Impurity-induced bound excitations on the surface of Bi2Sr2CaCu2O8

We have probed the effects of atomic-scale impurities on superconductivity in Bi2Sr2CaCu2O8 by performing low-temperature tunneling spectroscopy measu rements with a scanning tunneling microscope, Our results show that nonmagn etic defect structures at the surface create localized low-energy excitatio ns in their immediate vicinity. The impurity-induced excitations occur over a range of energies, including the middle of the superconducting gap, at t he Fermi level. Such a zero bias state is a predicted feature for strong no nmagnetic scattering in a d-wave superconductor.