  The presence of edges locally breaks the inversion symmetry of
heterostructures and gives rise to lateral (edge) spin-orbit coupling (SOC),
which, under some conditions, can lead to the formation of helical edge states.
If the edge SOC is strong enough, the helical edge states can penetrate the
band-gap and be energetically isolated from the bulk-like states. As a result
backward scattering is suppressed, dissipationless helical edge channels
protected against time-inversion symmetric perturbations emerge, and the system
behaves as a 2D topological insulator (TI). However, unlike in previous works
on TIs, the mechanism proposed here for the creation of protected helical edge
states relies on the strong edge SOC rather than on band inversion.
