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

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Numerical studies on Mott transitions caused by the control of the ratio between bandwidth and electron-electron interaction (U) are reported. By using the recently proposed path-integral renormalization group(PIRG) algorithm, physical properties near the transitions in the ground state of two-dimensional half-filled models with the nearest and the next-nearest neighbor transfers (-t and t', respectively) are studied as a prototype of geometrically frustrated system. The nature of the bandwidth-control transitions shows sharp contrast with that of the filling-control transitions: First, the metal-insulator and magnetic transitions are separated each other and the metal-insulator (MI) transition occurs at smaller U, although the both transition interactions U increase with increasing t'. Both transitions do not contradict the first-order transitions for smaller t'/t while the MI transitions become continuous type accompanied by emergence of unusual metallic phase near the transition for large t'/t. A nonmagnetic insulator phase is stabilized between MI and AF transitions. The region of the nonmagnetic insulator becomes wider with increasing t'/t. The phase diagram naturally connects two qualitatively different limits, namely the Hartree-Fock results at small t'/t and speculations in the strong coupling Heisenberg limit.