Observation of bound surface states in grain-boundary junctions of high-temperature superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain-boundary junctions (GBJ's) fabricated from the high-temperature superconductors (HTS) YBa2Cu3O7-delta (YBCO), Bi2Sr2CaCu2O8+delta (BSCCO), La1.85Sr0.15CuO4 (LSCO), and Nd1.85Ce0.15CuO4-y,(NCCO). In ail experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed that decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero-energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions. [S0163-1829(98)08441-0].