Table of contents

The authors describe the application of the reverse Monte Carlo simulation technique to the analysis of EXAFS data. This technique generates a series of three dimensional particle configurations which are consistent with the experimental EXAFS spectrum. No input information other than the density and the chemical composition of the sample is required. They present initial results for crystalline and amorphous silicon and preliminary results for silver bromide. Since the method works directly from an experimental spectrum it promises to be extremely powerful in modelling the structures of amorphous materials.

Computations of the frequencies of longitudinal and transverse phonon modes in a two-component metallic glass (Mg₇₀Zn₃₀) based on the pseudopotential approach used for the calculation of interatomic pair potential have been presented. The theory employed for these calculations is a self-consistent phonon theory as developed by Takeno and Goda (1971) for amorphous solids. The computed results are compared with molecular dynamics and neutron inelastic scattering experiments. It appears that the short-wavelength collective excitations detected in this glass arise from longitudinal phonon excitations and probably not from a diffuse Umklapp scattering from transverse acoustic waves.

The authors have calculated the total energy for a simple metal within pseudopotential theory to second order in the pseudopotential. Their work is based on a first-principles nonlocal pseudopotential. The results presented here contain an extra term accounting for the density dependence of the higher-order terms in the total energy. This term is required to equilibrate the system at the observed density and should be included in any low-order expansion of the total energy. A closed-form expression for the energy-wavenumber characteristic is given, from which the pair potential can readily be obtained by a Fourier transform. Also included is a simple and accurate expression for the local field, which gives the electron gas screening.

The authors have obtained a simple analytic pair potential and volume potential for simulations of simple metals in which the density dependence of the parameters is given explicitly. The potentials are based on a first-principles nonlocal pseudopotential theory with a correction term in the volume potential to allow for the effects of the density dependence of terms in the total energy beyond second order in perturbation theory. This correction term gives the pressure required to obtain the measured equilibrium lattice constant. In this paper the authors present a potential for Li and show that it gives the FCC structure at low temperatures, and gives good agreement with measured phonon dispersion curves and elastic constants.

The strain-optic contribution to the electro-optic effect in KCl doped with Br^-, Tl⁺, Cd²⁺ and Mn²⁺ ions has been evaluated from the independent measurements of the photoelastic and the electrostrictive coefficients and the true electro-optic coefficients have been calculated. The relatively small electrostrictive elasto-optic contribution in these doped KCl systems has been discussed in the light of the change in the ionicity of the bonds between the impurity ions and the Cl^- ions. The possible application of the systems KCl:Cd²⁺ and KCl:Mn²⁺ at high (microwave) frequencies has been discussed in comparison with KTaO₃.

The authors have studied the mechanical properties of two-size disc mixtures, in relation to their geometrical structure. These packings present an orientational order: the spatial extent of this order, depending on the composition of the sample, has been measured in terms of a characteristic length xi . The stress-strain law in an oedometric compression is particularly sensitive to the disorder of contacts between the discs, but also clearly depends an orientational order.

The internal friction (IF) measurement technique is used to study the magnetization of pure iron, nickel and Permalloy-42. The IF of samples undergoing a uniform magnetic field sweep is measured with a torsion pendulum. An IF peak is observed in the field range 0-10 Oe. The field sweeping increases the IF and hence the characteristic quantity is the difference between the IF measured in field sweeping and that in a static field, which is called dynamic internal friction. The dynamic IF shows a viscoelastic feature; it increases with increasing H/ omega where H is the field sweep rate and omega the angular frequency of the alternating strain. The behaviour of the dynamic IF shows that it is caused by the domain wall motion in field sweeping and thus is related to a magnetomechanical interaction.

The properties of the collective excitation and the motion of solitons originates in the inner local fluctuation and deformation of the structure of molecules in the organism, and protein has been discussed utilizing an analysis of inner excitation in the molecule and the sound oscillation caused by neighbouring correlation effects between molecules by considering the anomalous correlation interaction in this system. These new results show that the Davydov theory is only an approximate theory.

The density of states (DOS) is calculated for a three-dimensional Penrose tiling with icosahedral symmetry by taking two different approximations, the cluster approximation and the commensurate approximation. It is compared with the DOS for a model of the icosahedral phase of Al-Mn-Si (i-AlMnSi) in the cluster approximation. At low frequencies the DOS contains pseudo-gaps. The spectrum possesses scaling properties. The global contours of the DOS for i-AlMnSi are in reasonable agreement with the experiments.

The author presents numerical as well as analytical results for the reduction of the energy gap in the fractional quantum Hall regime due to disorder. The disordered substrate is treated perturbatively. It consists of randomly distributed long-range scatterers. It turns out that the reduction of the energy gap is determined by the mean value of the fluctuations of the potential alone and that it is proportional to the fractional charge of the quasiparticles.

Electroacoustic waves are investigated in the 180 degrees periodic domain structure. Some elementary methods are provided. The expressions for the Hamiltonian and the explicit equations for the dispersion of bulk modes as well as interface modes are obtained. The results are analysed and a possible elementary mode is proposed.

Systematic local density treatment (via the all-electron, full-potential, linear combination of gaussian orbitals fitting function (LCGTO-FF) algorithms) of the cohesive properties of mono- and dilayer Li when combined with recent calculations for bulk Li of similar high quality, yields a prediction of modest intraplanar lattice expansion (dilayer: 2.8%, monolayer: 1.6%) and substantial interplanar contraction (dilayer c/a=1.46 versus calculated bulk c/a=1.64). The differences between these predictions and the limited experimental data for Li overlayers on graphite (which exhibit about 6.1% expansion in nearest-neighbour separation) suggest possible substrate or bonding effects in the experiment. The total dilayer cohesive energy is 79.8% of the bulk cohesive energy while for the monolayer the fraction is 63.1%. The interplanar contribution to the dilayer cohesive energy is -0.29 eV. The dilayer uniaxial (c-axis) compressibility is 2.5 times as large as the calculated value for the HCP Li crystal (the latter value is in quite good agreement with available measurements). Mono- and dilayer energy bands (at the level of bare Kohn-Sham eigenvalues) are basically consistent with those calculated self-consistently for the crystal using the same LDA model. However, calculated work function values are larger than measured crystalline values, by more than 0.6 eV, for both the mono- and dilayers.

The aim of this paper is to evaluate the two-dimensional angular correlation of the positron annihilation radiation (2D ACPAR) in YBa₂Cu₃O₇. The calculation is performed in the independent-particle model using as electronic wavefunctions the molecular orbitals from a cluster calculation and a variational wavefunction for the positron. The anisotropic part of the 2D ACPAR and the folded distribution related to the theorem of Lock, Crisp and West (1973) are compared with experimental results.

An EPR study of a Gd³⁺-doped NH₄La(SO₄)₂.4H₂O(ALST) single crystal has been made between liquid-helium and room temperature. The behaviours of the overall splitting of EPR lines and EPR linewidth indicate the occurrence of a first-order phase transition at 201+or-0.5 K; this is the first-ever report of the phase transition undergone by ALST. The spin-Hamiltonian parameters for Gd³⁺ at a room temperature have been estimated. The intrinsic parameter b₂ for Gd³⁺ in ALST crystal has been calculated, using the superposition model of Newman, to be -4.281 GHz, with the exponent t₂=8.5.

⁵⁷Fe Mossbauer spectra and magnetic properties of YTi(Fe_1-xNi_x)₁₁ (x=0-0.6) have been studied. Mossbauer spectra show that the hyperfine fields of the three iron sites and the average hyperfine field increase at first, then decrease and have a maximum between the Ni concentrations x=0.2 and 0.3. The Ni atoms preferentially occupy the 8f site and the Fe atoms the 8i site at Ni concentrations of x<or=0.3 according to the Mossbauer subspectral areas. Magnetic measurements indicate that both the saturation magnetizations at room temperature and the Curie temperatures increase at first and then decrease with Ni substitution. The maximum of the saturation magnetization is 127 emu g^-1 at a Ni concentration of x=0.1 and that of the Curie temperature is 602 K at x=0.3. The concentration dependences of the saturation magnetizations and the Curie temperatures are calculated using M_s=11((1-x) mu _Fe+x mu _Ni) and T_f=(1/2N)(n_Fe-FeT_Fe-Fe+n_Ni-NiT_Ni-Ni+((n_Fe-FeT_Fe-Fe-n_Ni-NiT_Ni-Ni)²+4n_Fe-Nin_Ni-FeT²_Fe-Ni)¹2/), respectively. The results obtained are in accord with the experimental data.

Analysis of the effect of the electron-molecular vibration (EMV) coupling on the conductivity spectra of charge-transfer crystalline complexes is presented. Discussion is limited to the case of n-merized compounds, for which the localized electronic states should be treated explicitly. Theoretical conductivity spectra based on the linear response and vibronic adiabatic Mulliken theories are compared with the experimental data. An alternative derivation of the linear response conductivity expression is presented. In addition, a simple method of calculation of the EMV coupling constants is proposed.

A simple mathematical model has been developed to simulate line narrowing spectroscopy. The application to different garnets doped with chromium is presented. A frequency shift effect due to non-resolved homogeneous and inhomogeneous lines of the same magnitude is observed and explained. The broadening of Cr³⁺ emission lines in gadolinium garnets is analysed in terms of interaction between paramagnetic ions.

The layered oxides LiCoO₂, LiNiO₂, NaCoO₂ and NaNiO₂ have been studied by XPS, UPS and EELS. The interpretation of data is somewhat hampered by surface segregation, especially for the Na compounds. However, all these materials are shown by photoemission to possess a very narrow d band, in contrast to the perovskite oxides LaMO₃, and EELS reveals the presence of local, ligand-field-type excitations for LiCoO₂. The authors argue that this is a consequence of the layered nature of these oxides. PES also shows satellites in both core and valence regions; the valence satellite intensity appears to correlate more with ligand ionizations, rather than metal ionizations. They interpret these results in terms of semi-empirical band calculations, and an impurity model.

The author extends the renormalized density functional theory (RDFT), which was proposed by Groot and Van der Eerden (1987), by using the Percus condition and applies this to hard-sphere fluids which have a hard repulsive potential. The low-density expansion is used to calculate the fourth virial coefficient and the second-order pair correlation function for hard-sphere fluids, in which it disagrees with the exact virial expansion of the system. These results are compared with those of the standard integral equations and other weighted-density approximation theory approaches to the theory of fluids. The extended RDFT also shows that this fails at low temperatures for systems whose intermolecular potential has an attractive part, as do other weighted-density approximations.

Triplet and higher-order molecular distributions in condensed media are obtained from pair distributions by extension of a method based on the notion of spatial Markoffian-like processes for local ordered structures. The extension is achieved by defining the spatial process in terms of multivariate theta distributions, in full analogy with multivariate distributions of a temporal Gaussian process. This result enables one to obtain effective pair potentials from empirical radial distributions by solving the BKY integral equation relating the pair potential to pair and triplet distributions. The relative merits of a microscopic description of the condensed system in terms of effective pair potentials or in terms of local structures are discussed.

The interface equation of motion is derived in a system in which the local order parameter is coupled with the elastic strains. As a special case, the dynamics of spherical domains growing in a metastable matrix is studied. In a non-conserved system it is shown that larger domains grow dominantly around smaller domains when the shear modulus of the new phase is larger than that of the matrix, while the size difference of domains tends to diminish when they are softer than the matrix.

It is shown that longitudinal field muon spin relaxation may be used to probe the local charge density correlations in two-dimensional systems and heterostructures. Magnetic quantum oscillations in the local density of extended states show up in the relaxation time T₁. This method should reveal the variation of electronic parameters within the sample.

A detailed examination of the spectrum of the isoelectronic bound exciton (IBE) recombination attributed to Be pair centres in Si reveals the presence in the sidebands of a previously unreported sharp local mode phonon of energy 104.7+or-0.5 meV (844+or-4 cm^-1). The energy of this local vibrational mode is in excellent agreement with a recent prediction of 860 cm^-1 for the vibrational energy of a pair of Be atoms aligned along a (111) crystal direction where one of the atoms occupies a substitutional site and the other an adjacent tetrahedral interstitial site. This result provides the most convincing evidence to date for assigning the IBE spectrum to Be pair centres with this configuration.

Measurements of 2R_bA, twice the boundary residual resistance times the cross-sectional area, for superconducting/ferromagnetic (S/F) boundaries in sputtered sandwiches yield values (in f Omega m²) of (6.1+or-0.3) for Nb/Co, (4.8+or-0.6) for Nb/Ni and (12.4+or-0.7) for NbTi/Co. These values are similar to those previously reported for superconducting/normal (S/N) boundaries in foil-based sandwiches, and smaller than predicted by the standard model.

The elastic constants of a single crystal of Cr+5 at.% V were measured in the temperature range 77-480 K. The results can be used as representative of the nonmagnetic behaviour in determining the magnetic contributions to the single-crystal elastic constants of Cr and its dilute alloys. Using the results on the Cr+5 at.% V single crystal it is shown that there is a change of sign in the magnetic contributions to c₄₄ and ¹/₂(c₁₁-c₁₂) of pure Cr below the spin-flip transition temperature.

By considering the finite lifetime of excitation and the band structure of the finite chain in the polymer, the authors interpret the two-peak structure in the spectrum of the third-order nonlinear susceptibility of trans-polyacetylene. The numerical result of the theoretical calculation agrees with the spectrum observed in experiments.