Table of contents

Using energy dispersive diffraction techniques employing synchrotron radiation the authors measured the effect of pressure on the lattice parameters of La₂CuO₄. During compression the c/a ratio remains constant up to about 150 kbar showing that the effect of pressure is isotropic, in contrast with the magnetic properties which are highly two-dimensional. The results agree well with the predictions of band-structure calculations, suggesting that the bonding is governed by the La and O atoms and not by the Cu-O₂ layers.

A stage-3 FeCl₃ graphite intercalation compound has been successfully synthesised by the solution method from graphitised poly(p-phenylene vinylene) film. Graphitised film intercalated by the solution method is a highly conductive material with a metallic conductivity as high as 2*10⁵ S cm^-1, and this is higher than the values for FeCl₃-intercalated films prepared by vapour-growth technique. Metallic behaviour of the temperature dependence of the electrical conductivity and of the optical reflectance have been observed.

Single-particle and collective microscopic dynamics of a classical Yukawa fluid are analysed on the basis of simple memory function and mode-coupling theories. Thermodynamic states for different screening regimes are correlated in terms of an effective hard-sphere packing fraction. The longitudinal current fluctuations and the velocity autocorrelation function are found to cross over continuously from the behaviour characteristic of classical fluids with short-range interactions to the dynamics of a one-component plasma as the screening parameter of the Yukawa potential is reduced. In particular, a simple mode-coupling analysis is capable of reproducing the increasingly oscillatory behaviour of the velocity autocorrelation function found in recent molecular dynamics simulations.

The Monte Carlo method is used to study epitaxial overlayer systems on a substrate with substantial lattice mismatch. The equilibrium atomic configurations are obtained for a wide range of the ratio of intra-layer and inter-layer interactions with various degrees of mismatch. Different modes in the relaxed lattice configuration for different regimes of interaction ratio are revealed. In addition to periodic boundary conditions, a circular-box boundary condition is investigated, which elucidates island structures and size-dependent alignment of axes in the overlayer in agreement with experimental results on the van der Waals epitaxy.

The authors present an argument which derives electron pairing correlations in a system whose many-body interactions are all repulsive. They analyse the spin-¹/₂ Anderson lattice in the nearly atomic limit through the nonlinear hopping Hamiltonian developed in the companion paper (ibid., vol.21, p.4567-89, (1988)). In a mean-field analysis, they find competition between magnetism in the truly atomic limit and superconducting pairing correlations as the kinetic energy is increased. They find singlet pairing solutions with a constant gap on the Fermi surface at some 'distance' from the magnetic solutions. Further, they find the possibility of a weaker instability towards odd-parity triplet pairing, with zeros of the gap on the Fermi surface, between these two coherent phases.

The authors develop exact nonlinear fermionic transformations which diagonalise the spin-¹/₂ Anderson lattice in the atomic limit. The original fermions, one localised and one conduction, are transformed into two new fermions which obey perfect anti-commutation relations. They find many possible transformations of which they consider two in some detail. Firstly, they analyse a transformation which is useful in the Kondo limit. It is accurate when the two smallest energy scales in the problem are the conduction band width and the hybridisation between the localised and conduction electrons. Secondly, they analyse a transformation useful in the mixed-valence limit. Here the accurate limit is when the two smallest energies are the conduction band width and the energy separation between the two orbitals. In the second limit, when the band is less than half filled, they find an effective Hamiltonian in terms of only one relevant fermion. This Hamiltonian is fairly general in this type of problem and includes a non-linear-hopping term. The most crucial aspect is the particle-hole asymmetry with particles being much more mobile than holes.

The conductivity of the following six new low-dimensional compounds is reported: (Me₄N)(Ni(dmit)₂), (Et₄N)(Ni(dmit)₂), (Pr₄N)(Ni(dmit)₂), (Bu₄N)(Ni(dmit)₂), (Bu₄N)(Pt(dmit)₂) and (Et₄N)_0.5(Pd(dmit)₂). Semiconducting behaviour is found for all the compounds investigated, with activation energies between 0.1 and 0.5 eV. The crystal structures have been established by X-ray diffraction. Using these, the band structure has been determined. It is shown that the compounds investigated are semiconductors each having a correlation gap resulting from an effective electron-electron repulsion of 1.0 eV.

The phase-transition-like anomalies above T_c were investigated by internal friction, stress-strain curve and lattice parameter measurements. The relationship between the anomalies and superconductivity is studied. The 110-130 K anomaly is found to be closely connected with the 90 K superconducting transition, but no obvious relation was observed for the 210 K anomaly. In addition, a relaxation-type internal friction peak was measured at about 100 K.

The crystal structure of paratellurite, alpha -TeO₂, space group P4₁2₁2, a=4.8082(3) AA, c=7.612(1) AA, is refined from single-crystal X-ray data (547 unique reflections). The atomic arrangement in the structure is correlated with the experimentally observed optical rotation (OR) along the unique axis. A structural origin for the OR is identified and demonstrated through the development of model structures. Calculations of the OR and birefringence in alpha -TeO₂ are made (using the program of Devarajan and Glazer) and it is demonstrated that the explicit inclusion of the contribution of the lone pair of Te(IV) to the optical response is essential for good agreement with experimental data.