Single electron tunneling effect in YBCO films

We have measured the tunneling spectra of c-axis-oriented YBCO films prepared by the sputtering method. In some spots, the spectra show periodic current steps at the corresponding voltages of V = (2n + 1)δV/2 or V = nδV. We have measured δV with the variation of the sample-to-tip distance. The resulting voltage period δV = e/C increases as the sample-to-tip distance increases. This behavior is explained 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 experimental result, indicating that the surface layer of YBCO is in a normal state due to oxygen deficiency.