ACCURATE BOUNDS FOR CRITICAL-TEMPERATURES OF O(N) SPIN MODELS BY RENORMALIZED RANDOM-WALK REPRESENTATIONS

We obtain new accurate bounds for the critical temperatures of N-vecto r (Heisenberg) models, We apply a block spin type transformation to ra ndom walk representations of the spin models and we accurately estimat e the effects of longer random walks with a computer, The bounds are e qual to the results by Monte Carlo simulations within differences of 6 .3% (3D) to 3.3% (4D).