Injection-detection experiments to study quasiparticle losses in Nb, using a series array of Nb/Al junctions

Injection-detection experiments have been performed to study quasiparticle diffusion and quasiparticle losses in Nb strips using a series array of Nb/Al junctions. The I-V curve of the detector junction was measured as a function of the spacing between the (high-quality) detector junction and the simultaneously biased injector junction. For our sample geometry, the quasiparticle gas is described in terms of a one-dimensional diffusion model. It was found that trapping losses at the interface between the Nb base electrode and the anodised Nb2O5 dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D = 1.2 × 10−4 m2/s has been found. We estimate that this corresponds to an electron mean free path of l ≈ 7 nm. Due to the difference between the bandgaps of the base and top electrode and to the peak in the density of states at the gap edge, the excess detector current shows a characteristic voltage dependence for bias voltages below 0.5 mV.