Table of contents

We report the first direct measurements of spin/orbital magnetization ratios in a ferromagnetic material using X-ray diffraction. The simple technique, based on diffraction of elliptically polarized white synchrotron radiation, produces spin/orbital ratios that are independent of the precise photon polarization. Results from HoFe₂ are shown to be generally in very close agreement with relativistic spin-polarized band calculations and known moments, but the measured orbital moment at the iron site is anomalously large.

This is an investigation into the effects of laser surface melting (LSM) on martensitic stainless steel, commonly used for steam turbine blades. The principal result is that the magnitude of overlap between neighbouring melted tracks determines whether or not the surface residual stress is tensile.

We have studied the field dependence of the hyperfine splitting of holmium as a dilute substituent in a lamina of dysprosium prepared by molecular-beam epitaxy. The measurements were made by spin-echo NMR at temperatures below 2 K and in fields up to 8 T. In fields above 3 T we obtain sharp seven-line spectra which are characteristic of ¹⁶⁵Ho in fully polarized holmium ions in a ferromagnetically ordered host medium. The resolution is markedly superior to that obtained in bulk rare-earth alloys and has enabled us to resolve features hitherto unmeasurable. The form of the zero-field spectrum suggests that helimagnetic and ferromagnetic phases coexist in the zero-field-cooled material. Magnetization measurements support this interpretation.

It is shown that the analysis of surface layers by neutron reflection interferometry is considerably enhanced by performing the reflectometry with phase information. We discuss two methods of providing such information. One method involves physically adding an extra reference layer, whose amplitude and phase are known theoretically. The other uses the Lloyd's mirror configuration, in which a directly propagating ray that interferes with the reflected ray supplies the phase information. The methods have much in common with holography.

We have performed total-energy pseudopotential calculations for a variety of structures for Al adsorbed on the GaAs(110) surface. For the coverage of one monolayer we find only very small differences of the adsorption energy for a number of competing structures. In contrast to recent LCAO calculations, we find all these structures to be either metallic or to have only a very small band gap. Total energy calculations are also made for a Ga-Al exchange reaction and for different structural models for half a monolayer coverage including a 2*1 reconstruction. In all cases we find the adsorption energy to be less than the cohesive energy of bulk Al in FCC structure or the energy gain due to an exchange reaction, indicating the tendency for cluster formation, as observed experimentally, or penetration into the substrate. By means of density-of-states calculations it is shown that the Schottky barrier is already formed for the ordered half-monolayer case and is completed for one monolayer of Al on GaAs. The band gap is found to open for an asymmetric dimer in a 2*1 reconstruction which may explain recent experimental observations.

Bandgap radiation in the presence of gaseous oxygen induces surface and selvedge photo-oxidation of amorphous GeS₂. The phenomenon involves preferential oxidation of the Ge sites and may be understood in terms of the structural and electronic properties of GeS₂. Such photo-oxidized films exhibit very different properties towards the subsequent deposition of Ag, as compared with untreated films. The inverse system (GeS₂ deposition on Ag) exhibits markedly different interfacial properties, not previously recognized: this could have significant implications for the interpretation of bulk photodiffusion experiments.

The void formation energies in simple metals are calculated in the stabilized jellium model. The total energies of stabilized jellium spheres mimicking small clusters of simple metals are determined. The electronic structures are solved in both cases self-consistently within the local density approximation for electron exchange and correlation. The planar surface energies and the curvature energies are extracted from the results. The stabilized jellium model is shown to give a physically meaningful description of planar surfaces as well as surfaces with positive or negative curvature. The results for voids and clusters are discussed using the so-called liquid drop model and its generalization. They are used to estimate edge and step formation energies.

The dependence of electrical resistivity of Cu/Mn double-layered films, with Cu base layer thicknesses of 16.7-26.5 nm and Mn overlayer thicknesses of 1.5-7.5 nm, on the temperature from 100 to 330 K is studied. The most striking behaviour observed is that the temperature-dependent resistivity of the Cu/Mn films, the Mn overlayers of which have thicknesses greater than 5.0 nm, exhibits a well defined resistivity anomaly, while the resistivity of samples which have thicknesses smaller than 3.5 nm increases linearly with increasing temperature. According to our analysis the deposition of very thin Mn overlayers onto copper base layers causes an increase in the surface scattering of the Cu/Mn double-layered films with respect to those of the uncovered copper base films. No interface scattering could be detected in all the samples studied.

The electron gas confined in GaAs/Al_xGa_1-xAs quantum wells delta -doped in the barriers has been self-consistently studied in an effective-mass model. In agreement with experimental evidence, our calculations show how the limitations in 2D electron density found in modulation-doped structures can be overcome with the delta-doping technique. The role of different structural parameters of these systems in the practical outcome is also discussed.

Using the empirical tight-binding force method the bond lengths and lattice constants are calculated for unstrained Ge_xSi_1-x alloys and strained Ge_xSi_1-x/Si(001) and Ge_xSi_1-x/Ge(001) alloy layers. For the unstrained alloys, the results show that the lattice constants generally obey Vegard's rule and the bond lengths are approximately independent of the alloy composition, agreeing with experiments better than other theoretical models do. For the bond lengths in the strained alloy layers, our results are generally consistent with the results calculated by other theoretical methods for the strained superlattices. For the longitudinal lattice constants in the strained Ge_xSi_1-x/Si(001) alloy layers the result agrees quite well with that calculated by elastic theory.

For each of the compounds MnTi₂, FeTi₂, CoTi₂, NiTi₂, PdTi₂, PtTi₂ and CuTi₂, the differences between the total energies of the compounds in the MoPt₂-, MoSi₂- and NiTi₂-type crystal structures are calculated. Trends of structural energy differences are analysed by performing 'frozen-potential' energy difference calculations and site-projected-energy analysis. Our analysis reveals that the Ti sites with icosahedral symmetry determine the structural stability in transition metal (TM)-Ti₂ compounds. A connection with the stability of quasicrystal models is established.

The ferroelastic phase transition in (NH₄)₄LiH₃(SO₄)₄ at 232 K has been studied by birefringence measurements in the temperature range 190-300 K. The temperature dependence of the birefringence can be well described by the Landau theory for a proper ferroelastic phase transition of second order. The domain structure is compatible with the symmetry reduction from the tetragonal to the monoclinic phase.

Phase diagram calculations (isothermal sections) of three ternary alloys (Ti-Al-Mo, Ti-Al-Nb and Ti-Al-W) are performed from a knowledge of experimental binary phase diagrams. The CPA-GPM theory has been used to write the total free energy within the approximation of the CVM for the entropy. The parameters used to calculate the total free energy have been obtained by fitting the binary phase diagrams.

A computer simulation study of the lattice dynamics and vacancy migration of BCC Fe and in BCC Zr is presented. The results obtained are in good agreement with experiments. Special attention is paid to the temperature behaviour of the T_l 1/2 (110) phonon mode. This mode in Zr is shown to be related to the beta -to- alpha phase transformation as well as to the curvature of the Arrhenius plot. A phonon-enhanced mechanism with a temperature-dependent vacancy migration enthalpy is proposed as an explanation of 'anomalous' self-diffusion in BCC metals.

A d orbital in an octahedral (O_h)-site-symmetry crystal is proposed by considering the ligand charge penetration. The quantitative relationship between the reduction in the Racah parameters B and C and crystalline parameters is established by using a simple point-charge model. The d orbital is applied to calculation of the spectrum of Fe³⁺ ions in plumbojarosite. The calculated results are in good agreement with experiment.

Mesoscopic conductance fluctuations in a 2D electron channel due to correlations between classical boundary scattering processes are discussed. The contribution to conductance provided by the correlations is calculated and analysed; it is a consequence of the inherent roughness of the boundary. As a result stochastic oscillations for a weak magnetic field are exhibited. In the absence of bulk scattering the stochastic behaviour has a fractal structure, the fractal dimension being 1.5. It is shown that at higher temperatures classical mesoscopic fluctuations, discussed here, dominate over the universal conductance fluctuations. Unlike the universal conductance fluctuations, these classical ones are non-universal and reveal a dependence upon the geometry of a sample. Such non-universal behaviour of the classical conductance fluctuations is consistent with recent experimental data observed for ballistic conductors.

It is shown that the protein molecule may play a catalytic role to facilitate electron transport between the donor and acceptor molecules attached to it. The proton is transported along the backbone hydrogen bonds of alpha -helices and involves keto-enol transitions. After the proton is transferred, leaving behind an enol chain instead of the original keto chain, the protein may return to the keto chain, making the injected electron transfer in the backward direction. With such a resetting mechanism, this is a viable possibility for repeated electron transport along the alpha -helical protein molecule.

To obtain qualitatively different amorphous carbon networks we carried out molecular dynamics simulations at different mean atomic densities using the Tersoff potential. Differences among the radial distribution functions obtained as well as among the ratio of threefold- and fourfold-coordinated atoms according to the mean densities are discussed. The electronic stability of the models is investigated by means of their total densities of states which are characterized by significant defect bands. These bands are ascribed to the uncorrelated free-atomic p orbitals occurring in the models. The effect may be quantified by a simple pi bonding analysis which estimates the relative number of non-bonding and of bonding and antibonding pi states by calculating the number of unbonded hybrid orbitals and topological pi defects. For a justification, a number of local densities of states are calculated.

The coupled-cluster method is used to obtain the ground-state energy of the isotropic Heisenberg-biquadratic quantum spin-one chain as a function of the ratio of the magnitudes of the two terms in the Hamiltonian. Two different model states are used which are expected to be valid in different regimes. In both cases we use simple approximation schemes to obtain numerical results for the ground-state energy which are compared with results of exact diagonalizations of short chains. For both cases we are able to incorporate some of the long-range correlations explicitly, using the so-called full-SUB2 approximation schemes, and this leads to evidence of phase changes at certain points. These are discussed in the light of known and conjectured phase transitions in this system.

A new series of ternary uranium intermetallics U₂TSi₃ (T identical to Fe, Co, Ni or Cu) has been discovered. All these phases crystallize in the hexagonal AlB₂-type structure, except for U₂CuSi₃ which is of the tetragonal alpha -ThSi₂ type. The magnetic properties of the new compounds were investigated by means of DC magnetization and AC susceptibility measurements. The results show all the main characteristics of a re-entrant spin-glass behaviour at low temperatures in all the ternaries studied. Owing to some structural properties of these compounds they could be classified as non-magnetic atom disorder spin glasses.

Magnetoresistance ( Delta rho / rho ) measurements for Au₈₇Fe₁₃ are reported in the temperature range 4.2-200 K and for magnetic fields (H) up to 45 kOe. The remarkable feature of the experimental observations is the rapid decrease of mod Delta rho / rho mod below 15 K at low fields. This behaviour is associated with the breaking up of large clusters into smaller ones as a result of the dominating RKKY interaction in the system at low temperatures. The observed behaviour corroborates the neutron depolarization studies carried out earlier.

EPR measurements at 300 K and 1.6 K have been made on CsMgCl₃ crystals doped with Cr³⁺ and Cr²⁺ and co-doped with Li⁺ using X- and Q-band spectrometers. Together with signals from the pair centre, signals from three trigonal Cr³⁺ centres (I, III, IV) and a monoclinic centre (II) are observed from the as-grown crystal. Accurate spin-Hamiltonian parameters at 300 K and 1.6 K are obtained for all the centres using the matrix-diagonalization method. The signs of the b₂⁰ parameters have been determined by the depopulation effect at 1.6 K. From a comparison of b₂⁰ on the basis of the ligand-distortion model for the centres I, III and IV and intensity change by x-irradiation, the trigonal centre with b₂⁰=1049.4*10^-4 cm^-1 is ascribed to the uncompensated Cr³⁺ ion and that with b₂⁰=-2807*10^-4 cm^-1 is ascribed to the Cr³⁺ ion associated with the nearest Mg²⁺ vacancy. From a spin-Hamiltonian separation analysis the monoclinic centre II is ascribed to a Cr³⁺ ion associated with the nearest Cs⁺ vacancy.

We seek to explain the large T=0 intercept of the normal-state NMR relaxation rate of Cu nuclei in cuprate superconductors by T=0 local quantum fluctuations in the S= 1/2 Heisenberg model. Good quantitative agreement is obtained at both low and high T with published experimental results.

Visible and infrared absorption spectra of U⁴⁺ in UGeO₄ are described and interpreted in terms of the standard parametrization scheme. One of the two models considered is consistent with available interpretations of other uranium germanates and silicates. The second one originating from theoretical estimation of the crystal-field effect permits exceptional properties of the compound under consideration, in particular, an untypical assignation of the electronic energy levels and values of the crystal-field parameters in comparison with other tetragonal oxides.

The temperature dependence of the luminescence of the ZnTe:O isoelectronic trap is presented for different excitation energies and is interpreted within a simple model. Interactions that are responsible for the temperature dependence of the lifetime at different excitation energies are discussed.