Transient Transport in One-Dimensional Nanostructures
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This thesis describes the results of measurements of the transient response of quantum point contacts (QPCs), and shows strong effects of heating due to optical-phonon emission. By changing the gate voltage that controls the confinement potential of the QPC, we observed three different regimes of transient response—the 2-D, multi-mode 1-D, and few mode 1-D regimes, respectively. With the QPC properly formed, the transient voltage is dropped almost entirely across it and we observe clear evidence of heating in the transient response when the QPC is in the multi-mode 1-D regime. The heating is strongly suppressed in few-mode 1-D limit, however, where the current can be carried by only a small number (1 or 2) of subbands. Accompanying the suppression of heating, we find that the non-linear conductance inferred from the transient pulses becomes pinned near 2 e 2 /h over a wide range. Further experiments shows that the suppression of heating cannot be explained simply as arising as the current in the QPC drops below some critical threshold value. Motivated by these observations, I have developed a theoretical model to explain our key experimental results. The key feature of the model is that it considers how strong electron-phonon coupling within the QPC can modify the structure of its subbands. Through a phenomenological treatment of the electron-phonon coupling in the QPC, our calculations how that virtual fluctuations of optical phonons allow the lowest subband to become "protected" from scattering under strong bias, as it becomes split-off in energy from the other subbands. Under this condition, only the lowest subband can be accessed for transport through the QPC, allowing us to explain the pinning of the conductance near 2 e 2 /h. The demonstration of this protected 1-D mode is a completely new result, and we attribute its discovery to the fact that our transient studies provide us with the ability to image heating directly, in real time, something that is not possible for DC measurements. Since the phonon coupling mechanism that we have considered is quite generic, the protected mode that we have identified may also play a role in determining the operation of scaled MOSFETs as they are pushed progressively towards the regime of ballistic transport.