Table of contents

Infrared absorption lines at 628 and 601 cm^-1 in GaAs have previously been attributed to the vibrations of B_As^- acceptor centres. High-resolution spectroscopy now reveals fine structure on these lines similar to that reported for C_As^- centres due to different nearest-neighbour combinations of ⁶⁹Ga and ⁷¹Ga. This result provides a direct confirmation of the authors' earlier assignment of the lattice location of the boron atom.

Calculations of the one-phonon absorption associated with isolated substitutional phosphorus in silicon are presented. It is shown that with only one change of force constant, namely a softening of the stretch constant between phosphorus and its nearest neighbours, very satisfactory agreement with experiment can be achieved as long as apparent charges of neighbours of the phosphorus impurity are included and a sum rule for the apparent charges is satisfied. A calculation to illustrate the effects of ignoring apparent charges of neighbours (and thus violating the sum rule) is shown for comparison.

Flux measurements for desorption angles -60 degrees <or= theta <or=60 degrees for H₂/Ni(111) show a cos^4.6 theta distribution and for H₂/Ni(110) a cos^1.2 theta distribution. Peak locations and half-width of the spectra remain unchanged for all angles investigated. Rotating the crystals 90 degrees about the surface normal does not change the spectra significantly. For all surfaces investigated CO desorbs in pure cosine distribution.

The equation of state of perovskite-structure CsCaF₃ has been calculated using Gordon-Kim pair potentials in the quasi-harmonic approximation. Agreement with experimental results is reasonably good for phonon frequencies and equation-of-state properties. Recently, similar calculations for RbCaF₃ were found to be consistent with an observed displacive phase transition in this material. Significantly, the same theory predicts the absence of any displacive transition in CsCaF₃, in agreement with experiment.

The convergence properties of the multiple-scattering problem are examined for an electron incident upon a crystal lattice partitioned into Wigner-Seitz cells. While the partial-wave series is divergent when standard phaseshifts and outgoing-wave Green functions are used, it is also shown that the series is asymptotic, and is typically optimal for three or four partial waves.

First, the authors calculate the frequency-dependent dielectric function of black phosphorus by using the band structure and wavefunctions obtained by the self-consistent pseudopotential calculation. Then by using these results, they discuss the optical properties of black phosphorus such as KPS, UPS, plasmon frequency and reflectance in visible, VUV and soft X-ray regions. Experimental results for all these optical properties are explained consistently using the theoretical results.

Recently a scheme has been developed to correct for the incomplete cancellation of the self-Coulomb interaction by the self-exchange term inherent in the local-spin-density (LDS) approximation to the Hartree-Fock theory. A calculation of the electronic energy band structure of the lithium chloride crystal is performed using the LSD theory with the self-interaction correction (SIC). The method of linear combinations of atomic orbitals is employed to solve the Hamiltonian self-consistently with four different versions of the exchange-correlation functional. The calculated band gap is in good agreement with the experimental value but becomes much smaller if the SIC is omitted. The binding energy of the centroid of the valence band obtained from the SIC-LSD calculation is slightly larger than the observed value. The density of states of the lower part of the conduction band is compared with photoemission data. The nature of the SIC potential is discussed and a simplified scheme for incorporating the SIC in the density functional theory is presented.

The kinetics of bimolecular quenching of incoherent Frenkel excitons is investigated in the one-dimensional case. The equation for the distribution of distances between neighbouring excitons is derived and solved analytically in the limit of large times. It is found that the time dependence of the exciton concentration c has the form c approximately t^-12/. The analytical expressions are confirmed by the results of direct computer simulations.

It is shown that a presence of a magnetic field gives an extra structure to the wavefunction, which has to be complex. The authors study the phase of the wavefunction to construct a gauge-invariant vector related to the local current density. A numerical study for the disordered system in a strong magnetic field in two dimensions reveals a structure of the current with vortices, which behaves quite differently for localised and delocalised states.

The electronic structure of disordered systems with periodic lattice distortion is investigated. The nature of the eigenstates including localisation is analysed, and numerical results are presented. It is shown that the electronic structure, which is quite different from those of undistorted systems, is sensitive to the period and magnitude of the distortion. It is stressed that although the band structure may be regarded as being modified into new bands by the CDW gapping, the presence of band-mixing effects arising from disorder forces the electronic structure to be determined for the whole band rather than for some part of the Fermi surface.

A diagrammatic theory is established for the tight-binding model of independent electrons hopping in a disordered lattice as described by the Anderson Hamiltonian. The diagrammatic analysis starts from Anderson's locator expansion, gives a short review of the techniques already developed by several authors and finally arrives at a closed formulation of omega -dependent (R/A) particle-hole diagrams where retarded (R) and advanced (A) propagation differ by frequency omega and momentum q. Several different types of Bethe-Salpeter equations are considered and a new Ward identity valid for all omega , q is proved representing the key for the calculation of diffusional modes. It is shown that the formalism works equally well for weak and strong disorder.

For pt.I see ibid., vol.17, p.1897 (1984). The diagrammatic locator formalism for independent electrons hopping in a lattice with random site potentials, which was developed in a previous paper, is applied to yield a self-consistent treatment of the Anderson transition. The theory of Vollhardt and Wolfle (1980, 1982) based on an extended state formalism within an Edwards model is re-inspected for this model resulting finally in a self-consistent equation for the frequency-dependent diffusion coefficient, which displays the same structure as the equation already derived by these authors but is built on a somewhat different physical picture and may be evaluated for strong coupling equally well. Finally the level repulsion effect of Mott, which leads to a omega ² mod ln omega mod ^d+1 dependence in the real part of the electrical conductivity is discussed in this picture.

The authors extend the renormalised effective-medium approximation (REMA) developed by Sahimi et al. (1983) for bond-disordered random conductance networks to the site-disordered and correlated-bond percolation conductivity problems. This new approximation combines the position-space renormalisation-group method and the effective-medium approximation. Good agreement is found between the predictions of the REMA and the numerical data for correlated-bond and for site-disordered percolation conductivity problems in a simple cubic network.

The time evolution of an electron-impurity or electron-phonon interacting system in the presence of a constant electric or magnetic field is studied. It is shown that the quantum-mechanical Liouville equation reduces to the familiar Boltzmann equation in the limit of weak electron-impurity or electron-phonon interactions.

The author discusses the properties of a simple model of a spin glass in a uniform field. The field dependence of the field-cooled susceptibility and the zero-field-cooled susceptibility are discussed. An S-shaped M against H curve similar to the experiments in CuMn and chi against T curves similar to the experiments in AgMn are obtained.

The influence of the linear magnetoelectric effect on the magnetic polariton spectrum is considered in simple uniaxial ferromagnets. Dispersion equations for the magnetic polaritons are obtained by two methods. (i) From the condition for the existence of a non-zero solution of Maxwell's equations in which the internal dynamics of the system is included through electric, magnetic and magnetoelectric susceptibilities. (ii) From the analysis of the linear response of the system to an external magnetic field. The linear magnetoelectric effect is shown to lead, in general, to the irreversible character of polariton propagation, as well as to broadening (by about ten per cent) of the band forbidden for propagation of right-hand circularly polarised waves. The light scattering cross section on the magnetic polaritons is also discussed.

Absolute reflectivities were determined for Mossbauer radiation in pure nuclear Bragg reflections of a nearly perfect ⁵⁷FeBO₃ single crystal. The measured anomalous high reflectivities reached values of 25-35%. The results agree with the dynamical theory for resonance diffraction, where anomalous high reflectivities are explained as a consequence of the suppression effect.

Mossbauer spectra of the reflection from and the transmission through a thin nearly perfect ⁵⁷FeBO₃ single-crystal platelet were measured at different angular positions in the vicinity of a Bragg angle. In the transmission spectra an angular-dependent shift of the nuclear resonance dips was observed in common with a shift of the resonance peaks in the reflection spectra. In addition, a pronounced asymmetry of the transmitted intensity above and below the resonances was observed. The angular dependence of the transmission spectra reveals that in transmission under Bragg reflection conditions the coherent channel (diffraction) becomes as important as the incoherent channels (internal conversion and incoherent scattering). The Borrmann effect induced by nuclear resonance diffraction was observed for the first time.

The authors report the polarised Zeeman spectra recorded for the 0.839 eV photoluminescence arising from trigonal Cr²⁺ in GaAs. The experiment confirms the '⁵E to ⁵T₂ model' of Barrau and co-workers (see ibid., vol.16, p.4581, 1983) and offers a direct determination of the polarisation ratio: B/A=-2.

The electroluminescence spectrum of CaS:Er phosphor is found to exhibit nine bands covering the ultraviolet, visible and near-infrared regions. The most intense bands, which occur at 530 and 550 nm, are split into as many as seven sharp lines. Calculations based on the theory of crystal-field splitting indicate that Er³⁺ ions can occupy the sulphur-ion vacancy position as well as the Ca²⁺ substitutional site.

Thermoluminescence (TL) as well as absorption and EPR spectra of X-irradiated natural calcites have been obtained. Irradiation produces UV absorption bands and a decrease of the Mn²⁺ EPR spectrum. A correlation of each TL peak with the bleaching steps of UV absorption bands and the recovering in intensity of the Mn²⁺ EPR spectrum has been found. These experimental results support a new model for the radiation damage and thermoluminescence process in calcites. The main point in this model is that holes become trapped at impurities, and the electrons are trapped at dislocations in the form of CO₃^3-.

For pt.I see ibid., vol.16, p.6499 (1983). By means of a self-consistent tight-binding method, a realistic calculation of a Si-H-Ag junction is made. More general cases for different interlayer atoms are analysed by changing the parameters of the interface. The authors' results show that the barrier height of the junction may have slightly changed values (by up to 0.2 eV) for different atom electronegativities. For atoms of low (high) electronegativity, they find larger (smaller) barrier heights.

The theory of the ortho-para conversion of a hydrogen molecule on magnetic surfaces is developed with special account taken of the dynamical properties of the solid surface. The transition probability of the ortho-para conversion is obtained in terms of the convolution of the spectral functions, namely the Fourier transform of the correlation function for the relevant physical quantity. The correlation functions are estimated within the stochastic model. The temperature dependence of the transition probability is determined by the static spin fluctuation and the broadening of the spectral functions. A variety of resulting temperature dependences are discussed.