Superconducting Resistive-Transition Device for the Precision Measurement of Low AC Voltage

Single and multijunctions thermal converters are devices widely utilised for AC measurements at room temperature. However, low sensitivity and high noise of these devices limits their applications at voltage level above 10 mV. A cryogenic thermal transfer sensor has been developed to realise AC measurements at 1 mV voltage level. This device consists of a superconducting-resistive-transition Nb thin film, which detects the AC power, dissipated on a Cr film resistor. To improve the sensitivity, the sensor has been fabricated on a 500 nm thick square silicon nitride membrane with size ranging from 1x1 mm(2) to 3.5x3.5 mm(2). The device has been tested at liquid helium and a thermal conductance lower than 10(-6) W/K has been measured. From these data we expect an increasing of responsivity higher than three order of magnitude and a noise limited by intrinsic thermal noise.