Spatially continuous zero-bias conductance peak on (110) YBa2Cu3O7-delta surfaces

We report the observation of a pronounced high zero-bias conductance peak (ZBCP) in the conductance spectra of (110)-oriented YBa2CU3O7-delta epitaxial thin-film surfaces by low-temperature scanning tunneling microscopy/spectroscopy at 4.2 K. On firms with very low surface roughness, both the superconducting gap structure and the ZBCP could be clearly observed in the spectra and neither depended on the measurement position or on the distance between the tunneling tip and the sample surface. This result is in agreement with the recent theoretical prediction of the formation of bound states on surfaces of d-wave superconductors that lead to a ZBCP with a maximum for (110)-oriented surfaces.