Resistive SQUID calorimetry at low temperatures

Abstract

A resistive SQUID may be used to measure heatcurrent. We call an RSQUID used in such a way an RSqUID: It acts as a heat-current to frequency (q-f) converter of sensitivity fi = 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 AT is counted. The measured heat capacity Cm (to be corrected for the addenda) is given by Cm = n/(fl AT). In the step method the specimen and heater together are separated from the RSq UID by a thermal resistance R. The RSq UID is biased to a frequency f l. 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=fl +Sf. By this method the quantity Cm = n/(R6f) is obtained. In both methods the heat current is integrated by counting to obtain an amount of heat Q = n/fi, which may thus be measured very precisely if the number of cycles is sufficiently large (and fl(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 fi varied from 0.8 to 0.4 Hz n W-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.

The inductances of RF SQUIDS and the period of their response to applied magnetic flux

“…Thus it might be used to increase the maximum slew rate of a locked on SQUID when feedback is applied. The increase in amplitude would be valuable in applications of resistive SQUIDS where the amplitude represents the strength of the oscillating signal whose cycles are counted as, for instance, in the heat current meter described by Park and Vaidya (1980).…”

Self-induced Structure in the Current-Voltage Characteristics of RSQUIDs

“…As increases, the coupling between the two junctions at their Josephson frequencies around the loop through is progressively reduced. The reactance of half the loop inductance at a Josephson frequency is (2) where and so is just the ratio . For HTS RSQUIDs , so the magnitude of has little effect on the coupling.…”

“…The first RSQUIDs were RF-biased and were used for noise thermometry (e.g. [1]), calorimetry [2] and thermal measurements [3]. Harding and Zimmerman [4] described a DC-biased 2-junction RSQUID using mechanical niobium point-contact junctions, and Verschueren et al [5] made a Pb-Sn solder-blob version, but only relatively recently have thin-film versions of the DC RSQUID been reported [6]- [12].…”

Numerical Demonstration of Relaxation Oscillation in a Resistive Superconducting Quantum Interference Device With Two Nonhysteretic Josephson Junctions

“…Its configuration is essentially identical to that of resistive superconducting quantum interference devices (RSQUIDs), including two Josephson junctions. RSQUIDs are superconducting quantum interference devices (SQUIDs) with a partly resistive loop, and they have been developed for applications of noise thermometry [14], calorimetry [15], current-controlled oscillator [16], etc. The author numerically demonstrates that an asymmetrically biased two-junction RSQUID works as a relaxation oscillator.…”