Abstract
The admittance of a quantum point contact (QPC) is investigated in the ac regime. Resonance-like peaks superimposed on the commonly occurring admittance plateaus are observed. To explore the origin of these peaks, we perform measurements on the sample in perpendicular magnetic fields of different strengths. We find that as the magnetic field increases, the resonance-like peak located at the first admittance plateau disappears, while that at the third plateau becomes more pronounced. Under certain conditions, these peaks can evolve into 0.7 anomalous plateaus. We believe that these resonance-like peaks are caused by the presence of impurities in the QPC. To confirm this, a bias voltage is added to the common gate voltage to generate an asymmetric QPC confinement potential. We then observe an asymmetric evolution of the peaks for both positive and negative bias voltages. The effects of the magnetic field and the bias voltage can be described well by the results of numerical calculation using the finite difference method and taking into account the influence of the impurity. The results of this work should help provide better understanding of charge dynamic transfer and may also be useful for the study of QPC-based devices such as qubits and quantum dots.
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