Quasiparticle loss rates in Ta-based superconducting tunnel junctions

The quasiparticle (QP) lifetime in superconducting tunnel junctions (STJs) at sufficiently low temperature is usually found to be governed by loss processes associated with edges, interfaces or contacts. Such losses are closely related to fabrication issues and difficult to control. In our Ta-based STJs we observe variations in pulse decay time up to a factor of 2–3 for nominally the same devices. Nevertheless, experiments with STJs in which the thickness of the Al layers adjacent to the barrier is progressively increased, show a clear trend of increasing QP lifetime. Alternatively, introducing Nb capping layers below and on top of the Ta STJs gives a significant reduction of QP lifetime.