Tunneling spectroscopy of high-T_c superconductors

We have performed tunneling spectroscopy experiments on different kinds of high-temperature superconductors (HTS) and different types of junctions. Firstly, spectra were taken on surfaces of YBa2Cu3O7-δ (YBCO) thin films and Nd1.85Ce0.15CuO4-γ (NCCO) single crystals by low-temperature scanning tunneling microscopy/spectroscopy (LTSTM/S) in a superconductor-insulator-normal metal (SIN) configuration. Second, the tunneling spectra of grain-boundary junctions (GBJs) fabricated on bicrystal substrates were measured for the materials YBCO, La1.85Sr0.15CuO4-δ (LSCO), Bi2Sr2Ca-Cu2O8 + δ (BSCCO) and NCCO in a SIS configuration. The characteristics of the spectra in both systems could be related to the symmetry of the order parameter in the superconducting electrode(s). In particular, a zero bias conductance peak (ZBCP) was observed only in systems with a d-wave symmetry, but was completely absent in the s-wave system NCCO. © 1999 Elsevier Science Ltd. All rights reserved.