Table of contents

A theoretical investigation of a binary mixture of hard spheres confirms that stable superlattice structures, with a complex long-range order of the AB₁₃ type, can form in these simple systems for intermediate values of the diameter ratio, in agreement with recent computer simulations and experimental studies of colloidal suspensions. It is shown that the larger entropy of mixing of the AB₁₃ structure relative to that of the competing structures is responsible for its thermodynamic stability.

The two-dimensional Yukawa pair potential represents a simple model for the interaction of charge-stabilized colloidal particles confined between two charged plates. Static and dynamic correlations in the liquid state of this model are calculated using Brownian dynamics simulations. In particular, the pair, triplet and orientational structure are discussed as well as dynamical quantities like particle diffusion and the time-dependence of the nearest-neighbour orientational correlation. It is found that the decay of nearest-neighbour orientation is non-exponential in time. Its relaxation occurs on a time scale that is much larger than a time typical for the crossover of the translational diffusion to the hydrodynamic regime.

A model of condensed disordered systems, based on theories of random spatial and temporal processes for local atomic structures, is used to derive analytic expressions for the contributions of the diffusive atomic motion to the van Hove time correlation function G_D(r,t) and its spectral representations. The spatial random process determines the static atomic correlations in space whereas the temporal diffusion process contributes to the central (Rayleigh) peak of the dynamic scattering function S_D(k,w). The relaxation time of the intermediate scattering function F_D(k,t) exhibits the known phenomenon of slowing down in the neighbourhood of the first peak of S(k), but in the limit of small k it remains finite, contrary to the infinite slowing down of heat dissipation predicted by linearized hydrodynamics.

The theory of the atomic-like electronic structure of metals near their liquid-gas critical points has been generalized for noble and d transition metals. In these metals the valence state may change through excitation of d electrons into the low-lying displaced term which becomes favoured because of the interaction of overlapping atoms. The author uses percolation theory together with concepts of atomic-like screening and classical localization to discuss the metal-non-metal transition. Interplay between the valence state and the metal-non-metal transition is shown to exist. The electric conductivity and the low-frequency dielectric function near the transition point have been estimated for copper as functions of the density.

The authors use an interface potential approach to consider the adsorption of a fluid with long-range interactions onto a substrate of two adjacent spheres. The interplay between prewetting-like thin-thick transitions and capillary condensation is discussed.

Thorough analysis of experimental data (the authors' own as well as those from the literature) on alkali halides and their mixtures with a view to application of a generalized Lindemann criterion for melting is presented. The result of a fitting procedure is ionic radii to be used in phase diagram calculations. It is shown that a successful choice of the radii allows one to predict new phase diagrams reliably. The authors' criterion of miscibility was used to finalize the choice.

Perfect single grains of the AlPdMn icosahedral phase have been used for structure determination by X-ray and neutron diffraction. Owing to the large difference between X-ray and neutron scattering factors, information is gained on the atomic positions of the three elements. A model is proposed as deduced from a six-dimensional (6D) Patterson analysis. Six different atomic hypersurfaces are located on node and body-centre sites of the 6D lattice. The superstructure that leads to a face-centred lattice is mainly due to a strong chemical ordering, all the palladium being on the even node and odd body centre of the 6D cube. The resulting 3D structure contains icosahedral clusters similar to the external shell of the Mackay icosahedron, with two kinds of chemical decoration. The structure may also be described via a quasi-periodic stacking of fivefold planes. Each set of planes is characterized by an average chemical composition and local order. This kind of description helps in the understanding of quasi-crystal growth, formation of dislocations and dynamic properties.

A classification scheme, which is able to relate many crystalline compounds to the decagonal quasi-crystalline phases, is given based on a description of the atomic decoration in terms of a small number of subtiling units. This scheme is suitable for one-dimensional sequences, two-dimensional tilings and three-dimensional networks. The relative numbers of each type of tiling unit in the Penrose pattern are calculated. The paper first focuses on the two-dimensional aperiodic plane. In a way reminiscent of polytypism, a distinction is then made according to the way such planes are stacked periodically on top of each other, thus giving insight into the structure of the known decagonal phases. Emphasis is then put on the aperiodic scheme, which may be related to non-ideally ordered icosahedral phases. This way of describing the structure allows one to construct atomic models in three dimensions and to calculate accordingly the composition of the model structure starting from a known crystalline phase with identical or closely related composition. Many approximant and quasi-crystalline structures, not all yet observed experimentally, may be sorted out.

The effect of alloy composition on radiation damage by 2.95 MeV electrons at low temperature has been investigated in Ni(Al) and Ni(Al,Ti) solid solutions and in Ni₃Al intermetallic compounds (24.6-26.5 at.% Al) by residual electrical resistivity measurements. In the solid solutions the main result of irradiation was point-defect production. Frenkel-pair resistivities, deduced from the comparative analysis of the initial damage rates in the solid solutions and in pure nickel, were 6.5-9.4 mu Omega cm/%, similar to or slightly larger than the value of nickel. In Ni₃Al intermetallic compounds, the resistivity damage rates were enhanced by a factor of about 15 compared with pure nickel. Disordering contributes only a small part of the increase. Assuming an average of 3+or-1 replacements per displacement, a comparison of the initial damage rates in nickel and in the compounds leads to Frenkel-pair resistivities ranging from 97 to 109 mu Omega cm/%. Such high values may be correlated with the large ideal resistivity of these materials.

For pt.I see ibid., vol.4, p.10199 (1992). The recovery of short-range ordered Ni(Al) and Ni(Al,Ti) solid solutions and long-range-ordered Ni₃Al intermetallic compounds was investigated by residual electrical resistivity measurements as a function of composition and of fluence, after low-temperature (4-9 K) irradiation with 2.95 MeV electrons. In the solid solutions, long-range defect migration resulted in increases of local order, self-interstitials were found to be mobile above 110 K (at least in the binary alloys) and vacancies to be mobile above 330 K. The recovery spectrum of irradiated intermetallic compounds showed some similarity with that of pure nickel, with a smaller amount of fine structure. The mobility of self-interstitials (corresponding mainly to Ni-Ni dumbbells) occurs at a higher temperature (75 K) than in nickel (50 K) and does not induce significant ordering. By contrast, the Ni vacancies, which are the dominant vacancy species, are slightly less mobile than in nickel and promote some increase of long-range order.

The authors investigate the validity of Fourier's law in a two-dimensional monoatomic Toda lattice using the molecular dynamics method. The temperature profiles in the lattice exhibit an exponential behaviour with lattice position. The temperature dependence of thermal conductivity is necessarily derived from the spatial variation of the local temperature, and found to be inversely proportional to the local temperature. The validity of Fourier's law is confirmed by excluding the non-diffusive heat flow from the total heat currents.

Resonance Raman experiments on doped and photoexcited single crystals of mixed-halide MX complexes (M=Pt; X-Cl, Br) clearly indicate charge separation: electron polarons preferentially locate on PtBr segments while hole polarons are trapped within PtCl segments. This polaron selectivity, potentially very useful for device applications, is demonstrated theoretically using a discrete, ³/₄-filled, two-band, tight-binding, extended Peierls-Hubbard model. Strong hybridization of the PtCl and PtBr electronic bands is the driving force for separation.

The Hartree-Fock formulae are evaluated for a model consisting of an uncompensated electron gas in an infinite V-shaped potential well. This contains a diverging electrostatic term. The elimination of this term is discussed on physical grounds, as is the relevance of such an extreme model. Similarities with and differences from the case of an infinite square well are discussed. The exchange energy is found to be always unimportant.

The non-linear AC magnetic susceptibility of polycrystalline NdBa₂Cu₃O₇ superconductor has been studied to examine the intergrain contribution to magnetic properties at low field. A superconducting sample is subjected to a DC magnetic field and a small AC magnetic field in parallel. The complicated dependence of the magnetic susceptibility on the applied field is attributed to the critical state of trapped magnetic flux in the intergrains. The non-linear magnetic susceptibility of higher harmonics with frequency up to 4f was measured as a function of varying applied fields. Numerical results of calculations using a modified critical state model are in good agreement with the measured AC susceptibility data. The authors find the relation J_C approximately H^-n (n=2.5) for the dependence of the intergrain critical current density J_c on the applied magnetic field H in conformity with the previous results of YBa₂Cu₃O₇ where a smaller value of n=1.8-2.2 was obtained.

The author calculates the dynamical structure factors of a two-dimensional Heisenberg antiferromagnet at T=0, to order 1/(2S)², taking careful account of the umklapp processes. It is shown that, for the transverse spin part, the terms of order 1/(2S)² give rise to a broad sideband peak of three-magnon excitations, whose intensity is small but not negligible. For the longitudinal spin part, the terms of order 1/(2S)² modify noticeably the spectral shape of the continuum of two-magnon excitations as a function of frequency. These results could be observed in future neutron scattering experiments.

The temperature dependences of the anisotropy constant, spin-wave stiffness and spontaneous magnetizations for the two-dimensional (2D) Heisenberg model with various types of anisotropy and interactions have been calculated with the use of the low-temperature perturbation (spin-wave) theory. The authors argue that this theory is effective within a large portion (if not the main part) of the ferromagnetic phase. On the contrary, the Polyakov renormalization procedure is found to be inapplicable for the ferromagnetic phase at any anisotropy. They consider both the easy-axis and the easy-plane cases, taking into account one-site and exchange anisotropy as well as the higher-order magnetic anisotropy and the dipole-dipole interaction. For realistic models the latter is found not to change essentially the temperature dependence of the magnetic parameters. In particular, they did not find the reorientation (easy-axis to easy-plane) phase transition due to dipole-dipole interaction predicted by Pescia and Pokrovsky (1990). However, in general, the temperature dependence of the anisotropy constant proves to be much more sensitive to the type of the anisotropy and interactions than that of the spin stiffness and the spontaneous magnetization. In the easy-plane case the difference between the spin-wave stiffness for the in-plane and the normal polarizations is found to be extremely strongly temperature-dependent. At temperatures comparable with T_C this difference may be of the order of magnitude of the stiffness itself, while at T=0 it is of relativistic origin. For the case of a very small anisotropy the authors also propose a renormalization procedure to perform a partial summation of the perturbation series.

Iron-based amorphous alloys have attracted technological and scientific interest due to their soft magnetic properties. Recently it was found that amorphous alloys like Fe_73.5Cu₁Nb₃Si_13.5B₉ (FINEMET^TM) have a transition to the nanocrystalline state after proper annealing, thus exhibiting excellent magnetic properties. An attempt was made to investigate the crystallization behaviour of this alloy, which is not yet fully known. The investigation was carried out by combining several methods, namely Mossbauer spectroscopy, X-ray diffraction, scanning and transmission electron microscopy as well as microprobe analysis. The alloy was studied after annealing at various temperatures for various times. The corresponding phase analyses are presented. Even after increasing the time of annealing at 950 degrees C from 1 h to 90 h significant changes in the phases were found. It became evident that the question of phase composition can be solved only by a combination of different methods.

New measurements and Monte Carlo simulations of the total coefficients eta ₊ for the backscattering of positrons from elemental solids are reported as a function of atomic number Z between 13 and 82, incident energies E from 1 to 50 keV, and incident angles between 0 degrees and 65 degrees . The measurements and simulations show generally good agreement with each other and with the recent measurements of Massoumi et al (1992) and Makinen et al (1992). Both experiment and simulations suggest that the monotonic increase of eta ₊ with Z seen at high E is not observed for E below 10 keV. Where possible, the new results are compared with earlier measurements of electron coefficients.

The optical properties of trivalent-chromium-doped fluorophosphate glass (21.74 Al(PO₃)₃-57.8 BaF₂-19.96 AlF₃ (wt.%)) have been investigated in the 4.2-300 K temperature range using steady state and time-resolved laser spectroscopy. The luminescence of Cr³⁺ shows a strong thermal quenching which can be analysed in terms of the quantum mechanical single-configurational coordinate model. In order to establish a correlation between the glass matrix composition and the spectral and temperature behaviour of the Cr³⁺ photoluminescence, detailed time-resolved measurements have been performed. When compared with fluoride glasses, the results attained for the emission wavelength dependence of the lifetimes and excitation spectra show an enhancement of the site-dependent effects.

The fine-structure spin-Hamiltonian parameters b₂⁰, b₄⁰ and b₄³ of Fe³⁺ in the spinel crystals MgAl₂O₄, ZnAl₂O₄ and ZnGa₂O₄ have been analysed by means of the superposition model. This study indicates that the site symmetry for iron is C_3v instead of D_3d. Moreover, information on the lattice relaxation around this impurity has been obtained for the three host lattices.

Experimental evidence of a crossover from a thermally to a non-thermally activated flux motion is reported for TlBaCaCuO. It was obtained by measuring the time decay of both zero-field-cooled and remanent magnetization. The measured relaxation rates at low temperature are in good agreement with the prediction of the quantum collective creep theory for anisotropic superconductors in the low-field-single-vortex pinning.

The authors report on the magnetic relaxation of thin films of Fe of 1000 AA thickness separated by layers of Ag of 15 AA thickness. It is argued that the flattening of the magnetic viscosity at low temperature may be due to quantum diffusion of domain walls. Both the crossover temperature, T_c, from the classical to the quantum regime and the magnetic viscosity extrapolated to T=0 depend on the value of the applied field, in agreement with theories of quantum tunnelling in magnets.

The electrostatic model was applied to interpret the XPS core level spectra of Cd_1-xPb_xF₂. The energy positions of the levels are governed by both the Madelung energy and the intraionic Coulomb interaction with the valence charge. Moreover, some aspects of the local structure of the mixed crystals were taken into account. It was shown that the effective charge of Pb is markedly greater than that of Cd ions (e_Pb^*=0.70e, e_Cd^*=0.46e). This is the main reason for the slight increase of the binding energy of all core levels investigated. This also explains the lack of broadening of the XPS spectra of the mixed crystals.

The results of experimental investigations of the electrical conductivity, magnetoresistance and Hall effect in the semiconductor TlSe with a chain structure are presented. The measurements of electrical conductivity have been carried out in two directions, i.e. parallel and perpendicular to the crystal c axis (the chain orientation). Galvanomagnetic measurements were made in magnetic fields of up to 60 kOe at different orientations of the magnetic and electric fields. The impurity conductivity is metallic in character with a negative magnetoresistance at low temperatures. The authors show that the low-temperature data can be explained by the model considering TlSe as a disordered system with a low-dimensional conductivity near the percolation limit.

Phonon spectra of YBa₂Cu₄O₈ have been measured in the range 110-1000 cm^-1 and between 30 K and room temperature. All seven B_1u-symmetry infrared-active phonons are observed at 119, 135, 200, 314, 332, 500 and 605 cm^-1 at 290 K. The effect of decreasing temperature on phonons with frequencies >or approximately= 210 cm^-1 has been studied and it is found that the 500, 314 and 605 cm^-1 modes-and perhaps also the 332 cm^-1 mode-exhibit changes in frequency, intensity and linewidth at the superconducting transition temperature (80 K). The observed phonon renormalization effects are compared to those previously observed in YBa₂Cu₃O_{7- delta} . The authors' results suggest that polaron-phonon interactions may be important for the occurrence of superconductivity in these materials-e.g. via the Bose condensation of bipolarons.

The authors report on the theoretical investigation of the structural properties and superconductivity of VN under high pressure. The effect of pressure on the band structure is obtained by means of the self-consistent linear muffin-tin orbital (LMTO) method. From total energy studies they predict a structural phase transition in VN from NaCl- (B1-) type to CsCl (B2-) type at about 65.2 GPa. The superconducting transition temperature (T_c) is calculated using McMillan's formula. The calculated value for T_c at ambient pressure is 9.322 K, which is in agreement with the experimental value of 8.5 K. Values for T_c increase with increasing pressure, with a pressure coefficient of 27 mK kbar^-1, which is in good agreement with the experimental value of 12 mK kbar^-1.

The authors present a detailed study of the field- and temperature-dependent AC susceptibility of Pd containing between 0.35 and 2.4 at.% Fe which reveals a number of unusual features of this previously well studied system. While all of the samples examined become ferromagnetic at low temperature, there are marked differences in the critical behaviour at different compositions. Near 1.4 at.% Fe this system displays near-Heisenberg-model exponents ( gamma =1.39, beta =0.365, delta =4.8) with little exchange-bond disorder. By contrast, at both higher and lower composition, effective exponent values that vary with both temperature and field are found, a result which is consistent with significant exchange-bond disorder. At lower composition, fluctuations in the exchange-coupling strength can arise due to oscillations that occur in the conduction-electron polarization at large distances, but at higher composition an alternative mechanism for exchange competition must exist. The authors suggest that direct Fe-Fe near-neighbour exchange is the source of this competition, and discuss other properties of the PdFe system that support this suggestion.

A detailed study of the anisotropy in the magnetoresistance of PdFe alloys containing between 0.35 and 2.2 at.% Fe at 1.5 and 4.2 K is presented. Using these data, the composition dependence of the spontaneous resistive anisotropy at Fe concentrations above that necessary to establish a ferromagnetic ground state is established and compared with that reported recently in PdNi. The 'exponent' governing this concentration dependence appears to be different for systems in which the impurities either carry or lack an orbital moment.

Magnetic properties and ⁵⁷Fe Mossbauer spectra for the nitrides, RTiFe₁₁N_x, and for their parents RTiFe₁₁ (R=Y, Nd, Sm, Gd and Er) have been studied. The Curie temperatures of RTiFe₁₁N_x increase by 135-200 degrees C over those for RTiFe₁₁. The maximum Curie temperature is 745 K for GdTiFe₁₁N_x. The Fe atomic moments at room temperature are 1.84 mu _B and 1.52 mu _B for YTiFe₁₁N_x and YTiFe₁₁, respectively. Mossbauer spectra of the nitrides are fitted by using seven sets of subspectra, needed because the N atoms affect the hyperfine parameters. As compared to RTiFe₁₁, the average hyperfine field increases by 5-9 T and the average isomer shift by 0.10-0.18 mm s^-1 for RTiFe₁₁N_x. For most of the RTiFe₁₁ series and their nitrides, the average hyperfine fields have a linear relationship with (g_R-1) J_R which can be understood with the RKKY theory. The core electron polarization field, B_cp, 4s electron polarization field, B_4s, and transfer field, B_mp, have been analysed and compared for the Y compound. The N atoms bring about an increase in magnitude of B_cp and B_mp and a decrease in magnitude of B_4s, which leads to an average increase of about 9 T in magnitude for the hyperfine field of YTiFe₁₁N_x as compared to YTiFe₁₁. The proportionality coefficient between the average hyperfine field and the Fe moment for YTiFe₁₁N_x deviates far from the normal value of 15T mu _B^-1. This is attributed to a very large increase in the total conduction-electron polarization field.

A quantum mechanical calculation of the conductance of quantum wires in the presence of boundary scattering is presented. The scattering-matrix method is used to evaluate the transmission coefficients through electron waveguides with a random fluctuation in the local width of the channel. The conductance quantization of ballistic wires breaks down for strong disorder. The authors find that the conductance has a dip when a new propagating mode opens. The conductance is suppressed exponentially as the length of the wire is increased, demonstrating localization of electron states in the quasi-one-dimensional system. The dependences of the localization length on the parameters of the wires are examined. The probability distribution of the conductance due to coherent scattering from the rough boundary is found to possess a long tail.

The thermally activated conductivity of undoped a-Si:H films, prepared by glow discharge and magnetron sputtering, was measured by cooling samples down from 453 K to 306 K at a rate of 0.3 K min^-1. This resulted in systematic and reproducible deviations from straight lines in the ln sigma versus 1/T plot, and from these data the Fermi level shift was determined. By means of a density of states model adapted to each sample, combining electrical, optical and ESR measurements, the temperature-dependent defect density was calculated from the charge conservation law. The results are explained by a non-monotonic temperature-dependent defect density below the equilibrium temperature in undoped a-Si:H. The thermally induced changes increase with increasing hydrogen content of the samples. Films of identical hydrogen content show similar behaviour, independent of the preparation technique.

The isotropic Compton profiles of vanadium oxides VO, V₂O₃, VO₂, V₆O₁₃ and V₂O₅ were measured by using 59.54 keV gamma -rays. The Compton profiles calculated by using the molecular-cluster model were found to be improper in describing the experimental results of VO and VO₂. The molecular-cluster model seems to underestimate the metal-ligand covalency. The localization and delocalization of valence electrons when forming oxides will be discussed on the basis of the atomic superposition model.

The twin, intrinsic and extrinsic stacking fault energies together with the FCC-HCP structural energy difference are calculated for Al by means of the total energy pseudopotential method. The influence of supercell geometry is controlled by extrapolating the calculated data to infinite cell size. All calculations include full interplanar relaxations and the final inter-planar separations are presented and shown to vary systematically for the three stacking faults. The calculated stacking fault energies are shown to be consistent with a simple two-parameter model which describes the effective interactions between atomic planes.

The crystal structure of CuS has been confirmed experimentally using the powder diffraction method on the high-resolution powder diffractometer at the Rutherford-Appleton laboratory. The observed crystal structure is P6₃/mmc. Standard density functional calculations on CuS on a variety of crystal structures are also reported. The calculations also predict P6₃/mmc as the stable crystal structure. On the basis of the agreement between theory and experiment the authors are able to discuss the details of the bonding in this material.

The magnetocrystalline anisotropy energy (MAE) of the pnictide MnSb is investigated by means of the augmented spherical wave method. The spin-orbit coupling enters the variational step of the procedure and the force theorem is applied to determine the dependence of the MAE on the directions of magnetization. The authors numerical results agree well with the first-order development of anisotropy energy in uniaxial crystal structures. The easy axis of magnetization is then found to be in the basal plane of the hexagonal structure in agreement with experiment, and the first-order anisotropy constant is given.

Relaxation peaks with the same activation energy as the DC conductivity have been observed in the low-frequency dielectric properties of insulator thin films. In particular, the authors present results for electron-beam-evaporated aluminium oxide films and sputtered hydrated nickel oxide films. The presence of the relaxation peaks seems to be correlated to the amount of hydrogen atoms present in the films. They propose a phenomenological model for the relaxation process. It is assumed that the DC conductivity is due to charge-carrier transport in electronic states positioned at a certain energy above the Fermi level. The energy difference between the Fermi level and the transport states is equal to the activation energy. Empty localized states close to the Fermi level will act as traps and give rise to a relaxation peak in the imaginary part of the dielectric permittivity.

For original work see ibid., vol.4, p.L303, 1992. Tsekov and Radoev recently studied the velocity autocorrelation function in a hydrodynamic model using the generalized Langevin equation formalism originally due to Mori and Zwanzig. The more recent recurrence relations formulation of the generalized Langevin equation provides a simplification. It also lends a perspective on the model of Tsekov and Radoev and points out the model's limitations. A refinement of the model is possible by consideration of the Hilbert space geometry as developed by the recurrence relations formulation.

For original work see ibid., vol.2, no.16, p.3737 (1990). Normand and co-workers predicted anomalous temperature dependence of a few thermal averages induced by the bifurcation behaviour or the local breaking of symmetry. Against their prediction, based on the phi ⁴ atom potential model, the double-quadratic-well (DQW) potential model leads to different temperature dependences on the specific heat and the mean square displacement. It turns out that the anomalous temperature dependence of the phi ⁴ model arises from the anharmonic character of the potential barrier as well as the bifurcation behaviour. Making a distinction between the two kinds of anomaly, the authors present a plausible temperature variation of the bifurcation behaviour on the basis of the DQW model of the above two quantities.

The electronic contribution to the energy of icosahedral quasicrystals and related crystalline phases is discussed within a nearly-free-electron approximation. The Penrose approximant is used through the Elser and Henley scheme (1985) for modelling the Frank-Kasper phase atomic structure. The relative position of the Fermi sphere and the wide-gap reciprocal vectors for both structures is analysed. It is shown that the Jones-like arguments concerning the electronic concentration explain the stability of these phases in systems exhibiting the stable quasicrystal formation. Some assumptions about quasicrystalline alloys of other simple metals are suggested.

For original article see ibid., vol.3, p.9151 (1991). A recent article addresses the problem of 'universal' dielectric relaxation in dipole glasses. The term 'universal dielectric relaxation', applied historically to conducting glasses as well, is used here for power-law behaviour of the conductivity above and below the 'loss peak' in the imaginary part of the dielectric constant. But no general consensus exists that the conductivity may be represented universally in this form. No physical interpretation for the 'loss peak' frequency is given; Debye-like relaxation below this frequency arises from a coincidence in parameters. In the criticism of this article certain parallels with conducting glasses are noted, where corresponding physical conditions can be derived; the level of 'universality' in the physical origins and experimental manifestations of dielectric relaxation processes remains uncertain, however. Weron replies to the Comment.