Possible observation of multiple-particle tunneling in niobium tunnel junctions

Abstract

The I-V characteristics of strongly coupled symmetric niobium-based superconducting tunnel junctions are found to display steplike structures at voltages less than the gap voltage 2Δ/e. A thorough investigation into the influences of magnetic-field and temperature variations on the structures has been performed. In addition, measurements have been made that allow the homogeneity of the junction barrier to be determined. The experimental results indicate that the structures arise due to either self-coupling, multiple Andreev-reflection processes or multiple-particle tunneling. The data have been analyzed in terms of each of these theories. The results of this analysis appear to indicate that multiple-particle tunneling is the mechanism most likely to be responsible for the subgap structures. If this is the case, this would indicate that three-particle tunneling has been observed in niobium-based junctions. Specific features in the structures are also observed; the current steps do not appear at exactly the voltages expected, and some steps are sharper than others. A modified version of the theory describing multiple-particle tunneling is also presented. It is found that this model is in good agreement with the experimental data. In addition, it is able to describe all features of the structures and indicates the presence of two different, resolved gaps in the superconducting region next to the junction barrier.