Table of contents

The magnetic structure of an exchange-enhanced ferromagnetic PdFe alloy was determined by polarized and unpolarized neutron scattering measurements. The polarized neutron data indicate that the magnetic moments are accompanied by an oscillatory spin component perpendicular to the ferromagnetic moments. Further measurements of unpolarized neutrons support the assertion that the oscillatory spin component can be described by a rotation wave.

Platinum films in the thickness range 20-500 Å have been sputtered onto sapphire substrates at different substrate temperatures . The structural properties of these films have been studied by high-resolution x-ray scattering techniques. Information about the total thickness of the film and about the surface and interface roughnesses has been determined by means of x-ray specular reflectivity measurements. The films sputtered at have shown Laue oscillations about the Pt(111) Bragg peak. The average thickness of coherently stacked lattice planes has been compared with the total film thickness , obtained from the reflectivity measurements. The epitaxial relationship between the platinum film and the sapphire substrate has been determined using in-plane x-ray diffraction. It was demonstrated that Pt epitaxial films include two in-plane orientations related by a rotation. Both anisotropic compression in the direction and anisotropic expansion in the [111] direction were observed for these films. A correlated variation of some structural parameters with the substrate temperature was found.

We have constructed an effective model Hamiltonian in the Hubbard formalism for the Cs/GaAs(110) surface at quarter-monolayer coverage with all of the parameters extracted from constrained local-density-approximation (LDA) pseudopotential calculations. The single-particle excitation spectrum of the model has been calculated using an exact-diagonalization technique to help determine the relevant interaction terms. It is shown that the intersite interaction between the nearest-neighbour Ga sites plays the key role in determining the insulating nature of the system and must be included in the model, in contrast to suggestions of some previous work. Our results show that a reliable mapping of LDA results onto an effective model Hamiltonian can be achieved by combining constrained LDA calculations for the Hamiltonian parameters and many-body calculations of the single-particle excitation spectrum for identifying relevant interaction terms.

We have performed an experimental investigation of the angular distribution of LEIS spectra and the charge fraction for the 4 keV system at 6 and of incidence angle. A numerical model is developed in order to assure a quick adjustment of several parameters. The influence of the electronic stopping power, the charge exchange in close collisions, the Auger neutralization, the expansion of electronic density outside the bulk and the image charge effect are studied and discussed. A good agreement is found between the experimental results and the numerical calculation.

In this paper, we studied the photonic properties of dielectric fibres woven into three-dimensional (3D) structures. Such fibres can be fabricated on the micrometre scale, and hence the gaps are in the far-infrared to the infrared regime. The vector-wave transfer matrix method is applied to evaluate the photonic band structures. We have also employed the constant-frequency dispersion surface scheme to investigate the development of a full band gap. Such a 3D absolute gap is observed in a rectangular lattice, but at a fairly large dielectric constant for the fibres. Ways to improve on this have been suggested. Our study indicates that woven structures are promising materials for realizing the 3D photonic insulator in the infrared regime.

Motivated by the unexpectedly strong influence of the Te atoms on the structural and bonding properties of the transition metal tellurides, we have performed a detailed study of . Experimentally, this comprises a crystal structure determination as well as electrical resistivity measurements. The former analysis leads to an accurate update of the structural data reported in the 1960s, while the latter provides evidence for the mainly electronic character of scattering processes leading to the electrical conductivity. In addition, the electronic properties of have been calculated using the TB-LMTO method. The partial density of states reflects the close connection of the Ta zigzag chains and the Te-Te network. This finding explains the charge transfer in the system in a rather simple way. The orthogonal-orbital character of the bands proved the existence of -bonds. The Fermi-surface study supports the interpretation of the experimental resistivity measurements.

The crystallographic properties of the compounds and have been investigated by time-of-flight neutron diffraction. A Rietveld profile refinement of the data shows that these compounds crystallize in the hexagonal -type structure and that Ni atoms have a greater preference as compared with Cu atoms for occupying the 2e dumb-bell site. A site preference of Cu atoms for the 2c sites is observed. Magnetic measurements made on these compounds show that the trivalent character of the Ce atoms increases with Cu concentration.

We study the electronic eigenstates on two- and three-dimensional quasiperiodic lattices using a tight-binding Hamiltonian in the vertex model. In particular, we analyse how a quasiperiodic lattice influences the decay form and the self-similar structure of the wave functions. The investigation of the earlier-suggested power-law localization is performed by calculating participation numbers and the structural entropy of the wave function. We also present results for the multifractal analysis of the eigenstates by a standard box-counting method. The eigenstates of the two-dimensional Penrose lattice display multifractal character. In contrast, most eigenstates of the three-dimensional Amman-Kramer lattice are shown to be extended; localized states occur only in the band tails, where the spectrum appears to be fractal.

The 2D system of electrons confined to the lowest Landau level is described using a representation with a density matrix depending both on electron and hole coordinates. Condensation of the electron system into a fractional quantum Hall state is associated with clustering of particle and hole coordinates. The correlation of particle and hole coordinates is studied and ground-state wave functions are derived for . These wave functions prove to be accurate for the cases studied, i.e. for and 3/5, and are identical to pair wave functions (Morf R, d'Ambrumenil N and Halperin B I 1986 Phys. Rev. B 34 3037) for and 2/5.

The Ioffe-Regel criterion predicts the existence of a metal-insulator transition in a film series when the parameter satisfies the criterion ; here is the Fermi wavenumber and is the elastic mean free path of the carriers. According to this criterion, films having are metallic, while films having are insulating. We experimentally observe the metal-insulator transition in amorphous indium oxide films at . The values of were calculated from room temperature resistivity and Hall voltage measurements, while the metal-insulator transition was determined from low-temperature resistivity data using the `w'-criterion of Mobius and of Zabrodskii and Zinov'eva.

The self-consistent Hartree-Fock approximation and the deformable jellium model are used to describe the ground state of the two-dimensional electron gas. Improved precision in the calculations allows us to confirm the existence of a stable corrugated state at finite densities. This is strongly corroborated by an extrapolation of our results to the low-density region. A positive bulk modulus is obtained in this region. A comparison with experimental data for the melting point and our model calculation is made and agreement is found within a factor of 2.

Magnetoconductivity in highly doped (with and ) poly(p-phenylenevinylene) (PPV) has been studied. Oriented samples with different stretching ratios were measured. A room-temperature conductivity of is achieved in highly oriented PPV, depending on the dopant. is metallic and has a strong positive magnetoconductance while is insulating and its magnetoconductance predominantly negative and large. In the magnetoconductance is analysed with the localization-interaction model for disordered metals. The positive magnetoconductance of highly oriented is due to weak localization. Its size, temperature dependence and anisotropy is discussed. With a lower stretching ratio we find negative magnetoconductance.

Silver antimony diselenide thin films were prepared by a thermal vacuum evaporation technique onto quartz and glass substrates kept at room temperature . The as-deposited films were amorphous and transformed to a face centred cubic (FCC) polycrystalline nature with the lattice constant on post-deposition annealing above 423 K for one hour in argon atmosphere. The optical constants (the refractive index n, and the absorption index k) of the films were determined for several samples of different thickness (180 nm-270 nm), using spectrophotometric measurements of the transmittance T and reflectance R at normal incidence in the spectral range 500-2500 nm. These constants were also determined for preannealed films, (polycrystalline). The obtained values of both n and k were independent of the film thickness within the above-mentioned thickness range. The refractive index data fitted a single-oscillator model with high-frequency dielectric constants increasing from 13 for the amorphous films to 15 for the crystalline films. It was found that the high-frequency dielectric constant has the same values as the lattice dielectric constant . The analysis of the spectral behaviour of the absorption coefficient in the intrinsic absorption region revealed the existence of an indirect allowed optical transition with energy gap 1.2 eV for amorphous films and 1.03 eV for the crystalline films, respectively.

A detailed study has been carried out on the optical properties of intrinsic colour centres, created in LiF crystals under irradiation with low-penetration 3 keV electrons. The kinetics of the production of F and M centres, determined from optical absorption measurements, exhibits a typical three-stage structure and an evident saturation at high doses. The quantitative evaluation of the kinetic parameters is in excellent agreement with the predictions of the most comprehensive theoretical model. A comparison of the effects of irradiation at different temperatures, determined from absorption and photoluminescence spectra, allows one to deduce the conditions for preferential formation of or defects in LiF crystals.

Field emission from a metal nanotip is studied, because of its relevance to the modelling of the Fresnel projection microscope. This study includes the development of a new numerical approach to the axially symmetric Schrödinger equations.

The valence electronic structures of organopolysilanes having symmetrical alkyl side-chains, i.e., poly(di-n-alkyl silanes) , have been systematically studied by x-ray photoelectron spectroscopy (XPS) for the first time. The specimens used in this work are poly(di-methyl silane) , poly(di-ethyl silane) , poly(di-pentyl silane) , poly(di-hexyl silane) and poly(di-decyl silane) . By comparing the XPS spectra of the poly(di-n-alkyl silanes) with those of normal alkanes, it is found that the spectral features are strongly affected by two factors, i.e., the carbon 2s and 2p electrons in the side-chains and the silicon 3s and 3p electrons in the backbone. The shape of the spectrum from 0 to 5 eV, which is strongly affected by the structure of the valence band edge, shows no significant change with a change in the number of carbon atoms in the alkyl substituents. On the other hand, the splitting of the carbon 2s orbitals in the side-chains strongly affects the shape of the spectrum in the region from 12 to 20 eV. The results are also discussed on the basis of the theoretical calculations as well as the ultraviolet photoelectron spectrum reported previously.

It is known that surface formation energies obtained as differences between slab energies and an independently determined bulk energy will diverge linearly with the slab thickness. A recent paper by Fiorentini and Methfessel presented `a solution to this problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies'. Although their work is correct, the solution they propose is not new. In this comment, we provide some of the historical background neglected by Fiorentini and Methfessel.

We reply to the preceding comment by Boettger and coworkers.

There is a typographical error in equation (1) of this article, the correct equation should read:

The author is grateful to Serena H Chung and Richard D Braatz of University of Illinois at Urbana-Champaign for bringing this to his attention.

There were two typographic errors in table 2, page 2908, of this article: the value of B₁₁ for Ba/Ca(0,0,0) at 150 °C should read 0.62(16) and for Ti(0.5, 0.5, z) at 150 °C should read 0.48(20).