I. Microwave Impedance of Superconducting Weak Links. I. Effects of Nonequilibrium Quasi-Particle Distributions in Superconducting Weak Links.

Nonequilibrium properties of proximity coupled superconducting weak links are studied. The impedance of a current-biased weak link was calculated analytically on the basis of several different theoretical models for the supercurrent and the quasi-particle current, and the results were compared with experimental measurements. A very small tunnel junction was used to inject quasi-particles directly into the center of a proximity effect weak link. It was found that the critical current of the weak link was, over a wide range of experimental conditions, proportional to (-alpha square root of P), where P is the injection power and alpha is a constant. The final experiment consisted of using a small tunnel junction as a probe of the nonequilibrium quasi-particle chemical potential inside a proximity effect weak link. The nonequilibrium value of potential was produced by simultaneously passing a dc bias current and a microwave current through the weak link. For most values of microwave current measured value of the quasi-particle chemical potential was proportional to the dc supercurrent carried by the weak link.