L. Hao et al., MAGNETIC-FIELD AND MICROWAVE EFFECTS ON CRITICAL-CURRENT FLUCTUATIONSIN HTS GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS, IEEE transactions on applied superconductivity, 7(2), 1997, pp. 2840-2843
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.