Table of contents

Following Stephenson (1969-70), the authors consider d-dimensional magnetic systems with competing interactions in (d-m) directions. The ground state is assumed to be ferromagnetic. These systems may exhibit, above the critical temperature, disorder points of the first or second kind that they study in the framework of a simple random phase approximation. For disorder points of the first kind, they find that the correlation functions are anisotropic if m is odd; they are simply m-dimensional for disorder points of the second kind. Applying an extra parameter (concentration, pressure, . . .) allows one to consider how a disorder line intersects a critical line; in their model, their happens at a Lifshitz point for disorder lines of the first kind, or at a special point (displaying m-dimensional critical behaviour) for disorder lines of the second kind. Their analysis also suggests the possibility of a disorder point below the critical temperature. Experiments are briefly considered.

The thermal expansion of the Mn²⁺-F^- bond in KZnF₃:Mn²⁺ in the 15-300K temperature range has been studied from the variations undergone by the crystal-field spectrum. It is shown that the thermal expansion effects are clearly more significant for the Mn²⁺-F^- distance than for the Zn²⁺-F^- distance of the perfect lattice. In this way the linear thermal expansion coefficient alpha for Mn²⁺-F^- is found to be alpha =(38+or-14)10^-6 K^-1 at room temperature to be compared with alpha =(15+or-0.7)10^-6 K^-1 for the host lattice.

The authors present new analytic formulae for the phonon density of states and inverse decay length of a one-dimensional granular structure and show that for large grains with no disorder these are universal functions of a dimensionless frequency Omega and a velocity of sound ratio, C. The density of states at frequencies Omega <C is shown to exhibit a granularity-induced enhancement, which is unaffected by the presence of disorder. For the first time a one-dimensional analogue of a network glass below a smeared rigidity percolation transition is analysed. It is shown that this system also exhibits a low-frequency density-of-states enhancement, whose magnitude is determined by the velocity of sound ratio C alone.

In LEC-grown n-GaAs:Ti, a triple-peaked absorption band at 0.66 eV and a band at 1.01 eV are observed and interpreted as the ³A₂ to ³T₁(F) and to ³T₁(P) transitions of Ti_Ga²⁺. Sharp zero-phonon lines are seen in absorption at 0.566 and 0.569 eV, the former line also being observed as the zero-phonon component of a photoluminescence band. The absence of any Zeeman splitting of the lines up to fields of 10 T indicates that these lines do not arise from Ti_Ga²⁺.

Theoretical and experimental results on the structure and properties of grain boundaries in ionic crystals are reviewed and several models for grain boundary structures and their usefulness assessed. The results of calculations using computer simulation techniques for grain boundary structures and point defect properties are summarised. Experimental work on grain boundary structure and energies is discussed and compared with the available theoretical calculations, with suggestions for the apparent discrepancies where necessary.

The local density functional technique is applied to the ab initio calculation of ground-state properties of cubic silicon carbide. Norm-conserving pseudopotentials are used with the atomic numbers of the elements as the only inputs. Results for the equation of state, real-space distribution of electronic charge and selected phonons are presented. The pressure derivative of the bulk modulus, B'₀, is predicted and the Gruneisen parameters describing the pressure variation of the phonon modes are calculated. More than 1000 plane waves per unit cell are needed to achieve satisfactory convergence of these quantities. The calculated values agree with experiment to within a few per cent. The features of the electronic charge distribution are discussed, as is its variation under applied pressure.

A modified two-dimensional XY model, in which the energy of the nearest-neighbour interaction is a function with two minima has been considered. The extrema of Hamiltonian have been classified and transformations to a dual model and Coulomb gas have been performed. It has been shown that a large depth of an additional minimum of the interaction function can lead to the splitting of the phase transition into two. One of them (of the Ising type) is connected with appearance of solitons of an infinite length, and the second one with dissociation of pairs of vortices with half of the circulation quantum. A corresponding phase diagram is constructed. It is proposed that recursion relations of the Migdal-Kadanoff moving bonds approximation can be modified. This new version shows the presence of a line of true fixed points in the XY model, and can be applied for studying the modification of the XY model introduced in this paper. The modified XY model can be applied for describing the behaviour of superfluid ³He thin films.

The authors analyse long wavelength collective modes in metal-salt solutions using (i) the linearised equations of hydrodynamics and (ii) a simple model of generalised hydrodynamics at zero wavenumber. The metal-rich concentration range is examined by assuming first that the conduction electrons form a rigid neutralising background, and then by allowing for their polarisability within an adiabatic, linear screening approximation. The most interesting prediction of their analysis is that sound waves should be overdamped below a critical wavenumber for a certain range of concentrations in the salt-rich region; coupling between mass and charge density fluctuations transforms the Brillouin lines of propagating sound waves into a central peak of finite width, characteristic of a relaxation mode.

Up to now, all the models proposed to describe the reconstruction of W(100) assume that the atomic displacements are limited to the surface plane. The authors present here a 'quenched molecular dynamics' approach in which they minimise the total energy, in a tight binding framework, with respect to alternate lateral displacements of atoms and interlayer spacings involving the first four layers. One finds that this multilayer relaxation favours the zigzag reconstruction for W(100) with lateral displacements decreasing rapidly with the distance to the surface. Finally, they check the influence of various parameters, especially that of the d band filling, paying some particular attention to Ta(100).

A recently developed method for performing self-consistent parameter-free calculations on the electronic structures of open systems is for the first time applied on infinite, periodic polymers. Linear carbon chains with alternating bond lengths (polyyne) and with equidistant bond lengths (cummulene) are investigated. The results are compared with other calculations on those systems and on graphite and diamond.

The authors report measurements of the Hall effect in (TMTSF)₂PF₆ under a pressure of 10.5 kbar performed at temperatures from 0.15 to 77K and in magnetic fields up to 9 T parallel to the c* axis. The low-temperature data suggest a series of transitions between semimetallic (SDW) sub-phases above a threshold field. The giant oscillations of the Hall voltage observed above the threshold field suggest for (TMTSF)₂PF₆ that, unlike in (TMTSF)₂ClO₄, electrons and holes play important roles. A particularly sharp anomaly of the Hall voltage detected around 7 T may be the signature of a first-order transition between SDW states with transverse and longitudinal nesting, respectively, below and above the transition field.

The mean-field renormalisation group approach is applied to the anisotropic Ising model by considering clusters consisting of finite blocks of spins and clusters consisting of infinite strips of spins with the finite widths. The critical temperature and estimates of the critical exponent nu are obtained as functions of the anisotropy. A two-step renormalisation group procedure using a phenomenological renormalisation transformation is also presented. In general, the results for nu are consistent with universality arguments.

The phase diagram of Rb₂MnCl₄ in magnetic fields up to 8.3 T parallel to the tetragonal axis was determined by neutron scattering. Similar to K₂MnF₄, but unlike conventional three-dimensional antiferromagnets, the authors experimental data exhibit a bifurcation of the phase boundary uniaxial antiferromagnet/paramagnet into the phase boundaries uniaxial antiferromagnet/spin-flop phase and paramagnet/spin-flop phase at the multicritical point T*=50.5K, H*=5.55 T. The boundary lines between the uniaxial antiferromagnetic phase and the paramagnetic and spin-flop phases are well described by T_c(H)/T_c(0)= alpha (H)ⁿ, where alpha (H) is the field-dependent anisotropy and n approximately=0.04 is possibly a universal exponent.

Mossbauer relaxation spectrum of Fe³⁺ in LiAl₅O₈ shows an EFG through unequal separations of the pairs of outer lines on the two sides of the velocity scale, as is conventionally expected. The application of an external field (H_ext) of 8 kG, however, is found to make this feature of the spectrum disappear even though the EFG is unaffected. This is explained by taking into consideration the presence of relaxation effects. To this end, the author has successfully treated the Mossbauer relaxation spectrum theoretically for the two cases: H_cr<<H_Zeeman and vice versa. Furthermore, when H_ext is small the mod +or-1/2) levels are shown to relax rapidly even when the other Zeeman states of the Fe³⁺ ion relax slowly.

Measurements of ¹⁵¹Eu Mossbauer spectra and magnetic susceptibility show that the europium 4f configuration in EuIr₂Si₂ changes continuously as a function of temperature from 4f^6.2 at 4.2K to 4f^6.7 at 290K. By contrast, europium in EuRh₂Si₂, EuPt₂Si₂, EuRhSi₃ and EuIrSi₃ remains divalent. Chemical systematics of the appearance of intermediate valence in ternary europium silicides are discussed.

A Green function technique is developed to study the real and imaginary parts of the dielectric function epsilon (k, E) for ferromagnetic semiconductors above T_c. The dependence of the numerically calculated epsilon (k, E) on energy E, wave-vector k, band width W and temperature T is discussed. Using the Kramers-Kronig analysis the refractive index n, the absorption index kappa and the reflectivity R are obtained, and they are in very good agreement with the experimental data on CdCr₂Se₄ at room temperatures.

The structural phase transition of As₂O₅ at T_c=578K is associated with frequency shifts of the strongest Raman lines at 35.5, 66.2, 567.7 and 579 cm^-1 according to omega approximately ((T-T_c)/T_c)^0.5. Although the temperature evolution of the line profiles is grossly in agreement with a displacive transition mechanism, the saturation of the lowest phonon branch at 27 cm^-1 and the spontaneous increase of the linewidth at T_c by 1.2 cm^-1 indicate the influence of relaxational or order-disorder processes. The temperature dependence of the order parameter related to the sublattice deformation follows mean-field theory within experimental resolution. The possible influence of order parameter coupling on the structural phase transition is discussed.

Simultaneous measurements of Raman and Brillouin scattering spectra were performed on the incommensurate (IC) phase of quartz near the alpha - beta transition at T_c=572 degrees C. In the low-frequency region there are two strongly temperature dependent modes. One is the 7 cm^-1 mode observed in Raman spectra with considerable intensity only in the IC phase between T_i(=T_c+1.8K) and T_c. The other mode is the Rayleigh (elastic) component in Brillouin spectra which shows a sharp intensity maximum at about 1.0K above T_c, that is in the middle of the IC phase. This mode, coupled to the longitudinal acoustic mode, seems to separate the IC phase into two temperature regions. Raman activities of the amplitude and phase modes in the IC phase of quartz are studied and compared with the experimental results. In particular, possible differences between the '1-q state', for which the phonon condensation occurs in a single q-direction, and the '3-q state' are discussed. Then, assuming the existence of the '1-q state' in the IC phase, it is shown that the temperature dependences of the 7 cm^-1 mode and the Rayleigh peak qualitatively agree with those of the A symmetry amplitude and A symmetry phase mode, respectively.

The energy levels of Cr-related trigonal centre in GaAs are studied both experimentally and theoretically. New lines and absorption bands in the energy interval of 0.9<h nu <1.4 eV are discovered in the p-type GaAs:Cr by the method of light-induced modulation of absorption. These lines are ascribed to the intra-centre transitions with charge transfer at a (Cr-X) trigonal defect, (Cr^-X⁺) to (Cr⁰X⁰), where (Cr⁰X⁰) are metastable vibronic states with a large relaxation time which is found to be 10^-1 s at room temperature. The microscopic theory of such a double defect is developed with the assumption that X is the arsenic vacancy and the strong lattice relaxation is originated from the pseudo-Jahn-Teller effect due to the tunnelling of an electron between Cr_Ga- and V_As-related deep levels of the double-well effective potential.

The authors have reported the experimental results of thermoluminescence and thermally stimulated conductivity of colour centres in NaF and NaF:Mg²⁺ crystals irradiated by electron beams with heavy doses. There are six glow peaks in the thermoluminescence of undoped NaF which are at 97, 142, 160, 210, 254 and 372 degrees C respectively. A glow peak at 77 degrees C and a single thermally stimulated conductivity peak at 47 degrees C are observed after optical bleaching using F-band light. In the case of crystals doped with MgF₂, the peaks at 77 degrees C, 47 degrees C and another thermally stimulated conductivity peak at 250 degrees C are observed in the original sample without bleaching with F-band light. The features of these peaks and the role of Mg²⁺ in the thermoluminescence are discussed in detail.