Table of contents

It is shown how the solution of the mean-field equations for the chiral Potts model can be reduced to the iteration of an area-preserving two-dimensional mapping. By this method a variety of modulated magnetic phases can be observed.

The authors present experimental results which show the existence of a 'glassy crystal' phase obtained by the quenching of the 1-cyanoadamantane plastic phase. From the DSC data, the glass transition temperature has been obtained and the glassy crystalline 1-cyanoadamantane has been shown to relax during annealing at 160K. The 'glassy crystal' is cubic (FCC) and is characterised by the vanishing of some low-frequency Raman contributions of the plastic phase.

As the temperature is lowered in a system of interacting fermions in one dimension with static impurities, the ratio of the elastic and inelastic scattering lengths depends crucially on the interaction coupling constants. In one range, transport in the system is classical and the resistance decreases with decreasing temperature. In the other range of coupling constants the transport has to be treated quantum mechanically and Anderson localisation results.

The authors report X-ray photoemission measurements of the intercalant 3s core levels in the magnetic layer compounds Mn₁3/NbS₂ and Fe₁3/NbS₂. Multiplet splittings of 6.1 eV (Mn 3s) and 4.5 eV (Fe 3s) are observed, which suggest, in agreement with previous magnetic studies, that the intercalant ions adopt 2⁺ ionisation states; there is some evidence for hybridisation of the intercalant 3d orbitals with orbitals on the surrounding atoms, particularly in Fe₁3/NbS₂.

The authors study here the variation of electrical resistivity with temperature of mixed-valence systems using a two-band model. They find various types of behaviour (e.g. metallic and semiconducting) for a different set of parameters in the theory. Results are compared with recent experimental findings on various mixed-valence systems, e.g. SmS, SmB₆, CePd₃.

The scaling theory of conduction has created much interest in the problem of conduction in a disordered system. Basic to this theory is the assumption that a one-parameter scaling function exists. An experimental test of this is presented for two-dimensional transport in silicon inversion layers. The results are found to be inconsistent with such an assumption and the authors conclude that the function does not exist.

The authors report the results of measurements on the rate of heat loss from hot electrons in silicon inversion layers at low temperatures. The results are interpreted in terms of the generation of acoustic phonons and it is found that disorder has a significant effect on this mechanism. In the low-disorder, high-temperature limit the energy relaxation time tau _epsilon varies with electron temperature T_e as T_e^-4. In the high-disorder, low-temperature limit tau _epsilon varies as T_e^-2. The electron temperature is measured by the effect on the weak two-dimensional localisation which allows the experiment to be performed at low temperatures.

A notional experiment has previously been described concerning the current decay curves in two identical dielectric specimens, each previously subjected for a long period to a constant electric field, and short-circuited at different times, the specimen subjected to the higher field being short-circuited first. If the polarisation mechanism exhibits a distribution of relaxation times, the current decay curves may cross over each other; if on the other hand the mechanism is such that the current decay obeys a t^-n law, then the curves will certainly cross over. Such a cross-over gives rise to certain apparent conceptual difficulties which are easily dealt with if a distribution of relaxation times is postulated, but otherwise remain to be explained.

High-resolution neutron quasi-elastic scattering has been applied to the problem of aqueous solutions. It is shown that for ions for which the primary hydration shell is in slow exchange with the remaining water, the frequently used two-state model is not appropriate. The experiments provide unambiguous evidence that the dynamics of water molecules other than those in the first shell are affected by the presence of ions.

By means of a velocity field approach, expressions are derived for the velocity autocorrelation functions of the two ionic species in a molten salt. The theory is applied to NaCl, giving results which are in satisfactory agreement with computer simulation data. In addition to coupling to the momentum density fluctuations, the single-particle motion is also coupled to the charge density fluctuations, with a coupling constant which is proportional to the ionic mass difference. The latter effect is more significant for the lighter ion.

The third- and fifth-order couplings between elastic strains and the order parameter of a first-order transition have been considered in the Landau free-energy expansion. The thermal hysteresis becomes pressure-dependent owing to the fifth-order coupling. The behaviour of the thermal hysteresis and the pressure hysteresis observed in some transitions of (NH₄)₃H(SO₄)₂ and (NH₄)HSO₄ crystals has been explained.

The third- and fifth-order couplings between the order parameter and elastic strains for a second-order structural phase transition have been considered in the Landau phenomenological free-energy expansion and it is shown that the ratio of the contributions to the spontaneous strain due to the fifth-order coupling and the sum of third- and fifth-order coupling is directly related to the ratio of the temperature derivative of the order parameter and that of the spontaneous strain.

A simplified theory of incommensurate phase transitions in anisotropically stressed BaMnF₄ is proposed. The equilibrium properties and phase transitions induced by temperature and/or anisotropic stress are analysed in terms of the Landau-Ginzburg theory. A conjecture concerning the phase diagram is presented. The occurrence of critical end points or n=2 component 'renormalised' tricritical points is demonstrated within the renormalisation group approach.

The authors present a Monte Carlo study of a dilute system of interacting classical quadrupoles on an HCP lattice for quadrupole concentrations X=0.68 and X=0.53. Within their computation time limits which were 4*10⁵ MCS per quadrupole for a sample of 96 sites and 10⁵ MCS per quadrupole for a sample of 216 sites, they find a progressive growth of the local quadrupolarisations. However, an analysis of the orientational fluctuations sets a lower bound of the order of 100 on the number of correlated quadrupoles responsible for this growth.

Linear response formulae for the phenomenological coefficients L_AA, L_AB and L_BB for matter transport by single vacancies in a very dilute isotropic alloy of solute B in solvent A are studied by the matrix method of random walk theory familiar in the calculation of impurity correlation factors. A symmetry classification of vacancy jumps based on symmetry with respect to the diffusion axis is introduced and applied to the linear response formulae in order to reduce them to compact forms. For typical models these forms contain a small number of functions K_ij which are linear combinations of vacancy migration Green functions for paths between sites of symmetry types i and j that do not pass through the impurity. The K_ij are calculated by inversion of a certain matrix of jump probabilities. Detailed calculations are made for models with first- and second-nearest-neighbour vacancy-impurity interactions for BCC, FCC and diamond lattices. The calculations of L_AB and L_AA are seen to be straightforward extensions of those already familiar in the calculation of L_BB and the impurity correlation factor. This suggests that extensions to other crystal structures and migration mechanisms can be made.

The authors have obtained an exact solution to the problem of diffusion by the vacancy mechanism in the sudden jump approximation, using the set of rate equations developed by Dibar-Ure and Flinn (1973) for self-diffusion. They have extended the equations and solution to the case of impurity diffusion via vacancies. The solutions are valid for all times and also yield the Bardeen-Herring correlation factor f in terms of an integral. Although numerical solutions have been obtained only for the square lattice in two dimensions, the method is easily extendable to other two-dimensional lattices and lattices of higher or lower dimensionality.

The authors study crystalline systems which as far as the electrons are concerned behave like composites of diatomic molecules loosely coupled together. The electrons are in molecular states which weakly overlap between neighbouring molecular sites. They examine a linear chain of such molecules which are oriented orthogonally to the chain direction. They show that such a system is unstable under a deformation consisting in squeezing the diatomic molecules orthogonally to the chain direction in some periodic fashion along the chain direction.

Expressions for the first six frequency moments of the self-correlation function have been obtained appropriate for a 2D classical electron liquid interacting with a 1/r potential. The results of the moments and the Mori-Zwanzig formalism have been used to analyse the recent molecular dynamics data for the velocity autocorrelation function. It is found that a phenomenological form of the relaxation kernel with two relaxation times provides a good agreement with the molecular dynamics data.

The effect of gauge transformation on the two-dimensional system in a strong magnetic field is studied numerically to shed light on the localisation of states. The result for the gauge dependence of energy levels and wavefunctions for periodic boundary conditions shows that an extended eigenstate is sensitive to the gauge transformation while a localised eigenstate is essentially unchanged indicating an exponential localisation.

Using Migdal's recursion relations the authors investigate the existence of spin-glass phases for bond-diluted Ising and Heisenberg models with randomly distributed nearest-neighbour ferro- and antiferromagnetic interactions. For three-dimensional systems a spin-glass phase can only exist at zero temperature, while four-dimensional systems exhibit spin-glass phases at finite temperature. They show that very general considerations imply strong restrictions on the form of 1D recursion relations. As a consequence, the qualitative properties of all the systems they considered are very close.

The authors consider the statistical mechanics of a lattice of orbitally degenerate magnetic ions coupled by superexchange. In such systems the ordering phenomena involve orbital as well as spin moments and may lead to various ordered phases and phase transitions. In particular, it is shown that the characteristic product form of the superexchange coupling leads, by a sign-reversal mechanism, to orbital phase transitions that occur at temperatures small compared with the Neel temperature for spin ordering. To treat such systems they develop an approximation scheme based on a 'factorisation approximation' which separates the total problem into spin and orbital subproblems. Detailed calculations are given for the specific case of 3d ions with triplet crystal-field orbital ground levels in perovskites.

The orbital superexchange model is a semiclassical model in which the ions of simple cubic lattice each have three purely orbital states apiece, each of which couples preferentially along a different cubic axis. The authors investigate the statistical mechanics of this model by means of exact results, analytical approximations and expansions, and Monte Carlo calculations. They find that for a negative coupling constant there is a strongly discontinuous phase transition to a phase with simple ferro ordering, and a ground state with infinite degeneracy but vanishing entropy density. For a positive coupling constant there is no phase transition and a large zero-point entropy, S₀. They derive rigorous upper and lower bounds for S₀ and obtain a Pauling estimate that is in good agreement with the Monte Carlo value. The absence of ordering when the coupling constant is positive is due not to frustration but rather to an opposite effect which they call satiation.

Electron paramagnetic resonance data dealing with the first observation of a Mn²⁺ axial center in CaO are reported. The tetragonal EPR spectrum was found in additively coloured crystals at 100K the best-fit parameters to the spin-Hamiltonian were found to be : g/sub ///=2.0015 (5), g_{perpendicular to} =2.0020 (5), b₂⁰=+132.6 (4)*10^-4 cm^-1, b₄⁰=+3.5 (3)*10^-4 cm^-1, b₄⁴=+11.7 (8)*10^-4 cm^-1, and A=-67.2 (5)*10^-4 cm^-1. The hyperfine coupling constant is anomalously small and points towards a considerable covalent bonding. The analysis of the data presented in this paper strongly suggests that the axial spectrum is due to Mn²⁺ ions perturbed by an adjacent F centre. Assuming this model, the reduction in the value of the hyperfine interaction relative to that of the free ion was estimated in terms of the approach of Simanek and Sroubek which puts the main emphasis of such a reduction on the covalent effects of the 4s orbitals of the Mn²⁺ ion and the result compares reasonably well with the experimental determination.

A theory of dielectric response that accounts for the nonsteady-state nature of many-body interactions in transient processes is presented. It is shown that the theory leads to the observed dielectric properties. Comparisons are also made with existing theories as regards their predictions, consequences and shortcomings.

The effect of pressure on Raman spectra of solid bromine has been measured up to 33 GPa using a diamond anvil cell. Four of six Raman-active modes could be observed in first-order Stokes and anti-Stokes spectra at room temperature. Overlap of the two intramolecular peaks takes place at 24 GPa. The relative scattering intensities of the two intermolecular modes vary with pressure, exceeding finally those of the intramolecular modes.

A new Cu-related photoluminescence band in InP is reported. It consists of a sharp zero-phonon line at 1.2889 eV with sharp LO and 'gap mode' local phonon replicas to lower energy superimposed on a broad vibronic background. Zeeman studies show that the recombination process arises from an exciton bound to a neutral isoelectronic centre. Electron and hole g-values of 1.27 and 2.12 respectively are deduced, with both electron and hole having spin ¹/₂. The very strong diamagnetic shift of the spectrum is found to be equal to that of a shallow donor electron in InP. This demonstrates clearly that the hole is tightly bound in the short-range potential of the centre with binding energy E_B^h=127.5 meV and that the electron is then weakly bound in the Coulomb field of the hole (E_B^e=7.3 meV), so that the overall behaviour is of an isoelectronic donor. The thermal activation energy of the vibronic sideband of the luminescence is found to be 135+or-10 meV, in good agreement with the spectroscopic value of the hole binding energy.

Photoluminescence fatigue (PLF) of alpha -Ge_ySe_1-y (0.05<y<.0.38) is investigated at 4K for irradiation times of up to two hours. Previous analyses of PLF cross sections (F_f) are shown to be inappropriate. A model involving the fatiguing of an effective fraction of all PL centres is introduced. The spectral and compositional dependences of F_f are obtained with this new model, and the convergence of F_f and the cross section, F_pa, for photoinduced absorption (PA) is observed for y=0.33 material. An 'apparent' fatigue due to self-absorption at extrinsic centres created in the material after prolonged irradiation is shown to provide an important contribution to PLF for large integrated exposure. Previously reported discrepancies in F_f and F_pa at gamma =0.33 are attributed to neglect of this extrinsic centre PLF mechanism. Specific defect structures are suggested for this material using the tight-binding approach of Robertson (1980), and their radiative properties are discussed in terms of configuration coordinate models. The nonzero PL of gamma =0 material is accounted for without recourse to unknown impurities. Mechanisms for PA/PR and PLF are proposed and the convergence of F_pa and F_f is explained by gamma =0.33 in terms of the proposed defects.

Thermoluminescence (TL) emission spectra of gamma-irradiated KCl, KBr and KCl-KBr mixed crystals have been obtained at various glow peak temperatures. The variation of emission-band wavelength with composition at different glow peaks has been studied. It shows a minimum for the intermediate composition. Also, the addition of Br^- ions to the KCl lattice has more influence on the TL emission of a KCl-KBr mixed system than the addition of Cl^- ions to KBr.

Optical absorption measurements between 0.35 and 2.5 eV at 22 degrees C on hydrogenated microcrystalline silicon films prepared in a DC discharge at deposition temperatures (T_dep) between 110 and 450 degrees C, are presented and discussed. Anomalously high optical absorption was observed for films prepared at a floating potential, which is a few orders of magnitude higher than of single-crystal Si for samples prepared at T_dep between 260 and 350 degrees C and, in the photon energy range up to 2.0 eV, is also higher than that of amorphous Si. Samples prepared at a floating potential at T_dep<or approximately=180 degrees C and >or approximately=400 degrees C, and those prepared at a negative substrate bias, show a lower optical absorption, which for the latter films is similar to that of amorphous silicon. Annealing up to 800 degrees C under ultra-high vacuum resulted in a small but reproducible increase in absorption independent of T_dep. Similar annealing experiments followed by absorption measurements in high vacuum indicated no measurable effect of the adsorbed oxygen on the optical absorption. The angular dependence of the forward light scattering from the films was measured at various wavelengths and found to diffuse scattering and absorption in the films. Furthermore, quantitative estimates of the surface roughness of the films from reflection data and scanning electron micrographs were made, and these results are discussed with respect to the enhanced absorption and scattering in the films.