UNUSUAL STRONG-COUPLING EFFECTS IN THE TUNNELING SPECTROSCOPY OF OPTIMALLY DOPED AND OVERDOPED BI2SR2CACU2O8+DELTA

Tunneling spectroscopy measurements are reported on single crystals of Bi2Sr2CaCu2O8+delta using vacuum tunneling and point-contact methods. A reproducible dip feature in the tunneling conductance is found near /eV/ = 2 Delta, observed for both voltage polarities in the best reso lved spectra. With overdoping the position of the dip continues to sca le with a, and its magnitude decreases as a decreases. These results i ndicate that the dip feature arises from a strong-coupling effect wher eby the quasiparticle lifetime is decreased at a characteristic energy of similar to 2 Delta, consistent with an electron-electron pairing i nteraction.