Exactly soluble model of boundary degeneracy

Understand

We investigate the topological degeneracy that can be realized in Abelian fractional quantum spin Hall states with multiply connected gapped boundaries.

• Such a topological degeneracy (also dubbed as "boundary degeneracy") does not require superconducting proximity effect and can be created by simply applying a depletion gate to the quantum spin Hall material and using a generic spin-mixing term (e.g., due to backscattering) to gap out the edge modes.
• We construct an exactly soluble microscopic model manifesting this topological degeneracy and solve it using the recently developed technique [S.
• Ganeshan and M.