Non-Linear Differential Conductance of Quantum Point Contacts Due to Phonon-Controlled Disorder
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This thesis describes the results of measurements of the differential conductance of quantum point contacts (QPCs) at cryogenic temperatures and under non-linear biasing. By varying the source bias ( V sd ) applied to these devices we are able to activate different phonons in transport, with acoustic phonons dominating in the range below V sd ~36 mV and optical phonons being activated at higher biases. Of particular interest here is the behavior observed prior to optical-phonon excitation, where we observe a giant zero-bias peak in the differential conductance. According to this effect, the conductance drops from its initial value at zero bias, by a value of as much as 90%. Such striking behavior has not been reported previously and is discussed here in terms of a model of "phonon-controlled disorder". According to this picture, the disorder potential that electrons see as they transit through the device can be dominated by the phonons in the system, which present an effectively static random potential to any electron passing through the device. The degree of this disorder is determined by both the temperature and the applied voltage, and using this model we are able to account for our experimental observations. Our work therefore reveals how electron-phonon coupling in small devices can play a vital role in governing their operation, especially under conditions of non-linear excitation.