Detection of High-Frequency Phonons from the Inhomogeneous Gap States of a Nonequilibrium Superconductor

Measurements are reported, for the first time, for detection of nonequilibrium phonons emitted from the inhomogeneous gap states of a nonequilibrium superconductor by means of a phonon transmission technique. The observed phonon signal exhibits a clear spatial structure with a certain periodicity. The connection between the $I-V$ characteristic and the phonon signal is discussed.