We observe that the retrapping current of high-resistance, small-capacitance, superconducting tunnel junctions decreases sharply with decreasing temperature, and then has a puzzling, flat, nonzero minimum at low T, rather than continuing to decrease as quasiparticle damping freezes out. We explain the crossover by considering the combined admittance of the leads and of the junction (including the onset of pair-breaking tunneling at the gap voltage) at both dc and the Josephson frequency. Our analog simulations and analytic approximations both agree well with the data.

• Received 17 April 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1263