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 ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}.$ We find that $90\ifmmode^\circ\else\textdegree\fi{}$ 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 $45\ifmmode^\circ\else\textdegree\fi{}$ 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.