PbTe/PbSnTe heterostructures as analogs of topological insulators

We investigate theoretically the PbTe/Pb_1-xSnₓTe heterostructure grown in the [111] direction, specifically a quantum wall (potential step of width d) of PbTe embedded in Pb_1-xSnₓTe. For x large enough to lead to band inversion and for large d, there are well-known gapless interface states associated with four L valleys. We show that for d≈10 nm, the three pairs of states from oblique valleys strongly couple and become gapped with a gap ~10 meV. On the other hand, the interface states from the [111] valley are essentially uncoupled, and they retain their helical character, remaining analogous to states at surfaces of thin layers of three-dimensional topological insulators. This opens up a possibility of studying the physics of two-dimensional helical Dirac fermions in heterostuctures of already widely studied IV-VI semiconductors.