Table of contents

It is shown that by imposing two physical restrictions-(a) only one atom (Li or Fe) can occupy a given site; and (b) the nearest neighbours of Li should be Fe-only two structures can occur in the ordering process of LFS-type: that of LFS and another associated with k=0.

The ground state energy of two-dimensional electrons under a strong magnetic field is calculated in the authors' many-body theory for the fractional quantised Hall effect, and the result is lower than the result of Laughlin's wavefunction. The presence of the energy gap at fractional fillings provides a downward cusp in the correlation energy which makes those states stable to produce quantised Hall steps. The ground state has a broken symmetry and no pinning.

Presents detailed results on the low-temperature magneto-resistance of Sb doped Si. Evidence is presented for the existence of the T^1/2 correction due to the electron-electron interaction. It is suggested that the large change in density of states leads to a combination of localisation and interaction effects. The authors find a large rise in conductivity with decreasing temperature which they ascribe to an electronic phase transition which may be driven by the interaction.

The authors demonstrate the disappearance of plateaux of quantised Hall resistance with increasing frequency resulting in a decrease in the extent of localisation. At the highest frequencies this is accompanied by the evolution of structure corresponding to fractional quantisation. This is discussed in terms of an increased role for the electron-electron interaction as the effects of disorder diminish.

The authors report the observation of fractional quantisation obtained by increasing the frequency of measurement; this is found up to a Landau index of 10. The result is coincident with the suppression of integer quantisation and is interpreted as indicating a transition from a liquid state to a solid state behaviour as the time scale of measurement is reduced.

X-ray absorption spectroscopic (XAS) technique has been employed for the study of charge transfer in the ternary intermetallic compounds RM₂X₂ (R=Nd, Sm and Gd; M=Mn, Fe, Co, Ni and Cu; X=Si or Ge). It is found that each constituent R, M and X in RM₂X₂ compounds contributes electrons to the conduction band. Further, the XAS study in conjunction with Mossbauer measurements on the samples RFe₂Ge₂ (R=Nd and Gd) indicate that there is no charge transfer from R and Ge to the 3d band of Fe.

The new tetrachalcogenide compounds have a one-dimensional structure and two of these show transport properties which are characteristic of this type of conductor. The authors electron microscopy study permits them to show clearly and characterise the charge density waves which appear at 263 and 285K in (TaSe₄)₂I and (NbSe₄)₁₀3/I respectively. Moreover, they describe the behaviour of (TaSe₄)₃I and (NbSe₄)₃I when the temperature changes. These compounds undergo a space group change around 200 and 280K respectively. The results are connected to those obtained in other works by the study of transport phenomena or by band scheme calculations.

In a recent paper it was shown how to treat a static deformation of a crystal as a quantum mechanical perturbation. The perturbed eigenfunctions of the system are used here to evaluate the internal strain and its first-order contributions to the elastic constants and piezoelectric coefficients. For the harmonic lattice model it is shown that the internal strain and its aforementioned contributions are independent of the quantum phonon state and hence, for a given harmonic lattice, of temperature. The results for the harmonic model agree with the classically derived results of Miller and Axe (1967).

Previously reported experimental results on the Na Auger signal decay that occurs during electron irradiation of soda-silica glasses, at liquid nitrogen temperature, have been interpreted on the basis of a numerical integration of the continuity equation for the ordinary and field-assisted diffusion processes. In this context, it is shown that the diffusion process of Na is stimulated by the electron irradiation. It is suggested that the diffusion process is enhanced by oxygen- and sodium-ion pairs being created by bond breaking when electrons strike soda-silica glasses. This phenomenon occurs not only at the surface, as some previous work has suggested, but also in the bulk. Moreover, some critical remarks about previous theoretical models concerned with the incubation time, during which the Auger signal remains constant, are also reported.

The exact calculation of soft X-ray spectra and surface dipole moments based on the recursion method is discussed. Simple cubium is used as a test example and exact and explicit formula are given for the dipole moment of a surface atom as well as the numerical result of the soft X-ray spectrum. For ideal surfaces the authors propose using the recursion method in a hybrid way, where the periodicity parallel to the surface is explicitly included.

There is a strong unscreened Coulomb repulsion between the electrons in the impurity band of a lightly doped compensated semiconductor. This can be divided into a long-ranged 1/r part and a short-ranged part between two electrons on the same site. Both of these interactions may lead to gaps in the density of single-particle states, the Coulomb gap and the Hubbard gap respectively. The properties of these two types of gap are explored in this paper using a classical model of an impurity band. The model was originally used by Efros and Shklovskii (1975) to study the Coulomb gap for spinless electrons. Including spin and the Hubbard (on-site) interaction leads to some interesting changes in the form of the Coulomb gap. Under the conditions appropriate for the impurity band of a lightly doped crystalline semiconductor the gap is asymmetric and in most situations the coefficient of the parabolic dip in the density of states is different from that calculated by Efros. The case of a negative Hubbard U, as may be relevant for a non-crystalline semiconductor, is also discussed briefly.

For pt.I see ibid., vol.16, p.5031 (1983). The strain results of Baker and Currel (see ibid., vol.9, p.3819, 1976) are analysed in terms of the superposition model using a complete diagonalisation of the J=¹⁵/₂ ground-multiplet Hamiltonian matrix with several sets of static crystal-field parameters. Fitting errors of a similar order of magnitude to data uncertainties and values of superposition-model parameters consistent with those obtained from other experimental data are obtained using the static crystal-field parameters proposed by Baker and Currell.

An analysis of the internal strain degrees of freedom in zircon-structure crystals RMO₄ is given. These are related to local distortions in the neighbourhood of the R³⁺ ions, and hence to the orbit-lattice coupling coefficients. The corresponding spin-lattice coupling coefficients are obtained for distortions involved in Jahn-Teller phase transitions and comparisons with experimental values are made for TmVO₄ and DyVO₄.

The thermopower and thermal conductivity of a two-dimensional electron system in a quantising magnetic field have been studied. The authors' results are based on the Kubo formula and the self-consistent Born approximation (SCBA). The non-diagonal component of the thermopower exhibits a novel oscillatory structure. The Wiedemann-Franz law appears to be violated by the diagonal coefficients. The descriptive parameters are k_BT/h(cross) omega _c, mu /h(cross) omega _c and Gamma /h(cross) omega _c; mu and Gamma being the chemical potential and the Landau level broadening, respectively.

An analysis of the conductivity intercept sigma _int found by extrapolating at high temperature the activated regime of conduction in 2D inversion layers is presented. In particular the dependence of sigma _int on carrier concentration N_s and oxide charge density N_ox is studied through the energy dependence of the conductivity function sigma (E) near the mobility edge. It is shown that the linear dependence of sigma _int versus N_s is mainly associated with a component linear of sigma (E) in energy which is attributed to an equivalent minimum mobility above the mobility edge. It is emphasised that the concepts both of minimum metallic conductivity and of minimum metallic mobility are involved in the mobility edge question.

Insight into the resolution of the recently developed technique of scanning tunnelling microscopy (STM) is achieved by considering the transmission of free electrons through a constant potential barrier with corrugated boundaries representing the sampled surface and probing tip, respectively. The amplitudes of the reflected and transmitted waves are calculated via an extension of the so-called GR-method developed to treat scattering from a corrugated hard wall. Results for the distribution of current density, for the dependence of the tunnelling current on the horizontal and vertical positions of the scanning tip and for the resulting equicurrent lines (STM images) are presented for a two-dimensional model. Simple analytical approximations are shown to reproduce computed trends versus tip-sample separation, tip curvature and average barrier height.

An eight-millimeter variable wavelength electron spin resonance spectrometer has been used to study the linewidth and relative intensity in some antiferromagnetic insulators in the paramagnetic region above the Neel point. The materials that have been studied are polycrystalline Cr₂TeO₆ and Cr₂WO₆. The intensity data give T_N=94+or-1K for Cr₂TeO₆ and 44+or-2K for Cr₂WO₆. Both the intensity and linewidth data suggest considerable short-range order above T_N.

The results of reflectivity measurements for An₃P₂ and Zn₃As₂ in the energy range 0.5-5 eV for polarised light, e//c and e perpendicular to c, at room temperature are presented. By using a modified algorithm for the Kramers-Kronig analysis the optical constants of both compounds have been calculated. Utilising group theory the selection rules for the optical transitions in these compounds have been specified and the observed structure of the reflectivity spectra has been related to the possible interband transitions.

This study of EuVO₄ is the first application of hole-burning techniques to the investigation of defect sites in a rare-earth compound. Laser excitation of the Eu³⁺ absorption spectrum in the region of the ⁷F₀-⁵D₀ forbidden transition ( approximately=580 nm) reveals some 30 absorption lines. Each corresponds to a distinct crystallographic site in EuVO₄, as there are no crystal-field splittings in a 0-0 transition. Hole-burning of several of these lines indicates a wide range of quadrupole splitting parameters P and eta , as would be expected for different crystallographic sites. More detailed study of two of these hole-burning spectra shows that, once the parameters P and eta are determined, Zeeman spectra of selected holes directly determine the orientation of the quadrupole tensor relative to the crystallographic axes. In such a simple crystal structure as EuVO₄ it is not at all obvious that there could be as many as 30 different type of defect site, and further studies will be needed to explain this.

Single crystals of Rb₂MnCl₄ and Rb₃Mn₂Cl₇ were prepared from the melt. The ⁶A₁ implies/implied by ⁴T₁ (G) transitions were investigated by high-resolution absorption, luminescence and excitation spectroscopy. Sharp fine structure was observed in the overlap region of excitation and luminescence spectra at low temperature. This can be rationalised in terms of pure-exciton, exciton-magnon and exciton-photon transitions. Excitation transfer occurs as a result of the exchange coupling.

Anomalous X-ray scattering factors at photon energies close to absorption edges are calculated by applying the dispersion relation to absorption spectra. Absolute absorption coefficients are obtained from measured relative spectral distributions normalised to known absolute cross sections. The calculated scattering factors are corrected for relativistic contributions. Results for Ni, Cu, Zn, and Zr close to the K-edges and for Ta, W, Pt and Au near the respective L-edges are presented and compared with published scattering factors.