Table of contents

An edge dislocation in an Al₇₀Co₁₅Ni₁₅ decagonal quasicrystal was studied by means of a contrast experiment and a defocus convergent-beam electron diffraction technique. The Burgers vector of this edge dislocation in six-dimensional space was determined to be (¹/₂ 00000). The magnitudes of this edge dislocation in six-dimensional space and physical space are both 0.4 nm.

The authors present the results of low-temperature transport measurements on 'metallic' Si:Sb doping layers with nominal widths of 10 nm, 20 nm and 80 nm. There is clear evidence of weak localization and interaction corrections to the transport coefficients and for a 3D to 2D transition as the layer width decreases-the first observation of these effects in this system. Of particular interest is the fact that the experimental results are in good quantitative agreement with most aspects of the theory, despite the highly disordered and multi-sub-band nature of the samples.

A non-bulk termination with a ( square root 3* square root 3)R30 degrees periodicity was observed on the clean Pt₃Sn(111) surface. X-ray photoelectron diffraction and low-energy electron diffraction showed that this phase is a mixed Sn-Pt topmost layer which is enriched in tin with respect to the bulk composition. The data also indicate that the subsurface region is depleted in tin. The structure of the ( square root 3* square root 3)R30 degrees phase is the same as that of the phase observed in the case of tin deposited on the clean Pt(111) surface.

By growing dysprosium films on Cu(100), the electronic band structure of dysprosium metal has been investigated using synchrotron radiation photoemission. The author has found that 5d electrons become localized with increasing dysprosium coverage. The localization of the 5d band with increasing thickness leads to changes in the 5p multiplet oscillator strengths.

Epitaxial thin films of cobalt ranging from 1 ML up to 10 ML have been grown on a Cu(100) substrate and characterized by LEED. The cobalt is found to grow, adopting a slight tetragonal distortion of its high-temperature FCC phase. Further growth of 5 ML Cu on top of a 5 ML Co film results in an almost perfect Cu(100) surface.

The authors consider the Kapitza conductance due to absorption and emission of phonons by a lossy surface layer which has complex elastic constants and/or complex mass density. They find a series connection of two heat resistances, one between the layer and the solid, and another one between the layer and the helium. A propagator matrix formalism is used to calculate the phonon absorption coefficients, and hence the Kapitza conductance, as a function of angle, frequency and layer thickness in the limit of continuum acoustics with anisotropy. The resulting conductance is large enough to account for the Kapitza anomaly.

Following the method of Gibbs, the equilibrium equations for a solid and various fluids in contact, including capillarity and elasticity, are written for the general case. They are then applied to the example of a thin plate in contact with a drop of fluid. The classical Young's equation is modified.

Thin Si(001) films were studied by transmission electron microscopy (TEM). Arrays of single steps were observed as alternately arranged bright and dark lines. The image contrast was weak where the lines were parallel to an operating reflection vector. Strains of the Si lattice around the steps were calculated using surface stress on the 2*1 reconstructed surface and the results explained the observed image contrast semiquantitatively.

Using a self-consistent pseudopotential technique the authors calculate the distribution of screening charge at the Al(111) and (110) surfaces with perpendicular applied electric fields of up to 5 V AA^-1 (with the metal charged positively). With increasing magnitude of the field the centre of gravity of the screening charge (the electrical surface) moves into the metal, although much less so than for the jellium model at the corresponding density. The major peak in the screening charge density profile narrows as the field is increased, in qualitative agreement with the results for the jellium model at a similar density and range of applied fields.

The authors have developed a theory that views the total energy of an array of multilayers as a weighted average of bulk contribution corrected by a sum of single-interface and interface-interface interactions terms. They demonstrate that they are due to the same quantum-mechanical property of a degenerate Fermi liquid in a localized external potential as that which gives rise to the Friedel charge and magnetic oscillation around an impurity and the corresponding Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. They argue that the recently discovered oscillatory magnetic coupling between magnetic layers separated by paramagnetic metallic layers should be accompanied by elastic interactions that give rise to periodic strain modulations, Delta alpha / alpha , in the spacer layers.

A charge carrier in a layered system experiences an additional Coulomb potential due to its interaction with its own set of images. The latter arise due to the differences between the dielectric properties of the layer materials. The charge can, further, form bound quantum states in its own image potential. The physical situation is examined here for a double heterostructure. It is first shown that for sufficiently large layer thicknesses, depending on the layer materials, the ground state exhibits image-molecular features. This is further substantiated by the numerical solution of the Schrodinger equation which provides predictions for a wide range of layer thicknesses by taking account of the multiple images.

The authors develop a theoretical framework for investigating charge-transfer processes occurring during the scattering of an ion from a solid surface, in the presence of a pulsed laser field. The theory is based upon the many-electron formalism, previously used by their group, to investigate charge transfer based upon purely electronic mechanisms, but it is here extended to include the additional matrix elements needed to describe laser-assisted transitions. The model is applied to several systems with different scattering behaviours, with the conclusion that the effect of the laser field can be expected to be greatest when the target substrate is one with a relatively narrow valence band.

In this paper a transport equation (TE) is derived that matches closely (within the limitation of an infinite target) the transport model in the Monte Carlo code TRIM. Initially, the authors derive a TE that incorporates an arbitrary free-flight path length distribution function and a stopping energy. From this TE a coupled set of integral equations (for spatial moments up to order four) incorporating the liquid free-flight path model used in TRIM is derived. Also, for the gas-like model of the free-flight path length distribution, the equivalence is shown between the new TE and the LSS backward linearized Boltzmann equation extended by Brice to include an intermediate energy.

Perturbed angular correlation, Rutherford backscattering and channelling experiments were conducted to study the lattice location and annealing behaviour of 110 keV hafnium ions implanted into iron single crystals. It was found that a fraction of 11-25% of the implanted hafnium atoms are located at substitutional sites in an undisturbed environment, while about 50% are located at irregular lattice sites. The remaining fraction are located at or near regular lattice sites in a perturbed local environment. Trapping and detrapping of monovacancies by substitutional hafnium atoms at 200 and 250 K, respectively, as well as hafnium precipitation during annealing at 873 K was observed. The vacancy-hafnium binding energy was determined to be 0.17(3) eV.

For pt.I see ibid., vol.5, p.2171 (1993) The authors have used the perturbed angular correlation technique to study the interaction of interstitially diffusing nitrogen atoms with substitutional hafnium atoms implanted in iron. It was found that after post-implantation of 250 eV nitrogen ions at 450 K, isolated HfVN_x complexes with x=1, 2 are formed in the iron matrix. The authors propose a possible geometrical configuration for the HfVN₂ complex. Isochronal annealing experiments showed nitrogen dissociation from the HfVN_x complexes at 773 K. The dissociation energy was determined to be 2.7(2) eV.

An investigation was made of the energy transfer between Eu²⁺ and Mn²⁺ ions in CaCl₂ single crystals under photoexcitation. The studies performed at several impurity concentrations and the measurements of the excitation spectra and lifetimes of the observed europium and manganese emissions in the temperature range 11-300 K suggest that the Eu²⁺ to Mn²⁺ energy transfer process taking place in the authors slightly doubly doped crystals of CaCl₂:Eu:Mn occurs in small complexes of Eu-Mn which are formed in the lattice. The spectroscopic data obtained substantiate the ionic radius criterion proposed by Rubio et al. (1988) to predict pairing between two impurity ions in a solid material between which energy transfer is desired.

The author studies the effects of dielectric screening on scattering rates due to electron-phonon interactions in quasi-one-dimensional quantum-well wires. He considers the interaction of confined electrons with bulk polar-optical and acoustic phonons. Acoustic phonons are examined, both in the deformation potential and the piezoelectric couplings. Screening effects are included via a temperature-dependent dielectric function epsilon _T(q) in the random-phase approximation, by renormalizing the electron-phonon interaction. Numerical results are given for the GaAs system in the quantum-size limit where a lowering of screened electron-phonon scattering rates is found. The effects of screening are most significant for the electron-optical-phonon interaction.

The splitting between the two lowest vibronic states in the T(X)t₂ Jahn-Teller system is calculated using two approximations. The first is analytical and is based on an adiabatic one-sheet WKB approximation in which the splitting is due exclusively to tunnelling. The second method is numerical and uses cubic states appropriate to very strong coupling which have been derived previously by a transformation method. The latter calculation is in direct contrast to other numerical methods which use states appropriate to the weak-coupling limit. The second calculation includes both tunnelling and 'hopping' (non-adiabatic mixing processes). Thus, by subtraction of the results from the two methods, the contributions of tunnelling and 'hopping' to the inversion splitting can be obtained.

The phenomenon of oscillating photoconductivity as a function of photoinjection energy is studied by Monte Carlo simulation in quasi-one-dimensional quantum wire structures. It is demonstrated that the amplitude of the oscillations of photoconductivity may be so large that this leads to a negative absolute conductivity at injection energies that are multiples of the optical phonon energy. These oscillations are associated with inelastic optical phonon scattering, leading to an asymmetric electron distribution function established under conditions of intensive electron photoinjection to the subband bottom or close to energies that are multiples of the optical phonon energy. Simulation results suggest that quasi-one-dimensional quantum wires are ideal for the experimental observation of negative absolute conductivity at low lattice temperatures.

Two-dimensional negatively charged donor centres D^2- (i.e. the states formed by a neutral donor D⁰ binding two extra electrons) in a strong magnetic field with electrons confined to the spin-split zero Landau levels 0 down arrow , 0 up arrow are considered. When the admixture of higher Landau levels is neglected, the energies and eigenfunctions of the doublet S=¹/₂and quadruplet S=³/₂ states are obtained. It is shown that both the doublet and the quadruplet D^2- states, although localized, lie above the ground D^- singlet and triplet states, respectively, i.e. have negative binding energies and, hence, are thermodynamically unstable against separation of an electron.

The effects of the image potential on hydrogenic impurity binding energies in quasi-one-dimensional GaAs/AlAs quantum well wires with a rectangular cross section are investigated. The results have shown that, when the impurity ion image potential is considered, the variations in binding energy are considerable and, when the impurity ion and electron image potentials are considered simultaneously, the corresponding variations in binding energy are small. The results also showed that the image potential is important, especially when the cross section dimensions of the quantum wire become small.

The spectral response of the photovoltaic current has been studied in LiNbO₃ crystals of several stoichiometric compositions and reduction states. The height and shape of the spectra are found to depend substantially on stoichiometry. These dependences are parallel to those exhibited by the corresponding absorption spectra of the same crystals, and provide evidence that the photocurrent spectrum consists of several contributing bands. These results are compared with an analogous analysis of data on Fe-doped LiNbO₃. The photovoltaic transport length has been determined for specific component bands and found to be within the same order of magnitude for the measured stoichiometries and doping levels.

The oxygen 1s and cobalt 2p X-ray absorption spectra of CoO, Li-doped CoO and LiCoO₂ have been measured with 0.1 eV resolution. The cobalt 2p spectra are analysed with a ligand-field multiplet model and the inclusion of chase-transfer effects is discussed. The oxygen 1s spectra are interpreted as transitions to empty oxygen p states and it is concluded that the effects of correlations in the 3d band possibly are too small to be detectable. The symmetries and the electronic configurations of the cobalt ions in the oxides are determined. It is concluded that, in contrast to for example NiO and La₂CuO₄, the doping-induced states are possibly of ¹A₁ symmetry, which would imply that the quasi-particles have spin ³/₂ and are most likely trapped.