The two-phase issue in the O(n) non-linear $\sigma$-model: A Monte Carlo   study
  We have performed a high statistics Monte Carlo simulation to investigate whether the two-dimensional O(n) non-linear sigma models are asymptotically free or they show a Kosterlitz- Thouless-like phase transition. We have calculated the mass gap and the magnetic susceptibility in the O(8) model with standard action and the O(3) model with Symanzik action. Our results for O(8) support the asymptotic freedom scenario. 
B. Alles, A. Buonanno and G. Cella