Measurements on Melting Pressure,Metastable Solid Phases,and Molar Volume of Univariant Saturated Helium Mixture

A concentration-saturated helium mixture at the melting pressure consists of two liquid phases and one or two solid phases. The equilibrium system is univariant, whose properties depend uniquely on temperature. Four coexisting phases can exist on singular points, which are called quadruple points. As a univariant system, the melting pressure could be used as a thermometric standard. It would provide some advantages compared to the current reference, namely pure $^3$ He, especially at the lowest temperatures below 1 mK. We have extended the melting pressure measurements of the concentration-saturated helium mixture from 10 to 460 mK. The density of the dilute liquid phase was also recorded. The effect of the equilibrium crystal structure changing from hcp to bcc was clearly seen at $T=294$  mK at the melting pressure $P=2.638$  MPa. We observed the existence of metastable solid phases around this point. No evidence was found for the presence of another, disputed, quadruple point at around 400 mK. The experimental results agree well with our previous calculations at low temperatures, but deviate above 200 mK.     

Pressure of the Saturated Vapor of Nickel at High Temperatures

Two series of measurements of the saturated vapor pressure of crystalline and liquid nickel are carried out in the temperature range of 1270–1896 K using the integral version of the effusion method and Knudsen mass spectrometry. The enthalpy of sublimation of Ni at 0 K is calculated from the measurement results in accordance with the second _s H ₀ ⁰(II) and the third _s H ₀ ⁰(III) laws of thermodynamics. Good agreement between the vapor pressure values, determined by different methods under different experimental conditions, makes possible their joint treatment for crystalline and liquid nickel and leads one to recommend the value of _s H ₀ ⁰ = 425.3 ± 2.0 kJ/mol. The results obtained are compared with the available data of other authors.     

Intervalley Scattering of Hot Electrons in Germanium at Millikelvin Temperatures

A method is presented to determine the field dependence of the intervalley scattering rate for hot electrons in germanium, based on an analysis of electron straggle in a detector crystal fitted with segmented electrodes. Measurements in high-purity and in lightly doped n- and p-type crystals at millikelvin temperatures demonstrate the dominant role of impurity scattering at low field ( \(<\sim \) a few V/cm), whereas phonon scattering takes precedence at higher field intensities. An analysis of the experimental data by reference to past investigations of the acoustoelectric effect in germanium strongly suggests that the impurities involved are the dopant species in the neutral state.     

Magnetization Measurements of CeRu2Si2 at Millikelvin Temperatures

We have measured the magnetization of the heavy fermion compound CeRu ₂ Si ₂ at temperatures down to 300 K. We observed that the dc-magnetic susceptibility had the plateau due to the Pauli susceptibility between 50 mK and 2 K, and the upturn from this plateau toward lower temperatures. This excess susceptibility shows a Curie- Weiss behavior with a slight signature of saturation at the lowest temperature. This fact indicates that CeRu ₂ Si ₂ has the small residual magnetic moments in its ground state.     

Hot Carrier Trapping in High-Purity and Doped Germanium Crystals at Millikelvin Temperatures

A new set of experimental data is presented for the mean drift lengths and the drift velocities of hot electrons and holes as a function of the electric field in ultra-pure and in lightly doped (n- and p-type) germanium single crystals at mK temperatures. Measurements are made in the field range between \(\sim \) 0.1 and 15 V/cm, typical for the operation of cryogenic germanium detectors for dark matter search. The analysis of the experimental data strongly suggests that the dominant trapping centers are the dopant species in the neutral state.     

Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

We report heat capacity and magnetisation measurements of ³He adsorbed on the surface of graphite plated with three atomic layers of ⁴He. For ³He coverages n ₃>4 nm^?2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the ³He quasiparticles is 1.38±0.05 m₃. The ³He effective mass ratio increases with coverage to 2.4 at n ₃=4 nm^?2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n ₃=4.5 nm^?2, similar to that previously observed on four layers of ⁴He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n ₃ is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n ₃=7 nm^?2, we find that this behaviour is very sensitive to small changes in the ⁴He third layer coverage, around the completed third layer. Measurements of the ³He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.     

Hot Carrier Velocities in Doped and in Ultra-pure Germanium Crystals at Millikelvin Temperatures

The velocity laws of electrons and holes in germanium single crystals at millikelvin temperatures are determined as a function of the electric field in the 〈001〉 orientation, based on time-of-flight measurements in cryogenic coplanar grid Ge detectors. Results obtained in two n-type crystals (|N _a ?N _d |<10¹⁰ cm^?3 and doped to 10¹¹ cm^?3) are compared with the experimental data from previous investigations, and shown to be consistent with Monte-Carlo simulations of carrier transport.     

Investigating Metastable hcp Solid Helium Below Its Melting Pressure

We report a first attempt to produce metastable hcp solid helium below its melting pressure. A focused sound pulse is emitted along the c-axis of a mono-domain hcp helium-4 crystal starting from a static pressure just above the melting pressure. The sound pulse is made as simple as possible with one negative and one positive swing only. Density at focus is monitored by an optical interferometric method. Performed numerical simulations show that the crystal anisotropy splits the focused wave into two separate pulses, corresponding to a longitudinal wave along the c-axis and a radial one perpendicular to it. The amplification factor due to focusing remains nevertheless important. Negative pressure swings up to 0.9 bar have been produced, crossing the static melting pressure limit. Improvements in the detection method and in the focusing amplification are proposed.     

Properties of Metastable Solid Helium Below the Normal Melting Pressure

In this note we report on simple estimates of the properties of metastable solid helium that is at a pressure below the normal melting pressure. We calculate the nucleation rate for formation of the liquid phase and estimate the “spinodal” pressure at which the solid becomes macroscopically unstable.     

A Noise Thermometer for Practical Thermometry at Low Temperatures

The application of a magnetic-field-fluctuation thermometer (MFFT) is described for practical thermometry in the low-temperature range. The MFFT inductively measures the magnetic noise generated by Johnson noise currents in a metallic temperature sensor. The temperature of the sensor is deduced from its thermal magnetic noise spectrum by applying the Nyquist theorem, making the thermometer in principle linear over a wide range of temperatures. In this setup, a niobium-based dc SQUID gradiometer detects the magnetic field fluctuations. The gradiometer design optimizes the inductive coupling to the metallic temperature sensor, yet equally ensures sufficient insensitivity to external magnetic interference. In order to obtain a highly sensitive and fast thermometer, the SQUID chip is placed directly onto the surface of the temperature sensor. The compact setup of the gradiometer/temperature sensor unit ensures good conditions for thermal equilibration of the sensor with the temperature to be measured, a factor that becomes increasingly important in the temperature range below 1 K. The first direct comparison measurements of the MFFT with a high-accuracy realization of the Provisional Low Temperature Scale of 2000 (PLTS-2000) are presented. Special emphasis is given to the investigation of the linearity, speed, and accuracy of the MFFT.     

Transport Anisotropy and Impurity Scattering in Ge at Millikelvin Temperatures: Experimental Study

Anisotropy effects in hot carrier transport have been investigated in germanium crystals at mK temperatures in the electric field range pertaining to the operation of the Edelweiss dark matter detectors. Comparative measurements have been made on n-type specimens of different impurity contents, both ultra-pure (N _d ?N _a <10¹⁰?cm^?3) and doped to 10¹¹?cm^?3. At relatively high field intensities (?5?V/cm), similar features of electron and hole transport are observed independent of the concentration of electrically active impurities. Differences appear at lower field (down to a fraction of a V/cm) with regard to electron straggling especially, dependent on crystal purity. These experiments demonstrate the importance on carrier transport of impurity scattering at low field, whereas phonon scattering becomes the dominant factor at higher field intensities.     

Stressed Silicon Nitride Nanomechanical Resonators at Helium Temperatures

We have characterized the mechanical resonance properties (both linear and nonlinear) of various doubly-clamped silicon nitride nanomechanical resonators, each with a different intrinsic tensile stress. The measurements were carried out at 4 K and the magnetomotive technique was used to drive and detect the motion of the beams. The resonant frequencies of the beams are in the megahertz range, with quality factors of the order of 10⁴. We also measure the dynamic range of the beams and their nonlinear (Duffing) behaviour.     

Measurements of the Dielectric Constant of Solid Helium at Low Temperatures

A careful measurement of the dielectric constant of solid ⁴He was made to search for signatures associated with the reported onset of nonclassical rotational inertia and shear modulus stiffening. The samples studied include zero-pressure liquid, solid-liquid mixtures on the melting-curve and solid helium. It was found that the dielectric constant of solid ⁴He decreases smoothly with temperature below 300?mK, showing no measurable anomalies down to 50?mK. The new measurements do not show the anomalous upturn effect that was reported in our previous measurements.     

Kapton Capacitance Thermometry at Low Temperatures and in High Magnetic Fields

A sensitive Kapton foil capacitance sensor, with size of 9.5 mm×4.5 mm, has been developed and used as a thermometer at ultra-low temperatures down to 1.2 mK and in high magnetic fields. There is no visible magnetic field dependence up to 15 T. The sensor was calibrated with ³He melting pressure thermometer (MPT) and vibrating wire (VW) viscometer. With the silver powder sintered heat exchanger sandwich-like design, the thermal relaxation time is as short as a few minutes at the base temperature. The low temperature (below 1.2 K) reproducibility has been tested and is satisfied within experimental errors.     

A New Type of Carbon Resistance Thermometer with Excellent Thermal Contact at Millikelvin Temperatures

Using a new brand of commercially available carbon resistor we built a cryogenic thermometer with an extremely good thermal contact to its thermal environment. Because of its superior thermal contact the thermometer is insensitive to low levels of spurious radio frequency heating. We calibrated our thermometer down to 5 mK using a quartz tuning fork He-3 viscometer and measured its thermal resistance and thermal response time.     

NQR on Gallium Single Crystals for Absolute Thermometry at Very Low Temperatures

It is shown that NMR/NQR-thermometry on Ga single crystals can serve the need of absolute thermometry in a temperature range from below 20 K to above 1 mK. Thus, the sensitive range includes all phase transition temperatures around 1 mK of solid and fluid ³ He, a necessity for an extension of the ³ He-melting-pressure temperature scale towards lower temperatures. The experiments were performed in magnetic fields of 50 to 200 mT and at temperatures down to 200 K using single rf-pulses. Special care was taken for the orientation of the magnetic field with respect to crystal orientation which was obtained by in-situ pulsed NMR-experiments. It has been shown that in the investigated parameter range nonlinear effects in NMR due to high spin alignment had no significant influence. In pulsed NMR as well as in specific heat experiments it could be shown that spin-spin interactions are only weak in Ga and do not influence nuclear paramagnetism, at least at temperatures above 100 K. The advantages of using Ga instead other elements for population difference thermometry are discussed.     

Hydrogen–Helium Mixtures at High Pressure

The properties of hydrogen–helium mixtures at high pressure are crucial to address important questions about the interior of Giant planets e.g. whether Jupiter has a rocky core and did it emerge via core accretion? Using path integral Monte Carlo simulations, we study the properties of these mixtures as a function of temperature, density and composition. The equation of state is calculated and compared to chemical models. We probe the accuracy of the ideal mixing approximation commonly used in such models. Finally, we discuss the structure of the liquid in terms of pair correlation functions.