Tunneling spectroscopy study of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films using a cryogenic scanning tunneling microscope

We have measured reproducible tunneling spectra on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (${\mathit{T}}_{\mathit{c}}$∼85 K) thin films (thickness ∼2 μm) with a cryogenic scanning tunneling microscope. We find that the I-V curves are generally of three types. The most common type, featured in a large majority of the data, shows a region of high conductance at zero bias. The amplitude of this region is inversely proportional to the tunneling resistance between the tip and sample. It is possible that this can be explained in terms of Josephson effects within the films, although an alternative is given based on electronic self-energy corrections. Data showing capacitive charging steps are analyzed in terms of two ultrasmall tunnel junctions in series.. Theoretical fits to the data give specific values of the junction parameters that are consistent with the assumed geometry of the tip probing an individual grain of the film. The third type of I-V curves exhibits negative differential resistance. We conclude that this phenomenon is probably due to tunneling to localized states in the surface oxide. We also present and discuss data with energy-gap-like behavior; the best example gives Δ to be about 27 mV.