Investigations of the molar heat capacity at low temperatures in the TiH x system

Results from heat capacity measurements in the temperature range 1.8–10 K on TiH _x samples with different x values are communicated. From the coefficients of the electronic heat capacity the dependence of the density of states N(E _F) on the x value for the cubic and the tetragonal TiH _x phases is discussed, and is compared with similar results from susceptibility measurements. The estimated value for the electron-phonon interaction parameter excludes superconductivity for TiH _x . An additional lattice contribution to the heat capacity of TiH _x samples with x < 1.7 was observed and seems to be related to a low-frequency anomaly in the phonon spectra.     

Heat capacity of aqueous solutions of sulfuric acid at low temperatures

Results of an experimental determination of the specific heat of H₂O-H₂SO₄ solutions at low temperatures are presented and the heat capacity of a heterogeneous two-component system is analyzed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 27, No. 1, pp. 78–84, July, 1974.     

Heat capacities and magnetic moments of potential regenerator materials at low temperatures

Several samples of rare-earth oxides of perovskite structure were developed and investigated between 1.5 and 10 K. The measurements have shown, that these materials have large magnetic moments and most of them exhibit a magnetic phase transition below 4 K. The heat capacities of the samples have been measured with the adiabatic heat-pulse method. The dependence of the heat capacity on the preparation temperature of the samples has been studied. The results of the measurements are presented in this paper. The possibility of using these materials as regenerator materials in cryocoolers, e.g. pulse-tube refrigerators, is discussed.     

The heat capacity of ternary molybdenum sulfides. II. The heat capacity of Mo5SnGa x S6 (x < 0.3) at low temperatures

The heat capacity of Mo₅SnGa _x S₆ was measured in the temperature range from 2 K to 25 K with x = 0.0, 0.18, 0.22, and 0.25. Independent of the x-value a peak appears in the phonon spectrum of all investigated compounds at low frequencies. Similar to the increase of T with the Ga concentration, a rise of the coefficient of the electronic heat capacity was observed, probably as a consequence of both the increase of the electron-phonon interaction parameter and the density of states at the Fermi level.     

Electrical and magnetic properties of rapidly and slowly cooled 18cr(10–25)ni alloys at low temperatures

Electrical and magnetic properties of the 18Cr(10–25)Ni alloys cooled rapidly and slowly from 1100°C have been studied in the low temperature range. The results show that slow cooling leads to a more favourable situation for the cluster structure formation than rapid cooling.     

Conducting polymers at low temperatures and high magnetic fields

Advances in the synthesis of organic conducting polymer systems has increased the electrical conductivity of these systems by several orders of magnitude in the last decade. Several practical applications are envisioned for such systems, but a thorough understanding of the conduction mechanisms and identification of the charge carriers is lacking, making design and implementation for bulk synthesis difficult. In order to clarify our understanding of the electrical properties of these systems, the resistivity and magnetoresistivity of various polymers doped near the metal - insulator transition, such as polyaniline protonated by camphor sulfonic acid (PANi-CSA) and polypyrrole doped with PF₆ (PPy-PF₆), have been studied down to 25 mK in magnetic fields up to 16 T.     

A single electric relaxation time in Ba(1-x)Sr(x)TiO(3) nanoparticles at low temperatures

It is shown that the dielectric response of Ba(0.77)Sr(0.23)TiO(3) nanoparticles at temperatures below 200?K has a frequency and temperature dependence in agreement with the Debye theory with a single relaxation time, which exhibits the Arrhenius law. By contrast, at temperatures above 210?K the dielectric response exhibits a broad range of relaxation times characteristic of relaxor-ferroelectrics. We suggest that the single relaxation time at low temperature originates from a frustration effect, in analogy with frustrated antiferromagnetism.     

Electronic conduction in bulk Se1−x Te x glasses at high pressures and at low temperatures

The electrical resistivity of bulk Se_1−x Te _x glasses is reported as a function of pressure (up to 8 GPa) and temperature (down to 77K). The activation energy for electronic conduction has been calculated at different pressures. The samples with 0⩽x⩽0·06 show a single activation energy throughout the temperature range of investigations. On the other hand samples with 0·08⩽x⩽0·3 show two activation energies in the different regions of temperature. The observed behaviour has been explained on the basis of band picture of amorphous semiconductors.     

Transport properties of (La,Pr)Sn3 alloys at low temperatures

We report on measurements concerning the transport properties of polycrystalline (La_1–x Pr _x )Sn₃samples with impurity concentrations 0.07 x 0.2 in the temperature range 0.3 T 12 K. The experimental results of the electrical resistivity, the thermopower, and the thermal conductivity show pronounced anomalies which are referred to crystal-field effects and a Kondo effect of excited levels. A quantitative comparison with existing theories confirms the importance of the aspherical Coulomb scattering in addition to the isotropic spin-exchange scattering of the conduction electrons by the magnetic ions. Taking into account the Kondo effect in a phenomenological ansatz, we are able to fit the magnetic part of the resistivity. Furthermore, we present the first experimental proof of the theoretically predicted influence of crystal-field split impurities on the thermal conductivity in dilute magnetic alloys. The thermopower is qualitatively discussed by a model including both the effect from inelastic scattering on the crystal-field levels and the Kondo effect. Finally the temperature dependence of the Lorenz number is compared with a calculation in the framework of crystal-field theory.Work supported by the Deutsche Forschungsgemeinschaft.     

Russian germanium resistance thermometers at low temperatures in magnetic fields

A type of germanium resistance thermometer manufactured at the Semiconductor Institute, Kiev, USSR, displays much lower magnetoresistive effects than most other types of cryogenic semiconductor resistance thermometers. Measurements are reported on the temperature dependence of the electrical resistance of these thermometers (1.5–30 K at zero magnetic field) and their zero-field stability when exposed to thermal cycling between 20 and 288 K.     

The influence of pressure on phase transitions at low temperatures: SrTiO3 and (Ba x Sr1−x )TiO3

The reciprocal dielectric constant of SrTiO₃ is found to follow a quadratic temperature law ^–1=A ₀+T ² between 10 and 75 K. The strong increase of the maxima pressure shifting in the dielectric constants of SrTiO₃ and (Ba _x Sr_1–x )TiO₃ at low temperatures is explained as a consequence of the deviations from the Curie-Weiss law due to quantum effects. Other examples of phase transitions showing a similar behavior are discussed.     

Galvanomagnetic effects in extremely strong magnetic fields at very low temperatures

Transverse and longitudinal galvanomagnetic effects in nondegenerate semiconductors in an ultraquantum magnetic field and at superlow temperaturesTm u ² are investigated, wherem is the electron mass along the magnetic field andu is the sound velocity. One-phonon processes are forbidden by energy and momentum conservation laws, and the two-phonon process is the basic relaxation process. The phonon relaxation on electrons is also considered. Nonlinear effects in the presence of a strong electric field are investigated when the effective electron temperature approximation is valid. Under certain conditions the electron temperature grows and one-phonon processes become allowed, with the current-voltage relation showing changes due to this phenomenon.     

Heat flow between nuclei and conduction electrons in metals at very low temperatures

Magnetic field and thermal gradients not only affect the heat flow between different parts of metallic samples but can also lead to anomalies in the observed energy transport between the electronic and nuclear energy reservoirs. Nuclear spin-lattice relaxation is considered in conditions of applied field and temperature such that the contribution of nuclear spins to the heat capacity dominates by far that of the electrons. The model presented is discussed with respect to recent steady state heat flow experiments on copper at submillikelvin temperatures in which a non-Korringa like, anomalously weak coupling between electrons and nuclei was observed. We show that in heat flow experiments at sufficiently low conduction electron temperatures, T _e , when thermal gradients are present in the sample, the apparent Korringa constant takes the form = ₀ (1 + (kT _e ²) ^–1 ) where depends on the heat current and the nuclear heat capacity, ₀ is the actual Korringa constant, and K is the thermal heat conductance of copper which in part depends on the applied field B ₀ as B ₀ ^–1 assuming that the Wiedemann-Franz law holds. In fact, a B ₀T _e ^–2 dependence was reported for the anomalous nuclear spin electron coupling which we therefore conclude could be explained by the finite thermal conductivity in an inhomogeneously heated sample. From a quantitative comparison of our model and the reported values, K would have to be one order of magnitude smaller than that expected for the used sample of high purity. Difficulties in comparing the over-simplified model with a real experiment are discussed and a comment concerning the effect of impurities in Pt-NMR thermometry at very low temperatures is given.     

Correlation of magnetic properties to oxygen and potassium stoichiometry in single-crystal Ba1−x K x BiO3−y

Recent theoretical calculations have suggested the coupling of electrons to high-energy oxygen phonons as an explanation of superconductivity in the Ba_1?x K _x BiO_3?y system. We have synthesized high-quality single crystals of the material and have examined the behaviors of critical field and critical current parameters as a function of changes in the oxygen content and in the Ba/K ratio. We have determined, via positron lifetime spectroscopy and singlecrystal X-ray measurements, that the oxygen stoichiometry in this system can be varied without significant impact on the metal atom sublattice. These results facilitate an investigation of the dependence of critical parameters on dopant and defect levels in this system.     

Microstructure and magnetic properties of Fe(x)Ni(1-x) alloy nanoplatelets

Plate-like nanoparticles (or nanoplatelets) of Fe(x)Ni(1-x) (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) alloy were successfully synthesized through a simple sonochemical method. The shapes of the alloy nanoplatelets with different Fe atom contents are almost same. Their average diameters are about 50 nm, and their average thicknesses are several nanometers. The obtained Fe(x)Ni(1-x) alloy nanoplatelets are single-phased and have a face-centered cubic (FCC) crystal structure. The lattice constants of the alloy nanoplatelets are larger than the corresponding bulk value and increase with increasing Fe content. The surface oxidation of the alloy nanoplatelets leads to the lattice expansion. The alloy nanoplatelet powders are all ferromagnetic, and their saturation magnetizations are slightly lower than the corresponding bulk value. The saturation magnetic field and the coercivity increase with increasing Fe content. Magnetic hysteresis loops along the directions deviating different angles from the nanoplatelets plane are obviously different, indicating that the easy-axis is in the in-plane direction and the magnetization reversal is incoherent mode. The micromagnetic simulation results for the array composed of thirty-six Fe0.6Ni0.4 alloy nanoplatelets fit well with the measured data.     

Non-destructive studies on tensile and fracture properties of molybdenum at low temperatures (148 to 423 K)

Tensile and fracture properties of molybdenum have been studied using the automated ball indentation technique. Tests have been carried out at several temperatures in the range of 148 to 423 K at a constant strain rate. Tensile properties determined from these tests agreed well with published results from conventional tensile tests. Temperature dependence of indentation energy to fracture, the fracture toughness parameter specific to this technique using critical stress-to-fracture concept, showed a sharp transition from brittle to ductile condition. These results complement the previous studies on pressure vessel steels, and clearly demonstrate that automated ball indentation technique is a reliable and non-destructive method for determining tensile and fracture properties of materials.