Properties of Variable Thickness Microbridges.

The development of Josephson arrays as coherent radiation sources or detectors requires a knowledge of general characteristics of microbridges such as uniformities; linear response of the current step to small external r.f. currents; the strength of r.f. radiation; amount of noise, etc. A comprehensive study of some of the physics behind these devices and a quantitative measurement of their properties are the subjects of this dissertation. The RSJ model with the additional r.f. and noise terms is compared with experimental data. In this thesis, constriction type variable thickness indium microbridges and niobium-copper-niobium SNS microbridges are studied. Data on the first step-height of indium microbridges agrees with the RSJ model very well at low external r.f. frequency and near T(,c). However, deviation from the RSJ model is seen at high r.f. frequency or away from T(,c). In the step response measurement at low external r.f. power, the conversion proportionality deteriorates at low temperatures in the indium microbridges. In noise measurements, excess noise is detected at low temperatures in both the indium and SNS microbridges. The microbridges with noise obey the frequency modulation theory very well. The current -noise spectral density in indium microbridges is found to have a flat noise bandwidth beyond 1 GHz. The Dayem-Wyatt effect in these indium microbridges is found in accordance with the Aslamazov-Larkin mechanism. The "foot" structure of the nonequilibrium effect is not observed in the SNS microbridges studied. In the SNS microbridges, the linear step response to r.f. does not degrade at low temperature. Excess noise in the SNS bridges is slightly smaller than that of indium bridges, and the strengths of their r.f. radiation are comparable. In the study of an SNS array which has various bridge lengths and can be current-biased individually, an effective electrical length larger than the physical length of the bridges is observed, which indicates that quasiparticle dynamics might have extended into the banks. The rugged nature, wide operating temperature range and featureless I-V characteristic show that the SNS is promising for the construction of a practical array.