ORIENTATION DEPENDENCE OF TUNNELING SPECTRA IN YBCO AND NCCO

We performed a crystal orientation dependent study of the tunneling sp ectra in high-temperature superconductors. The comparison of the tunne ling spectra for different orientations and materials as measured by L ow Temperature Scanning Tunneling Microscopy/Spectroscopy (LTSTM/S) gi ves information about the relation between tunneling spectra and order parameter symmetry. Our data - obtained on well characterized, high q uality (100), (110), and (001) epitaxial YBa2Cu3O7-delta thin films an d Nd1.85Ce0.15CuO4-y single crystals - can be interpreted by assuming zero-energy surface states resulting from the sign change of the pair potential in d-wave superconductors. The observed spatially continuous and bias voltage independent zero bias conductance peak (ZBCP) on (11 0) YBa2Cu3O7-delta surfaces is then due to the d(x2-y2)-wave symmetry. Compared with that, Nd1.85Ce0.15CuO4-y never exhibited any ZBCP as ex pected because of its s-wave superconductivity.