QUASI-PARTICLE TUNNELING IN HTS BICRYSTAL GRAIN-BOUNDARY JUNCTIONS

We have measured the temperature and voltage dependence of the quasipa rticle tunneling (QPT) in YBa2Cu3O7-delta (YHCO), Bi2Sr2CaCu2O8+x (BSC CO), Nd1.83Ce0.17CuOx (NCCO) and La1.85Sr0.15CuO4-delta (LSCO) grain b oundary Josephson junctions (GBJs). At voltages well above the gap vol tage V-g, the measured conductance vs. voltage curves show a temperatu re 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 ar e 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, which explains both the large subgap c onductance at low temperatures and the strongly reduced conductance pe ak at the gap voltage, most likely is related to the high density of l ocalized states within the grain boundary barrier.