The current-voltage and noise properties of high temperature superconductor SNS and grain boundary junctions

The study of the noise from Josephson Junctions is an intense field of research. Despite this, no clear picture of the current transport in these devices has emerged. Without the detailed understanding of the mechanisms of current transport in High Temperature Superconductor junctions, the design of superconducting electronics will not move significantly forward. The work in this thesis has examined two main types of High Temperature Superconducting Josephson Junctions: grain boundary and SNS. Extensive measurements of the correlation of critical current and normal state resistance noise have been performed using two main switching techniques. These techniques have been designed, developed and extensively tested during this project. It was found that the critical current and normal state resistance noise were uncorrelated for bi-epitaxial junctions, but had a finite correlation of, on average, 0.3 for a 22.5 deg misoriented bi-crystal. In addition preliminary measurements on a step edge junction indicate that the correlation coefficient is approximately 0.15. Comparisons were made between SNS junctions with a PrBCO interlayer in two different junction geometries; those of sandwich junction (c-axis) and ramp edge junction (a-b plane). For the ramp edge geometry, the properties of the junction can tentatively be ascribed to pinhole shorts between the superconducting electrodes and the presence of low angle grain boundaries. The sandwich junction had normal current transport that was dominated by the interlayer material, but supercurrent transport that could be due to pinhole shorts. In addition, both geometries, especially the sandwich junction, showed large amounts of normal state noise making these junctions unsuitable for device applications. The effect of shining laser light onto a junction was investigated. The effects on the I-V characteristic were unobservable with the experimental resolution available. The effect of the light on the critical current noise of the junction was to reduce the noise peak slightly when compared to the unilluminated case- the reduction was of the order of 10%, comparable with experimental resolution. (author)