Comparative Studies of the Steady State and R.F. Properties of Proximity Effect Bridges

Author: McNamara, Robert Patrick

Year: 1978

Degree: Dissertation (Ph.D.)

Advisor: Mercereau, James E.

Committee Member: Unknown, Unknown

Option: Applied Physics

DOI: 10.7907/rfth-6762

Abstract

Experimental investigations on proximity effect bridges at zero and finite voltages in the μV range are reported. Measurements have been made of the bridge/film material properties (resistivity, coherence length, penetration depth) and geometry (length, width) which have been correlated to the bridge steady state properties. The steady state properties examined include the current distribution of a thin film/ proximity effect bridge, the magnetic field modulation of the critical current as a function of film geometry, and the critical current density of a proximity effect bridge. A model which correlates the bridge/film material and geometric properties has been developed to calculate the bridge critical current density for the temperature regime T > Tn, the bridge transition temperature.

In the nonequilibrium (finite voltage) case, one particular bridge property has been examined, the bridge rf response to external microwave irradiation. Comparison is made between the bridge rf response and predictions based on the resistively shunted junction (RSJ) model for the behavior of a weak link. Large discrepancies were found to exist between the RSJ model and the measured bridge response. At low microwave frequencies (< 10 GHz), the supercurrent-voltage relationship for these devices was found to be

Is (T,V) = I0 exp (-|V|/V*) sin (2e/h ʃ Vdt

This result appears to be consistent with a new model which is based on a concept of voltage induced pair breaking in the link.

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