Table of contents

Quartz is an insulator with an extremely wide band gap in the vacuum ultra-violet. However, under irradiation from high-energy electrons or X-rays, samples of high purity emit a luminescence band in the blue, corresponding to a Stokes shift of approximately 7 eV. This large Stokes shift has been ascribed to the self-trapping of an exciton in an otherwise perfect lattice owing to the distortion it induces; the authors review the evidence for this assignment, and describe electronic-structure calculations which reveal the structure of the distorted configuration and also explain various experimentally determined properties of the centre. The self-trapping process they postulate is a novel one as it is driven primarily by the electron component of the exciton.

A metallic point contact, consisting of a tungsten whisker pressed onto a tungsten film, is used to inject high-frequency phonons into a ruby crystal kept at 1.5 K. The effective phonon temperature near the point contact is measured via the temperature dependences of the position and width of the R₁ line of the Cr³⁺. High Spectral resolution is provided by the technique of fluorescence line narrowing. The temperatures observed range up to 300 K, demonstrating the effectiveness of the point contact as a generator of high-frequency phonons.

A 63-atom cluster is employed, in a self-consistent semi-empirical molecular orbital total energy algorithm, to calculate the binding energies of potassium atoms adsorbed on a Si(100) (2*1) reconstructed surface at various possible adsorption sites, namely, sites H, B and C. The authors results strongly suggest that the B site is the most stable one. The results of a half-monolayer coverage calculation show that the double-layered structure of potassium atoms is feasible and in accordance with recent experimental observations. However, the bond length between Si and K, d_Si-K, is found to be around 15% lower than the experimental determination. From the Mulliken population analysis, the amount of charge transferred from Si to K atoms varies from 0.01 to 0.32 of the electronic charge at various adsorption sites.

The authors measure to low temperatures (10 mK) the conductivity behaviour in high magnetic fields of an Sb-doped Si sample of just-metallic character. The sample had previously been assessed by Long and Pepper (1984) as showing evidence for a new electronic phase. Their measurements further highlight the anomalous low-temperature behaviour, and they discuss the results in the context of a recently published theory of the effect, and in comparison to other magnetoresistance data on silicon-based doped semiconductors. A further possibility emerges from this discussion, in that the anomaly may be appearing under experimental conditions such that the usual limiting mechanism on the conductivity is 'frozen-out' by the high magnetic fields used.

The non-equilibrium Green function technique is applied to derive a quantum kinetic equation for transport in quantum wires. Calculations are done for short-range impurity scattering neglecting localisation effects. In linear response the expression obtained for conductance is equivalent to the Kubo-Greenwood formula. Scattering-induced level broadening is included by self-consistent solution for the retarded Green function. For typical scattering strengths, level broadening suppresses oscillations of conductance in agreement with experimental observations. Width fluctuations of only 2% remarkably reduce quantum size effects, in arrays of parallel wires. A non-zero temperature (T<10 K) is not limiting for observation of confinement effects in typical samples.

Enhancements of the low-temperature thermopower of amorphous melt-quenched (Zr_0.64Ni_0.36)_1-xAl_x alloys with x=0, 0.05 , 0.10, 0.15, 0.20 and 0.25 are shown to be in good agreement with the predictions for the electron-phonon enhancement. The results imply that the low-frequency part of the Eliashberg function alpha ²( omega ) F( omega ) is proportional to omega ⁿ with n>1 for these alloys. These alloys are weak-coupling superconductors, and the addition of Al reduces the electron-phonon coupling constant lambda despite the tendency to strong coupling in simple amorphous metals.

Variable-range-hopping conduction in lightly doped semiconductors is analysed when strong magnetic and electric fields are applied in the crossed configuration. It is shown that the temperature dependence of the critical electric field above which the transport becomes activationless is a power law that depends on both the shape of the density of states spectrum and the asymptotic behaviour of the electronic wavefunction. An experimental study of transport in n-type GaAs under these conditions is in agreement with a model of hopping of exponentially localised and interacting electrons in a system where intermediate elastic scattering events are important.

The constant-field influence on the electron scattering processes in semiconductors is shown to lead to correction of the static conductivity. This correction is non-zero in particular for the absence of electron heating. In the case of acoustic phonon scattering, the correction is small. For optic phonon scattering and ionised impurity scattering, the corrections can be of the order of a few per cent and 10-20%, respectively.

In this paper a new quasi-classical expansion is defined. It consists of the asymptotic expansion of wavefunctions with respect to the Planck constantl, which multiplies the time derivative in the Schrodinger equation. It is proved that such an approach provides a systematic and unequivocal scheme for studies of multiphoton processes in which a large number of photons are exchanged with a far-infrared laser field. The domain of validity of this approach is also discussed.

A theoretical approach is derived to describe a Josephson junction of complex form with self-crossings. Results of the approach are used to study properties of granular superconducting systems with Josephson junctions between grains (Josephson networks or lattices). The authors show that properties of Josephson lattices depend on the grain size (L). If L is smaller than the Josephson length ( delta ), then the lattices are described by the energy functional of the XY model in which internal magnetic fields are taken into account self-consistently. On the basis of the functional they find the low critical field (H*_cl) and the pinning energy of vortices. They study also a case of high magnetic fields (H>>H*_cl). In this case the lattices are also described by the XY model but the coupling strength between grains may depend strongly on H.

The magnetic susceptibility data recorded from rutile-related iron antimonate of composition FeSbO₄ in low applied magnetic fields show features such as susceptibility peaks and irreversibility which are characteristic of spin-glass-like behaviour. A small peak at approximately 70 K and a broad peak at approximately 25 K were observed in the zero-field-cooled (ZFC) magnetic susceptibility data. Neutron powder diffraction patterns recorded at temperatures between 160 and 80 K show that antiferromagnetic correlations develop within the (001) plane. The correlation length of the spins increases to a maximum of approximately 30 AA with decreasing temperature within this temperature range. A saturation of the antiferromagnetic correlation length is observed at temperatures below approximately 70 K. The results show that the magnetic behaviour of iron antimonate is considerably more complex than the canonic examples of random field Ising systems such as Mn_1-xZn_xF₂ or canonic insulator spin-glasses such as Eu_1-xSr_xS. The authors suggest here that the peak in the ZFC magnetic susceptibility data at approximately 70 K may be associated with an inhibited antiferromagnetic transition which may be related to the thermally activated dynamics recently observed in different kinds of random magnetic systems. The broad peak at approximately 25 K may be associated with the dynamic crossover of the magnetic moments without a modification of the antiferromagnetic correlations.

The 'HAUP' method, described by Kobayashi and Uesu (1983-85), for the measurement of optical activity in birefringent crystals is extended to include dichroic effects and analysed qualitatively in a novel way using intensity contour maps. The elimination of the parasitic ellipticities of the polariser and analyser is achieved by repeating the measurement in a second crystal setting, rather than physically exchanging the prisms, which has been reported as unworkable.

The temperature dependence of the luminescence from Eu ²⁺ activators in BaFBr:Eu²⁺ photostimulated between 10 and 300 K in the optical absorptions of F centres produced by room temperature X-irradiation, revealed two thermally-activated processes. The photostimulated luminescence (PSL) increases sharply in intensity when excited between 60 and 100 K. After the low-temperature PSL is exhausted by continuous F centre excitation, it can be almost entirely regained by warming the crystal to 300 K. This thermal cycle can be repeated several times. It is shown by EPR that no Eu³⁺ is formed upon X-irradiation at 300 K. These results are explained with a model for the PSL mechanism which involves a loose aggregate of Eu²⁺, F, and hole centres.

The authors discuss the electronic structure of the FCC phase of sputter-deposited Fe-Cu-Ag ternary alloy films and determine theoretically their X-ray photoelectron spectra using the tight-binding linear muffin-tin orbital method and the coherent potential approximation. The calculated results are in good agreement with the experimental data.

The magnetic susceptibility of chain and disordered structures of high-T_c superconductive particles dispersed in textured polymerised magnetic fluids has been calculated by means of the Maxwell-Garnet theory using an iterative procedure. The critical magnetic fields, H_cl, for various structures are compared with the critical magnetic field H_cl of the bulk.

The authors report the first observation of the de Haas-van Alphen effect in the heavy-fermion superconductor CeCu₂Si₂. Magnetic oscillations are observed above and below a magnetic phase transition which, depending on the field orientation, occurs between 2.9 and 7.2 T. Angle-resolved measurements are made in the c-a plane on a group of frequencies of 130-550 T which are characterised by quasiparticle masses in the range 4.5-6 m_e. They find no evidence to support the view that a considerable part of the Fermi surface is composed of 'light' electrons. Further, it is concluded that the superconducting state is formed out of a fully developed coherent Fermi liquid.

Two species of Fe³⁺ ESR have been observed in LiNbO₃:6% Mg codoped with 530 p.p.m. Fe. Their axial crystal field parameters, D, differ by a factor of about two. As indicated by a superposition model analysis, it is likely that the centre with the large D has to be attributed to Fe_Li³⁺, and that with the low D to Fe_Nb³⁺. This assignment is also supported by recent shell model calculations of the incorporation mechanism of impurities in LiNbO₃.