Impurity-Induced Bound Excitations on the Surface ofBi2Sr2CaCu2O8

We have probed the effects of atomic-scale impurities on superconductivity in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ by performing low-temperature tunneling spectroscopy measurements with a scanning tunneling microscope. Our results show that nonmagnetic defect structures at the surface create localized low-energy excitations in their immediate vicinity. The impurity-induced excitations occur over a range of energies, including the middle of the superconducting gap, at the Fermi level. Such a zero bias state is a predicted feature for strong nonmagnetic scattering in a $d$-wave superconductor.