title: Nonperturbative effects of a topological theta term on principal chiral nonlinear sigma models in 2 + 1 dimensions.
abstract: We study the effects of a topological Theta term on 2+1-dimensional principal chiral models, which are nonlinear sigma models defined on Lie group manifolds. We find that when Θ=π, the nature of the disordered phase of the principal chiral model is strongly affected by the topological term: it is either a gapless conformal field theory, or it is gapped and twofold degenerate. The result of our Letter can be used to analyze the boundary states of three-dimensional symmetry protected topological phases.
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: Cenke
authors.initials: C
authors.lastname: Xu
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: Andreas W W
authors.initials: AW
authors.lastname: Ludwig