Barrier properties of grain boundary junctions in high-T/sub c/ superconductors

We have measured the quasiparticle tunneling (QPT) in grain boundary Josephson 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 voltage range provided strong evidence for an insulating barrier with a high density of localized states. The average barrier height /spl phi/ and thickness d of the different materials was extracted from the parabolic background conductance at high voltages. In the low voltage regime we observed a zero-bias conductance peak, that is discussed in terms of spin-flip and Kondo-type scattering at the interface (Anderson-Appelbaum model).