Quasiparticle scattering time in niobium superconducting films

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean-free paths in a nonequilibrium state; that is, the highly dissipative flux-flow state driven up to the instability point. From the measured current-voltage curves we derive the vortex critical velocity v* and the quasiparticle energy relaxation time ?? for several temperatures within the Larkin-Ovchinnikov model. Numerical calculations of the quasiparticle energy relaxation rates are also carried out to support the experimental findings. We observe a reduction of the quasiparticle relaxation time along with an almost constant behavior as a function of the temperature below Tc as the electron mean-free path decreases. These features can be associated with an increasing contribution from the electron-phonon process arising from a larger quasiparticle energy scale involved at the vortex instability. Furthermore, for thicker samples the recombination process dominates over scattering processes in the total quasiparticle relaxation.