Temperature dependence of the Josephson linewidth of resistively shunted double junction HTS SQUIDs

Double Josephson junction HTS resistive SQUIDs have been fabricated using an on-chip shunt Au resistor across a gap in an YBa₂Cu₃O_7-δ loop, providing a low shunt resistance of R_s ~26 μΩ at T⩽77 K. The dependence of the heterodyne oscillation linewidth of the R-SQUID on the thermal noise across the shunt resistor is under investigation with a view to the development of an absolute noise thermometer. Signal powers ~nW have been measured and the device has been shown to be tunable from 5 to 50 MHz. The measured oscillation linewidth (~30 kHz at 17 K) is believed to be the narrowest reported for such an HTS device, however non-thermal broadening of the linewidth is also observed, as has been reported for single junctions at GHz frequencies. We describe measurements on such devices over a range of temperatures and bias conditions