Abstract
We report a conductance asymmetry of quantum point contacts (QPC) at near-zero applied current biases. The asymmetry is measured at low temperatures as a rectified dc voltage response to an applied small amplitude ac current. When measured as a function of negative gate-voltage, the rectified signal oscillates. For some of the samples this oscillatory behavior scales linearly with the applied ac current amplitude, which indicates only one step-like non-linearity in differential resistance vs. current. The value of the step is of the order of a few tens of Ohms on a background of several kOhms, and embedded in intrinsic random telegraph signal (RTS) noise. The step could be verified also by a direct measurement of differential resistance vs. dc current. We believe a two-level impurity configuration near the constriction is responsible for the step-like differential resistance.
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