NONLINEAR SIGMA-MODEL AT THETA=PI - EUCLIDEAN LATTICE FORMULATION AND SOLID-ON-SOLID MODELS

We study the strong-coupling expansion of a Euclidean lattice formulation of the two-dimensional O(3) sigma-model at nonzero topological angle theta, mapping it onto a solid-on-solid model with well-known critical behavior. The results are consistent with those predicted by the mapping of the Hamiltonian formulation of the sigma-model onto quantum spin chains. The model has a gapless phase at theta = pi and weak coupling in which a condensate of Coleman's "half-asymptotic" particles exists.