High T c refractory thin-film microwave SQUID’s

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Abstract
Thin‐film SQUID’s fabricated from the high Tc refractory material NbN have been studied at 9.4 GHz. Devices with granular weak links display a Josephson critical current dependence over a wide range of temperatures below Tc. We have observed open loop signal‐to‐noise ratios as high as 20 000 corresponding to a minimum detectable energy of 1×10−30 J in a 1 Hz bandwidth over a temperature range of several °K. In two samples, the response time was found to be limited by the intrinsic gap response time, τg∼h//2Δ (T). These devices are rugged, stable, cycle well, and have good SQUID properties at both 9.4 GHz and 20 MHz.