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The cell parameter of cubic iron monosilicide, epsilon -FeSi, has been determined at 25 pressures between 0 and 9 GPa by time-of-flight neutron powder diffraction, using the POLARIS diffractometer at the ISIS spallation neutron source. NaCl was used as an internal pressure standard. Fitting of the data to a third-order Birch-Murnaghan equation of state with K^'₀ fixed at a value of 4.0 gave an isothermal bulk modulus K₀, of 160(1) GPa.

The chemisorption and etching of fluorine on the Si(111)7*7 surface has been studied using Si₉₀H₇₄F_n (1<or=n<or=5) clusters and the atom superposition and electron delocalization molecular orbital (ASEDMO) method. At low coverage, the fluorine atoms are adsorbed onto the dangling bonds. As more fluorines are added, however, a fluorine-bonded adatom is found to move from its original threefold site (one fluorine atom), to an adjacent bridge site (two fluorine atoms) and then to a neighbouring on-top site (three or more fluorine atoms), while the rest atoms remain fixed near their original threefold equilibrium sites. The lowest etching energy is found to be 0.35 eV for SiF₄ desorption from a rest atom site.

The chemisorption of chlorine on the Si(111)7*7 surface has been studied using clusters and the well known AM1 semiempirical molecular orbital method. Both the equilibrium structures corresponding to n Cl atoms (1<or=n<or=7) chemisorbing onto the adatom and rest atom positions of the Si(111)7*7 reconstructed surface, and the etching energies for an adatom or rest atom bonded to x Cl atoms (0<or=x<or=4), have been obtained. We find that as successively more chlorine is chemisorbed, an adatom moves from its original threefold site (one chlorine) to an adjacent bridge site (two and three chlorines), and then to a neighbouring on top site (four or more chlorines). A rest atom bonded to up to four chlorine atoms, on the other hand, remains dose to its threefold clean surface position. Our results also predict, in agreement with experiment, that all of the SiCl_x species (1<or=x<or=4) will occur on an Si(111)7*7 surface exposed to atomic chlorine at low temperature. Increasing the temperature will first result in all of the SiCl₃ and SiCl₄, and most of the SiCl₂, desorbing from the surface. This will be followed by the desorption of the remaining SiCl₂, leaving only SiCl on the surface. The final stage of the desorption process involves the removal of the SiCl. Whilst the etching energies appropriate to a rest atom site are generally lower than the corresponding adatom values, the minimum etching energy is found to correspond to the desorption of an SiCl₄ complex involving an adatom of the Si(111)7*7 reconstructed surface.

We investigate the behaviour of a hydrogen atom in interaction with the surface of a polar semiconductor. The proton is considered as static and we calculate the binding energy of an electron that cannot penetrate the semiconductor. The interaction of the electron with the surface is described microscopically in terms of electron-phonon and electron-exciton interactions. The electron ground-state energy is calculated using a variational Green function Fock approximation. We find that, when the proton approaches the surface, the electron binding energy decreases dramatically. This decrease is governed by three factors: the deformation of the electron wavefunction that has to vanish at the surface, the screening of the proton charge by the induced dipole moments on the surface and the attraction of the electron to the surface by its image potential. The first effect is found to be the most important when the proton is outside the semiconductor. When the proton is inside the semiconductor, the electron is in a surface image state and the last effect is dominant.

An elastic body moving in a quenched random force environment with a singular force correlator experiences an additional friction force which is inversely proportional to velocity. This dynamical friction mechanism is important, e.g., for a moving interface which is subjected to quenched random pinning forces. For sufficiently strong pinning forces, interface motion becomes unstable at values of driving force smaller than some critical one and the interface velocity drops. We expect in this case a first-order depinning transition with hysteresis in the dependence of the interface velocity on driving force. Instability in the interface motion remains under perturbations like an additional periodic driving force or thermal fluctuations. Interface motion instability can also take place in systems with small viscosity, when inertial effects are important.

The influence of size on the dielectric behaviour of ferroelectrics is discussed using phenomenological Landau theory. Three types of free-standing geometry are used in our calculations-film, cylinder and sphere. For films, there is a size-driven phase transition (i.e. a transition from a ferroelectric state to a paraelectric state as the thickness of the film is decreased) so long as the surface ferroelectricity is weaker than that of bulk. The polarization becomes zero below a critical size at which the susceptibility has a maximum. Otherwise the susceptibility decreases as the film thickness decreases and no size-driven phase transition exists. However, for cylinders and spheres there is always a size-driven phase transition, and so the dielectric susceptibility is always enhanced at small size. The sphere geometry has the largest critical size amongst the three geometries. In order to fit the experimental measurements on fine-grained samples, we renormalize our calculations for the sphere geometry using a Gaussian distribution function to represent the variation of particle size. The renormalization rounds the peak in the susceptibility and noticeably shifts its position to smaller size if the standard deviation of the size distribution is comparable with its mean. The critical size judged from dielectric measurements could therefore be smaller than that of an isolated sphere. Our calculations are in qualitative agreement with experimental measurements on the susceptibility of barium titanate and lead titanate.

We study the influence of surface roughness on the infrared reststrahlen band in insulating materials. We model the surface features with either truncated spherical bumps on, or truncated spherical pits in, the substrate. The polarizability of the truncated sphere-substrate system has been calculated from a multipolar expansion, taking into account up to 16 orders. Two types of resonance have been identified. Substrate-related absorption peaks occur at the transverse optical or longitudinal optical phonon frequency, depending on the direction of the incident electric field. Additional absorption peaks occur in the vicinity of the resonance frequency of a sphere. They are dependent on the detailed geometrical configuration.

Using angle-resolved photoemission and inverse photoemission, the electronic structure of films of the molecular icosahedron orthocarborane (C₂B₁₀H₁₂) on Cu(100) has been studied. The measured gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals is in good agreement with modified neglect of differential overlap calculations. An unoccupied exopolyhedral state has been found and is ascribed to molecular orbital hybridization. This hybridization is consistent with the observed dispersion of the orthocarborane molecular orbitals. This molecular orbital dispersion indicates a surface Brillouin zone edge with an average radius of 0.7 AA^-1, and suggests that the molecules adopt local close-packed ordering in the overlayers, with an intermolecular distance of 5.58 AA.

Ab initio band structure calculations within a density functional formalism were performed to compute the binding energy curves for atomic hydrogen interacting with high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.23, 3.12 and 3.24 eV, for on top, bridge and threefold sites, so the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on top sites. Starting from the ab initio results for the H-Cu(111) system, a LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favoured approaches correspond to a H₂ molecule parallel to the Cu surface, and have activation barriers of 0.6 eV, located at the corner between entrance and exit reaction channels. The LEPS potential was used in quasiclassical trajectory calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The dynamical results show that (a) when H₂ is in its ground vibrational state, the dissociative adsorption probability, P_alpha , increases from 0 to 0.90 along a roughly sigmoidal curve when increasing the collision kinetic energy from 0.4 to 1.3 eV; (b) the vibrational energy of H₂ can be as effective as the translational one in promoting chemisorption; (c) dissociation of H₂ is inhibited by rotational motion at low j values, but enhanced by high values of j;(d) P_a scales with normal component of collision energy; (e) no azimuthal corrugation of the surface is observed.

A structural and thermodynamic study of the DyNi_5-xAl_x-hydrogen system has been performed. It was found that the hexagonal crystal structure of the prototype compound DyNi₅(CaCu₅ type; space group, P6/mmm) exists up to DyNi₃Al₂. Beyond this composition and up to DyNi₂Al₃ another related hexagonal structure (YCo₃Ga₂ type; space group, P6/mmm) is stable. All alloys have been exposed to hydrogen gas at pressures up to 15 MPa and temperatures between 77 and 700 K. Under these conditions, ternary alloys having the CaCu₅ structure react with hydrogen. Their crystal structures remain the same but the hydrogen uptake increases the unit-cell volume by up to 13%. The remaining ternary alloys as well as the binary DyNi₅ do not exhibit any significant hydrogen absorption. The pressure-composition isotherms were measured. The entropy and the heat of formation have been extracted for systems that exhibit an equilibrium pressure plateau. It was found that the hydrogen capacity at ambient temperature and the equilibrium pressure decrease with the increasing content of aluminium.

Using the harmonic approximation as a model to describe a bilayer of a Wigner crystal in wide quantum wells, we calculate the ground state properties and the phonon spectrum of this system as a function of the density and layer separation. A fitting formula is given for the ground state energy.

Time-dependent second-harmonic generation experiments in K_1-xLi_xTaO₃ crystals for the first time yield a Li concentration dependence of the activation energy of the Arrhenius-like reorientation process of the Li⁺ ions. The experimental data support the assumption of a Li⁺ off-centre ion tunnelling via excited and ground vibrational states. With increasing Li concentration, the tunnelling reorientation is suppressed in favour of a thermally activated over-barrier hopping. The suppression of the tunnelling reorientation can be explained by a model of cooperative tunnelling in a percolation dipole cluster and destruction of the tunnelling resonance condition by internal electric dipole fields.

We propose a theoretical model for the low-frequency noise observed in a quantum point contact (QPC) electrostatically defined in the 2D electron gas at a GaAs-AlGaAs interface. In such contacts electron scattering by soft impurity or boundary potentials coherently splits an incoming wave function between different transverse modes. Interference between these modes has been suggested to explain observed non-linearities in the QPC conductance. In this study we invoke the same mechanism and the time-dependent current due to soft dynamical impurity scattering in order to analyse the low-frequency (telegraph-like) noise which has been observed along with a non-linear conductance. For the simplified case of a channel with two extended (current carrying) modes, a simple analytical formula for the noise intensity is derived. Generally we have found qualitative similarities between the noise and the square of the transconductance. In comparison with the more traditional d.c. transport measurements we believe that noise measurements can provide additional information about the dynamical properties of QPCs.

The possible existence of non-chain superstructures in YBa₂Cu₃O_6.375 is studied by Monte Carlo simulation in the framework of the lattice gas model with screened Coulomb potential pair interactions. It is found that this model is capable of describing the occurrence of such structures only if the interactions are cut off at a certain distance. Then, only the superstructure proposed from the neutron diffraction experiments of Sonntag et al is found to be stable at finite temperature. A phase diagram with coexisting chain, non-chain and disordered structures is obtained.

The nuclear magnetic resonance of ²³Na (I=³/₂) in an NaMnCl₃ single crystal grown by the Czochralski method has been investigated by employing a Bruker FT NMR spectrometer. From the experimental data, the quadrupole coupling constant e²qQ/h=155+or-2 kHz and asymmetry parameter eta =0 are determined at room temperature. The principal Z axis of the EFG tensor of the ²³Na, the largest component, is parallel to the crystallographic c axis. The temperature dependence of the quadrupole parameters is also investigated in the temperature range of 140-380 K. The quadrupole coupling constant of ²³Na is found to be linearly increasing as the temperature increases.

The differential cross-section (DCS) for an electron Raman scattering (ERS) process in a semiconductor quantum wire (QW) of cylindrical geometry is calculated for T=0 K and neglecting phonon-assisted transitions. Electron states are considered assuming complete confinement within the QW. We also assume single parabolic conduction and valence bands. Two kinds of spectrum are discussed: emission spectra DCS as a function of emitted photon energy) and excitation spectra (DCS as a function of incident photon energy). In both cases we analyse the DCS for different scattering configurations. We study selection rules for the processes. Singularities in the spectra are found and interpreted. The ERS studied here can be used to provide direct information about the electron band structure of the system.

In this work Mossbauer spectra of CsCo_1-x-y⁵⁷Fe_xMg_yCl₃ (where x approximately 1% and y=0.07, 0.3, 0.7, 2.6%) in powdered form have been taken for a range of temperatures from 250 K down to 5.5 K. The spectra of these compounds taken below 21 K could be analysed in the same way as the spectra of CsCo_0.09Fe_0.01Cl₃ where the fits to the spectra are consistent with the magnetic phases of pure CsCoCl₃. CsCoCl₃ is a one-dimensional Ising-like antiferromagnet and moving domain walls or 'solitons' have been observed in the one-dimensionally ordered Co²⁺ chains from approximately 75 K down to 9 K. At 9 K full three-dimensional order is formed. However, experiments on Mg-doped CsCoCl₃ indicate that the Mg suppresses this transition so that it may be possible for solitons to be present below 9 K. No unequivocal evidence to indicate the presence of solitons below 9 K in Mg-doped CsCoCl₃ could be found in the Mossbauer spectra. It was found that the addition of the Mg lowers the temperature of the transition to the partially disordered phase, T_N1, from 21.1+or-0.3 K (no Mg) to 19.6+or-0.3 K (2.6 at.% Mg). The spectra of CsCoCl₃ doped with 2.6 at.% Mg showed differences that resulted in soliton relaxation rates which were approximately a factor of two higher than the rates determined for the other compounds. In all compounds the soliton relaxation rates determined above 9 K were found to be between one and two orders of magnitude below the theoretical prediction for a non-interacting soliton gas and it is suggested that this model is not appropriate for CsCoCl₃.

Samples of RFe₁₀V₂C_x with R=Y, Gd, Dy or Er were prepared by arc melting. X-ray diffraction and thermomagnetic analyses showed that interstitial carbides with the ThMn₁₂ structure can be formed only for a small amount of C (x<or=0.3). The effects of C addition on lattice expansion and Curie temperature are much smaller in the carbides prepared by arc melting than in those prepared by gas-solid reaction. Neutron diffraction shows that the C atoms occupy the interstitial site in the structure.