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PbTe/PbSnTe heterostructures as analogs of topological insulators

We investigate theoretically the PbTe/PbSnTe heterostructure grown in [111] direction, specifically a quantum wall (potential step of width d) of PbTe embedded in Pb_{1-x}Sn_{x}Te. 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 \approx\ 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 semicondcutors.