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Title: Fabrication of, and transport in, mesoscopic superconducting devices
Author: Hart, A. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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We have fabricated and studied mesoscopic-scale microfabricated superconducting devices whose size is comparable to the Cooper pair coherence length. In the course of this research, we have performed a series of new experiments with mesoscopic superconducting devices based on aluminium, microfabricated using electron-beam lithography and lift-off techniques. Discussion of fabrication issues forms a significant portion of this thesis. Initially, we present novel devices featuring a single tunnel barrier which was artificially deposited, obviating the need for troublesome shadow-mask evaporations. We have investigated their electrical characteristics at low temperature (300mK) and they were found to behave in a manner similar to that described by the BTK theory of 1982. Some calculations are presented and barrier parameters extracted. Modifications to the geometry of the devices appeared to produce a change in their electrical characteristics, reminiscent of the often-discussed Zero Bias Anomaly (ZBA), and an explanation is tentatively proposed. Mesoscopic multi-layered superconductor/insulator devices based on an improved version of this barrier technology are then presented. The size of the device action region was less than 600nm in all dimensions in the smallest of these devices. The fabrication process is discussed, and extensive results from electrical measurements at low temperatures are presented, investigating the effects of barrier thickness, lateral device size, magnetic fields, temperature, bias voltage and RF illumination on device size conductance. Clear differences in behaviour between "large" (14μm), "medium" (1.6μm) and "small" (0.5μm) devices were observed. Some relevant theories are outlined and the results are interpreted. Finally, a third class of device was fabricated. Mesoscopic squares of superconducting aluminium with a normal-metal measurement probe at each corner, these devices were subjected to extensive magnetoresistance measurements at temperatures as low as 50mK and at fields of up to 9T. Several interesting phenomena were observed including magnetoresistance fluctuations, so great in some cases that the device exhibited a novel re-entrant superconductivity: a loss and subsequent re-gaining of the superconducting state as magnetic field was swept upwards, as predicted recently by Spivak and Zhou. Current sweeps about such re-entrant points showed relatively typical behaviour for a superconductor.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available