Publications

Optical absorption evolution is studied for 150-800 mu m thick diamond films grown by microwave plasma enhanced CVD technique and annealed in vacuum at temperatures 900-1600 degrees C. A noticeable internal degradation (darkening) of the films takes place at T > 1300 degrees C. Changes in the concentration of different CH, groups are revealed from the IR spectra starting at T > 1000 degrees C. Up to 25% of the total amount of hydrogen in as-grown films is found to be unbonded. An interesting effect of reduced absorption in UV range due to the formation of new C-H bonds by reaction of primarily unbonded hydrogen is observed. The diamond darkening correlating with the bonded hydrogen loss process, is ascribed to the appearance of a graphite-like phase (being under a high stress) presumably at grain boundaries, that is revealed by the appearance of a specific absorption peak in UV spectra.

We synthesized Fe3-xCoxO4 (x=0-0.6) using the hydrothermal method at 300 degrees C in order to demonstrate substitution effects on the hyperfine magnetic fields and populations of the tetrahedral (A) and octahedral (B) sites of magnetite. Our Mossbauer spectroscopy results were first analyzed using the site model, which provided direct evidence for the presence of the Co substitution in the (B) sublattice. This was accompanied by a systematic increase of the hyperfine magnetic field at (B) sites. The mechanism we propose relies on the substitution of Fe2+ by Co2+ in the (B) sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on the content x of cobalt substitution. Finally, the Mossbauer spectroscopy results were also analyzed using the hyperfine magnetic field distribution model for direct comparison. For the first time, it was demonstrated that the held values provided by the site model fall within the standard deviation obtained from the distribution model. (C) 2000 Elsevier Science Ltd. All rights reserved.

In this review are presented the main results of a complex research project devoted to preparation and investigations of physical properties of [(BaCuO2)(n)/(CaCuO2)(m)](N) artificial superconducting superlattices. The paper starts with the layer-by-layer pulsed laser deposition technique developed for this project and discuss the structure of this new material. The superconducting properties and the effects of structural disorder on them are also presented.

Thermal relaxation and the relaxation induced by UV radiation in thin amorphous films of composition Ge(x)As(40.x)Se(60) (0 less than or equal to x less than or equal to 40) have been investigated by diffraction de rayons X. The structural results allowed to suggest the models for the atomic scale transformations.

The dephasing and condensation kinetics of bosonic excitons after coherent pulse excitation is studied in the vicinity of a Bose-Einstein condensation (BEC). For a system of excitons interacting with acoustic phonons various excitation conditions an investigated. Significant qualitative changes of the polarization dephasing in the vicinity of the BEC are discussed. Furthermore the kinetics of excitons, interacting via a two particle interaction, is studied. Both the bosonic mean-field terms and the scattering terms are calculated on a microscopic level. As in the phonon-model the dephasing rate of the exciton-exciton scattering diminishes considerably in the vicinity of the BEC. All results are suited as experimentally observable signatures of an excitonic BEC.

The influence of the mesa size on the structural and luminescence properties of Ge island layers deposited by low-pressure chemical vapour deposition was studied. Diode structures containing one Ge island layer were grown selectively on patterned wafers with variable area. The analysis shows that the electroluminescence spectra could be separated into three contributions lying at low-, medium and high-energy positions, respectively. The absolute peak intensity of each signal depends on the mesa size and on the injected current density. The signal at the low-energy position was assigned to an effect due to the facets of the diodes; the two other signals originate from islands on the (001) plane having different Si contents or strain states on the (001) plane.

The polarization behaviour of Armco iron in aqueous solutions of HCl with and without inhibitors has been studied by the potentiostatic method. At lower overvoltage values, the dissolution process is controlled by activation, while at higher overvoltage values, the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using four surfactants. Under the critical micelle concentration (CMC), the inhibition of these four surfactants is negligible. At a concentration higher than the CMC, the inhibiting action of tetradecyl trimethyl ammonium iodide (TTAI), tetradecyl trimethyl ammonium bromide (TTAB), hexadecyl trimethyl ammonium bromide (HTAB) and dodecyl trimethyl ammonium bromide (DTAB) increases rapidly. The process of inhibition was attributed to the formation of adsorbed film on the metal surface, that protects the metal against corrosive agents. Protection efficiency improved when the inhibitor concentration and the length of the alkyl chain were increased. The four quaternary ammonium salts tested were adsorbed on the Armco iron surface according to a Langmuir isotherm. (C) 2000 Elsevier Science S.A. All rights reserved.

A procedure for computing the resolution function for a crystal neutron diffractometer, taking into account the spatial configuration of the experimental setup and the monochromator curvature, is presented. A brief general description of the matrix computation technique is given. The program DAX, which employs this technique, is presented.

The evolution of the critical current density of Li-doped YBa2Cu3O7-x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 less than or equal to B less than or equal to 6 T) temperature (5 less than or equal to T less than or equal to 85 K) and neutron fluence (0 less than or equal to Phi less than or equal to 9.98 x 10(17) cm(-2)). At fluences lower than 10(17) cm(-2), a second peak in j(s) vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10(17) cm(-2), the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B-infi. A dependence of B-infi on fluence is proposed.

The effect of electron irradiation having the energy of 75, 100, and 200 keV on structural modifications of Bi-2212 superconducting samples has been studied. For the last energy, the irradiation time from zero to 150 min was used. At a constant energy of the electrons, the observed phenomena consist in the disappearance of the incommensurate unidimensional modulation, in the decreasing of spots intensity and their elongation along the equivalent crystallographic axis a, and even spot splitting with the occurrence of double extra spots, with the increase of the irradiation time. After electron irradiation with energy of 75 and 100 keV, the structural modifications lead to some spot patterns consisting of some planar lattices (in some cases a pseudotetragonal one) that are twisted on each other at different angles (8 degrees, 13.6 degrees, 19 degrees, etc.) around the axis of the incident electron beam. For the irradiation at increased doses of thin microcrystals having reduced lateral dimensions, the electron diffraction spots were arranged in discrete or partial continuous Debye rings or continuous concentric Debye rings characteristic for the polycrystalline state. After electron irradiation with energy of 200 keV, the effects of electron irradiation on Bi-2212 samples depend strongly on irradiation fluence rate and time and consisted in the following: disordering defects in the diffraction patterns (disappearance of some spots, spot intensity modification, streaks occurrence, spot elongation); alteration and disappearance of incommensurate structural modification; conversion of single crystal particle areas into polycryslalline material; and quasi-amorphization. A simple approach based on the evaluation of the displacement yield of in-plane oxygen atoms vs. irradiation time for the different incident energy and electron fluence rates could explain the general trend of irradiation damage in HTS materials.