MAGNETIC-FIELD AND MICROWAVE EFFECTS ON CRITICAL-CURRENT FLUCTUATIONSIN HTS GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS

The excess noise levels in HTS grain boundary Josephson junctions, whe n expressed as critical current fluctuations \delta I-c\, vary linearl y with I-c as the temperature changes. By contrast, when the critical current is suppressed by an external magnetic: field, the measured flu ctuations \delta I-c\ remain nearly constant while I-c itself undergoe s order-of-magnitude changes. This effect, which does not seem to have been widely recognized, is examined with respect to the spatial modul ation of the critical current density within the junction. Calculation s based on this model are in good agreement with our experimental meas urements. In related studies, we measure the excess noise levels betwe en Shapiro steps in a microwave-irradiated junction, and find them to agree with a modified version of the Likharev-Semenov equation for the rmal noise.