Table of contents

The differential broadening of the 5p_1/2 and 5p_3/2 levels of Xe physisorbed on W(100) is interpreted in terms of differences in the image polarization energies of p_z and p_x,y orbitals.

The data from the numerical experiment in two-dimensional random lattices by the authors is used to provide evidence that the states expand like (E_c-E)^{- eta} with eta >1 near the mobility edge.

Using electron energy-loss spectroscopy at grazing incidence on graphite crystal, we have observed: (i) a strong dispersion-from 5 eV up to 19 eV-of the volume loss associated with an excitation direction parallel to the c axis and a wavevector greater than the Ferrel wavevector cut-off (2.1 AA^-1< mod q/sub /// mod <3.5 AA^-1); (ii) a variation of the mean inner potential seem by the incoming electrons when the penetration depth is varied from 4 AA up to 7 AA; and (iii) the existence of two losses at 6.2 eV and 20.1 eV which may correspond to surface-plasmon excitations parallel to the layer surface.

It is argued that, since the average phase of the wavefunction is uniquely fixed by charge neutrality, for the purpose of calculating dangling-bond surface states in narrow-gap semiconductors, a self-consistent barrier can be replaced by a suitably displaced abrupt barrier. This yields a simple scheme which can be used to calculate the eigenvalue E(k) for any desired k. Calculations are carried our for Si and Ge, and compared with self-consistent calculations where available.

For transition-metal compounds the scattering of metal d states into empty nonmetal p states is suggested to be the mechanism responsible for the soft-mode anomalies found in the phonon spectra. This idea is confirmed by investigating the effects of carbon vacancies on the electronic structure of NbC using the X alpha scattered-wave method.

Measurements of the variation of the dielectric constant with temperature at 11 GHz and between 0.4 and 14K have shown that the coupling between electromagnetic waves and the low-energy excitations present in vitreous silica strongly depends on the concentration of OH^- ions. This result can be explained by an increase of the density of states of the coupled excitations with the OH^- content.

A general expression for the structure factors of Bragg reflections in HCP crystals which includes non spherical distortions of the atomic charge density and an anharmonic classic effective potential is presented. Group theory is used to expand these quantities in terms of appropriate combinations of spherical harmonics. This expression should be useful in the interpretation of X-ray and neutron diffraction data. The main outcome is that the 303(2n+1) forbidden reflections are now allowed due to the presence of both the antisymmetric component of the charge and an anharmonic temperature factor. While the effect of the first term will probably be too weak to be detected by X-ray techniques, it is argued that high-temperature X-ray or neutron scattering data should show a measurable T² behaviour for the 3031 and 3033 reflections.

The phonon dispersion curves of FeCl₂ are calculated using the static-displaced-shell model, introduced by Cran and Sangster for MnF₂ (ibid., vol.7, p.1937 (1974)). The model takes into account the effect of static intra-ionic polarization. The model is fitted to the acoustic dispersion curves measured by Yelon, Scherm and Vettier (Solid St. Commun., vol.15, p.391 (1974)), and to the dielectric constant epsilon _{perpendicular to} ^{( infinity )}. A second version of the calculation also includes fitting to optical frequencies interpolated from Lockwood's infrared and Raman spectroscopic measurements on MnCl₂ and CoCl₂ (1973). The fit to experimental data is fairly good and the parameters assume reasonable values.

HgIn₂ Square Operator Te₄ has a structure which may be regarded as being derived from that of zincblende by the incorporation of an ordered array of vacancies. The k=0 vibrational modes of this compound have been studied using infrared reflectivity and Raman scattering measurements, the latter using Brewster-angle geometry. A semi-empirical theoretical model of the Keating type achieves a reasonable fit to the observed mode frequencies as well as the elastic constant data of Saunders and Seddon (1970) and is used to predict dispersion relations. Qualitative features of the spectrum related to the role of the vacancy are discussed.

A general method is presented for the evaluation of multipole-multipole force constants, and the temperature dependence is calculated by representing each H₃ triangle as a superposition of a ring of charge q_r and a triangle of charge q_t, with q_t increasing and q_r decreasing as the temperature falls through T_c. Changes in these multipole forces with decreasing temperature depress the initial gradients of the transverse acoustic modes by coupling with the librational doublet, with the latter exhibiting soft-mode behaviour. The calculations also predict that the stability of the structure is very dependent on the magnitude of the interaction between clusters, in agreement with theory, and that the chloride structure is more stable and has a higher transition temperature than the iodide, both of which are in agreement with experiment.

Sequences of maximum and minimal variational principles for the lattice thermal conductivity coefficient obtained in an earlier paper by the author (Phys. Stat. Solidi (b), vol.68, p.213 (1975)) have been derived from canonical Euler equations. An example of a linear electrical network, which is analogous to a linearized Boltzmann equation, is also considered, and sequences of complementary variational bound-results are obtained for the power dissipation coefficient. The first term of the sequence of upper bounds for the lattice thermal conductivity is calculated and compared with some earlier reported bound-results for Ge.

The two-dimensional electron gas (2DEG) is a widely used model for the electrons in the inversion layer of certain MOSFET devices. The sensitivity of a number of physical quantities of the 2DEG to a proper treatment of many-body effects has been examined. This was achieved by calculating the plasmon dispersion, pair correlation function, exchange and correlation energy and the self-energy at the Fermi level using three different approximations to the many-body problem, namely: (i) the RPA; (ii) the Hubbard approximation (HA); and (iii) the self-consistent STLS approximation of Singwi et al. (1968). The RPA and HA were found to be less satisfactory approximations for a 2DEG than for a 3DEG.

The Pariser-Parr-Pople Hamiltonian is used within the framework of the restricted and unrestricted Hartree-Fock crystal orbital methods to compute the energy-band structures, charge and spin density distributions in the one-dimensional TCNQ^- columns of the NMP-TCNQ crystal. As a consequence of electronic correlation effects the ground state of the system is shown to be a magnetic insulator. Different mechanisms which reduce the interelectronic repulsion in organic systems are discussed and the possibility of the application of a simple Hubbard-type Hamiltonian to molecular crystals is investigated.

The localization of a non-interacting electrons is studied for systems in which the atomic sites are taken to be completely randomly distributed in space. The method used is based on weak-scattering arguments in tight-binding s and p bands in which the product of wavevector k and mean free path L is calculated. The criterion for the onset of localization is taken as the limit kL equals unity and estimates of the densities at which all states are localized are made for a particular model. The results are compared with Kikuchi's numerical calculations.

A method based on an interactive solution of the Boltzmann equation is presented for the calculation of the warm electron coefficient beta in polar semiconductors. The effect of electron-electron collisions is incorporated by using a Maxwellian energy distribution function for the carriers. With the aid of the method the warm-electron coefficients in InAs, InSb, InP and Hg_0.8Cd_0.2Te are computed taking account of all the relevant scattering mechanisms. The effects of band nonparabolicity, p-function admixture and electron screening are included in the calculations. The sensitivity of beta on the choice of the scattering parameters is studied and the available experimental data are analysed in the light of the calculated values. Agreement between theory and experiment is found to be satisfactory in the case of InAs only.

The thermoelectric power ( theta ) of Ag₄KI₅ and Ag₄RbI₅ is measured between room temperature and few degrees below their melting points using silver electrodes. The behaviour of theta with temperature can be expressed by the equations - theta =0.088(10³/T)+0.310(mVK^-1), (40-240 degrees C) and - theta =0.086(10³/T)+0.338(mVK^-1), (40-225 degrees C) respectively, for the two solids. A theoretical analysis of the data gives 0.088 and 0.086 eV for the heats of transport of Ag⁺ ions which are very close to the respective activation energies, in accordance with the prediction of the free-ion theory of Rice and Roth (1972) as well as that of the earlier theory on the heats of transport.

EPR experiments on SrFCl single crystals doped with Mn²⁺, Eu²⁺ and Gd³⁺ ions at room temperature are reported. It resulted that the crystal field parameter b₂⁰ for SrFCl:Mn²⁺ and SrFCl:Eu²⁺ is more than one order of magnitude larger than that of the system SrFCl:Gd³⁺. For the systems investigated the experimental values of b₂⁰ are compared with the theoretical ones, obtained from lattice-sum calculations and it was found that there is reasonable agreement between theory and experiment only for SrFCl:Gd³⁺. It is suggested that in the systems SrFCl:Eu²⁺ and SrFCl:Mn²⁺ the second degree crystal field parameter is determined by a displacement of the impurity along the tetragonal crystal axis. Furthermore, a possible breakdown of the point ion lattice model, resulting from over-lapping charge clouds for SrFCl:Mn²⁺, is indicated.

Theoretical expressions are obtained for the infrared absorption coefficient of a one-dimensional monoatomic lattice containing one diatomic substitutional impurity. The diatomic molecule interacts with the environment introducing high rotational barriers that reduce the molecular rotation to a libration coupled to the host-lattice vibrations (libron-phonon coupling). This is equivalent to the explicit consideration of static and dynamic interactions on the lineshape function. Numerical results of the absorption coefficient of CO trapped in linear chains of Ne, Ar, Kr and Xe interacting via molecular-pair potentials are discussed. The near-infrared spectrum is qualitatively in agreement with the measured vibrational frequencies.