Table of contents

The temperature dependence of the longitudinal and shear ultrasound wave velocities in (As₂S₃)_1-x(PbS)_x glasses has been determined from 77 to 300K using a pulse echo interferometer. Elastic constants of the prepared glasses at room temperature have been computed from the experimental data. Both longitudinal and shear ultrasound wave velocities in these glasses show a linear temperature dependence with a negative temperature coefficient.

Inhomogeneous currents in the systems with pairing of electrons and holes (for T=0 and T approximately T_c) are considered. All types of interband transitions are taken into account. The equation obtained for the order parameter phase is similar to the equation for the physical pendulum with rotating plane of oscillations. The lifetimes of the current states in the excitonic phases are discussed.

Some recent observations of unusual angular dependences for EPR spectra of exchange-coupled pairs of Cr³⁺ ions are explained by extending the strong-exchange spin-Hamiltonian formalism for pairs of magnetic ions to include second-order effects. These effects arise from the mixing of different spin multiplets by single-ion terms in the pair spin-Hamiltonian. Specific formulae for the second-order corrections are derived for single-ion terms of the form B₂⁰O₂⁰ and B₂²O₂². In the cases where such terms are small but not negligible compared with the magnitude J of the Heisenberg exchange, these formulae permit a determination of J which is more accurate than that obtained by the usual method of measuring temperature dependences of spectral intensities. Values of J=0.92, 1.15 and 0.96 cm^-1 have been determined from the published data for Cr³⁺ pairs in CsMgCl₃, CsMgBr₃ and CsCdBr₃ respectively, compared with earlier estimates approximately=5, 10 and 17 cm^-1 based on temperature dependence measurements.

The dispersion of the Verdet constant of a single-crystal terbium gallium garnet (TGG) was measured using a pulsed magnetic field up to 70 KG. The data show a large Verdet constant for TGG throughout the visible region. The observed hysteresis effect is briefly discussed.

The magnetic properties of GdMg₃ and TbMg₃ have been found to depend very strongly on the Mg concentration. With increasing Mg concentration the strong ferromagnetism is lost, giving way to antiferromagnetism. For the magnesium rich compound TbMg₃ the magnetic structure has been determined by neutron diffraction. Ferromagnetic (111)-planes are stacked antiferromagnetically in a direction perpendicular to these planes.

The problem of determining the phases of the diffracted waves when only their intensities can be measured is a question that has received a great deal of attention in the case of X-ray diffraction but has so far been largely ignored in low-energy electron diffraction. By applying a technique similar to the one employed in the Kramers-Kronig analysis of optical reflectivity data it is found that under certain circumstances a good approximation to the phase can be found. The error involved depends on the location of the zeros of the reflectivity in the complex energy plane. In a test calculation for Ni (100) it is found that these zeros are favourably placed for the clean, unrelaxed surface but that any departure from this leads to a serious deterioration in the accuracy of the phase determination.

A theory for the isothermal compressibility of monatomic liquids at temperatures close to their triple points is presented. Assuming that the atoms interact through pairwise potentials, an exact relation for the direct correlation function of the fluid in terms of that of a reference fluid is derived. This is used to construct approximations for the long wavelength behaviour of the liquid structure factor a(q). By dividing the pairwise potential into a reference part, representing the short range repulsive forces, and a perturbation part, representing the long range 'attraction', the compressibility can be expressed in a mean field fashion in terms of an effective hard sphere contribution modified by the perturbation potential. The theory has been used to calculate the long wavelength limit a(q=0) in liquid Ne, Ar, Kr and CH₄ and in a wide range of liquid metals.

Fine structure is observed on the GR1 zero-phonon lines in the cathodoluminescence spectra from lightly irradiated ( approximately 10²¹ 2 MeV electrons m^-2) natural semiconducting diamond. Temperature dependence measurements over the interval 4 to 77K indicate that the structure derives from the excited state of the centre. Four closely spaced lines are observed in the T to E transition and it is proposed that the spectrum is due to a combination of luminescence from perturbed and unperturbed GR1 centres. The perturbation is specific, rather than random, splitting the excited T state into three levels; the energy separations between the lowest and the two higher levels are 1.4 and 3.5 meV. Measurements on heavily irradiated samples suggest that the perturbation may be associated with the radiation damage itself-perhaps due to the mutual interaction between close separation GR1 centres.

Ultrasonic studies of the elastic constant, c₆₆, of DHCP Pr are reported. Using CdS thin-film transducers the authors have measured c₆₆ as a function of temperature and as a function of an applied magnetic field at 4.2K. c₆₆ is found to be strongly affected by a field applied in the basal plane, whereas the application of a field along the c-axis has almost no effect. This behaviour reflects the anisotropy of the magnetic susceptibility of the hexagonal ions, indicating that they dominate the magnetic effects on c₆₆. Neglecting magnetic contributions from the cubic ions, the observed behaviour of c₆₆ is in agreement with calculations based on the magnetic Hamiltonian for the hexagonal ions which has been deduced from the field dependence of the magnetic excitations.

An atom-atom interaction force model has been used to calculate the frequencies of long-wavelength lattice vibrations in high-symmetry layer crystals-MoS₂, PbI₂, GaSe and GaS. The weak interlayer force has been neglected. Assuming that each layer is electrically neutral we need to perform the lattice sums only in the two directions parallel to the layers. It is found that in the case of MoS₂ and PbI₂ crystals, good results obtained by expressing the lattice sums in terms of two nearest-neighbour forces only. In the case of GaSe and GaS crystals the lattice sums are evaluated in terms of four nearest-neighbour forces. The larger difference between the calculated and observed values in GaSe may be due to long-range electrostatic forces.

Using a model Hamiltonian initially formulated by Pytte and Feder (1969) and later by Gillis and Koehler (1974) for describing a displacive-type ferroelectric, the effects of stress and strain have been included and the thermal expansion coefficient as well as the pressure coefficient of the transition temperature have been calculated. The thermal expansion coefficient ( alpha _i) shows a finite jump at the transition temperature and the jump Delta alpha _i is proportional to the pressure coefficient of the transition temperature dT_c/dP_i.

Structural (gyrotropic) phase transitions from optically inactive to optically active (gyrotropic) phases are analysed for nonmagnetic materials. The following basic results of group theoretical analysis are gathered for those equitranslational gyrotropic phase transitions which are associated with a single REP (irreducible or physically irreducible representations): e.g. the macroscopic transition parameter, the spontaneous components of the gyration tensor, the polarisation and deformation tensors newly onsetting at the transition, the indices of faintness, faint interactions, and switching forces. A simple characterisation of the domain structure of the ferroic phase is given in terms of these tensors.

Several microscopic models of desorption reactions are studied. The emphasis is on the predicted energy and angle dependence of the desorbed flux. The model of Lennard-Jones, Devonshire, and Strachan (1936) is extended to three dimensions and the model of Kramers (1940) is adapted to a surface configuration. The latter model shows good qualitative agreement with experiment if it is coupled with an activation barrier to adsorption, as suggested by Van Willigen (1968).

The first calculation of the ground state of the two-dimensional excitonic molecule has been performed. The authors have used the transformation of the coordinates of four bodies with equal masses which excludes the motion of the centre of mass, and allows the isolation of one weakly interacting quasiparticle and a core of two strongly interacting quasiparticles in an effective three-body problem. Since the core problem is exactly solved and the form of the variational wavefunction of the system takes into account the essential part of the exchange interactions, satisfactory values are obtained for the binding energy of the two- and three-dimensional biexcitons even when using rather simple trial functions. The binding energy is E₂=0.12 au for a two-dimensional biexciton and E₃=0.013 au for a three-dimensional biexciton.

Electron energy-loss spectra of clean NiO (100), CoO (100), and MnO surfaces have been obtained using primary electrons of 100-900, 30-800, and 300-700 eV, respectively. The observed electronic transitions are interpreted on the basis of the primary electron energy dependence and comparison with optical data. The peaks below approximately 10 eV are due to the O^2-2p to M²⁺3d and 4s(M=Ni, Co, and Mn) transitions. It is shown that the onset of the interband transitions is caused by the 2p to 3d charge-transfer excitons. The peaks between 10 and 50 eV are attributed to the one-electron excitations of O^2-2p to M²⁺4p and M²⁺3d to 4p, and to the single- and multiple-bulk plasmon excitations associated with O^2-2p, M²⁺3d, and coupled 2p and 3d electrons. The 3p-core excitation spectra are observed beyond 50-60 eV.

Angle-resolved photoelectron spectroscopy has been applied to study atomically clean surfaces of SnSe₂ prepared by cleavage in vacuum. The polar variation of the emission, as well as its dependence on the azimuthal angle, has been measured at room temperature. The results may be easily understood in terms of the band structure of the material and are compared with theoretical calculations.

A model for the high-temperature transport properties of one-dimensional conductors with very narrow bands and strong electron-phonon couplings is discussed. It is argued that electronic states can be viewed as effectively localised over some characteristic time scale and that the conductivity is due to dynamic rearrangement of the localisation region. For a series of charge-transfer complexes like tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ) it is argued that a consistent interpretation of the almost identical temperature dependences of the high-temperature conductivities and yet apparently totally dissimilar temperature dependences of the thermopowers is possible if one regards both the donor and the acceptor stacks as conducting. In particular, for TTF-TCNQ where most of the relevant parameters are known, quantitative calculations of the magnitudes and the temperature dependences of the conductivity and the thermopower are presented. The results are in excellent agreement with the observed values.

The cyclotron resonance of electrons and holes in red HgI₂ has been observed below 4.2K using a cross modulation technique at a microwave frequency of 137 GHz. Cold polaron masses of carriers moving within the layer planes were found to be m_{e perpendicular to} =(0.37+or-0.02)m₀ and m_{h perpendicular to} =(1.03+or-0.10)m₀, where m₀ is the mass of the free electron. Masses of carriers moving parallel to the crystal c axis were deduced from a combination of cyclotron resonance and mobility measurements, as m_e//=(0.31+or-0.03)m₀ and m_h//=(2.06+or-0.51)m₀. Polaron effects are discussed and carrier band masses deduced. The masses are found to agree with band structure calculations, and suggest a reinterpretation of the experimental exciton spectra.

Bias and temperature dependence data are presented for the large zero bias resistance peak observed in In-Ge:As (irradiation compensated) Schottky barrier tunnel junctions. A model based on variable range phonon assisted tunnelling is discussed and shown to be consistent with several important features of the data.

The electron paramagnetic resonance (EPR) spectra of Fe²⁺ and Fe³⁺ ions in single crystals of SrO are reported. Fe³⁺ ions occupy sites of orthorhombic symmetry and their EPR spectra have been fitted with an S=⁵/₂ spin-Hamiltonian in which the dominant second-order terms have coefficient values of B₂⁰=(884.7+or-0.3)*10^-4 cm^-1 and B₂²=(-422.0+or-0.5)*10^-4 cm^-1. Fe²⁺ ions occur at sites of trigonal symmetry, and the single observed resonance line may be described by a spin-Hamiltonian with S'=¹/₂, g_{perpendicular to} =0, g/sub ///=3.992+or-0.005 and A/sub ///(Fe⁵⁷)=30.2+or-0.6*10^-4 cm^-1. Variable temperature experiments indicate that the Fe²⁺ resonance occurs between the two levels of a doublet lying 12+or-1 cm^-1 above a ground state singlet. A lattice distortion calculation, based on a model of polarisable point ions, and which incorporates the crystal field stabilisation energy, is applied to the cases of Fe²⁺ and Mn²⁺ ions substituted into alkaline earth oxide host lattices. The calculation is used to support an interpretation of the spectroscopic data for SrO:Fe in which the trigonal Fe²⁺ site arises from an off-centre displacement of the Fe²⁺ ion along a (111) axis.

The authors have conducted reflectivity measurements of the layer materials SnS₂, SnSe₂, CdI₂, PbI₂, BiI₃ and BiOI between 8 and 40 eV. Band structures have been calculated for SnS₂, CdI₂, PbI₂ on the basis of an LCAO approach and these are compared with the experimental data. Transitions from the metal d core states are discussed in terms of an atomic-like picture.

Green functions are derived for the displacement gradients associated with the acoustic vibrations of an elastically-isotropic semi-infinite medium which has a single flat surface. The frequency dependence of the Brillouin-scattering cross section is calculated for arbitrary scattering angle and optical absorption length. The contributions of Rayleigh surface waves and of bulk acoustic waves modified by the presence of the surface are evaluated for some illustrative examples.

The far infrared spectrum of a saccharose single crystal shows the polarisation of the vibrational modes. The low frequency mode (49.5 cm^-1) polarised along the binary axis, has allowed the authors to explain the temperature dependence of the pyroelectric coefficient.

The lifetimes of luminescence from liquid argon, krypton and xenon excited by energetic electrons have been studied with such a high electric field that all of the observed decay characteristics have to be attributed to the self-trapped exciton luminescence. In all cases the decay shows two exponential components. The lifetimes of the fast components are in the range from 2 to 5 ns, and major second components have lifetimes of 860+or-30 ns for argon, 80+or-3 ns for krypton and 27+or-1 ns for xenon. These lifetimes are interpreted in terms of two self-trapped exciton states, one predominantly singlet in character, the other triplet; this is the first strong evidence for the triplet assignment. The variation in triplet-state lifetimes is attributed to the different spin-orbit couplings.