Table of contents

It is shown that the short-wavelength collective excitations recently detected in the metallic glass Mg_0.7Zn_0.3 using inelastic neutron scattering arise from a diffuse Umklapp scattering from transverse acoustic waves.

The cooperative Jahn-Teller effect in systems containing octahedrally coordinated ions with the E_g electronic ground state is discussed. The analysis of the energy of interacting Jahn-Teller distortions shows that in some cases a complex configuration, which may be classified neither like ferro- nor like antiferroelastic ordering represents a stable state. As an example, the system with BCC symmetry is considered.

Measurements of the linear coefficient of thermal expansion of KTaO₃ at temperatures from 2K to 30K give values of the Gruneisen parameter gamma reaching approximately 40 at helium temperatures, falling to 20 at 20K. These are larger than usual values ( approximately 2) for acoustic (or optic) modes but considerably smaller than gamma _f predicted from the pressure dependence of the dielectric constant.

Wegner's statement (1980), that the asymptotic form G(E) varies as (E-E_c)^{- gamma} (with E_c real) of the averaged retarded Green's function (of an electron in a random potential) entails necessarily gamma <or=1, is shown to hold only in the physical sheet of the complex E plane. If E_c is an essential singularity, then for unphysical sectors around E_c the possibility gamma >1 cannot be excluded. The effect of thermodynamic limit on the analytical properties of G(E) is discussed.

The temperature dependence from 4.2 to 300K of the thermopower of a stage-2 graphite-FeCl₃ intercalation compound is reported. The thermopower is found to be positive over all the temperature range and exhibits a temperature dependence quite different from that of pristine graphite.

The excitonic phase in the system of spatially separated electrons and holes in high magnetic fields proves to be absolutely unstable because of the negative sign of isothermal compressibility. Instead of this phase, electron-hole liquid drops may exist in the system.

It is shown that the surface tension of the dilute Ising model at T=0 is proportional to the area a(p) of the cross section of the infinite percolation cluster at its narrowest point. Some arguments are also given to establish the expression mu _p=(d-1) nu _p between the percolation critical exponents mu _p, related to a(p), and nu _p, related to the connectedness length.

Reports a high-temperature itinerant ferromagnet CeRh₃B₂ which orders magnetically at 115K. The magnetic ordering seems to be intrinsic and presumably arises from the Rh d-band. Similar compounds of La, Pr and Nd do not show any magnetic ordering down to 77K.

Using the exact Goldhirsch (1980) boson expansion of spin ¹/₂ operators and coherent states representation the author obtains an exact semiclassical equation of motion for the anisotropic Heisenberg chain. Comparison is made with the classical case and with other approaches in the quantum case. The possible contribution to the quasi-elastic neutron scattering cross section is estimated.

The transferred isotropic hyperfine interaction A_s for Mn²⁺ in the fluoroperovskite compounds is discussed in terms of the model recently suggested by Baker (1979), which takes into account the local distortion produced in the lattice when the paramagnetic impurity ion enters substitutionally. It is found that the ligand displacement gives an important contribution to this interaction.

A linear chain of XY dipoles coupled to the elastic degrees of freedom of the lattice is studied. For second-order coupling, in the presence of an external electric field, the dipolar kink-soliton excitations lead to soliton-like deformation of the lattice. First-order coupling mediates dipolar kink-solitons excitations which themselves induce kink-soliton deformation of the lattice.

The time development of the spectral transfer of optical excitations in inhomogeneously broadened systems is investigated as a function of transfer range, concentration of optically active ions and the dimensionality of the system. The equation of motion method is employed to calculate the spin-wave density of states for the equivalent ferromagnet and this is transformed to obtain the time dependence of the narrow band fluorescence. It is found that neighbours out to the fifth shell in a simple cubic lattice are significant for dipole-dipole interactions. Reducing the effective dimensionality of the model leads to a considerably slower spectral redistribution of the fluorescence.

GaP containing boron but a negligible concentration of silicon has been irradiated in stages with 2 MeV electrons. Infrared absorption measurements have been made at each stage to monitor the concentration of the radiation-induced interstitial defect B(1), which involves a single boron atom. A previous uncertainty relating to the possible involvement of silicon impurities in the defect in other GaP and GaAs crystals has been resolved. The defect is now shown to have the structure of a boron atom complexed with an intrinsic interstitial atom.

The authors report measurements of the X-ray absorption spectra near the Ta L edges of different polytypes of TaS₂ and TaSe₂ and TaSe₃. Analysis of structure seen near the absorption edges and associated with transitions from well defined core levels to conduction band states has enabled the extent of the p character of the predominantly d character conduction band to be probed. The results show that conduction band in the Ta chalcogenides studies has a significant amount of p character, and a lower limit of between 6 and 10% has been determined. The amount of p character varies between the various compounds in the following way: 1T-TaS₂<TaSe₃<2H-TaS₂ approximately 1T-TaSe₂<2H-TaSe₂. The Ta X-ray absorption edges have also been monitored during and after intercalation of the chalcogenides with hydrazine. Small changes in p character are observed and the results indicate that intercalation modifies the conduction band, not only through charge transfer but more significantly through changes in hybridisation of the band.

Single crystal X-ray diffraction data were collected and interpreted for the plastic phase of bicyclo (2.2.2) octane (CH(CH₂CH₂)₃CH) at T=265K. In order to determine the structure of this phase, several refinement procedures are tried: Frenkel models (jump model and free rotation around threefold axes), decomposition of the orientational probability of molecules on symmetry-adapted functions. The best fit (R_w=4.7%) is obtained with the last model. The maximum of the orientational probability occurs when the threefold molecular and crystal axes are coincident. Around each (111) direction the molecular rotation is a 'quasi' free rotation.

The low-temperature structure of stoichiometric silver beta alumina, AgAl₁₁O₁₇, is described for the first time. A powder neutron diffraction study at 4.2K reveals that there is a single three-fold silver location of unit occupancy in each mirror plane. The silver site lies between the mid-oxygen and Beevers-Ross positions where the Ag-O contact distances can be minimised. The Ag-O interactions, in particular the cooperative movement of the spacer oxygens, are responsible for the superlattice which has been described previously. The Beevers-Ross, anti-Beefers-Ross and interstitial aluminium sites are vacant and the c axis is expanded relative to the non-stoichiometric parent.

The diffuse neutron scattering function S(Q, omega ) from a single crystal of deuterated adamantane in its 300K plastic phase has been measured throughout the 110 plane of the reciprocal lattice, and over the range of energy transfers h(cross) omega up to 12 meV in which intermolecular vibrations occur. The results are presented as contour maps of S(Q, omega ) over the reciprocal zone at fixed energies. They reveal the translational and librational phonon modes which have been intensively studied to be a minor feature of the scattering at energy transfers above a few meV. The underlying reciprocal lattice structure is soon lost, and the scattering is dominated by a smoothly varying dynamic structure factor. At the higher energies this becomes increasingly isotropic in scattering vector with liquid-like oscillations in Q and a major peak at 35 nm^-1.

Measurements of the effects of hydrostatic and uniaxial pressures on ultrasonic wave velocities have been used to determine the six third-order elastic constants of PbTe. The hydrostatic pressure dependences of the elastic stiffness constants of SnTe and Ge_0.08Sn_0.92Te have also been obtained. Estimation of the third-order constants of PbTe on the basis of a Born-Mayer potential has established that nearest-neighbour repulsive forces are predominantly responsible for the large negative value of C₁₁₁; however, this ionic model does not generally account well for the observed third-order elasticity of this mixed ionic-covalent compound. The Gruneisen mode gammas in the long-wavelength limit have been calculated for PbTe, SnTe and Ge_0.09Sn_0.92Te; the large anisotropy found has been explained in terms of the strong influence of the nearest-neighbour repulsive forces on C₁₁₁. The elastic stiffness constants of SnTe have been measured as a function of pressure up to 15*10⁸ Pa, quite close to the phase transition pressure of 18*10⁸ Pa. C₄₄ softens somewhat over about 9*10⁸ Pa.

The corundum structure ( alpha -R₂O₃ where R=Al, Ti, V, Cr, Fe) has been chosen for the first detailed study of inner displacement and internal strain in a moderately complex system. It is shown that the inner displacements of sublattices of ions are governed by two independent internal strain tensors: these relate to O-O and R-R displacements and have eight and six independent components respectively. These results are used to give a proper description of the deformation of octahedral clusters of O^2- ions surrounding an R³⁺ cation when the crystal is subject to uniaxial stress. Such clusters and their deformations are important in the understanding of a variety of recent experiments but have been the subject of controversy and erroneous description. The major components of the internal strain tensors have been estimated using a pseudo-HCP model of the anion structure.

Based on a critical view of existing theories, the influence of a secondary order parameter eta (elastic strain, polarisation) on the incommensurate-commensurate transition is reinvestigated. The necessity to regard the intrinsic nature of eta in the variational treatment, which yields the Landau free energy, is emphasised. Restricting themselves to a one-dimensionally modulated structure, the authors prove the statement of Bruce et al. (1978), that the coupling to the strain makes the transition discontinuous. On the contrary, for the coupling to the ferroelectric polarisation P, they did not find change of the transition both for P parallel and perpendicular to the modulation. The electric susceptibility chi exhibits a Curie-Weiss law. They argue that thermal fluctuations do not change these conclusions, but lead to a weaker singularity of chi .

Structural phase transitions in K₂CO₃ have been investigated using light scattering techniques. The Raman spectrum of the high-temperature hexagonal phase consists of a very broad structureless band which suggests that the system is highly anharmonic. Below the hexagonal-monoclinic transition at T_cl=693K the authors observe a soft mode which is assigned to librational motion of the CO₃ groups. Evidence of a cell doubling transition at T_c2=526K is found in the number of Raman active modes observed at low temperature, although a soft mode is not observed at the transition.

The general expression for the energy functional relevant for non-linear hydrodynamics of superfluid ³He is investigated. A number of new terms which had not been considered before are found. There is little change in the formal equations of hydrodynamics but the explicit content is significantly modified.

The theory of non-retarded and retarded ferromagnetic cavity polaritons is developed. These are the excitation modes of a cavity formed between the parallel planes of two semi-infinite ferromagnets. It is shown that the cavity modes can propagate within a limited angular range. The dispersion relation of the cavity polaritons is derived first within the magnetostatic approximation, and is then extended to include retardation effects. It is shown that retardation introduces a lower cut-off on the polariton wavevector. As the cavity width is varied from zero to infinity, the polaritons are shown to develop from bulk magnons into surface magnons. For a finite cavity width, the associated polariton field configuration is non-symmetric, being larger on the wall where the propagation direction agrees with that of the allowed surface mode direction.

Transverse magnetoresistance measurements have been made at low magnetic fields in the temperature range 77-300K in a series of n-type CdS single crystals annealed in molten Cd and In. The magnetoresistance in the Cd-annealed samples is found to be positive, obeying square law dependence on the magnetic field ( Delta rho / rho ₀ varies as H²), but the magnetoresistance in samples annealed for a long time (>96 h) is much less than expected for free electron conduction. The magnetoresistance in the In-annealed samples is negative for short annealing times obeying the relation - Delta rho / rho ₀ varies as H^s with s between 1 and 1.5 and decreasing with temperature. As the annealing time is increased the magnitude of negative magnetoresistance decreases and becomes positive for annealing times in the range of about 50 to 75 h. However, for annealing times exceeding about 75 h, the magnetoresistance again becomes negative. The results have been discussed on the basis of a significant contribution of the impurity band conduction to the transport phenomenon in the In-doped samples in which the concentration of ionised impurities and the compensation ratio are high due to the presence of complex acceptor systems.

A computer simulation on the dilute Ising ferromagnet, where the magnetic atoms are distributed randomly in a two-dimensional matrix of non-magnetic atoms, is carried out. The interaction between the magnetic atoms is assumed to be long range. The numerical results for the temperature dependence of the magnetic specific heat show broad and round peaks. This rounding is too large to be attributed to the small size of the system. It is concluded that most of the rounding can be attributed to the spatial distribution of the local field due to the random distribution of the magnetic atoms. On the other hand, the results for the magnetic susceptibility show rather sharp peaks.

The concentration dependence of the ferromagnetic ordering temperature, T_c(p) in a dilute bond Ising model,, is studied for several long-range interactions near the limit p to 0, T_c to 0. The critical region of the random Ising model transition, occurring at all finite T_c, is shown to shrink to zero as p approaches zero. For dipolar interactions, spin glass ordering may occur at sufficiently small p.

Powdered and single crystal specimens of dysprosium manganate, DyMn₂O₅, have been studied by neutron diffraction at temperatures between ambient and 4.2K. Magnetic transitions are observed at 44K, 18K and 8.4K. Both manganese and dysprosium moments are ordered below 8.4K, where the magnetic structure has been refined. Measurements above 8.4K indicate that this transition is to a second phase in which both moments are again ordered, but the details of the structure of this phase are not yet known. The relationship of the low-temperature phase to the magnetic structure of some isomorphous rare-earth manganates is discussed.

The polarisability for a two-dimensional electron gas is calculated in the presence of impurities by a Green function formalism. This leads to a system with finite mean free path due to electrons scattering off impurities. The calculated polarisability is found to be strongly dependent on the mean free path. The main feature is the suppression of the sharp corner at wavevector 2k_F for finite mean free paths, and the pure metal result is recovered for the infinite mean free path. A possible application of the results of the transport properties of semiconductor inversion layers is discussed.

A variational method in the induced covalent bond theory of hydrogen chemisorption on a metal surface is presented, which interpolates between the strong- and weak-coupling limits previously examined. In the formulation, the variational wavefunction for the localised state is transformed, with the aid of the Green function and symmetry, into a single function of energy, which simplifies the variation considerably. It is applied to a simple model of hydrogen chemisorption on the (100) surface of a simple cubic s-band metal. The binding curves are obtained and it is shown that in both of the strong- and weak-coupling limits they are in good agreement with those calculated by the perturbation method so that the variational function is appropriate in almost all the range of the coupling strength. It is demonstrated how the binding energy increases of the interelectronic Coulomb interaction as well as a decrease of the bandwidth of the substrate.