Quasiparticle tunneling in HTS grain boundary Josephson junctions

The quasiparticle tunneling (QPT) in YBa₂Cu₃O_7-δ (YBCO), Bi₂Sr₂CaCu₂O_8+x (BSCCO), Nd_1.85Ce_0.15CuO_x (NCCO) and La_1.85Sr_0.15CuO_4-δ (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 Γ is assumed for the gap. The large Γ 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 scattering at the interface (Anderson-Appelbaum model).