Transport and noise characteristics of submicron high-temperature superconductor grain-boundary junctions

We have investigated the transport and noise properties of submicron $YBa2Cu3O7−δ$ bicrystal grain-boundary junctions prepared using electron beam lithography. The junctions show an increased conductance for low voltages reminiscent of Josephson junctions having a barrier with high transmissivity. The voltage noise spectra are dominated by a few Lorentzian components. At low temperatures clear two-level random telegraph switching signals are observable in the voltage versus time traces. We have investigated the temperature and voltage dependence of individual fluctuators both from statistical analysis of voltage versus time traces and from fits to noise spectra. A transition from tunneling to thermally activated behavior of individual fluctuators was clearly observed. The experimental results support the model of charge carrier traps in the barrier region.