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We describe a compact calorimeter that opens ultra-low-temperature heat capacity studies of small metal crystals in moderate magnetic fields. The performance is demonstrated on the canonical heavy fermion metal YbRh 2 Si 2 . Thermometry is provided by a fast current sensing noise thermometer. This single thermometer enables us to cover a wide temperature range of interest from 175 µK to 90 mK with temperature-independent relative precision. Temperatures are tied to the international temperature scale with a single-point calibration. A superconducting solenoid surrounding the cell provides the sample field for tuning its properties and operates a superconducting heat switch. Both adiabatic and relaxation calorimetry techniques, as well as magnetic field sweeps, are employed. The design of the calorimeter results in an addendum heat capacity which is negligible for the study reported. The keys to sample and thermometer thermalisation are the lack of dissipation in the temperature measurement and the steps taken to reduce the parasitic heat leak into the cell to the tens of fW level. [ABSTRACT FROM AUTHOR]

The dual-mode auto-calibrating resistance thermometer (DART) has recently been proposed for highly accurate temperature measurement based on noise thermometry. In this paper, it is demonstrated that calibration and operation of the DART at part-per-million (ppm) level should be possible with the hardware developed. For this purpose, we have extensively tested a representative signal path comprising the basic DART components. This includes a low-noise amplifier connected to a 24-bit ÎŁ Î" ADC and a metrology-grade voltage reference. A Josephson arbitrary waveform synthesizer (JAWS) generates a pseudo-noise consisting of low-distortion multitones superimposed on a low-frequency square-wave reference voltage. Using this signal, a fast and efficient calibration scheme for the signal path gain is demonstrated. The reference voltage stabilizes the gain at ppm level. We observed gain fluctuations within ± 2 ÎĽ V V âˆ' 1 over a period of 19 d, a temperature coefficient of âˆ' 0.5 ÎĽ V V âˆ' 1 K âˆ' 1 , and insignificant nonlinearity within an uncertainty band of ± 2 ÎĽ V V âˆ' 1 for rms input levels between 5 µV and 80 µV. The behavior of the signal path with a 300 Ω resistor as a noise source was also investigated. From the observed stability of the voltage reference and flatness of the noise gain between 10 kHz and 225 kHz, we estimate that the presented hardware components are suitable for temperature measurements with systematic uncertainties well below 10 ÎĽ K K âˆ' 1 . [ABSTRACT FROM AUTHOR]

The effects of nuclear radiations on conventional thermocouples (type K, C and N) mainly used in irradiation experiments may create significant drifts of the signals. In order to solve these difficulties, the Commissariat à l'Energie Atomique has started to develop and qualify in laboratory conditions a miniature device, which combines a noise thermometer and thermocouples, for a future application in a research reactor. In this paper, a particular approach of a combined noise thermometer-thermocouples is described. Present measurements, based on a correlation and comparison method, are realized in laboratory at 300^\circC which represents the mean temperature in materials irradiation experiments. Referred measurement uncertainties are lower than \pm 1.5^\circC corresponding to the accuracy tolerance of conventional thermocouples at this temperature. [ABSTRACT FROM AUTHOR]

Abstract: Currently, the CODATA value of the Boltzmann constant is dominated by a single gas-based thermometry measurement with a relative standard uncertainty of [P.J. Mohr, B.N. Taylor, D.B. Newell, CODATA recommended values of the fundamental physical constants: 2006, Rev. Mod. Phys. 80 (2008) 633–730]. This article describes an electronic approach to measuring the Boltzmann constant that compares Johnson noise from a resistor at the water triple point with a pseudo-random noise generated using quantized ac-voltage synthesis. Measurement of the ratio of the two power spectral densities links Boltzmann''s constant to Planck''s constant. Recent experiments and detailed uncertainty analysis indicate that Boltzmann''s constant can presently be determined using Johnson noise with a relative standard uncertainty below , which would support both historic and new determinations. To cite this article: S. Benz et al., C. R. Physique 10 (2009). [Copyright &y& Elsevier]