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 (τᵢ) and elastic (τₑ) scattering times in the center Al film (Tc=1.28 K). The current peak at Vd1=±[|Vg|+(Δ₁-Δ₃)e] and the current step at Vd2=±[|Vg|-(Δ₁+Δ₃)e] provide complementary data on τᵢ(E,T*) and τₑ(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; eVd1 and eVd2 yield Δ₁* which through the BCS gap equation yields T₁*. We assume T₁*=T₂*=T* and T₃=Tₛ the substrate temperature which is measured directly. The measured generator junction power per unit area Pg, T*, and Tₛ yield Pg=11(T*4-Tₛ*4) mW cm^-2, in good agreement with the calculations by Kaplan for an Al-sapphire interface. A fit of our data for τᵢ with theory gives a value of (1.0±{}0.14) ×{} 10^-7 sec for τ₀, the characteristic electric-phonon scattering time.