Table of contents

We apply elements of the self-consistent theory for localization, reciprocity and conservation of energy, in order to find a theory for the enhancement factor in Coherent Backscattering of classical waves in 3D. An application is presented for the classical Milne problem.

We report experimental measurements on the mixing properties of a powder. We study a 2D model granular medium confined in a half-filled rotating drum. Using an image-processing device, we follow the trajectories of tracer particles in a monodisperse assembly of beads. Tracer particles with different size ratios exhibit a violent segregation effect: a smaller particle has a tendency to stay in the centre and a larger one will rather dwell on the edges. Furthermore, for a tracer of identical size, we evidence a specific dispersion property where the centre and the edges are competing attractors of the mixing dynamics.

We study the excited-state quantum numbers in the extremely neutron-rich nucleus ⁸He and show that the existing experimental data admit spin-parity J^π = 2⁺, which is standard for first-excited states in nuclei with even numbers of protons and neutrons, as well as the unique case of J^π = 1^-. To admit discrimination between these two cases, the existing experimental inelastic data have to be extended to smaller angles. We also try the sensitivity of the elastic scattering of halo nuclei at energies about 100 MeV/u to details of the halo distribution.

Neutron angular distributions from violent break-up reactions of ¹¹Li and ¹¹Be have been measured at 28 MeV/u and 280 MeV/u and at 41 MeV/u and 460 MeV/u, respectively. The derived neutron momentum distributions show a narrow component in transverse momentum that is within uncertainties independent of beam energy and target charge. This component is suggested to be simply related to the momentum distribution of the loosely bound halo neutron(s) in the projectiles.

This work analyses formation of the optogalvanic signal for NeI(5s₁-2p₄) optical transition by using data obtained from direct measurements of the electron temperatures and concentrations and from an estimation of the frequencies of electron-atom and electron-ion collisions. Our results show negligible contribution of the electron temperature change to the measured optogalvanic signal. Therefore, the appearance of the optogalvanic signal may be attributed to the increased electron concentration (due to the enhanced ionization from 5s₁ Ne atomic level) under resonant laser illumination.

Salt-free micellar solutions exhibit strong electrostatic correlations even at very low surfactant volume fraction. By applying a flow, micellar growth can be induced suggesting the existence of a hydrodynamic/electrostatic coupling. Moreover, neutron scattering experiments under shear and rheological experiments seem to indicate orientational rather than positional nature of the electrostatic correlations.

A homeotropically aligned nematic (MBBA) with negative dielectric anisotropy subjected to electric and magnetic fields has been investigated. If an electric field perpendicular to the nematic layer is applied, first the Fréedericksz transition takes place spontaneously breaking the rotational symmetry and subsequently the electrohydrodynamic instability (EHC) sets in. An additional magnetic field in the plane of the layer induces a preferred director orientation. The effects of the magnetic field on the EHC threshold voltage, on the critical wave vector and on the roll angle are examined as a function of the applied frequency. The data are compared with theoretical results. Furthermore, accompanying domain walls are characterized.

Using the Non-linear Schrödinger Equation, we have found a new mechanism for the generation of vortices in a superfluid. This mechanism could be relevant for explaining the experimental creation of vortices by vibrations. This follows from the break-up of dark solitons in more than 1 D, which are linearly unstable, as shown by Kusnetsov and Turitsyn, Ž. Eksp. Teor. Fiz., 94 (1988) 119 (Sov Phys. JETP, 67 (1988) 1583).

Discommensurations (DC) and domains in incommensurate charge density wave (CDW) states of the quasi-two-dimensional (PO₂)₄(WO₃)_2m (m = 12) bronze have been observed directly by dark-field transmission electron microscopy with use of the CDW satellite reflections at temperatures below ∼ 500 K. Two types of domains are found. The narrower ones have a fairly constant width of ∼ 45 Å; they are bounded by a pair of discommensurations. They are separated by the broader ones, whose widths vary from ∼ 45 Å to ∼ 140 Å. All broader domains have the same phase. The two types of domains are disoriented one with respect to the other about a direction parallel to the DCs by an angle of ∼ 1 degree. This domain structure does not depend significantly on temperature in the range of existence of the CDW metallic phase.

We have performed reflectance and EELS measurements on the quaternary superconductor YNi₂B₂C. Linking together the results of these two types of experiments we derived the dielectric function in the energy range between 10 meV and 50 eV. The data were analysed by using a generalized Drude-Lorentz model, where retardation effects resulting from the electron-phonon interaction were taken into account to describe the free-carrier response. For the d.c. coupling constant a value of λ_tr ∼ 1.2 was obtained.