Table of contents

The use of the ab initio Hartree-Fock local orbitals method in solid state computations has been limited primarily to the cases of simple solids such as alkali halides, solid rare gases and the ceramics. This usage has been mostly limited to the study of energy band structures and to the case of impurities and point defects in these systems. The only exceptions to these statements are energy band calculations for solid methane and calcium, and a study of the edge dislocation in RDX. In this report, techniques to extend the utility of the local orbital methods for the determination of geometry are explored, with a particular emphasis on the study of molecular solids. The method may be extended to include elevated pressures and also allows the study of point or extended defects and impurities simultaneously. It is also possible, using previously defined methods, to obtain the energy band structures for such systems. In this initial report a study of the equilibrium lattice properties of solid methane is made.

The electronic structure of diamond-like amorphous carbon is investigated using a tight-binding molecular dynamics method. The study shows that bonding and antibonding of pi -states associated with pairs and quartets of threefold 'defected atoms' embedded in the fourfold matrix yield a band gap of about 2 eV as observed in optical absorption experiments. These states are in general very localized. However, coupling between the threefold 'defects' can produce delocalized states near the band gap, which is essential for electronic conductivity in the material.

We present here a computationally feasible and fast technique for obtaining the electronic structure of random alloys which allows us to incorporate effects like clustering, short-range order and off-diagonal disorder arising out of size mismatch and consequent lattice distortions. The method combines the augmented-space technique with the recursion method and the tight-binding LMTO. AgPd alloys are studied to illustrate our procedure.

We have studied the electronic and structural properties of the so-called endohedral complex Na⁺@C₆₀ which consists of a C₆₀ molecule with a sodium ion trapped inside. Based on the Su-Schrieffer-Heeger (SSH) model and the analysis of physical pictures of this kind of complex, we have introduced two reasonable terms into the Hamiltonian to describe the interaction between the ion and the C₆₀ cage. Using the simplex optimization method, we have calculated the lattice distortion and its effect on the energy potential curve. Our results show that this effect, which has been neglected by other first-principles calculations, is very large and important. We discuss the physical meaning of our results.

Single-crystal and powder X-ray diffraction experiments, together with Raman spectroscopy, show that the 'hexagonal perovskite' CsCuCl₃ undergoes a phase transition at pressures between 1.65 and 3.1 GPa. The transition pressure is strongly dependent upon the form of the sample, being lower in powder specimens than in single crystals. The transition is first order in character and is accompanied by a 1.8% volume charge and a reduction in the length of the c-axis from the six-layer repeat of the room pressure phase to a two-layer repeat. These observations, together with the change in Raman modes at the transition lead us to conclude that the high-pressure structure contains statically disordered Jahn-Teller-distorted CuCl₆ octahedra, in contrast to the known high-temperature phase, which contains dynamically disordered distorted octahedra, and the low-pressure, ambient-temperature structure, which displays an ordered array of distorted octahedra.

The moments method was shown to be very useful for determining the linear response of very large harmonic systems. In the presence of anharmonicity or electron-phonon coupling, the phonon self-energy appears in the one-phonon propagator. The phonon frequency is then renormalized and the system is damped. In this paper, we show that it is possible to extend the moments method to compute the response of these damped systems. The method involves introducing a two-dimensional density functional that can be calculated by the moments techniques and which allows the computation of the response function. We illustrate these results via applications to several models: systems with constant damping, systems of coupled relaxation modes and oscillators, and systems with electron-phonon coupling.

We describe results of Raman and infrared (IR) studies of single crystals of pure C₆₀, CS₂ solvated C₆₀ (C₆₀1.5CS₂), (BEDT-TIF)₂ C₆₀ (BEDT-TIF or ET is bis(ethylenedithio)tetrathiafulvalene) and C₇₀. We find that changing the crystalline environment of C₆₀ has little effect upon the vibrational frequencies of the intramolecular vibrations but has some effect on relative intensities of peaks. By comparing our IR and Raman spectra using the published inelastic neutron scattering and surface enhanced Raman data we have been able to identify the complete set of 46 fundamental intramolecular vibrational frequencies of C₆₀ and 64 of the 122 fundamentals of C₇₀. Theoretical calculations for C₆₀ are not accurate enough to completely assign the symmetries of the identified fundamentals. We comment on effects of oxygen and laser power on the Raman spectrum of C₆₀.

In soft-mode structural phase transitions the Ginzburg temperature interval in which fluctuations and the interactions between them become important is often observed to be small on the scale of the transition temperature. We consider the size of the Ginzburg interval (GI) in framework and 'cogwheel' structures using the concept of 'rigid unit modes'. Such materials, as well as being very displacive, i.e. close to the soft-mode limit, have an extremely anisotropic phonon spectrum. Modelling these two properties with a suitable effective Hamiltonian for the degrees of freedom driving the transition we find that the GI can range from very small to large, depending on the balance between displaciveness and anisotropy. For the two perovskites SrTiO₃ and LaAlO₃ and the 'cogwheel' structure K₂SeO₄, we obtain values of the model parameters describing displaciveness and anisotropy from experimentally measured phonon dispersions and find, for the size of the GI, quantitative agreement with experiment. We also estimate typical values for the model parameters and the size of the GI for framework silicates, using quartz and cristobalite as examples. Finally, we use computer simulations to confirm the results of our theoretical analysis over a wider range of model parameters.

High-pressure X-ray diffraction studies on BaFCl and BaFBr up to 50 GPa, on SrFCl up to 42 GPa, and on CaFCl up to 27 GPa were performed at room temperature using a diamond anvil cell and synchrotron radiation. Data for the equations of state are given. The c/a ratios of the tetragonal structure of these compounds show different variations under pressure and structural phase transitions are observed for BaFCl and BaFBr at transition pressures of about 21 GPa and 27 GPa, respectively. A simple hard-sphere model is presented, which describes the observed structural variations almost quantitatively and accounts for the structural phase transition.

In this paper we present a comprehensive study of the tetrahedral semiconductor ZnSe crystallizing in the zincblende structure. The electronic structure of ZnSe has been determined first using the ab initio self-consistent linear muffin-tin orbital method with a local-density form of the exchange-correlation functional. Then it has been adjusted to reproduce the experimental energy positions of the Zn 3d bands and of the optical gap. The adjusted electronic structure, densities of states and interband optical properties are presented and compared with previous calculations. Good agreement with experimental photoemission and bremsstrahlung isochromat measurements was found after including an energy-dependent lifetime broadening. We measured the reflectivity of ZnSe with high resolution from 4 to 30 eV and the high-energy region of the spectra has been interpreted on the basis of the present calculation.

A treatment for the phonoriton problem based on the model introduced by Wang and Birman is proposed. It is shown that the model describes an interacting phonoriton system rather than a free one. The phonoriton spectrum is then calculated by using the Matsubara Green function technique. It is seen from the result that the phonoriton-phonoriton interaction narrows the gap in comparison with the gap in the spectrum of free phonoritons obtained by Wang and Birman. The damping of phonoriton states due to the interaction is explicitly calculated as well.

The bend region of a two-dimensional quantum channel of constant width is shown to generate a well-type effective potential in the problem of electron wave scattering. For narrow channels the longitudinal modes of electron propagation should be considered as approximately independent. In the one-mode approximation the coefficient of electron reflection is calculated and its non-monotonic dependence on the incident electron momentum and the channel bend angle is determined. The local maxima in the reflection coefficient decrease exponentially with increasing electron momentum and increase with increasing channel bend.

We calculate the Green function, particle-hole susceptibility and Cooper pair susceptibility of a 2D Luttinger liquid that results from an electron gas with long-range current-current interactions. The Haldane bosonization scheme allows this model to be written in terms of boson operators, from which the required fermion operators are constructed. We find weakly decaying power law tails in the spectral function and a Kohn anomaly in the 2k_F particle-hole susceptibility, but no divergence in the Cooper amplitude.

The Curie temperatures of magnetic 3d elements and yttrium intermetallics are investigated. Using densities of states obtained by self-consistent LMTO band-structure calculations, it is shown that the Curie temperatures in strong and nearly strong ferromagnets cannot be understood in terms of extensions of the theory of very weak itinerant ferromagnetism such as the Mohn-Wohlfarth approach. In particular, a knowledge of the partial densities of states D up arrow (E_F) and D down arrow (E_F) around the Fermi level is not sufficient to deduce the Curie temperature. Reasons for this are the non-analytic character of the density of states and k-space delocalization of the thermal excitations.

The spin-Hamiltonian parameters for the 3d³ ion in an orthorhombic centre are calculated within the framework of the electrostatic model. Expressions for the three components of the g tensor and the fine structure parameters b₂⁰ and b₂² are given in terms of the crystalline field energies and spin-orbit coupling parameters. The calculations are performed including the contributions from the ⁴T₂ states in the ground term and the ²T₂ states in the excited terms. The relationship between g/sub ///-g_{perpendicular to} and b₂⁰ is derived. The basis of the spin-Hamiltonian separation for the orthorhombic centre of 3d³ ions is shown by electrostatic theory. The proper coordinate system for the orthorhombic centre is also discussed.

The form of the transverse depolarization function of an I spin due to dipolar interaction with a system of S spins is analysed for local field interactions without using the high-temperature approximation. This analysis is valid in the absence of diffusion and in the presence of a large, static external magnetic field. It is relevant to muons implanted in metals and to the free induction decay of dilute I nuclei in a system of S nuclei, for which the local field model is valid for short times. An analytic expression for the transverse depolarization function is obtained using the Boltzmann distribution for the occupation of Zeeman states of the S spins at a temperature T. The behaviour of the depolarization function depends on the ratio of the external magnetic field B to the temperature T and three regimes are identified. For small B/T the familiar high-temperature result is independent of B/T and sample shape. For intermediate values of B/T the depolarization function depends on B/T and has an envelope that is independent of sample shape and a time-dependent phase that is dependent on sample shape. For larger values of B/T the depolarization function is again independent of B/T with the phase remaining dependent on sample shape. The results indicate the effects that could be observed at values of B/T larger than those corresponding to the high-temperature approximation and provide the values of B/T at which these would be apparent.

Seven TbFeCo based magneto-optic phase enhanced quadrilayers have been designed and fabricated to improve the detection of the polar Kerr effect in order to maximize the SNR associated with the read process in a magneto-optic memory. Each system consisted of ZnS/TbFeCo/ZnS/Al/glass and was fabricated using RF and DC sputtering techniques as well as conventional thermal evaporation. The experimental performance of these structures as a function of angle of incidence is reported and compared with theoretical predictions. Results obtained with incident radiation polarized in both P and S states indicate the success of the design philosophy and the fabrication procedure.

The impact excitation rates, and consequently the emission intensities, of various energy states of Tm³⁺ in ZnS thin-film electroluminescence (TFEL) have been quantitatively evaluated by calculating the radiative transition rates and the impact cross section, together with a Baraff distribution function of hot-electron energy. The results show that the direct impact excitation rate of the ¹G₄ state of Tm³⁺ is very small, while that of the ³F₄ state is fairly large, resulting in a weak blue light intensity relative to the IR light in TFEL. Different theories of the electron distribution function are compared and discussed.