  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.
