Properties of normal metal/dielectric/high-T-c junctions obtained by in-situ oxidation

We describe a novel method by which normal metal/insulator/high-T-c planar tunnel junctions can be obtained. In this method the thin dielectric layer is produced by the in-situ oxidation of a metal with a strong electronegativity, through diffusion of oxygen from the high-T-c oxide. When applied to high-T-c thin films, the resulting junction is in the underdoped regime, In a-axis YBa2Cu3O7-delta/Al2O3/Ag junctions, in which T-c has been reduced down to about 50 K, the gap structure extends up to more than 40 meV, suggesting a mean-field T-c significantly higher than the measured one.