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.     

Nuclear spin-lattice relaxation in metals at low temperatures

In NMR experiments performed on metals at low temperatures, the analysis to obtain the nuclear spin-lattice relaxation time ₁ can be quite complicated. The electronic heat capacity ultimately becomes less than that of the nuclear spins, and the characteristic times for heat flow through the various resistances between the nuclear spins and the thermal bath can become long relative to ₁. This makes it necessary to solve the full set of coupled differential equations describing the thermal relaxation. This problem has been examined because ₁ is used as a thermometric parameter, and we consider the cases when the radiofrequency field penetrates the sample completely (powders and foils) and when only the skin-depth nuclei are excited (bulk specimens). Using our theoretical model, we analyze experimental results for platinum powder and a thallium bulk sample and obtain consistent values for =₁ T over a wide range of experimental conditions, even when thermal bottlenecks occur and the apparent relaxation time is much longer than ₁. These values are (Pt)=(34.2±0.6)×10^–3 sec K and (Tl)=(4.37±0.08)×10^–3 sec K.On leave from Department of Physics, University of British Columbia, Vancouver, British Columbia, Canada.     

The g-factor of conduction electrons in Cd3P2 at low temperatures

The g-factor of conduction electrons in Cd₃P₂ at low temperatures is calculated as a function of carrier concentration, taking into account the InSb-like electronic energy-band structure of this material. In the case of typical samples with 10¹⁷, 5 × 10¹⁷, and 10¹⁸ electrons per cm³, we have determined the values of the magnetic field at which the extrema in the Shubnikov-de Haas and/or de Haas-van Alphen oscillations should occur, and suggested the experimental conditions under which effects due to spin splitting may be observed.On leave of absence from Centre de Recherches sur les Très Basses Températures, CNRS, and Service Basses Températures, Centre d'Etudes Nucléaires, Grenoble, France.     

Long-time tunneling in amorphous metals at helium temperatures

The long-time heat release in amorphous Co₆₉Fe_4.5Cr₂Si_2.5B₂₂ and Fe₈₀B₁₄Si₆ after cooling fromT ₁ (3.12T ₁292 K) toT ₀=1.3 K was measured. The power released is proportional tot ^–1 (0.5t60 hr). At lowT ₁, the experimental results are in good agreement with the standard tunneling theory. The resulting densities of states of two-level systems are close to those in vitreous silica. For higherT ₁, the observed heat release may be explained assuming the existence of a maximum energyE _f in the distribution function.On leave of absence from the Technical University, Dresden, GDR.On leave of absence from the Central Institute for Solid State Physics and Material Research of the Academy of Sciences of the German Democratic Republic, Dresden, GDR.On leave of absence from the Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia.     

Fibre-epoxy composites at low temperatures

The thermal and mechanical properties of carbon, glass and Kevlar fibre reinforced epoxy composites are discussed, with particular reference to the behaviour of these materials at cryogenic temperatures. The effects of production techniques and various fibre arrangements are determined.     

Ranque effect at low temperatures

Methods and results of experimental research on the Ranque effect at a gas temperature reduced to 80° K.