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) YBa₂Cu₃O_7-δ (YBCO), Bi₂Sr₂CaCu₂O_8+δ (BSCCO), La1.85Sr0.15CuO₄ (LSCO), and Nd1.85Ce0.15CuO_4-y (NCCO). In all 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.