NEW UNIVERSALITY CLASSES FOR 2-DIMENSIONAL SIGMA-MODELS

We argue that the two-dimensional O(N)-invariant lattice sigma-model with mixed isovector/isotensor action has a one-parameter family of nontrivial continuum limits, only one of which is the continuum sigma-model constructed by conventional perturbation theory. We test the proposed scenario with a high-precision Monte Carlo simulation for N = 3,4 on lattices up to 512 x 512, using a Wolff-type embedding algorithm. The finite-size-scaling data confirm the existence of the predicted new family of continuum limits. In particular, the RP(N-1) and N-vector models do not lie in the same universality class.