Table of contents

The position-space renormalisation group technique is applied to the randomly directed bond percolation problem. There are three types of bonds in the lattice considered. The bond is undirected with probability u. The bond randomly oriented along one of the two possible directions is present with probability q. The bond is broken with probability 1-u-q. A recursion relation fixed point different from that for the usual undirected bond percolation problem is found. The critical exponents and the percolation threshold probabilities are obtained for percolation in the square lattice.

The hypernetted-chain integral equation is solved numerically in order to determine the pair correlation function, structure factor, internal energy and free energy of a system consisting of polystyrene spheres trapped at the water-air interface and interacting via dipole-dipole repulsion. The electric dipoles associated with the polystyrene spheres are perpendicular to the surface and the interaction in the plane of the surface is of the form phi (r)= epsilon ( sigma / mod r mod )³. The calculation is performed over a large range of the coupling parameter Gamma = epsilon ( sigma /r₀)³/K_BT. The results obtained are in very good agreement with those of molecular dynamics computation recently reported by Kalia and Vashishta (1981).

Two TSC peaks were obtained by measuring insulated undoped and SrCl₂-doped KCL. It is shown that shape, area, maximum position and activation enthalpy obtained by fitting of the peak at higher temperatures suits the theory of surface polarisation by blocked point defects.

The large relative error of the local density approximation for the transverse-photon-electron or Breit energy in atoms and solids is shown to arise from spurious self-interaction of the 1s electrons. Orbital self-interaction correction dramatically reduces this error. On the other hand, the large relative error of the local density approximation for the Breit contribution to the 2s electron removal energy is almost unchanged by self-interaction correction.

The two-level correlation function in a disordered metallic particle with a restricted volume is calculated. The method of a supersymmetry reducing the problem to a zero-dimensional sigma -model is used. The result of the calculation exactly coincides with the one obtained from Dyson's hypothesis.

The concept of gauge equivalence is introduced for the classical one-dimensional Heisenberg ferromagnets with single-site anisotropy. The classical continuum version of the one-dimensional anisotropic Heisenberg antiferromagnet (ferromagnet) of spin ¹/₂ is shown to be gauge-equivalent to the nonlinear Schrodinger equation with the repulsive (attractive) interaction, which is consistent with the Yang and Yang statement on thermodynamical equivalence between the corresponding quantum systems.

Magnetic susceptibility, Mossbauer spectroscopy and powder neutron diffraction experiments have shown that iron(III) phosphate, FePO₄, undergoes a novel spin reorientation transition at 17K between two antiferromagnetic phases. The ordered magnetic moments lie in the basal plane of the trigonal unit cell between the Neel temperature (24K) and 17K, below which temperature the spins align along the unique (c) axis.

Using the cluster Bethe lattice method and the quasi-spin model for itinerant magnetism, the authors show that in a one-band model there is no metallic-antiferromagnetic phase for a half-filled band at T=0.

The phase diagram for the amorphous alloys, Fe_yNI_80-yP₁₄B₆, has been determined by AC hysteresis loop and AC susceptibility measurements.

Gd³⁺ in glasses exhibit sharp g approximately=6.0 features in the EPR spectra. It is demonstrated that these features can be adequately accounted for by using the 'rhombic' spin Hamiltonian H=g_c beta H.S+D(S_z²-(S/3)(S+1))+E(S_x²-S_y²) in the strong crystal field case. Computer simulations of the experimental Gd³⁺ spectra in phosphate glasses result in the following estimates of mean values of D and E: mod D₀ mod approximately=1 cm^-1, mod E₀ mod approximately=0.2 cm^-1. The short-range order is rather well preserved at Gd³⁺ sites responsible for the g approximately=6.0 features.

Proton magnetic resonance studies have been carried out on a polycrystalline sample of parachlorophenoxy acetic acid in the temperature range 77 to 400K. The experimental second moment (11.5 G²) agrees within the experimental error with the value calculated using the available crystal structure data, indicating that the lattice is rigid below 112K. Second moment fall, which is observed in the temperature range 112-125K, is attributed to the rapid reorientation of the CH₂COOH group. This reorientation is associated with an activation energy of 7.90 kcal mol^-1, which indicates that the intramolecular reorientation breaks the strong hydrogen bonds in the dimeric unit.

Presents experimental evidence for a ferromagnetic ordering of the proton spins in Ca(OH)₂. This ordering was produced by adiabatic demagnetisation in rotating frame (ADRF) and occurred at negative spin temperature. The transverse and longitudinal susceptibilities were measured and were in agreement with theoretical predictions. Further evidence for the ferromagnetic type of ordering is contained in the relaxation behaviour of the dipolar energy after ADRF. The observed relaxation behaviour was found to be due to spin diffusion between the ferromagnetic domains. From these relaxation data the size of these domains could be calculated.

An infrared absorption peak at 1450 cm^-1 develops during the annealing, at temperatures greater than 500 degrees C, of electron-irradiated type I diamonds. This is shown to be caused by a defect involving a single nitrogen atom. This atom is probably an interstitial which has been displaced from a substitutional site by exchange with an irradiation-produced carbon interstitial.

Field-theoretical renormalisation-group technique developed by Ohta et al. is applied to the dynamics of a single polymer chain in a good solvent. The main results are: (i) the dynamic scaling is explicitly demonstrated, and (ii) the scaling function of the polymer relaxation time with the associated effective dynamic exponent which describes the crossover from the ideal gaussian chain to the regime with the excluded-volume and hydrodynamic interactions is obtained in the first orders in epsilon with epsilon =4-d,d being the spatial dimensionality.

The temperature dependence of the six principal stiffness components c₁₁ to c₆₆ and the corresponding attenuation was determined by the pulse-echo technique in the range from 150K to room temperature. Step-like variations in c₁₁ and c₃₃ at the upper transition at T₄ approximately=200K into an incommensurate phase as well as peaks in their imaginary parts are attributed to an interaction process with energy-density fluctuations arising from the soft part of the phonon spectrum around the incommensurate ordering wavevector. A relatively low and slowly softening value of c₆₆ in this temperature range points to a bilinear interaction with a low and temperature dependent zone-centre phonon mode. In the incommensurate phase a variation proportional to a power of the order parameter was observed. Despite some similarities to the elastic behaviour of K₂SeO₄ there are distinct differences in the coupling of phasons to one of the shear waves, and no effects could be seen indicating interactions with solitons, although effects from amplitons a well as phasons cannot be excluded.

Raman scattering data for ZrS₃ are reported as a function of hydrostatic pressure. The assignment of optically active modes is presented, based on these new results. An anharmonic resonance phenomenon is observed and explained by a simple model. The unidimensional character of ZrS₃ is discussed.

Using the technique of phase analysis, localisation of amplitude in molecular dynamics is examined for disordered linear systems with dilute isotropic impurities (up to 10⁶ particles and 2*10³ impurities). A localisation parameter is introduced related to the magnitude of standing waves between the impurities. Estimated values of the growth parameter for eigenfrequencies as well as non-eigenfrequencies are consistent with a simple analytic expression obtained through a random-phase-propagation approximation. Starting from a value 1.0 at omega ²=0, the growth parameter is a monotonically increasing function of omega ². At any given omega ² not=0, the author's results indicate localisation in the limit of infinite impurities (total number*concentration) without an abrupt Anderson transition.

The density of states and inverse participation ratios have been obtained numerically by directly diagonalising the tight-binding Hamiltonian on the square and cubic lattices with Gaussian randomness in the off-diagonal elements. The density of states, which has a singularity at the band centre, is well approximated in both dimensions by the loop expansion of Oppermann and Wegner (1979) taken to one-loop order. All the eigenvectors appear to be localised in two dimensions.

The author has calculated the electronic energy structure of alpha -alumina in corundum structure ( alpha -Al₂O₃) using a first principles extended tight-binding method. The interest in the electronic properties of alpha -Al₂O₃ is expanding because of well-documented interest in chemisorption of oxygen on aluminium and the oxidation of aluminium itself. To understand bonding in this compound the author has compared his results with the known electronic structure of alpha -quartz ( alpha -SiO₂). The latter has been investigated much more thoroughly and has similar properties to alpha -Al₂O₃. The authors have noted certain trends in going from alpha -Al₂O₃ to alpha -SiO₂. These trends are quite similar to those reported in a cluster model calculation on AlO₄ and SiO₄. The author has also calculated the total and orbital densities of states and have compared these with X-ray photoemission spectra. It has been found that the calculated results do not agree fully with the existing experimental data. The author also briefly comments on the controversial O-Al bond length for oxygen chemisorbed on Al(111).

First-order time-dependent perturbation theory is used to derive expression for scattering rates in the presence of high electric fields. Analysis in terms of Houston functions suggests that scattering rates are approximately field independent, but a more accurate treatment in terms of Airy functions shows that at sufficiently high fields, the resonant width of the scattering vertex is limited by the domain of integration, and a field dependent suppression of scattering occurs. This effect occurs first for small-wavevector phonons, and appreciable effects are expected for fields exceeding 10⁴-10⁵ V cm^-1. It is also suggested that the complicated retarded transport equations recently introduced into hot electron theory are not necessary to treat the intra-collisional field effect in the weak phonon coupling limit at fields of interest in conventional semiconductor transport. At fields where suppression of scattering is significant, it is found that additional scattering into field-induced band tails is also important.

Transverse magnetoresistance is measured at magnetic fields up to 6.5 T at temperatures of 77K and 4.2K for highly oriented graphites; four samples of kish graphite with different crystallinity. The effect of the structural perfection of graphite on the field dependence of the exponent n in he magnetoresistance Delta rho / rho ₀ against the magnetic field B relationship ( Delta rho / rho ₀ varies as Bⁿ) is examined. It is found that the relationship varies systematically with structural perfection. A minimum of n occurs in the low-field region for all samples at 77K and 4.2K. The curve of the exponent versus field is calculated on the basis of a multiple-carrier group model within the framework of Slonczewski-Wiess-McClure band model with the energy band parameter gamma ₃=0. Good agreement between the calculated and experimental curves is obtained. The minimum of n can be attributed to the market dependence of the effective mass of carriers on k₂ along the Brillouin zone edges.

Spin glass behaviour has been observed in a very large concentration range in both CdCr_2xIn_2-2xS₄ and ZnCr_2xAl_2-24 spinel systems. It is a result of the presence in these systems of competing interactions, ferromagnetic between first-nearest neighbours and antiferromagnetic between higher-order neighbours. The ratios between the interactions are such that the non-dilute x=1 compounds are ferromagnetic for the first system and helimagnetic for the second. The entire phase diagrams of both solutions have been determined by very low field AC and DC susceptibility measurements. The results for the ferromagnetic system are compared with theoretical predictions derived from a bond-diluted Ising model using the replica method and a second-order variational approximation.

Presents a detailed study of the NMR spectra of solid H₂ in the intermediate ortho-concentration regime (0.20<or=X<or=0.55). This system displays a glassy orientational behaviour resulting from quadrupolar interactions and site disorder. The authors obtain the distribution functions for the local quadrupolarisations which describe the extent to which the molecular orientations are frozen. They observe broad distributions that evolve only smoothly with temperature. This behaviour suggests a continuous growth of quadrupolarisation on cooling without any abrupt transition.

New experimental results from photoluminescence (PL) data are presented for the Cu-related 'characteristic-orange-luminescence' (COL) emission in GaP. The PL spectrum shows a sharp electronic line at 2.1774 eV at 2K, together with a very complicated phonon wing from interaction with continuum modes as well a large number of rather sharp inband resonance modes. The nature of these discrete low-energy phonon modes is discussed in the paper, in relation to a plausible model for the identity of the centre. The phonon coupling causes the electronic line in the PL spectrum to disappear completely above 45K, while the entire COL emission is thermally quenched by an electronic process above 140K. The thermal activation energy deduced from this electronic quenching is consistent with the thermal emission of the exciton as a single entity. A qualitative estimate of the contributions of discrete and continuum phonon modes to the quenching of the zero-phonon line is also presented. The presence of a low-energy discrete mode of 2.6 meV makes a quantitative evaluation difficult, since coupling to this phonon mode is observed only in the excited vibronic state, when it is thermally excited. Comparison between the discrete phonon energies on the anti-Stokes wing and those obtained from absorption measurements reported previously shows that these agree within experimental accuracy. The phonon energies in the excited vibronic state are reduced compared with the values for the ground state, illustrating anharmonic effects in the electron-phonon coupling.

EPR and luminescence measurements on uranium-activated sodium fluoride are reported. Six types of U⁵⁺ EPR centres, obtained in these crystals after x or gamma irradiation, having tetragonal, rhombic or monoclinic symmetry are discussed. Tentative charge compensation mechanisms are proposed. The connection with parent U⁶⁺ luminescent centres is also discussed. It is proved that for the main luminescent centre obtained in oxidising conditions (5636 AA line) hexavalent uranium has five oxygens and one fluorine as near neighbours.

The techniques of deep level transient spectroscopy (OLTS) and optically detected magnetic resonance (ODMR) have been used to study ZnSe single crystals subjected to extended heat treatment in molten zinc. The zinc-fired material has been found to contain a set of electron traps at 0.34 eV below the conduction band and to exhibit a broad luminescence emission band, peaking at 810 nm, which ODMR shows to be due to a donor-acceptor recombination process. An energy level scheme consistent with these observations is presented together with evidence to suggest that the donors, identified with the 0.34 eV traps, are due to complex centres involving residual impurities.

Group theoretical techniques are used to analyse the geometrical dependence of natural circular dichroism (CD) and circular dichroism induced by a linear interaction with either a magnetic (MCD) or electric field (ECD). For each of these effects the dependence of the spectra on the relative orientations of the crystal, radiation, and the external field, is given for each point group. At arbitrary orientations the spectrum is a linear superposition of a definite number of irreducible spectra. For example, MCD has axial symmetry in the groups D_n, D_nb, C_nv(n not=2), D_2d, D_3d while ECD is always forbidden in cubic symmetry, and also for non-cyclic groups if the field and radiation are parallel. The results apply equally to the analogous transmission effects, i.e. natural optical activity, electro-optical activity and the magnetic-field-induced Faraday effect.

An epitaxial monolayer of nickel was deposited onto a clean, flat copper (111) surface and a data set of I(V) curves collected at three diffraction geometries. Comparison between theory and experiment using the Pendry R factor gives the relaxation of the topmost layer to be -2% +or- 1% of an interlayer spacing or -0.04 Å +or- 0.02 Å. From this result a nearest-neighbour distance for Ni-Cu is calculated which is about mid-way between that for Ni-Ni and Cu-Cu. Decreasing the surface Debye temperature of the nickel layer to 242K from the bulk value of 456K improves agreement between theory and experiment. The pendry R factor is found to be particularly sensitive to this parameter. The result for the top-layer relaxation using the Zanazzi and Jona R factor is -3% +or- 2% or -0.06 Å +or- 0.04 Å.