Unusual strong-coupling effects in the tunneling spectroscopy of optimally doped and overdoped Bi2Sr2CaCu2O8+δ

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 resolved spectra. With overdoping the position of the dip continues to scale with a, and its magnitude decreases as a decreases. These results indicate that the dip feature arises from a strong-coupling effect whereby the quasiparticle lifetime is decreased at a characteristic energy of similar to 2 Delta, consistent with an electron-electron pairing interaction.