Resistive SQUID calorimetry at low temperatures

A resistive SQUID may be used to measure heat current. We call an RSQUID used in such a way an RSqUID: It acts as a heat-current to frequency (q–f) converter of sensitivity =f/q. Two methods of heat capacity measurement by the use of an RSqUID are described. In the direct method the number of cycles of oscillation when the temperature of the RSqUID and specimen is changed by T is counted. The measured heat capacity C_m(to be corrected for the addenda) is given by C_m=n/(T). In the step method the specimen and heater together are separated from the RSqUID by a thermal resistance R. The RSqUID is biased to a frequency f₁. The heater is switched on and a number of cycles is obtained which represents the amount of heat required to bring the specimen to equilibrium in the new steady state, in which f=f₂+f. By this method the quantity C_m=n/(Rf) is obtained. In both methods the heat current is integrated by counting to obtain an amount of heat Q=n/, which may thus be measured very precisely if the number of cycles is sufficiently large [and (T) calibrated accurately]. The first RSqUID constructed for heat capacity measurement is described. In this RSqUID the high thermal resistance of press contacts has been avoided and soft solder as well as niobium used for superconducting material. The sensitivity varied from 0.8 to 0.4 Hz nW_–1 between 2 and 7 K. The results of measurements made between 2 and 7K by the step method with this RSqUID in a rather unsophisticated cryostat achieved a relative accuracy of ~0.1% with samples of mass about 1 g. Results of measurements made on samples of pure copper and indium are described.