Table of contents

In certain surfactant solutions, such as aqueous cetyltrimethylammonium bromide (CTAB) in 0.1 M KBr, the amphiphiles are found to assemble reversibly into long, flexible worm-like micelles. Above a few per cent surfactant, these form an entangled viscoelastic fluid, reminiscent of a polymer solution. The authors review some recent experimental and theoretical progress concerning the equilibrium statistics and dynamics of these systems, which differ from normal polymer solutions in that the chains can break and reform reversibly.

The structure of succinonitrile in its orientationally disordered phase was reexamined through extended X-ray diffraction measurements. It was solved by using both the analytic procedures of symmetry-adapted functions and a Frenkel model assuming discrete orientations. A possible translation-rotation coupling was included in this latter case via an offset vector e. The study confirms that the nitrogen atoms are localised along the fourfold axis of the cubic cell but evidences a strong offset of the centre of mass for gauche conformations. It is shown that this can be explained by steric hindrance between some configurations of neighbouring molecules.

A simple model for the formation of polytypes in ZnS is presented. It is based on theoretically calculated stacking fault energies for this material. Polytypes are explained as metastable structures occurring during the wurtzite-zincblende phase transition. A Monte Carlo simulation of the process results in structures very similar to those observed in experiment. The same mechanism also explains the main features of X-ray data obtained from disordered highly twin-faulted cubic crystals of ZnS.

AC (10²-3*10⁵ Hz) measurements have been performed on Sawyer premium-quality synthetic quartz oriented along the z axis, in the temperature range between 240 and 1500 K. The samples were both 'as-received' and X-rayed, in the case of the lower-temperature measurements. The results confirm that the conductivity, in the temperature range considered, is essentially due to extrinsic phenomena, i.e. to the dissociation of Al-Na substitutional centres and to the migration process of alkali ions. An interpretative scheme is proposed, which involves the detailed structure of the attractive potential of Na⁺ ion in the vicinity of an aluminium impurity and the increase in the radius of the z-axis channels in the beta -quartz-to-tridymite transition.

The solution behaviour of hydrogen in the A-15 compounds V₃Ga and Ti₃Ir was studied by hydrogen absorption and X-ray diffraction measurements. It was found that the presence of the hydrogen did not change the structure of the A-15 host crystals. The lattice parameter, the partial enthalpy of solution and the partial entropy of solution were determined for both compounds investigated as a function of the hydrogen content.

The excitation of luminescence and the generation of point defects by monochromatic synchrotron radiation at near liquid helium temperature (LHT) was investigated in NaF crystals. Emission bands at 298 and 523 nm are ascribed to the sigma and pi bands of the intrinsic STE emission. This is supported by the temperature dependence of these bands. The growth of the F-centre concentration during VUV irradiation was monitored by a highly sensitive laser-induced luminescence method. F-centre generation efficiency, as well as the excitation spectra (110-160 nm) of the main emission bands, showed a strong maximum at 123 nm. This excitation maximum coincides with the long-wavelength tail of the first exciton absorption band. A weaker excitation maximum was recorded in the alpha -band region. These findings agree well with the predictions of the previously proposed model.

Degenerate InAs single crystals have been irradiated by thermal neutrons below 6 K. The Shubnikov-de Haas effect and the electrical resistivity have been measured as a function of the neutron dose and the annealing temperature. The effects of transmutation doping and simultaneous introduction of lattice defects have been analysed in terms of the conduction electron density and the scattering rates tau _p^-1=pne²/m* and tau _x^-1=2 pi k_BX/h(cross) (where X is the Dingle temperature). The measured conduction electron density after irradiation and the thermal annealing agreed well with the values calculated from the experimental and materials parameters. The effects of radiation damage may qualitatively be explained assuming neutral In vacancies to be the most common type of defect in the thermal-neutron-irradiated InAs. A comparison with similar experiments on InSb is given.

The authors study a one-dimensional non-linear lattice with two sites per unit cell, placed respectively in a double-quadratic and a parabolic substrate potential. The phonon stability and phase diagram of this structure which can represent a possible model of some hydrogen-bonded diatomic chains are determined and some special features of the model are found.

The accommodation of misfit in epitaxial interfaces by homogeneous strain and by misfit dislocations (MD) is studied by means of the 1D model of Frank and van der Merwe in which the elastic interactions are replaced by a real pair-wise potential. A relevant feature of the real potential is its inflection point beyond which the potential is not convex. The latter leads to a significant difference in the physical behaviour in comparison with the system with a convex potential. An alternation of long, weak and short, strong bonds occurs in expanded epilayers. This transition from undistorted to distorted structures is found to be of second-order with respect to both the misfit and the substrate modulation and the critical exponents are equal to 1/2. The commensurate-incommensurate transition is continuous in compressed chains. In expanded chains it is also continuous as long as all the atoms in the ground state sample the convex part of the potential. Above a certain value of the interfacial bonding this condition is violated and the commensurate-incommensurate transition changes into a first-order transition. The energy of MD interaction is considerably smaller in expanded than in compressed chains. It is suppressed additionally by the distortion of the chemical bonds between the MD. The energy of a single MD depends strongly on the absolute value of the misfit; it grows in compressed and decreases in expanded epilayers. At zero value of the misfit the energy of a positive MD (empty potential trough) is smaller than the energy of a negative MD (two atoms in a trough).

A variational wave function to describe the ground-state properties of the attractive Hubbard model is presented. The function is complementary to the Gutzwiller wave function for positive U. The results are in agreement with the canonical transformation which is known to relate the attractive and repulsive Hubbard models for all electronic densities.

Magnetic susceptibility and neutron diffraction measurements prove the 3D rather than the 2D or 1D character of magnetic interactions in Bi₂CuO₄. It has been shown that, at T_N=44.6+or-0.3 K, Bi₂CuO₄ undergoes a transition to an antiferromagnetic state. The magnetic unit cell coincides with the chemical cell (k=0) and is of type I. The value of magnetic moment is either (0.53+or-0.1) mu _B or (0.75+or-0.1) mu _B depending on whether the magnetic moments are oriented parallel or perpendicular, respectively, to the c axis. The effective magnetic moment of the Cu²⁺ ion is (1.80+or-0.2) mu _B. The results of theoretical susceptibility calculations for the BCT lattice model and for the SQ lattice model together with the molecular-field interactions have been presented and compared with the experiment.

Powder neutron diffraction patterns of alpha -MnMnO₄ have been recorded between 2 and 127 K in 0.48 K intervals. A subtle structural transition from C2/m to P2/m (or lower) symmetry is observed at T_s=79.1(2) K and antiferromagnetic order occurs below T_N=10.7(2) K. The magnetic structure consists of collinear Mn²⁺ moments of 4.71(3) mu _B close to the (101) direction of the a*b*2c magnetic supercell. The moment reduction due to covalency is compared to that in other oxo salts of d⁵ cations. Ferromagnetic alignment of the four spins within Mn₄O₁₆ clusters of edge-sharing MnO₆ octahedra is observed and weaker antiferromagnetic interactions between clusters are mediated by molybdate groups. A potential exchange mechanism involving spin transfer from three Mn²⁺ ions onto one O^2- is proposed to explain the unusual ferromagnetic interaction. Refinements of the crystal structure using each of the 260 diffraction profiles reveal an exchange-strictive structural distortion around T_N that is consistent with the ferromagnetic interactions within the clusters.

Monte Carlo simulations of two-dimensional fluids with a truncated Lennard-Jones interaction in the NVT ensemble are analysed with a block density distribution technique, for N=256 and N=576 particles. It is shown that below T_c (critical temperature) the block density function develops a well defined two-peak structure. From the locations of these two peaks the densities of the two coexisting phases can be reliably estimated. In the one-phase region the width of the single-peak is used to extract information on the compressibility, by extrapolating the results for finite block size versus inverse block linear dimension to the thermodynamic limit. Studying the temperature dependence of the fourth-order cumulant of the block density distribution at the critical density for various block sizes, the location of the critical temperature is found from the intersection of the cumulants, just as in the simpler case of Ising models. The authors' results suggest that finite-size scaling techniques can be used to analyse the critical properties of Lennard-Jones fluids and related systems.

The authors report the first application of molecular dynamics simulation to the calculation of the thermal conductivity of ionic solids. Results are presented for the thermal conductivity of solid NaCl and KCl at a range of temperatures between room temperature and the melting point, and also for a number of transport coefficients in the liquid state. The calculations are based on simulations in full thermal equilibrium, and make use of the Green-Kubo relations. The simulations reproduce the experimental measurements with satisfactory accuracy (generally to within 10%), except for the solids near the melting point.

The spectral density of an electron in a Penrose lattice is investigated numerically. it is found that the profile of the spectral density of the energy versus the wavenumber plane exhibits a dispersion-relation-like pattern. The positions and the 'intensities' of the stationary points of the dispersion relation are accounted for by the ordinary structure factor, S(Q), and the 'optical structure factor' S_opt(Q); the Penrose lattice is a 'non-Bravais type' quasilattice and the eigenstates near the top of the band are considered to be 'optical modes'.

The authors calculate the Hall resistivity in the low frequency limit for a model of independent electrons in the lower tail of the density of states of the disorder broadened lowest Landau level. It is shown that the Hall coefficient remains finite for omega to 0 even in the lowest localization regime for 2D as well as for 3D systems in contrast to the characteristics of magnetic freezing out. They relate their theoretical results to recent experiments on magnetic field induced MI transitions in doped semiconductors.

DC resistivities and optical dielectric constants have been obtained from thin Ag films for a range of thicknesses as a function of temperature, both before and after annealing. Discrepancies between DC and optical frequency relaxation times at lower temperatures lead to the conclusion that a significant contribution to the residual imaginary dielectric constant results from phonon assisted absorption. The effect of electron scattering on the phonon assisted absorption is also considered.