Double tunnel junction experiments with epitaxial niobium and tantalum films

We have used three-terminal double tunnel junction devices to investigate non-equilibrium quasiparticle and phonon transport in epitaxial superconducting films of niobium and tantalum. We show that the two junctions are coupled both by quasiparticle diffusion through the film and by the diffusion of recombination phonons through the sapphire substrate. By separating these two processes and making a random walk simulation of the quasiparticle diffusion we are able to estimate the effective quasiparticle lifetime in our niobium and tantalum films. This is not limited by recombination. Furthermore the quasiparticle lifetime in tantalum is less limited by the other (as yet unidentified) loss processes than that in niobium. This suggests that superconducting tunnel junction particle spectrometers utilising tantalum absorbers may display better energy resolution than those with niobium absorbers which are currently being investigated.