Observations of quasi-particle tunneling and Josephson behavior in Y sub 1 Ba sub 2 Cu sub 3 O sub 7 minus x / native barrier/Pb thin-film junctions

Low-leakage, thin-film planar tunnel junctions made of Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}/ native barrier/Pb were fabricated. The Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} films were prepared by {ital in} {ital situ} molecular beam epitaxy aided with an activated oxygen source. The as-grown, smooth superconducting perovskite film surface exhibits quasi-particle tunneling characteristics very similar to the etched bulk single-crystal data. The results in agreement are a linear dependence of the normal-state conductance on voltage, a gap-like structure at {similar to}20 mV, asymmetric modulations up to 50 mV, and a finite zero-bias conductance at low temperature. Junctions of lower resistance show, at temperatures below {ital T}{sub {ital c}} of Pb, the development of a supercurrent at zero bias and associated hysteretic subgap structure, with a typical {ital I}{sub {ital c}}{ital R} {similar to}0.5 mV. Josephson-like behavior occurred in response to applied magnetic field and microwaves.