Table of contents

The identification of the 9R rhombohedral structure as the low-temperature phase of lithium offers a resolution of the longstanding order-of-magnitude discrepancy between theory and experiment for the electron-electron scattering resistivity. In the 9R structure, the Fermi surface of monovalent lithium has many intersections with the Brillouin zone boundaries. This implies polyvalent-like behaviour for the electron-pseudo-wavefunctions, which leads to an order-of-magnitude increase in the calculated electron-electron scattering resistivity.

The theory of electromagnetic wave propagation in a cholesteric liquid crystal is developed, in the case when the free-space wavelength of the radiation is small compared to the pitch of the crystal. An arbitrary direction of the light is considered. Reflection and refraction at a crystal face that is perpendicular to the cholesteric helix is studied. Explicit formulae are found for the electric and magnetic fields of the reflected and refracted beams.

Neutron and X-ray diffraction measurements have been made on series of eight mechanically alloyed CuTi amorphous alloys having atomic compositions between 75 and 30 at.% copper. The neutron measurements have shown that the alloys have a small impurity of hydrogen, which is attributed to contamination of the parent titanium powder. After appropriate corrections, the structure factors for these alloys have been found to be broadly similar to those of both glassy and molten CuTi specimens. The radial distribution functions provide some evidence of detailed differences between the atomic-scale structures of the molten, glassy and mechanically alloyed samples, which will need to be confirmed by measurements on samples of higher purity. The possible influence of gaseous impurities on the MA process is discussed.

The authors reconsider vesicle budding, this time including (besides bilayer bending elasticity) the energy of monolayer stretching, i.e. area dilation of one monolayer and compression of the other. Monolayer stretching during a shape change of the vesicle affects the spontaneous curvature of the membrane but, if measured values of material parameters are inserted in the equations, it turns out to be a minor perturbation. They also treat the budding of infinitely extended flat membranes where the spontaneous curvature is strictly constant during the deformation. The calculated shapes are compared to photographed buds in very large phosphatidylcholine membranes.

Recently it has been shown that the nearly free electron model for describing the electrical, magnetic and optical properties of expanded liquid caesium has already broken down by the time three times the critical density is reached, i.e long before the transition from the metallic to the non-metallic state occurs, which seems to coincide with the critical point of the liquid-vapour phase transition (T_c=1924 K, p_c=92 bar, rho _c=0.38 g cm^-3). The authors discuss the deviations from the nearly free electron model within the framework of a consistent quantum statistical approach to the electrical conductivity, which leads to a generalized Ziman formula, taking into account electron-electron scattering, arbitrary degeneracy and screening. Further possible mechanisms that might contribute to the behaviour of the electrical conductivity, such as the formation of bound states are also discussed.

Diffuse backscattering of electrons at a single crystal surface is often due to the adsorption of atoms or molecules whose presence breaks the translational symmetry of the substrate surface. Two different approaches to the diffuse LEED problem have been given. The first, due to Saldin and Pendry, considers only a single molecule adsorbed at the surface. The second, due to the present authors, considers a partially ordered overlayer of molecules and takes up the aspect of the diffuse LEED problem. The first aim of this paper is to compare both approaches. In the case of adsorbates randomly and sparsely distributed at the surface, the authors show that they are equivalent. However, each of them uses a different concept for the renormalized t-matrix of the adsorbate. The second aim of this paper is to provide a theory of multiple scattering of electrons from a molecule, from which the authors can derive a suitable expression for the t-matrix that one can readily use in the diffuse LEED theory. Application to the simple case of the CO molecule is examined here.

For pt.I see ibid., vol.4, p.1671, 1992. The aim of this paper is to investigate the relative influence of geometrical and order parameters on diffuse LEED intensities. In most diffuse LEED studies relative to molecules adsorbed at single crystal surfaces, there are probably several chemisorption sites or several adsorbed species. For this reason, this situation is examined in detail here. The elastic diffuser LEED intensity from a single crystal surface Pt (111) partially covered with CO molecules is calculated for different values of the parameters which characterize the adsorbate/substrate model. These parameters are separated into: geometrical parameters related to the local arrangement of atoms near chemisorption sites, and order parameters related to the statistical distribution of the occupied chemisorption sites. In the present case, the geometrical parameters are the C-Pt and the C-O bond lengths and the location in the 2D unit cell of the surface lattice (bridge and top sites). The order parameters are the site occupancy pair correlation functions or the quantities which determine them: surface coverage, surface temperature, pair interaction potential between adsorbates, etc. Even in the case of a short-range order, the sensitivities of diffuse intensities to both kinds of parameters are found very similar. On the other hand, in the situation where several kinds of chemisorption sites exist, the diffuse LEED intensity is not the product of a form factor and a structure factor. So the use of the Y function method, which avoids the structure factor determination, is not possible. Thus, a determination of the statistical distribution of adsorbates at the substrate surface cannot be circumvented.

This paper is devoted to problems of self-organization of surface periodic structures on a Si crystal under the action of nanosecond excimer laser radiation. One kind of structure, the so-called 'dead zones', on Si crystal surfaces after UV excimer laser irradiation is discussed in detail. A possible mechanism of the formation of regions lacking indulation ('dead zones') is due to an interaction between the capillary waves (grad T_x not=0, along the surface) and the thermocapillary waves (grad T_z not=0, towards the interior of the liquid silicon).

The transport properties of bismuth are investigated within the pseudo-parabolic model by using a variational technique. The deformational potentials are accordingly taken to depend on the energy of electrons. The energy dependence considered is shown to be a general characteristic of the two-band model and is not confined to the case of bismuth. The convergence of the method is studied both numerically and analytically. Analytical expressions are obtained in the low-temperature range (T<77 K) by using two variational parameters. For higher temperatures (77 K<or=T<or=300 K) the resistivity and thermoelectric power coefficients are calculated numerically to sufficiently high accuracy.

The crystallographic phase transition which 1,2,4,5-tetrachlorobenzene undergoes at about 188 K from a high-temperature monoclinic (P2₁/a) structure to a low-temperature triclinic (P1) structure has been studied by means of the ³⁵Cl nuclear quadrupole resonance transition frequencies measured as a function of temperature from 77 to 300 K. The transition is shown to be first order displacive, determining the order parameter and its temperature dependence, and deducing a critical exponent beta =0.309+or-0.012 which indicates that the system behaves as a 3D Ising-like model where the important forces are those of short range. Also, the measured spectra are shown to be consistent with the measured (P2₁/a) and proposed (P1) crystal structures, producing an assignment of the spectra to the various chlorine sites in both primitive unit cells. Additionally, a wide temperature range (approximately 165-205 K) where phases coexist in a stable form is found and analysed.

The authors determine the extent to which Lifshitz tail states can be defined in a condensed Bose system. This provides a justification for the inert-layer picture in the study of helium on Vycor. They argue that, in three dimensions, hard-core bosons can be trapped by a sufficiently deep potential well despite a strong tendency for all particles to participate in the Bose-Einstein condensate. This is demonstrated by a variant of the conventional spin-boson problem. The authors find that phase fluctuations in the condensate tend to depress particle exchange between the trap and the rest of the system and hence has an enhancing effect in terms of Lifshitz-tail localization. However at low densities (near onset of condensation) they argue that the condensate will be relatively enhanced compared to the inert layer model, in agreement with experiment.

Semi-empirical effective charge calculations were made of crystal field effects and the corresponding energy level splittings for NNN-F^- compensated centres in a number of RE³⁺: fluorite structure systems. While the results obtained confirm that such centres were indeed observed in a number of experimental investigations of these dopant: lattice systems, in at least one instance (the B centre in the Er³⁺:CaF₂ system) such identification appears to be untenable. Model reconstructions were also made of local lattice distortions in the centres in the Eu³⁺:SrF₂ and Er³⁺:SrF₂ systems. The two centres are structurally related but the corresponding local distortions appear to be qualitatively different, this result is discussed in the context of other recent work in the field. Finally, the procedure developed was used to improve the phenomenological calculation of B_q^k values for the trigonal entre in the Eu³⁺:BaF₂ system, demonstrating its utility in such applications by significantly improving final fit quality.

Measurements of the electrical resistivity in high-purity, single-crystal palladium are presented for the temperature range 17-300 K. A polynomial in temperature is used to describe the data. The results are compared with previous work and the differences are attributed mainly to impurities.

The authors consider the semiclassical description of the fractional quantum Hall effect based on cooperative ring exchange. They generalize the original picture to the n=1 Landau level and look at its validity. They find that the Wigner-crystal-like configurations which are the starting point for the picture are unstable for filling fractions nu ₁>or=1/3. For nu ₁<1/3 the effects of ring exchange are quantitatively very similar to those in the n=0 Landau level.

The authors predict the presence of symmetric-antisymmetric oscillations of the magnetoplasmon energy as a function of magnetic field in a double-quantum-well structure. Because electrons in the localized states cannot effectively screen the scattering potential from the impurities, in a strong magnetic field, the Landau-level width increases much faster than the square root B dependence in a weak field. As the barrier width increases, the oscillation peaks at the smaller odd integral filling factors are smeared out before those at the larger odd integral filling factors. This leads to the quenching of the symmetric-antisymmetric magnetoplasmon oscillation peaks.

Measurements of AC resistivity have been carried out on sol-gel-derived glasses in the system Sb₂O₃-(1-x)SiO₂ with x=0.07 and 0.23 for frequencies from 2 kHz to 100 kHz in the temperature range 80-400 K. The AC resistivities show a sharp minimum at around 310 K. This is ascribed to a reduction in the ratio of (Sb⁵⁺) to (Sb³⁺) in these glasses as a function of temperature. Another minimum is observed at temperatures of 178 K and 209 K for glasses 1 and 2, respectively. The frequency exponent s for AC resistivity also shows a maximum as a function of temperature at around these temperatures. The correlated barrier hopping model has been used to explain the trend of s in the temperature range 170-290 K whereas the overlapping long polaron tunnelling mechanism appears to give the correct variation in the temperature range 100-170 K. Both the glasses show anomalously large values of dielectric constant in the range 1000-6000 at a frequency of 2 kHz at around 310 K. Analyses of all these data indicate that there are antimony-rich and antimony-deficient layers present in this glass system.

The extreme profile of a magnetic field penetrating into a non-uniform Josephson junction is computed in the case of periodically arranged pinning centres by use of a numerical solution of a chain of the sine-Gordon equations with boundary conditions at the pinning centres. It is found that the extreme profile obtained is approximately described by the Bean critical-state theory. At sufficiently small spacings the authors find no stochastic regime for a Josephson junction of a finite length. This shows two possibilities: either the front of the profile is extremely extended or at magnetic fields larger than H_cl, only a uniform vortex distribution is realized.

Using a plasmon exchange model, analytic expressions for the effective interaction between charge carriers in a multilayered system containing one, two, three and four conducting layers per unit cell has been calculated. The interaction is mediated by one pseudo-optical plasmon band and L-1 proper narrow acoustic plasmon bands where L is the number of layers per cell. It is found that the pseudo-optical plasmon band has a proper optical (high energy) limit, equivalent to bulk plasmon modes, as well as an acoustic (low energy) limit. In part II (ibid. vol.4, p.1814, 1992) the authors show that this interaction can lead to superconductivity in the high-T_c superconductors.

For pt.I see ibid., vol.4, p.1799, 1992. A theory for high-T_c superconductivity has been developed using the Eliashberg model on the basis that the attractive interaction is provided by the Plasmon-mediated effective interaction between the charge carriers. Calculation of T_c using these interactions, presented in paper I, shows that T_c should rise, with a saturation effect, with the number of CuO layers per unit cell, which is in agreement with observations in the Tl- and Bi-based compounds. Furthermore, this theory shows that a short coherence length is an essential requirement for superconductivity at high temperature.

Electron paramagnetic resonance studies of Rb_1-x(NH₄)_xH₂PO₄ (x=0, 0.5, 0.8, 0.9, 0.95 and 1.0) containing an AsO₄^4- probe gave a clear indication of the fluctuations between ferroelectric (FE) and antiferroelectric (AFE) regions in the mixed crystals right from room temperature. This has become possible because the AsO₄^4- probe has different ⁷⁵As hyperfine coupling constants in pure RDP (FE) and APP (AFE) crystals (x=0 and 1). This fluctuation is manifested as the averaging of the ⁷⁵As hyperfine coupling constant, with compositional dependence of the form (A)mixed crystal= (1-x)A_FE+xA_AFE. The characteristic fluctuation time T<or=3*10^-8 s. The existence of this fluctuation in the mixed crystal also shows up in the washing out of ¹H superhyperfine structure from the close protons which is of the order of 30 MHz.

The mixed-spin-relaxation rates of substitutional paramagnetic Fe²⁺ (⁵D) ions in ZnCO₃ have been explicitly evaluated between 10 and 35 K. These results are then compared with the experimental data obtained earlier from the analysis of the Mossbauer magnetic hyperfine spectra of the above system at liquid-He temperature. It is found that at low temperatures the mixed-spin-relaxation process provides a competing mode for the exchange of energy between electronic spins and phonons.

Guanidinium hexafluoroaluminate (C(NH₂)₃)₃AlF₆ crystallizes in a cubic system in the space group P2₁/a3,a=13.953(2) AA, Z=8. The guanidinium cation does not exhibit any symmetry; the hexafluoroaluminate anion has 3 symmetry and occupies two inequivalent (4a) and (4b) positions. The ions are connected by relatively strong N-H...F hydrogen bonds. Proton and fluorine spin-lattice relaxation times as well as second moments have been measured over a wide range of temperatures. The analysis of all cross-relaxation effects for the system of four unlike spins has been performed. Activation parameters for the C₃ reorientation of the cation and for the NH₂ group motion have been derived. Unlike in other guanidinium salts, anion reorientation is hindered by a much higher energy barrier than cation reorientation.

A ¹⁶⁹Tm Mossbauer investigation of thulium pyrogermanate is reported. All spectra recorded over the temperature range 4.2 K-740 K are pure quadrupole doublets. This observation and the detailed temperature dependence of the quadrupole interaction are inconsistent with recent claims of a fivefold local symmetry at the Tm³⁺ site. Initial attempts at analysing the data in terms of the full crystal-field Hamiltonian appropriate for C₁ symmetry suggest that the electronic ground state consists of a pair of non-magnetic singlets well isolated from the next state.

The sub-surface region of semi-insulating gallium arsenide, after annealing at a range of temperatures, has bean investigated with a positron beam. A positron diffusion model analysis of the Doppler lineshape parameter as a function of positron energy shows significant variations in the surface region due to annealing. A comparison with infrared spectroscopy measurements suggests that the observed changes are due to positron trapping at Ga vacancies associated with the formation of Ga_As antisite defects. After annealing at 900 K, a layer of positively charged As vacancies induces an electric field which in turn prevents positrons from diffusing to the surface.