Table of contents

A realistic physical model for conducting polymers is proposed which includes a new symmetry breaking term for charge conjugation. The origin is an additional electron-lattice interaction. Both kink and polaron solutions are discussed. The optical absorption for the kink is calculated.

It is argued that the recent admission of the zero-field splitting and the higher-order Zeeman terms of odd order into the spin hamiltonian for S-state ions is incorrect. The admission appears to be based on misconceptions concerning the nature of 'spin' in the case of S-state ions and a confusion of the states of an actual Hamiltonian and an effective (spin) Hamiltonian.

The paradox of linear momentum in ferromagnetics is explained in terms of the exchange of the momentum between coherent and incoherent degrees of freedom in condensed matter.

An equation is presented which describes the conductivity (resistivity) of a wide variety of binary macroscopic mixtures as a function of the conductivities (resistivities) of the components, the volume (area) fraction of each, and two parameters. Unlike in an earlier version, the granular shape of at least one component is no longer required to be roughly spherical (circular). One parameter is the critical volume (area) fraction. The other, the exponent, is a combination of effective demagnetisation coefficients and the critical fraction. The equation, which is valid for all volume fractions, is shown to fit a variety of experimental data very well. Some deductions about the shape of the grains in the media can be made by analysing the experimental results.

The dynamics of alpha -relaxation of undercooled fluids close to the liquid glass transition is analysed within the recently developed mode coupling theory. Asymptotic expansions for the master functions are obtained from the scaling equation for short and for long times. The Cole-Cole expression is found as the simplest extrapolation of the short time results predicted by the scaling equation, and it is in turn related to Kohlrausch relaxation preceded by beta -relaxation processes. A formula for an effective Kohlrausch exponent is derived. Detailed numerical results for alpha -relaxation curves are presented for a schematic model which are compared with the Cole-Cole and Kohlrausch fits as well as with some experimental data.

The authors investigate bifurcations initiated by either soft-mode or hard-mode instabilities in autonomous dynamical systems, with strong emphasis on symmetry considerations. The analysis is performed within the Landau theory of phase transitions extended to driven systems. The bifurcation behaviour is described in terms of an n-component order parameter which transforms either as an irreducible representation of the symmetry group of the system (symmetry-induced bifurcations) or as a reducible representation consisting of an irreducible representation occurring twice (coupling induced bifurcations). Symmetry criteria are derived for bifurcations to stationary and to time-periodic structures. The types of bifurcation may be separated into different classes characterised by the distinct matrix groups which are induced in the order parameter space. They state a number of general stability criteria, and give a complete classification of bifurcations occurring for order parameter dimensions n=1, 2 and 3.

The authors report an investigation of the electrical conductivity of p-InSb samples at low temperatures via an ultrasonic velocity measurement technique. Because there is a piezoelectric interaction between the free carriers and the ultrasonic waves, the acoustic velocity variations are used to deduce the electrical conductivity for the range 4-20 K. Two conduction regimes-those of carrier activation in the valence band and impurity hopping-are confirmed. Ultrasonic frequency and magnetic field effects are observed for both regimes and compared with the predictions of known models of conduction. The electromechanical coupling constant of InSb is also determined. The relative velocity measurements were performed on p-InSb samples having different excess impurity concentrations using an acoustic interferometer having a sensitivity of 2 p.p.m.

The spectrum of conduction electrons of a narrow-band semiconductor in paramagnetic and antiferromagnetic phases for an arbitrary value of localised spins is investigated by the diagram technique for the Hubbard operators. It is shown that there can be both blue and red shifts of the optical absorption edge depending on the type of lattice.

The transmission spectra of later-deposited PbSe films have been measured over the wavelength range 2-5 mu m at different temperatures over the range 90-310 K. The dispersion of the refractive index and the spectral dependence of the absorption coefficient for samples with various carrier concentrations have been obtained. The values of the optical dielectric constant epsilon _infinity , the susceptibility effective mass m_sigma and the energy gap at different temperatures have been determined. From the concentration dependence of m_sigma at the Fermi level, the values of the longitudinal and transverse effective masses at the top of the valence band have been found. The authors' interpretation of the spectral dependence of the absorption coefficient leads them to the conclusion that an absorption additional to free-carrier and interband absorption takes place in the samples. In their view this additional absorption can be attributed to a defect resonant level in the valence band.

The authors have obtained the absorption and emission spectra for the Sm²⁺ impurity in the mixed fluorides, Ca_1-xY_xF_2+x and Ca_1-xLa_xF_2+x. From these spectra they conclude that the Sm²⁺ ions tend to be coordinated by one or more extra F^- ions in addition to the normal eightfold coordination expected in CaF₂. Measurements of the emission lifetime and intensity as a function of temperature lead them to propose that the luminescence quenching of Sm²⁺ in the mixed fluorides is due to a competition between ionisation of the 4f⁵5d level with relaxation into the ⁵D₀ (4f⁶) emitting state. In addition, the thermal activation energy for Sm²⁺ emission quenching in various alkaline-earth halides is consistent with the calculated optical separation of the 4f⁵5d level from the conduction band of the host, lending further support to the ionisation mechanism of emission quenching.

Radiation effects of X-rays in zircon (ZrSiO₄) single crystals were studied at RT, by analysing the kinetics and spectrum of the thermoluminescence between 20 degrees and 420 degrees C. The results indicate that some of the electrons produced by the irradiation are captured by Dy³⁺ impurities apparently substituting for Zr⁴⁺ ions, while others are trapped at Si⁴⁺ lattice ions. The hole traps include O^2- lattice ions near Si³⁺ or Dy²⁺ electron centres, as well as other unidentified impurities or point defects. A TL peak at 77.5 degrees C which consists of a broad emission band around 385 nm typical of the SiO₄^4- group is ascribed to the recombination of an electron and a hole trapped at adjacent Si⁴⁺ and O^2- ions respectively. Its measured activation energy (0.89 eV) is in agreement with the theoretical value (0.87 eV). A series of TL peaks having activation energies of 0.82-1.60 eV and showing the spectral lines of Dy³⁺ are attributed to electron-hole recombinations at the RE site. The main TL peak at 109 degrees C (E=1.18 eV), which contains both the 385 nm band and the Dy³⁺ lines, might involve a recombination process which excites both the SiO₄^4- group and the RE ion.

The Cd K-shell X-ray absorption spectrum (26.6 to 27.6 keV) of polycrystalline Cd metal has been recorded at temperatures from 20 to 300 K. The spectrum of a Cd single crystal was also measured at temperatures from 4 to 300 K with the crystal in two orientations-one optimising the EXAFS from the first Cd distance (2.97 AA), the other optimising the EXAFS from the second Cd distance (3.29 AA). The visibility of the second Cd-Cd distance varies greatly in the spectra on account of the large anisotropy in the thermal motion. The approximate Debye-Waller factors for the thermal motion along the two near-neighbour directions are derived in a harmonic approximation and compared to other measures of the relative thermal motion.