Influence of the oxygen content on the tunneling characteristics ofYBa2Cu3O7−δ

We have formed planar tunnel junctions between Pb films and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals with different surface oxygen stoichiometries. Oxygen losses have been induced by heating the crystals at 400 °C in vacuum for different times. Low-leakage tunnel junctions have also been obtained in the case of the most ‘‘degraded’’ samples. As the oxygen deficiency increases, the Y-Ba-Cu-O gaplike structure gradually broadens and shifts towards higher energies. For thermally untreated samples, tunneling spectroscopy gives information on the local ${\mathit{T}}_{\mathit{c}}$.