Evidence for strong electron-phonon interaction from inelastic tunneling of Cooper pairs in c-direction in Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8) break junctions

A reproducible fine structure at subgap voltages in the I(U)-characteristics of Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8) break junctions has been observed and investigated. The structure is detectable only in the presence of an a.c. Josephson current. The position of the dips, composing the structure in the dI/dU-characteristics, is independent of the gap parameter Delta , the temperature T and the geometry of the contacts. The overall form of the fine structure is in good agreement with the Raman scattering spectra of the phonon modes in this material. We attribute this structure to an inelastic (phonon assisted) tunneling of Cooper pairs, which is accompanied by the emission of coherent Raman-active optical phonons at resonance voltages U sub(res) identical with h-bar omega sub(phon)/2e. These results hint for strong electron-phonon interaction in this material.