Table of contents

Theophilou's extension (ibid., vol.12, p.5419, 1979) of the Hohenberg-Kohn theorem to excited states is shown to follow from specialisation of the familiar Hohenberg-Kohn theorem to the ground state of an appropriately defined non-interacting super-Hamiltonian.

Exact calculations of the low moments of the resistance for models of random one-dimensional conductors are given. The fluctuations in resistance grow with length more rapidly than the average resistance, so that the latter is not representative of the probability distribution. The probability distribution is calculated in the limit of large disorder.

A numerical study of transmission through a one-dimensional lattice of random delta functions is presented. Results verify analytical predictions about the statistics of transport properties: averages of resistance and conductance are unrepresentative, but averages of the inverse localisation length obey the central limit theorem. Dependence of the inverse localisation length on disorder is discussed.

A model for amorphous chalcogenide semiconductors predicts an intrinsic photoinduced paramagnetic band of metastable states which may absorb light and give rise to an electron spin resonance. In addition, a wide band of non-magnetic (two-particle) metastable states may be occupied, especially in doped samples. The bandwidths are evaluated and the optical spectrum is deduced in good agreement with experiment.

Measurements of the AC susceptibility of an insulating manganese aluminosilicate spin glass in the presence of a small static field are reported. The susceptibility decrease is proportional to H², and has a strong maximum near the spin glass freezing temperature. The results are discussed in terms of a superparamagnetic domain model.

The interaction of an intense microwave electric field with Cr³⁺ ions in ruby is investigated. The authors report solutions for the electric and magnetic polarisation vectors oscillating at the second-harmonic frequency of the driving field and a comparison with experimental data.

A new equivalent-neighbour model is introduced in which the spins within a hypercube of volume (2R+1)^d interact equally with the spin at the centre of the hypercube. The lattice dimensionality d and the range of the interaction R are variables. The concept of the coincidable occurrence factors is introduced. Connections between the coincidable occurrence factors and the high-temperature lattice constants are shown. A systematic way of deriving the high-temperature lattice constants through the use of the coincidable occurrence factors is developed. The high-temperature lattice constants are derived up to eighth order. The extension of d and R from integers to real values is discussed. The present model can also be extended to an anisotropic one in which the ranges of interactions are different in different lattice directions.

Experimental results on elastic neutron diffraction and diffuse scattering in the plastic phase CrBr₄ (T>320K) are reported. A new determination of the structure in this phase is made from 30 independent reflections. Intense diffuse scattering is observed as anisotropically distributed in the vicinity of the (220) Bragg peak. It is shown that this diffuse scattering is due to orientational correlations. No critical behaviour of the intensity is observed at T=T_c. Inelastic analysis of the diffuse scattering indicates the existence of three quasi-elastic components with HWHM of 0.03, 0.25 and 0.5 THz respectively.

For pt.I see ibid., vol.13, p.2833 (1980). The object of the paper is the study of the intermolecular correlations in the orientationally disordered phase of Cbr₄. It is shown that, contrary to what takes place in the plastic phase of methane, very strong correlations of steric origin occur between disordered molecules. The theoretical calculation of the diffuse scattering, based on a graphical series expansion in the Q vector, shows that the correlations of steric origin produce an intense cigar-shaped diffuse scattering, centred near (2.1, -2.1, 0) and oriented along the (111)* direction. This is in good accordance with the known experiments.

Expressions for the anharmonic Debye-Waller factor are derived for all temperatures for atoms on sites possessing arbitrary point-group symmetry. The results are obtained by perturbation expansion about the harmonic case (to second order), enabling the Debye-Waller factor to be expressed in terms of the harmonic one-phonon Green function. The final expressions, valid for the fully interacting Hamiltonian, are presented in such a way that they may be readily specialised to the commonly used one-particle potential model. Representative terms in the Debye-Waller factor are investigated in detail for this model.

The authors set up a model for an intermediate valence equivalent to the 'atomic' limit of the Anderson Hamiltonian. Detailed analysis of this model shows that most of the essential characteristics of valence fluctuators are already present in the crudely simplified Hamiltonian. The spin-spin for the 4f charge-charge correlation functions are studied and it is shown that it is possible to define a spin fluctuation frequency omega _sf and a charge fluctuation frequency omega _cf. omega _sf and omega _cf differ considerably for some values of the parameters of the model. The magnetic susceptibility and the specific heat are calculated as functions of temperature and it is shown how the results simulate the behaviour found in valence fluctuators.

It is proposed that the interactions of a hydrogen molecule with a solid matrix can be represented by a crystal field. Results are presented for a crystal site of high symmetry.

A theoretical study of the dependence on electric field E of the free-carrier trap time tau , the bound-carrier release time tau _b, and the probability phi of avoiding geminate recombination of optically created carrier pairs, is carried out for an ideally one-dimensional electronic system. The results are qualitatively different from those in a three-dimensional system. A critical field E_c=kT/ew exists, determined by the distance apart w of the traps, which for a pure system is a small field. For E<E_c, tau , tau _b and phi are independent of field and controlled by carrier diffusion. For E>E_c however, tau , tau _b and phi ^-1 vary as E^-1 and are controlled by carrier drift. This latter result has been put forward by separate authors for tau and phi before (Haarer and Mohwald, 1975; Haberkorn and Michel-Beyerle, 1973 and Blossey, 1974). The paper extends their results, and also shows how they are related to each other. For fields large enough to saturate the drift velocity, tau again becomes independent of field.

The electrical properties of TiSe₂ single crystals doped with several impurities have been investigated as a function of the concentration. The electrical resistivity at low temperature is distinctly affected by Nb, Cr and Ni in connection with a localisation of carriers and of complex scattering mechanisms. Crystals of the Ti_1-xNb_xSe₂ system have been grown for the whole range of concentration x. They present properties typical for the 1T and 2H phases. Substituted impurities like Sc and Y appear to behave as acceptors and impurities like Re or Cu as donors.

Solutions are presented to the non-linear equations of motion of classical antiferromagnetic chains in a continuum description. Results have been obtained for, in addition to isotropic exchange, various combinations of single-ion anisotropies (Ising and xy-like) and external magnetic fields (supporting and breaking the anisotropy). Among the solutions are both sine-Gordon solitons, representing antiferromagnetic domain walls and pulse solitons with continuously varying amplitude. Solitons in the xy antiferromagnet in asymmetry-breaking external field are discussed with respect to their observability in TMMC. They are found to contribute two different central peaks to the dynamical structure factor.

The temperature dependence of the magnon energies in two-dimensional antiferromagnets with easy-plane anisotropy has been investigated by means of a Green function approach. A preliminary treatment of the non-diagonal effects of the anisotropy is made and the Hartree-Fock magnon energies are obtained by a first-order perturbation theory in the anisotropy. These results are applied to the planar antiferromagnets K₂FeF₄ and Rb₂FeF₄. In the first compound, satisfactory agreement is found with the experimental temperature dependence of the magnetisation as well as the one-magnon and two-magnon Raman scattering data. The case of Rb₂FeF₄ (where only zone-boundary features are accounted for) is critically discussed.

Explicit expressions are derived for the Green functions between a pair of spin operators at any two sites within a Heisenberg ferromagnetic slab of finite thickness. These Green functions are found to contain a description of the bulk spin waves and surface spin waves, together with their spectral weighting factors. The results are employed to evaluate the magnetisation in the ferromagnet as a function of temperature and distance from the surfaces.

Reflectivity measurements have been made on bulk p- and n-type GaSb at 300K and 77K. The results have been analysed to obtain values of carrier concentration and mobility and compared with Hall measurements. Since carrier concentration measurements on n-type GaSb are complicated by the occupation of the satellite L_1c minima, the Hall and reflectivity measurements have been analysed using two-carrier models and compared in terms of total carrier concentration. Good agreement was obtained between the two types of measurement. Calibration curves relating carrier concentration to plasma frequency have been obtained for 300K and 77K and the measurements were consistent with a small positive temperature coefficient for the energy gap between the Gamma _1c and L_1c minima. The reflectivity technique and calibration curves have been used to assess n-type Te-doped epitaxial layers of GaSb. Te concentrations of approximately 10^-6 at.% produced layers with n approximately 5*10¹⁶ cm^-3, the total carrier concentration tending to saturate at approximately 10¹⁹ cm^-3 for Te concentrations of approximately 10^-3 at.%.