Hidden edge Dirac point and robust quantum edge transport in InAs/GaSb quantum wells

The robustness of quantum edge transport in InAs/GaSb quantum wells raises an issue on the fate of topological phases of matter under time-reversal symmetry breaking. Here a peculiar band structure evolution in InAs/GaSb quantum wells is revealed: the electron subbands cross the heavy hole subbands but anticross the light hole subbands. The topologically protected band crossing point of the helical edge states, i.e., the Dirac point, is pulled to be close to and even buried in the bulk valence bands when the system is in a deeply inverted regime, which is attributed to the existence of the light hole subbands. A sizable Zeeman energy gap verified by the effective g-factors of edge states opens at the Dirac point by an in-plane or perpendicular magnetic field, however it can also be hidden in the bulk valance bands. This provides a plausible explanation for the recent observation on the robustness of quantum edge transport in InAs/GaSb quantum wells subjected to strong magnetic fields.