Influence of Josephson Effect on Proximity Coupled Superconductors

Experimental investigations to examine the influence of Josephson effect on proximity coupled superconductors are reported. The results are interpreted with a proposed physical model that incorporates spatial gradients in both the phase and the amplitude of the proximity induced energy gap. Calculations based on this model successfully describe and explain most of the experimental observations. This model also indicates the evolutton of weak coupling (Josephson effect with two superconductors) to strong coupling (phase slip in one-dimensional superconductor) in superconductivity. The experimental systems utilized herein were thin film structures fabricated from hard superconductor composite layers. The structure forms an extended conducting region between two superconductors. These structures have well defined geometry down to optical resolution, clean boundary conditions, controllable material properties, reliable and reproducible characteristics, and temperature recyclability which enabled systematic investigations to be carried out. A major influence of Josephson effect on proximity coupled superconductors is the distinction between the phase in the current-phase relationship and the phase in the voltage-phase relationship in both the stationary and nonstationary situations. This distinction leads to the possibility of multiple states of proximity induced superconductivity. The transition between these states is identified as a phase slip process involving phase change of less than 2π. This phase slip in the nonstationary regime results in a supercurrent with an a.c. component (similar to the Josephson oscillating supercurrent) and a d.c. component (the excess current in superconducting weak links). They were experimentally tested with spatial perturbation via external magnetic field and time perturbation via external radiation field. The observations in this nonequilibrium situation are consistent with the predictions from the quasiequilibrium description.