Table of contents

A coupled-mode theory of spin fluctuations in the d-dimensional Heisenberg magnet at infinite temperature is used to predict the time dependence of various spin correlation functions. The real-space spin autocorrelation function is shown to have a long-time behaviour approximately (1/t)^d theta / where theta =(4+d)/2. Properties at intermediate values of the time are extracted from the theory by numerical analysis. In this time window, the reciprocal-lattice spin autocorrelation function, G(q, t), is, to a good approximation, an exponential function of time. The decay rate is proportional to q^alpha , where q is the wavevector. Analysis of our numerical data indicates that the exponent alpha depends weakly on d, and it is significantly different from the value 2 which is compatible with a spin diffusion model. In the asymptotic limit, defined by q to 0, t to infinity , and q²t to 0, G(q,t) is a function of a single variable=(tq^theta ). This result rules against the validity of a diffusion model also in the asymptotic limit.

The cathodoluminescence (CL) spectrum of the oxygen-plasma-anodized film on Ge (111) was measured in the energy region from 2.40 eV to 4.60 eV for the first time. Several photoemission bands located at 4.25 eV, 3.50 eV, 3.20 eV and 3.00 eV were obtained, and the reflection spectra were also studied, the result of which confirms these four structures. Some other discussions of the influence on CL measurements are also given.

This review presents the state of the art of the study of germanium-related colour centres in silica glass and silica-based fibres. Most attention is concentrated on the dominant colour centres such as the Ge-related oxygen-deficient centres and paramagnetic Ge(n) centres, all of them having ultraviolet absorption bands. The hypothetical models and formation mechanisms proposed so far for these colour centres are discussed in detail. The origins of the models and formation mechanisms and their weak and strong points are analysed. The origin of the less well studied Ge-related colour centres in the visible spectral range (GeH, GeX, drawing-dependent defects) is also discussed.

We use a simplified treatment to calculate the melting temperature of metals under a high pressure presented by Schlosser et al (1989) using Lindemann's law, the Debye model and the assumption that the Gruneisen parameter divided by the volume is constant. Although the same assumptions are made in this paper, the procedure for calculating the melting temperature under a high pressure for seven metals (Al, Ag, Au, Cu, Na, K and Rb) demonstrates that the method used in this paper has the same efficiency as that of Schlosser et al but that our method is much simpler.

Dynamic dielectric and mechanical responses in the splitting region of PnBMA are analysed with the help of two Havriliak-Negami-function fits. The alpha and beta processes in the Arrhenius diagram are parallel to each other over a wide temperature range, and do not merge.

Frozen-in trans and gauche conformational disorder of symmetrical difluorotetra-chloroethane (DFTCE) produces the tendency of this plastic crystal to form an orientational glassy state on cooling below T_g=86 K. To throw light upon the freezing mechanism in DFTCE neutron diffraction and Brillouin scattering as well as infrared and dielectric measurements are employed. The overall picture of the results obtained can be characterized in terms of a classical glass-transition process with a relaxation frequency obeying the Vogel-Fulcher law. However, a more sophisticated picture of the freezing process is indicated by a quantitative analysis of the new Brillouin data, e.g. the temperature dependence of the sound velocity. The behaviour could be understood on the basis of a mode-coupling theory.

We present results of calculations of the equilibrium density profile and related surface properties for liquid-vapour interface of simple fluids. The interface has been considered to be (i) planar and (ii) spherical, with some attention paid to the stability of small liquid droplets. These calculations are based on density functional theory, in particular the gradient approximation, in conjunction with the simplified random phase approximation Bhatia-Young model for the bulk liquid.

The electronic structure has been calculated for large approximant crystals (up to 51752 atoms in the periodically repeated cell) to icosahedral AlCuLi quasicrystals. The results show that a deep structure-induced pseudogap at the Fermi energy is a generic property of the quasicrystal and its crystalline approximants. The physical mechanism for the formation of the gap is discussed on the basis of a quasiperiodic generalization of the nearly-free-electron model.

The electron-phonon interaction of an oxide spinel LiTi₂O₄ is calculated on the basis of a realistic electronic band structure, which is obtained by the tight-binding model so as to reproduce the first-principles bands. The lattice dynamics of LiTi₂O₄ is studied by taking account of the effect of the electron-phonon interaction. Due to the characteristic dependences of the electron-phonon interaction on wavevectors and vibrational modes, a remarkable phonon frequency renormalization is found in the frequency range of 30-80 meV over a wide region of the Brillouin zone. By using the electron-phonon interaction and the renormalized phonon frequencies, the electron-phonon spectral function alpha ²F( omega ) is calculated to provide a basis for understanding the superconductivity of LiTi₂O₄. The superconducting transition temperature, gap functions, tunneling spectra and other thermodynamic properties are studied by solving the Eliashberg equation. The calculated results are in good agreement with observations.

Changes are considered in the electronic spectrum of crystals caused by their elongation and expansion. Situations are examined when, owing to crystal deformation, the electrons are localized in alternate planes or alternate atomic sublattices. It has been shown that in both cases forbidden gaps may, under certain conditions, arise in the electronic spectrum. The results of the work are discussed in the light of experiments in which it has been found that the increase of delta in LaCuO_{3- delta} and BaPb_0.75Bi_0.25O_{3- delta} leads to the metal-semiconductor transition.

We theoretically investigate the influence of electron-electron interaction and the Pauli exclusion principle on electron transport in highly doped homogeneous n-GaAs. The electron-electron interaction potential is split up into a long- and a short-range part. Electron-electron interaction due to the long-range part is treated as inelastic scattering at coupled plasmon-phonon oscillations; electron scattering processes due to the short-range part are described as elastical collisions of identical particles. The minimal wavelengths of coupled plasmon-phonon oscillations are given by lower limits. Velocity-field characteristics are computed for field strengths of 0<E<or=30 kV cm^-1 and electron concentrations of 10¹⁶ cm^-3 and 10¹⁸ cm^-3. A small influence of electron-electron interaction and the Pauli exclusion principle is found at n=10¹⁸ cm^-3. The large dip in mobility in GaAs at n=10¹⁹ cm^-3 is shown to be caused by the exclusion principle.

Steady-state and transient photoconductivities have been investigated as a function of electric field, light intensity and temperature in single crystals of partially polymerized polydiacetylene, 1,4-bis(3-quinol)-1,3-butadiyne, with conjugating side groups. The polarization dependence of the steady-state photocurrent was also investigated. The one-dimensional Onsager theory of geminate recombination qualitatively accounts for the experimental results. Deviation from the theory is observed in the temperature dependence of the transient peak photocurrent at temperatures below 100 K.

We propose a model that provides, for the first time, a quantitative explanation for the anomalous resistance peak in the superconducting transition of mesoscopic metal wires and loops. We assume a normal-current path in parallel with a normal-superconducting-normal quasi-particle tunnel barrier forming at the node area of the dangling voltage probes. The model allows a quantitative fit to the dependence of the resistance anomaly on the external parameters, such as temperature, bias current, and an external magnetic field.

Based on the spin fluctuation theory, we propose a way to analyse the magnetic properties of nearly ferromagnetic metals by introducing quantities corresponding to the saturation magnetization in the ground state and the Curie temperature for weak ferromagnets. We also show how to evaluate the spin fluctuation parameters from experiments. As an example, magnetic properties of paramagnetic Y(Co_1-xAl_x)₂ are discussed.

In this paper we have applied the method of connected-moments expansion (CMX) to investigate the ground state of the spin-half (S= 1/2 ) anisotropic Heisenberg antiferromagnet for linear chain, honeycomb, square, simple cubic and body-centred cubic lattices. For the case of isotropic exchange interaction, a detailed comparison has shown good agreement between the CMX results and those obtained by other methods. The convergence of the CMX is fairly rapid, and becomes better as the coordination number increases or the anisotropy parameter of exchange interaction decreases. Hence the CMX seems to be a practical tool for calculating the ground-state energy of a spin system since only the second- or third-order estimation will be needed in actual practice.

The magnetocrystalline anisotropy of YFe_12-xMo_x and YPe_12-xMo_xN_y (x=1-3, y approximately=1) have been investigated by X-ray diffraction and the singular point detection technique. The uniaxial anisotropy of the YFe_12-xMo_x series decreases with increasing Mo_x concentration with a clear turning point at about x=1.6. The anisotropy characteristic of the nitride YFe_12-xMo_xN_y changes into a canted-type anisotropy for x<1.5 and a planar anisotropy for x>or=1.5. The anisotropy reduction for YFe_12-xMo_x is mainly due to the substitution of Mo_x atoms on the 8i sites, while the further change in anisotropy for YFe_12-xMo_xN_y may be due to a change in the 3d orbital moment or to a change in crystal-field interaction by the interstitial N_y atoms.

We investigate the stability of a domain wall in the striped phase with simultaneous charge and spin ordering in doped planar antiferromagnets. The line defects gain magnetic energy but the localization of holes costs kinetic energy. A Hartree-Fock approximation is used to estimate the energy cost of a static kink in the domain wall. Such kinks can gain kinetic energy by delocalizing along the line. We find that for values of t>or approximately=J this energy gain surpasses the cost in magnetic energy so that a quantum melting of the domain walls through the proliferation of kinks is predicted for this parameter regime.

The wavevector-dependent spin autocorrelation function of a classical Heisenberg model on a square lattice is calculated from the coupled-mode theory of spin dynamics. This theory is consistent with the spherical model for static spin correlations; as the temperature, T, approaches zero the inverse correlation length kappa approximately exp(-const/T). For a ferromagnetic exchange coupling, the decay rate of long wavelength fluctuations, Gamma (q), is proportional to q²T^1/2 in the limit (q/ kappa ) to infinity , whereas in the opposite, hydrodynamic limit Gamma (q) varies as q²(T 1n( kappa /q))^1/2. At the wavevector for incipient antiferromagnetic ordering, the decay rate is proportional to kappa T^1/2, while the corresponding decay rate near the Brillouin zone centre is proportional to (q²T^1/2/ kappa ). The coupled-mode equations for ferromagnetically and antiferromagnetically coupled models are solved numerically on a fine grid of wavevectors. The spin autocorrelation function, and its power spectrum are surveyed over a wide range of temperatures and wavevectors.

The sign, mode magnitude, and distribution of the electric quadrupole interaction (EQI) in single-crystal ⁵⁴MnFe have been examined along an easy (100) direction for the first time using transient NMR on oriented nuclei (NMRON) techniques. Single-passage experiments yielded the sign of the EQI as being negative, and the modulated adiabatic passage on oriented nuclei (MAPON) technique yielded a mode magnitude and distribution for the EQI of P/h=-4.4(4) kHz and Delta P/h=4.0(5) kHz respectively. The resulting mode electric field gradient is V_zz=-1.11(10)*10¹⁹ V m^-2.

Mossbauer absorption spectra have been recorded for powder and single-crystal samples of the antiferromagnetic compound Rb₂FeCl₅.H₂O over the temperature range 1.4-293 K and also at 4.2 K in applied magnetic fields up to 10 T. Electrostatic hyperfine interaction parameters have been deduced from the paramagnetic spectra. At 4.2 K an external field of 1.74 T applied along the (magnetic easy) a axis reoriented the Fe spins to the c axis. The spin-reorientation phase transition was found to be very sharp and with a small hysteresis. Comparisons are made with recent studies of (NH₄)₂FeCl₅.H₂O and K₂FeCl₅.H₂O.

A method for determining the infrared dielectric constant function of an isotropic material from measurement of the reflectance from powdered samples has been implemented. The method is based on an expression for the average dielectric constant of a heterogeneous system, which shows percolation features. The results obtained through the use of this method compare well with published results from single-crystal reflectance data, and show that damping factors in powders are moderately higher than those measured for single crystals. The method works best if samples are made of small microparticles (less than 2 mu m), pressed to filling factors less than 0.6. Some precaution should be exercised with plastic ionic substances. The method can be used to determine the dielectric constant of a large number of compounds that cannot be obtained as centimetre-sized single crystals.