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Title: Disorder in superconductors in reduced dimensions
Author: Constantino, N. G. N.
ISNI:       0000 0004 8503 9237
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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Superconducting nanowires have been identified as dual elements to the Josephson junction. This duality is attributed to the existence of quantum phase slip (QPS) phenomena, where the magnitude of the superconducting order parameter fl uctuates to zero. This has provided scope for equivalents to well-established applications of Josephson physics, such as a quantum current standard. Existing literature indicates an in uence of disorder on the rate of QPS events in superconductors, but few studies have looked specifically at quantifying this disorder as dimensions are reduced for QPS materials. We have investigated disorder in two superconductors of particular significance for QPS phenomena, NbSi and NbN. We have engineered compositions of these materials and grown thin-films using magnetron sputter deposition. We developed a novel top-down nanowire fabrication technique to overcome factors limiting the minimum widths achievable using standard lift-off techniques and used this method to reduce our films to nanowires for the investigation of dimensional effects. We present low-temperature transport behaviour in our superconducting thin- films and demonstrate the dependence of the critical temperature on both the sheet resistance and film thickness. These illustrate a trend in disorder from which we extract the Finkel'stein disorder parameter γ, the mean free path, and the BCS and Ginzburg-Landau coherence lengths in our films. As films are reduced to nanowires, we show the infl uence of noise on measurements of superconductivity in nanowires and demonstrate the importance of filtering. We demonstrate that the relationship between the critical temperature and dimension is also observable for a reduction in width of the superconductor when reduced to nanowire dimensions. Finally, using characteristics we have extracted from our investigations, we present a feasibility study on the integration of these nanowires into a voltage-biased QPS junction circuit, dual to the current-biased Josephson junction. Using QPS theory, we predict how our materials are expected to behave in such a circuit and present measurements of a prototype device.
Supervisor: Warburton, P. A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available