Disorder-induced helical phase in magnetic topological insulators*

In magnetically doped thin-film topological insulators, aligning the magnetic moments generates a quantum anomalous Hall phase with a single chiral edge state. We study the de-magnetization process and show that disorder from randomly oriented magnetic moments can produce a quantum-spin-Hall-like phase with counter-propagating helical edge modes protected by a unitary reflection symmetry. This phase is analogous to the quantum spin Hall effect observed in the zeroth Landau level of graphene, which is also protected by a unitary symmetry (spin rotation). We show that introducing superconductivity, combined with selective breaking of reflection symmetry by a gate, allows for creation and manipulation of Majorana zero modes via purely electrical means.