Table of contents

Self-consistent calculations of the electronic states of V⁰, V^- and V^2- gallium vacancies in GaP have been carried out within the framework of the multiple-scattering X alpha molecular cluster model. The results obtained are used to interpret a recent electron paramagnetic resonance experiment which was assumed to be related to the V^2- centre.

The one-dimensional quantum sine-Gordon Hamiltonian with a density rho _s of solitons is studied. rho _s( mu ) is the order parameter for the lock-in transition, which happens when the chemical potential mu equals the single soliton energy and temperature is T=0. The soliton density rho _s( mu ,T) and the critical behaviour at T=0 are studied in the classical limit (quantum coupling beta to 0) and in the quantum system with beta ²=4 pi ; the critical exponent for rho _s is 0 and ¹/₂ respectively. An important application of these results is the temperature dependence of an incommensurate charge transfer rho _e in one-dimensional conductors due to Umklapp scattering. rho _e(T) can be determined by the function rho _s( mu ,T).

The commensurate-incommensurate transition of a two-dimensional classical sine-Gordon system is studied by the equivalent one-dimensional quantum system. The latter is expanded around its classical limit, corresponding to a low-temperature expansion of the two-dimensional system. It is found that both mass and wavefunction renormalisation are required. The critical exponent for incommensurability is beta =0, but if the momentum cut-off is kept finite one obtains beta =1. The classical limit is a singular point and the results are reliable when they are not too close to the transition.

The layer compound RbAlF₄ is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction. The structures of the three phases are determined by neutron powder profile refinement. The high-temperature phase has the ideal TlAlF₄ structure P4/mmm which corresponds to the tilt system a⁰a⁰c⁰. The intermediate room-temperature phase is confirmed as P4/mbm, the tilt system is a⁰a⁰c⁺. The low-temperature phase is consistent with the space group Pmmn which corresponds to both a_p⁺b⁰c⁺ (near the transition temperature) and a_p⁺b_p⁺c⁺ (at very low temperature). The 553K phase transition is a non-ferroic phase transition while the 282K transition is ferroelastic. According to Aizu the low-temperature phase is a 4/mmmFmmm species. The temperature behaviour of the structural parameters is discussed, particularly in the framework of ferroelasticity.

For pt.I see ibid., vol.15, p.183 (1982). The linear birefringence (LB) of RbAlF₄ is measured in the temperature range 5-630K. The discontinuous structural phase transitions (SPT) at T_cl=559.4K (P4/mmm to P4/mbm) gives rise to a small jump of the tetragonal LB of delta ( Delta n_ac)=6*10^-7. The continuous SPT at T_c2=282.1K (P4/mbm to Pmmm) is accompanied by orthorhombic LB, Delta n_ab, rising from zero at T_c2 to 3.4*10^-4 at T=0, whereas n_ac changes by about 8*10^-4. The critical exponent of the order parameter obtained from Delta n_ab, beta =0.324, does not change on application of uniaxial stress up to 8 MPa. The deformation of the Delta n_ab curve due to internal strains is discussed and the switching of ferroelastic domains by uniaxial stress is demonstrated. The relationship between birefringence, spontaneous strain and the order parameter is discussed.

The incoherent dynamical structure factor and the diffusion coefficient of the nearest neighbour (NN) jump diffusion model is studied. Diffusion of a specific particle in a static but random arrangement of background particles is considered. The virtual crystal approximation and coherent potential approximation procedures are used to find solutions. First five exact frequency moments of the spectral density are calculated and compared with those given by these theories. In simple cubic and body-centred cubic lattices four first moments are exactly reproduced by the CPA solution.

The Luttinger theorem has been used by Martin and Allen (1981) to comment on the validity, or otherwise, of various approximations used in the theory of intermediate valence. Their arguments are examined in detail.

The change in charge density due to a phonon and its connection with the electron-phonon interaction and the change in potential is discussed. It is shown that these are analytic functions of q in metals and where Z^eff is zero. The charge density and the potential can be uniquely split into a part that rigidly follows an ion and a part that deforms as the ions move; the composite is called 'the generalised pseudo-atom' and is shown to be neutral. The behaviour of the potential near q=0 in metals is discussed. Using a relation between the ionic potential, the dielectric function matrix and the electron-phonon interaction, a useful expression for the dynamical matrix is easily derived. The discussion is generalised from the Fourier representation to other representations. It is shown that finite-range rigid-atom potentials give incorrect results near q=0 and that in cubic metals with one ion per unit cell or with rocksalt structure, deformation occurs, but only in Fourier components outside the first Brillouin zone. The occurrence and possible results of deformation are discussed.

The electron-phonon interaction in polyacetylene is studied by means of a simple model that can be solved to a high degree of accuracy. Some one-electron properties, such as the density of states, are calculated. The unusual features induced by the one-dimensionality of the system are discussed, as are its implications for the striking transport features of the polymer.

It is suggested that the combined effect of a resonant magnetic field (dynamic Zeeman effect) and of the fluctuations due to spin-spin and spin-lattice interactions could give rise to inelastic components in the emission spectrum of a paramagnetic system. It is also shown that a similar effect can be expected in the second harmonic emission. It is found that if the Rabi frequency is much larger than the relaxation rates, both the resonant and the second harmonic spectra show the two side lines typical of the atomic resonant fluorescence. The results are illustrated by means of a semiclassical model. Plots of expected spectra as a function of the frequency are given for some realistic parameters of paramagnetic samples.

The 2D layered Ising model is introduced as a member of a special class of models which may be periodically frustrated. A simple equation is obtained for the critical point and this is related to ideas about bond moving.

Expressions for the free-energy and the critical point of a general, periodic 4*1 layered Ising model are given. The free-energy expression is used to obtain, analytically, the ground-state properties of several frustrated versions of the model. This is complemented by an investigation of the ground-state configurations to uncover the physics behind the analytical results.

Measurements of magnetic susceptibilities of rare earth tungstates RE₂(WO₄)₃ (with RE=Ce, Pr, Nd and Gd) are reported in the temperature range 3-300K at a magnetic field of 1.2*10⁶ A m^-1. All the tungstates are found to be ferrimagnetic with ferrimagnetic Neel temperature lying in the range 20-30K (30K (Ce), 25K (Pr), 20K (Nd) and 20K (Gd)). Values of effective magneton numbers agree fairly well with those of free tripositive rare earth ions. The results are discussed on the basis of a newly proposed model.

NMR of ⁵⁵Mn nuclei in the double-layer antiferromagnet K₃Mn₂F₇, detected through its effect on the antiferromagnetic resonance, yields for the zero-point spin reduction Delta ₀(K₃Mn₂F₇)=(0.57+or-0.06) Delta ₀(K₂MnF₄)+(0.43+or-0.06) Delta ₀(KMnF₃), and without reference to other compounds Delta ₀(K₃Mn₂F₇)=0.12+or-0.03. The results are in agreement with spin wave theory.

The spin-packet width Delta _h of the inhomogeneous resonance lines of dilute ruby samples (Cr₂O₃:Al₂O₃) has been measured as a function of the orientation of the external field H₀ with respect to the crystal c axis. The results show that the broadening of the packet lines arising from the fluctuations of the superhyperfine interaction occurs within a few degrees around the parallel orientation. Moreover, this broadening mechanism is shown to be peculiar to the low-field lines of very dilute samples, since no angular dependence of Delta _h was detected either in high magnetic fields or in concentrated samples. The relationship between the reported results and the results of spin-echo, photon-echo and stirring effect experiments is examined and the lack of a rigorous theory is pointed out. The authors also discuss some advantages of the method used here for measuring Delta _h with respect to the traditional ones.

Mossbauer spectroscopy of ⁵⁷Fe nuclei has been employed to determine relative recoilless fractions and hyperfine fields in a number of ultrafine iron and iron oxide particles prepared under a variety of conditions. The f-factor data are compatible with a model that assumes that some of the microcrystals undergo complete recoil upon absorption or emission of the gamma ray; this number depends on the temperature and on the supporting medium. The iron oxide hyperfine fields fit a collective magnetic excitation model proposed earlier by Morup and Topsoe However, for alpha -iron particles, some as small as 10 nm, the hyperfine field is the same as the bulk value.

The kinetic energy transport which is associated with the flow of free carriers in a semi-infinite spatially dispersive solid state plasma and which is induced by a TE polarised electromagnetic wave penetrating the surface at an oblique angle is considered. The angular splitting and the ratio between the magnitudes of the material and electromagnetic Poynting vectors are determined as functions of the frequency and the angle of incidence of the electromagnetic wave. Numerical calculations of the energy flow, based on near-local and fully non-local microscopic classical transport theories are presented for (far) infrared light in a heavily doped semiconductor (n-InSb) and (near) visible light in a metal (Al).

A theory for the surface tension of liquid metals is developed on the basis of the direct perturbation expansion to second order in the electron-ion pseudopotential. A simple and easily tractable expression for the surface tension is derived from an expression for the Helmholtz free energy which is generalised to a liquid metal with a planar surface. The variational method is used for determining both the electron and ion density profiles at the surface and the bulk radial distribution function is assumed for describing the correlation between ions in the transition zone.

Raman scattering measurements on hydrogenated microcrystalline silicon prepared in a hydrogen plasma at deposition temperatures between approximately=65 and 400 degrees C are presented and discussed. The shifts of the crystalline (c) and 'amorphous-like' (a) components of the spectra to lower frequencies with decreasing crystallite size have been correlated with the lattice expansion and the finite dimensions of the crystallites in these films. The roles of hydrogen and of the compressive stress in the samples have been investigated by annealing experiments and a deposition of the samples under negative bias of the substrate, respectively. These results point to a probable mechanism of the crystalline-amorphous transition in silicon. The data presented allow an assignment of the amorphous-like feature in the Raman spectra to surface-like modes at grain boundaries of the crystallites. Strong arguments are given that suggest that the 480 cm^-1 peak in the Raman spectra of X-ray amorphous silicon is of the same origin and is hence associated with some shearing modes of Si clusters rather than a broadened density of states. Results on the depolarisation ratio of Raman scattering in the microcrystalline and X-ray amorphous films are also presented and discussed.

The characteristic photoluminescence lines in GaN undoped epitaxial layers have been investigated in a large number of samples at low temperature. The lines observed in the high-energy part of the photoluminescence spectra are called I₁, I₂, I₃ and I₄ with decreasing photon energy. I₁, situated at 3.476+or-0.002 eV and I₄ at 3.311+or-0.002 eV are reported here for the first time. Their dependences on temperature and excitation intensity have also been measured. These lines are proposed to be due to bound exciton emission at localised centres and to donor-acceptor pair recombination caused by imperfections and nitrogen vacancies in the crystal. The thermal activation energies are determined as Delta E₁=1.9 meV, Delta E₃=3.4 meV and Delta E₄=14.4 meV. A new value for the lowest transverse free exciton in GaN at 3.479+or-0.002 eV is suggested.