Publications

The Letter presents an attempt to include the concentration dependence in the Landau-de Gennes theory for the nematic to isotropic phase transition for nematic lyotropic liquid crystals by introducing a critical concentration for the lyotropic solutions. This allows to describe phase transitions of micellar solutions as depending on temperature and concentration. An estimation of phenomenological coefficients of the CsPFO/water system is made, and we obtained the trends of the order parameter curves in function of temperature and concentration in good agreement with experimental results from literature. (C) 2001 Elsevier Science B.V. All rights reserved.

Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350-1450 degreesC. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.

The CO consumption of tin dioxide, thick film sensors and their sensor signals have been investigated in order to study the GO-water interaction on the SnO2 surface. The mechanism of CO combustion, and the influence of the sensor's operation temperature (200-400 degreesC) and ambient humidity (0-50% r.h.) on the combustion has been characterised in detail. Here, the overall CO consumption of the sensors has been divided according to the contributions of the various individual sensor parts: alumina substrate; electrodes: heater; and the SnO2 based sensitive layer. It can be shown that CO combustion is a linear function of CO concentration. The influence of the substrate on total CO consumption has been proven to be significant. (C) 2001 Elsevier Science B.V. All rights reserved.

An Fe+-type center with axial (001) symmetry has been studied by electron nuclear double resonance (ENDOR) in the microwave X and Q bands. The Fe+(I) center is produced only after irradiation with x or gamma rays at 80 K of SrCl2:Fe2+ crystals grown in a Cl-2 atmosphere. It has a 3d(7) F-4 Gamma (8) ground state with spin S =3/2. As shown by the correlated analysis of the ENDOR data and electron paramagnetic resonance superhyperfine structure, the Fe+ ion is situated in the center of a tetragonally compressed cube of eight nearest Cl- ions. The simplest model of the Fe+(I) center, compatible with the magnetic resonance results, consists of an interstitial Fe+ ion with two substitutional monovalent cations, very likely K+ ions, in two opposite nearest-neighbor Sr2+ sites along the tetragonal axis of the center.

The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 degreesC when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 degreesC, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed. (C) 2001 American Institute of Physics.

The RD48 (ROSE) collaboration has succeeded to develop radiation hard silicon detectors. capable to withstand the harsh hadron fluences in the tracking areas of LHC experiments. In order to reach this objective, a defect engineering technique was employed resulting in the development of Oxygen enriched FZ silicon (DOFZ), ensuring the necessary O-enrichment of about 2 x 10(17) O/cm(3) in the normal detector processing. Systematic investigations have been carried out on various standard and oxygenated silicon diodes with neutron, proton and pion irradiation up to a fluence of 5 x 10(14)cm(-2) (1 MeV neutron equivalent). Major focus is on the changes of the effective doping concentration (depletion voltage). Other aspects (reverse current, charge collection) are covered too and the appreciable benefits obtained with DOFZ silicon in radiation tolerance for charged hadrons are outlined. The results are reliably described by the "Hamburg model": its application to LHC experimental conditions is shown, demonstrating the superiority of the defect engineered silicon. Microscopic aspects of damage effects are also discussed. including differences due to charged and neutral hadron irradiation. (C) 2001 Elsevier Science B.V. All rights reserved.

X-ray diffraction and conversion electron Mossbauer spectrometry (CEMS) measurements of Fe87Zr6B6Cu1 samples in different steps of the crystallization process have been performed. The results have been compared with those obtained by means of the transmission Mossbauer technique. The X-ray diffraction patterns and CEMS spectra of the samples present systematically larger crystallized fractions than those corresponding to spectra obtained by transmission Mossbauer spectroscopy (TMS). As these techniques offer us information about different regions of the sample, the differences among the obtained results have been related to an inhomogeneity of the crystallization process into the sample induced by thermal treatments. (C) 2001 Elsevier Science B.V. All rights reserved.

Epitaxy of Fe on GaAs by the MBE technique has a long tradition in magnetism research. This paper deals with the growth of epitaxial Fe-GaN-Fe trilayer structures whose intriguing magnetic properties were exploited for the evaluation of GaN as a spin-dependent tunneling barrier. The trilayers were grown on semi-insulating (0 0 1) GaAs using an ultra-high vacuum deposition chamber equipped with electron cyclotron resonance (ECR) microwave plasma nitrogen source. (C) 2001 Elsevier Science B.V. All rights reserved.

The resistive transitions R(T) and the current-voltage (I-V) in fields up to 5 T were measured in a melt-textured YBa2Cu7-delta (YBCO) sample consisting of two single-grain domains separated by an inter-domain border, and in a single-grain sample cut from the same pellet and oxygenated in such conditions that ensure the optimum oxygenation of the grains. From comparing the two sets of results, we were able to suggest that the inter-domain border consists of oxygen-deficient YBCO with thickness smaller than 6 mum, having a slightly lower critical temperature, a higher anisotropy factor and without strong pinning centers. From applications point of view, our work shows that, for improving the superconducting properties of large multiple-domain melt-textured pellets. an oxygenation process much longer than that required for optimum grain properties is needed. (C) 2001 Published by Elsevier Science B.V.

A classical model of electrons interacting with a longitudinal optical (LO) continuum phonon field via the Frohlich coupling is studied. Simulating a spatially homogeneous excitation by initially,distributing the electrons on a lattice, the solution of the many-body problem reduces to the evaluation of one nonlocal integro-differential (vector) equation, which can be solved numerically. According to the two parameters governing the behavior-electric field and coupling constant-two different regimes can be distinguished, a diffusive and a ballistic one. In the critical range of transition from one regime to the other the influence of the electron density becomes crucial. It is shown that the electrons-via the interaction with the phonon field-emit radiation in the terahertz range around the LO-phonon frequency.