Quasiparticle tunneling in HTS grain boundary Josephson junctions

The quasiparticle tunneling (QPT) in YBa2Cu3O7-delta (YBCO), Bi2Sr2CaCu2O8+x (BSCCO), Nd1.85Ce0.15CuOx (NCCO) and La1.85Sr0.15CuO4-delta (LSCO) grain boundary Josephson junctions (GBJs) has been studied. At voltages above the gap voltage V-g, the measured conductance vs. voltage, G(V), curves show a temperature independent parabolic shape allowing the determination of the effective height and thickness of the insulating grain boundary barrier. At \V\ < V-g, the G(V)-curves are well fitted by a BCS density of states and temperature dependence of the energy gap, if a large broadening parameter Gamma is assumed for the gap. The large Gamma value most likely is related to the high density of localized states within the grain boundary barrier. Furthermore, a zero-bias conductance anomaly is observed which can be analyzed in terms of spin-flip and Kondo-type scatttering at the interface (Anderson-Appelbaum model).