ANDREEV BOUND-STATES IN HIGH-TEMPERATURE SUPERCONDUCTORS

Andreev bound states at the surface of superconductors are expected fo r any pair potential showing a sign change in different k-directions w ith their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation uf Andre ev bound states in high temperature superconductors (HTS) employing tu nneling spectroscopy on bicrystal grain boundary Josephson junctions ( GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YB a2Cu3O7-delta (YBCO), Bi2Sr2CaCu2O8+delta (BSCCO), and La1.85Sr0.15CuO 4 (LSCO) show a pronounced zero bias conductance peak that can be inte rpreted in terms of Andreev bound states at zero energy that are expec ted at the surface of HTS having a d-wave symmetry of the order parame ter. In contrast, for the most likely s-wave HTS Nd1.85Ce0.15CuO4-y (N CCO) no zero bias conductance peak was observed. Applying a magnetic h eld results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic fiel d. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.