Imaging nonequilibrium phonons in Nb

Phonon imaging techniques are applied to superconducting Nb. Due to the sup erconducting energy gap, the electronic scattering of phonons in metals is greatly reduced and ballistic phonon transport is observed. The non-equilib rium phonons which ape generated in the experiment are accompanied by non-e quilibrium quasiparticles. These quasiparticles modify the observed phonon propagation and also recombine to generate additional phonons which are det ected. Thus, our experiments provide information not only about phonon prop agation in a Nb lattice, but also about the phonon-quasiparticle interactio ns and the dynamics of the non-equilibrium quasiparticles, (C) 1999 Elsevie r Science B.V. All rights reserved.