Energy-dependent quasiparticle group velocity in a superconductor

We have measured the energy dependence of the quasiparticle group velocity in a superconductor. We use a normal-insulator-superconductor tunnel junction to inject quasiparticles at known energies into a superconducting Al film. The quasiparticles diffuse throughout the superconductor and their flux is subsequently measured at a known distance from the injector junction by a thermal detector. We measure an increase in the flux of quasiparticles reaching the detector when their energy is raised and explain this result using the predictions of Bardeen, Cooper, and Schrieffer for the energy dependence of the quasiparticle group velocity.