HIGH-SPIN STATES IN THE HUBBARD-MODEL - GENERALIZED HUND COUPLING AND A CROSSOVER TO STRONG U-REGIME

The occurrence of high-spin ground states in the strongly correlated electron system is clarified in one and two dimensions. Even for infinitestimal Hubbard U, the ground state with a spin-polarized Fermi surface periodically arises as the number of electrons is varied due to a 'k-space Hund's coupling'. A non-perturbative crossover to strong U regime, which is accompanied by a series of level crossings and involves the stability of the Nagaoka state, is also demonstrated.