Steady state measurements of the quasiparticle lifetime in superconducting aluminium

Abstract

We report details of the steady state measurements of the quasiparticle lifetime (τ) in superconducting aluminium which were published previously. We begin with a synopsis of the relevant background, followed by an explanation of our experimental procedure. Included are some aspects of the experiments not primarily related to the lifetime measurements. It is shown that the process responsible for the excess current found by Burstein and Taylor in aluminium tunnel junctions for Δ < Vⁱ < 2Δ is one which breaks Cooper pairs and injects new quasiparticles. Here Vⁱ is the voltage across the injector junction and Δ is the energy gap. At large injection currents τ decreases as predicted, but for Vⁱ > 4Δ we observe enhanced injection of quasiparticles. We show that the effect of heating on the lifetime measurements is negligible at low temperatures and can be accounted for near the transition temperature (T_c). Near T_c there is another recombination process in which the excess energy is absorbed by another quasiparticle. Very near T_c there are indications that τ becomes very long. As the temperature decreases much below T_c, τ approaches a constant value due to recombination in regions of flux penetration.