Table of contents

The inversion of specific heat data to obtain the phonon density of states has been an important problem for many years. The authors present a new very simple and flexible iterative procedure and apply it to Einstein and Debye models. Excellent results are obtained and the method could be simply extended to more complex situations using improved numerical accuracy.

The energies of the excited states of a donor impurity in GaAs/GaAlAs multi-quantum wells are calculated using a model which is principally three-dimensional modified by the square well potential. Landau-like basis states are chosen and the effects of the Coulomb potential included using the techniques discussed by Simola and Virtamo (1978). The results are found to be in very good agreement with the authors earlier calculations, using an alternative method, for those states which are hydrogen-like in weak fields. However, additional states are also obtained that are not hydrogenic in nature. These states are commonly termed 'metastable', in that they only exist in strong fields. The predictions of the model are shown to be in good agreement with experimental data obtained previously in photoconductivity experiments.

The field assisted moderation of positrons (e⁺) has been achieved by charging the surfaces of argon and krypton rare gas solid (RGS) films. Following deposition the RGS surface was coated with a thin layer of molecular oxygen which serves to trap some of the charge which was then applied by low energy electron bombardment. Energy spectra of both the low energy e⁺s and secondary electrons emitted from a charged argon surface were found to be consistent with the production of a surface potential of approximately -20 V. This resulted in a threefold enhancement of the slow e⁺ yield and a reduction of around a factor of five in the emission of secondary electrons. Electron bombardment of RGS films not exposed to oxygen showed no such surface charge effects.

The electronic energy subbands and minigaps in lateral-surface superlattices (LSSLs) produced by deposition of AlAs and GaAs fractional layers on (001) vicinal GaAs substrates were calculated variationally. The results were compared with those calculated with other theories. It was found that energy minigaps induced by periodic structures on LSSL interfaces are much smaller than those induced by periodic structures inserted in the middle of LSSLs, and the approximation of infinitely high potential barriers between GaAs and AlAs interfaces is not as good in calculating the energy subbands in LSSLs as it is in the case of quantum wells.

High-resolution statical and dynamical spectroscopy investigations of Nd³⁺ in a YAlO₃ laser crystal have been carried out at 77 K. New satellite lines have been detected around every normal transition. Up to four such satellites have been resolved at 77 K, with shifts depending on the transition. In the ⁴F₃2/(1) from or to ⁴I₉2/(1) transition four clear satellites have been detected with lifetimes varying between approximately=10 and approximately=105 mu s. These satellites are most probably pair lines, with the ion-ion interaction of dipole-dipole type.

The authors present a method for calculating the chemical potential of arbitrary chain molecules in a computer simulation. The method is based on a generalization of Siepmann's method for calculating the chemical potential of chain molecules with a finite number of conformations. Next, the authors show that it is also possible to extend the configurational-bias Monte Carlo scheme developed recently by Siepmann and Frenkel (1992) to continuously deformable molecules. The utility of their technique for computing the chemical potential of chain molecules is demonstrated by computing the chemical potential of a fully flexible chain consisting of 10-20 segments in a moderately dense atomic fluid. Under these conditions the conventional particle-insertion schemes fail completely. In addition, they show that their novel configurational-bias Monte Carlo scheme compares favourably with conventional Monte Carlo procedures for chain molecules.

In the present study, the effects of charge bidispersity on static structure factors of dilute charged aqueous colloidal suspensions, for compositions covering the full molar fraction range, are investigated using static light-scattering experiments and integral equation theory. The suspensions under study are made from different charged polystyrene particles with an average diameter of 100 nm. The experimentally-observed static structure factors are then compared with theoretical results obtained from the hypernetted chain approximation. The values of both charges are determined by fitting the scattering data of the two pure components and are kept constant for the mixtures. In order to fully describe the experimental data, the effects of the intrinsic polydispersity have to be taken into account. It is found that the height of the main peak of the measured static structure factor, S^M(q), changes monotonically as a function of the composition.

The authors develop a finite-size-scaling theory describing the joint density and energy fluctuations in a near-critical fluid. As a result of the mixing of the temperature and chemical potential in the two relevant scaling fields, the energy operator features in the critical density distribution as an antisymmetric correction to the limiting scale-invariant form. Both the limiting form and the correction are predicted to be functions that are characteristic of the Ising universality class and are independently known. The theory is tested with extensive Monte Carlo studies of the two-dimensional Lennard-Jones fluid, within the grand canonical ensemble. The simulations and scaling framework together are shown to provide a powerful way of identifying the location of the liquid-gas critical point, while confirming and clarifying its essentially Ising character. The simulations also show a clearly identifiable signature of the field-mixing responsible for the failure of the law of rectilinear diameter.

By means of Mossbauer spectroscopy the authors have studied the dynamics of nano sized particles when these are dispersed in decalin in the form of a crystalline matrix or as a supercooled liquid. In the case of a crystalline matrix the particles perform Brownian oscillations at temperatures close to the melting point of the decalin. In the case of a supercooled liquid ordinary Brownian motion is observed.

A systematic analysis of the spectral moments method is presented and developed to compute the response functions of very large harmonic systems. Convergence of the algorithms is discussed and solutions are proposed to improve the results obtained. New developments are proposed. They concern, on the one hand, the determination of the Green functions or correlation functions of the system, and the localization of eigenvectors and, on the other hand, the determination of the spectral density of very large homogeneous matrices by a very simple and powerful technique. These results are illustrated by several examples taken from the main subjects studied by the authors: conducting polymers, fractals and quasi-crystals. Then comparison with other methods is discussed.

The dynamic and scattering properties of the Sierpinski gasket are studied in systems up to as large as N=2,391,486 atoms (level=13), using the spectral moments method. Two models, with scalar and vectorial forces, are developed. The effects of disorder are also investigated. The density of states on the scalar perfect Sierpinski gasket is found to be in agreement with previous results. For the vectorial perfect model, the authors find that the density of states exhibits self-similar properties. For the disordered systems, results show that the density of states exhibits two regimes. For the disordered vectorial model, the density of states is proportional to omega in the low-frequency regime. A cross-over is found, and on short length scales the density of states is proportional to omega ^alpha . Determination of the correlation functions shows that, although the density of states follows the Debye law, the low-frequency region does not correspond to an acoustic regime, which is in agreement with the lack of translational invariance. A microscopic theory of the scattering of light by fractals is developed and comparisons with recent results obtained in Raman scattering measurements of silica aerogels are reported. The results confirm that, in the fracton regime, the Raman intensity behaves with a power law, with the value of the exponent depending on the scaling properties and the susceptibility derivatives.

In the first part of this paper is reported a Raman study performed on several electrochemically prepared poly(3-methyl thienylene) (P3MeT) films. The sample dependence of the Raman spectra is analysed. A correlation between the intensity, the frequency and the width of the Raman lines and the nature of the films under consideration (defined in terms of thickness, conjugation length, amount of structural defects and level of doping) is shown. Some Raman criteria are defined and used in order to characterize P3MeT samples from Raman spectroscopy. These Raman results are used to characterize a class of materials that are attracting much interest in electrochemistry, electrocatalytic and analytical chemistry: the poly(3-methyl thienylene)-heteropolyanion (P3MeT-PHA) complexes, with PHA=(SiW₁₂O₄₀)^4- or PHA=(PMo₁₂O₄₀)³. Two results can be emphasized: (i) the strong increase of the structural disorder of the P3MeT chain related to the intercalation of the PHA; (ii) the low value of the doping level or, in other words, the poor charge transfer between the PHA and the P3MeT chain. These results question the high value of the conductivity measured in this class of compound ( sigma around 10 S cm^-1) and suggest that the role of the heteropolyanions in the conductivity process cannot be ignored.

Changes in the structure of the amorphous alloy Fe_73.5Cu₁Nb₃Si_13.5B₉ were investigated after annealing for 1 h in a temperature range from 450-800 degrees C using X-ray diffraction scattering and Mossbauer effect spectroscopy. Between 520 and 550 degrees C nanocrystalline Fe₈₀Si₂₀ grains with the DO₃ structure (diameter of about 10 nm) are embedded in an amorphous grain boundary phase. Above 650 degrees C the grain size increases and the amorphous grain boundary phase disappears. The Fe-B phases form and a new paramagnetic phase is observed. Furthermore the kinetics of the amorphous-to-nanocrystalline phase transition of this alloy was examined by X-ray diffraction observing the development of crystallization with time at a fixed annealing temperature of 520 degrees C. The beginning of crystallization appears at times less than 2 min, most grains developing in the first 10 to 20 min while after about 5 min the grain size remains constant with a diameter of about 10 nm.

The results of an extensive simulation study of non-stoichiometric sodium beta "-alumina are presented. The simulations are performed using the molecular dynamics method on large periodic systems (3213 and 3204 ions) of differing stoichiometries at a series of temperatures between 200 and 1200 K. The simulated systems include all the ions in the spinel blocks, and are based on a realistic model for the interactions between the ions. The results presented give new insight into the dynamics of the diffusing ions, the non-Arrhenius behaviour of the ionic conductivity, the formation of the proposed vacancy superlattice, and the anomalous diffuse scattering observed in X-ray studies.

The X-J model is a strong-coupling limit of both the natural tight banding model of perovskite superconductors and the Anderson lattice model of heavy fermions. The application of the model to perovskite superconductivity is straightforward, but its use to describe heavy fermions is more speculative. The straight-line motion of charge carriers in the model is sympathetic to antiferromagnetic correlations along the path traversed, although the motion destroys the long-range antiferromagnetic order by exchanging the two sublattices in passing. Antiferromagnetism is destroyed in both the square lattice geometry relevant to a CuO₂ plane and the triangular geometry relevant to an isolated layer of CeAl₃. A paramagnetic phase with shorter range correlations than suggested by the Heisenberg model seems preferred by the charge-carrier motion in these two-dimensional examples.

First-principles electronic structure calculations have been performed for the intermetallic compound YCo₅. This compound is known to have unusually large cobalt orbital magnetic moments and one of the largest magnetocrystalline anisotropies among itinerant ferromagnets. By including spin-orbit coupling and orbital polarization in the theoretical treatment the orbital magnetic moments and the magnetocrystalline anisotropy energy were calculated. It was found that in order to obtain reasonable agreement with experiments the inclusions of orbital correlation (here in the form of orbital polarization) is essential. The different contributions from the two inequivalent cobalt sites to the orbital magnetization and the anisotropy are discussed.

Measurements of the thermopower in the charge density wave compound (NbSe₄)₁₀I₃ show a large hysteresis between the cooling and heating cycles below and above the Peierls transition temperature. The magnitude of the observed hysteresis depends on the lowest temperature reached in the cooling regime. This effect is attributed to the interaction between charge density wave deformations and a quasi-periodic structure of lattice defects.

The authors consider the possibility that superconductivity (SC) may arise in narrow-band metals owing to electron-electron correlations. Consideration is given also to the superconductivity suppression mechanisms which is caused by the same correlations (spin fluctuations, or transition to dielectric phase). A new approach is proposed to describe high-T_c SC by introducing the Nambu field in the functional integral scheme. A stable superconducting state is realized in a rather small region of parameters: hole concentration and Hubbard repulsion. A method is proposed for approximating an interacting system by means of some simple multichannel models.

The electron Green functions are calculated for a semi-infinite itinerant electron ferromagnet in the framework of the infinite-U Hubbard model close to half-filling. It is shown that the ferromagnetic state is saturated in the surface as well as in the bulk for small enough hole concentration. At the same time the empty states near the Fermi level (and the occupied ones for the case of excess electrons) are depolarized due to spin polaron effects.

The magnetic susceptibility of the superparamagnetic particle modulated along its magnetic easy axis by the RF field has been calculated. The temperature behaviour of the pole of this function at the modulation frequency may be considered as a realization of the stochastic resonance in this system.

It is shown that the non-uniform phase can exist in Jahn-Teller antiferromagnets under a magnetic field near the boundary of the antiferromagnetic and pseudospin-flop phases. The existence of non-uniform phase results in characteristic peculiarities in the field dependence of the magnetic moment and orthorhombic deformation.

Spin reorientation process in the quasi-one-dimensional easy-plane antiferromagnet on a stacked triangular lattice has been considered theoretically and studied in an experiment on CsMnBr₃. The second-order phase transition associated with the flip of two pairs of sublattices has been shown to persist if the magnetic field is canted at an angle phi from the basal plane. The field and angular dependences of the magnetic torques measured at T=1.8 K, although in qualitative agreement with the classical theory, demonstrate a strong effect of quantum fluctuations.

Density operator theory of nuclear spin relaxation due to fluctuating magnetic dipole or electric quadrupole interactions is based on a differential equation for the density operator which, in the weak collision limit, depends on the spectral density functions of the dipolar or quadrupolar fluctuations. It is shown that the differential equation for relaxation in the rotating frame may be expressed in a similar form to that for relaxation in the laboratory frame. Expressions for magnetization recoveries in the rotating frame can then be simply deduced from the laboratory frame expressions by replacing the spectral density functions in the laboratory frame results by linear combinations of spectral density functions. The method is applied to single-exponential relaxation for the dipolar mechanism and to single- and multiple-exponential relaxation for the quadrupole mechanism for both longitudinal and transverse magnetization recoveries.

Two kinds of polyethylene samples, low-density polyethylene and high-density polyethylene, have been studied using positron lifetime measurements in the temperature range from 96 to 370 K. Glass transitions and other secondary transitions are observed and the glass transition temperatures are determined. It has been found that the orthopositronium lifetime and its intensity are sensitive to the transitions and the number of free-volume holes reaches a minimum near the glass transition temperature T_g. The molecular motions, the structural transitions and the crystallinity effects are discussed in terms of the properties of the free volume.

Decay processes in solid gadolinium and europium after 4d to 4f resonance excitations have been studied by means of Auger and photoelectron spectroscopy. Several photon energies covering the whole resonance region were used. Excitations corresponding to the narrow absorption peaks at the low-energy side of the main resonance were found to decay mostly via autoionization involving 5p or 5s orbitals. However, the N_4,5O_2,3N_6,7 and N_4,5N_6,7N_6,7 resonance Auger processes were also found to play a significant role. After excitations corresponding to the main 4d to 4f giant resonance the autoionization leading to a 4f^-1 one-hole state proved to be the strongest decay channel although the N_4,5O_2,3N_6,7 and N_4,5N_6,7N_6,7 normal Auger transitions were also observed.