National Institute Of Materials Physics - Romania

In this paper the quasi-energies and the reduced Floquet vectors corresponding to a particle in 2-dimensions interacting with a time-dependent, monochromatic, linearly polarized electro-magnetic field, and in the presence of a weak, static, spatial-periodic field, are calculated. The later is treated using the stationary perturbation theory in the extended Hilbert space adapted for the eigenvalue equation of the Floquet Hamiltonian. The results have similarities to those of the quasi-free electron model.

An X-band electron paramagnetic resonance (EPR) study of nominally pure, diamond-like cubic boron nitride (c-BN) crystalline powders, has led to the first identification of a spectrum attributed to two related paramagnetic species. The composite EPR spectrum can be observed only in dark brown colored powders known to contain excess of boron. It consists of two superimposed lorentzian components, called D1 and D2, centered at g1 = 2.0063 and g2 = 2.0084, with peak-to-peak linewidths of 3.3 and 17.9 mT, respectively. The temperature dependence of the integrated intensities, their linewidths and intensity ratio D2/D1 allows one to conclude that the narrow line D1 originates from EPR transitions inside a S = 1/2 ground doublet and the broad line D2 from transitions inside the excited levels of another center. Evidence suggests that both centers are boron related paramagnetic species. (C) 2000 Elsevier Science Ltd. All rights reserved.

The X-ray crystal structure of trinuclear iron acetate [Fe3O(CH3COO)(6)(H2O)(3)](2)[ZnCl4]. 2H(2)O was determined. The crystal has a ionic structure. It is monoclinic, a = 25.363(7), b = 14.533(4), c = 15.692(4) Angstrom, beta = 103.11(2)degrees, space group C2/c, and R = 0.0685. The structure of the cationic complex [Fe3O(CH3COO)(6)(H2O)(3)](+) is typical of trinuclear iron(III) compounds containing a mu(3)-O bridge: the iron atoms are situated at the vertices of an almost equilateral triangle with the O atom at the center. Each Fe atom is coordinated by four O atoms of bridging carboxy groups, the mu(3)-bridging O atom, and the water molecule in the trans position to the latter O atom. Mossbauer spectroscopy evidence indicates the high-spin state (S = 5/2) of the iron(III) ions. (C) 2000 MAIK "Nauka/Interperiodica".

The paraelectric ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl2O4 was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accom panied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400-670 K) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6(3)22, a approximate to 5.22 Angstrom, c approximate to 8.8 Angstrom) and of the FE phase (space group P6(3), a = 10.4469(1)Angstrom, c = 8.7927(1)Angstrom) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.

Conduction properties as well as trapping phenomena in nanocrystalline porous silicon films are investigated in relation to their microstructure. Theoretical models to describe the phenomena are proposed. The main results presented in this paper are: the two-scale porosity of our porous silicon films, the decisive role of the quantum confinement to the conduction phenomena and the prominent part played by the oxidation on the surface effects.

ITO thin films were deposited by spin coating method starting from indium and tin propoxide in the frame of a sol-gel process. Films with the thickness corresponding to 68 nm for one layer deposition were produced and characterized by X-ray diffraction, transmission electron microscopy and infrared reflection spectroscopy. Low conductivity polycrystalline films exhibiting cubical structure (bixbyite-type) and good adherence were obtained.

In this paper is presented an extension of the density dependence in the Landau-de Gennes theory for the nematic to isotropic phase transition, taking into account this dependence for three phenomenological coefficients in the free energy expansion. In this manner, a better agreement between the calculated thermodynamic quantities and their experimental values was obtained near the nematic-isotropic phase transition. More. the theory leads to a better description of the temperature dependence of the order parameter in the same temperature range.

A series of CuGa1-xInxTe2 bulk quatemary compounds (x=0.17, 0.33, 0.50, 0.67, 0.83) has been synthesized from the corresponding stoichiometric melts by a rapid cooling procedure. The same procedure was used to obtain the ternary bulk compounds CuGaTe2 and CuInTe2 with the aim to compare, according the Vegard law, the match between molar fraction of the indium (x) and the lattice parameters for the aforementioned quatemary compounds. The nature of the crystalline phases, the local structure homogeneity and stoichiometry of these compounds have been investigated by x-ray diffraction (XRD) and scanning electron microscopy (SERI) techniques. The analysis revealed the presence of the chalcopyritic structure for all the samples.

Pb(ZrxTil .x)O-3 (x = 0 divided by 0.8) graded thin films were fabricated by sol-gel method on Pt/ SiO2/ Si substrates using metal alkoxide solution. The samples were crystallized by conventional thermal annealing in air for 30 min. at 700 degreesC. The ferroelectric properties of the graded structure were investigated Specific unusual behavior in the ferroelectric loops like large offset on the polarization axis was observed and disscused.

The morphology and structure of diamond films, deposited on cubic boron nitride (c-BN) crystals by microwave-plasma-enhanced chemical vapor deposition, is studied by high-resolution scanning electron microscopy and micro-Raman spectroscopy. The c-BN crystals, with sizes of 200 to 350 mu m and grown by a high-temperature/high-pressure technique, were embedded in a copper holder, and used as substrates in deposition runs of 15 min to 5 h. The nucleation centers for diamond appear as well-shaped cuboctahedral crystallites, having diameters of approximately 100 nm. With increasing deposition time the diamond crystallites grew larger, forming islands on the c-BN faces. In some cases, epitaxial growth was observed on the (111) c-BN faces where coalesced particles gave rise to very smooth regions. A number of diamond crystals with peculiar shapes are observed, such as a pseudo five-fold symmetry due to multiple twinning. Moreover, both randomly distributed carbon tubes, about 100 nn in diameter and 1 mu m in length, and spherically shaped features are observed in samples prepared under the typical conditions of diamond deposition, this effect being ascribed to the influence of plasma-sputtered copper contamination. Quite unusual diamond crystals with a deep, pyramidal-shaped hole in the middle grew on the copper substrate between the c-BN crystals. (C) 2000 Elsevier Science S.A. All rights reserved.