Changing anyonic ground degeneracy with engineered gauge fields

Abstract

For systems of lattice anyons such as Majoranas and parafermions, the unconventional quantum statistics determines a set of global symmetries (e.g., fermion parity for Majoranas) admitting no relevant perturbations. Any operator that breaks these symmetries explicitly would violate locality if added to the Hamiltonian. As a consequence, the associated quasidegeneracy of topologically nontrivial phases is protected, at least partially, by locality via the symmetries singled out by quantum statistics. We show that it is possible to bypass this type of protection by way of specifically engineered gauge fields, in order to modify the topological structure of the edge of the system without destroying the topological order completely. To illustrate our ideas in a concrete setting, we focus on the ${\mathbb{Z}}_{6}$ parafermion chain. Starting in the topological phase of the chain (sixfold ground degeneracy), we show that a gauge field with restricted dynamics acts as a relevant perturbation, driving a transition to a phase with threefold degeneracy and ${\mathbb{Z}}_{3}$ parafermion edge modes. The transition from the ${\mathbb{Z}}_{3}$ to the topologically trivial phase occurs on a critical line in the three-state Potts universality class. We also investigate numerically the emergence of Majorana edge modes when the ${\mathbb{Z}}_{6}$ chain is coupled to a differently restricted gauge field.