National Institute Of Materials Physics - Romania

The processing of electrical contacts of low electrical resistance constitutes a serious problem in the case of high temperature superconducting ceramics. Bad electrical contacts generate an important heat flux if the current overcomes several ampere. Moreover, the contacts tend to detach during the experiment. To avoid this difficulty, we made a special device, based on the elastic steel blades at low temperatures (external adjustment permits total surmount of the detach tendency). If the sample has a convenient geometry (bar bells) and the electrical contacts are made inside of the little silver paste island, the resistance of the current contacts is below 0.1 Omega.

There was a wide range of structures which are neither monocrystalline nor amorphous. Setting up a general model for such structures is very difficult because lattice distortions have different reasons and configurations. Therefore only approximations can be simulated. One of the most useful approximation is the model of paracrystals mainly developed by R. Hosemann. His idea was to postulate the distance to the neighbour atom or particle by a certain statistics. Starting with the linear "paracrystal" it is very easy to generate a statistical distribution of atoms or panicles. But even for two dimensional paracrystals the problem arises that the diagonal statistics does not obey the general idea of the paracrystal. Several attempts have been made to model the two or three dimensional paracrystal but only the present authors potential field model offers the possibility to simulate those structures. The basic idea is very simple. To each of the atoms (or particles) positive center Gaussian functions are attributed together with a certain number of negative Gaussian traps; there interaction leads to the position of the neighbour atoms. Using the Monte Carte method each of the neighbours can occupy a statistically generated position. So the three dimensional paracrystal can be generated. Several possibilities of additional parameters enable to adapt the model to the wanted conditions.

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu= 1.8:0,4:1.8:0.2:2.2:3.0, the precursor powder for synthesis of Bi-2223 superconducting phase, was obtained by spray-frozen, freeze drying technique. Samples of the nitrate precursor powder were placed in a heated furnace and extracted in air when temperature of the powder has attained values of 640, 660, 710, 750, 760, 778, 790, 815 and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

Metal/ Ferroelectric/ Semiconductor (MFS) heterostructures have been of interest in the last decade due to possibility of their potential integration in classical silicon technology and because their potential use in memory devices. Research in this field has lead to new generations of sensors and actuators, laser modulators and deflectors, non-volatile random access memories (NRAM's). Unfortunately many at these types of devices remain in the prototype phase because of the impossibility to know and control phenomena which appear at the ferroelectric/ semiconductor interface. The present paper presents some results on the investigation of a PbTiO3/ Si heterostructure. A study of the film occurring at the interface and how it influences the ferroelectric properties of the heterostructure can clarify some problems of the reproducibility of the parameters of a MFS heterostructure.

Anodic film oxide was deposited in a alcohol-glycol-water (AGW) solution on n-GaAs for passivation purpose in stripe technology for laser diodes. The characteristics of anodic oxide were measured by scanning electron microscopy (SEM), Rutherford backscattering (RBS) analysis and elastic recoil detection (ERD) techniques. The result indicates a complex oxide structure in the phase base Ga2O3:As2O3 (1:1) joint together with carbon bonds. Due to the presence of carbon in anodic oxide, laser diodes are exposed to rapid degradation during operation.

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.