Table of contents

The localized and resonance modes in polyethylene below 700 cm^-1 induced by specific conformational defects (kinks and folds) were calculated using Lifshitz's Green functions method. This method together with an exact evaluation of the unperturbed Green function yields smooth curves for the resonance modes and delta peaks for the localized modes. A series of peaks, characterizing kinks and folds, was found.

The imaginary part of the RPA dielectric constant gives a contribution to The exchange-correlation energy which, when added to the Robinson et al. exchange approximation leads to a better reduction factor to the Slater exchange. The authors present results from band and atomic calculations to prove this point.

Auger electron spectroscopy has been applied to investigate surfaces of some layer chalcogenides prepared by cleavage and by argon ion bombardment. The creation of defects leads to large increases in the reactivities of basal faces. The sticking coefficient for oxygen is found to be orders of magnitude greater for nonbasal surfaces than for basal faces, the anisotropy factor being at least 10¹¹ for TiTe₂ and NbSe₂.

The authors calculate the excitation spectrum of van Vleck paramagnets including all crystalline field energy levels. It is shown that transitions among high-lying levels lead to satellite peaks in the line of the low- lying excitations. The effect occurs in special regions of q space and is strongest for temperatures close to the magnetic ordering temperature.

The change in the gamma radiation intensity pattern from ⁵⁴Mn nuclei oriented in antiferromagnetic CoCl₂.6H₂O is observed through the spin-flop transition. The critical field for the transition and the new direction of spin orientation are determined.

A method is developed to investigate the behaviour of a liquid under the action of a very high shearing force using computer simulated molecular dynamics. Values for the viscosity are calculated but these require more extensive computation for statistical accuracy. A short calculation is, however, sufficient to establish the nature of the processes involved, and to plot the velocity distribution curve; these graphs are presented. The method abandons the usual homogeneous isotropic cyclic conditions used so far in calculation of transport coefficients and can be applied to arbitrarily large shear.

The probability distribution of the dilational strain is calculated for the idealized situation of a random array of straight edge dislocations in a finite spherical piece of material. The result is exactly separable into two contributions, one of which corresponds to the distribution in a region close to a single dislocation, and the other is a gaussian distribution. A physical understanding of these two distributions is presented, which makes it possible to extend the results semiquantitatively to the strain probability distributions in real situations, and to conclude from energy considerations that dislocations grown into crystals may often be effectively random.

Thermal expansion in glasses above and below 1 K is briefly discussed in connection with known results relative to other thermal properties.

The valence bands of MgF have been computed by the tight binding method. The potential used consists of a sum of ionic potentials with the exchange term in the Slater approximation. The resulting wavefunctions were constructed first as linear combinations of the 2s and 2p orbitals for F^- ions and then with the 1s, 2s and 2p functions for F^- and Mg²⁺ ions. Differences between the results are discussed. The top of the valence band is the state Gamma '₂ mainly formed from p_z functions of F^- and the total width of the valence band is about 6 eV.

Formulae are derived for the electronic Green function of a system, and in particular of a crystal, when this is expressed as a linear combination of basis functions which are not mutually orthogonal. Expressions are derived for the electronic density of states and the electrical conductivity of a system in this representation, and the differences in these properties between perfect and imperfect crystals, and between different types of crystal, are examined. Application of the method to a simple cubic crystal with the Mathieu potential shows that, as for a one dimensional system, the use of optimized in place of fixed atomic orbitals as basis functions leads to a great increase in accuracy.

Three-terminal measurements of capacitance and dissipation factor have been made on samples of d-camphor, dl-camphor and l-camphor at frequencies between 200 Hz and 10 kHz over the temperature range -196 degrees C to +20 degrees C. The dielectric relaxation processes in d-camphor, below the transition temperature, have been studied and the change in the frequency of maximum dielectric loss, which occurs during the transition, has been estimated. It is believed that these results are the first to provide direct evidence of the increased freedom of molecular rotation which occurs during the transition. Electrical conduction appears to commence immediately on completion of the transition and increases with increasing temperature.

The optical modulation of an electron beam by a laser beam in a dielectric crystal is shown to involve at least two processes. The transmitted beam is modulated by a first order process involving the absorption and emission of photons, while the diffracted beams are modulated by a second order process involving an additional transfer momentum to the crystal lattice. It is suggested that observed effects in the diffracted beams result from multiple scattering in the relatively thick specimen. The calculation are discussed in relation to other published theories.

The ground state energy and wavefunction of Zn^- impurity in silicon is calculated using a model in which a realistic pseudopotential description of both the impurity potential and the host crystal band structure is taken into account. The frequency dependent photoionization cross section is obtained via the wavefunction in favourable agreement with experiment. Comparison with other acceptors in silicon is given and ground state energies of B, Al, Ga and In in silicon are also presented.

The thermally modulated absorption (TMA) spectra of Fe ions in micas, vivianite and olivine have been measured in the visible and near infrared region at room temperature and they agree with the corresponding static absorption spectra. The micas show a strong negative TMA peak, located between 12000 cm^-1 and 14000 cm^-1, corresponding to Fe²⁺ to Fe³⁺ intervalence charge transfer and two negative-going peaks at 11000 cm^-1 and 9000 cm^-1 for spin-allowed absorption of Fe²⁺ ions in the crystal field. The TMA spectrum of vivianite is essentially similar to that of micas. The olivine shows a strong negative-going peak at 9700 cm^-1 for the spin allowed absorption of Fe²⁺. The positive background is consistent with the phonon-assisted transition of Fe ions in the near infrared region.

A comparison has been made between the infrared local mode absorption spectra of boatgrown and pulled crystals of GaAs:Si. The samples were compensated either by electron or neutron irradiation or by copper diffusion. Boron is present in the pulled material and previously unreported lines have been assigned as follows: ¹¹B(Ga) (517.0 cm^-1), ¹⁰B(Ga) (540.2 cm^-1), B(Ga)-Si(As) pairs (349.0, 570.9, 596.0, 661.0 and 684.8 cm^-1). Two further lines at 762.7 and 796.0 cm^-1 may be due to interstitial boron atoms produced by the irradiations. It is thought that other lines may be due to the centre B(Ga)-Si(As)-Si(Ga).

Measurements have been made of the Raman scattering and infrared absorption peaks arising from excitation of magnon pairs in the system (Mn_1-cZn_c)F₂. Single crystals were investigated with c varying from 0 to 1. The measurements are compared with predictions of both a mean field model and an Ising cluster model. Approximate mean field values are obtained for the first, second and third moments of the two-magnon peak. The Ising model has been applied in a way that takes account of the proximity in pairs of the manganese atoms responsible for the two-magnon scattering and absorption and it gives a mean frequency that is higher than the mean field result although not quite as high as the experimental results.

The optical absorption spectrum of Tm³⁺ in TmVO₄ has been studied to obtain information on the cooperative Jahn-Teller phase transition which occurs in this material at 2.14+or-0.04 K. The temperature dependence of the absorption lines below 2.14 K was in good agreement with the predictions of molecular field theory along the crystallographic c axis the field dependence of the spectra at 1.4 K provided direct evidence for the reduction of the Jahn-Teller distortion to zero over the field range 0 to approximately 5kOe, an important consequence of the theoretical treatment of the problem. The observation of the optically uniaxial to biaxial transition associated with the crystallographic phase transition provided additional evidence for the proposed model of behaviour, and showed that the distortion is of the symmetry type Gamma ₃⁺.

The double group D₂⁺ is investigated from both Bethe and Opechowski viewpoints, and is shown to be isomorphic to the quaternion group. The use of matrix groups in quantum mechanics is discussed, with particular emphasis on the application of exact theorems of group theory when dealing with approximate eigenfunctions. The discussion is illustrated by a treatment of D₂ and D₂⁺ as matrix symmetry groups for certain 2*2 matrices.

The hamiltonian for an ion which has a dynamic Jahn-Teller interaction with the many normal modes of a crystal can be separated in such a way that nearly all the coupling strength is concentrated in an interaction with a set of vibrations of one frequency only. All the rest of the hamiltonian, including the consequence of the effective vibration not being a normal mode, is expressed in a simple form, and treated as a second order perturbation on the energy levels of the first part. This procedure gives a good approximation to the lower states of the Jahn-Teller system, and can be used in studying phonon scattering. Some results are derived for the case of an E electronic doublet at a cubic crystal site.

The theory of acoustic attenuation due to the spin-phonon interaction in a dense paramagnetic crystal is presented for a system with S=¹/₂. The acoustical attenuation is related directly to the structure of the phonon Green function which is calculated by a diagrammatic technique. A detailed analysis of the attenuation is made in the case when the paramagnetic ions are in sites of octahedral symmetry in a cubic crystal and with the magnetic field directed along a fourfold axis. An explicit expression is given in the situation of experimental interest, namely, for transverse wave propagation along a cubic axis when the propagation and polarization vectors are respectively perpendicular and parallel to the magnetic field.

This paper considers the effect on both conventional nuclear orientation, and on the combined nuclear magnetic resonance/nuclear orientation (NMR/ON) techniques, when a small electric quadrupole interaction is added to a simple magnetic nuclear hyperfine hamiltonian. It is shown that such an addition is difficult to detect by conventional nuclear orientation and can lead to substantial misinterpretation of experimental results. Quantitative examples are given. The effect of such an addition on the NMR/ON resonance line spectrum is discussed in detail and it is shown that, for the linewidths found in ferromagnetic metals, this combined technique can detect electric quadrupole couplings eQV_zz of order 0.1% of the magnetic dipole coupling. A discussion of possible sources of such quadrupole interaction and dilute impurities in ferromagnetic metals is included.