National Institute Of Materials Physics - Romania

We show that, due to band mixing, the eigenstate localization within the disordered Landau bands gets an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a two-dimensional lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift disappear when the interband coupling is switched off.

FexOy-SiO2 nanocomposites were prepared by sol-gel method by using two SiO2 sources and Fe(SO4)(2)center dot 7H(2)O as raw materials. The amorphous gels were thermally treated up to 1000 degrees C. The initial gel and the thermally treated samples were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) and infrared spectroscopy. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The total amount and the size distributions of the hematite nanoparticles can be controlled via the initial precursors and subsequent by annealing conditions. (C) 2006 Elsevier B.V. All rights reserved.

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba1-xSrxTiO3 are very important candidates for a wide range device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x=0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 divided by 1260 degrees C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between - 150 and 150 degrees C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 degrees C/ min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 divided by 2 GHz). The Curie temperature of BST samples with x=0.35 and x=0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported. (C) 2007 Published by Elsevier B.V.

A comparative study of the sol-gel films prepared in the SiO2-P2O5 system starting with triethylphosphate, triethylphosphite and phosphoric acid as P precursors was performed. The work addresses basic aspects of physics, chemistry, and engineering of oxide films for applications in microelectronics, sensing, nano-photonics, and optoelectronics by establishing the influence of different precursors on the composition, structure and optical properties of the obtained films. The influence of the type of substrate (glass and indium tin oxide coated glass) and of the thermal treatment (200 and 500 degrees C on their structure and properties was studied. By spectroscopic ellipsometry, X-Ray photoemission spectroscopy and atomic force microscopy measurements the high vaporization of the phosphorous during the densification of the films by thermal treatment was noticed when P-alkoxides were used. The phosphoric acid that forms chemical bond with silica network during the sol-gel process leads to better incorporation of P in the silica network as compared to the P-alkoxides. (C) 2006 Elsevier B.V. All rights reserved.

The sol-gel deposition method has been successfully applied to obtain Pb(Zr0.2Ti0.8)03 thin films on platinized silicon wafers. Addition of different amounts (7-15 wt.%) of organic macromolecular polyvinylpyrrolidone in the precursor solution prior to spin coating proves to be an excellent method for obtaining porous films. The crystal structure of as deposited films was analyzed by X-ray diffraction. The porous films show perovskite phase after annealing at 650 degrees C. The surface morphology has been studied by Atomic Force Microscopy and Scanning Electron Microscopy. The surface profile indicates a roughness of the film of 5 nm and no microcracks on the surface. The ferroelectric behavior was proved for each film, by hysteresis loops and by the "butterfly" shape of the capacitance-voltage characteristics. The remnant polarization and the coercive field decrease while the amount of added PVP increases. (C) 2006 Elsevier B.V. All rights reserved.

Small particles of iron - based magnetic spinel oxide are technologically interesting because of their magnetic properties. The synthesis from solution, generally achieved by coprecipitation, offers some advantages: simple and rapid preparation, control of particle size and composition, various possibilities to modify the particle surface state allowing to make homogeneous and stable dispersion in liquid or solid media. This work reports the thermal studies of magnetic iron spinel oxide nanoparticles. For this study, the Fe2+-Fe3+ mixture (Fe2+/Fe3+=0.5) was introduced in an alkaline solutions of which the pH was kept constant. The influence of the particles sizes on the Y -> aFe(2)O(3) transformation is analyzed.

Magnesium doped (Zr0.8Sn0.2)TiO4 material has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1270 divided by 1315 degrees C for 2 hours. The effect of sintering temperature on structural and dielectric properties was investigated. The 0.2 wt.% MgO addition improves the sintering process and well sintered samples with a high value of bulk density were achieved. The material exhibits a dielectric constant epsilon(r) similar to 36.6 and high values of the Qxf product, greater than 61000 at microwave frequencies. The dielectric properties make the magnesium doped (Zr0.8Sn0.2)TiO4 material very attractive for such microwave applications as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc.

High harmonics ac susceptibility measurements joint with neutron irradiations on quasi-bi-dimensional high temperature superconductor (quasi-2D-HTSC) are good tools to study the flux dynamics and its interaction with pinning processes in these superconductors. Flux neutron intensity of 5 * 10(17) n * cm(-2) shows a deep change in the flux pinning dynamics in Bi-Sr-Ca-Cu-O (BSCCO) system. Third harmonic susceptibility signal increases in amplitude after the neutron irradiation, followed by a rise of the pinning and critical current. Moreover, after the irradiation, the measurements underline the demise of the anomalous peak effect (PE) associated with a three-dime.nsional/two-dimensionaI (3D/2D) flux lattice transition.

Simple investigation of the electrical resistance does not permit to draw up the conduction mechanism of thin SnSe2 layers. The presence of reducing gas atmospheres, respectively CH4, CO and H2O could change the concentration of the free charge carriers, beside electron affinity changes induced by the appearance of surface dipoles. Work function and electrical resistance were simultaneous investigated for thin SnSe2 films. The experimental results permit the evaluation of electron affinity and band-bending behavior. Conduction mechanism can be discussed in terms of surface or bulk contribution.

We determined the Berezinskii-Kosterlitz-Thouless transition temperature T-KT at the superconducting layer level and the mean-field critical temperature T-c0 in oxygen-deficient YBa2Cu3Oy films (y similar to 6.5, 6.55, and 6.65). We used the T dependence of the quasi-two-dimensional I-V exponent for T <= T-KT, and the analysis of the resistive transition in the framework of the Ginzburg-Landau two-dimensional Coulomb gas model for T>T-KT. Both procedures give essentially the same results, with T-c0 remaining in the domain of the electrical resistivity drop.