SINGLE-ELECTRON TUNNELING EFFECT IN YBCO FILMS

We have measured the tunneling spectra of c-axis-oriented YBCO films p repared by the sputtering method. In some spats, the spectra show peri odic current steps at the corresponding voltages of V = (2n + 1)delta V/2 or V = n delta V, We have measured delta V with the variation of t he sample-to-tip distance. The resulting voltage period delta V = e/C increases as the sample-tu-tip distance increases. This behavior is ex plained by single electron tunneling. The simulated conductance, with the assumption of a normal state density of states for both the metal grains and the YBCO surface layer, shows a good agreement with the exp erimental result, indicating that rite surface layer of YBCO is in a n ormal state due to oxygen deficiency.