Quasiparticles Injection Effects for Nb/Al/AlOx/Al/Nb Tunnel Junctions at 0.4K using Stacked Josephson Junctions

We have studied on the quasiparticles (QPs) injection effects for Nb/Al/AlOx/Al/Nb tunnel junctions at 0.4 K. A device structure was a stack of an injector Josephson junction (JJ) and a detector JJ. At a low temperature, there is small number of thermally excited QPs and we expected to observe easily the QPs injection effect of the detector JJ using the injector JJ. In this study, the structure of the Nb/Al/AlOx/Al/Nb tunnel junctions was the same for X-ray detectors using superconducting tunnel junctions. For constant-QPs current injection, we expected a small-signal current gain exceed the unity. We used pulsed-QPs current with changing pulse duration. For long duration, I-V curves for the detection junctions were sum of original current and QP current. On the other hands, for short duration time, the I-V curves showed different shapes from those for the long duration time.