Evidence for strong electron-phonon interaction from inelastic tunneling of Cooper pairs in c-direction in Bi2Sr2CaCu2O8 break junctions

A reproducible fine structure at subgap voltages in the I(U)-characteristic s of Bi2Sr2CaCu2O8 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-characterist ics, is independent of the gap parameter Delta, the temperature T and the g eometry 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 mat erial. We attribute this structure to an inelastic (phonon assisted) tunnel ing of Cooper pairs, which is accompanied by the emission of coherent Raman -active optical phonons at resonance voltages U-res = (h) over bar omega(ph on)/2e. These results hint for strong electron-phonon interaction in this m aterial. (C) 1999 Elsevier Science Ltd. All rights reserved.