FERROMAGNETISM IN THE 2-DIMENSIONAL T-T' HUBBARD-MODEL AT THE VANHOVEDENSITY

Using an improved version of the projection quantum Monte Carlo techni que, we study the square-lattice Hubbard model with nearest-neighbor h opping t and next-nearest-neighbor hopping t' by simulation of lattice s with up to 20 x 20 sites. For a given R = 2t'/t, we consider that fi lling which leads to a singular density of states of the noninteractin g problem. For repulsive interactions, we find an itinerant ferromagne t (antiferromagnet) for R = 0.94 (R = 0.2). This is consistent with th e prediction of the T-matrix approximation, which sums the most singul ar set of diagrams.