Temperature dependence of submillimeter wave response in the point-contact Josephson junction

The authors have measured the magnitude of the first Shapiro step in both bridge-type (Ta-Ta and Ta-Sn) and tunneltype (Ta-SnOx-Sn) point-contact Josephson junctions in the SMMW (Submillimeter Wave) region. By varying the temperature continuously with the frequency of the SMMW fixed (ω=2π/lambda, lambda=699, 570 and 469 μm), we determined I1 /SUP MAX/ /I /SUB c/ minutely as a function of the normalized frequency ω/ω /SUB g/ (T), where I1 /SUP MAX/ is the maximum (half) height of the first Shapiro step as a function of the ac field power, I /SUB c/ is the critical current, and ω /SUB g/ (T)=4Δ /SUB Ta/ (T)/h or 2(Δ /SUB Ta/ (T)+Δ /SUB Sn/ (T))/h. By fitting the shape of the Riedel peak at ω/ω /SUB g/ (T)=1 with the theory of the tunnel junction, we have determined the damping factor delta. They obtained delta=0.02 for the Ta-SnOx-Sn tunnel - type junction, and delta=0.05 for both the Ta-Sn and Ta-Ta bridge - type junctions, respectively. At ω/ω /SUB g/ (T)>1.5, we have found that I1 /SUP MAX/ /I /SUB c/ tends to saturate in both the tunnel-type and the bridge-type junctions as ω/ω /SUB g/ (T) increases. For the tunnel-type junction, this agrees with the very slow decrease predicted by the theory of the tunnel junction. But for the bridge-type junction, this slow decrease is in marked contrast to the very rapid decrease found from the previous experiments, where the Shapiro step was measured for a number of frequencies at a fixed temperature