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Calculations of the positron trapping efficiency at vacancies and their clusters in the transition limit within the optical potential approach are presented. The results are compared with the results of the approach based upon Fermi's golden rule reported in the literature. No significant differences are found either for metals or semiconductors. The calculations show that the elastic scattering of positrons at vacancies and their clusters is more important than trapping. The absorption coefficients for positrons at vacancies and their clusters in aluminium and silicon are evaluated.

The Mori integrodifferential equation of motion has played a key role in the study of transport and dynamical properties of classical dense fluids. We have derived a hyperbolic secant form of the memory function from the Mori equation of motion using a Markovian approximation and an ansatz for its higher-order memory function. The validity of the memory function has also been investigated.

We report on computer modelling studies of the structure of the equiatomic liquid alloy KPb over a temperature range from 873 K up to 1873 K. In the crystalline state, this semiconducting alloy contains tetrahedrally coordinated polyanions Pb₄^4-. Recent neutron diffraction results indicate the survival of polyanions in the expanded liquid alloy even up to the highest temperature measured, as indicated by the persistence of a prepeak in the static structure factor S(Q) at a momentum transfer of Q approximately=IAA^-1 up to 1873 K. Applying the reverse Monte Carlo technique, we demonstrate that there is a structure consistent with the experimental data, which consists of small clusters with a partially tetrahedral arrangement or an open loose structural network of lead atoms, typically with three or fourfold coordination. The calculations show that the peak at approximately I AA^-1 observed in the diffraction patterns occurs predominantly in the partial structure factor S_PbPb(Q), which is related to correlations between Pb clusters, for all temperatures. The Pb-Pb bonds in the clusters are fluctuating. With increasing temperature the tendency of Pb atoms to form particular clusters becomes a more general tendency to microphase separation.

A semi-empirical model for the atomic-scale structure of liquid Si and Ge has been constructed on the basis of atomic arrangements with the structure of the normal and high-pressure phases of solid Si and Ge. It has been found that liquid Si and Ge have quite similar atomic orderings, exhibiting the local tetrahedral features of the considered solid phases of Si and Ge where the features of the high-pressure phase predominate over those of the normal phase. The present model picture has been found to account well for the peculiar character of the solid-liquid phase transition in Si and Ge.

We present a microscopic model for the aggregates of amphiphilic molecules, based on a simple density functional approximation for the free energy. The different molecular aggregates are described as self-structured density distributions at the relative minima of the grand potential energy. We search for these structures with planar and spherical geometries, and obtain the phase diagram for bilayer membranes, and the curvature energies for vesicles and different types of micelles. The study of a global phase diagram, to get the density of micelles and isolated amphiphilic molecules, at equilibrium with free membranes, requires me link between two description levels of micelles: as self-structured density distributions, or as molecular clusters in the solution of amphiphilic molecules in water. This is done with the help of a simple harmonic model which provides an appropriate choice of the configurational unit cell for micelles.

The static centre of mass structure factor of liquid deuterium, already determined by means of time of flight neutron diffraction (as reported in Physical Review E in 1993), has been re-investigated with a two-axis diffractometer at a reactor source. A detailed analysis of the data is presented, with special attention devoted to the correction for inelastic scattering effects. The comparison of the two sets of data reveals the presence of discrepancies, which cannot be attributed to statistical inaccuracies. The density and the temperature derivatives of the structure factor have also been measured with high accuracy. For these quantities, the two experiments are in good agreement with each other. This suggests that the discrepancy in the structure factor is due to systematic effects which vanish in a differential measurement.

The structural behaviour of copper(I) iodide, CuI, has been investigated between room temperature and its melting point (T_M=878 K) using neutron powder diffraction. Detailed measurements have been made in the vicinity of the two known structural phase transitions gamma to beta and beta to alpha , which are observed at 643+or-2 K and 673+or-8 K. Within the zinc-blende-structured gamma -phase (space group F43 m) increasing disorder of the Cu⁺ ion sublattice is observed as temperature approaches the gamma to beta transition, in addition to a non-linear thermal expansion. The hexagonal beta -phase (space group P3m1) is observed as a single phase in the temperature range 645-668 K, but on first heating it is found to coexist with a rhombohedral phase. This transient phase was observed in isolation for only a short time but this was sufficient to show that its structure was that of CuI-IV (space group R3m), which had only been observed previously at elevated pressures. The high-temperature phase alpha -CuI has Fm3m symmetry with Cu⁺ ions distributed randomly over all the tetrahedral sites within the cubic close-packed I^$sublattice.

DTA and ESR methods were applied for studying photostimulated changes in molecular arsenic (y-As). It is shown that under the effect of illumination/irradiation the stability of y-As decreases, which is due to the formation of stable fragments of the polymeric phase (a-As). Using the principles of orbital symmetry conservation, it is shown that polymerization takes place most favourably from a staggered 'face to face' configuration of As₄ molecules. Photopolymerization may proceed without activation. As a result we get a formation of an intermediate molecular cluster As₈ of 'chairlike' symmetry D_2h.

By using a mean-field approximation we give a semianalytical method for reducing data on metal-hydrogen systems, which allows for hydrogen-hydrogen (H-H) interactions beyond the level of pair interactions. Interactions up to the tripler level are explicitly included here, although the method allows for interactions up to any finite order. The model fits low-concentration data at one temperature and then predicts pressures, lattice expansion and various energies as functions of concentrations for a range of temperatures. A Monte Carlo simulation is used to lend support to the approximate validity of the mean-field approximation for calculating the entropy in the regime of low concentrations. We find an increasingly negative average triplet interaction energy as the H concentration is increased. Although the mean-field approach is not an adequate description of clustering phenomena leading to the alpha - beta phase transition, nevertheless our finding of a more negative triplet energy with increased concentration may indicate the importance of beyond-pair interactions in the phase transition.

The effects of strain and quantum confinement in Zn_1-xCd_xSe/ZnSe strained-layer superlattices grown by molecular beam epitaxy on the GaAs(100) substrates were studied using photoluminescence (PL) at 4.4 K. The sample with 30 periods and total thickness of 3600 AA was directly grown on the substrate without a buffer layer. The PL spectrum shows a single peak that is attributed to the free excitons between the lowest electron subband and ground heavy-hole subband of the Zn_1-xCd_xSe wells. The blue shifts of the excitonic peaks in the PL spectra induced by the effects of strain and quantum confinements were calculated on the basis of deformation potential theory and Bastard's method, respectively. In addition, the temperature dependence of the PL features of the sample was studied both theoretically and experimentally in detail. The experimental results coincide with the theoretical predictions very well.

Ab initio investigations of the electronic structure of disordered body-centred cubic and of amorphous Nb-Mo alloys are presented. We find that the crystalline Nb-Mo alloys are a rare example of an alloy system that conforms with the expectations of the rigid-band model. The electronic density of states of the amorphous Nb-Mo alloys differs drastically from that of the crystalline phases: it is rather close to the simple Friedel model of a rectangular density of states. The origin of the striking differences in the band structure and their consequences for the electronic properties, e.g. Collver-Hammond correlation in the superconducting transition temperatures, are discussed.

The excitation spectra of the recombination luminescence of YAG single crystals doped with isovalent impurities of Sc and La are studied. The excitation band of the excitons trapped near impurity ions is observed at 6.7 eV, adjacent to the YAG intrinsic absorption edge. The processes of energy transfer and storage as well as their relation to the problem of the origin of the 4.9 eV emission band (supposed to be self-trapped exciton emission) are studied and discussed. The overlapping of the spectral region of direct creation of excitons with the interband transition edge (causing a metastable character of excitons) is shown. The features of relaxation processes are discussed from the viewpoint of the specificity of the electron structure of complex oxides.

We study the static density response function of the 2D quantum electron solid with analytic and quantum Monte Carlo techniques. The 'longitudinal' response function at small and intermediate momentum transfer from the simulation is well approximated by the analytic results using phonons alone, provided the Debye-Waller factor is included. The 'transverse' response function and the large momentum transfer longitudinal response function from simulations are larger than the analytic results from contributions with phonons alone.

The binding of two holes in a coupled-chain model described by the t-t'-J Hamiltonian is studied. Using the Bethe Ansatz analysis, it is shown that two holes form antibound states in a triplet spin state. This result can be extended to the case of more than two holes. The eigenvalue problem when the two holes are in a singlet state is solved exactly and one finds that the two holes can bind for all values of J. In this case, antibound states are found for sufficiently large values of J.

The magnetization in S=1 one-dimensional Heisenberg antiferromagnets Ni(C₂H₈N₂)₂NO₂(CiO₄) (NENP) and CsNiCl₃ was measured by means of the Faraday rotation in ultrahigh magnetic fields up to 150 T. A distinct saturation of the magnetization was observed in both samples. From the saturation field, the exchange coupling constants are evaluated as J=45.5 K and 25.8 K for NENP and CsNiCl₃, respectively. The experimental magnetization curve is in good agreement with a theoretical curve for an S=1 one-dimensional Heisenberg antiferromagnet at finite temperatures.

AC susceptibility, zero-field-cooled (ZFC) magnetization and field-cooled (FC) magnetization have been reported for the Zn_0.5Co_0.5Fe_1.1Cr_0.9O₄ disordered ferrite system. A polarization analysis of the transmitted neutron beam in both ZFC and FC conditions has also been reported for Zn_0.5Co_0.5Fe_2-xCr_xO₄(x=0.9 and 1.0) systems. The low-field AC susceptibility shows a broad peak at a temperature (well below the Neel temperature) where a strong irreversibility between the ZFC and FC curves starts. The presence of a ferrimagnetic-like domain structure over the entire temperature range (below the Neel temperature) is confirmed from the presence of significant depolarization up to 12 K, the lowest measured temperature. The observed features of chi AC, magnetization and neutron depolarization measurements have been analysed using a model of thermally activated domain wall movement of ferrimagnetic arrangement of local canted spins.

Electron spin resonance spectra, denoted W26 and W25, observed in type Ia diamonds showing the H3 and H4 optical absorptions, respectively, are shown to be due to excited states of the defects responsible for these absorptions. Zero-field and hyperfine parameters have been determined and are presented in this paper. The models proposed earlier by Loubser and van Wyk for the H4/W25 defect, and thus by extension also for the B nitrogen aggregate, are confirmed by the present work. The ESR results also support the the model proposed by Davies for the H3 centre.

The low-frequency hyper-Raman spectrum of K_1-xLi_xTaO₃(x=0.011,0.016, and 0.036) is measured as a function of temperature between 10 and 200 K. At each Li concentration x and temperature T, the spectrum in the wavenumber range below 150 cm^-1 is adequately described by the sum of a damped harmonic oscillator lineshape and a central delta function representing the soft-mode and hyper-Rayleigh scattering, respectively. The x and T dependence of the hyper-Rayleigh intensity I_HRL(x,T) reflects the growth and coalescence of polarization clusters induced in the incipiently unstable host lattice by the off-centre displacements of the Li ions. The concept of a cluster coalescence temperature T_cc(x) is introduced and verified for I_HRL(x,T). From the increase of I_HRL(x,T) between T_cc(x) and 10 K, the low-temperature limit of the dipolar correlation radius is estimated. While I_HRL(x,T) is a smooth and single-valued function of temperature for x=0.011 and 0.016, it exhibits a thermal hysteresis loop with rather steep edges for x=0.036, in accordance with a previous conjecture that the condensation of the Li subsystem changes its character at a crossover concentration around 0.022. In contrast to I_HRL(x,T), the soft-mode frequency Omega ₀(x,T) turns out to be insensitive to the coalescence of polarization clusters because it monitors quadrupolar rather than dipolar correlations. Up to x=0.036, the Li-induced increase of the soft-mode damping constant y(x,T) cannot be explained in terms of an unresolved soft-mode splitting due to the tetragonal symmetry of the polarization clusters. Instead, the influence of symmetry reduction and disorder on the selection rules of multiphonon damping processes has to be considered. As in I_HRL(x,T), new features are observed in y(x,T) when passing from x=0.016 to x=0.036.

Structural and optical properties of amorphous and crystalline ternary AgSbS₂ thin films as well as the effect of annealing temperature on these properties have been investigated. From the study of X-ray diffraction patterns, it is clear that the as-deposited films (300 K) are amorphous in nature, whilst those heat treated under vacuum at moderate temperatures (373 K) for 1 h have been transformed from the amorphous structure to a crystalline structure and the crystallinity increases with increasing temperature. At 500 K a new crystalline plane, (200), appears, characteristic of the binary alpha -AgS₂ besides AgSbS₂. The optical constants, absorption coefficient alpha , reflective index n, optical energy gap E_g, valence band density of states g_i and dielectric constants epsilon ' and epsilon ", were computed for amorphous and crystalline AgSbS₂ films of thickness 169 nm. The analysis of the absorption coefficient data revealed the existence of two optical transition mechanisms, indirect and direct transitions, depending on the value of wavelength range. These energy gaps were found to have the values (1.38-1.17 eV) and (2-1.77 eV) in the temperature range from 300 to 473 K for indirect and direct energy gaps respectively.

The precipitation kinetics of the Au-26.5 at.% Pt alloy, quenched within the miscibility gap and annealed isothermally at 900 K, was studied by positron annihilation spectroscopy (PAS) using the lifetime technique as well as the Doppler broadening technique. Information on the size of the platinum-rich precipitates was obtained from small-angle neutron scattering (SANS) measurements. The results of the PAS measurements were interpreted by assuming that a fraction of the positrons, depending on the morphology of the material, is trapped at misfit dislocations on the boundaries of platinum-rich precipitates. The positron trapping rate per unit specific area of the interphase boundary was obtained by combining PAS data with SANS results. The discussion points out the importance of this parameter and of the positron lifetime data for a characterization of the local structure of the interphase boundary. By assuming that the growth kinetics of the platinum-rich precipitates is governed by a t^1/3 law, we obtain from our data quantitative information on the shape of the size distribution of the precipitates.

The electronic structure of the inorganic misfit-layer compounds (SbS)_1.15(TiS₂)_n (n=1,2) has been investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and X-ray absorption spectroscopy (XAS). These compounds are built of alternating modulated double-layers of SbS with the NaCl-type structure and sandwiches TiS₂ (for n=1) or paired sandwiches TiS₂ (for n=2) with IT-TiS₂ structure. The XPS spectra of the Sb 3d core levels show a splitting of 1.1 eV, indicating two states for Sb, corresponding to the two types of Sb atoms present in the crystal structure. The XPS of 2s and 2p of sulphur atoms are a superposition of the spectra of S in SbS and in TiS₂. The valence band spectrum is mainly from the S 3p bands; a small peak just below the Fermi energy is attributed to electrons in the Ti 3d t_2g conduction band. The Ti 2p X-ray absorption spectrum shows structure, due to multiplet and crystal field interactions. The electronic structures of the misfit-layer compounds can be considered to be a superposition of the electronic structures of the two subsystems SbS and TiS₂, but with energy bands broadened due to the incommensurate modulation.

Doppler broadened positron annihilation spectroscopy has been used to investigate the effects of pulsed magnetic fields on the defect structure of heavily cold-worked nickel samples. The experimental variables investigated are processing temperature (-196 to 100 degrees C) and the relationship between the external field direction and sample texture. The key results are: (i) the maximum extent of magnetically induced recovery occurs at an intermediate temperature of around -80 degrees C, and (ii) the extent of magnetically induced recovery can be increased if the sample is subjected to multiple magnetic treatments conducted at different field/sample orientations.

An ordering transition at 155 K is reported for the stage 2 InCl₃ graphite intercalation compound. It was detected as a sharp decrease in the resistivity during the first cooling of a sample. It was observed also by differential thermal analysis during both warming and cooling. X-ray diffraction from intercalated single crystals showed an ordered phase at room temperature after a sample was cooled below the transition temperature. The ordered phase has a (4 square root 3*4 square root 3)R30 degrees unit cell with respect to the hexagonal graphite lattice.