Table of contents

Paramagnetic resonance studies of SiO₂:Ge crystals X-irradiated in a microwave cavity at 4K show a dominant hole centre stable below 35K. It is suggested that the hole is stabilised in an oxygen 2p orbital by a nearby germanium atom and that germanium is an amphoteric impurity in quartz. The relevance of the measurements to the mobility of hole polarons in crystalline quartz is discussed.

The authors develop a relation for the fracture strength of a porous solid or random network near the percolation threshold. Near the percolation threshold the fracture strength vanishes as (p-p_c)^{( tau +v(2-d))}2/, where tau and v are the elasticity and coherence length exponent respectively and d is the fractal dimension of the percolating cluster. Employing this relation, they have determined the elasticity exponent in a recent molecular dynamic investigation to be approximately=2.

X-ray diffraction, dielectric and differential scanning calorimetry studies have been performed on (CH₃NH₃)₃Sb₂Br₉ crystals. The first-order structural phase transitions at 168 and 131K have been revealed to be related most probably to a freezing of the reorientational motion of CH₃NH₃⁺ cations.

Measurements are reported for the recovery of stress-induced dichroism in the 9 mu m oxygen infrared absorption band for samples given various treatments prior to stressing. Enhanced hopping rates are observed in samples deliberately contaminated with copper or iron and the degree of enhancement up to a factor of 10² depends on the conditions of the stressing treatment. It is also shown that the enhancement is a transient effect. The results are tentatively explained in terms of an oxygen-vacancy interaction with the vacancies being generated by site switching of the metal atoms.

The thermal conductivity of ³He-⁴He mixtures is calculated in the temperature range 50 mK to 500 mK allowing for the presence of three-phonon processes. By comparing the theory with experiment, the authors are able for the first time to obtain information about the rate at which phonons of high energies decay via the three-phonon process.

The authors consider, using a combination of variational and 1/N techniques, an actinide impurity whose lowest ionic configurations are f² and f³ embedded in a free electron matrix. They show that the ground state is a singlet, in agreement with recent experiments on dilute uranium systems.

The total resonant tunnel current through a double-well structure is calculated. Remarkably, the current is found to increase as the widths of the outer potential barriers increase until a maximum value is attained when the widths of the outer barriers are half the width of the barrier between the potential wells. The double well is the simplest system in which resonant tunnelling involves coupling between two localised energy levels in addition to couplings between the continuum of energies in the electrodes and discrete energy levels in the potential wells so it serves as a model for tunnelling through superlattices. The results reveal the importance of including the effect of the coupling between the quantum wells and the electrodes when optimising the performance of superlattice tunnelling devices.

The oscillatory structure in the low-temperature reverse bias characteristics of n⁺GaAs/(AlGa)As/n_-GaAs/n⁺GaAs tunnelling devices is explained in terms of the modulation of the resistivity of the undepleted region of the n^- layer above the substrate by means of energy relaxation of hot electrons arriving from the high-field depletion layer beyond the barrier.

Sharp transitions have been observed in the photoconductivity of GaAs-GaAlAs multi-quantum wells at energies greater than the energy gap of GaAs, corresponding to both allowed and forbidden excitonic transitions between electron and hole sub-bands in the quantum wells. Transition energies have been analysed using the Bastard envelope function model, with the energy band offset and well width as variable parameters, and the model has been found not to give an accurate description of the energy levels involved. A simpler empirical approach has been used to yield limits on the distribution of band discontinuity between conduction and valence bands; these limits are found to disagree with the previously accepted value Delta E_c=0.85 Delta E_g, suggesting the valence band discontinuity is greater than previously thought.

It has been shown that the model of chemisorption-induced surface segregation, initially proposed by the authors to interpret the variation of the proton Knight shift as a function of Ni concentration in the chemisorption system H/Cu_1-xNi_x, can in addition interpret the temperature dependence of the Knight shift against Ni concentration curves.

The authors have measured the inelastic neutron scattering spectra of three samples of molten CsCl with different isotopic chlorine compositions (including the naturally occurring one) using an incident wavelength of 4.1 AA, and also of molten CsCl with natural Cl using 1.08 AA. The experiments were performed on the time-of-flight spectrometers IN6 and IN4 at the Institut Laue-Langevin. By a first-order subtraction they have substantially separated the contributions from mass (acoustic mode) and charge (optic mode) current correlations. A quasi-dispersion curve has been derived from the positions of the peaks in these functions. The acoustic and optic branches are well separated in energy over the measured region (1 AA^-1<Q<4 AA^-1) with relatively flat dispersion. The modes are strongly overdamped and although the energy of the low-Q optic mode corresponds to the crystal LO there is no evidence of a side peak in S_qq(Q, omega ). Thus the observed optic mode is not propagating, even at the lowest values of Q. Indeed the IN4 measurements show that S_qq(Q, omega ) is very broad (full width 18 meV) at low Q while S_mm(Q, omega ) is much narrower. Small maxima in the widths of S_T(Q, omega ) at Q approximately=1.7 AA^-1 indicate the possible existence of a soft acoustic mode. Classical de Gennes narrowing may be observed at Q approximately=2.0 AA^-1, corresponding to a change in diffusion mechanism. The statistical accuracy of the spectra was too low and the differences between scattering lengths too small to enable a second-order separation to be made using results from all three samples.

The structure of molten NiCl₂ has been investigated by the scattering of neutrons from isotopically enriched samples. The three partial structure factors relating to Ni-Ni, Cl-Cl and Ni-Cl correlations and the three corresponding radial distribution functions have been determined from the experimental data. The results are compared with those obtained from earlier studies of 2:1 molten salts and are discussed in the context of polarisation and ion size effects.

Biphenyl is studied as a simple archetypal incommensurate material, with particular emphasis on the physical origin of its incommensurate nature, which persists to 0K. The authors present a simple picture of the enmeshing of the phenyl rings of biphenyl with those of neighbouring molecules within the crystal. Since the molecules are located on a triangular lattice (in the ab plane), twisting the molecules produces 'frustration' in the mutually intermeshed rotations of the phenyl rings. This leads directly to the singly incommensurate (phase III) of biphenyl, and several analogies are mentioned. The model is supported by calculations on the actual structure, using a model intermolecular potential. The symmetry of the incommensurate phase III is discussed more fully and accurately than previously, without the use of four-dimensional space groups. The librations and translations of the molecules, associated with the modulated structure, are given qualitatively by symmetry and the enmeshed rotations model, quantified by calculations, and used to explain the differences between the twisting phase transitions in biphenyl and p-terphenyl.

The technique of high-resolution X-ray scattering has been used to study the structurally modulated phases in proustite (Ag₃AsS₃) in the temperature range approximately 61 to approximately 27K. The wave-vector of the modulation in the incommensurate phase ( approximately 61 to approximately 49K) has components (¹/₃- delta ₁)a* and (¹/₃- delta ₂)c* (adopting the hexagonal setting of the rhombohedral cell). At 57K delta ₁=0.0063(3) and delta ₂=0.0125(3), and both decrease with decreasing temperature and make a discontinuous jump to zero at T_c= approximately 49K. The ordering of the structure in this temperature range is shown to be subject to very long equilibrium times-of the order of several hours. Fifth- and seventh-order satellites have been observed in the commensurably modulated phase ( approximately 27 to approximately 49K), but their intensities decrease to zero on warming through the approximately 49K transition.

A theoretical study has been made of the defect structure of lithium hydride and lithium deuteride. A potential model is obtained describing the statics and dynamics of these crystals. Intrinsic defect energies are calculated using the Harwell HADES program which is based on a generalised Mott-Littleton method. The results are in good agreement with the experimental data, and suggest that the vacancy and interstitial migration mechanisms of anions and cations are all comparable in their contribution to ionic conduction.

A quantitative theory of distortion-induced infrared and Raman intensities due to forbidden modes of molecular units at polar site symmetries in crystals has been discussed. This relates the intensity of such modes with the amount of distortion in the unit which can be evaluated easily from the crystal structural data. It is shown that even for the distortion commensurate with C₁ symmetry, not all forbidden modes of the unit attain first-order or even second-order activity in all cases. An example of the IR absorbance due the v₁ mode of SO₄^2- ion in (NH₄)₂SO₄ crystal has been discussed with a view to demonstrating the usefulness of the results of this theory in the study of phase transitions in crystals.

A theoretical study is given of the effect of the Umklapp process quasi-conservation of wavenumber on the propagation of high-frequency thermal waves in dielectrics. The study is performed in the presence of both normal and Umklapp processes. The relaxation times characterising these types of processes are assumed to have the same wavenumber dependence. The effect is found to be very strong when Umklapp processes play the major role.

The results of two different calculations of the mean width E_g of the Coulomb gap are presented. The calculations are performed for the 'lattice model' used by Efros and Shklovskii (1975) in which the fractional site occupation K and the disorder energy W₀ are parameters which may be varied independently. The authors determine approximations to the ground state first by stabilising the system against single-electron transfers, and second by direct minimisation of the total energy. E_g is found to depend on W₀ as E_g varies as E_c((E_c/W₀+E_c))¹(d-1)/ where d is the dimensionality, and is either two or three. The dependence on K of E_g differs slightly for the two methods.

The interaction of a relativistic electron beam, travelling parallel to the surface of a semi-infinite medium, is analysed. The specific energy loss of the beam and the probability of excitation of surface and bulk modes in the medium are calculated. Beams both external and internal with the medium are considered, and the predictions are compared with well known non-retarded limits. A detailed analysis of the expressions derived is provided for electron beams interacting with specific dielectric materials. The effects of retardation are seen to be large, particularly for beam energy losses in regions where the real part of the dielectric constant is large. The retarded excitation probability is typically about 10-30% larger than the non-retarded result, for electron energies greater than about 100 keV. The image force is calculated for an external beam, and the radiative de-excitation of the medium is also analysed.

The authors study in detail the short-range order at low temperature in a one-dimensional Ising model with competing interactions by studying various staggered susceptibilities. From this they get a 'phase diagram' which divides regions of various dominant short-range orders. This phase diagram is shown to resemble qualitatively the low-temperature phase diagram of the anisotropic next-nearest-neighbour interaction Ising (ANNNI) model in higher dimensions. They argue that this method of studying a corresponding one-dimensional problem may be useful to understand the phase structure of complex higher-dimensional systems.

Disordered spinel ferrites exhibit relaxation behaviour in their Mossbauer spectra. The results of the low-field magnetisation measurements on a representative system show for the first time that the system undergoes a ferrimagnetic transition at T_N and then, on cooling, enters into a cluster spin-glass state below T_f with the magnetisation of different clusters randomly frozen. The observed relaxation features in the Mossbauer spectra could be related to the instability in the ferrimagnetic state at higher temperatures.

A theoretical analysis of the magnetostriction in doped narrow-gap semiconductors is presented in the framework of a simplified model. It is shown that, owing to the field and strain dependences of the impurity levels, impurities contribute to the magnetostriction. A rough estimate of this effect for highly doped InSb yields a strain of 10^-8 at 5 T.

Chlorine nuclear quadrupole resonance lineshapes are reported for the antifluorite K₂OsCl₆ for a series of temperatures below the temperature of the cubic to tetragonal structural phase transition. The data serve to clarify the identification of the transition temperature T_c. A detailed analysis of the magnitude spectra shows that: (i) the data are consistent with the occurrence of a commensurate structure phase transition at T_c; (ii) additional strains are introduced into the crystal as a result of the structural alteration.

X-ray Cu K absorption edges in the compounds CuCl, CuBr and CuI have been studied using a Cauchois-type X-ray spectrograph. The structures observed in the edges have been correlated with the plasmon shift. It has been observed that as the plasmon shift increases the positions of the structures (XANES) also shift to the high-energy side. It has also been observed that the separation between the structures increases as the plasmon shift increases. These correlations have been explained on the basis of molecular orbital theory.