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

We describe a novel method by which normal metal/insulator/high-T sub(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 sub(c) oxide. When applied to high-T sub(c) thin films, the resulting junction is in the underdoped regime. In a-axis YBa sub(2)Cu sub(3)O sub(7- delta )/Al sub(2)O sub(3)/Ag junctions, in which T sub(c) has been reduced down to about 50 K, the gap structure extends up to more than 40 meV, suggesting a mean-field T sub(c) significantly higher than the measured one.