Table of contents

A detailed theory is presented for the recently observed Ga isotope fine structure of local vibrational modes of C and Si impurities on an As site in GaAs. It is found that the magnitude of the splitting of the C line implies a substantial increase in the bend/stretch force constant ratio at the impurity.

The authors study the behaviour of the axial next-nearest-neighbour Ising (or ANNNI) model in a magnetic field using low-temperature series. An infinite sequence of phases of mean wavevector q= pi /(2j+1)a, j=2,4,6...is shown to exist in the vicinity of a multiphase line.

The dynamic mean field theory of the spin glass phase is generalised to n-component spins. Exact averaged equations of motion are derived within a purely relaxational model. It is shown that dynamic stability enforces a violation of the fluctuation-dissipation theorem for any finite n and T<T_c. The marginally stable state is characterised by algebraic decay of correlations t^{- nu} with an exponent which depends continuously on temperature and the number of components, nu (T,n)=¹/₂-3 mod T-T_c mod / pi n(n+2)+O( mod T-T_c mod ²)+O(1/n³).

Magnetic ordering of the Hubbard model is investigated with the help of a variational ansatz for the ground state. It is shown that only antiferromagnetic and special ferromagnetic states are stable against the paramagnetic one for the band fillings n close to n=1 and sufficiently strong Coulomb interaction.

The leading sub-extensive terms in the free energy for the TAP mean field theory of Ising spin glasses are evaluated. For T=T_c and for T=0 they are shown to be convergent provided only that N^-1( beta J)² Sigma (1-m_i²)²<or=1.

The first stage of F- and M-centre colouring has been investigated at room temperature in quenched samples of europium-doped NaCl as a function of the impurity concentration in the range 30-500 PPM. Attention has been paid to determine whether a correlation exists between the change in the concentration of Eu²⁺-cation vacancy dipoles and the colour centre formation in order to get a deeper insight into the nature of the interstitial halogen traps. The effects of x irradiation are (a) to reduce the number of (I-V) dipoles, (b) to increase the number of trapped electrons, (c) to convert the doubly valent europium ions to the monovalent, as well as to the tripositive state, and (d) to increase the rate of aggregation of dipoles. The number of dipoles destroyed by irradiation was found to be equal to or to exceed the sum of the number of europium ions which had changed their valence state and the number of F centres produced. The results presented in this paper suggest that the (I-V) dipoles act as the dominant traps for the radiation-induced interstitials. It was also ascertained that the M-centre density is proportional to the square of the F-centre density, the proportionality constant being clearly dependent on the inverse of the impurity concentration.

Measurements of the elastic properties of Li-doped KTaO₃ by the resonance technique and by the pulse-echo method show a softening of 1/s₁₁ and c₁₁, respectively, at temperatures below 120K. The strong dispersions observed between the respective measuring frequencies (300 kHz and 15 MHz) and the ultrasonic attenuation point to strain relaxation in which reorientation of Li by thermal activation is the rate-limiting process. A comparison of these data with nuclear spin relaxation and dielectric relaxation provides evidence for a condensed state at low T, and yet evidence against long-range order.

Infrared absorption studies have been carried out on the vibrations of sodium ions in the mirror plane of both non-stoichiometric and nearly stoichiometric sodium beta -alumina. Absorption by the mixed crystals Na_x(NH₄)_1-x beta -alumina has also been measured for 0<x<1. Measurements were made between 2 and 300K and covered the region 20-200 cm^-1. The results are related to configurations of sodium ions in the mirror plane and they are used in conjunction with recent calculations by Walker and Catlow (1982) to discuss models for ionic conduction.

Presents results of a detailed theoretical survey of the defect structure and transport properties of non-stoichiometric beta Al₂O₃. The authors methods are based on a generalised mott-Littleton approach which includes a full explicit treatment of lattice relaxation around defect species. Their calculations show that the anti-Beevers-Ross site is the stable configuration for excess Na⁺ ions. These migrate by an interstitialcy mechanism. In grossly nonstoichiometric beta Al₂O₃, however, the migration mechanism may more closely resemble the vacancy mechanism operative in beta "Al₂O₃. The authors' results account for the observed differences between the conductivities of the stoichiometric and non-stoichiometric phases. They differ considerably from those of earlier theoretical studies of Wang et al. (1975); this is attributed to improvements in the interatomic potentials and the inclusion of lattice relaxation in the authors study.

Electronic structure calculations are reported for the 3d pyrite-type sulphides Fes₂, CoS₂, NiS₂, CuS₂ and ZnS₂, and for marcasite-type FeS₂. The energy bands are compared with the experimental data available. FeS₂ is calculated to be a semiconductor, with an indirect gap of 0.7 eV (pyrite) or 0.4 eV (marcasite) separating filled t_2g and empty e_g sub-bands. In ZnS₂ the d band falls below the main, fully occupied S p bands to give a semiconducting gap approximately=2 eV to the lowest metal s and S₂ p sigma * bands. The intermediate compounds CoS₂, NiS₂ and CuS₂, within the present limitations of delocalised electrons and paramagnetic exchange potentials, are found to be metals; in these the d states move steadily down through the S p bands. Sulphur s and p valence bands extend through energy ranges 11-19 eV and 2-9 eV respectively below the Fermi level, in good agreement with photoelectron spectra.

The influence of polar electron-phonon interaction on electronic properties of zero-gap semiconductors is examined in the one-phonon and zero-temperature approximations. The remarkable renormalisation of electronic dispersion for pure and doped semiconductors is obtained. For sufficiently doped n-type samples (E_F⁰> omega ₀-longitudinal optical phonon frequency), the broadening of the valence band may influence the shape of the interband absorption edge. The results are compared with previous theories for standard semiconductors and for single polarons in the valence band edge in zincblende-type semiconductors.

Measurements of thermostimulated luminescence and electric conductivity are presented. They show that the electron-hole recombination by tunnelling in SrCl₂ crystals doped with K⁺ ions is quenched for certain positions of the doping ion with respect to the V_k centre. This quenching is interpreted by a lattice relaxation which differs for the different states of the electron-hole pair. The mobility of the V_k centres by 180 degrees jumps is explained.

Measurements of the Seebeck coefficient or thermoelectric power (TEP) of HgSe at high pressures confirm the existence of an inverted band ordering, similar to that in HgTe. The pressure-induced phase transition from the zincblende- to the cinnabar-type structure, which occurs around 10 kbar, is manifested as a large change in TEP. The temperature behaviour of the resistivity has also been studied in both of these structural modifications. These studies indicate that the energy gap in the high-pressure semiconducting phase decreases with increase of pressure; this is similar to what is observed in the elemental semiconductors Se and Te.

Neutron inelastic scattering has been used to determine the imaginary part of the susceptibility chi "(Q,h(cross) omega ) in uranium arsenide. The measurements were performed on a time-of-flight spectrometer using a polycrystalline sample. In the paramagnetic state a broad (in energy) spectrum is found which can be characterised as a Lorentzian centred at zero energy with a half width ¹/₂ Gamma =17+or-3 meV. No evidence is found for any crystal-field levels below 40 meV. At the ordering temperature (T_N=126K) the overall susceptibility decreases, as expected, but the dynamical part may still be characterised as a Lorentzian whose half width decreases gradually to 8+or-3 meV at 5K. Such a response in the ordered state, in which an apparently localised moment of 2.2 mu _B resides on each U atom, is quite surprising, but is in agreement with experiments on single crystals in which no well defined excitations were found. At all temperatures the susceptibility peaks at the magnetic zone centre, showing the dominance of antiferromagnetic correlations. The results are discussed in terms of the systematics of the uranium rocksalt compounds and the need for realistic calculations of the ground state.

The ladder approximation for metamagnets with exchange and single-ion uniaxial anisotropy and small interplane coupling is developed. The self-energy for zero wavevector is given as an explicit function of the Hamiltonian parameters. The result so obtained is kinetically consistent: taking the dynamical interaction fully into account amounts to the exact low-temperature solution of the model, as expected. The theory presented here is of practical interest, because a lot of magnetic insulators are just metamagnets; that is, they are arrays of ferromagnetic layers with small interlayer coupling. The calculations are developed explicitly for rhombohedral and hexagonal lattices.

The magnetic phase diagrams of Mn(Br_1-xCl_x)₂.4H₂O obtained from differential susceptibility measurements, with the magnetic field H applied along the easy axis, are presented. It is found that the Neel temperature T_N and the spin-flop-paramagnetic transition field at T=0 varies linearly with the concentration x, between the values observed for the unmixed compounds MnCl₂.4H₂O and MnBr₂.4H₂O. The authors also found that the region in the H against T plane in which the spin-flop phase exists increases with increasing x. From these results they conclude that the anisotropy varies with the concentration, and that the ratio H_A/H_E between the anisotropic exchange field H_A and the isotropic exchange field H_E assumes values between 0.25 and 0.73. These are respectively the values for the ratios in Mncl₂.4H₂O and MnBr₂.4H₂O.

The magnetic susceptibility of the degenerate free electrons in the crust of a neutron star is computed for a range of densities, temperatures, and field strengths. It is shown that when the temperature is low enough (typically less than 10⁷K for densities of about 10⁷ g cm^-3 and 10¹² G fields), the susceptibility undergoes large de Haas-van Alphen oscillations. The crust is then unstable to the formation of layers of domains of alternating magnetisation. Associated with these domains are magnetic field fluctuations of a few per cent amplitude and anisotropic magnetostrictive stresses which may be large enough to crumble the crust. It is argued that these domains are unlikely to directly influence the surface properties of the neutron star but may possibly be coupled indirectly to observable effects.

The spin packet lineshape of the inhomogeneous resonance lines of dilute ruby samples is experimentally investigated by the saturation transient method. By comparing experimental and theoretical results of the saturation behaviour of these samples both during the transient and the steady state, the authors show that the spin packet lines are well described by a gaussian shape. The relationship between their results and conflicting predictions issuing from different theories is also examined.

Reports the results of Zeeman measurements up to 22 T on the 0.84 eV photoluminescence arising from trigonal Cr in GaAs. The measurements are interpreted in terms of effective Hamiltonians for the ground and excited states, both of which have trigonal symmetry. It is demonstrated that recent criticisms of the authors model (by Voillot and co-workers) (1980-81) are without substance. The authors model is also shown to give an accurate description of both the intensities and energies of the lines seen in high-resolution optical absorption measurements.

The CaSO₄:Dy TL emission spectrum between 400 nm and 800 nm is studied using real-time spectroscopy. The emission spectrum is found to be independent of exposure between 460 R and 10⁴ R, and in the temperature range between room temperature and 400 degrees C the TL emission spectrum is solely the result of characteristic trivalent Dy transitions. Optical absorption bands observed in CaSO₄:Dy are correlated with optical transition of the Dy²⁺ and Dy³⁺ ions to determine that Dy³⁺ is photo-reduced during irradiation and oxidised during subsequent heating. The effect of thermal treatments on the behaviour of the SO₃^- and SO₄^- radicals produced by ionising radiation, as determined in independent ESR results, is discussed in terms of the concurrent changes in the Dy impurity that results in the thermally stimulated release of luminescence. It is concluded that the model of the TL process most consistent with the various experimental results is one in which the Dy impurity acts as an electron trap, with charge recombination at the radical sites resulting in energy transfer to Dy³⁺ ions which de-excite by luminescent emission.

X-ray K-absorption edges in the compounds CuCl₂.2H₂O, CuS, CuO and CuSO₄.5H₂O have been studied using a Cauchois-type X-ray spectrograph. Peaks observed in the edges have been assigned to the electronic transitions 1s to 4s and 1s to 4p. The observed energies of the peaks have been compared with the spectroscopic energy levels of the Z+1 ion.

The authors obtain a homogeneously thick film of close-packed Na with close-packed planes parallel to the surface by evaporating Na onto Cu(111) held at LN₂ temperature. Comparison between measured LEED intensity spectra, recorded at LN₂ temperature, and theoretical ones, calculated under the assumption that the crystal structure of Na is FCC or HCP, favours the HCP structure. The authors UPS results indicate that if Na is evaporated onto Cu(111) held at RT, it will form three-dimensional clusters on top of a Na film only few atomic layers thick.

A comprehensive study of laser induced optical anisotropy in amorphous GeSe₂ films is presented. This anisotropic structure of the amorphous network can be reoriented reversibly by changing the incident-beam polarisation. A model is constructed that accounts for experimental findings, in particular for the logarithmic time development of the laser-induced anisotropy.