Speaker
Description
By definition, a d-dimensional symmetry protected topological (SPT) phase must have nontrivial d-1 dimensional boundary states. The boundary of a large class of (3+1)d SPT phases can be described by a (2+1)d nonlinear sigma model (NLSM) with a topological Wess-Zumino-Witten (WZW) term. We will demonstrate that a stable strongly interacting (2+1)d conformal field theory (CFT) could emerge in the quantum disordered phase in this boundary system, due to the existence of the WZW term. This CFT is stable in the sense that any symmetry allowed perturbation will be irrelevant. In order to perform a controlled calculation, we choose to study the NLSM whose target manifold is the Grassmannian U(N)/[U(n) x U(N-n)], which permits a WZW term in (2+1)d for any N and fixed n, and hence permits a large-N generalization. Through a large-N, large-k, and epsilon generalization of this model, we indeed identify a stable CFT fixed point in the quantum disordered phase through a (quasi) controlled renormalization group calculation.
