Quantum spin Hall effect and electric field-induced topological phase transition in germanene

Speaker: Harold Zandvliet – Twente University

Host: Ingmar Swart

Abstract:

Germanene, i.e. the germanium analogue of graphene, shares many properties with graphene. Both materials host Dirac fermions, but there are also a few salient differences: (1) the spin-orbit coupling in germanene is much larger than in graphene and (2) the honeycomb lattice of germanene is not flat, but buckled. The large spin-orbit gap in germanene makes this material the ideal candidate to exhibit the quantum spin Hall effect. We show that germanene is a quantum spin Hall insulator with a gapped interior and topologically protected helical edge states. The buckling of the lattice of the honeycomb allows to tailor the quantum state of matter of germanene. By applying a perpendicular electric field the band gap in germanene first closes and then reopens again. The reopening of the gap is accompanied by a topological phase transition of germanene from a two-dimensional topological insulator to a trivial band insulator. This electric field-induced tailoring of the quantum state of matter of germanene allows to controllably switch the topologically protected helical edge states on and off. The latter makes germanene the material of choice for the realization of a topological field-effect transistor.