Table of contents

We describe three-dimensional magnetic resonance imaging (MRI) of a liquid foam. Difficulties in imaging arise from the rate of evolution of the foam and the magnetic susceptibility mismatch at the boundaries between liquid and air. We used a protein gelatin foam, for which we improved relaxation rates and matched the susceptibility. A three-dimensional spin-echo sequence yielded strongly T₂ weighted signals. The reconstructed data represents the first such full three-dimensional image of a liquid foam.

Carbon nitride films have been prepared by ion beam assisted deposition with high N-ion fluxes up to the order of 1 mA cm^-2. Both Auger electron spectroscopy and Rutherford backscattering spectroscopy measurements show that an average nitrogen concentration of up to 45 at.% has been incorporated into the films. The nitrogen concentration in some local areas reaches as high as 59 at.% examined by energy-dispersive X-ray analysis. In such a high nitrogen content area, Laue spots of the predicted beta -C₃N₄ structure have been observed for the first time by transmission electron microscopy. Under continuous electron irradiation, beta -C₃N₄ grains refine to dimensions of <0.1-0.4 mu m in diameter.

In a double-well quantum Hall system, electronic motion in either of the two layers introduces a new (pseudo-spin) degree of freedom. The system exhibits a time-independent quantized response to the combined effect of two electric fields, one in each layer, of opposite directions. This response is inherently related to itinerant quantum Hall ferromagnetism.

We have investigated the structural and vibrational properties of hydrogen-covered GaAs(110) and InP(110) surfaces in the framework of the density-functional theory. The plane-wave pseudopotential method combined with the slab supercell description for the surfaces has been employed to determine the relaxation geometries of H:GaAs(110) and H:InP(110) for the half- and one-monolayer coverages. Our results are in good agreement with all available experimental data. For both surfaces and for all coverages we have determined the surface vibrations by means of an ab initio linear-response formalism. The calculated bond-stretching frequencies compare very well with the results from high-resolution electron-energy-loss spectroscopy. The half-monolayer coverages on InP(110) exhibit vibrational states which are related to the mechanism which leads to the structural decomposition of the surface experimentally observed at high hydrogen exposures.

Chemisorption of an isolated CO molecule at various adsorption sites above the epitaxial Pd overlayer on Ta(110), and on related systems, is studied by using a self-consistent tight-binding scheme. Basic assumptions of the model are discussed in detail. The sigma donation and especially the pi backdonation are generally a good deal reduced at the bimetallic surface as compared with the pure Pd(111) surface. Simultaneously, the bond between CO and the surface becomes weaker. However, for CO in the atop position the effect is considerably smaller than for sites with twofold or threefold coordination and the atop site can therefore become preferred. A simple bonding picture explaining the gross trends is suggested. Core-level shifts induced in surface atoms by the adsorption are briefly discussed.

We report an angle-resolved photoelectron spectroscopy study of the layered semiconductor ZrSe₂, and of changes in its electronic structure induced by in situ intercalation with Cs. The results show that the valence band structure of ZrSe₂ is initially of 3D character, but is transformed to become essentially 2D upon Cs intercalation. The observed changes are supported by self-consistent LAPW band calculations, and are not compatible with the rigid-band model. Changes in the Se 3d core level lineshape are attributed to an intercalation-induced increase in the carrier density in the lowest conduction band, which produces a different screening of the core hole.

We have investigated the coupling of hole intersubband transitions and phonons in p-type modulation-doped GaAs-Al_xGa_1-xAs multiple quantum wells by means of resonance Raman spectroscopy. Magnetic fields with flux densities up to B=14 T were applied, oriented parallel to the growth direction. For B=0 T the spectra can be interpreted in terms of a Fano interference of the zone-centre quantum well LO phonon with a quasi-continuum of hole transitions between the lowest and the second excited heavy-hole subband. For B>4 T we observe the coupling of a hole intersubband transition with interface phonons. The character of the latter modes is deduced from a comparison of the excitation energies with values calculated within a dielectric continuum model as well as from their behaviour under photoexcitation. Our measurements indicate an interaction of the Frohlich type, where the strength of the coupling seems to increase with the in-plane wavevector of the excitations. The ionized impurities in the barriers dominate the relaxation of wavevector conservation in the scattering process.

In this contribution, we have calculated angle-dependent photoelectron spectra from the Si(100) surface along the Gamma XWK bulk mirror plane by excitation with unpolarized He- gamma radiation. For this theoretical investigation the full-potential photoemission theory has been used, which is a straightforward generalization of the one-step model of photoemission in the case of anisotropic, space-filling cell potentials. The crystal potential employed for the determination of the theoretical data has been calculated self-consistently within the full-potential Korringa-Kohn-Rostoker band structure method. The comparison with the corresponding experimental data shows a very good agreement for all bulk transitions.

A combination of near-edge and surface extended X-ray absorption fine structure (NEXAFS and SEXAFS), normal incidence X-ray standing wavefield absorption (NIXSW) and (laboratory-based) X-ray photoelectron spectroscopy and diffraction (XPS and XPD) has been used to investigate the local phases and associated structures resulting from the interaction of methylthiol (CH₃SH) and dimethyldisulphide ((CH₃S)₂-DMS) with Ni(111). The interaction was followed by adsorbing at low temperature and observing the effects of sequential heating cycles to up to 373 K. At the lowest temperature DMS appears to be intact on the surface, but at slightly higher temperatures, up to at least 223 K, a second molecular species is formed which is also obtained by the thiol adsorption, even at 153 K. This is identified as methylthiolate (CH₃S-) and appears to be bonded (through the S atom) to the hollow sites on the surface with a S-Ni bondlength of 2.24 AA; essentially equal occupation of both (fcc and hcp) hollow sites is found. Both XAFS and XPD indicate that this species shows a substantial tilt angle of the S-C symmetry axis away from the surface normal; the situation is somewhat similar to that reported for the same species on Pt(111) on the basis of C K-edge NEXAFS and vibrational spectroscopy, and comparative S K-edge NEXAFS measurements for Pt(111) are also reported here. The tilted geometry found on both Ni and Pt surfaces contrasts with the perpendicular orientation found on Cu(111). Heating the Ni(111)surface to 373 K converts the thiolate species to a third state which we identify as atomic sulphur. Both SEXAFS and NEXAFS show that this final species has the same local geometry as that found in earlier studies of atomic S on Ni(111): in particular, while the S-Ni nearest-neighbour spacing is similar to that of the thiolate, the S in this phase appears to penetrate the top Ni layer.

The anion coordination environments around Zr⁴⁺in ZrF₄-BaF₂-BaCl₂-EuF₃ system glasses were examined by Zr K-EXAFS, Raman scattering, and IR absorption spectroscopies. Experiments were performed with 58ZrF₄.(39-x)BaF₂.xBaCl₂.3EuF₃ glasses in which the x values were changed from zero to 29 in batch composition. These spectroscopic analyses reveal that the anion coordination environments around Zr⁴⁺ are hardly changed by the successive substitution of Cl for F. These results agree with our previous study using molecular dynamics simulations.

Catastrophic dechannelling resonance (CDR) occurs when the half wavelength ( lambda /2) of a planar channelled ion beam matches the path length per layer, s, of a strained layer superlattice (SLS). CDR has been used for probing important properties of SLS. The dependence of CDR on various parameters, like incident angle, minimum impact parameter for channelling and superlattice strain in strained layer superlattices, are studied using a shell structure potential, which is based on the atomic shell structure of the target (SLS) atom. We have taken a GaAs_0.09P_0.91/GaP (s=48.2 nm) superlattice as our target and a ⁴He ion beam as the incident probe beam. Comparison with other frequently used Thomas-Fermi-type potentials and good agreement with experimental results shows the usefulness and accuracy of this shell model potential.

We examine shock-induced detonation in a three-dimensional model of a nitromethane crystal. The crystal may contain a defect in the form of a vacancy cluster, or small void. Three regimes are identified: the shock can be weak enough that no chemical bonds are broken; the shock can be so strong that a detonation front is established in the perfect crystal; or the shock can be of intermediate strength, where chemical activity is dependent on the existence of the defect. In all regimes, the defect increases the reaction rate and causes a hot spot to appear.

Inelastic neutron scattering studies of Zr_0.85Ca_0.15O_1.85 at 300, 1200 and 1500 K show LA and TA phonons rapidly broadening with phonon wavevector q in all observed directions ((00 xi ),( xi xi 0),( xi xi xi ), (- xi /2, - xi /2, xi )). Optical phonon branches could not be observed. Both results can be related to the underlying static disorder. Comparative studies of well ordered CeO₂ show that the influence of anharmonicities is negligible in both these oxides up to temperatures of 1500 K. The phonon linewidths of Zr_0.85Ca_0.15O_1.85 are exceptionally large at 1200 K. This can be related to phase diagram boundaries occurring in this temperature regime. The temperature dependence of the linewidths is associated with the hindered cubic:tetragonal phase transition. The line broadening is markedly anisotropic. Elastic constants c₁₁, c₁₂ and c₄₄ were derived from the data at 1200 K.

The peculiarities of polariton light scattering under strong anharmonic interaction of the fundamental vibration with the complex transition (i.e., polariton Fermi resonance) are investigated. It is shown that the polariton Fermi resonance influences the polariton band shape and dispersion curves. A detailed comparison with experimental results for a BaTiO₃ crystal is carried out.

We study the stability of the soliton-like polaron states in a molecular chain by means of a perturbation approach. We demonstrate that, under the influence of thermal fluctuations in the host lattice, the soliton gradually decays into delocalized ('excitonic') states. The soliton lifetime depends strongly on temperature and the coupling strength.

Drift experiments on optically excited electrons and holes in beta -rhombohedral boron in drift of fields up to 146 V cm^-1 at room temperature show that the propagation of both carriers is of the soliton type. The velocities tend to saturate even at the rather low drift fields used. Characteristic mobilities are mu _e=0.11 and mu _h=0.076 cm² V^-1 s^-1. Different ratios of the retrapping to the recombination rate, 15 for electrons and 1.2 for holes, are explained by the different modes of action of the electronic states involved. For electrons the mean drift distance between generation and recombination exceeds 3 cm. The representative diffusion constant of electrons and holes D=6*10² to 7*10³ cm² s^-1 depends on the specific conditions. The diffusion covers carrier velocities between 10^-5 and 10² cm s^-1. The decay of photoconduction can be exactly described by the re-excitation of trapped electrons and their subsequent recombination.

We present a theoretical investigation of electron impact ionization processes under a high electric field within the framework of the Lei-Ting balance equation approach in the case where the hole gas is approximately in thermal equilibrium. An expression for the impact ionization coefficient for an arbitrary conduction band is obtained. As an example, the impact ionization coefficient of bulk InSb is calculated in the approximation of a parabolic conduction band and the result is compared with existing experimental data.

Low-temperature nuclear orientation has been used to study the magnetic behaviour of ⁵⁴Mn and ⁵⁷Co impurities in the AuFe alloy, below (14 at.% Fe-spin-glass state) and above (18 at.% Fe-re-entrant state) the percolation threshold. The anisotropy of the gamma -rays was monitored in the direction of the external magnetic field, as a function of the external magnetic field B_ext up to 8.5 T and of the temperature in the range 4-40 mK. In Au₈₆Fe₁₄ we have found total misalignment of the Mn and Co spins when B_ext to 0 and a rather easy alignment of Mn and Co with the application of B_ext, which is typical for a spin-glass behaviour. On the other hand, both Mn and Co spins in Au₈₂Fe₁₈ show an alignment (mean value approximately 32 degrees ), even at B_ext to 0, and are turned only slowly by B_ext which corresponds to the re-entrant state. The difference in the Mn and Co results can be interpreted as an indication of the existence of local correlations between the Fe spins, supporting the necessity of using the finite-range model for the re-entrant state.