Direct Measurement of Quasiparticle-Lifetime Broadening in a Strong-Coupled Superconductor

We have measured the quasiparticle recombination time in the strong-coupled superconductor ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$ directly by measuring the lifetime-broadened energy gap edge. This is done by measuring the $I\ensuremath{-}V$ characteristics of a superconducting tunnel junction of the type ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$-insulator-${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$. Agreement with the calculated value is excellent.