Table of contents

Results on the fluorescence lifetimes and relative fluorescence yields of ultraviolet excited type I diamonds in the temperature range 80-300 K are presented and discussed.

The author presents the results of calculations of the energy of formation of Schottky defects in some alkali halides and alkaline earth oxides. It is shown that the basic Born-Mayer model of an ionic solid, which treats each ion as a point polarizable dipole, remains very adequate for defect energy calculations so long as the repulsive potential is chosen to give correctly the static dielectric constant of the substance it represents. The derived values for the alkali halides are in good agreement with experimental ones, particularly for those substances for which the latter are well documented, namely the potassium halides.

A model Hamiltonian with 2-point, 4-point,..., 2n-point infinite-range interactions chosen in a particular way is introduced and the complete thermodynamics is obtained exactly in an elementary manner. The critical exponents alpha '=1-1/n, beta =1/2n, gamma = gamma '=1, delta =2n+1, Delta '=1+1/2n are derived and a minor violation of scaling is found.

The nature of the spectrum of the normal processes collision operator for phonons is studied in the framework of the theory of Hilbert space. It is shown that in the limit V to infinity the continuous part of the spectrum extends down to the value zero. A rigorous proof regarding the absolutely continuous part of the spectrum of the collision operator in the gas model approximation is given. The only eigenvalue is zero and the continuum covers the range (0, infinity ).

The feasibility of using the CNDO method for the calculation of the band structure of crystalline solids has been investigated by means of calculations on model systems comprising various configurations of lithium and fluorine atoms. These include a one dimensional linear chain and various planar and cubic structures containing up to 27 atoms. These results show that the typical band structure associated with ionic crystals is reproduced and that a comparison between these results and those of previous experimental and theoretical studies gives encouragement as to the applicability of this method to aspects of the properties of crystalline solids.

For pt. III see ibid, vol. 4, 2611 (1971). Electron-phonon interaction effects in tunnelling junctions are considered in the framework of the formalism proposed in Pt. I (see abstr. A52955 of 1971). This formalism is simplified and then used to study electron-phonon interaction in metal-insulator-metal junctions. The interactions inside the metallic electrodes are shown to yield a complicated mixture of dispersive and dissipative lineshapes in the second derivative d²I/dV². Such a structure originates in a destructive interference which also leads to a strong reduction in the magnitude of the effect. On the other hand, the interactions in the insulator yield dissipative lineshapes, due to the opening of a new inelastic tunnelling channel. Such a contribution is reinforced by the absence of destructive interference inside the insulator. It is proposed that the observed structures arising from electrode phonons are due predominantly to the penetration of metallic phonons inside the insulator, rather than to selfenergies localized in the metal itself.

The authors extend the theory of impurity band hopping conduction to lower temperatures. The theory of Miller and Abrahams (1960) is critically examined and found to be unsatisfactory, especially at low temperatures. Alternative approaches based on percolation theory are discussed in detail; the successes and difficulties of simple analytic estimates are noted. Finally attention is draw on to the importance of the transition region between high and low temperature regimes.

A method of solution of the steady state Boltzmann equation for small fields using a spherical harmonic expansion of the nonequilibrium part of the distribution function has been derived for acoustic phonon scattering in nondegenerate semiconductors. The method is easy to apply for arbitrarily complicated scattering functions and has some other advantages over previous treatment. It is possible to extend the method to include other scattering mechanisms and the effects of a magnetic field. Applications to silicon and germanium are discussed. As a by-product, a new sum rule for Clebsch-Gordan coefficients is derived.

The authors study a system of itinerant, uncorrelated, electronic states forming a set of conduction (and valence) bands weakly hybridized with a system of highly correlated, ionic-like, electronic states. The latter are localized around lattice sites and correlated so as to give ionic configurations with a well defined total angular momentum J. The effective coupling that arises between the localized moments as a consequence of hybridization is studied. The result, in fourth order perturbation theory, shows that two 'different' interactions occur: a Ruderman-Kittel-Kasuya-Yosida interaction, characteristic of metals, and an Anderson type superexchange, characteristic of insulators. Which of the two dominates depends on the number of conduction electrons, position of the Fermi level and position of the localized levels with respect to the Fermi energy. In general both have to be included. Three simple models are examined: a free-electron-like conduction band (with an application to metallic Ce), free-electron-free-hole-like conduction bands, and an s-like BCC tight binding band. For this last example the equilibrium configuration of the moments is computed from the classical 'Heisenberg' energy. Although no specific application is made to magnetic semiconductors, it seems likely that many compounds in this group (Eu chalcogenides, for instance) could serve as examples of the electronic system treated.

The formula of Herbert and Jones (1971) relating the distribution of eigenvalues to the range of localization of an eigenstate for the Anderson model in one dimension is discussed. An explicit formula for the localization distance is given for Lloyd's model in one dimension. The formula, which is essentially a dispersion relation is generalized to the case of the Schrodinger equation in one dimension.

The thermoluminescence spectrum and phosphorescence decays of Harshaw KCl samples gamma irradiated at room temperature have been studied up to 400 degrees C. For the six peaks found, the order of the kinetics of recombination, the pre-exponential factor and the activation energy have been obtained. It has been observed that the area under the thermo-luminescence curve is always proportional to the F centre concentration in the sample before heating. It was also found that there is an annealing step of F centres corresponding to each thermoluminescence peak, when the temperature of the sample is raised at a constant rate. It is concluded that the F centres play the role of recombination centres in the annealing process, the interstitials being the mobile entities moving towards the recombination centre. At some stage in the process light is emitted. The spectral analysis of this light shows that the emission band has its maximum at 440 mu m for all the thermoluminescence peaks.