Table of contents

Most papers on methyl group tunnelling use a model in which the motion of a rigid rotator is hindered by a potential of threefold symmetry. There are a number of difficulties with such a description and, in this paper, the model is replaced by one which describes the motion of three protons, which have mutual interactions and which are hindered by an external potential. The analysis makes considerable use of symmetry ideas and arrives at conclusions which are substantially different from those of the previous model. A comparison of the two models shows that, in the absence of dipolar interactions, both will have an orbital singlet level with spin of ³/₂, but that the level described as ²E in the rigid-rotator model will not occur in the present model and that there will be no corresponding level with fourfold degeneracy. Instead there will be two separated orbital singlets, each with spin ¹/₂. The inclusion of the dipolar interaction splits the quartet into two doublets, so that finally there are four separated low-lying energy levels, each being doubly degenerate.

Conventional wisdom is that thermal transport at milliKelvin temperatures is appreciable only in substances containing conduction electrons. This paper discusses the possibility of attaining temperatures of 0.02-0.2 mK by isentropic demagnetisation of the insulating crystals, semi-dilute (Y,Ho)VO₄ and PrVO₄, together with, measurement of the final temperature. It is shown that thermal transport to the surface of powders of diameter approximately 0.08 mm takes place in times of order 1-30 min by diffusion through spin-spin interaction between the enhanced nuclear moments. Cross-relaxation between the vanadium nuclear spins (⁵¹V,I=⁷/₂) and the lanthanide nuclei ¹⁶⁵Ho, I=⁷/₂ and ¹⁴¹Pr, I=⁵/₂, is also considered.

A fully self-consistent scheme based on the two-component density-functional theory and an embedded-cluster model within the framework of the discrete variational method has been developed for calculating positron states in solids. As an example, aluminium is tested. The changes of the electronic structures and positron states for an aluminium vacancy due to the electron-positron interaction are presented. The effects of the atomic relaxations on the positron annihilation characteristics are considered. The results are also compared with former calculations and the conventional scheme.

The magnitude and behaviour of the conductivity in Ca_xAl_1-x is of considerable interest as an example of a system with high resistivity where theories of weak and strong localisation can be tested without the complicating presence of transition metals and a corresponding high density of d states. In this paper the authors present computations of the density of states, the spectral function and the electrical resistivity using hard sphere structural models containing 216 atoms with periodic boundary conditions and pseudopotentials from a previous theoretical calculation. The computed resistivity follows quite well the general behaviour observed experimentally but it is rather larger and the authors show how this discrepancy can be accounted for by relatively small changes in the pseudopotential.

The authors predict that the excitations of a superconductor with a spatially inhomogeneous order parameter Delta (r) can be localised by Andreev scattering. Results of calculations on two models are presented. The first contains fluctuations only in the magnitude of Delta (r), while the second has fluctuations only in the phase of Delta (r). For both models, the authors' results suggest that all quasi-particle states are localised in one dimension. In particular they show that for quasi-particles incident on the superconductor from a normal wire, the reflection coefficient for Andreev scattering can approach unity, even for energies far above the maximum value of mod Delta (r) mod . This leads to new charging effects which can be used as an experimental probe for order-disorder transitions in Delta (r).

Polarised neutron scattering and magnetisation measurements on dilute PdMn alloys have been analysed with a local environment model of ferromagnetic interactions and atomic short range order. The analysis allows the authors to calculate several magnetic properties of PdMn such as exchange constants of ferromagnetism, range of magnetisation clouds and chemical short range order parameters. Critical temperatures of ferromagnetism, which are low in dilute PdMn alloys compared to those in other typical giant moment systems such as PdFe, were calculated using the Monte Carlo simulation method and the results, which included atomic short range order, were compared with experimental data. The comparison shows a good agreement between experimental and calculated values for the concentrations considered.

The magnetocrystalline anisotropy of the pseudobinary (Pr_1-xNd_x)₂Fe₁₄B compounds has been measured by using the singular point detection (SPD) technique. The measured temperature and composition dependence of the anisotropy field has been compared with the data obtained in CEF approximations using different sets of (A_n^m) parameters for Nd₂Fe₁₄B and Pr₂Fe₁₄B and by considering a linear combination of the anisotropy in the mixed system. The comparison is made by calculating the anisotropy constants, then the anisotropy field values, starting from the (A_n^m) parameters. Experimental and calculated data concerning the occurrence of spin reorientation transition (SRT) and of type 1 and type 2 first-order magnetisation processes (FOMP) as a function of composition have also been compared.

The problem of interpreting Mossbauer spectra of antiferromagnetic powders subject to applied magnetic fields is considered. A mean-field model for systems with uniaxial anisotropy is used to fit simultaneously spectra taken with applied fields of different strengths. The model predicts a variety of asymmetric broad line profiles depending on the strengths of the exchange and anisotropy terms. The method is applied to the 4.2 K spectra of goethite and haematite subject to longitudinal applied fields of 0, 3, 6 and 9 T. Favourable agreement is obtained with the results from single crystals, showing that the method can be reliably used for a variety of antiferromagnetic materials.

On the basis of a classical model of spins, the authors discuss the temperature dependence of both the width and the resonance frequency of the EPR lines in a one-dimensional Heisenberg antiferromagnet with a Dzyaloshinsky-Moriya antisymmetric exchange interaction as a main perturbation term. They find that the width does not grow with decreasing temperature toward T_N, while the resonance frequency is independent of temperature. These results are completely different from those for a system with symmetric perturbation terms, such as the dipolar, anisotropic exchange and single-ion anisotropy terms; in such systems, with decreasing temperature toward T_N the linewidth broadens and diverges, while the resonance frequency goes up or down according to the direction of an external field with respect to the chain axis.

A derivation, based on first principles, is provided of the Bhatia-March formula for S_cc(0) of a binary fluid. A natural simple generalisation (a completion of the underlying van der Waals-like description) is then proposed. Finally, the application to Na-Cs is considered and the generalised formula is shown to provide a good description not only of S_cc(0) but of the excess internal energy and entropy.

The variation in excess compressibility beta _S^E and excess dielectric constant in '^E as a function of mole fraction x at room temperature have been determined for a liquid mixture of morpholine (as the solute) with n-butyl alcohol (as the solvent). The excess compressibility was found to be negative for this liquid mixture and is explained in terms of solute-solvent interactions. The excess dielectric constant was found to be abnormally positive for this liquid mixture at a mole fraction of around 0.25 and is explained in terms of the alignment of dipoles.

A new metastable surface state on the boundary of ³He-B is found analytically in the vicinity of the first-order transition line between ³He-B and ³He-A. This state consists of an A-phase layer separated from the B phase in bulk liquid by a conventional A-B interface. As distinct from all other surface states obtained by calculation so far, this state has a spontaneous mass supercurrent along the surface.

A gated low-mobility GaAs-Al_0.3Ga_0.7As delta-doped heterojunction of large area is used to demonstrate a transition from diffusive to hopping transport as the electron concentration in the two-dimensional conducting layer is reduced. At helium temperatures (1.3<T<4.2 K), the hopping conductivity is well described by the variable-range-hopping law sigma approximately sigma ₀ exp-(T₀/T)^1/2 expected when electron-electron interactions are strong. At very low temperatures (T approximately=100 mK), conductance fluctuations are seen, indicating that under these conditions size effects are important and a description of transport by a macroscopic model of conduction is inappropriate.