Characterization of YBCO/Au/YBCO resistors for HTS Josephson noise thermometry

It has been proposed that an absolute thermometer based on the linewidth broadening of the Josephson oscillation could be realized in the temperature range 10 K to 50 K with the use of high-temperature-superconducting resistive superconducting quantum interference devices (HTS R-SQUIDs). Although this technique has been successfully demonstrated with LTS devices in the temperature region below 1 K, there are unresolved problems which have so far prevented its translation to higher operating temperatures. A crucial component of a resistive SQUID noise thermometer is the shunt resistor, which is required to have a value <0.1 mΩ. The often excessive and unpredictable contact resistance between normal metal and HTS superconducting thin films has up to now hindered the successful implementation of an HTS resistive SQUID. YBCO-Au-YBCO shunt resistors with suitable resistance values of ~20 μΩ have recently been manufactured and their characteristics under cryogenic operating conditions have been measured using the Josephson frequency-voltage relationship