Quasiparticle relaxation times in clean Al films

Double tunnel junctions S1-S2-S3 (Al-Al-Pb:Bi, mounted on sapphire substrates in vacuum) have been used to measure the inelastic (tau/sub i/) and elastic (tau/sub e/) scattering times in the center Al film (T/sub c/=1.28 K). The current peak at V/sub d/1= +- (Vertical BarV/sub g/Vertical Bar+(Δ1-Δ3)/e) and the current step at V/sub d/2= +- (Vertical BarV/sub g/Vertical Bar-(Δ1+Δ3)/e) provide complementary data on tau/sub i/(E,T*) and tau/sub e/(E,T*) in S2 over the energy range 1.5Δ(0)<E<5.5Δ(0) and the temperature range 0.6<T*<1.1 K; eV/sub d/1 and eV/sub d/2 yield Δ(1 which through the BCS gap equation yields T(1. We assume T(1=T(2=T* and T3=T/sub s/ the substrate temperature which is measured directly. The measured generator junction power per unit area P/sub g/, T*, and T/sub s/ yield P/sub g/=11(T/sup asterisk4/-T/sup asterisk4//sub s/) mW cm-2, in good agreement with the calculations by Kaplan for an Al-sapphire interface. A fit of our data for tau/sub i/ with theory gives a value of (1.0 +- 0.14) x 10-7 sec for tau0, the characteristic electric-phonon scattering time