Table of contents

The stability of the slave-boson mean-field theory is explicitly demonstrated for the N_f=2 Anderson lattice Hamiltonian. A recent claim to the contrary is shown to be in error.

It is known that the low-energy density of states in electronically disordered systems exhibits a crossover from mod E mod ^d(5-d)/4 exp(-const mod E mod ^2-d/2) to the form mod E mod ^d exp(-const mod E mod ²) when the correlation length of the random potential exceeds the de Broglie wavelength. This crossover occurs for a spatially correlated gaussian random potential with large correlation length. The gaussian behaviour in a long-range correlated potential is derived using a variational principle originally proposed by Luttinger.

Magnetotransport effects have been studied in modulation-doped In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunctions grown by molecular beam epitaxy. Mobilities in excess of 15 m² V^-1 s^-1 were obtained after exposure to light and a novel photoconductivity effect has been observed. More than one sub-band was occupied in each structure. The magnetic depopulation effects have been modelled using variational Hartree and WKB approaches. The experimental data indicate the importance of lifetime broadening of sub-bands even in these high-mobility systems.

A method for the generation of random network models for the structure of amorphous semiconductors is critically re-examined in terms of the structure factor of the models in reciprocal space. It is found that, while the authors' previous models show a feature that is retained from the original diamond cubic structure (as pointed out by Guttman, Thorpe and de Leeuw), this may be suppressed simply by taking the randomisation process further.

It is shown that the main fluctuation effects in re-entrant superconductors are due to exchange rather than electromagnetic interaction between the magnetic moments of impurity atoms and conduction electrons.

Infinite degeneracy of the ground state, a 'soft line' in the elementary excitation energy spectrum, and no long-range order in 3D at finite temperature are all found for suitable exchange competition in Heisenberg helimagnets. The authors show here that a similar scenario arises in a Heisenberg model on a rhombohedral lattice with antiferromagnetic in-plane nearest-neighbour (NN) exchange coupling J and any inter-plane NN coupling J' if mod J' mod <3 mod J mod . Moreover, for small J' the in-plane spin configuration is similar to a 120 degrees phase, but any phase relation between the planes is possible. In the absence of anisotropy there is no long-range order, with a possible algebraic decay of the correlation function. A similar unorthodox behaviour is expected at intermediate temperature if the anisotropy is small enough to leave a structure of the excitation spectrum reminiscent of the 'soft line'. The authors think that these somewhat surprising properties of a rhombohedral antiferromagnet can be useful for understanding the magnetic behaviour of the beta -oxygen.

The theory of high-energy electron diffraction from modulated structures is discussed. A perturbation theory is used in which the change of potential caused by the modulation is treated as being weak in comparison with that of the average structure. An expression for the single scattering diffraction amplitude is derived and analysed in terms of its symmetry. The large thickness limit of this expression is also considered. It is shown that it takes a simple form, enabling information about the nature of a modulated structure to be obtained from diffraction experiments. Double diffraction is also briefly discussed to show that such effects are experimentally distinguishable from single diffraction.

For pt.I see ibid., vol.19, p.3497 (1986). The extraction of quantitative information from large angle, satellite dark field, convergent beam diffraction patterns taken from the incommensurately modulated ternary phase NiGeP is described. Expressions for the incommensurate satellite structure factors are derived in terms of an initially unconstrained phonon eigenvector. The little-co-group of the modulation wave-vector is used to derive the normal modes of the unmodulated lattice and the appropriate eigenvectors for two of the modulations observed in experiment. By fitting the variation of satellite diffraction intensity with incident orientation to that observed experimentally, the eigenvector components of one of these are determined.

For pt.I see ibid., vol.19, p.3391 (1986). The author continues a study of the quasi-elastic peak observed by coherent neutron scattering in super-ionic CaF₂, and investigates here the connection between this peak and the motion of point defects. The work is based on molecular dynamics simulations which the author found in a previous paper to agree well with experiment. The point defects in the simulated system are examined using a technique due to Dixon and Gillan (1978), which the author has extended so that fluctuations in their density can be monitored for different wave-vectors. It is argued that the motion of the defects will govern the low-frequency fluctuations of the ion densities, and the author shows that the correctness of this idea can be explicitly tested by an examination of the cross-correlations between defect and ion densities. Numerical calculations demonstrate that the quasi-elastic peak does arise entirely from the motion of the defects. The collective fluctuations of the ions are influenced in an important way by the lattice distortion surrounding the defects, and the strong wave-vector dependence of the quasi-elastic intensity arises directly from this distortion.

An existing consistent set of model potentials for alkali halides formulated for convenient use in defect applications is extended to incorporate hydrides. The additional parameters required are fitted to the limited available experimental data for perfect alkali hydride crystals. Calculations of local-mode frequencies for isolated H^- substitutional (U-centre) and interstitial impurities, H^-H^-, H^-D^-, H^--cation-impurity and H^--anion-impurity pairs in alkali halides are reported. These calculations, which are based on large periodic defect cells, involve no further adjustment parameters and are in good agreement with results of infrared absorption measurements found in the literature.

The volume behaviour of cerium monoxide CeO, obtained by synthesis under high pressure, has been determined as a function of pressure up to 25 GPa at room temperature in a diamond anvil cell. Cerium monoxide retains its cubic NaCl-type structure over the whole pressure range investigated but a rapid volume decrease of approximately 8% occurs between normal pressure and 3 GPa with a mean bulk modulus B₀ approximately 30 GPa. Above 3 GPa, the volume variations are described by a Birch equation of state with a bulk modulus B₀=171 GPa, a pressure derivative B₀'=4.8 and an initial relative volume calculated at normal pressure of 0.937. The low bulk modulus at low pressures can be interpreted as resulting from a volume-dependent Kondo interaction.

The authors elucidate the violation of the Harris criterion that often occurs in real-space renormalization group (RSRG) treatments of quenched random systems. The system studied is the bond-diluted q-state Potts model on the diamond Berker lattice, since it allows either sign for the pure specific heat exponent alpha which is calculated exactly and satisfies a generalised hyperscaling relation involving the fractal dimension. It is argued that it is necessary to allow the bond probability distribution to evolve, under length scaling, from its initial binary form. The width of the distribution (excluding the weight of the missing bonds) is found to play a crucial role and is used as an additional parameter. These conclusions are confirmed by actual calculation for this system of the RG flow in a generalised parameter space, in which the random fixed point is located. The associated fixed-point distributions are discussed, and it is demonstrated that the generalised view allows the Harris criterion to be satisfied.

Using data available from literature for hydrogen in the liquid and solid phases the authors constructed a consistent equation of state valid over a large part of the operating range of the diamond anvil cell. A large part of the experimental data is reproduced within 0.1% and practically all data within 0.5%. From this equation of state they derived the thermodynamic properties of molecular hydrogen. The volume, entropy, Gibbs free energy and enthalpy are tabulated as a function of temperature and pressure for P<or=1 Mbar and 100K<or=T<or=1000K.

The authors examine a model of the disordered, non-iterating two-dimensional electron gas in a strong, perpendicular magnetic field, in which the Fermi energy is supposed to lie in the nth Landau level, for large n. The model is found to be exactly solvable in the limit n to infinity due to the vanishing of quantum interference between scattered wave packets. Three choices of random potential are examined: gaussian white noise; a uniform spatial distribution of zero-range scattering centres of fixed strength and with a lorentzian distribution of strengths. In all cases, all eigenstates are extended. Although the absence of quantum interference suppresses localisation, all the potentials lead to modulations in electron mobility across an impurity band and variations in the conductivity tensor with electron density which are reminiscent of the quantum Hall effect.

Several experimental methods-absorption, photo-emission, and transport measurements-were used to determine the energy position of the Fe²⁺(3d⁶) donor state in the band structure of the semi-magnetic semiconductor Hg_1-v-xCd_vFe_xSe. For v less than or equal to 0.40 the Fe²⁺(3d⁶) level is a resonant donor located in the conduction band.

The authors report the results of a fully relativistic band-structure calculation made using the KKR method for SmTe with the NaCl structure in the low-pressure phase. The exchange parameter was obtained using the free-electron approximation scaled by the exchange parameter alpha . They have solved the full Dirac equation for the interior of the muffin-tin spheres and the energies were evaluated at several symmetry points and other general points in the ¹/₄₈ wedge of the FCC Brillouin zone. The energy band indicates a gap throughout the whole BZ with excited f-type bands located below the d-type conduction band, and the indirect band gap determined by the band structure for the Gamma to X direction to be 0.67 eV in good agreement with the experimental value of 0.63 eV.

The authors propose a model to explain the experimental results for the electrical resistivity in polycrystalline samples of SnTe. In some cases the grain boundaries act only as carrier scattering centres, in others there is evidence of a thermally activated current associated with the polycrystallinity. They give expressions for the different contributions to the resistivity and propose a model based on the superposition of two currents which gives rise to the composition in parallel of two resistivity terms associated with the currents. Finally the authors give their parameters.

It is predicted that the polariton and plasmon modes of a solid state plasma which are simultaneously excited when light is incident on a metal at an oblique angle can be strongly coupled by an optical lattice anisotropy stemming from interband transitions. A theoretical study of the dispersion relations of the coupled plasmon-polariton modes is presented. It is shown that the simple hydrodynamic (or near-local) model for the description of non-local effects in metals gives an accurate description of the coupling around the longitudinal plasma edge. It is demonstrated that the polariton dispersion relation is strongly modified at the longitudinal plasma edge by the Coulomb field of the plasmon. The optically interesting situation stemming from the fact that one of the principal axes of the conductivity tensor is fixed by the scattering plane whereas the principal axes of the dielectric tensor of the lattice can be rotated by rotating the crystal is investigated. Numerical results based on free-electron data for hexagonal Zn are obtained. It is proposed that the polariton-plasmon coupling is used for measurements of optical lattice anisotropies at frequencies around the plasmon plasma edge.

Optical characteristics of H₂-reduced LiNbO₃ have been studied. The optical transmission in the spectral range 0.3 to 5 mu m is progressively narrowed as the sample is reduced. An absorption band centred at about 2.48 eV is assigned to small polarons. This interpretation is consistent with the published defect structure, transport and spectroscopic reports on reduced LiNbO₃. A semi-quantitative analysis of the 2.48 eV absorption band is carried out by employing the Reik and Heese theory (1967) for optical properties of small polarons.

The rise time behaviour of photocurrents in a PTS crystal excited by rectangular light pulses has been measured and the rate constant K(t) of charge carrier relaxation has been evaluated. At short times (t<or approximately=10^-4 s) and long times (t>10^-2 s) K(t) follows a power law characteristic of transport controlled by tunnelling barriers. At intermediate times K attains a constant value that increases with light intensity according to K_opt approximately I^0.85. The effect is explained in terms of an interaction between a delocalised optical excitation of a PTS chain and a charge carrier localised at a defect. It is concluded that photo-simulated barrier crossing is the rate-determining step for charge carrier transport in presence of a photon field.

The Glauber kinetic equations have been solved numerically for the relaxation of the magnetisation in a one-dimensional random bond Ising chain with periodic boundary conditions. The chain relaxed from the completely orientated state. Calculations have been performed for the short time relaxation of the magnetisation using the Fernandez-Medina picture. These are in agreement with the author's numerical data. Irreversible behaviour similar to that occurring in laboratory spin glasses has been observed. Finally in the long time regime they perform a lower bound calculation for the magnetisation which yields non-exponential relaxation of the form M(t) approximately exp(-(t/ tau )¹2/). The numerical data give the same non-exponential relaxation (with a different tau ) provided that a natural prefactor of ¹/₃ is introduced. Hence they find M(t) approximately ¹/₃exp(-(t/ tau ')¹2/).

The magnetic properties of two orthopyroxenes Fe_xMg_1-xSiO₃ with x=1.0 (synthetic sample) and x=0.87 (natural compound XYZ) have been investigated by means of susceptibility and magnetisation measurements, neutron diffraction and Mossbauer spectroscopy. A collinear antiferromagnetic ordering is observed below T_N(x=1.0)=45K and T_N(x=0.87)=26K characterised by a wave-vector k=(010) and by an alignment of the moments along the b axis. Inside one ribbon formed by two non-equivalent M₁ and M₂ sites the moments are parallel to each other resulting from a preponderant ferromagnetic nearest-neighbour coupling. The Fe²⁺ ions on M₁ and M₂ sites are quite distinct: large orbital contributions were found in FeSiO₃ for the moments on the M₁ site leading to m₀(M₁)=4.1 mu _B (neutron result) and to low values of the hyperfine field H_hf(M₁)=70 kOe (Mossbauer data), compared with m₀(M₂)=3.3 mu _B and H_hf(M₂)=315 kOe on the M₂ site. For x=0.87 a distribution of H_hf around these mean values is present. The strongly anisotropic character of these systems is reflected by the large difference observed between the susceptibilities along the orthorhombic axes even at T=300K, and by the metamagnetic transition that occurs at H_c(x=1)=50 kOe and H_c(x=0.87)=35 kOe.

A phenomenological model is introduced to describe the frequency dependence of the AC susceptibility, chi ( omega ), of random magnetic materials. The model is applied to CuMn, (Eu, Sr)S, holmium borate glass and aluminosilicate glass. On the basis of the model criterion the various systems are classified, and it is possible to distinguish between spin glasses and superparamagnets. The authors' approach is to divide the spin system into two temperature-dependent parts: one with correlated spins (clusters) and one with non-correlated (free) spins. Thereby they obtained meaningful values for the activation energy Delta E_a and the intrinsic time constant tau ₀ which follows from an Arrhenius law. In those cases where the authors were able to combine susceptibilities over twelve decades in frequency their analysis showed an enormous evolution towards long times for the cluster relaxation times near and below T_f.