EXACT-SOLUTIONS OF FRUSTRATED ORDINARY AND CHIRAL 8-SITE HUBBARD MODELS

This contribution addresses the question of whether or not the ground state of a frustrated spin-1/2 Heisenberg model can be smoothly related to the ground state of a simple tight-binding model at half-filling in an appropriately chosen magnetic field. This continuity is considered explicitly for an eight-site square-lattice Hubbard model with nearest- and next-nearest-neighbor hopping, which approaches a frustrated Heisenberg model with nearest- and next-nearest-neighbor antiferromagnetic exchange coupling in the limit of large Hubbard U. In addition, a counterexample to the widely held belief that the half-filled band of the ordinary Hubbard model is nonmagnetic has been found in the regime where the hopping parameters and Hubbard interaction are all the same order of magnitude.