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Negatively charged centres () formed in semiconductors by attaching a second electron to a shallow neutral hydrogenic donor () have been analysed in the quasi-zero- and quasi-one-dimensional limits. On the basis of the present and already known results for higher dimensions, it is proposed that ions support one and only one stable configuration which evolves continuously from the three-dimensional to the zero-dimensional limit.

We report the first example of amorphization of garnets at high pressure and ambient temperature. Synchrotron x-ray diffraction and fluorescence spectroscopy at high pressure were performed on polycrystalline gadolinium scandium gallium garnet (GSGG) doped with and . X-ray diffraction measurements reveal a loss of long-range order beginning at GPa. This is followed by a change in local environment for dopants at 70 GPa as indicated by the loss of fluorescence emission. The amorphous phase is retained at ambient conditions, after decompression. We present possible mechanisms for these two distinct transformations in GSGG.

We present magneto-optical Kerr effect measurements of epitaxial Co/Cu(001) films studied in situ as submonolayer quantities of Cu are deposited under ultrahigh-vacuum conditions. An enhancement of the magneto-optical response coincides with changes in the remanence behaviour, which are shown to be due to a variation in the ratio of the uniaxial and cubic in-plane magnetic anisotropy strengths. The data are consistent with the preferential growth of Cu atoms at atomic steps due to the substrate: these Cu `wires' reduce the step-induced uniaxial anisotropy and their electronic structure strongly affects the magneto-optical response.

We have investigated photon stimulated ion desorption from physisorbed on highly oriented pyrolytic graphite using VUV synchrotron radiation in the energy range 13 - 40 eV. In contrast with gas phase photodissociation measurements, only ions are detected. This result is attributed to the image potential barrier to desorption from the surface and the unequal distribution of kinetic energy amongst the molecular photofragments. The yield of desorbed ions shows a threshold at 20.5 eV and a resonance at 24 eV and appears to be enhanced in the monolayer regime. This behaviour differs significantly from that of the gas phase, and is attributed to selective quenching of excited molecular electronic states on the surface and to chemical reactions between specific molecular dissociated fragments and the substrate.

Recent simulations of the interaction between planar surfaces and model atomic force microscope (AFM) tips have suggested that there are conditions under which the tip may become unstable and `avalanche' toward the sample surface. Here we investigate via computer simulation the stability of a variety of model AFM tip configurations with respect to the avalanche transition for a number of fcc metals. We perform Monte Carlo simulations at room temperature using the equivalent-crystal theory (ECT) of Smith and Banerjea. Results are compared with recent experimental results as well as with our earlier work on the avalanche of parallel planar surfaces. Our results on a model single-atom tip are in excellent agreement with recent experiments on tunnelling through mechanically controlled break junctions.

Using the lattice gas model and transition-type-dependent type Monte Carlo method, we calculated the chemical surface diffusion coefficient with nearest-neighbour repulsion () and next-nearest-neighbour attraction () () directly from Fick's first law. Our results show that the chemical diffusion coefficient depends on the structure of the adsorbate layer. When , the ordered lattice gas phase is established near c = 0.50. The presence of the ordered domains will greatly reduce the chemical surface diffusion coefficient.

We have used the adiabatic bond charge model within a repeated slab scheme to study lattice dynamics on the GaAs(110) surface. The results provide a more detailed analysis of the surface phonon modes than is available from existing theoretical and experimental techniques.

A simple model is proposed for calculating the reaction yield of a surface reaction of molecules chemisorbed on regular surfaces. An application to benzene from acetylene tricyclization on Pd(111) is performed. The analysis relies on random adsorption of acetylene on the clean crystal surface. The benzene yield is calculated in terms of the acetylene coverage. Analytical results are compared with computer simulation ones and experimental data. The predicted value for the maximum reaction efficiency as well as the behaviour as a function of the acetylene coverage are in fair agreement with experimental measurements.

Scanned-energy-mode photoelectron diffraction has been used to determine the local adsorption structure of CO on Ni(111) at low temperature at a coverage of approximately 0.25 ML, corresponding to a nominal phase. CO molecules are found to occupy the same two (`hcp' and `fcc') inequivalent hollow sites as in the 0.5 ML phase, and while the results do indicate a preference for the hcp hollow site directly above a second-layer Ni atom in the low-coverage state, this preference is of marginal statistical significance. The similarity of this parameter in both the and phases can only be reconciled by assuming significant local disorder. It does, however, indicate that the local binding energies at the two hollow sites are very similar and that the presence of both sites in the phase is not simply a consequence of near-neighbour adsorbate - adsorbate repulsion.

The spin-flip rate of scattering resulting from the presence of Fe atoms in a very thin Cu film is determined as a function of the temperature between 1.6 K and 40 K by means of weak electron localization. The spin-flip scattering rate shows a maximum at 18 K which can be interpreted as resonant Kondo scattering. From a comparison of the Suhl - Nagaoka approximation with the present data a Kondo temperature is found.

We have determined the anisotropy of electron transport in a series of Cu/Si multilayers, by measuring their low-temperature transverse magnetoresistance due to weak localization (WL). For each sample, the measurements were carried out for two different orientations of the applied magnetic field: parallel and perpendicular to the plane of the sample film. Using an analysis based on an anisotropic 3D theory of WL we find that the ratio of the in-plane and out-of-plane components of the electron diffusivity, , varies from essentially 1 to more than 2 as the ratio of the Cu to Si nominal layer thicknesses is increased. X-ray diffraction studies indicate the formation of a copper silicide at the interfaces of the multilayers, and a simple stratified conductor model based on this provides an estimate of the anisotropy that is consistent with that measured through the magnetoresistance. In some samples the magnetoresistance shows departures from the expected WL behaviour that are consistent with the presence of superconducting fluctuations.

Positron mobility and lifetime measurements have been carried out on semi-insulating Fe-doped InP samples with Au contacts used for electric field application. The lifetime measurements, with electric fields directed towards the Au - InP:Fe interface, reveal no component associated with interfacial open-volume sites and thus give no evidence of any positron mobility. The mobility measurements, made using the Doppler-shifted annihilation radiation technique, however, reveal a temperature independent positron mobility of about in the range 150 - 300 K. These observations, together with results from I - V analysis, are discussed with reference to two possible band-bending schemes. The first, which requires an ionized shallow donor region adjacent to the Au - InP interface, seems less plausible on a number of grounds. In the second, however, an negative space charge produces an adverse diffusion barrier for positrons approaching the interface together with a non-uniform electric field in the samples capable of explaining the observed mobility results.

We present the results of a valence band (VB) photoemission study of a alloy in a photon energy range covering the Cooper minima of the Pd 4d and Au 5d photoionization cross sections, ( = 90 - 280 eV). The spectra show the presence of the Pd 4d-derived virtual bound state (VBS) which we find to lie between the Au 5d band and the Fermi level with a binding energy of 1.45 eV. The intensity of this peak goes through a minimum at = 140 eV. The results are discussed in terms of the spatial distortion of the Pd 4d wavefunction in the alloy. Subtractions between the alloy and metallic Au spectra at different photon energies reveal marked differences in the Au 5d part of the valence band. These differences are ascribed to modifications in the density of states caused by the presence of the impurity and to the variation in the photon energy dependence of the cross section of different parts of the VB spectrum.

Lineshapes in x-ray core-level photoemission (XPS) reflect the local electronic environment of the atomic species from which they originate, and the excitations of the material system which reduce the emerging photoelectron's kinetic energy. For metallic systems, we show that detailed analysis of such lineshapes provides important information on the local conduction band electronic structure at distinct sites of the same atomic species. A form is derived for the core-level lineshape which is based on the spectrum of excitations in the conduction band, predominantly the formation of electron - hole pairs which screen the core-level hole produced in XPS; this is more general (but less rigorous) than the Doniach - Sunjic formulation. The lineshape is used as the basis for a numerical data analysis package - SHAPER - which extracts lineshape information from experimental XPS data by least-squares fitting. Various lineshapes derived from hypothetical conduction band profiles, and the reliability of the fitting process, are examined.

SHAPER, a computer package for x-ray photoemission lineshape analysis, is applied to experimental data on and some of its transition metal intercalates, revealing that charge transfer into the conduction band resulting from intercalation affects the core-level lineshape by modifying the density of states at the Fermi energy. The observations are discussed in terms of the rigid-band model of intercalation. In order to emphasize the interactive character of the fitting process, and its strengths and limitations, a range of fits of increasing complexity is presented.

High-resolution x-ray photoemission (XPS) studies of the lineshapes of the Ta 4f core levels in 1T- and are reported. These materials show charge density wave (CDW) behaviour at room temperature, and the local electron densities at different atomic sites are shown to produce different local screening of the core-level photohole, so the components of the doublet or triplet 4f emission line resulting from the CDW-induced shifts in the local potential at different atomic sites have different core line asymmetries. The complex lineshapes are analysed using the iterative fixing procedure, SHAPER, allowing connections to be made between the observed spectra and the site-dependent density of states.

We have carried out measurements of the temperature dependence of the emission yield of Si atoms from the Si(100) surfaces induced by nanosecond laser pulses of several photon energies. The yield is plotted as a function of , where is the photon energy and is the band-gap energy at temperature T where the yield is measured. We find that the Si emission yield is within the noise level for E = 0.9 - 1.15 eV (region I), increases to a saturation level for E = 1.15 - 1.42 eV (region II) and increases further for E = 1.42 - 2.15 eV (region III). The results are compared with those for GaP, GaAs and InP.

The crystallographic and magnetic structures of (x = 5,7) compounds at room temperature have been refined by Rietveld analysis of the high-resolution neutron powder diffraction data. The analysis indicates that (x = 5,7) compounds have -type structures (R3m). The Ga atoms only occupy preferentially 18h and 18f sites in compounds, but Ga atoms occupy preferentially 18h, 18f and 6c sites in compounds. The magnetic moments of all of the Fe atoms of compounds display ferromagnetic alignment, lie in the plane perpendicular to the sixfold axis and exhibit planar magnetic anisotropy. In contrast, the magnetic moments of all of the Fe atoms of compounds lie parallel to the sixfold axis and exhibit easy-axis magnetic anisotropy.

The stabilities of different kinds of ordered structure in complex perovskites have been studied by the eight-point cluster variation method (CVM). The grand potentials of these structures have been calculated and compared with that of the disordered state. It has been proved that the -type ordered structure will be stable in the system when the nearest-neighbour interaction is much stronger than other longer-distance interactions. The - and -type ordered structures result from the next-nearest- and the third-neighbour interactions respectively. The grand potentials of the - and -type ordered structures reach minima at the composition x = 0.5, whereas the grand potential of one of the -type ordered structures reaches its minima at around x = 0.25 and x = 0.75. The formation of different kinds of ordered structure in real material systems has been discussed.

We report the results of a series of calculations of the electronic structure of ordered, equi-atomic CuAu. We have investigated the effects of the creation of superzone boundaries on the Fermi surface of CuAu I when the long-period superlattice, the CuAu II phase, is formed. The new boundaries destroy appreciable regions of Fermi surface, thus favouring the formation of the latter phase. The positions of the boundaries are related directly to the period of the long-period superlattice and we have investigated their dependence on the electron/atom ratio and pressure. We find that the results are in very good agreement with previous experimental measurements of the average domain size in CuAu II. Thus, we provide strong evidence that Fermi surface topology plays an important role in determining the period of the long-period superlattice in CuAu II.

We calculate the optical absorption spectra associated with transitions from the n = 1 valence subband to the donor level in a dielectric quantum well. It is found that the impurity binding energies evaluated from photoluminescence experiments are lower than the practical binding energies, because of the effect of the image potential on the exciton binding energy in the dielectric quantum well. The broadening of absorption spectra due to the impurity distribution and the fluctuation in well width is also discussed.

The second moment of the local polarization distribution W(p) has been measure in as a function of temperature by a nuclear-quadrupole double-resonance technique. The temperature variation of is well described by a dynamic disorder of randomly asymmetric O - H...O hydrogen bonds. The variance of the Gaussian distribution of random fields which induce the asymmetry of the O - H...O hydrogen bonds is equal to .

In previous polarized neutron inelastic scattering experiments on Fe - Ni alloys [1] a `forbidden' mode was found at the Invar composition. This mode has the same dispersion as the transverse acoustic phonon, but was observed in longitudinal scans in which the momentum transfer was along the () direction. The origin of this mode, which seems to be confined to the Invar composition, is not yet understood. This paper reports the results of an inelastic neutron scattering experiment using high-energy incident neutrons in which the mode has been found, and the wavevector dependence of its energy measured, in the first Brillouin zone. The intensity of the mode is greater in the first zone than in that centred on (200) and since the phonon cross section is minimized at small wavevectors it was concluded that the mechanism giving rise to the excitation is predominantly magnetic in origin. This conclusion receives support from the decrease in intensity of the mode with increasing temperature. On the basis of these results it is postulated that the `forbidden' mode arises from a dynamic modulation of the magnetic moments associated with the compressive strain in the phonon excitations rather than from magnetovibrational scattering. These results together with the earlier polarized neutron measurements, indicating a frequency-dependent moment, are able to account for the anomalous thermal variation of the magnetization.

The magnetic properties of two tris-{-propanetriolato(3-)} tricuprate clusters: ( 1) and ( 2); and of a polyolato metallate cluster of sixteen ions: ( 3) are investigated. The Cu - O - Cu intracluster superexchange constants determined by measurements of the magnetic susceptibility are antiferromagnetic. They depend strongly on the corresponding Cu - O - Cu bonding angle and take values up to 400 . The interplay of exchange narrowing and dipolar, hyperfine and g-anisotropy broadening for clusters of strongly exchange coupled ions is studied using electron spin resonance (ESR) at 9.5 GHz and 245 GHz.

The easy magnetization direction of lies within the basal plane over all magnetic ordering temperatures. A spinning-sample magnetic alignment technique was used to align the basal planes of fine single crystallites of (R = Y or Gd and x = 0 or 1) to be parallel to each other. The degree of magnetic alignment was checked by x-ray diffraction. The magnetocrystalline anisotropy field of has been determined by a singularity appearing in the curve of versus H measured on a magnetically aligned sample when an external field is applied perpendicular to aligned basal planes. The temperature dependence of the anisotropy fields was measured from 4.2 K up to the corresponding Curie temperatures. It was found that the Curie temperatures of compounds with R = Y or Gd were enhanced upon partial substitution of Fe by Ga. However, the magnetic anisotropy fields of Ga-containing compounds are smaller than those of the corresponding pure compounds at low temperatures.

The first spin-resolved photoemission experiment on an iron - boron amorphous alloy using a synchrotron source is presented. The experimental spin polarization of the d band of has been compared with three theoretical predictions and found to be in best agreement with self-consistent spin-polarized calculations based on a supercell LMTO approach. The observed average spin polarization of the valence band is approximately twice that of 10 eV secondary electrons. Hysteresis loops for determined from the 1 eV and 20 eV secondary-electron asymmetry are similar to those determined using the magnetooptic Kerr effect, but show a lower coercivity. The differences are attributed to a combination of different sampling depths of the two techniques and to the surface inhomogeneity of the sample.

The optical absorption spectra of the rhombohedral DyOF were measured at selected temperatures between 9 and 300 K. The emission spectra of the ion in the LaOF and GdOF matrices were obtained at 77 K and room temperature. The energy level scheme of ( electron configuration) was simulated with a Hamiltonian of 20 parameters. The diagonalization of the energy matrices including simultaneously the free ion and crystal field (c.f.) interactions was carried out for the point symmetry of the site. Good correlation with an rms deviation of was obtained between the experimental and calculated energy level schemes of 153 Kramers doublets. The c.f. parameters show only slight distortion from symmetry. A comparison to , , , , and electron configuration) in other REOF matrices showed smooth evolution of the c.f. effect which was discussed in terms of the possible interactions involved. The x-ray powder diffraction pattern of DyOF between was analysed by the Rietveld profile refinement method. The structural data were used to calculate the c.f. parameters by the modified electrostatic point charge model yielding , and values close to the experimental ones, whereas the , , and values were too large. Using the experimental free ion and c.f. wave functions, the paramagnetic susceptibility of DyOF as a function of temperature was simulated. Above the Néel temperature (3.6 K), good agreement was obtained between the calculated and experimental susceptibilities in the paramagnetic range.

Approximately stoichiometric InP films have been prepared over a range of substrate temperature . The structure of the deposited films has been investigated by means of transmission electron microscopy (TEM), extended x-ray absorption fine structure (EXAFS) and x-ray photoelectron spectroscopy (XPS) experiments. TEM measurements reveal that samples deposited below approximately are amorphous while films prepared above this temperature are microcrystalline in nature. Both EXAFS and XPS measurements indicate that bond angle disorder in the atomic network decreases as is raised. Changes in the optical absorption properties are observed as is increased, with particularly marked changes noted over the relatively narrow temperature range ; the bandgap (defined as the photon energy at which the absorption coefficient is equal to ) increases from 1.1 to 1.25 eV while n(0.5) (the refractive index at 0.5 eV photon energy) decreases from 3.7 to 3.3 over the above temperature range. The changes in optical properties are correlated with the structural data.

Conduction-band electronic states of amorphous photodissolved by Ag and Cu metals have been examined by means of soft-x-ray core absorption spectroscopy. Both the Ge and the Se spectra of the photodoped samples are similar to the corresponding spectra of undoped in contrast with the Se 3d core absorption spectra, which suggests that the prominent change in the conduction band density of states by Ag or Cu photodoping is caused through the Se 4p contributions. The core absorption spectrum has a clear peak which would originate from the Cu - Se covalent bonds. These results do not contradict the structural model described by Fischer-Colbrie et al.