NMR on gallium single crystals: Not yet an absolute thermometer at very low temperatures

A twin crystal of Ga was investigated in the temperature and field ranges of 0.5 to 10 mK and 60 to 250 mT, respectively. In these fields the magnetic interaction is too large to be treated as a perturbation of the quadrupolar interaction. Therefore, the eigenvalues of the Schrödinger equation were calculated numerically, and then used to determine the crystalline axes by pulsed NMR on ⁶⁹Ga and ⁷¹Ga. The temperature dependence of the NMR line intensities was measured using ¹⁹⁵Pt-NMR for thermometry. The observed intensities do not follow the theoretical expectations for a spin system of low nuclear polarization. In addition, a temperature-dependent frequency shift was observed. These results suggest that the effect of interactions between the spins, as well as more complicated spin dynamics, need to be considered in order to use gallium as an absolute thermometer at temperatures below 1 mK.     

Magnetization measurements of polycrystal ZnMn alloys at very low temperatures

The magnetization of ZnMn alloys with Mn concentration ranging from 68 to 1 ppm has been measured from 2 K down to 10 mK. The results show large departures from free spin behavior due to impurity-impurity interactions and some concentration independent effects.     

Mössbauer study of very dilute Pd(Fe) alloys at very low temperatures

The magnetic behaviour of very dilute (7 ppm and 0.6 ppm) Pd(Fe) alloys has been studied by Mössbauer spectroscopy in external fields up to 6 T in the temperature range between 0.018 and 300 K. No magnetic ordering was observed even at the lowest temperature. Above 0.067 K the magnetization exhibits free-spin behaviour. The saturation moment was found to be independent of temperature. Magnetic hyperfine patterns observed below 0.067 K were interpreted in terms of paramagnetic hyperfine interactions in the regime of slow electronic relaxation.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

The electrical resistivity of amorphous alloys at low temperatures

Measurements of the temperature- and magnetic field dependence of the electrical resistance of some metallic glasses are presented. The data obtained for Cu₅₇Zr₄₃, Cu₄₀Zr₆₀ and Pd₃₀Zr₇₀ demonstrate that deviations from the high temperature behaviour extrapolated to low temperatures are caused by superconducting effects. The paraconductivity which is strongly enhanced in amorphous alloys is shown to agree quite well with theoretical models. The normal state resistance does not saturate down to temperatures of about 2 K. It still exhibits a negative temperature coefficient of the resistivity.     

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).     

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.     

Recovery of copper alloys after neutron irradiation at low temperatures

The effects of fast neutron irradiation near 4.2°K on dilute Cu, Cu₃Au, α-CuZn, β-CuZn and CuNi alloys were studied by electrical resistivity measurements. For Cu₃Au, the recovery stage at ～ 100°K becomes more distinct with increasing long-range order, and is attributed to migration of interstitials. The ratio of the number of replacements to the number of displacements (C _R /C _F ) is estimated to be about 50 for neutron irradiation, in contrast to the previously reported value of ～2 for electron irradiation. For α-CuZn, a large recovery stage at ～150°K is found and attributed to annihilation of interstitids whose migration produces ordering. For β-CuZn, migration of interstitials also produces ordering. It is also suggested that in β-CuZn, the ratio C _R /C _F is larger for neutron irradiation than for electron irradiation as in the case of Cu₃Au. The results on CuNi alloys are presented without explanation.     

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.