I. Microwave Impedance of Superconducting Weak Links. II. Effects of Non-Equilibrium Quasi-Particle Distributions in Superconducting Weak Links

Author: Franson, James Dean

Year: 1977

Degree: Dissertation (Ph.D.)

Advisor: Mercereau, James E.

Committee Member: Unknown, Unknown

Option: Physics

DOI: 10.7907/56t9-7j16

Abstract

Three separate but somewhat related experiments having to do with the nonequilibrium properties of proximity coupled superconducting weak links are reported.

The first experiment consisted of calibrated measurements of both the real and imaginary parts of the microwave impedance of proximity effect weak links. The impedance of a current-biased weak link was calculated analytically on the basis of several different theoretical models for the supercurrent Is and the quasi-particle current Iqp, and the results were compared with the experimental measurements. For dc bias currents Idc less than the critical current Ic, the measurements were consistent with the usual resistively shunted junction (RSJ) model. However, for Idc > Ic, the Fourier component of Is at the Josephson frequency was found to be less by a factor of ~ 0.65 ± .1 than would be expected from the RSJ model, but consistent with the predictions of the phase-slip model. The phase-dependent part of the quasi-particle current, if it exists at all, was found to be no larger than 10% of the total quasi-particle current.

In the second experiment 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 Ic of the weak link was, over a wide range of experimental conditions, proportional to exp(-α√P), where P is the injection power and α is a constant. This result was somewhat surprising since, on the basis of simple heating or various microscopic theories, Ic would be expected to depend exponentially on the power itself. It was suggested that the order parameter inside the weak link is sufficiently small that the excitation of collective modes is more important than the excitation of quasi-particles, and that the transverse collective modes are excited by the noise voltages produced by the injected power.

The final experiment consisted of using a small tunnel junction as a probe of the nonequilibrium quasi-particle chemical potential μqp inside a proximity effect weak link. The nonequilibrium value of μqp was produced by simultaneously passing a de bias current less than Ic and a microvwave current I through the weak link. For most values of Iμ the measured value of μqp was proportional to the dc supercurrent carried by the weak link, as reported in a similar experiment by Yu and Mercereau. In addition, for certain values of Iμ, the measured value of μqp became a discontinuous function of the supercurrent. A physical mechanism which may be responsible for these effects was proposed and the calculated value of μqp was found to be in qualitative agreement with the data, including the newly discovered discontinuities.

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