Quasiparticle tunneling in HTS bicrystal grain boundary junctions

Abstract

We have measured the temperature and voltage dependence of the quasiparticle tunneling (QPT) in YBa2Cu3O7−σ (YBCO), Bi2Sr2CaCu2O8+x (BSCCO), Nd1.83Ce0.17CuOx (NCCO) and La1.85Sr0.15CuO4−σ (LSCO) grain boundary Josephson junctions (GBJs). At voltages well above the gap voltageVg, the measured conductance vs. voltage curves show a temperature independent parabolic shape. From these curves the effective height and thickness of the insulating grain boundary barrier is derived. At voltages below the gap voltage, the conductance vs. voltage 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, which explains both the large subgap conductance at low temperatures and the strongly reduced conductance peak at the gap voltage, most likely is related to the high density of localized states within the grain boundary barrier.