Table of contents

A new compound of overall composition NdCo₈B₃Si with a tetragonal structure has been synthesized by arc-melting, and identified by X-ray powder diffraction and analytical electron microscopy.

The authors present semi-empirical calculations of the atomic geometries and electronic charge distributions of beta -carotene homologues of different chain lengths. They find defects in charged and photoexcited chains that are similar to the defects found in the degenerate polymer transpolyacetylene, and show how confinement affects these defects as the chains are shortened. The results exhibit a generalized form of charge-conjugation symmetry in which the properties of a negatively charged defect are related to those of a positive one and vice versa.

The authors investigate the kink dynamics in a discrete Phi ⁴ chain with dissipation and external field. The kink, dressed and parameterized by its position X(t), is introduced as a distinct degree of freedom in the discrete chain. By using the projection operator method with Dirac's second class constraints, they show that the asymmetric kink motion is modulated by the Peierls-Nabarro (PN) force whose barrier is a decreasing function of the external field. It is also seen that the average value of the PN force is an increasing function of the external field. The dressing of the kink profile is analysed numerically. It results in a considerable increase of the PN force amplitude.

Inelastic incoherent neutron scattering studies of vibrational spectra (2-400 meV) were carried out on the atomically ordered phase of palladium-silver hydrides synthesized under a high pressure of gaseous hydrogen. The results showed a large difference betwen the Pd-H and Ag-H interactions in the hydrides. The optic-phonon group was found to be split into two peaks. One peak was positioned at about the same energy as the peak from the transverse optic modes in PdH_x whereas the other had a much higher energy and originated from the Ag-H interaction. The experimental spectrum of nearly stoichiometric PdAgH_0.86 was fitted using a conventional Born-von Karman model. The value of about 93 meV was predicted for the local hydrogen vibrations in dilute Ag-H solid solutions.

The multifractality of graphite and amorphous carbon fractal clusters with distinct morphologies was studied by analysing the growth probability distribution (GPD) using the harmonic measure. The results showed that the multifractality of the graphite clusters is similar to that of diffusion-limited aggregation (DLA), and that a single phase transition to non-multifractals occurred at a negative scaling factor, beta . However, two critical points at which the multifractality was broken down were observed in the GPD of the multifractal-like amorphous carbon clusters. Such a multifractal behaviour was also found in the GPD of dissolution patterns generated by etching of porous materials, and the GPD of magnetic-microsphere aggregates, which suggests that the behaviour is common to many fractal clusters.

The effective response of nonlinear random resistor networks consisting of two different types of resistor is studied numerically and compared with some simple approximations. One type of resistor is assumed to be ohmic, while the other is assumed to have a non-linear I-V response of the form i= chi ^{upsilon beta +1}. The effective response is calculated by solving Kirchhoff's equations for the voltages at each node of the network. Numerical results, for beta =2 and beta =4, are compared to theoretical predictions of a recently derived Clausius-Mossotti approximation for such networks. The Clausius-Mossotti results are found to provide a good description of the results of the simulations in cases of low contrast between the two components or a small fraction of the non-linear component in the network. It is also found that the range of validity of this non-linear Clausius-Mossotti approximation is larger than that of the classical Clausius-Mossotti approximation for linear two-component random resistor networks. An extension of Bruggeman's effective-medium approximation to this case is found to give somewhat better agreement with the numerical results.

Among many proposed field theories for thermal phenomena, thermo-field dynamics (TFD) is very useful and simple tool to study non-equilibrium many-body systems, especially for a complicated system composed of several subsystems at different thermal equilibria. A complete perturbative calculation scheme in TFD for such systems is presented. In the TFD method, explicit investigation will be made of the energy loss rate (ELR) and resistivity of hot electrons in semiconductors. The ELR formula is shown to be different from that obtained by the Keldysh method. The possible ambiguity in the Keldysh method is discussed.

Non-collinear spin configurations in an external magnetic field are widely studied but many features await a satisfactory understanding. The phase diagram for a three-sublattice configuration is known when the zero-field helix is supported either by lattice frustration or by exchange competition, but generic helix configurations need further effort. Here the authors consider the classical square planar model with competing exchange interaction up to third neighbours. In particular, they are interested in the helix configuration with the spin-spin turn angle Q=²/₅ pi for which they give a low-temperature expansion of the free energy within the harmonic approximation and Monte Carlo simulation. The results support the existence of a first-order helix-fan transition at finite temperatures for a field of about half the saturation field, in agreement with a previous guess based on zero-temperature energy calculation.

The authors have studied energetic and structural ground-state properties of the one-dimensional Falicov-Kimball model in the strong-coupling limit. Using standard perturbation theory they recover some results already obtained (the phase segregation at large interactions) and present some new results concerning the ground state, e.,g., the analytic expression for the boundary, below which the segregated configuration cannot be the ground state, and the phase diagram of the model calculated for special periodic (aperiodic) configurations of ions and for the case when the ion concentration goes to zero.

Optical absorption measurements were made to determine energy gap values as a function of temperature on samples of Cu₂MnGeS₄, Cu₂MnSiSe₄ (both antiferromagnetic), Ag₂MnGeSe₄ and Ag₂MnSiTe₄ (both ferrimagnetic) with the orthorhombic structure, and of Ag₂MnSiTe₄ with the cubic structure (ferrimagnetic). These data were analysed to determine the contribution triangle E to the energy gap due to magnetic effects in the temperature region around the critical temperature T_N. It was shown that the magnetic contribution increased the energy gap for the antiferromagnetic cases and decreased the energy gap for the ferrimagnetic ones. The variation of triangle E with T was found to agree very well with the theoretical predictions of Alexander et al.

The authors report a study of the photophysical properties of poly(p-phenylene vinylene), PPV, prepared in a way that gives an especially high degree of intrachain order. Optical absorption, photoluminescence, photoinduced absorption, and photoconductivity excitation spectra are presented and compared to data reported for less well ordered PPV. Spectral red shifts, sharpening of spectral lines, and a transfer of oscillator strength into the vibronic ground states of the electronic transitions are observed. Photoinduced absorption due to long-lived charged excitations, previously reported for less ordered PPV, could not be detected in this material. Photoconductivity excitation spectra show a steep rise at the absorption edge with no appreciable offset between the onsets for photoconduction and absorption. A very slow photocurrent component is observed, which the authors associated with the trapping and subsequent thermal release of photocarriers.