YBCO/Pb tunnel junctions: reproducibility, cyclability and role of the oxygen content at the YBCO surface

Planar tunnel junctions were realized on liquid-etched YBa₂ Cu₃O_7-δ (YBCO) single crystals with a native barrier and a Pb film as counterelectrode. Reproducibility of the tunneling characteristics has been tested on more than 150 junctions. Cyclability was good for six thermal cycles over the course of 6 mo. The main features on the dV/dI-vs-V curves were gaplike structures at ±4-5 mV and ±19 mV and well-defined minima at ±36 mV. The amplitudes of these structures showed little change when the YBCO single crystals were exposed for 24 h to room atmosphere prior to the Pb evaporation. By heating the samples for 1 h at 400°C in vacuum before the counterelectrode was applied to complete the junction, the structures broadened and shifted toward high energies. For all junctions, subgap leakage currents were less than 5% at 4.2 K, and the Pb gap and phonon structures were clearly shown on the tunneling characteristics, indicating a good-quality tunnel barrier