Formation of an Andreev bound state at the step edges of Bi₂Sr₂CaCu₂O_8+δ surface

We have used a scanning tunneling microscope to measure the density of electronic states (DOS) in the vicinity of step edges and twin boundaries on the surface of Bi₂Sr₂CaCu₂O_8+δ. We find that 90° twin boundaries do not perturb superconducting DOS at all; however, near step edges, within a coherence length in the direction perpendicular to step, we find strong filling-in of the energy gap in the DOS. In this region of suppressed superconductivity, for step edges oriented at a 45° degree relative to the CU-O bond direction, the spectra show evidence for the formation of an Andreev bound state, which results in a resonance in the DOS at the Fermi level.