Type-II Fracton Models with Gapped Boundaries

Type-II fracton systems are exotic models whose pointlike excitations are strictly immobile at zero temperature and have locally indistinguishable degenerate ground states, and unusual error correction properties. We study gapped boundaries of such models, including Haah’s code. After enumerating the set of possible gapped boundaries, we study the ground state degeneracy and logical operators in the presence of these boundaries. Although these properties are complicated on the torus, they simplify dramatically for certain open boundary conditions. We discuss the interpretation of the degeneracy in terms of surface line defects, i.e. the domain walls between distinct gapped boundaries.