Violation of the integral quantum Hall effect: Influence of backscattering and the role of voltage contacts

Heterostructure (GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As) Hall-bar devices with short cross gates are studied in the quantum Hall regime. A potential barrier introduced by the gate causes back scattering of edge currents, making steplike structures in the curves of the Hall and the diagonal resistances versus the gate bias voltage. The Hall resistance on either side of the gate deviates largely from expected quantized values, indicating a nonequilibrium occupation of edge states and its stability over a macroscopic distance (50 μm). An essential role of imperfect voltage contacts in probing the nonequilibrium edge currents is noted.