Breakdown of the quantum Hall effect in InAs/AlSb quantum wells due to counterflowing edge channels

We investigated magnetotransport in the two-dimensional electron gas (2DEG) present in InAs/AlSb quantum wells. The filling factor Ng underneath a gate electrode was reduced relative to the bulk filling factor Nb. For Ng<Nb the resistance at the minima of the Shubnikov–de Haas oscillations failed to vanish, and the corresponding plateaus in the Hall resistance deviated from their expected quantized values Rxy=h/(e2Ng). The results are explained by the presence of anomalous edge channels at the 2DEG boundaries, which flow in opposite direction compared to the regular ones. They result from the band bending due to the combination of Fermi level pinning at the exposed InAs surface and the electrostatic potential generated by the gate electrode. This picture is supported by a comparison between the measured resistances and the resistances calculated with an edge channel model.