Quasiparticle relaxation and phonon emission in superconducting tunnel junctions

Abstract

Using Sn-I-Sn tunnel junctions as generators and detectors of high frequency phonons, the temperature dependence (down to 0.3°K) of the derivative of the detected signal as a function of generator bias is reported. For T≤1°K we have observed the expected BCS singularity in the signal at a generator bias of 4Δ. An additional singularity has been observed at a bias of 2Δ when the number of injected particles greatly exceeds the number thermally excited. Unlike the 4Δ peak, the temperature of occurrence of the 2Δ peak depends markedly on generator impedance, (injection rate) as expected. The data provide strong evidence that Tewordt's model (strictly valid for T=0°K), in which an excited quasiparticle relaxes predominantly in a single step to the gap edge before recombination, is correct in the temperature range T/T_c < 0.3 and for low injection rates.