Table of contents

The Raman scattering of TlTiOPO₄ has been studied from 110 K up to above the ferroelectric-to-centrosymmetric phase transition at T_c approximately=850 K. At low temperatures the analysis of the spectra includes the electrostatic interactions involved in the polar modes which are responsible for the variations in the frequencies and in the intensities as a function of the wavevector direction. Most of the non-polar (A₂) and polar (B₁, B₂ and A₁) modes among the 189 expected ones have been tabulated. The frequencies of the lines related to internal vibrations of the tetrahedral PO₄ groups do not significantly depend upon the temperature. On the contrary, a soft transverse mode belonging to the A₁ irreducible representation has been observed in the ferroelectric phase. The square of its frequency decreases linearly with the temperature and vanishes at T_c. This mode is connected with the Tl⁺ motion but its coupling with internal TiO₆ modes is evidenced through the temperature variation of Raman lines at higher frequencies.

A model for the description of magnetoelastic interactions in orthorhombic RECu₂ intermetallic compounds is presented and compared with available experimental data on TmCu₂ and ErCu₂.

The validity of Fourier's law is investigated by taking account of the temperature dependence of the thermal conductivity for the one- and two-dimensional diatomic lattices. The thermal conductivity is found to be proportional to the inverse of temperature for both the one- and two-dimensional lattices. Fourier's law is confirmed by excluding the nondiffusive or ballistic energy flow from the total energy current.

A variant of the technique of electron state calculation in a ferromagnet, based on derivation of the Dirac equation by the relativistic Korringa-Kohn-Rostoker method, is presented. Using the (l mu ) representation allows one to retain the same structure of the dispersive matrix as in the absence of spin polarisation.

Gradient corrections to the local density (LD) potential proposed by Perdew and Wang (1986) and to some extent by Langreth, Mehl and Hu (1985) have been used in self-consistent LMTO band calculations in order to determine groundstate and band properties in some 3d, 4d and 5d transition metals, in the alkali metal Li and in Ce. The effect compared with LD is to expand and soften the lattice, which in general is too drastic for an improvement especially for 4d and 5d metals. Magnetic properties in Fe and Ni do not improve while the effect on some general band properties are only slightly better than in LD calculations. Despite some cases of improvement the authors conclude that the overall results obtained via gradient-corrected potentials are not yet sufficiently good to replace the LD potential.

The thermopower of the Ni_xB_100-x-ySi_y amorphous alloy system with a metalloid concentration ranging from 20 to 39 at.% has been measured from 77 up to 300 K. The thermopower, as well as the specific heat, clearly shows filling of the d band on increasing metalloid content. The thermopower is negative for x>or approximately=70 and increases rapidly as x decreases. For x<or=67 the thermopower is positive and decreases slightly with decrease in x. The high-temperature slope of the thermopower, the residual resistivity and the room-temperature coefficient of resistivity are simultaneously explained within the framework of the extended Ziman theory. In performing the corresponding calculations the authors have chosen the phase shifts deduced in such a way that they reflect the d-band filling on alloying (deduced from the specific heat measurements). The have also used Hall effect measurements which determine the number of conduction electrons. For x>or=78 the Ziman contribution to the thermopower is concealed by the large extra term which can be ascribed to magnetic fluctuations.

It is shown that at low temperatures the effective Hamiltonian, or spin-Hamiltonian, methods of magnetic resonance can be extended to the many-body problem of the magnetic excitation spectrum of singlet ground-state systems. This method of description is applied to the excitations of Cs₃Cr₂Br₉ and theoretical methods for calculating the effective-Hamiltonian parameters are discussed. It is shown that at low temperatures the magnetic interactions in perfectly crystalline single ground-state systems do not contribute to the excitation linewidths.

The magnetic phase diagram of (Cr_1-xMo_x)_0.986Si_0.014 for 0<x<or=12%, as determined from electrical resistivity, ultrasonic wave velocity and thermal expansion measurements, has both commensurate and incommensurate spin-density-wave regions. The results are discussed in terms of the two-band imperfect nesting model, taking changes in the nesting parameter into account. The phase diagram is fairly well explained by assuming changes in the nesting parameter on alloying to be the dominant effect. Effects of d-band broadening, impurity scattering and changes of density of states on alloying seem to play a relatively lesser role in changing the Neel point and the magnetic phases on alloying. Mo appears to act as an electron acceptor in the alloys.

The three-dimensional Ising model in a transverse field including the fourth-order interaction is treated by means of the Green function technique to calculate the excitation spectrum and the damping above and below T_c. The coupling between the transverse soft mode and the relaxing longitudinal mode produces a central mode in the dynamic structure factor. The temperature dependence of the dynamic structure factor for zero wavevector is calculated numerically taking parameters appropriate to LHP and LDP.

A consistent theory for coherent multiple isotropic scalar and Rayleigh vector scattering is presented using a point interaction model for the scatterers. A diagrammatic expansion is employed and new interesting coherent effects are discussed. Dependent-scattering corrections are calculated for the dielectric constant and the scattering mean free path. Intensity diagrams are considered and, in particular, the contribution of closed light paths (loops) is investigated. A new coherent feature is identified: a forward cone but, because of its density dependence, it will be difficult to detect this cone experimentally.

X-ray photoemission measurements have been performed on the Eu/Pd and Eu/Ta systems. Small amounts of Eu, approximately 0-10 AA, were evaporated on polycrystalline substrates of Pd and Ta. Eu was found to form overlayers on Ta, whereas on Pd a mixed interface layer was observed. Europium is always divalent on Ta, whereas both divalent and trivalent Eu were observed in the Eu-Pd interface, the 2+/3+ intensity ratio being dependent on the amount of deposited Eu. A situation of inhomogeneous mixed valence, in which divalent and trivalent Eu ions occupy different sites, exists in the interface. The Eu atoms at the surface were always found to be divalent.

Critical wetting in fluids with short-ranged forces is investigated for dimensions d<or=3. By combining statistical mechanical sum rules and capillary-wave theory with a standard scaling hypothesis for the singular part of the interfacial tension the authors show that, whilst the thickness l of the wetting film and the interfacial roughness xi _{perpendicular to} diverge with the same critical exponent for d<3, the critical amplitudes are such that the ratio l/ xi _{perpendicular to} diverges as (3-d)^-1/2 in the limit d to 3^-. Their analysis suggests that the extent of the critical regime becomes vanishingly small in the same limit and provides further insight into the crossover from d<3, as strong-fluctuation regime with universal exponents, to d=3, where l approximately xi _{perpendicular to} ² and non-universal exponents occur.

Photoemission spectra of the valence-band of Hume-Rothery-type Ag₄₅Cu₃₅Ge₂₀ and Ag_37.5Cu_37.5Ge₂₅ metallic glasses are reported. In the present measurements and in contrast to previous measurements on liquid-quenched samples of these alloys the authors avoid any sputter-cleaning procedure prior to the spectroscopy by vapour quenching the samples in the preparation chamber of the spectrometer. Whereas in the liquid-quenched samples no indications of distinct structure-induced effects in the electronic density of states near E_F have been observed, in the new samples the existence of a minimum can clearly be seen and so the earlier conclusion is partly corrected by this paper. Structural influences on the density of states exist and can be detected by photoelectron spectroscopy in electronic states with large dispersion such as s and p states. In addition they show that the decrease of the density of states towards E_F cannot be discussed in terms of sp-d hybridization effects as stated previously.

The electronic properties of liquid Ag_1-xS_x have been measured as a function of temperature and concentration near the stoichiometric composition Ag₂S. The unusual negative temperature dependence of the conductivity at stoichiometry found previously has been confirmed, and shown to extend over the composition range 0.3250<or=x<or=0.3345. The concentration dependence of the conductivity shows a sharp peak at the stoichiometric composition reaching a value of 250 Omega ^-1 cm^-1 at 1000 degrees C; such a feature has never been observed in a liquid semiconductors with a comparable conductivity. The thermopower of liquid Ag_1-xS_x is negative and shows a broad minimum centred around the composition x=0.315.

Measurements of the heat capacity, C_p, from 0.6 to 15 K can be represented by C_p=(0.32+or-0.02)T³ mJ mol^-1 K^-s ( theta _D⁰=265+or-10 K) below 10 K. Linear thermal expansivities parallel ( alpha /sub ///) and perpendicular ( alpha _{perpendicular to} ) to the symmetry axis from 2 to 120 K show marked anisotropy; alpha _{perpendicular to} is negative below 10 K (while alpha /sub /// remains positive) but becomes positive and much larger than alpha /sub /// above 50 K. The Gruneisen parameter, gamma , increases from approximately=1 at 3 K to +1.4 at 30 K and is approximately=0.9 room temperature.

The author applies a recently developed quasi-classical expansion for multiphoton processes in strong fair-infrared laser fields in order to show that the Coulomb threshold significantly suppresses ionization rates. He also observes a new oscillatory behaviour of differential ionization rate probabilities which is due to the non-analytic behaviour of the Coulomb scattering wavefunction.