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) (Formula presented) (YBCO), (Formula presented) (BSCCO), (Formula presented) (LSCO), and (Formula presented) (NCCO). In all experiments the tunneling direction was along the (Formula presented) 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. © 1998 The American Physical Society.