Spatial structure of a tunnel-injected nonequilibrium superconductor

The measurements on the spatial distribution of phonons emitted from a tunnel-injected nonequilibrium superconductor using a phonon transmission technique are reported. The injector junction emitting nonequilibrium phonons is an in-line-type $\mathrm{Sn}-I-\mathrm{Sn}$ junction almost free from the geometrical effect of current flow. The phonon sensor is either a small tin or lead junction. The periodically modulated structure with the period about 100 μm is found, which is profoundly enhanced in case that a lead sensor is used. The nature of the transmitted excitations through liquid helium is also investigated.