Table of contents

In this paper the earlier ionic model of Akdeniz and Tosi (1987) for solutions of polyvalent cation chlorides in molten alkali chlorides, allowing for the possible formation of long-lived tetrahedral chlorocomplexes in chemical equilibrium with free species, is extended to octahedral Y-alkali halide and La-alkali halide mixtures. Complex stability is evaluated as a function of the alkali chloride solvent and of solute concentration, c, the relevant free energies being determined from charged-hard-sphere interactions, with a statistical treatment of the melt based on the mean spherical approximation. The binding free energy of the complexes is calculated from ionic-model treatments of species in vacuo. The results are compared with the Raman scattering experiments.

The authors present an attempted consistent description of the interplay of coherent and incoherent electronic processes during the transient non-linear optical response of a laser-pulse-excited electron-hole plasma in a semiconductor. By applying the density-matrix formalism of Stahl's 'coherent band-edge dynamics' they follow the evolution of the carrier system, going all the way back to the instant of the photogeneration of the individual electron-hole pairs. The basic objects of the theory are expectation values of the various carrier-carrier pair correlation functions in r space. Their time evolution is given by Bloch-type rate equations with phenomenological time-dependent damping terms, which in turn are obtained from a parallel r-space ensemble Monte Carlo simulation of the detailed scattering dynamics of the non-equilibrium carrier-phonon system. They apply this novel algorithm to experimental scenarios recently realized in the sub-picosecond laser-pulse spectroscopy of hot carriers in GaAs-type polar semiconductors.

Incommensurate potentials present quite peculiar physical properties due to the deterministic break of translational symmetry; among others the authors mention generalized metal-insulator transitions, mobility edges and the singular behaviour of localization length. The authors stress here how these aspects can be fruitfully described by the joint use of the renormalization procedure and an appropriate envelope function formalism. The application to some significant examples clearly reveals that their approach is well suited to treat more complex and realistic incommensurate potentials.

¹³⁹La NQR spectra and relaxation measurements in the La_2-xSr_xCuO₄ (LSCO) system are presented for x<0.02. In this compound the introduction of electronic holes in the planar antiferromagnet (AF) La₂CuO₄ by Sr doping and/or excess oxygen drastically reduces the temperature of ordering T_N whereby for x>or=0.02 the long-range AF order is totally suppressed. For 0.02<x<0.08, LSCO systems display 2D short-range AF correlations in regions of mesoscopic size leading to an unconventional spin glass phase at low temperature (T<15 K). For x<0.02 the system is in a doped Neel state, and at low temperature critical effects are observed, probably involving magnetic quasi-particles introduced by holes. Furthermore the authors find for T>200 K a relaxation contribution due to excess O, and this phenomenon is related to phase separation which leads to a superconducting phase for appropriate O doping of La₂CuO₄.

Measurements of the conductivity and proton spin-lattice relaxation times T_1p and T₁ in polyanilines (PANI) prepared through a novel polymerization-condensation synthesis are presented. The new method of preparation yields a significant increase in the electrical conductivity, which may be related to the lesser amount of decomposition products, leading to a more uniform distribution of dopant protons along the chain, thus allowing the charge carriers to move along a longer path. The effects of the increase of the effective length of the chains on the relaxation rates are studied in terms of their frequency and temperature dependences. From an analysis of the data in the light of a diffusion equation for the magnetization, it appears that the intra-chain diffusion rate is larger than that in conventional PANI, whereas the cut-off frequency calculated by comparing T_1p and T₁ gives an upper limiting value of the inter-chain diffusion rate of about 10⁷ rad s^-1.

³⁵Cl NMR spectra and relaxation measurements in the triangular two-dimensional lattice of VCl₂, with antiferromagnetic (AF) interactions between V²⁺ S=³/₂ ions are presented. The spectra are characteristic of the ¹/₂ to or from -¹/₂ central line, broadened by the powder distribution of the second-order quadruple interaction; the electric field gradient at the Cl site is derived The ³⁵Cl relaxation is driven by the dipolar hyperfine interaction with the magnetic ion, and from the temperature dependence of the relaxation rate, W, information on the correlated V²⁺ spin dynamics is obtained. The absolute value of W and its temperature dependence for T to T_N⁺ are not consistent with the behaviour expected for conventional 2D Heisenberg systems. It is argued that peculiar spin dynamics, similar to that related with Z₂ vortices, with a possible crossover to a 3D or a 2D Ising system, may be responsible for these effects.

Experimental studies of the atomic scale structure of ionic and semiconducting solid solutions have shown that the bond-length mismatch is partially accommodated by microscopic lattice distortions and that the values of the nearest-neighbour bond lengths are intermediate between those of the corresponding pure compounds and the average Vegard value. Here the authors investigate theoretically the atomic scale structure of alloys using a generalized Born-Mayer model in the ionic case and an ab-initio pseudopotential approach for semiconductors. They also present results for the atomic scale structure of semiconductor heterojunctions and discuss how much interfacial and bulk strains can influence the electronic properties of the interface.

It is shown that, from recent measurements, the static structure factor S(k) in noble gas fluids can be determined with the precision of a few parts per thousand. Examples for argon at low density and krypton at high density are discussed and comparison with theoretical calculations are reported. The low-density case is analysed and compared with theory in terms of virial expansions, while at high density a refined modified hypernetted chain (MHNC) theory, which includes the three-body interaction, is used for the calculations. From the comparison the authors can conclude that precise information on the detail of the structure of the fluid and its connection with the microscopic interaction potential can be derived.

The pair distribution function of an electron gas of intermediate degeneracy is investigated through various approximations of the generalized mean field type. Special emphasis is put on the difference between the quantum system and the corresponding classical one. The short-range correlations exhibit an interesting non-monotonic temperature dependence. It is demonstrated that the existence of this effect does not depend on the particular choice of the approximation. For a two-dimensional system the local-field-correction formalism is extended to finite temperatures and the same phenomenon is found.

The authors determine the phase-stability region of a Lennard-Jones fluid using an Ornstein-Zernike integral-equation approach. The theory, which is made self-consistent on the thermodynamic side, is complemented by an entropy-based criterion for the freezing of the liquid.

From a combination of NQR, NMR and AFNMR spectra in Cu_1-xLi_xO information on the electric and magnetic electron-nucleus interactions is obtained. In particular, it is argued that Li⁺ enters the CuO lattice up to x approximately=4%, with a relatively small effect on the Neel temperature T_N. ⁶³Cu NQR and AFNMR, and ⁷Li NMR spin-lattice relaxation measurements are used to study the spin dynamics in the paramagnetic (PA) and in the antiferromagnetic (AF) phase and the effects associated with the creation of holes upon lithium doping. The Li doping modifies in a dramatic way the spin fluctuations in the AF phase. These effects are related to the fluctuations in the local Cu²⁺ spin configuration due to the diffusional motion of the extra holes in the electronic bands. A satisfactory explanation of the temperature and frequency dependences of Li relaxation rates is given in terms of a correlation time tau _h for the hole motion controlled by an energy gap E(x) and quantitative information on tau _h and E(x) are thus derived.

Using a variety of interionic potentials, the authors compute the configurations of adsorbed NaCl and KBr monomers on NaCl(001) and KBr(001) surfaces. In the authors' best estimate, the monomer adsorbs with the cation nearly on top of the surface anion and is tilted by 35-55 degrees from the vertical.

The application of sophisticated theories combined with refined neutron diffraction experiments has revolutionized the authors understanding of the simpler ionic liquids over the past decade. There are, however, substantial areas where more work is required, and the aim of this review is to set these topics in the broader context.

The author discusses typical problems that can presently be investigated by first-principles molecular dynamics simulations. To this end, they present the results of ab initio simulations of liquid and amorphous semiconducting alloys and surface reconstructions, which are representative of the most recent advances in the field. The main limitations of first-principles molecular dynamics in its present formulation are also discussed and recent methodological developments that try to overcome these limitations are briefly described.

A brief review is presented of some recent statistical mechanics work and numerical simulations of concentrated colloidal suspensions. The topics under consideration include the phase diagram of sterically stabilized, bidisperse suspensions, sedimentation equilibrium of interacting colloids, and an 'ab initio' description of charge-stabilized suspensions.

In structurally disordered materials there is strong inelastic light scattering in the low-frequency (Raman) region, say omega <or=50 cm^-1. In glasses, i.e. statically disordered solids, it is ascribed to vibrational excitations, an increased density of states and suppression of momentum conservation. New experimental evidence is cited for an analysis of the resulting broad peak ('boson peak') in terms of a correlation length. For the pseudo-binary (As₂S₃)_x(P₂S₅)_1-x glasses, Raman spectra are presented and analysed. In the frequency region up to the boson peak, the scattering intensity, corrected for thermal occupation, increases much more rapidly with increasing T above the experimental glass transition temperature T_g than below T_g; I versus T is linear in both regimes (x=0.5). A way of connecting this increase with the gradual glass to or from liquid transformation above T_g is discussed.

The paper briefly reviews the development of the atomistic method of calculating the properties of defects in ionic crystals, as it derives from the early work of Mott and Littleton (1938). The physical principles and assumptions of the method are described, although the main emphasis is on the development of the technique. It is pointed out that appeal to the quasi-harmonic approximation has made it possible to extend the technique so that the characteristics of defects can now be successfully obtained as functions of temperature. The insights into atomic transport phenomena which these calculations provide are illustrated by three examples.

The early proposal that magnetic fields applied to impure semiconductors would assist Wigner electron crystallization has been verified experimentally in GaAs/AlGaAs heterojunctions. These experimental studies have established some of the basic features of the melting curve of the two-dimensional Wigner electron solid as a function of Landau level filling factor. Using thermodynamics plus the anyon model, remarkable magnetic behaviour of the electron liquid in equilibrium with the Wigner electron solid is exhibited and interpreted. In addition to anyon magnetism, the momentum and statistical distribution functions for anyons are treated.

Spectroscopic studies on H-bonded systems (like H₂O) in confined geometries are presented, with an emphasis on the role played by the hydrogen bond on the properties of the systems. The reported examples (polyethylene oxide-H₂O liquid mixtures and soybean lecithin-based polymer-like gels) put into evidence that the dynamical properties (vibrational, translational and relaxational) of the H₂O molecules in these environments deeply differ from those in the bulk.

The long-standing problem of determining the order parameter of the lines of critical points in binary mixtures is addressed and solved within the framework of the hierarchical reference theory of fluids which has already proven successful in one-component systems. An exact relation is given, expressing the order parameter in terms of easily accessible experimental data, like the partial molar volumes. The competition between two fixed points is identified as the mechanism leading to the strong corrections to scaling seen experimentally.

Depolarized dynamic light scattering (DDLS) measurements are performed on an aqueous suspension of spherical particles possessing an intrinsic optical anisotropy. The particles are prepared by emulsion polymerization of a polytetrafluoroethylene copolymer, and are electrically charged. The correlation function of the depolarized scattered intensity contains both the translational and rotational self-diffusion coefficients D_s and D_r. The DDLS measurements allow derivation of the dependence of D_s and D_r on the volume fraction and on the ionic strength. In particular, the experimental data show for the first time the effect of direct interparticle interactions on the rotational Brownian motion of a colloidal sphere.

This paper discusses the electrical and optical properties and, in particular the neutron scattering measurements of the dynamic structure factor in the metal-non-metal transition range. This transition occurs when the dense liquid evaporates to the dilute vapour or when the fluid is expanded by heating to its liquid-vapour critical point. The shape of S(Q, omega ) changes considerably on approaching the transition from the high-density liquid side, indicating a change in the interparticle interaction and the molecular structure. S(Q, omega ) of rubidium in the density range between the melting point density and three times the critical density is characterized by the existence of well defined acoustic-phonon-like collective density excitations at high momentum transfer, whereas S(Q, omega ) at a density of about twice the critical density is consistent with excitations of an optic-type mode in which two species tend to move in opposite directions.

The author extends to the computer simulation of off-lattice models a phenomenological description of finite-size effects, already successfully used in the simulation of lattice systems. The density fluctuations are studied in subsystems of the simulation cell in the canonical ensemble. The density distribution functions of the subsystems are analysed by using finite-size scaling. Results are presented for the two-dimensional Lennard-Jones fluid with N=4096 particles. The study of the reduced fourth-order cumulants of the density distribution functions allows him to obtain the critical temperature and density. He compares the results with previous values obtained by different methods. He shows that finite-size scaling concepts can be extended to off-lattice models. A much larger computer effort would be necessary, however, in order to get reliable estimates of critical exponents and amplitudes.

A brief review of the present status of the mode coupling theory for supercooled liquids and liquid-glass transitions is given.

The effects of electron-electron interactions on the magnetic properties of alkali metals are reviewed. At the densities of the solid or liquid near the melting point, magnetic properties are well described by a generalization of the Stoner model. This approach accounts for enhancement of the magnetic susceptibility and deviations from the Korringa relation between the NMR Knight shift and the corresponding contribution to nuclear spin-lattice relaxation. When expanded by heating the liquid toward the liquid-gas critical point, the magnetic properties and optical reflectivity of caesium develop new characteristics best described by correlation enhancement of the effective mass. Behaviour of the Korringa relation deviation implies a change in the spin correlations from essentially ferromagnetic in the dense liquid to antiferromagnetic in the dilute metal. The similarity of these effects to those observed in the high T_c superconducting cuprates is discussed.