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

Abstract

We have measured the magnitude of the first Shapiro step in both bridge-type (Ta-Ta and Ta-Sn) and tunnel-type (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/λ, λ = 699, 570 and 469μm) . we determined I_{1}^{\max}/I_{c} minutely as a function of the normalized frequency \omega/\omega_{g} (T), where I_{1}^{\max} is the maximum (half) height of the first Shapiro step as a function of the ac field power, I c is the critical current, and \omega_{g}(T)=4\Delta_{Ta}(T)/h or 2(\Delta_{Ta}(T)+\Delta_{Sn}(T))/h . By fitting the shape of the Riedel peak at \omega/\omega_{g}(T)=I with the theory of the tunnel junction, we have determined the damping factor δ. We obtained δ =0.02 for the Ta-SnOx-Sn tunnel- type junction, and δ=0.05 for both the Ta-Sn and Ta-Ta bridge - type junctions, respectively. At \omega/\omega_{g}(T)>1.5 , we have found that I_{1}^{\max}/I_{c} tends to saturate in both the tunnel-type and the bridge-type junctions as \omega/\omega_{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 measuared for a number of frequencies at a fixed temperature.