Spontaneous symmetry breaking from anyon condensation (1811.00434v2)

Abstract: In a physical system undergoing a continuous quantum phase transition, spontaneous symmetry breaking occurs when certain symmetries of the Hamiltonian fail to be preserved in the ground state. In the traditional Landau theory, a symmetry group can break down to any subgroup. However, this no longer holds across a continuous phase transition driven by anyon condensation in symmetry enriched topological orders (SETOs). For a SETO described by a $G$-crossed braided extension $\mathcal{C}\subseteq \mathcal{C}^{\times}_{G}$, we show that physical considerations require that a connected \'etale algebra $A\in \mathcal{C}$ admit a $G$-equivariant algebra structure for symmetry to be preserved under condensation of $A$. Given any categorical action $\underline{G}\rightarrow \underline{\sf Aut}{\otimes}^{\sf br}(\mathcal{C})$ such that $g(A)\cong A$ for all $g\in G$, we show there is a short exact sequence whose splittings correspond to $G$-equivariant algebra structures. The non-splitting of this sequence forces spontaneous symmetry breaking under condensation of $A$. Furthermore, we show that if symmetry is preserved, there is a canonically associated SETO of $\mathcal{C}^{\operatorname{loc}}{A}$, and gauging this symmetry commutes with anyon condensation.