Light emission by tunneling Cooper pairs across a vacuum gap

We have observed visible light emission by tunneling Cooper pairs across th e vacuum gap of a scanning tunneling microscope (STM) formed by a Nb tip an d a Nb sample at 4.7 K. Besides the usual STM light emission that is excite d by single electron tunneling, we have observed a new peak at the photon e nergy of (h) over bar omega = 2eV(0) for a bias voltage V-0. This peak is o bserved only when the system is cooled below the superconducting transition temperature of niobium (9.3 K). We conclude that the new emission is excit ed by inelastic tunneling of Cooper pairs in superconducting Nb. (C) 2000 E lsevier Science Ltd. All rights reserved.