METAL-INSULATOR-TRANSITION AND MAGNETIC-ORDERING IN HUBBARD MODELS NEAR THE NAGAOKA LIMIT

We study the metal-insulator transition and magnetic ordering in the H ubbard model using the particle-hole mapping. The analysis simplifies near the ferromagnetic limit. We find that the two-dimensional Hubbard model has a charge excitation gap at half filling for any finite U in this region on both the bipartite square lattice and the nonbipartite triangular lattice. In some cases, the system goes through a first-or der phase transition to become a paramagnetic metal as S-z is lowered. We also discuss the extension to the doped case. We find that in the large U limit, a single doped hole has a bandwidth of order of J rathe r than t at S-z = 0.