TUNNELING OBSERVATION OF PHONON CONTRIBUTION TO THE PAIRING INTERACTION IN OXIDE SUPERCONDUCTORS

Tunneling conductance spectrum measurements were carried out on BSCCO(2212) and La2-xSrCuOy (x=0.2) single crystals. Above the definite gap structure suggesting s-wave pairing, remarkable fine structures were reproducibly observed ranging to the bias limit of 0.2 eV. The conductance derivative peaks were compared with the phonon density of states peaks of the oxides reported by Renker et al, and a reasonable peak to peak correspondence turned out. Each peak area reflecting the coupling intensity alpha2(omega)F(omega)/omega was estimated and the character of the ion oscillation mode is discussed. Beyond the Debye phonon energy further significant peaks are recognized in BSCCO. Some of them correspond to multi-phonon energies, however with unexpectedly strong intensities.