BARRIER PROPERTIES OF GRAIN-BOUNDARY JUNCTIONS IN HIGH-T-C SUPERCONDUCTORS

We have measured the quasiparticle tunneling (QPT) in grain boundary J osephson junctions for different high temperature superconductors. The investigation of the temperature and voltage dependence of the QPT in different applied magnetic fields (from 0 to 12 T) over a wide voltag e range provided strong evidence for an insulating barrier with a high density of localized states. The average barrier height phi and thick ness d of the different materials was extracted from the parabolic bac kground conductance at high voltages. In the low voltage regime we obs erved a zero-bias conductance peak, that is discussed in terms of spin -flip and Kondo-type scattering at the interface (Anderson-Appelbaum m odel).