Tunable quantum spin Hall effect in double quantum wells

The field of topological insulators (TIs) is rapidly growing. Concerning possible applications, the search for materials with an easily controllable TI phase is a key issue. The quantum spin Hall effect, characterized by a single pair of helical edge modes protected by time-reversal symmetry, has been demonstrated in HgTe-based quantum wells (QWs) with an inverted band gap. We analyze the topological properties of a generically coupled HgTe-based double QW and show how in such a system a TI phase can be driven by an interlayer bias voltage, even when the individual layers are noninverted. We argue that this system allows for (layer-)pseudospin-based physics similar to that in bilayer graphene but with the crucial absence of a valley degeneracy.