Design principles and coupling mechanisms in the 2D quantum well topological insulator HgTe/CdTe.

We present atomistic band structure calculations revealing a different mechanism than recently surmised via k · p calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that 2D interface (not 1D edge) TSs are possible. We find that the transitions from a topological insulator at critical HgTe thickness of n = 23  ML (6.453 [corrected] Å) to a normal insulator at smaller n is due to the crossing between two interface-localized states: one derived from the S-like Γ₆(c) and one derived from the P-like Γ₈(v) light hole, not because of the crossing of an interface state and an extended quantum well state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi₂Se₃.