Measurements of quasiparticle thermalization in a normal metal

Abstract

We have probed the inelastic relaxation of gap-edge quasiparticles in a superconductor to conduction electrons in an adjoining normal metal. Quasiparticles are injected into a superconducting Al film by a normal-insulator-superconductor tunnel junction at a temperature of about 100 mK. These quasiparticles diffuse throughout the superconductor and those that do not recombine are trapped in an adjoining Ag film. We find that trapped quasiparticles transfer more than 80% of their excitation energy to conduction electrons in the Ag over a broad range of electron and phonon temperatures and explain this result with a model that incorporates electron-electron, electron-phonon, and phonon-electron interactions.