Phonon and Quasiparticle Dynamics in Superconducting Aluminum Tunnel Junctions

The effect of the $2\Delta$ recombination phonons in modifying the effective lifetime ${\tau }_{\mathrm{eff}}$ for quasiparticle recombination has been determined for the first time in a double-junction experiment by comparing measurements taken both with and without a helium film covering the junctions. For a double junction made from evaporated aluminum films 300 Å thick deposited on a glass substrate, we find $\frac{{\tau }_{\mathrm{eff}}}{{\tau }_R}=8.6\mathrm{}\pm 1.1$ in the absence of the helium film, and, at $\frac{\Delta }{\mathrm{kT}}=6\mathrm{}$, ${\tau }_R=1.4\mathrm{}\pm 0.1$ μsec.