Table of contents

Non-empirical calculations of the electronic structure of NiO and related compounds have proven to be problematic, in that NiO is an insulator both above and below the antiferromagnetic ordering temperature (T_N approximately 525K) with a gap of approximately 4 eV. This is so despite the partly filled d-band on the Ni ion. Conventional hand theories predict that NiO is a metal for T>525K and a narrow-gap semiconductor or a metal for T<525K. It is argued here that the failure of band theory is due to the use of an over restricted form of the Hartree Coulomb potential, and the use of Koopmans theorem for the narrow, partly occupied 3d band. In this letter the author considers an approach based upon systematic improvements to a valence bond description for the narrow 3d states. He predicts a wide gap insulator for all T and find other results which are in reasonable agreement with experiment.

The authors report the first-order Raman spectra of one-dimensional solid solution ZrS_2.5Se_0.5. The Raman active modes have been correlated with those of ZrS₃ and ZrSe₃.

A phenomenological scaling method is used to obtain exact renormalisation-group transformations for pure and bond-diluted transverse Ising chains at zero temperature. An exact discussion is given of basic critical properties of the two systems, and the exponents characterising the quantum and percolative critical behaviour of the correlation length of the dilute chain are found to be nu lambda = nu rho =1.

The non-local susceptibility is calculated for a semi-infinite, simple cubic insulator with two narrow bands: the conduction ( alpha ) band and valence ( beta ) band. The electrons in the conduction band and the holes in the valence band hop between nearest-neighbour atoms only. The interactions between electron-hole pairs on the same lattice site and on nearest-neighbour lattice sites are also included. As a model for the surface the authors ignore interactions between electron-hole pairs on opposite sides of a plane which bisects the crystal but contains no atoms itself. The hopping integrals of the electrons and holes across this plane are neglected as well. It is shown that the Coulomb interaction between two electron-hole pairs on the same lattice site never produces excitons (bound electron-hole pairs). Excitons are produced by the nearest-neighbour exchange interaction. The equation of motion of the exciton green function is solved analytically. Under suitable conditions, surface excitons localised on the surface of the insulator may be produced.

Hyperfine interaction data from ENDOR measurements on paramagnetic centres in silicon give important information of electron wavefunctions. A comparison is made between data on the positive and the negative charge states of the divacancy. For this comparison a phenomenological description is developed. The difference between wavefunctions of lattice defects like the divacancy, and coulombic centres like substitutional single and double donors leads to the supposition that in lattice defects electrons experience a potential which results from an extended lattice distortion. The data of the divacancy wavefunctions are related to other experimental observations on the divacancy. Correction terms to the dipole-dipole part of the hyperfine interaction of an LCAO scheme are elaborated. They can account qualitatively for experimental deviations from the simple LCAO model. Quantitatively, the corrections do not allow for further identifications of hyperfine interactions with next-nearest-neighbour lattice sites.

The problem of resonant energy transfer (RET) is addressed for diluted rare-earth and transition-metal ions in a solid. Such processes involve the redistribution of excitation among sites with degenerate states. A new spectroscopic double selection technique is presented which is capable of detecting RET for inhomogeneously broadened multilevel systems. The method has been applied to LaF₃:Pr₃⁺ for concentrations from 5 to 80 at.%. The experiments indicate that no significant RET occurs in this system. Reasons for this result are discussed, and other interesting materials to which this technique may be applied are mentioned.

The theory of the vibrational or phonon Raman spectrum of molecules or solids in the nonresonant regime is formulated for systems with electronic degeneracy. A selection rule is established, extending that of Child (1963), previous disagreement on this selection rule is reviewed. Symmetric and antisymmetric parts of the scattering amplitude are proved to combine incoherently, as observed by Kiel and co-workers (1969). Tables of Raman scattering contributions in nondegenerate systems for oriented scatterers by Loudon (1964) and for orientationally averaged scatterers by McClain (1971) are generalised to systems with electronic degeneracy. This should greatly facilitate the assignment of spectral lines in systems with Jahn-Teller or Kramers degenerate ground states, and constitutes the first step in determining a strategy for the economical extraction of the information available through Raman spectroscopy. An alternative recoupling is considered following work in quantum beat spectroscopy. This will prove advantageous when only linear polarisation is used, or when particular intermediate state symmetries are important.

The author studies the effect of crystal thickness on the vibrational properties of films and bicrystal interfaces by the Green function method. An expression for the surface Green function is obtained which depends on the crystal thickness by using a short-range interaction potential between the first and second neighbour atoms. The cases of acoustical and optical-like phonons are discussed.

The authors have measured the total hydrogen concentration (H) and the hydrogen concentration in SiH (H₁), SiH₂ (H₂) and other sites (H'). From uniform depth profiles of total hydrogen concentration they conclude that hydrogen concentration is not limited by diffusion. They report here the variations of (H), (H₁), (H₂) with annealing temperature for substrate temperature T_s=150 degrees C, 190 degrees C and 250 degrees C and deposition rates ' nu ' ranging from 20 to 120 AA min^-1. The authors lose hydrogen only above the annealing temperature at which hydrogen saturates the Si matrix; (H) saturates at a value (H_s) which is independent of T_s and nu , at each annealing temperature T_A. Annealing induces simultaneous variations of the hydrogen concentration on all the sites. All these results are interpreted by a model according to which the hydrogen is linked to the a-Si matrix defects, filling first the deeper ones and then the more shallow defects. According to this model the total hydrogen concentration is limited by the total Si matrix defect concentration.