Tunneling measurements in Y-Ba-Cu-O point contact junctions

Tunnelling experiments in high-Tc superconductors (YBa2Cu3O7-δ) using point-contact junctions are discussed. Different phenomena are observed in these junctions depending on the nature of the electrodes and the experimental conditions, including superconducting tunnelling and normal background conductance. Direct evidence can be obtained of some features of the superconducting status in these materials: an oxygen deficient layer has been evidenced at the surface exhibiting distributed gap values in addition to a natural tunnelling barrier which seems present anyway. Measurements on large-area-point-contact junctions of polycrystalline YBa2Cu3O7-δ samples, both monophasic and polyphasic, are reported. In order to interprete in a quantitative way the measurements, a phenomenological model has been introduced, which decomposes the experimental conductance in a background component and in a superconducting-tunnelling component, smared out in voltage by a large amount of broadening