Abrikosov vortex motion and elementary pinning force in a SNS Josephson junction

Abstract

<p>Procedures have been developed to determine the location of a single Abrikosov vortex in a Superconductor - Normal metal - Superconductor (SNS) Josephson junction and study its motion under the influence of a Lorentz force. A vortex in a SNS junction generates characteristic magnetic field inside the junction and this field, in turn, induces a specified phase across the junction. This phase caused by the vortex changes the critical current characteristics of the junction, that then can be used to locate the vortex inside the junction. A single vortex was successfully trapped in the junction by the field cooling process and the location was determined by the diffraction pattern. Motion of the vortex was induced by the transport current, I[subscript] p, and the vortex was found to move in discrete jumps. By tracing the vortex after successive depinning events, many pinning centers could be identified. From the minimum depinning current, the elementary pinning force associated with an individual pinning site of the Pb - Bi(4 atomic percent) superconducting layer has been measured and found to be of order of 10[superscript]-8 dyne (or 10[superscript]-4 dyne/cm) at T/T[subscript] c = 0.95. The force is asymmetric and different from one pinning site to another. For the given SNS junction, the pinning force of a pinning center is dominant over all other forces associated with the vortex in the junction. In addition, from the experiment the temperature dependence of the pinning force is found to be f[subscript]p ~ (1 - T/T[subscript] c)[superscript]3/2 near T[subscript] c. There are two ingredients for the vortex depinning experiment. First, the N layer of the junction must be thick to reduce the field energy and dipole coupling force of the vortex. Secondly, vortex can be depinned at higher temperatures, at which depinning current is smaller than vortex nucleation current. ftn * DOE Report IS-T 1218. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.</p>