Table of contents

The velocity autocorrelation function for a viscous fluid is studied, in the limit of long wavelengths in terms of a non-linear integral equation for a wavevector and frequency dependent contribution to the kinematic viscosity. The results are consistent with the recent predictions by Forster, Nelson and Stephen (1976) using a renormalization-group method.

The growing interest in microwave field effect transistors points to the need for assessing rapidly the high-frequency potentiality of binary compounds and mixed crystals. Though the two relevant parameters, the breakdown field and the electron mobility, are difficult to calculate from first principles, their product is related only to the effective mass and the LO mode frequency, and this product gives a good measure of the peak electron velocity. Velocities are calculated for a number of materials, and comparisons made with experiment.

A simulation of the transmission of electrons through thin films of aluminium, copper and gold is made for a primary energy of 1 keV and for normal incidence. The model includes the essential contributions to the attenuation of the primary electron beam and allows the determination of the range of electrons, the variation of the transmission coefficient with the sample thickness and the shape of the energy distribution of the electrons. The results show a reasonable agreement with experiment.

Far-infrared reflectivity measurements are reported for n- and p-type SnS single crystals between 10K and 300K using polarized radiation in the frequency range 50-350 cm^-1. Kramers-Kronig and least-squares fitting analyses are used to determine the values of the phonon frequencies and dielectric constants for light polarized parallel to the a, b and c crystal axes. Transmission measurements on thin samples are also reported which confirm the reflectivity measurements.

For pt.II see ibid., vol.9, p.227 (1976). It is shown by means of symmetry arguments that the models proposed in the preceding papers are generalizations of the usual ones. The implications of this result on the concept of crystal symmetry are discussed in detail with special emphasis on treating the crystal as a finite dynamical system.

It is shown that finite strain coordinates may be obtained from the unique factorization of a non-singular matrix into the product of a triangular matrix, with positive diagonal elements, and an orthogonal matrix. These finite strain coordinates are the six elements of the triangular matrix. They are the usual strain elements of infinitesimal strain theory, if a suitable geometrical restriction is applied. The use of this restriction and of these coordinates facilitates a discussion of the stability of a phase, which differs from previous treatments. It is shown that it is necessary to ensure that the mechanical equilibrium condition of vanishing total moments be maintained throughout any variation, by the use of geometrical constraints. The well known difficulty of the so-called trivial instability of a compressed strut does not occur in this treatment, and the thermodynamic stability of a phase can be tested against any variation of intrinsic elastic strain whatsoever. A fundamental error in a recent criticism by the author of the Coleman-Noll inequality is acknowledged, and this inequality is briefly discussed.

Vapour pressure measurements on dilute Ar-O₂ alloys, which have the HCP structure, have been used to measure the free energy difference between the HCP and FCC forms of solid Ar near its triple point. The value of the relative free energy difference Delta (T) is found to be 1.20+or-0.27*10^-4 at T=80.72+or-0.04K falling to zero at T=89.3+or-1K. The vapour and solid concentrations corresponding to the FCC-HCP phase boundary have also been determined at 80.72 and 72.14K.

The dynamics of the A₁ phase of ³He in appreciable magnetic fields is discussed within the framework of irreversible thermodynamics. Special attention is paid to the spin part of the order parameter, which, though not a macroscopic variable in a strict sense, may have an influence of the dynamics in NMR experiments. Coupling effects between the spin and the orbit part of the dynamics, induced by the external magnetic field are considered. Among the authors' predictions are a magnetic field dependence of first, second and fourth sound, a finite lifetime of one branch of the orbit 'waves' even for k=0, and, possibly, a further structure of the transverse NMR spectrum due to the internal motion of the spin-order parameter.

X-ray structure factors and directional Compton profiles are calculated for diamond (cubic carbon) using a self-consistent field unrestricted Hartree-Fock procedure on a cluster of five carbon atoms. A Mulliken population analysis procedure is used to isolate results to be associated with the central atom. The choice of a high spin state for the cluster yields reasonable agreement with the experimental and theoretical results for crystalline diamond.

The variational approach to the problem of multimode Jahn-Teller coupling of impurities of solids is extended to finite temperatures. A trial density matrix is constructed which is isomorphic with that derived from the cluster model but differs from the latter in that the effective quasi-molecular frequency and coupling strength are allowed to be temperature dependent. The behaviour of these parameters as functions of temperature is determined in a self-consistent manner from a thermodynamic variational principle. Simplified lattice models are employed to exemplify the method for various Jahn-Teller systems.

The electrical conductivity of La_1-xSr_xVO₃ (0<or=x<or=0.4) has been measured in the temperature range 170K<or=T<or=1600K. At low temperatures and for small or zero values of x, the material behaves like a conventional impurity semiconductor with conduction resulting from the excitation of carriers from states 0.14 eV above a valence band edge. At high temperatures, the conductivity increases, with an activation energy of 0.63 eV. For x>or=0.05, the conductivity is determined by the strontium concentration with the mobility remaining constant at 0.15 cm² V^-1 s^-1. A metal-nonmetal transition of the Anderson type is observed in both the conductivity and thermopower for x approximately=0.2, and a dramatic increase in mobility at this composition is deduced. The thermopower activation energies are lower than the corresponding conductivity activation energies, but there is little evidence for strong polaronic effects. A band model involving impurity Hubbard bands and the t_2g band of the vanadium ion is established which accounts for both the electrical and optical properties of LaSrVO₃.

Carrier yields were measured using short electron pulses of energy up to 50 keV and clearing fields up to 20 kV cm^-1 over the temperature range 323 to 85K and found to vary with temperature approximately as T². This behaviour favours the mechanisms of geminate recombination rather than that of columnar recombination as being responsible for the low carrier yield in electron-bombarded anthracene. The electron mobility normal to the ab crystallographic plane is 0.38+or-0.02 cm² V^-1 s^-1 in the temperature range 323 to 85K and agrees well with other published values. The observed continued flow of electron-drift-induced charge below 195K is attributed to a trap release mechanism.

For pt.II see ibid., vol.9, p.809 (1976). The exchange-correlation energy, V_xc, for the jellium model of an interacting electron gas with surface is studied by the application of the random phase approximation. The leading coefficients of the r_s expansion for the surface energy are evaluated. It is shown that V_xc is not well described by a finite gradient expansion.

The authors combine the local density of states method with the bond orbital model to study several surfaces of silicon. Relaxation, hydrogen chemisorption and a 2*1 reconstruction have been considered for the (111) surface. In addition, surface-state band structures and total densities of states are reported for both the ideal (100) surface and a model of its 2*1 reconstruction, using a slab model. By examining local densities of states which have been obtained in the same way for all the surfaces considered, it is possible to see the strong similarities in most of the valence band as well as the differences in the details of the dangling-bond surface states. The effect of the surface on the lower part of the valence band is seen to be insensitive to relaxation and reconstruction.

Electron irradiation of NaCl crystals has revealed structure in the luminescence spectra. Variations in electron energy, lifetime studies and changes in irradiation temperature enable one to resolve several of the component features. The spectra comprise overlapping emission bands centred near 2.35, 2.65 and 2.85 eV which are interpreted as the result of exciton decay at impurity sites. The intrinsic emission at 3.4 eV is also noted. Detailed studies also reveal the presence of excited excitons which have different lifetimes in the bulk and in the surface layers. The higher states of the surface exciton can be preferentially stimulated with an electron beam at the energy of excitation of the sodium K_alpha energy level. The luminescence is correlated with defect formation and ejection of atoms from the surface.