Abstract
Measurements of five variations of a toroidal point-contact dc superconducting quantum interference device (SQUID) are presented. The energy resolution and other parameters of these SQUIDs are examined and compared with the predictions of the Resistively Shunted Junction model. For these SQUIDs, the measured minimum energy resolution was approximately 2×10−30 J/Hz. Excess noise in the point contacts was found to limit the energy resolution of the SQUIDs. A comparison between the typical junction parameters and noise obtained for our niobium-niobium point contacts and those of others is given. TheI–V characteristics of the junctions showed the effects of Joule heating. The white voltage noise spectral density was found to have an approximately parabolic dependence on the average voltage for bias currents larger than the critical current. While this parabolic dependence is consistent with heating effects in the junctions, the amplitude of the noise cannot be explained in terms of a heating model. The low-frequency noise of the point contacts has also been investigated.
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