Publications

An elastic model for the twisted nematic texture observed in nematic cells containing octyloxybenzoic acid is proposed. According to the model the twisted textures are due to the presence of cybotactic groups which are responsible for the chirality of the liquid crystal. We show that by assuming for the density of cybotactic groups a critical temperature behaviour close to a special temperature, the experimental data can be well interpreted by our mechanical model.

The dependence of the displacement damage induced by pions in GaAs has been evaluated, in the energy range 50 -1000 MeV, using the Lindhard theory and parametrised values of the pion-nucleus interaction. The curves corresponding to the NIEL produced by protons, neutrons, and pions in Si and in GaAs intersect, in the limit of calculation errors, in the region 50 - 80 MeV, at a NIEL of 2 - 5.5 MeV cm(2)/g. The energy dependence of the NIEL of pions in Si and GaAs is discussed in correlation with the simulated pion spectra at new hadron colliders.

We investigate the far-IR absorption of short-period parallel quantum wires in a perpendicular quantizing magnetic field. The external rime-dependent electric field is linearly polarized along the wire modulation. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. We consider the effects of a metal gate grating coupler, with the same or with a different period as the wire modulation, on the absorption. The evolution of the magnetoplasmon in the nonlocal region where it is split into several Bernstein modes is discussed in the transition from narrow to broad wires and isolated to overlapping wires. We show that in the case of narrow and not strongly modulated wires the absorption can be directly correlated with the underlying electronic band structure.

An EPR study of the Tl+-doped PbCl2 crystals x-ray irradiated at low temperature reveals the presence, besides known spectra attributed to hole-trapped Tl2+ centers and Pb-2(3+) self-trapped-electron (STEL) centers, of an additional strongly anisotropic set of lines consisting of an intense doublet and several weaker satellite lines spread over a large magnetic-field range. The quantitative analysis of the EPR spectrum shows the corresponding paramagnetic center to be a bent molecular ion (PbTl)(2+) resulting from electron trapping at a pair of substitutional Pb2+ and Tl+ ions. Its formation suggests the electron trapping at pairs of neighboring cations is a more general characteristic of this material, with possible relevance in photolysis and exciton localization properties.

We compute numerically the time dependent retarded Green function of the polaron within the self-consistent RPA approximation. The results show an approximately Gaussian behaviour at t = 0 changing at later times its concavity to an exponential decay? as it has been predicted in the approximate form of an inverse hyperbolic cosine function. The result contrasts with the non-selfconsistent RPA: where the exponential decay is only a transitory behaviour and the asymptotics is rather oscillatory. Our conclusions are significant in the context of the quantum kinetics with LO-phonons, where the transition from an intitially coherent scattering kinetics to a Markov kinetics with energy conservation is controlled by the time behaviour of the retarded Green function.

A Mossbauer and magnetic study of FeRh has been undertaken in order to understand better the remarkable properties of both melt-spun BCC and ball-milled FCC-phases of the anti-invar FeRh alloy. (C) 1998 Elsevier Science B.V. All rights reserved.

The EPR spectra of Mn2+ doped K2ZnCl4 show an anomalously large broadening of the Zn site Mn2+ lines between 152.5 K and 145 K, i.e. just above the transition to the low temperature commensurate phase of A1a1 symmetry. The line broadening is related to a distribution of rotations of the ZnCl4 tetrahedra and supports the existence of a second incommensurate phase in this temperature range as reported by Gesi. (C) 1997 Published by Elsevier Science Ltd.

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculated in the four-probe Landauer-Buttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of the density of states at a given magnetic flux are correlated with the behaviour of the mentioned resistances as function of the Fermi energy. The Hall insulator and quantum Hall regimes are evidenced. The magnetic field dependence of the configurational averages of the longitudinal and Hal; resistance is studied in a purely quantum-mechanical approach. Both, negative and positive magnetoresistances are found.

The phase transition from the solid state to the liquid crystal state was investigated for the copper(II) salts of the fatty acids with 10, 12, 14, 16 or 18 carbon atoms in the chain, by measuring the temperature dependence of the conductance and capacitance of a cell containing the material under investigation. The temperatures deduced from these dependences agree reasonably with those found by DSC measurements and with those to be found in the literature. Differences in dielectric behaviour between the samples are qualitatively discussed.

Samples obtained from the hot conditioning and hot performance procedures have been investigated by GEMS. Various types of substrates were considered. The surface phases were assigned to magnetite, alpha-Fe, wustite and to spinel solid solutions. The data have been analyzed taking into account the compound stoichiometry, the number of iron unequivalent positions appeared in each identified phase and the relative ratio of the formed products.