Table of contents

A quadrupolar model with infinite-range random interaction is considered in the frame of a replica-symmetric approach. The equations for orientational and glass order parameters have no trivial solution and do not indicate phase transitions. The orientationally ordered and glass regimes grow continuously on cooling. The entropy at T=0 is positive.

Numerous channel inclusion compounds of the nitroxide radical: (tetramethyl-2,2,6,6 oxo-4 piperidyl-1) oxyle- or tano (C₉H₁₆NO₂) with linear chain molecules (alkanes, alkenes substituted alkanes . . . polymers), have been investigated by X-ray diffraction and DSC. In the high-temperature phase, chains are disordered in the channels (orientation and position). At low temperature, lock-in transitions exist. Structural results for tano-heptane are given above and below the transition (233 and 173K). Tano molecules show a chiral disorder in the two phases.

The authors study a model of a fault by considering it as a dynamical array of asperities and show its formal analogy to probabilistic cellular automata. They use the fact that when the stress on a given asperity exceeds a critical value, it is transferred in a finite amount of time to its neighbouring ones. They also introduce conditional probabilities which allow for a dynamic description of the fault. In the limit of a broad stress distribution, they obtain exact solutions for the equations and show how initial distributions of failed asperities lead to stress propagation through the fault.

Kinetics of first order C-IC transitions are considered by adopting a discommensuration model with attractive interactions. The author found situations in which critical nuclei are bound complexes consisting of differing numbers of simple stripples. A marked difference is exhibited between transitions from C to IC states and reverse transitions.

The spin relaxation time in a two-dimensional hole gas has been studied from investigation of anomalous magnetoresistance in the inversion layer in a silicon film on sapphire substrates. The spin relaxation mechanism in two-dimensional systems has been observed to be due to the spin splitting of the ground electric sub-band, associated with heavy holes, because of the spin-orbit coupling in crystals without an inversion centre. The lack of symmetry centre in the inversion layer is due to the external electric field, which forms a hole gas near the silicon surface and gives rise to the asymmetry of the potential well, where carrier motion is quantised. Analysis of the experimental data has allowed the authors to determine the value of spin splitting as a function of hole density.

Resistivity increase near the ferroelectric phase transition and the resistivity enhancement towards 0K for Pb_1-xGe_xTe crystals with small x and presented. Hall coefficient dependence on temperature and hydrostatic pressure points out the changes of the carrier scattering rate as an origin of the examined phenomena. The assumption about the off-centre displacement of the small Ge²⁺ ion from the regular cation site of the PbTe matrix is adopted to propose the possible mechanism of the mobility decrease.

As cubic rare-earth chalcogenides in the intermediate valence state approach the valence transition (when B to 0 because C₁₁ to 2C₁₂ with C₁₂ negative) crystal stability requires that Poisson's ratio should have a lower limit of -1.0 for a stress applied along any crystal symmetry axis. In agreement with this prediction, as the yttrium content in Sm_1-xY_xS solid solutions is reduced towards the critical concentration x_c (=0.15) for the valence transition, the Poisson's ratios for stress applied along any symmetry axis do tend towards -1.0. To determine the effect of pressure on the elastic behaviour of an intermediate valence crystal in the Sm_1-xY_xS series, the elastic constants and their hydrostatic pressure derivatives of the intermediate valence state material Sm_0.58Y_0.42S have been measured by the ultrasonic pulse echo overlap technique.

It is well established that the phonon spectra of intermediate-valence materials are anomalous. This paper discusses the properties of SmB₆, by investigating the energy changes which occur when a static strain, of the same symmetry as an X-ray phonon, is imposed. It is shown that energy changes proportional to the square of the displacement can arise from changes in the energies of localised (4f) states and from interplay of the changes in the conduction band states and the transfer processes between localised and conduction states. These effects are peculiar to intermediate valency.

The behaviour of the new material Na₂UBr₆ has been studied as a function of temperature by differential thermal analysis, electrical conductivity and neutron diffraction. The large increases in electrical conductivity at 652 and 663K accompany structural phase transitions, with the space group changing from P3m1 at room temperature to Pmnn at 652K and finally to Im3m at 663K. As the material is heated, libration of the UBr₄ octahedra becomes important, allowing Na⁺ ions to hop between equivalent holes along the z axis in the orthorhombic phase. At higher temperature, the UBr₆ octahedra begin hindered rotation, forming a simple body-centred cubic lattice with the Na⁺ ions moving between all available holes. The libration of octahedra, dynamically opening up channels for cation migration is an especially interesting feature which allows this material to conduct at a lower temperature than would otherwise be possible.

Field-dependent scattering rates for the electron-phonon interaction are calculated using time-dependent perturbation theory. An application to GaAs is discussed where it is found that the total scattering rate via the emission of polar optic phonons is directly proportional to the length of the field region.

The first successful application of the full alternant molecular orbital (AMO) method to an extended three-dimensional system is reported. The system is that of the electron gas constrained to a cubic Hartree-Fock Fermi surface. At all values of r_s the AMO energy lies below the restricted Hartree-Fock energy, so an RHF to AMO transition found by Laskowski and co-workers (1973) when the AMO state was restricted to one coupling parameter is removed. This quantitative result is consistent with an inequality due to Rosengren and Johansson (1973). An unexpected additional result is that the AMO energy lies below the spherical RHF energy for r_s>49 indicating the onset of a spin-polarised ground state of the uniform electron gas in rough accord with Ceperley's recent Monte Carlo calculations (1978).

A strong-field coupling scheme of 'once-diagonalisation' is proposed to calculate the spin-forbidden bands of the Fe²⁺ ions located in the D₂ symmetrical eightfold site in silicate garnet. The method is to establish the energy matrices of the strong-field terms of the D₂ point group of the d⁴(d⁶) electron configuration directly, so as to diagonalise simultaneously the crystal field (as a whole) and the electrostatic repulsion energy between the electrons. According to this scheme, making use of the observed data of the spin-allowed spectrum, the authors have, using only one adjustable parameter B (with C=4B), obtained all the spin-forbidden bands. Their results are in quite good agreement with the twenty bands observed. As to the spin-allowed spectrum, the assignment of Newman et al. (1978) is used in their calculations and supported by their theoretical results.

The authors show that the conventional model for the basic physics of conduction in granular metals is unable to account for low-temperature field-effect measurements on discontinuous metal films. They discuss the physics of electrical contact between metals and insulators and show that it results in potential disorder that is large in comparison with other relevant energies. They explore the idea of electrostatic relaxation in the insulator and show that it correctly accounts for the small field effect that is observed and for its dependence on temperature and the activation energy for conduction. They have verified the role of electrostatic relaxation by direct experiments in which the relaxation processes are modified with the predicted consequences. They assert that potential disorder must play a dominant role as regards electrical transport in all granular systems.

The transition temperature for the triangular-lattice Ising model with first- and second-neighbour interactions has been calculated using the interface method. All interactions are assumed to be antiferromagnetic with an arbitrary interaction strength ratio. The authors include the possibility that the second-neighbour interactions are anisotropic and present explicit results, shown in figures 3, 4 and 5, for three special model classes. These special cases correspond to next-nearest-neighbour interactions in one, two or three directions, the nonzero couplings having equal strengths. The results agree with the exact critical temperature in limiting cases where it is known. They call attention to an entanglement problem that one may also encounter in other applications of the interface method if the arrangement is not chosen with care. Going beyond the interface method they discuss the exact limiting behaviour when one of the two interactions is very weak. When the first-neighbour interaction approaches zero, in particular, they argue that the transition temperature should vanish exponentially for the one-direction model, should have an algebraic singularity of order ⁴/₇ for the two-direction model and should vanish linearly for the three-direction model.

A Landau-type free energy serves as the basis for a phenomenological theory of the generalised susceptibility and spin dynamics associated with the helical spin-density wave phase of MnSi. The effect of including magnetoelastic interactions in the model leads to the prediction of nonanalytic wavevector dependence for the static susceptibility, as a result of elastic anisotropy. Spin-wave frequency dispersion relations are calculated, in the long-wavelength regime, for the spin-density-wave and paramagnetic phases in the presence of an applied magnetic field. The results show a variety of interesting behaviour including the appearance of gaps in the frequency at certain values of the wavevector, as a consequence of spin-orbit coupling effects and the incommensurability of the magnetic ordering. The imaginary part of the dynamic susceptibility matrix is also derived and comparisons are made with recently reported neutron scattering data, which exhibits anisotropic behaviour associated with a symmetry-breaking term in the free energy that determines the sense of the helicity of the spin-density wave.

Ferromagnetic resonance measurements of the magnetic anisotropy and resonance linewidth are reported for single-crystal specimens of the cubic Laves-phase intermetallic compound GdAl₂ and for R_xGd_1-xAl₂ where R is a heavy rare-earth ion. The doping levels of the samples were: for R=Tb, x=0.0025, 0.01, 0.02, 0.05; for R=Dy, x=0.02; for R=Ho, x=0.02; for R=Er, x=0.01, 0.03. Microwave frequencies close to 9.3, 14.3 and 17.3 GHz were employed. The anisotropy fields H_K(100) and H_k(111) in the two principal crystallographic directions are reported for temperatures that range from 6 to 150K. Single-ion theory for the anisotropy at 0K, with crystal-field and exchange-field parameters taken from the literature, gives quite close agreement with experiment for Tb-doped samples, fair agreement for Er doping and disagreement for Dy and Ho. Doping causes a modest increase in the linewidth of resonance but this is much less severe than is observed for the substitution of similar amounts of the R ions in insulating ferromagnetics. Neither of the two models that have been employed to explain the observations in these insulating samples is successful in explaining the behaviour of the linewidth in the intermetallics and this is invoked as evidence that the R ions relax very much more rapidly in these latter than in the insulators. The linewidth passes through a minimum close to the Curie point.

The effect of free-carrier screening on the indirect optical transitions in semiconductors, both intra- and interband, is studied in detail. The formalism allows for slowly and rapidly varying (in space) static perturbations of the crystal potential (ionised impurities, chemical disorder in alloys, etc.), as well as for the difference between the static and high-frequency dielectric constant of the crystal.

Light scattering spectra have been measured for molten MgCl₂, CaCl₂, SrCl₂ and BaCl₂. The temperature dependence of the spectra has been studied for MgCl₂ and CaCl₂. Anhydrous samples were prepared from hydrated salts by dehydration under high vacuum (CaCl₂ and BaCl₂), by dehydration in the presence of NH₄Cl followed by sublimation (MgCl₂) and by dehydration and zone refining (SrCl₂). The spectra, while of the same general form as those found for the alkali halides, show one marked difference in that MgCl₂ exhibits a well resolved peak in the polarised spectrum. The spectra of CaCl₂, SrCl₂ and BaCl₂ show incompletely resolved shoulders which are more pronounced than those found in the spectra of the molten alkali iodides. A new method of analysis has been applied to the data from the molten alkaline-earth chlorides. Fourier transforms have been computed and the spectra have been examined in the time domain. The subsequent analysis is based on the hypothesis that the spectra are associated with the correlation functions of the charge-rather than the mass-currents. Thus there is little or no influence from Rayleigh-Brillouin scattering but it is anticipated that there will be a three-line spectrum associated with the charge currents. These are shown to be the equivalent of the first-order Raman effect while the second-order Raman effect gives rise to high-frequency exponential tails.

For pt.I see ibid., vol.17, p.4705 (1984). The authors have measured the inelastic neutron scattering spectra of three samples of molten SrCl₂ with different isotopic chlorine compositions using the IN6 time-of-flight spectrometer at the Institut-Langevin in Grenoble. de Gennes narrowing of the quasielastic peak is observed at the position of the first peak in S₊₊(Q). The power spectra of the total longitudinal current correlation functions are more complex than those for the molten alkali halides. Using second-order differences in scattering between the three samples (the first use of second-order isotopic substitution in this type of inelastic scattering experiment) they have identified broad acoustic (mass-current) and optic (charge-current) modes similar to those found in molten RbCl and CsCl. The energies of these modes are comparable with those in the crystal. In addition there are two well defined modes (A₁, A₂) that are neither acoustic nor optic in nature but seem to be related to the structural disorder of anions. A quasidispersion curve derived from the peak positions shows that for h(cross) omega >10 meV these two branches are approximately parallel with a slope equal to the sound velocity. At lower energies one of the modes (A₁) lies outside the (Q, omega ) range of the spectrometer while the other mode (A₂) has a vertical dispersion at Q approximately=2.1 AA^-1 and is identified with diffusive broadening of the first peak in S_--(Q). The A modes are found to occur upon disordering of the anion sublattice in the superionic phase of the solid state and so may be modes of the vacancy/interstitial clusters that have been proposed to account for the superionic behaviour. This would imply that such clustering effects also occur in molten SrCl₂.

An estimate of the contribution to the linewidth of vibrations at surfaces from vibrational energy and phase relaxation is presented. It is found that phase relaxation is not important, while decay via emission of two phonons is very important when energetically possible. Thus the Ni-CO stretching vibration for CO on Ni (100) is probably damped via the emission of two phonons. High-frequency modes, such as the C-O stretching vibration, are probably damped via the excitation of electron-hole pairs.