Deformation of Cd-based alloys single crystals at very low temperatures

The temperature dependence of the critical resolved shear stress — CRSS on Cd-Ag andCd-Zn single crystals was studied at very low temperatures 1·5–80 K. The deformation experi-ments were made by a creep technique. The CRSS for Cd-Ag alloys was determined from re-constructed shear stress — shear strain curves, while the method of one sample was applied tothe determination of ₀-T dependence for Cd-Zn alloys. The difference in the temperaturedependences of ₀ for both Cd-based alloys can be caused by different methods for determiningthe CRSS.     

NMR at very low fields

Although nuclear magnetic resonance in low fields around or below the Earth's magnetic field is almost as old as nuclear magnetic resonance itself, the recent years have experienced a revival of this technique that is opposed to the common trend towards higher and higher fields. The background of this development is the expectation that the low-field domain may open a new window for the study of molecular structure and dynamics. Here, we will give an overview on the specific features in the low-field domain, both from the technical and from the physical point of view. In addition, we present a short passage on the option of magnetic resonance imaging in fields of the micro-Tesla range.     

Excitations in Metmyoglobin crystals at low temperatures

Specific heat and dielectric constant of Metmyoglobin crystals at low temperatures are measured and found to be similar to those of amorphous dielectrics. This provides evidence for the existence of two level systems in the protein. Possibility of many conformational substates and the frozen in disorder in the protein is proposed as the origin of these excitations.     

Thermal hardening of Ni3Ge alloy single crystals with an L12 superstructure at low temperatures

The mechanical properties and the dislocation structure of Ni₃Ge alloy single crystals have been experimentally studied at low temperatures. It is found that the flow stresses increase beginning with 4.2 K, and the observed rise in the stresses depends on the orientation of the strain axes of the crystals. The dislocation structure is investigated thoroughly. It is revealed that the mean density of dislocations and the interdislocation interaction parameter α anomalously increase as the temperature increases in the range 4.2–293 K. The mechanisms providing an explanation for the temperature anomaly of flow stresses and the α parameter are considered. The activation energy of thermal hardening is evaluated. It is assumed that the low activation energies of thermal hardening are due to the motion of dislocations at velocities close to the velocity of sound at these temperatures.     

Spin relaxation in metals at very low temperatures

A general theory of spin relaxation in metals is developed from a statistical mechanical point of view. The theory is valid for all temperature domain and the multiple-time characteristics of the relaxation process are completely determined: The relaxation times are strongly dependent on the temperature and magnetic field. At very low temperatures, behaviours of the relaxation times are quite different from the usual ones showing a saturation effect. Temperature varations of the relaxation times for I ? 1 (I the magnitude of spin) are qualitatively different from those for $\text{I =}\text{1}\text{2}$. Namely, in the former case, the largest relaxation time has a maximum as a function of inverse temperature.     

On the mobility of very pure semiconductors at very low temperatures

The mobility μ of a very pure semiconductor at very low temperatures is investigated in terms of a model where electrons are scattered by charged impurities distributed uniformly in space, and the electron-electron interaction is taken into account by the Debye-Hueckel screening in the interaction potential. The equation for the current relaxation rate Γ, derived previously by the proper connected diagram expansion, incorporates the quasi-particle effect in a self-consistent manner. The solution of this equation at high carrier concentrations n yields the so-called Brooks-Herring formula. At lower concentrations, the solution deviates significantly from the latter. The solution is in general smaller than the standard expression for the rate based on the Boltzmann equation; and this is consistent with the existing conductivity data available. At the very low concentrations e.g. n = n₃ = 10¹³cm^?3 or lower for Ge, the mobility calculated is inversely proportional to the square-root of the impurity concentration n_s, and has a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$-dependence (T: temperature).

$\text{μ = 0.3597\&z.xl;h}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{k(k}{}_{\mathrm{B}}\text{T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{(ze)}{}^{\mathrm{?1}}\text{n}{}_{\mathrm{s}}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{m}{}^1{}^{\mathrm{<mtext>?3</mtext><mtext>4</mtext>}}$

, where k is the dielectric constant. The conductivity data directly comparable with this formula are not available at present. However, the quasi-particle effect which led to this peculiar concentration-dependence should also show itself in the cyclotron resonance width; there, experiment and theory both show the ${}_{\mathrm{n}}{}_{\mathrm{s}}$-dependence for very pure semiconductors.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Thermal properties of smoky quartz at very low temperatures

The existence of a distribution of low-lying energy levels associated with holes trapped at substitutional aluminium impurities is confirmed by measurements of low temperature specific heat and dielectric loss. A large linearly temperature-dependent contribution to the specific heat is observed at temperatures below 1 K, down to 30 mK. A localization temperature describing the width of the distribution of the energy levels is estimated to vary between 3 and 6 K.

We determined the thermal conductivity down to 30 mK and attribute the decrease below the boundary-scattering value to irradiation damage generally, arguing that it is not attributable to resonant scattering.     

Diffusion and localization of muons at very low temperatures

Conclusion The measurements of muon diffusion below 2 K have provided many new data and ideas in the metals Cu and Al, although the understanding is far from complete. Especially intriguing are theAlMn data with their very clear temperature and concentration dependence over a wide temperature range. My feeling is that we now approach well-defined situations where the basic properties of muon diffusion and localization can be seen. Still I would hesitate to draw any definite conclusions regarding the existence of e. g. coherent motion, and I strongly hope that the existing theories of low-temperature diffusion can be further developed, hopefully yielding less extreme results for muons in the very low temperature range.     

Free induction decay with radiation damping II. Possible limitation of NMR thermometers at very low temperatures

Solution of Bloch equations with a term including the radiation damping for the pulse excitation of a two-level spin system shows, that the shape of the envelope of the free induction decay can be, under certain conditions, temperature dependent. This result is quantitatively exact for spin systems obeying these equations. Qualitatively, we show that this fact can give a limitation to the widespread use of NMR thermometers at very low temperatures (in the region of and below 1 [mK]).     

Phase transition studies of paramagnetic crystals at low temperatures

Magnetic, thermal, electrical and optical properties of a series of paramagnetic compounds of general formula ABF₆, 6H₂/6D₂O and A(ClO₄)₂, 6H₂O where A=Co, Na, Zn, Hg and B=Si, Ti, Zr, showing structural transition from room temperature hexagonal with one molecule in the unit cell to low temperature monoclinic with two molecules in the unit cell, are reviewed.     

Concentration dependence of the critical resolved shear stress of cadmium-silver alloy single crystals at low temperatures

The increase of the critical resolved shear stress of cadmium single crystals by addition of silver has been investigated in the temperature range of 77 K to 199 K. At all temperatures the critical resolved shear stress increases withc ^2/3, wherec is the atomic concentration of silver as solute, and it decreases with increasing temperature. The concentration dependence of the critical resolved shear stress is explained according to the theory ofLabusch (phys. stat. sol.41 (1970), 659).     

Anisotropic heat conduction in cubic superconductors at very low temperatures

Anisotropic heat conduction is predicted in defect-free superconducting lead and niobium in the temperature range T/T_c ? 1. This anisotropy is predicted to result from phonon focusing which is a general property of elastically anisotropic crystals.     

Anomalous temperature dependence of paraelectric relaxation at very low temperatures

Hydroxyl doped KCl crystals were subjected to two dimensional stress in a (001)-plane so that the paraelectric hydroxyl dipoles can be described by a double-well system with a small tunneling splitting. In the temperature range from 0.025 to 0.3 K this system exhibits a dielectric relaxation rate that increases with decreasing temperature.     

Plastic-strain localization in barium fluoride single crystals at elevated temperatures

The strain distribution is studied in BaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate in the temperature range T = (0.22–0.77)T _m. At T > 0.5T _m, the plastic strain in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The degree of localization increases with temperature. Localized-shear microbands are shown to be oriented along {001}〈110〉 slip systems. The phenomenon of serrated yielding is detected, and stress jumps (serrations) are established to correlate with the formation of shear zones.     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

The ¹H NMR line-width and spin-lattice relaxation time T₁ of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T₁ near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T₁ around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH₃ groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the ¹H NMR relaxation behavior, and that ¹H nuclei play important roles in the phase transitions of the TSCC single crystal.     

Deuteron NMR spectra of ammonium ion isotopomers at low temperatures

Partially deuterated ammonium compounds contain ammonium ion isotopomers with relative abundances given by the binomial distribution of protons and deuterons. All isotopomers with deuterons contribute characteristic deuteron NMR spectra at 5K. Experimental NMR spectra were separated and respective contributions of isotopomers were determined. The derived contributions agree with expected values for a given deuteration in the case of ammonium hexafluorophosphate. In ammonium hexachlorotellurate both NH2D2+ and about 50% of NH3D+ ions are rigid, while the remaining NH3D+ perform limited jumps. NHD3+ and ND4+ ions undergo tunnelling rotation, NH3D+ ions perform either jumps about C2 axis or limited jumps, but some stay rigid in ammonium hexachlorostannate. NH2D2+, NHD 3+ and ND4+ undergo rotational tunnelling. In the case of ammonium perchlorate, the NH3D+ ions perform either jumps about C3 axis or limited jumps whilst some remain rigid. Very low values of activation energies were derived for all spectral components from the temperature dependence of their spectra, up to about 20K, which indicates an incoherent tunnelling nature of the observed dynamic processes. The diverse mobility of NH3D+ ions appears to be the most interesting and new feature.