Table of contents

For original paper see ibid., vol.11, no.4, p.651 (Feb. 1978). For comment see ibid., vol.11, no.18, p.L747 (Sept. 1978). A detailed consideration of the X-ray scattering pattern for silver-chalcogenide-type superionic conductors is presented, in reply to the comment on the previous paper. The authors maintain that both cation-cation and cation-anion interference terms contribute to the diffused haloes and that these can be separated by the standard techniques used in the structural analysis of amorphous materials and binary liquid alloys.

Strontium chloride shows a broad anomaly in its specific heat at a temperature T_c approximately 730 degrees C well below its melting temperature of 873 degrees C, which arises from the development of disorder in the anion sublattice. Neutron diffraction experiments are performed to investigate the nature of the time-averaged disorder at temperatures up to 820 degrees C. From the data it is concluded that at 820 degrees C about 3 per cent of the Cl^- ions leave regular lattice sites and occupy interstitial positions.

A recent neutron scattering experiment designed to detect charge fluctuations in potassium bromide is analysed by use of a simple theory. For q=0.5 AA^-1 and onwards the theory does not indicate any optic-type mode, but at q=0.3 AA^-1, a distinct optic-type mode has been found.

Potential barriers of identical, rectangular shape but randomly spaced are shown to leave delocalised an infinite, discrete spectrum of eigenstates of the Schrodinger equation in one dimension.

Effects of surface plasmon dispersion and single-particle effects on the image force are studied for parallel incident particles. The inclusion of such effects avoids the unphysical logarithmic divergence of the parallel force at the surface. The probability of surface plasmon excitation is reduced with respect to the undispersed model.

Schottky barrier heights are analysed for those metal-semiconductor junctions for which no chemical compound is formed between the metal and the semiconductor. At these junctions the results show that the barrier height is slightly dependent on the properties of the metal as well as on the oxide layer which could be formed at the interface.

The applicability of the n-replica method to an Ising system with positive and negative bonds of equal strength is investigated. It is shown that this method does reproduce the correct low-temperature and low-(negative bond) concentration series for the ground-state energy. It is essential, however, to take the low-temperature limit after the n to 0 limit. For a square lattice, the slope of the critical line near the pure-system transition point is found to be 2 square root 2/ln( square root 2+1). The onset of spin-glass order is conjectured to be associated with a negative bond concentration at which unfrustrated plaquettes no longer percolate the lattice. The estimated value of 16% is in agreement with Monte Carlo results.

A random Ising model with random positive, negative and zero nearest-neighbour exchange coefficients (with probabilities p, q=p and r respectively) is considered at zero temperature. The s state Potts model in the limit s to ¹/₂ is shown to describe the statistics of those clusters of bonds which contain no frustrated plaquettes. The phase transition of this model is interpreted as corresponding to a 'Mattis spin-glass' ordering on these clusters, yielding an upper bound for the concentration p at which some spin-glass ordering must occur.

EPR of carefully annealed crystals of SrF₂ and BaF₂ containing Ce³⁺ revealed a large number of sites of low symmetry, which appear to arise from a small distortion of the tetragonal site Tg(F_i^-). One of these sites in SrF₂ has been analysed in detail; it corresponds to a rhombic site of different symmetry from that previously observed in crystals with the fluorite structure.

The absorption spectrum for transitions from the ground to the first excited state in GaP:Ni(d⁸) contains a zero-photon line at 0.664 eV and phonon side-bands with a prominent peak at 41 meV. A model is proposed for the energy-level scheme which is consistent with published data on luminescence.

Using the static structure factor S(q) and the fourth moment of the dynamical structure factor S(q omega ), the author investigates under what conditions a classical liquid can support short-wavelength density waves. It is concluded that particularly the alkali metals lithium and potassium, like rubidium, should show sharp resonance in S(q omega ) for intermediate q values.

For pt.I see ibid., vol.10, no.9, p.1431 (May 1977). A modified form of the electron-gas approximation is proposed for the calculation of interionic potentials in the solid state. On the basis of these potentials perfect lattice and defect properties of a number of solids are calculated and compared both with experiment and with the results derived from other theoretical methods. The cohesive energies, lattice constants and compressibilities of a wide range of oxides are considered, while the defect energies for NaCl, MgO, MnO, CaF₂ and MgF₂ are examined in detail. From the results presented it is concluded that for solids such as the alkali and alkaline-earth halides and the alkaline-earth oxides, which are largely ionic, the modified electron-gas approximation is a reliable non-empirical method for the calculation of interionic potentials.

The authors have made detailed calculations of the energies of formation and migration of vacancy defects in those alkali halides with the rock-salt structure, using recently derived potentials based on the shell model. The computed Schottky and cation-vacancy activation energies are in good agreement with experiment. The calculated cation and anion activation energies are very similar; the anion values thus agree more nearly with the results of diffusion measurements than with the larger values deduced by assuming that the separate contributions of the two types of vacancy can determine the curvature of the plot of log( sigma T) against T^-1 ( sigma is the conductivity and T the temperature). The observed curvature of the conductivity plot may therefore indicate that some other process contributes to ion transport.

Structural, dielectric and infrared spectral studies of mixed oxide systems of general formula (M₁2/La₁2/)_xBa_1-xTiO₃, where M=Li and K, and 0<or=x<or=1.0, have been reported. The extent of solid solubility can be represented as 0<or=x<or=0.5 for M=Li and 0<or=x<or=1.0 for the K-series. (Li₁2/La₁2/)TiO₃ crystallises with a tungsten bronze type (tetragonal) structure and exhibits an anomalously high dielectric constant (peak value: 37000) and simultaneous existence of both ferroelectric and antiferroelectric ordering. When M=K and x=0.3, the compound is a tetragonal perovskite showing a dielectric anomaly at approximately 400K. It does not produce any hysteresis loop in the polarisation versus electric field plots up to 5 kV.

The structural and electrical properties of single crystals of the systems Ta_xTi_1-xSe₂ and V_xTi_1-xSe₂ have been investigated. Single crystals have been grown by chemical transport reactions over the whole concentration range. Their crystallographic structures can be described on the basis of the 1T polytypic form. The electrical properties of the mixed crystals are interpreted in comparison with the stoichiometric samples. The most interesting effects occur in vanadium-doped TiSe₂ samples where the temperature of the resistivity anomaly is lowered by additions (0.001<or=x<0.01). For larger x(0.01<or=x<or=0.05) the anomaly disappears and a semiconducting phase is observed down to low temperatures. For increasing x, the temperature dependence of the resistivity has a metallic character.

The alloy transfer matrix approximation for a one-dimensional system with off-diagonal disorder is discussed. A general method for determining dispersion relations within this approximation is proposed and shown to lead to reasonable results for the asymptotic decay lengths of states. However, a calculation of the conductivity shows that all states within the band are extended, revealing a basic shortcoming of the approximation.

A variational procedure has been used to build up Wannier functions to study Ge, GaAs and ZnSe. The Wannier functions, charge density and band structure are calculated self-consistently. With a simple trial set of localised orbitals the results for the valence bands are satisfactory. The authors have applied to Ge an extension of the method for the conduction bands. Once the interactions between valence Wannier functions are computed, they develop a simple approach which includes a small number of neighbours. The effect of the ionicity on the different features they have studied can be easily analysed.

A molecular cluster model of the tetrahedrally coordinated semiconductors is discussed within the framework of the self-consistent-field multiple-scattering method. A method to avoid the dangling bond effects is discussed and the model is particularly investigated with respect to gallium arsenide. It is shown that the model leads to a fairly consistent description of the bulk electronic structure.

The authors have studied the process of formation of bound states associated with the substitutional nitrogen impurity in GaP_xAs_1-x as a function of the change in band structure with alloy composition. The host crystal band structure and the impurity potential are represented by local pseudopotentials, using the virtual crystal approximation. The problems of finding a self-consistent potential are dealt with in some detail and it is demonstrated that the results obtained by perturbation theory (screening) may differ from those obtained by large-scale computer calculations typically by 10-15%. The magnitude of this correction is affected by processes similar to those participating in the formation of deep localised states.

The transfer matrix approach has been utilised to derive a general expression for the energy levels due to isolated islands of impurity in the one-dimensional Kronig-Penney model. The present method is used to interpret the structure observed in the spectral density of a disordered diatomic lattice.

The drift mobility mu _d of melt-grown BaTiO₃ single crystals has been determined in the 20 degrees C to 80 degrees C temperature range, using the Spear method. At room temperature, mu _d=(3+or-1)*10^-7 m² V^-1 s^-1 with a resistivity of 4*10⁸ Omega m. The mobility thermally activated with an activation energy of 0.15+or-0.04 eV. The results are interpreted in terms of non-adiabatic small-polaron hopping theory in the semi-classical and quantum mechanical cases. The related polaron and band parameters are deduced; taking into account the LO phonon energy h(cross) omega ₀, the half-bandwidth J lies between 0.03 and 0.06 eV. Hall mobility, Seebeck coefficient and absorption coefficient are obtained as functions of parameters h(cross) omega ₀, E_a and J and compared with existing experimental data.

A set of coupled integral equations for the correlation functions of the 3-D Ising ferromagnet is generalised to the case of a ferromagnet with the additional feature that beside the conventional two-spin interaction terms (J₂S_i^zS_j^z) it contains four-spin interaction terms (J₄S_i^zS_j^zS_k^zS_l^z) in its Hamiltonian. The second equation is linearised yielding the two-spin correlation function which is found to be the same as the two-spin correlation function for the pure two-spin interaction ferromagnet. The Ward identity is shown to be satisfied. The approximations which reproduce the Weiss and spherical results in a generalised form are indicated. Using a generalised non-uniform magnetic field method, a relation between the transition temperature for the combined two- and four-spin interaction ferromagnet and the transition temperature for the pure two-spin interaction is derived.

The authors investigate the properties of ferromagnetic spin systems with higher-order exchange interactions, in particular with four-spin terms of the form (S_i.S_j)(S_k.S_l) and (S_i.S_j)(S_j.S_k). Using the Green function equation of motion method, and simple decoupling approximations, they obtain renormalised spin-wave energies, magnetisation curves, and critical temperatures for various strengths of the four-spin interactions. For sufficiently large values of these interactions the transition becomes first order. For comparison they also give the results of spin-wave and mean-field theories.

The author has considered the dynamics of the interaction of a 180 degrees ferromagnetic domain wall with a planar defect characterised by an abrupt change in both the magnetic anisotropy and exchange energies. It is shown that the minimum velocity for the soliton or domain wall to penetrate the barrier is proportional to the fractional change in the characteristics of the medium as defined by the defect, the maximum or critical domain wall velocity, and the hyperbolic tangent of the ratio of defect to domain wall width. For iron, the author computes the theoretical value of approximately 3000 cm s^-1. The resonant domain wall oscillation frequency due to a grain boundary in iron is computed as approximately 400 MHz in good agreement with typical experimental values of approximately 500 MHz.

A theoretical treatment of the radiative detection of single-pulse and double-pulse (free induction decay) nuclear resonances of oriented nuclei is given. The relationship of these results to the observation of multipole effects in the line shapes and to nuclear reaction-NMR experiments is discussed.

Mossbauer effect measurements have been made on single-crystal and powder specimens of GdVO₄ using the 86.5 keV transition in ¹⁵⁵Gd. The temperature dependence of the hyperfine field below the Neel temperature agreed well with order parameter measurements using the magnetoelectric effect. All available magnetic data were analysed to give a nearest-neighbour exchange constant of -0.061+or-0.002K and next-nearest-neighbour exchange constants of -0.002+or-0.002K (inter-sublattice) and -0.002+or-0.001K (intra-sublattice). Covalency effects were shown to have a significant effect on the EFG and CEF parameters.

The time dependence of delayed radiation is investigated when a pulse of synchrotron radiation undergoes resonance Bragg scattering by the nuclei in a crystal that have an isomeric level with a Mossbauer transition. Instead of the usual exponential law exp(-t/ tau ₀) characteristic of an isolated nucleus, a time dependence in the form ( tau ₀/t)² exp(-t/ tau ₀) was discovered at small deviations alpha from the Bragg angle. The acceleration of the decay is connected with the collective nature of the excitation of the nuclei in the crystal-the nuclear exciton formation. The exponential law remains at large alpha but the intensity decreases as 1/ alpha ². In the case of large divergence of the incident beam the law ( tau ₀/t) exp(-t/ tau ₀) is obeyed. The frequency distribution of the reflected pulse and the possibility of formation of a resonance structure with sufficient resolution are analysed in detail.