DOUBLE TUNNEL JUNCTION EXPERIMENTS WITH EPITAXIAL NIOBIUM AND TANTALUM FILMS

We have used three-terminal double tunnel junction devices to investig ate non-equilibrium quasiparticle and phonon transport in epitaxial su perconducting films of niobium and tantalum. We show that the two junc tions are coupled both by quasiparticle diffusion through the film and by the diffusion of recombination phonons through the sapphire substr ate. By separating these two processes and making a random walk simula tion of the quasiparticle diffusion we are able to estimate the effect ive quasiparticle lifetime in our niobium and tantalum films. This is not limited by recombination. Furthermore the quasiparticle lifetime i n tantalum is less limited by the other (as yet unidentified) loss pro cesses than that in niobium. This suggests that superconducting tunnel junction particle spectrometers utilising tantalum absorbers may disp lay better energy resolution than those with niobium absorbers which a re currently being investigated.