Interface states in band-inverted semiconductor heterojunctions.

We study the interface-bound midgap states in semiconductor heterojunctions, where the constituents have opposite band-edge symmetry and overlapping gaps. Even including the effects of band offsets, graded interfaces, and far-band corrections, the spectrum remains gapless and essentially linear as a function of in-plane momentum. Optical transitions between interface levels have an unusual spectrum and polarization dependence, which may have device applications. We remark on the possible existence of ``anomalous currents'' in these systems.