Characterization of niobium point contacts showing Josephson effects in the far infrared

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Abstract
The high‐frequency behavior of niobium cat‐whisker point contacts has been studied using radiation from an optically pumped far‐infrared laser. When the point contacts are classified on the basis of their high‐frequency performance, their dc I‐V curves fall into recognizable groups. We find that the ac Josephson effect has a strong correlation with the gap‐related structure on the I‐V curve, but none at all with the apparent excess current observed in all the contacts. For high‐performance junctions, these and other features of the I‐V curves are very reproducible from contact to contact, allowing a comparison with the available theories. The experimental evidence seems to suggest that our point contacts are best modeled as extremely small metallic constrictions.