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Highly porous thin films with columnar microstructures are applicable to many optical, chemical, and electronic devices, and can be fabricated using the glancing angle deposition method for physical vapour deposition onto tilted substrates. In a recent advancement of this method, it was shown that decoupling of the vapour incidence direction from the column growth direction provides significant flexibility to engineer the pore structure of the films. Here we elaborate on the decoupling principle by applying it to chiral thin films with a periodic microstructure, and demonstrating how it leads to improved film uniformity and the elimination of column broadening. We also show the effects of such depositions onto substrate seed layers with intentional defects. Substrate based defects normally transfer to thin films as air filled defects, but here we present for the first time that a simple adjustment of the deposition parameters can invert the normally air filled defects to become solid, evaporant filled defects. This defect engineering capability provides new opportunities for the deployment of glancing angle deposition thin films to photonic bandgap crystals and microfluidic devices.

The concept of magnetic stresses in ideal MHD plasma theory is reviewed and revisited with the aim of demonstrating its advantages as a basis for calculating and understanding plasma equilibria. Expressions are derived for the various stresses that transmit forces in a magnetized plasma and it is shown that the resulting magnetic forces on a finite volume element can be obtained by integrating the magnetic stresses over the surface of the element. The concept is used to rederive and discuss the equilibrium conditions for axisymmetric toroidal plasmas, including the virial theorem and the Shafranov shift. The method had pedagogical merits as it simplifies the calculations, improves the physical understanding, and facilitates an assessment of the approximations made in the calculations.

New measurements and Monte Carlo simulations of the total coefficients eta + for the backscattering of positrons from elemental solids are reported as a function of atomic number Z between 13 and 82, incident energies E from 1 to 50 keV, and incident angles between 0 degrees and 65 degrees . The measurements and simulations show generally good agreement with each other and with the recent measurements of Massoumi et al (1992) and Makinen et al (1992). Both experiment and simulations suggest that the monotonic increase of eta + with Z seen at high E is not observed for E below 10 keV. Where possible, the new results are compared with earlier measurements of electron coefficients.

Molecular dynamics simulations are reported of the melting of small inclusions of rare gases in cavities 10-20 AA in radius in metals containing a few hundred gas atoms. Such systems enable the authors to study the melting phenomena of clusters of atoms constrained by the cavity walls. The authors have looked at the positions of the atoms, the phase diagrams, and the importance of fluctuations for bubbles of different sizes. Melting in these systems takes place over a finite temperature range as in simulations of isolated clusters. The geometric structure factors have been calculated and compared with recent X-ray diffraction measurements of Andersen et al. on krypton inclusions in aluminium. Their inability to reproduce the experimentally observed increase in the transverse diffraction peak width with temperature supports the suggestion of Andersen et al. that this increase is due to a roughening transition of the aluminium facets of the cavities.

The authors present a theoretical analysis of positron trapping into large voids in metals. They show that, even if the trapping rate vanishes as the positron energy goes to zero, a significant amount of trapping of non-thermalized positrons ensures a substantial trapping probability even at very low temperatures.

New measurements and Monte Carlo simulations for 1-30 keV positron implantation in Al are reported. The annihilation lineshape was measured as a function of positron energy E and the thickness of Al overlayers on a glass substrate. Median implantation depths vary as 117(6) E1.64(2) AA, with E in keV. The implantation profiles differ significantly from the widely accepted Gaussian derivative form. The excellent agreement between experiment and simulation confirms the power of Monte Carlo method in positron (and by implication electron) spectroscopies.

The authors have calculated the positron wavefunctions and annihilation rates in voids in aluminium as functions of void radius using density functional theory with the non-local approximation to the positron-electron correlation potential developed by Jensen and Walker (1988). Their calculations show the gradual evolution of the positron state, as the radius increases, from a state extending over the whole of the defect volume in monovacancies to a state localized at the void surface in large voids. They find that for voids with radii larger than about 5 AA the lifetime is independent of void size.

The authors have made a detailed study of the thermalization of positrons implanted into metals, using aluminium as an example. This is done by solving the Boltzmann equation for the positron momentum distribution in a homogeneous medium, allowing the positrons to scatter off electrons and phonons. They obtain both the time-dependent and steady-state solutions. The former gives the time evolution of the positron momentum distribution and the average energy and energy loss rate as functions of time after implantation. The full statistical description of the slowing-down process and the inclusion of both electron and phonon scattering mean that the authors' energy loss rates are more accurate than earlier results, which are considerably lower than the authors'. The steady-state solution gives the momentum distribution from which positrons annihilate. Their formulation allows them to evaluate the influence of non-thermal trapping into defects, such as vacancies, on measurable parameters in positron experiments. The results show that, even if resonances are present in the momentum-dependent trapping rates, the differences between the results from the full calculation and for trapping from a thermal momentum distribution are exceedingly small. Thus the authors' conclude that non-thermal trapping is not important for positron studies of vacancies in metals.

The concept of magnetic stresses as transmitters of magnetic forces is revisited with the aim of showing that it can facilitate calculations and the understanding of magnetic interaction at an undergraduate level. A simple derivation of magnetic stresses in vacuum and magnetized plasmas is presented. As an illustrative example magnetic stresses are used to calculate the forces between parallel conductors.

A comprehensive set of calculations of positron lifetimes in bulk metals demonstrate that experimental lifetimes are reproduced well by a simple first-principles theory in which the annihilation rate (the inverse of the lifetime) is evaluated in a local density approximation. It is shown that the method presented avoids assigning unrealistic enhancement factors for annihilation with valence and d-band electrons, in contrast to previous calculations in which partial annihilation rates with different types of electrons are evaluated separately.

Previous calculations of the angular correlation of annihilation radiation (ACAR) spectra for positron surface states on aluminium have resulted in varying degrees of anisotropy with respect to momentum components parallel and perpendicular to the surface. However, all experiments to date have yielded almost perfect isotropy. The present calculations show that the ACAR spectrum of a positron localised in a surface monovacancy is nearly isotropic, providing some evidence that this defect-trapping state may be responsible for the observations.

Reports on a simple experimental technique which can be used for determination of the plasma where the plasma potential prevails; these ions are accepted by the analyser plate when phi c= phi pl. Part of this technique has been used by Andersen, Jensen, Michelsen and Nielsen (1971) in measurements of the ion velocity distribution function in a Q-device. It is suggested that this technique might be useful in satellite measurements of the ion energy distribution function in the solar wind.