White-noise measurements and Joule heating in superconducting point contacts

Abstract

We report measurements of the white noise in superconducting niobium point contacts. These measurements, the junction parameters, and the method of fabrication of the point contacts are compared with those of other experiments. The power spectral density was found to have an approximately parabolic dependence on the average voltage V rather than the linear dependence expected in the shot-noise limit. We also found that the magnitude of the excess noise was larger than has been previously reported. Because of these differences, the noise data presented add to the existing body of data. The V2 dependence of the power spectral density suggests that Joule heating may have been responsible for the excess noise. Joule heating was determined to be present in the junctions from the relatively small critical currents I0 ≲2 μA and the effect that a resonant cavity had on the Josephson oscillation. However, we show that Joule heating cannot account for the level of the excess noise observed in present or previous data.