Non BCS IV characteristics of superconducting Bi₂Sr₂CaCu₂O_8+δ single crystals

We present vacuum tunneling characteristics obtained on Bi2Sr2CaCu2O8+δ single crystals using a low temperature scanning tunneling microscope. We demonstrate very reproducible tunneling spectroscopy where the IV characteristics do not depend on modifications of the tunneling barrier obtained either by scanning the tip along the sample surface or by varying the tip/sample spacing. The IV curves and the differential conductance spectra show significant features which are not consistent with single gap BCS-like superconductivity. In particular the peaks at the gap edges and the low bias excitations in the experimental density of states are too large compared to the predictions of an isotropic BCS gap. Furthermore, we observe a dip in the differential conductance spectra at about -70meV sample bias, a structure which does not appear at positive sample bias