Table of contents

The authors show that a particular interstitial structure in c-Si has both ring structures characteristic of a-Si and low-energy excitations of a tunneling type and a bond-switch (soliton) type. These excitations exist for reasons of symmetry not because of anomalous bond angles or lengths. They suggest these defects may act as nuclei for amorphisation under irradiation and offer a possible explanation of the observed relation between tunneling level systems and the extent of amorphisation in quartz.

EPR spectra at 9.1 and 30 GHz are reported for the iso-electronic ions Gd³⁺ and Eu²⁺ in glasses of composition 0.50 ZrF₄, 0.20 BaF₂, 0.055 AlF₃, 0.045 LaF₃ and 0.10 NaF or LiF. The spectra are similar to the 'U' spectra familiar in many oxide glasses, indicating very low and disordered site symmetries with a broad distribution of crystal fields. For europium the mean crystal-field parameter, b₂⁰ approximately=0.075 cm^-1, is 30% larger than that for gadolinium, b₂⁰ approximately=0.058 cm^-1, suggesting that the two ions occupy significantly different sites. Whereas the trivalent gadolinium will substitute readily into La³⁺ sites, it is likely that divalent europium ions replace Ba²⁺ in sites where they are more weakly bound and have a higher fluorine coordination.

FT FIR spectra of solid chlorine at ambient temperature have been measured using a diamond anvil cell. The effect of pressure on the frequencies of the IR-active modes is compared with previous results on Raman-active modes in the pressure range up to 6.6 GPa.

The exponent eta in the expression G(r) approximately r^{-d+2- eta} at the coherence critical temperature has been measured in random three dimensional arrays of Josephson junctions. The authors obtain eta =0.027+or-0.015 for 'pure' systems and binary 'alloys', the latter being made of grains of different materials. The experiment suggest a particular simple form for the scaling function of the para-coherent conductivity.

High-field magnetisation measurements and Mossbauer experiments, with and without an applied field, have been performed on Li_1.125Ti_1.25Fe_0.625O₄ spinel in the temperature range 4.2-300 K. A coherent interpretation of the results is based on both Fe spin canting at A and B sites and on a transverse-component relaxation between preferential directions in the temperature range T_N-T_f.

The magnetic excitations in quasi-periodic layered media composed of alternating magnetic and non-magnetic films are studied. The array of layers forms a Fibonacci sequence, so the Bloch theorem is inapplicable. The spectrum of spin waves is calculated by the transfer matrix method in rational approximation up to the tenth Fibonacci generation. A distinctive feature of the mini-band structure is found in these excitations.

The authors solve Maxwell's equations for a medium with a random dielectric constant by an iterative method that takes into account all two-photon processes by summing all the maximal crossed and ladder diagrams. Their analytic solution for the back-scattering intensity I leads to an inverse linear dependence on wavenumber (I varies as 1/Q), in agreement with recent experiments. The shape of the peak preserves the main features that are found by the approach that replaces Maxwell's equations by a diffusion equation but takes into account interference from time-reversed photon trajectories.

An X-ray diffraction study was carried out together with luminescence measurements for NaCl:Sr²⁺:Eu²4u+ single crystals under annealing treatment at 200 degrees C. It was found that the precipitation of secondary SrCl₂ phase into the NaCl matrix is responsible for the presence of an Eu emission band peaking at 407 nm in the optical fluorescence spectrum of specimens annealed for a long period.

Based on the potential-energy profiles of hydrogen in the Ge lattice calculated by the complete neglect of differential overlap (CNDO) method, the authors are able to investigate the states of atomic hydrogen (H⁰), proton (H⁺), and molecular hydrogen (H₂) in the Ge lattice. Their calculations show that molecular hydrogen is the most stable form and the energy surfaces obtained for H^o and H⁺ are fairly complex and different from one another. These results are compatible with experimental data which suggest that molecular hydrogen is the dominant species at lower temperatures. The results are also discussed in relation to the work performed by Mainwood and Stoneham on interstitial hydrogen in silicon.

Laser-pumped electron-hole-drop clouds in semiconductors are modelled phenomenologically as a three-dimensional spinodal decomposition within a single-component non-ideal gas of the van der Waals type. The experimentally significant low-excitation discontinuous solutions of the continuity equation are doubly degenerate in the decomposition variables so an optimisation principle is required for specification of the stable gas and liquid spatial patterns, including the drop and gas densities and the dispersion of the cloud. The authors develop the scaling laws by invoking the criterion of maximum path probability which justifies the location of a stability point at a bivariate maximum in the entropy production rate. Predictions of gas density and cloud dispersions in germanium as a function of temperature are compared with experiment and show fair to good quantitative agreement. Current experiments do not, however, provide a firm basis for distinguishing between a spinodal model with optimisation of the entropy production rate only and a hybrid model with optimisation of both the Helmholtz free energy and the entropy production rate.

The density functional theory of freezing is extended to the crystallisation of multicomponent systems into substitutional solid solutions. The formalism is applied to the case of binary mixtures of hard spheres. As the ratio of diameters alpha = sigma ₁/ sigma ₂ is lowered, the fluid-solid phase diagram evolves from a spindle shape (1> alpha >0.94) into an azeotropic diagram (0.94> alpha >0.92) and finally into a eutectic diagram (0.92> alpha >0.85). The influence of inter-atomic attractions is examined within the van der Waals mean-field approximation. The resulting phase diagrams are even richer than for the bare hard-sphere mixtures, and turn out to be very sensitive to the combination rule for the attractive energy between opposite species.

The structural phase transition in GeTe has been investigated by neutron diffraction in the temperature range 295-716 K. Accurate positional and thermal parameters have been obtained as a function of temperature both in the rhombohedral hR2 phase(binary analogue of grey arsenic structure A7) and in the cubic CF8 phase (NaCl-type, B1). The temperature variation of the rhombohedral distortion and the relative shifts of the Ge and Te substructures show that the structural phase transition in GeTe is a displacive type.

Quantities relevant to the chemical and tracer diffusion coefficients, namely the tracer correlation factor, the mobility correlation factor and the site-availability factor are calculated for an interacting lattice gas model using a Monte Carlo method. The origin and nature of the mobility correlation factor is discussed. Calculations are performed for diffusion by nearest-neighbour hopping between the tetrahedral sites in a BCC host, for each of four different site-blocking models (no double occupancy, multiple blocking to first, second or third neighbour). Comparisons are made with previous experimental work on NbH_x and TaH_x where it is found that for x<0.25 the measured correlations are greater than those calculated for any model, which is consistent with the presence of additional long range attractive interactions known to be important in this region; for 0.25<x<0.6 the results are consistent with multiple blocking to between the second and third neighbour, tending to the former at higher temperatures, while for x>0.6 the atoms are considerably more mobile than predicted by the third-neighbour blocking model. This is to be expected because at these concentrations the assumption of complete site exclusion is unrealistic.

General formulae for memory functions of a single molecular exciton in an initially unrelaxed periodic lattice are made explicit by partially resuming a perturbation series to infinity. The decay time of the memory functions determining the loss of coherence is estimated and it is shown that it may exceed the usual estimates obtained from optical data using the lowest-order Kenkre-Knox relations by several orders of magnitude. Hence, the exciton motion may remain coherent and insensitive to temperature over much longer intervals following the exciton creation than is usually assumed.

A model calculation of the stopping power of the degenerate electron gas for slow protons is presented. The screened potential of a slow proton is calculated from an ansatz for the induced density fluctuation, giving a finite value at the proton site. Phase shifts satisfying Friedel's sum rule are obtained via Ladanyi's variational method. An analytical formula for the screening parameter as a function of the mean electronic separation is given. A comparison with the results from using the self-consistent density functional formalism is made.

A new percolation theory for n-electron hopping in the low-temperature (large-n) limit is presented. Applied to the DC conductivity sigma the theory yields sigma varies as T^P. While p can vary from system to system, it is typically large, of the order of 20. In another application the glass temperature, T_e, of the electron glass is calculated. Here T_e is the minimum temperature for which the ergodic hypothesis may be satisfied within reasonable experimental times. For the specific case of impurity conduction systems in germanium (concentration 10¹⁶ cm^-3, compensation ¹/₂) T_e=0.6 K. A related calculation of the entropy yields a low-temperature behaviour S varies as T², and shows that the time evolution of the entropy is smooth.

The valence bands of Hf_xTi_1-xSe₂ have been studied by angle-resolved photoelectron spectroscopy for x=0.3 and 0.5. These results indicate the existence of small band gaps between valence and conduction bands in both these alloys. The authors interpret existing Hall effect data, which show evidence for high-mobility holes for the Hf_0.3Ti_O.7Se₂ sample, in terms of thermal excitations across the small band gap at high temperatures. They model the temperature and pressure dependence quantitatively, and determine the band gap of Hf_0.3Ti_0.7Se₂ to be 104 meV at atmospheric pressure with a pressure derivative of -6 meV kbar^-1. The new model is relevant also to TiSe₂, but the conclusion that this material is semi-metallic in its high-temperature phase is unaffected.

The authors have carried out Monte Carlo simulations of a system of classical three-dimensional spins interacting with their nearest neighbours in a FCC lattice. Spins randomly occupy one half of the lattice sites and interact via antiferromagnetic Heisenberg exchange as well as via quenched, random Dzyaloshinskii-Moriya (DM) anisotropy. They calculate the relaxation times ( tau ) against temperature (T) in the range 0.25<or=kT/J<or=2.0 for systems of 4L³ sites with L=4, 6 and 8, and for various values of the strength of DM anisotropy (D): D=0.01, 0.3 and 0.5. The expression tau =exp(A/(T - T₀)^y) fits the data rather well in all cases, A is about 0.015 and is independent of L, D and T, y varies slightly between 1.25 for D=0 and 1.45 for D=0.5, T₀ is below 0.1 J/k and the best fit to our data indicates that T_O=0. They conclude that no phase transition exists when one half of the lattice sites are occupied by spins, and also that the effect of DM anisotropy is only to shift slightly the value of the exponent y in the expression of tau (T).

A second fixed-frequency RF source is introduced within the technique of Raman heterodyne detection of nuclear magnetic resonance and used to burn spectral holes in the I₂=+or-¹/₂' implies/implied by +or-³/₂' transitions of Eu³⁺ in KEu(WO₄)₂. At 1.8 K the hole has a width (FWHM) of 350 Hz whereas the homogeneous width of the transition determined from Raman heterodyne-detected nuclear spin echoes is 220 Hz. At 4.2 K the width of the RF hole is 1300 Hz and the homogeneous width is 660 Hz. The holes are asymmetric in shape and displaced by about 100 Hz to the low-energy side of the burn frequency; this is attributed to cross-relaxation between the resonant ions and ions with adjacent resonant frequencies. The effect of the burn frequency and burn power on the shape and depth of the holes is reported. In a magnetic field, burning in one Zeeman component also gives auxiliary holes in the other Zeeman components. The hole spectrum gives an increase in resolution of an order of magnitude compared with the regular Raman heterodyne scheme.

Proton spin lattice relaxation (T₁) in (CH₃)₄NCdBr₃ at different Larmor frequencies (10, 20 and 30 MHz) has been studied in the temperature range 77 to 400 K. The variations in T₁ at high temperature are independent of frequency and show a maximum due to spin rotation-interaction. The other features are interpreted as being due to isotropic tumbling of the tetramethylammonium ion and random reorientation of the CH₃ group. The CW spectrum remained narrow up to 77 K and develops a wing structure at low temperatures. This observation is attributed to a possible tunnelling motion of the CH₃ group, which has rather low activation energy as demonstrated by the study of T₁.

Acoustic second-harmonic generation is studied in non-degenerate piezoelectric semiconductors, such as n-type InSb, with a uniform DC magnetic field B directed along the acoustic wave. The effect of electron scattering in solids has been taken into consideration, so the electron relaxation time cannot be neglected. Coupling mechanisms for the electron-phonon interaction are taken into account in this investigation through both deformation potential and piezoelectric couplings. It is found that the second-harmonic generation due to the piezoelectric coupling appears to be comparable with that due to the deformation potential coupling only in the approximate range of frequencies omega =6*10^10- 4*10¹¹ rad s^-1. Outside this range of frequencies, the deformation potential coupling becomes more significant than the piezoelectric coupling for the second-harmonic generation in semiconductors.

Domain walls are studied in a one-dimensional discrete model derived from the classical Landau expansion of the free energy. For ferroelectric and antiferroelectric crystal structures the shape of the walls is calculated exactly and their activation energy is determined. It is shown that for this energy the discreteness of the lattice plays an important role, particularly in the antiferroelectric state.

The reflective infrared spectra of zinc and magnesium fluoride are investigated from 40 to 5000 cm^-1 between 4.2 and 300 K. These spectra are analysed by two methods: the first is a classical analysis of an oscillator response and the second is a Kramers-Kronig analysis. This second method shows in the MgF₂ spectrum the presence of a supplementary band near 350 cm^-1 which is not explained by the group theory in this case. For ZnF₂, the comparison of the two methods shows a three-oscillators simple model gives an imperfect representation of the spectrum of E_u symmetry. In the case of MgF₂, the frequency shifts of the transverse modes with temperature confirm the Raman scattering data. The transversal frequencies of ZnF₂ for the A_2u and E_3u modes present a maximum near 80 K. It could be possible that this result was the consequence of the presence of a para-ferroelectric transition situated at lower temperature.

The resonant energy transfer in ruby between single ions and fourth-nearest-neighbour pairs was investigated at various temperatures and concentrations, with the application of an external magnetic field. The results give direct experimental evidence of the resonant mechanism, which dominates the transfer rate. Spin-memory effects are observed during pair relaxation after transfer. A simple model is presented which satisfactorily accounts for the observed dependence of the transfer on the field.