Formation of an andreev bound state at the step edges of (formula presented) 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 (formula presented) We find that (formula presented) 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 (formula presented)\ 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.