Jack H. Colwell

Measurements of absolute temperature below 0.75 K using a Josephson-junction noise thermometer

In order to extend the international temperature scale of 1990, ITS-90, below its lower limit of 0.65 K, we have developed a temperature scale ranging from 6 to 750 mK. Values of absolute temperature are defined on this scale by an R-SQUID noise thermometer. A review is given here of the decade of our experience in the operation of this thermometer and in modeling its systematic errors. The reproducibility of noise temperature values was assessed using superconductive fixed points and the melting curve of3He. To assess the accuracy of the noise thermometer, it was compared with another absolute thermometer (based on nuclear orientation) at the lowest temperatures and with an internationally recognized scale above 0.5 K. The noise thermometer temperatures were used to calibrate two paramagnetic salt thermometers, the result being the construction of a temperature scale that is smooth to approximately 0.01%. On the basis of these comparisons, temperature values defined by the R-SQUID are deemed to be reproducible to within 0.1% and accurate to within 0.3%.

Temperature and pressure variation of the refractive index of diamond

The temperature and pressure variations of the refractive index for a Type IIa diamond have been measured at audio frequencies using capacitance techniques. Measurements have been made at zero pressure over the 5.5-340-K temperature range and at pressures up to 1.4 x 10(8) Pa (1.4 kbar) at room temperature. At room temperature, (1/n)(dn/dT)(p) = +4.04 x 10(-6)/K and (1/n)(dn/dp)(T) = -0.36 x 10(-12)/Pa. In addition, the curvature in the refractive index with temperature has been determined. The first-order derivatives are compared with previous experimental data and the recent theoretical calculations of Van Vechten and Yu and Cardona.

Molecular Reorientation and Nuclear‐Spin Conversion in the Solid Deuteromethanes at Low Temperatures

The heat capacities of the solid deuteromethanes have been measured down to 0.3°K, tracing out the low‐temperature heat‐capacity anomalies which had been detected in the earlier work of Colwell, Gill, and Morrison. The anomalies in CH3D and CHD3 have maxima at 0.43 and 0.48°K, respectively, but the CH2D2 heat capacity is still increasing with decreasing temperature at 0.3°K. A small anomaly was found in CD4 but this is ascribed to chemical and isotopic impurities present in the sample. The heatcapacity anomalies are of the Schottky form and are believed to arise from the reorientation of the molecules on the lattice sites. This results in the removal of the orientational degeneracies of the molecules which give rise to the zero‐point entropies of R ln4 for CH3D and CHD3 and R ln6 for CH2D2. In molecules having identical protons, conversion between the different nuclear‐spin states also occurs. This is substantiated in the case of CH3D where the measured entropy change in the range of the measurements, 0.3...

The Performance of a Mechanical Heat Switch at Low Temperatures

A mechanical heat switch used in calorimetry experiments down to 0.3 K is described and its performance analyzed in sufficient detail to be useful for design purposes. The thermal conductance of the indium‐copper switch contacts was determined as a function of temperature and applied force over the range 0.3–2 K. These results are compared with other conductance measurements to evaluate choices of metals for heat switch contacts.

The equivalence of the superconducting transition temperature of pure indium as determined by electrical resistance, magnetic susceptibility, and heat-capacity measurements

The superconducting transition temperature of a well-annealed polycrystalline sample of pure indium was determined by three different techniques. The heatcapacity and susceptibility transitions are approximately 1 mK wide and span the same temperature interval. The resistance transition is less than 0.2 mK wide and occurs at the center of the other transitions. By choosing the midpoint of each transition the three measurements are identical to within 0.1 mK.

Low‐Temperature Heat Capacity of NdCl3 and PrCl3

Heat capacities of PrCl3 and NdCl3 have been measured between 0.3° and 4°K. The dominant feature of the heat capacity curves is a broad anomaly indicating a region of extensive short‐range magnetic ordering. The anomaly in NdCl3 has a maximum at 0.47°K and fits closely that of a linear Ising chain model. In addition, two small peaks appear at the same temperatures as the sharp spikes found in the magnetic susceptibility. PrCl3 has a sharp peak in the heat capacity at 0.4°K which is presumably associated with long‐range ordering. The broad anomaly in PrCl3 has a maximum at 0.8°K. The magnetic entropy change for the temperature region of the measurements is approximately 80% of R loge2 for both salts.

Current-voltage characteristics of nanoampere Josephson junctions

We have studied the current-voltage characteristics of small area tunnel junctions at temperatures below 1 K. The junctions were made in an edge geometry with a Nb base electrode and had areas less than .05 μm2and critical currents in the nA range. Although the measured I-V characteristics resemble those of ordinary hysteretic junctions, the supposed zero-voltage portion of the curve proved to have a finite slope and to deviate from zero voltage. For these junctions it is apparently possible for occasional 2π phase slips to occur without switching to the usual voltage state. This behavior can be explained either by macroscopic quantum tunneling or by a model in which the effective shunt conductance of the junction is frequency dependent.

The heat capacity of cerous magnesium nitrate and some related materials between 0.3 and 4 K

Cerous magnesium nitrate (CMN) is the preeminent electronic paramagnet in use in cryogenic physics for magnetic thermometry and adiabatic cooling. In demagnetization experiments designed to establish the thermodynamic temperature relations for CMN, an inexplicable heat capacity anomaly was found to occur above 20 mK and is shown here to persist to temperatures near 1 K. The anomaly is small but its presence interferes with and may cause errors in the analysis of thermometric data. We have measured the heat capacity of CMN, lanthanum magnesium nitrate (LMN), cerous nitrate hexahydrate, and a saturated aqueous solution of CMN (CMN liquor) in the temperature range 0.3–4 K in an attempt to find the source of the anomaly. The LMN heat capacity shows no anomaly and is used to approximate the lattice heat capacity of CMN. At low temperatures the CMN heat capacity, exclusive of the lattice contribution, is some 2 1/2 times larger than the magnetic heat capacity predicted by other investigations. At high temperatures an exponentially increasing heat capacity due to the first excited electronic level is observed and indicates a splitting which is in accurate agreement with the spectroscopic value. There is evidence that the lattice heat capacity in CMN is about 1% smaller than in LMN, which is probably the result of the crystal-field interaction with the electronic states of the cerous ions. The lattice terms and theT−2term of the magnetic heat capacity for cerous nitrate have been determined, the latter being 25 times larger than the predictedT−2term in CMN. The CMN liquor measurements indicate that this sample had probably become a glass on cooling. The lattice heat capacity is considerably larger than could be predicted from the separate components and there is no indication of the exponential term which would be observable if appreciable crystalline CMN were present. These measurements help to define the nature of the anomalous heat capacity and remove from consideration some possible explanations, but they do not reveal the cause of the anomaly.

Influence of the Josephson junction on the impedance and noise of a resistive superconductive quantum interference device

The impedance and noise of a resistive superconductive quantum interference device (R‐SQUID) have been measured as a function of the dc and rf currents applied to it. The Josephson junction was adjustable so that data were also taken for several values of the junction critical current. The results were compared with the predictions of a resistively shunted junction (RSJ) model which takes into account the influence of the Josephson junction on the impedance and noise. The agreement was found to be quite good and demonstrates that the noise in the circuit is well understood. Use of the R‐SQUID as a noise thermometer below 1 K is assessed in terms of corrections due to the RSJ model. It is demonstrated how the dc and rf currents may be adjusted so that the total noise of the R‐SQUID is reduced to within 0.1% of the Johnson noise generated by the resistor alone. Under these conditions, the R‐SQUID may be used as a noise thermometer to determine thermodynamic temperature to this inaccuracy from 6 to 700 mK.

Development of a temperature scale below 0.5 K

Abstract We summarize our most recent results on the development of an absolute temperature scale below 0.5 K. It is the most recent of several experiments at our laboratory and represents both our best effort and the one in which all available thermometers were simultaneously and fully operational. Implications of our results for other work will be discussed.