SCANNING SQUID MICROSCOPE MEASUREMENTS ON JOSEPHSON JUNCTION ARRAYS
The first part of my dissertation work involved the design, construction, and operation of a scanning microscope that uses a superconducting quantum interference device (SQUID) as a probe. This system can produce two-dimensional (2D) images of the magnetic flux above samples as large as 1cm by 1cm. The microscope has a spatial resolution on the order of 40....m and a flux resolution on the order of a micro flux quantum. Next, I used this scanning SQUID microscope system to study the distribution of currents in 2D arrays of SNS Josephson junctions and attempt to estimate the penetration depth for perpendicular magnetic fields,... ..is an important parameter in that it is a determining factor for the possibility of a Kosterlitz-Thouless phase transition in arrays. Raster scanned images of the flux above an array were produced for various temperatures and currents. The first method I used to determine ..was transforming the 2D flux images into images of current distribution. Intrinsic problems with the transforation algorithm led to a second approach of fitting a single flux scan to the Biot-Savart law. For the samples studied, ..was determined to be on the order of the array lattice constant. Finally, we investigated the usefulness of Fisher, Fisher, and Huse dynamical scaling to determine the occurrence of a Kosterlitz-Thouless transition in 2D systems. We simulated current-voltage (IV) curves for a 2D Josephson junction array using appropriate parameters and expressions above and below the Kosterlitz-Thouless-Berezinskii transition temperature (TKT). We also included a contribution arising fromfinite-size induced free vortices. The curves were scaled for different voltage cutoffs to simulate the minimum sensitivity of a voltmeter. We found that the value of the dynamical scaling exponent, z, for the best scaling fit and the optimal value of the transition temperature, depended upon the voltage cutoff level chosen; in effect the fit depended upon how much of the finite-sized linear portion of the IV curve that we included.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:scanning squid microscope josephson junction arrays kosterlitz thouless transition perpendicular penetration depth dynamical scaling
Date of Publication:01/01/2000