Table of contents

The Raman spectra of the mixed crystal SnS_2-xSe_x show a two-mode behaviour with local and gap modes developing for concentrations of 25% for S and Se respectively, which is approximately a factor of 3 higher than the corresponding concentrations in mixed zincblende crystals. The difference is consistent with the predictions of percolation theory for two- and three-dimensional crystals.

A qualitative calculation is made on the pseudopotential of an adparticle interacting with the order parameter of a substrate near a second-order phase transition. The surface effect is taken into account. For negative surface energy the pseudopotential is shown to have a logarithmic divergence as a result of the onset of long-range order at the surface. An application of the results to desorption phenomena is discussed briefly.

The classical anisotropic Heisenberg chain (with J_{perpendicular to} /J/sub ///= alpha ) is treated by a simple scaling theory valid in the low-temperature limit (K identical to J/sub ////K_BT>>1) for any alpha between 0 (Ising) and 1 (isotropic case). The correlation lengths are given in terms of universal single-variable scaling functions Phi /sub //, perpendicular to /: 1/ xi /sub //, perpendicular to /=(cosh^-1(1/ alpha )) Phi /sub //, perpendicular to /(K(1- alpha ²)¹2/+F( alpha )). The term F( alpha ), given in the text, is negligible except when alpha approximately 0. The asymptotic forms of the functions are: Phi /sub ///(x) approximately Phi _{perpendicular to} (x) approximately A/x for x<<1; and Phi /sub ///(x) approximately Be^-2x, Phi _{perpendicular to} (x) approximately C, for x>>1 (where A, B and C are constants), giving the characteristic 'Heisenberg-like' and 'Ising-like' behaviours occurring on either side of the crossover which takes place at x approximately 1. Exact critical and crossover exponents are deduced.

The stored-energy spectrum of LiF, NaF, KCl, KBr and KI samples irradiated at room temperature have been obtained. The capability of alkali halides to store energy by irradiation increases as the cation size decreases. It has been found that most of the observed energy release is not related to annealing processes of the radiation induced anion Frenkel pairs. The existence of damage in the cation sublattice with which this energy might be related is considered.

A formalism to obtain a general relation between a tensor (Born-Von Karman) and a valence force-field is presented. The derived formulas can be used to cast additional physical meaning on the parameters of the force field adopted. The case of diamond is explicitly worked out and formulae given by Musgrave and Pople (1962) are extended to include an additional important parameter of the valence force-field.

Renormalisation group recursion relations and matching techniques are used to evaluate to O( epsilon ) the crossover scaling function describing the competition between isotropic short-range and dipolar critical behaviours in the two-point correlation function of a magnetic system above T_c in zero field. The scaling function, obtained in closed form, interpolates between the small-q (q xi <<1) and large-q (q xi >>1) limits where asymptotic expressions are written. The results for the effective critical exponents are compared with neutron scattering measurements.

Presents new methods of characterising localised states. In one method, the wavefunction amplitudes are plotted as spatial maps in both two-dimensional and three-dimensional random systems. These amplitudes are obtained from local Green functions expressed in the continued fraction form and the Haydock-Heine-Kelly scheme (HHK) is used in numerical evaluations. Another method examines the convergence of the local Green function as a function of the truncation parameter n, where n is the number of steps down the continued fraction. The behaviour of the coefficients in the HHK continued fraction can be used in a Mobius transformation analysis to give information about localisation. The model systems studied are the random binary alloys and the Anderson Hamiltonian. Extended states are shown to be very likely present even in the two-dimensional Anderson model at small enough W.

Necessary conditions for the existence of extended states in the upper band, in the lower band and simultaneously in both bands of a random binary alloy are given. The classical percolation idea is modified for the quantal problem. The effect of lattice dimensionality is also discussed. The predictions are examined numerically by displaying spatial maps of the amplitudes of the wavefunctions and by other methods. A tight-binding model Hamiltonian is used in these studies.

In this work the authors show how a model disordered problem can be simply reformulated and the averaged Green functions calculated in an orderly and systematic fashion by methods of quantum field theory. For convenience and definiteness, the authors consider a theory described by a random binary (tight binding) Hamiltonian with a diagonal and off-diagonal disorder.

A simplified cluster Bethe lattice method is proposed for the calculation of the electronic density of states in n-component alloys. The method is applicable to the study of the effect of the short-range order in coordinately disordered alloys. As an example, the theoretical results for a ternary alloy are obtained and compared with the results from exact calculation of the 300-site model.

An atomic orbital study is presented for the electronic structure of NbS₂Cl₂, NbSe₂Cl₂, Nb₃Se₅Cl₇, Nb₂Se₉ and I_0.33NbSe₄. Each crystal structure is built up from Nb₂X₄ clusters (X=S,Se) containing a strong metal-metal bond. Band structures and densities of states are displayed and interpreted in terms of the different geometrical arrangements of the atoms. In particular the crystal stabilities are discussed with reference to the origin of the semiconducting gap for each compound. Metal-metal bonding contributes very strongly to the narrow bands at the edges of the gap.

Specific heat, magnetisation and susceptibility measurements on a single crystal of ytterbium gallium garnet (YbGaG) have been performed between 44-600 mK for the specific heat measurements and from 70 mK up to 4.2K for the magnetisation and susceptibility measurements. Specific heat measurements indicate that the magnetic ordering takes place in two steps. Initially there exists a magnetic ordering of unknown nature represented by a large broad hump with a rounded maximum. At about 180 mK, and below 54 mK, a long-range magnetic ordering occurs evidenced by a lambda-type anomaly in the specific heat. The changes in entropy, together with the temperature and field dependence of magnetisation, indicate that the ground state is a well isolated Kramers doublet. Therefore a molecular-field model of dipolar interactions cannot explain the unusual magnetic behaviour of YbGaG.

The temperature dependence of the g factor and the hyperfine coupling constant A of Mn²⁺ in HgSe has been measured with an EPR spectrometer in the temperature range 4.2-360K. It was found that the temperature dependence of g was due to the thermal expansion of the lattice while the temperature dependence of A was mainly caused by lattice vibrations.

The EPR spectrum of Ti²⁺ in ZnSe has been observed after firing the crystals in molten zinc. The spectrum consists of sharp double-quantum lines superimposed on broader single-quantum signals, and shows isotropic hyperfine structure due to ⁴⁷Ti and ⁴⁹Ti and anisotropic superhyperfine structure due to ⁷⁷Se. The spin-Hamiltonian parameters are determined. The g-value and the ⁷⁷Se hyperfine constants show that there is a large covalent contribution to the selenium-titanium bond.

ENDOR measurements are reported for the ¹⁹F nuclei in the first four shells of KMgF₃ and KZnF₃ containing Yb³⁺ ions in the cubic site. The signs of the hyperfine and transferred hyperfine interaction parameters are determined. An estimate of the crystalline lattice local distortion in the vicinity of a paramagnetic impurity is carried out.

High-resolution far infrared spectroscopy has been performed on the lowest-lying energy levels of Cr²⁺ in a MgO host crystal in magnetic fields up to 25 kG at temperatures between 1.2K and 4.2K. The relevant electromagnetic radiation from 2 cm^-1 to 10 cm^-1 was produced by using the higher harmonics of klystron radiation, leading to a tunable radiation source for the region between 2 cm^-1 and 25 cm^-1. From the extrapolated resonance frequencies at zero magnetic field of the electron paramagnetic resonance spectra and the measured temperature dependence of these resonances, an energy level scheme for the Cr²⁺ ion in MgO has been constructed which is in close agreement with theoretical considerations. The relative positions of the low-lying energy levels can be described with the two parameters 3 Gamma =17.2 cm^-1 and D=2.87 cm^-1. In addition, the sign of the anharmonicity parameter B was found to be positive. The magnetic field dependence of these low-lying energy levels (described by a g-factor in the case of linear dependence) differs significantly from theoretical predictions. The linewidth of the resonances suggests an upper limit for the lifetime of the T₂ level of 3*10^-9 s.

The Raman spectra of triethylenediamine at T=298K were collected and interpreted. The strong width of the A_g mode was connected at the amplitude of the libration about the three-fold molecular symmetry axis (the N-N axis). The agreement between experimental and calculated frequencies is discussed. The symmetry attribution of internal modes allows the authors to decide in favour of an untwisted molecular conformation with space group P6₃m.

Photoluminescence and photoluminescence excitation spectra are presented for ZnSe:Ni, GaP:Co and GaP:Ni. Comparison of the excitation spectra with published absorption spectra leads to the identification of Ni²⁺(d⁸) as the luminescent charge state in ZnSe:Ni, and Co²⁺(d⁷) as the radiative ion in GaP:Co. In GaP-Ni this procedure demonstrates that while Ni²⁺(d⁸) may dominate the near-infrared absorption spectrum, the photoluminescence band with zero-phonon line (ZPL) at 5354 cm^-1 is attributable to Ni⁺(d⁹) as concluded previously by Kaufmann et al. The photoluminescence excitation spectrum of GaP:Ni is shown to contain peaks corresponding to absorption by the inverse of the ²E-²T₂ emitting transition of Ni+(d⁹) and dips corresponding to competing absorption by the ³T₁(F)-³T₁(P) transition of the Ni²⁺(d⁸) charge state which is also present.

By means of exact calculations, the complete recoil spectrum of a suddenly-perturbed N-electron Fermi sea is evaluated. Its threshold shape is compared with the theory of Nozieres and de Dominicis (1969) and Doniach and Sunjic (1970) which is believed to be asymptotically valid for small excitation energies and N to infinity . For N approximately=10, the N= infinity threshold behaviour is apparent. However, the distance from threshold over which the asymptotic threshold shape remains universally valid for all final-state interactions is limited to a small percentage of the Fermi energy. The present work provides a simple method for computing complete X-ray photoemission spectral (XPS) lineshapes for independent-electron metals. It also offers potential explanations of: (i) the incompatibilities between XPS threshold exponents and exponents extracted from other data; (ii) the occurrence of nearly-symmetric XPS lines; and (iii) the lack of mirror symmetry of absorption and emission edges.

The method of factorisation is used to obtain a surface Green function to study the virtual surface states which arise when a linear field is applied to the surface of a nearly-free-electron crystal. Such an approach enables the normalisation problem encountered in previous treatments to be avoided. For a given surface termination, it is found that, although the surface-state energy depends on the gradient of the linear field, its actual location in the energy gap is governed by a 'competitive balance that is struck between the field gradient and the crystal surface potential. A singularity is found in the real part of the inverse surface Green function, which results in the surface-state energy being confined to a certain region of the energy gap, the actual region depending on the value of the surface termination parameter. A density-of-states calculation was performed to show the influence of the applied field on the position and lifetime of the virtual surface state.

Reports an investigation of normal ( theta =0) and near-normal ( theta <or=6 degrees ) photoemission from Cu(100) and Cu(111) surfaces using He I (h(cross) omega =21.2 eV) and He II (h(cross) omega =40.8 eV) radiation. The experimental spectra are obtained at both good energy resolution ( Delta E<or=100 meV) and good angular resolution ( Delta theta <or=1 degrees ). The data are in excellent agreement with calculations based on a one-step photoemission model using the LEED formalism.

The photoemission of Cr films deposited under UHV conditions has been investigated in the photon energy range from 30 eV to 230 eV. The 3p-3d intershell interaction gives rise to a strong maximum in the 3d partial yield above the 3p threshold.