INJECTION DETECTION EXPERIMENTS TO STUDY QUASI-PARTICLE LOSSES IN NB,USING A SERIES ARRAY OF NB AL JUNCTIONS/

Injection-detection experiments have been performed to study quasipart icle diffusion and quasiparticle losses in Nb strips using a series ar ray of Nb/Al junctions. The I-V curve of the detector junction was mea sured as a function of the spacing between the (high-quality) detector junction and the simultaneously biased injector junction. For our sam ple geometry, the quasiparticle gas is described in terms of a one-dim ensional diffusion model. It was found that trapping losses at the int erface between the Nb base electrode and the anodised Nb2O5 dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D = 1 .2 X 10(-4) m(2)/s has been found. We estimate that this corresponds t o an electron mean free path of 1 approximate to 7 nm. Due to the diff erence between the bandgaps of the base and top electrode and to the p eak in the density of states at the gap edge, the excess detector curr ent shows a characteristic voltage dependence for bias voltages below 0.5 mV.