HIGH CRITICAL TEMPERATURE SNS JOSEPHSON MICROBRIDGES.

摘要

This thesis work presents a new technique for reproducibly fabricating superconductor-normal metal-superconductor (SNS) Josephson microbridges which can be used with a wide variety of superconductors and normal metals. We have used this technique to make the first high-T(,c) SNS microbridges and dc SQUID's, employing Nb(,3)Ge or Nb(,3)Sn for the superconductor and Cu for the normal metal. These devices have the widest operating temperature range of any Josephson device, among other advantages. They have nearly ideal characteristics: sharply defined critical currents (I(,c)) with a voltage rise resembling a square root behavior, large I(,c)R(,n) products, and heating restricted to large dissipation. The modulation of the critical current in an applied magnetic field is nearly ideal and sharp constant voltage steps are observed in the current-voltage characteristic when microwaves are applied.; These properties are compared with the Likharev-Usadel model for the I(,c)R(,n) product, modified to take into account out geometry. We find reasonable agreement with the predictions of the model, both as a function of temperature and as a function of bridge length. The latter has been made possible for the first time due to the reproducibility of our technique. New boundary conditions are presented for this model. The dynamic behavior is described by the time-dependent Ginzburg-Landau theory. We show calculations for the current-voltage characteristics and new results which include the effect of microwaves. Both of these are in excellent agreement with the measured characteristics.; Further possibilities for using this fabrication procedure in studies of basic physics and useful applications are investigated.