Table of contents

The temperature dependence of the dielectric constant of opal containing dispersed sodium nitrate was measured in the temperature interval from 370 to 600 K. Features associated with the ferroelectric and melting - freezing phase transitions in sodium nitrite particles were observed. A low-temperature shift of the Curie point of compared to that of the bulk was found on cooling, and significant broadening (up to K) of the melting - freezing phase transition was observed. A giant enhancement of the dielectric constant (up to at 100 Hz) of was found at high temperatures, when the sodium nitrite particles melt, transforming to electrolytic drops.

We consider a generalized mean spherical approximation for the pair-connectedness function of a dipolar hard-sphere fluid. Based on its analytical solution, we propose an exponential approach to the continuum percolation of dipolar fluids. The mean cluster size and the critical percolation density so obtained agree well with previously reported Monte Carlo simulations. The Kirkwood factor calculated among connected dipoles, a magnitude that can be taken as a measure of the dipolar ordering inside the cluster, also compares well with the simulation results.

A three-dimensional model of the atomic arrangement in liquid Se has been constructed by the reverse Monte Carlo method without applying structural constraints. The model has been analysed in terms of ring size statistics, and near-atomic-neighbour and bond-angle distributions. It has been found that, on an atomic scale, liquid Se may well be described as a branched network of short chains which also involves many rings. The relationship between the structural features of liquid Se, revealed by the present reverse Monte Carlo simulations, and those of solid crystalline and amorphous Se, previously well established, has also been discussed.

The liquid Mg - Bi system exhibits strong compound formation at the `octet' composition . We present results of first-principles molecular dynamics simulations of this alloy system at different compositions: the pure Mg and Bi liquid components, the stoichiometric liquid, and a Mg-rich composition . For the pure liquids, our results are in excellent agreement with experimental diffraction data. For , a significant modification of the characteristics of the local ordering is found w.r.t. the crystalline -phase: the ordering in the liquid is much more ionic. This structural modification is consistent with the structure of the superionic -phase, that was reported recently by Barnes et al 1994 J. Phys.: Condens. Matter 6 L467. Our simulations cannot reproduce the `reverse' metal - nonmetal transition observed upon melting, the computed conductivity being much larger than found in experiments. Instead, for the Mg-rich alloy, the calculated conductivity approaches closely to the experimental value.

The velocity of sound and high-energy -ray attenuation in Ag - Se alloys have been measured as functions of temperature over the whole composition range. The sound velocity in an alloy with a miscibility gap shows an unusual temperature dependence which is characteristic of the critical behaviour associated with the two-melt phase separation. Using these quantities, the molar volume, volume expansion coefficient and adiabatic compressibility have been deduced, which show a distinct cusp at . The results suggest the existence of stable ionic association in the liquid. No indication in the temperature dependences of these thermodynamic functions was found which would correlate with the unusual temperature dependence of the electrical conductivity.

Recent measurements for expanded Hg, Cs and Rb have shown that the liquid - vapour coexistence curves for these metals do not obey the `law of rectilinear diameters'. It is shown here that a two-state Van der Waals equation with a density-dependent cohesive energy will lead to compositional fluctuations that result in a breakdown of the `law'. Calculations predict that Hg and the alkali metals will exhibit similar behaviour near the critical point. However, in the case of Hg the 6p - 6s band-gap closure, which occurs at higher than critical densities, leads to an anomalous behaviour not observed in the alkali metals. The influence of this transition on the rectilinear behaviour is treated by introducing a further modification of the two-state model.

We derive and discuss sets of sum rules relating to the density fluctuation operator and to the single-particle creation and annihilation operators in a superfluid of charged bosons. The physical interpretation of the particle - particle and particle - density sum rules hinges on the single-particle excitation spectrum at long wavelengths having a gap equal to the plasma frequency in the presence of the Bose - Einstein condensate. This spectral property is shown to follow from the Hugenholtz - Pines relation for the chemical potential in terms of the half-diagonal two-body density matrix. Data on this density matrix are obtained by quantum Monte Carlo methods and are used to check the self-consistency between the Hugenholtz - Pines relation and the value of the chemical potential calculated from the ground-state energy. We also tabulate the contributions from the plasmon and from multiparticle excitations to various matrix elements and sum rules at long wavelengths.

We report some new results from our study of two-dimensional (2D) Bose systems, particularly the collective excitations in 2D charged Bose gas, in the presence of disorder. The effects of disorder are taken into account through collisions the Bose particles suffer due to disorder; these collisions are considered in the relaxation-time approximation within a number-conserving scheme in the random-phase approximation (RPA) at T = 0. The dependence of the plasmon dispersion on the boson interparticle potential, and the critical wave vector below which the plasma excitations cannot propagate, are investigated. The static structure factor S(q) in the presence of disorder is evaluated in closed form. We consider single- and double-layer systems and compare our results with the corresponding electron gas systems. A double-layer system in which one layer is disordered while the other is disorder-free exhibits results analogous to the `drag effect'. Many-body extensions beyond the RPA are also discussed.

For the first time, we present the calculation of the nonlinear optical coefficient of the (NAB) crystal from a systematic and quantitative standpoint. Based on the dielectric theory of complex crystals and the Levine bond charge model, the method of calculation of the second-order nonlinear optical tensor coefficients of complex crystals has been given systematically. The chemical bond parameters and linear and nonlinear susceptibilities of the NAB crystal have been calculated in detail, and the calculated value of is esu, which agrees with the measured value of esu.

The crystal structure of at 298 K has been determined (monoclinic, space group Cc, a = 19.464(2) Å, b = 17.736(2) Å, c = 8.116(2) Å, , Z = 4). The structure consists of one-dimensional polyanionic chains extended along the c-axis, and n-propylammonium cations located in elongated cavities formed by polyanions. The cations are connected to chlorine atoms by hydrogen bonds. Differential scanning calorimetry, the temperature dependence of the lattice parameters and static electric permittivity studies revealed a first-order phase transition at K of an `order - disorder' type. It is related to the ordering of n-propylammonium cations that occurs on decreasing the temperature. Debye-like dispersion of the electric permittivity between 30 MHz and 900 MHz is observed over a wide temperature range above . The activation energy of the reorientation of the n-propylammonium cations is found to be 0.29 eV.

Krypton atoms incorporated in sputtered a-silicon films are investigated by means of Mössbauer spectroscopy. The hyperfine parameters of the source were determined by taking a spectrum against solid krypton. Mössbauer spectra were taken for films containing krypton concentrations up to 7 at.%. A Debye temperature of 116(4) K has been measured for a sample containing 2.83 at.% Kr. The isomer shifts found for all spectra and the high Debye temperature indicate that krypton resides in small highly pressurized precipitates.

We present here an application of the augmented-space recursion technique for binary disordered alloys. The method allows us to incorporate effects like clustering, short-ranged order and off-diagonal disorder arising out of size mismatch and consequent lattice distortions. We base our calculations on the TB-LMTO Hamiltonian of Andersen and co-workers. We study three alloy systems: AgPd, CuZn and FeTi, and compare our results with earlier work.

The effects of conduction-band (CB) impurities in heavy-fermion systems are studied on the basis of the periodic Anderson model in the framework of a slave-boson mean-field theory within the single-site coherent-potential approximation (CPA). We provide a simplified CPA formalism for the treatment of mixing disorder induced by CB alloying. The density of states for conduction and localized f electrons can be calculated self-consistently in the whole impurity-concentration range. The concentration dependence of the specific-heat coefficient, static magnetic susceptibility and resistivity at low temperatures as well as the Kondo temperature in the alloys is obtained. The results indicate that the experimental observations in upon doping with Au, Ag and Al can be qualitatively explained as the effects of mixing disorder.

The problem of the existence of bound states outside the continuum of the bands due to the presence of a nonlinear impurity is studied within a two-band tight-binding model in one dimension. The nonlinear impurity deviates from the ordinary sites by an on-site energy of the form , where is the probability of finding the particle at the impurity site, is referred to as the strength and is the nonlinearity of the impurity. For , there exist threshold values for below which there is no bound state. For , there is always one bound state above the upper band for ; and two bound states, one with energy inside the band gap and another below the lower band, for . For , there is always a bound state above (below) the upper (lower) band for . For , there is always a state inside the gap; while for , there exists an upper bound for above which no bound states are found inside the gap.

We report ultrasonic and thermal expansion measurements on quasi-one-dimensional . The elastic constant exhibits strong anomalies over a wide temperature range around the Peierls transition. Anisotropic behaviour of the slow shear mode propagating along and perpendicular to the chain direction is observed around . Elastic constants, like electrical resistance, are very sensitive to a small Nb content substituted for Ta. Anomalous behaviour of at low temperatures is reported. We also present measurements of the anisotropic thermal expansion coefficients.

We have used the variational approach to calculate the in-plane electronic thermal conductivity of high- superconductors along the lines of a two-fluid model. To account for the layered structure of these compounds, an anisotropic electronic energy spectrum with a Josephson-like coupling between the superconducting planes within a Lawrence - Doniach model has been assumed. Both elastic and inelastic electronic scattering processes are considered. The theoretical results obtained are in quantitative agreement with experimental data on single crystals and polycrystals. These results suggest an electronic origin of the peak observed in the thermal conductivity of these materials.

The effect of annealing on the magnetic phase diagram for the amorphous La - Fe alloy system has been investigated. The amorphous La - Fe alloys in the as-prepared state show re-entrant spin-glass behaviour, and the ferromagnetic region is extended by annealing. The Curie temperature and the magnetic moment per Fe atom are increased, and in contrast the spin freezing temperature and the high-field susceptibility are decreased by annealing. The re-entrant spin freezing temperature of the amorphous alloy is not observed in the temperature range down to 2.5 K. The magnetization curves show relatively good saturation after annealing. The change in these magnetic properties on annealing is correlated with the structural change.

The absolute chirality of potassium dithionate, , , trigonal, space group P321, a = 9.782(1) Å, c = 6.298(2) Å, , Z = 3, R = 0.024, wR = 0.031 for 2586 unique observed reflections, has been determined at room temperature through a combination of single-crystal x-ray diffraction, including anomalous scattering, and optical study. Using a classical point dipole - dipole theory, good agreement has been obtained between the calculated and observed optical activity and refractive indices at room temperature along [001], , at a wavelength of 632.8 nm). The polarizabilities of the different atoms have also been determined by means of this theory. It has been found that in order to fit the optical data, it is necessary that the oxygen atoms must show quite large anisotropic polarizabilities. The structural features responsible for the optical rotation have been identified. In this way, a visual correlation between the sign of the optical activity and the arrangement of atoms in the crystal structure has been established.

By means of a conventional Raman scattering technique, Raman spectra of proton-exchanged (PELN) and (PELT) crystals were examined as a function of PE time. The experimental result was that the relative intensities reach maxima and minima in PELN and PELT with about 12 and 8 h proton treatment, respectively, when y-surface cracks are visible. This phenomenon, together with the different extraordinary index changes reported previously, reflects the microstructural difference between PELN and PELT. This difference mainly arises from both the different displacements of positive ions along the optical z axis in bulk and the different proton distributions in the exchange layer due to PE.

The positron lifetimes in a source have been measured. The fraction of positrons annihilated in salt and in the Kapton supporting foils has been determined as a function of the atomic number Z of the sample. All of the experimental results have been compared with calculated values obtained from a computer program based on a Monte Carlo algorithm. An algorithm has been proposed for the determination of the source correction in lifetime spectra in positron annihilation lifetime spectroscopy.

In the previous paper by Rybicki et al in 1995 we presented the Monte Carlo calculations of the time-of-flight (TOF) hopping-transport current profiles in non-uniformly defected crystalline layers, and a marked influence of the spatial non-uniformity of the total hopping centre distribution has been shown in the case of extremely thin layers. In the present paper we report using a Monte Carlo simulation the dependence of the TOF transient currents on the layer thickness L, in the wide range . The characteristic current maxima occurring in layers with hopping centre density which increases with increasing x (x is the distance from the injecting contact) just before the effective TOF, reported previously for very thin layers, persist in much thicker samples (L = 10 000a). The relative height of the current peaks initially increases with increasing layer thickness and tends to a saturated value for thicker layers.

We make a comparative study of two schemes that predict the ground states of a quantum dot with parabolic confinement in a strong magnetic field. One method is based on the composite fermion (CF) approach, and the other was recently proposed by Dharma-wardana. The prediction of these two schemes differ in general, and we compare the results of both methods against exact diagonalization calculations.

Kawamura et al have used the exact diagonalization results for quantum dots containing N = 7 and N = 10 electrons to review the predictions of cusp structure in the interaction energy given by two recently proposed schemes. Here we revisit the issues raised by them.