Table of contents

Material flow takes place in nematic liquid crystals when a d.c. electric field E is applied. Some measurements of the mean velocity as a function of E and temperature are presented for different geometric conditions of the nematic material p-methoxy-n-p-benzilidene butylaniline (MBBA).

The exchange contribution to the band gap is calculated in a simple model to illustrate the various processes and approximations involved. It is shown how the exchange contributes a large and approximately constant fraction of the semiconductor band gap. The changes involved in using a dynamic electron-electron interaction are calculated and found to be significant for this model.

A simple model is proposed to account for the temperature dependence of the current from a field emitter at constant potential in He II. Experimental data suggests that a new conduction mechanism, for which an effective mobility mu _V=(8.0+or-0.4)*10^-3 cm² V^-1 s^-2 can be defined, is being observed below 0.5 K.

The techniques of the renormalization group and the epsilon -expansion are used to discuss static critical effects near a structural phase transition. In the case of the transition in strontium titanate the theory suggests that the static critical exponents are those of the classical isotropic Heisenberg antiferromagnet.

A closed form is presented for the one-dimensional Green function of a particle in a uniform electric field pervading all space. The Green function is found to be analytic in the whole complex energy plane; some interesting implications of this analyticity are discussed. Further analysis employing more realistic boundary conditions at infinity indicates, however, that no physical system (no matter how large) is adequately represented by the uniform field model for arbitrarily weak field intensities, though the limitation is of no practical importance.

For pt. I see abstr. A37362 of 1971. In an interacting spin-phonon system, coherences in the phonon scattering from different ions lead to coupled spin-phonon modes and the thermal resistance arises from the finite lifetime of the coupled excitations. The phonon Green function is derived here by means of a diagrammatic method employing the drone-fermion representation for spin-¹/₂ operators. Comparison of the phonon lifetime with previous treatments is made, with particular attention being given to the dependence on phonon frequency and ionic concentration. The relationship to the single-ion perturbation-theory calculation is then established and the implications of these results for the analysis of thermal conductivity data are discussed.

The systematic Laue method has been used together with a method of interfilm correlations to produce quantitative maps on a relative scale of the thermal diffuse scattering of X-rays by hexamine (hexamethylenetetramine) for the kl/2, 3kl/2 and 5kl/2 interlayers at room temperature. It is estimated that the error in the final results is probably less than 20%. The method is compared critically with the use of interlayer Weissenberg photographs and of diffractometers to measure X-ray diffuse scattering. It is concluded that it has some advantages over both these methods.

For pt. I see ibid., 1489-99. The X-ray thermal diffuse scattering from hexamine (hexamethylenetetramine) has been calculated on the basis of several lattice dynamical models. Experimental results do not distinguish between these models, although they do show the shortcomings of the difference Fourier transform approach. Absolute measurements of diffuse intensity to an accuracy of better than approximately 10% may enable a distinction to be made. Theoretical maps of the scattering at 298 K and 100 K calculated on the basis of one model are presented for the kl/2, 3kl/2 and 5kl/2 interlayers on an absolute scale. A comparison showed that there is a discrepancy at the point (1/2)80. This may be due to the omission in the theoretical calculation of contributions to the scattering by processes involving more than two phonons.

The dynamical matrix of a molecular crystal is set up in terms of Wilson's internal coordinates and of external coordinates describing the translational and librational vibrations of the molecules as rigid units. The treatment assumes the harmonic approximation. No extra parameters are introduced in the present formalism beyond those required for the isolated and rigid-molecule approximations. Perturbation expansions are given for the corrections to the isolated and rigid-molecule normal-mode frequencies. It is shown that the interaction between internal and external vibrations is a second-order effect only.

The model studied consists of a two-dimensional triangular lattice of which some sites are occupied by the centres of molecules, the remainder being unoccupied. The calculations of Bell and Lavis, using a first order approximation based on a triangle of sites, transitions are obtained, one of a vapour-liquid type and another of a liquid-solid type, the solid being less dense than the liquid. It is thus possible to obtain a complete phase diagram for vapour, liquid and one solid phase. The close-packed state is shown to be the low temperature form of the liquid phase.

Accurate measurements are reported of the heat capacity of a polycrystalline specimen of 99.999% pure tellurium between 1.5 and 20 K. The limiting low-temperature value for the corresponding Debye temperature Theta ₀^c is 152+or-1 K.

The principal linear coefficients of thermal expansion, alpha /sub /// and alpha _{perpendicular to} , have been measured from 2 to 30 K and 55 to 90 K. Above 10 K they agree with earlier work; from 2-5 K, alpha /sub /// approximately=-0.19*10^-8T³ and alpha _{perpendicular to} approximately=0.23*10^-8T³...K^-1. Limiting values (T to 0) of the principal Gruneisen parameters are gamma /sub ///=-0.7, gamma _{perpendicular to} =1.8 and gamma =2.1.

Results are reported of exact numerical calculations of the eigenvalues and eigenstates of a one-dimensional random Anderson model. The complete set of eigenstates is obtained for lattices containing up to 500 sites, and the degree of localization of the states is examined as a function of the eigenvalue of the state, the degree of randomness, and the number of lattice sites. Curves of degree of localization versus eigenvalue appear to show a central 'band' of extended states separated by a 'critical energy' from a 'tail' of localized states. However it is shown that the separation is only apparent, in that it is caused by the spatial extent of the eigenstates becoming comparable with the size of the finite lattice.

For generalized coherent potential approximations a theorem is derived which gives the necessary and sufficient condition for the equivalence of the localized state (or locator) expansion and the band state (or propagator) expansions. This condition is that the self-energy or coherent potential (and hence the locator) be cluster-diagonal. In the ordinary CPA this is satisfied sine the coherent potential is site-diagonal, but in most of the attempted generalizations of the CPA to clusters now in the literature this condition is not met. An exception is the molecular CPA model of Tsukada which is discussed as an example. Furthermore it is pointed out that when the self-energy is cluster-diagonal the multiple-scattering approach of Nickel and Krumhansl and that of Cyrot-Lackmann and Ducastelle and the diagrammatic technique all merge to a single result.

Using the Empirical Pseudopotential Method electronic charge densities are calculated as a function of position in the unit cell for ZnS in a zincblende and ideal wurtzite structure. A comparison of the charge densities reveals the presence of a net polarization in the ideal wurtzite structure. Two representative k points are found whose total charge density is in very good agreement with the charge density obtained by summing over many points in the irreducible part of the Brillouin zone.

The available experimental data on the conduction band structure of GaAs and InP is reviewed. The sub-band energy gaps and density of states in the minima involved in the high electric field transferred electron effect are discussed and preferred values given. The X_1C- Gamma _1C gaps in both GaAs and InP apparently increase at low temperatures. High pressure Hall effect data on InP are analysed in terms of possible pressure coefficients, effective masses, and energy gaps. The effective masses in the X_1C and L_1C minima are higher than expected from direct comparisons with other III-V compounds, but are in general agreement with the basic predictions of k.p theory.

The ionic thermocurrents (ITC) of undoped, NaF- and YF₃-doped CdF₂ were measured between 90 and 320 K on samples polarized at room temperature and below. It is shown that the two peaks observed below 200 K are due to two states of Na⁺-anion vacancy and Y³⁺-fluorine interstitial complexes with different binding energies and activation energies of reorientation of about 0.22 and 0.34, and 0.21 and 0.36 eV respectively. It is further shown that these ITC peaks are enhanced due to a space- charge formation.

The equations describing parametric conversion rates associated with acoustic waves amplified in the acoustoelectric effect are solved for two cases of downconversion and two cases of upconversion. The sources of energy, called the pump modes (p), are taken to be the incoherent waves amplified in the electronic gain process in a band around the frequency for maximum gain-the waves which receive energy by the parametric process are termed bucket modes (b). The four processes considered are for downconversion (p,pb) and (p, bb) for upconversion (p p, b) and (b b, b).

After correction for the moment reduction due to covalency and zero point spin deviation, the form factor closely follows the calculated free ion curve. The results have been compared with some recent theoretical calculations and ENDOR measurements.

The authors studied the magnetic susceptibility of the two- dimensional antiferromagnet Rb₂NiF₄ in the temperature range 77-671 K. The values of the paramagnetic Neel temperature, the Van Vleck susceptibility and exchange integral were calculated from the analysis of the susceptibility measurements and were found to be close in magnitude to and consistent with those reported for the iso-structural K₂NiF₄. The optical absorption and magnetic circular dichroism (MCD) spectra for the ³A₂ to ¹T₂, ³T₁^b transitions were measured in the range 50-295 K and the zeroth, first and second moments of MCD were estimated.

This centre consists of an electron trapped at an oxygen vacancy with a magnesium ion as one of its nearest neighbours. The EPR spectrum shows the expected tetragonal symmetry, and hyperfine interaction with the 10% abundant ²⁵Mg nucleus confirms the structure of the defect. Optical absorption bands associated with transitions from the '1s' ground state to the '2p' excited state split by the tetragonal field have been identified. Both bands show vibronic structure at low temperatures and their vibronic properties are compared with those of the F⁺ band. Annealing studies and a comparison of the ratio of F_A⁺ to F⁺ centres in various crystals show that the EPR and optical spectra are due to the same centre. A simple theoretical model gives the correct relative shifts of g/sub /// and g_{perpendicular to} , but not their absolute values.

For Pt.I see abstr. A84418 of 1972. The authors measured the tellurium hyperfine field in uranium monotelluride from ¹²⁵Te Mossbauer spectroscopy, and the uranium ordered magnetic moment in the same sample by neutron diffraction as a function of temperature. The temperature dependence is essentially the same except close to the Curie temperature, where effects due to short-range magnetic order may be present. The isomer shift of UTe is +0.24+or-0.05 mm s^-1 with respect to the source, and a value for the Debye temperature of about 160 K is obtained. The value of the tellurium saturation hyperfine field is 130+or-5 kOe. The values of the anion hyperfine field in UTe and in the antiferromagnet USb are discussed in terms of the long-range nature of the magnetic exchange interactions in those compounds.

The Mossbauer spectra of two compositions of hexagonal Ni_1-xS doped with 2%⁵⁷Fe are obtained at various temperatures from 4.2 to 300 K at T_c=244 K in the first and 221 K in the second, a first-order phase transition occurs between an antiferromagnetic nonmetal and a paramagnetic metal; the magnetic susceptibilities, thermoelectric powers and conductivities change sharply in magnitude with a transition width of 4-5 K. The magnetic hyperfine splittings are fitted by a least squares procedure to a Brillouin function with J=2. The values of the spin disordering temperature T_B at which the function cuts the zero-magnetization axis are discussed.

The lineshape for the transition to a vibronically coupled doublet from a singlet state has been calculated using an analytic approach proposed by M. Wagner. The result obtained is compared with another numerical method.

Experiments support the hypothesis that the continuous X-ray spectrum arises from a coherent electron scattering by a crystal lattice. Structure is observed in the spectrum which is interpreted as evidence that some sharp lines are present in the spectrum due to the periodic nature of the scattering potential, although the intensities are too weak to permit resolution of individual lines.

A simple model is put forward which demonstrates that the interpretation of directional effects in photoemission from layer crystals should be rather more straightforward than for non- layer materials. MoS₂ is taken as an example and the available experimental data are discussed in terms of this model.