Effect of the quasiparticle-pair interference current on thermal fluctuations in weakly coupled superconductors and on the operation of rf-biased SQUIDs

Careful studies of the effect of thermal fluctuations on the I-V characteristics of two different types of weakly coupled superconductors were made. Measurements on externally shunted, oxide-barrier tunnel junctions were found to be in complete quantitative agreement with a theory due to Ambegaokar and Halperin in the limit of β/sub c/ identical with 2eI/sub c/C/sigma₀² h much less than 1 where the theory is valid. Similar measurements in the region of β/sub c/ approximately equal to 1 were found to be in qualitative agreement with a theory due to Kurkijarvi and Ambegaokar. Assuming the Ambegaokar and Halperin theory is applicable, measurements on Notarys-Nercereau normal metal underlay weak links indicate the presence of a phase-dependent conductivity predicted by B. D. Josephson in 1962. The magnitude of this conductivity was found to be in agreement with that predicted by theory, however, the sign of the conductivity was found to be in disagreement. A study of the operating characteristics of rf-biased thin-film superconducting quantum interference devices (SQUIDs) has also been made and a set of performance parameters developed to optimize the behavior of these devices. The behavior of these SQUIDs has been compared with a theory due to Hansma in order to look for the effect of the phase-dependent quasiparticle-pair interference current. The measurements were found to be qualitatively different than predicted by Hansma's theory. (author)