Table of contents

It is shown that the dislocation theory of melting gives a value for the melting temperature of a two-dimensional electron solid that is in good agreement with the value obtained in the molecular dynamics calculations of Hockney and Brown (1975).

The modified tight binding approximation (MTBA) is re-examined in the light of recent criticisms. The role of the interband transitions, developed in the MTBA, is emphasised with particular reference to transport properties of transition metals.

A practical scheme for obtaining total and local densities of states from non-orthogonal basis functions is described, and results for bulk SrTiO₃ are presented by way of an example.

Spin dependent recombination (SDR) and so-called photoinduced EPR, have been measured for the same samples as a function of orientation for the first time. It is suggested that photoinduced EPR is better described as photoconductive resonance (PCR). Several centres were found, one of subsurface (bulk) origin and at least two others associated with surfaces. One SDR line is the same as a PCR line, but the other is not. The sensitivity of SDR is at least 100 times greater than EPR. The magnitude of SDR agreed with theoretical calculation.

It is shown that the nonlinearity originating from the nonparabolic momentum energy relation can lead to amplification of an electron plasma wave in semiconducting plasmas. The growth rate of the instability is presented. Applications to InSb are discussed.

Corrections are reported in the data of Wood and Dalton (1972) for the fifth-order term in the high-temperature series expansion for the zero-field susceptibility of the anisotropic general spin Heisenberg model. These corrections are consistent with the data of Ishikawa and Oguchi (1971) for the special case of the simple quadratic lattice.

The binding energy and wavefunction associated with an antisite P(Ga) defect in GaP are presented and compared with the spin-Hamiltonian parameters of Kaufmann et al. (1976). Whereas calculations concerning an equivalent phosphorus interstitial failed to produce an s-like bound state, the wavefunction of the antisite defect appears to be localised enough to account for the observed spectrum. The wavefunction in the wave vector space is also presented and its composition indicates that deep centres on Ga site are less likely to be efficient radiative recombination centres in accord with the predictions of Morgan (1969).

A series of the compounds of the type Ni_xZn_1-xMn₂O₄ (x=1.0, 0.8, 0.6, 0.5, 0.4, 0.2, 0.0) have been synthesised and their characterisation by X-ray diffraction analysis indicates a tetragonal matrix at x<or=0.5 and at x>or=0.6 the symmetry becomes cubic. Low-temperature magnetic susceptibility results show a tendency to ferrimagnetism at x<or=0.4 with the exception of ZnMn₂O₄, while at x>or=0.5 a complex magnetic behaviour is observed.

It is shown that in samples submitted to a magnetic field gradient, after a spin-lock experiment, where a pi /2 RF pulse is followed by a lock field of amplitude omega ₁, only those spins are measured with off-set frequencies close to omega ₁. In this way spatial selection is possible.

The high-resolution excitation spectrum is determined. It appears in the ⁴A₂ to ⁴T₂ excitation spectrum as a zero-phonon line accompanied by a weaker line at 50 cm^-1 on the high-energy side. The effect of the uniaxial stress on these lines is determined. The linear dichroism absorption spectrum under pressure for these lines is obtained.

The Debye-Waller factors of polycrystalline As, Sb and Bi, obtained by neutron diffraction in the temperature range of 5K up to almost the melting point, are compared with lattice dynamics model calculations and results from X-ray and Mossbauer effect measurements. Within experimental accuracy the thermal motions appear to be isotropic and almost harmonic except for Bi at high temperatures. Moreover, the temperature dependence of the lattice constants and the positional parameter z were determined.

The structure of molten BaCl₂ has been investigated by applying the technique of neutron diffraction to isotopically enriched samples. The three partial factors relating to Ba-Ba, Cl-Cl and Ba-Cl correlations have been successfully extracted from the experimental data. It is concluded that the correct description of the melt is in terms of Ba²⁺ ions and Cl^- ions rather than BaCl⁺ complexes and Cl^- ions as was earlier supposed. The observed structure of the liquid is related to the fast ionic conductivity known to characterise the high temperature (cubic) phase of solid BaCl₂.

For pt.I see ibid., vol.9, p.3141 (1976). Two different pseudopotential methods are used, one approximate and one retaining all two-centre integrals, to study F-centres. The more detailed method is tested on the F-centre in cubic NaCl, where it gives isotropic hyperfine constants for the first two shells of ions which are within 20% of the experimental values. In the wurtzite crystals, the calculated transition energies are significantly larger than those measured. The hyperfine constants are sufficiently close for the discrepancy between theory and experiment to be attributed to lattice distortions, but the observed difference between the values for axial and basal nearest neighbours is not predicted. A detailed treatment of lattice distortion is probably essential for both this difference among the hyperfine constants and for an accurate estimate of the transition energies.

The microwave acoustic attenuation has been measured in samples of single-crystal iron doped MgO, as a function of temperature, and at 0.58 GHz and 1 GHz. The attenuation of (100) propagating compressional waves exhibits a peak at approximately 30K which is attributed to relaxation between the ground state levels of Fe²⁺ ions on cubic sites. An analysis of the form of the peak enables the relaxation time to be determined as a function of temperature and an excited state at 110+or-2 cm^-1 is derived in agreement with infrared spectroscopic measurements. The magnitude of the attenuation of the (100) compressional mode and the absence of a peak in the transverse (100) mode is in fair agreement with a model which supposes that the Fe²⁺ ions are subject to random static strains, which are predominantly compressional in nature.

Electrical conductivity has been used as a characteristic quantity to follow the growth of the crystalline phase in the amorphous solid and liquid phases of SSe₂₀. The nature of the growth phases during both the amorphous-crystal and liquid-crystal transition have been discussed and compared to that of pure Se. An attempt has been made to fit the experimental data to the known theories of transformation kinetics.

A three-electron recombination process in semiconductors is analysed by employing second-order perturbation theory. Taking the unperturbed states to be planewaves and ignoring exchange effects, the recombination coefficient is calculated to be approximately /10^-51 to 10^-54 cm⁹ s^-1. The overlap of the modulating parts of the Bloch functions reduce this estimate by approximately 10^-3.

The hot-electron distribution function has been calculated in the magnetic quantum limit for crossed field configuration. The theoretical model of Kurosawa and Yamada (1973) has been improved by including electron-electron scattering. The broadening of the energy levels is also considered. In addition our model allows the calculation of the time evolution of the hot-electron system. Results for the stationary and time-dependent distribution function are found to be in good agreement with experiments performed on n-InSb at liquid helium temperatures.

An evaluation of the ionic-thermocurrent technique has been performed by monitoring the relaxation of divalent impurity-cation vacancy dipoles formed from various dopants incorporated into monocrystalline sodium chloride. The observations have been compared with the predictions of the relaxation of a nearest-neighbour impurity-vacancy dipole. Investigation has also been made of the sensitivity of the technique to detecting a response from impurity-vacancy dipoles existing in a complexed or aggregated state.

The critical behaviour of d dimensional Ising models in a transverse magnetic field, Gamma , is investigated using a quantal generalisation of the Landau-Ginzburg-Wilson free-energy functional introduced by Hertz and Young (1976). A variational approximation of the Hartree type allows a detailed analysis: behaviour near the critical line, Gamma = Gamma _c(T) is governed by the exponents of the d dimensional spherical model (which are mean-field-like for d>or=4). However, the point ( Gamma ,T)=( Gamma _c(0),0) is multicritical in nature, and characterised by the exponents of the (d+1) dimensional spherical model. Crossover scaling functions, valid near the multicritical point, are found for the longitudinal susceptibility and free energy. They involve the crossover exponent phi _T; for 2<d<or=3 this is equal to the (d+1) dimensional correlation length exponent, nu _d+1=1/(d-1); for 3<d<or=4, we find phi _T=¹/₂(4-d), while for d>4, no crossover occurs. These results are shown to be consistent, for all d>2, with the finite-size scaling analogy suggested by Suzuki (1976) and others.

The low-temperature ordered magnetic moments of uranium monopnictides are analysed using a two-level model appropriate to fourth-order terms in the crystal-field potential. It is concluded that in UN, uranium has a 5f² electronic configuration whereas 5f³ is possibly more appropriate for the other uranium monopnictides. The 5f² configuration satisfactorily accounts for the temperature dependence of the sublattice magnetisation of UN in the ordered region, as well as the paramagnetic susceptibility between 53K and 1000K.

A general upper bound to the perimeter polynomial is demonstrated for the site percolation problem and it is concluded that at percolation threshold p=p_c the limiting mean perimeter-to-size ratio of (1-p_c)/p_c exists for large clusters. The standard deviation of the perimeter-to-size distribution approaches zero as n^-12/. These properties are illustrated by an exact solution of the percolation problem on the expanded cactus. The connection between the asymptotic form of the perimeter polynomial and critical exponents are discussed and three recent suggestions for this asymptotic form are compared.

Measurements of the magnetic susceptibilities between 4.2 and 1.1K of the rare earth zinc nitrates are reported. It is shown that the results for the Van Vleck temperature independent susceptibilities for cerium magnesium nitrate and cerium zinc nitrate are consistent with the published low lying level schemes for these two salts.

For pt.I see ibid., vol.11, p.345 (1978). The metamagnetic phase transition and the associated phase diagram of the anomalous antiferromagnet CeSb were determined in a neutron diffraction study of the magnetic ordering of CeSb single crystals in applied magnetic fields parallel to the (001) and (011) directions (H<5 T). Because of the anisotropic exchange interactions that favours a (001) easy direction of magnetisation, the magnetic properties are similar to those of the Ising spin systems i.e. antiferromagnetism, ferrimagnetism and ferromagnetism occur at low temperatures and at increasing magnetic fields. The observed magnetic structures do not correspond to the stable configurations expected from the molecular field theory of the face-centred cubic lattice. The change from a first-order transition at the Neel temperature in zero field to second-order transition at high fields points to the existence of a tricritical point, presumably at T approximately 16K and H approximately 0.3 T.

The nature of the freezing process in two- and three-dimensional Ising spin glasses with bonds J_ij=+or-J has been investigated. Monte Carlo simulations indicate that the characteristic cusp in the susceptibility chi is a non-equilibrium phenomenon resulting from the finite observation times available for the computation of chi (or its experimental measurement). This picture of the freezing process, which is illustrated by explicit calculation for the one-dimensional Ising spin glass, implies chi =1/k_BT for all temperatures, or that the Edwards-Anderson order parameter q vanishes. Investigation of the ground states of certain periodic models in two and three dimensions show that every spin belongs to a finite cluster which may be turned over with no cost of energy, so that q is rigorously zero for such models. It is argued that this result holds also for the fully random models.

Without application of an external electric field, the temperature dependence of the shear angle u_xy, which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma -radiation in KH₂PO₄ and RbH₂PO₄. Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a first-order one. The modified Slater's theory provides for RbDP a good fit to the experimental data u_xy(T) in the whole temperature range investigated. The fact that beta =0.5 near T_c can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.

Extended Huckel theory is applied to (111) surfaces of Si and Ge. The calculated dangling-bond surface state energies and energy dispersions are compared with those by previous calculations. For the relaxed Ge(111), the strong mixing of the dangling-bond surface state with an s-like 'back-antibonding' surface state appearing at the two-dimensional conduction band edge at the surface is found to be closely related to the striking lowering in energy of the dangling-bond surface state near the centre of the surface Brillouin zone, in contrast to the ideal Ge and the ideal and relaxed Si(111) surfaces.

It is shown that a magneto-Raman effect for rare-earth ions in crystals should exist, and a theory has been developed to describe it. The theory has been developed for a system of energy levels which consists of two doublets and is applicable universally to any system with similar levels. The main results come from the determination of the power change in unresolved lines and indicate special changes in optical polarisation which occur on the application of a magnetic field in a Raman experiment. Changes in power output in a paramagnetic system in which a coherent Raman effect is excited have been considered, and it is shown that there should be an increase in output on the application of a magnetic field.

The thermoluminescence and the thermal stability of the F and V-centres induced by irradiation in KCl:Ca and KCl:Sr samples have been studied. It has been found that the thermal annealing of both types of colour centres and the thermoluminescence are different aspects of the same basic phenomenon. Namely, as previously proposed, the thermoluminescence is due to the recombination with F-centres of mobile interstitials thermally released from V-centres; at some stage in this process light is emitted. No evidence of thermally stimulated currents has been found. The two types of impurity induce a different pattern of first-order glow peaks. This indicates that each impurity induces several interstitial traps. Their activation energies and pre-exponential factors are given.