insulator phase is st abilized between MI and AF transitions. The region of the nonmagnetic insul ator becomes wider with increasing t'/t. The phase diagram naturally connec ts two qualitatively different limits, namely the Hartree Fock results at s mall t'/t and speculations in the strong coupling Heisenberg limit.