Table of contents

Recent advances in LEED theory have made it possible to use intensity/energy spectra to investigate the contents of the surface unit cell. Previously diffraction patterns could yield only the size and shape of the surface cell. The authors report the first structure to be deduced in this manner: that of c(2*2) sodium on nickel. Amongst other things, it is deduced that the spacing between the sodium and topmost nickel layer is 2.9+or-0.1 AA.

It is shown that irradiation of either n or p type gallium phosphide by 2 MeV electrons at 300 K reduces the concentration of boron initially present on gallium lattice sites. Local mode absorption bands due to a defect centre involving a single boron atom have been observed at 849 (¹¹B) and 882 (¹⁰B) cm^-1. It is thought that the boron atom occupies an interstitial site in these defects.

Translational self diffusion in three molecular crystals has been studied as a function of pressure using nuclear spin-lattice relaxation time measurements. Activation volumes are obtained which are consistent with a vacancy diffusion mechanism.

The author shows that the energy derivative of the Dirac matrix for a spherically symmetric potential can be derived in a simple manner. (see also abstr. A11276 of 1972).

The nuclear Overhauser effect observed on the two lines in the proton spectrum of ²⁹Si¹HCl₃ allows the determination of the spin-lattice relaxation time of ²⁹Si. The temperature dependence of (T₁)_29Si is interpreted in term of spin- rotation interaction.

An analysis is given of the fundamental properties of an iterative numerical method, free of any inherent physical approximations, for evaluating electron transport effects in solids. A formulation applicable to both semiclassical and quantum calculations is used. The method is shown to reduce, in effect, to evaluation of the time development of an electron state, and its application to both steady state and time response calculations is considered. Conditions for iterative convergence are established that are less restrictive than those previously supposed.

A numerical solution of the equations describing the acoustoelectric oscillator has been performed. For short oscillators (<30 mu m) there is no correspondence with experimental results. In the nonlinear region, however, the results are in qualitative agreement with what is found in 30-100 mu m long crystals. Reasons for the discrepancy are discussed in view of recent theories.

Some magnetic properties of the Hubbard Hamiltonian in the limit I to infinity are discussed using the single particle Green function and the transverse dynamic susceptibility. Conditions for ferromagnetism in a model density of states are derived in various approximations. Results are compared with those of other authors.

The addition of an anisotropic spin-orbit coupling term to the crystal field Hamiltonian predicts extra contributions to the energies of the electronic states. An estimate of the magnitude of these contributions for Yb³⁺:CaF₂ and Pr³⁺:PrCl₃ predicts a very small effect.

In an attempt to include the effects of charge transfer states into the existing theory for optical transitions in ions embedded in an alkali halide matrix and possessing a closed s² outer electron shell, group theoretical arguments are employed in order to provide the simplest possible generalization of the existing theory whilst introducing the minimum number of new parameters necessary for its description. Expressions are developed which permit not only an experimental estimate of these new parameters, but also show their utility in accounting for known experimental results which appear anomalous in terms of the existing theoretical structure. Finally further experiments are suggested on specified impurity ion systems in which the advantages of the present formulation over that previously adopted can be subjected to experimental investigation.

The energy level matrices associated with transitions of the type d¹⁰ to d⁹s are constructed employing the strong crystal field formalism. The resulting expressions eliminate the difficulties of the formulation given by Knox (1963) at the expense of introducing additional parameters. Means of estimating these are described. Finally arguments are presented which show that the reported characteristics of the spectra attributed to Cu⁺ ions in the potassium halides are consistent with the assignment of this ion to an interstitial position.

The spontaneous emission spectrum in n-type GaAs excited by a 40 kV electron beam at 300 K is theoretically determined for different values of the doping concentration. The procedure of determination is justified through reasonable agreement of the doping dependence of the peak emission energy and the linewidth of spontaneous emission as obtained from the computed cathodoluminescence spectra with that observed experimentally. Moreover, it is shown that the doping dependence of the low energy tail slope of the computed cathodoluminescence spectrum and the appearance of humps on the high energy side of this spectrum beyond a certain level of doping are also in agreement with the experimental observation. An attempt is also made to explain the origin of such humps.

Stress-induced dichroism in the emission spectrum of the CaO F⁺ centre has been used to identify the symmetry of the vibrational modes coupling to the ²T_1u excited electronic state of the defect. The polarization of the main one-phonon sidebands under (001) and (110) stress shows that a peak at a separation of h(cross) omega =205 cm^-1 from the zero phonon line is due predominantly to A_1g modes and a sharp peak at h(cross) omega =302 cm^-1 to a composite of E_g and T_2g modes. A separation of the E_g and T_2g components shows that the frequencies of these modes are equal to within about 15 cm^-1, and that they couple equally strongly to the electronic state. The relevance of this result to the lattice dynamics and Jahn-Teller effect of the F⁺ centre is discussed.

Paramagnetic resonance spectra attributed to six Fe³⁺:Ag⁺ associated centres designated FA(I) to FA(VI) have been observed in CdS single crystals. FA(I) may be described with an orthorhombic spin Hamiltonian H=g₀ beta H.S+D(S_z²-(1/3)S(S+1)) + E(S_x²-S_y²) with S=5/2, g₀=2.018, D=0.52 cm^-1 and E/D=0.17. The x and z axes lie in the 1120 plane, the x axis making an angle of (5+or-1) degrees with the c axis. The other centres are less well defined. The resonance is not photosensitive. The results further confirm that close association occurs between iron and luminescent impurities in CdS.