We have measured the temperature dependence of the superconductive critical magnetic field of three samples of beryllium from three different sources. The irregular geometric shape of one sample prevented a complete analysis of the data, but the data for the other two samples were well represented by the BCS model of superconductivity. The values of T c and H c (0) are found to be sample dependent. For the purest sample studied, T c =24.34±0.02 mK, H c (0) =107.7±0.2 μ T and ( γ / V )=32.9±0.3 μ J/cm 3 K 2 . By a very small extrapolation of the data as a function of purity, it is inferred that the critical parameters for pure Be are: T c =24.38±0.02 mK, H c (0) =107.9 μ T± 0.2 μ T and ( γ / V )=33.0±0.3 μ J/cm 3 K 2 .

We have applied the Ginzburg-Landau equations to calculate the behavior of the diamagnetism of macroscopic samples consisting of a normal metal in contact with a superconductor. In particular, the calculation focuses on the temperature region above the superconductive transition temperature T-cN Of the normal metal. We have compared these calculations with experimental measurements of the temperature (0.006-1 K) and magnetic field (10(-9)-10(-6) T) dependence of the diamgetism for several samples of Be (T(cN)similar to 23 mK) and W (T(cN)similar to 15.5 mK) in contact with the superconductor Al (T-cS= 1.18 K). The agreement between the predictions and measurements is quite good and confirms the approach of using the Ginzburg-Landau model to calculate the proximity effect in macroscopic normal systems above their superconductive transitions.

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%.

We have measured the white and additive noise generated by an R-SQUID noise thermometer whose temperature was varied from 6.3 mK to 0.738 K. We have also conducted room temperature simulations of the effect of white and additive noise on the circuit used to measure the noise for the R-SQUID. We generally found that the measured noise in all cases was fit to within the 0.1% statistical measurement imprecision by a model for the R-SQUID based on frequency modulation. This conformity is sufficiently good that any deficiencies in either do not lead to systematic inaccuracies in the noise temperature scale which exceed 0.1%. Comparisons of the R-SQUID with several other thermometers indicate that the overall agreement is at worst 0.2%.

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.

The recently-developed international temperature scale, ITS 90, does not extend below 0.65K. However, a considerable body of research, particularly that devoted to defining the physical properties of liquid 3He, has been conducted below this temperature without benefit of a common temperature scale. Accordingly, we have been developing a temperature scale in this region. The primary thermometer used in this effort is a special form of noise thermometer consisting of a resistively- shunted Josephson junction, or an R-SQUID. The R-SQUID offers advantages over conventional analogue noise thermometry in that the influence of the measuring circuits on the measured noise temperature is considerably reduced. A review is given of the decade of experience gained at NIST in the operation of this thermometer and in modelling its systematic errors. The reproducibility of the temperatures measured using the R-SQUID noise thermometer will also be presented, as well as comparisons of the noise temperature scale with scales defined by other thermometers.

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.

We present the results of an extensive study of the effect of a filter upon the performance of a resistive SQUID noise thermometer used to define an absolute temperature scale below 1 K. Agreement between the theory and the experimental results indicates that the scale defined by this noise thermometer is accurate to 0.1%

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.

In this article we try to relate the general features of several types of heat capacity curves to the underlying energy level spectrum. We are interested in heat capacity “anomalies” (i.e., heat capacities which occur in addition to the usual lattice vibration and conduction electron contributions) and restrict ourselves to systems which can be described on an independent particle basis. We begin by considering Schottky anomalies and show the effects on the heat capacity of varying the spacings and degeneracies of the energy levels. The relation between the Schottky anomaly arising from the equally spaced nondegenerate levels of the Brillouin function and the heat capacity of the Einstein function is discussed. The effects on the heat capacity of a marked local change in the density of states of an otherwise uniform energy-level spectrum is considered. The heat capacities of the free rotor and the molecular field model are used to exemplify particular points of the discussion.

The construction of an adiabatic calorimeter for the temperature range between 10 and 350 K is described. Emphasis is given to the construction of the cryostat and the regulation of the shields. For a measurement of heat capacity from 5 to 100 K about 3 to 4 dm3 of liquid helium is needed. The performance of the apparatus was checked by measuring the molar heat capacity of a standard sample of synthetic sapphire and of a sample of n-heptane. Results are within 0.2 per cent of the accepted literature values.