Degree Type

Dissertation

Date of Award

1998

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

D. K. Finnemore

Abstract

Thermal depinning of a single vortex trapped in a superconducting Josephson junction is studied as a function of an external magnetic field perpendicular to the junction, in order to examine the conditions necessary for establishing a vortex free junction. If a thick normal metal barrier is used in a cross-strip SNIS Josephson junction, then a single vortex can be nucleated in one of the superconducting strips and systematically moved from one location to another. The location of the vortex can be determined because there is a unique connection between the location of the vortex in the junction and the Fraunhofer-like diffraction pattern of the junction. In zero applied field, it was found that the superconducting order parameter Delta/Delta0 when the vortex depins and starts to move around the film at 8.804 K is equal to 0.19. This implies that the superfluid density is 3.4%. In an applied magnetic field of 10 mG perpendicular to the junction, the vortex moves at 8.707 K where Delta/Delta 0 ~ 0.6. The shielding currents from the applied perpendicular field makes the vortex mobile at a lower temperature;The sharp steps seen previously in I0, the critical current density at zero applied field, during the nucleation process in Pb junctions, did not occur in the Nb junctions used in the present work. Once the vortex has been nucleated and begins to move, a rather a semi-continuous step-like behavior was observed both during the field cooling process and the transport current nucleation process. This was because the vortices were nucleated close to the junction's edge where the there is a small spatial variation of I0. This in turn suggests a lower free energy closer to the edge of the junction due to spatial variation of the film thickness, with the films being thinner close to the edge.

DOI

https://doi.org/10.31274/rtd-180813-11494

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Dimitris Kouzoudis

Language

en

Proquest ID

AAI9911614

File Format

application/pdf

File Size

87 pages

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