National Institute Of Materials Physics - Romania

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5781 articles found

3591. Vortex-system ordering during magnetisation measurements in YBa2Cu3O7-delta films at low temperatures

Authors: Miu, L; Miu, D; Jakob, G; Adrian, H

Published: SEP 1 2007, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 460, 1207, DOI: 10.1016/j.physc.2007.04.050

Zero-field-cooling dc magnetisation relaxation measurements performed on optimally doped epitaxial YBa2Cu3O7-delta films with the external magnetic field H oriented parallel to the c-axis reveal the ordering of the creeping vortex-system in the low temperature T domain, due to the macroscopic currents induced in the sample. This dynamic ordering manifests itself through the appearance of a maximum in the T variation of the normalised vortex-creep activation energy. The location of this maximum in the H-T plane describes a well-defined current induced order-disorder line. Our results suggest the absence of a static elastic vortex glass in disordered high-temperature superconductors at high H, and the current induced ordering of the vortex-system as a possible origin for the current-voltage characteristic scaling in terms of an elastic vortex glass-vortex liquid transition. (c) 2007 Elsevier B.V. All rights reserved.

3592. Surface-analytical and EPR studies on sediment samples in Herastrau Lake, Romania

Authors: Seletchi, ED; Negrila, C; Duliu, OG; Turcas, CV

Published: SEP 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2953, DOI:

X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been used to investigate the elemental composition, chemical and electronic state of elements that exist within the Herastrau Lake (Bucharest, Romania) sediments. XPS spectra were obtained by irradiating the sediment samples with an Al source of X-rays (1486.6 eV). C, N, O, F, Na, Mg, Al, Si, S, Cl, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, and Pb have been determined by XPS and AES techniques. The analysis of Electron Paramagnetic Resonance (EPR) spectra revealed an additional free radical peak in the center of Mn2+ sextet, at a g = 2.0038, attributed to SO3- species as well a very intense and broad signal due to Fe3+ ions.

3593. Couplings control in a compact microstrip dual-mode resonator

Authors: Banciu, MG; Ioachim, A; Ramer, R; Militaru, N; Lojewski, G

Published: SEP 2007, MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, 49, 2295, DOI: 10.1002/mop.22722

A compact microstrip dual-mode resonator with slots in the ground plane is presented in this paper. Several methods for coupling between the dual modes are investigated. One of the methods involves using slots in the ground plane in order to enhance the external coupling and to introduce the necessary perturbation to the initially symmetrical structure. The proposed device requires a surface area about 6 tunes smaller than the area needed for a conventional patch dual-mode resonator. (C) 2007 Wiley Periodicals, Inc.

3594. Transport through a multiply connected interacting mesoscopic system using the Keldysh formalism

Authors: Dinu, IV; Tolea, M; Aldea, A

Published: SEP 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.113302

We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green's function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green's function is not necessary and an exact current formula can be expressed in terms of retarded Green's functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameter space where the conductance is temperature independent. We include a comparison with the results from Ng's ansatz [Phys. Rev. Lett. 70, 3635 (1993)], which are less accurate, but can be used also in the absence of the above mentioned symmetry.

3595. Influence of geometrical properties on light emission of ZnO nanowires

Authors: Enculescu, I; Sima, M; Enculescu, M; Enache, M; Vasile, V; Neumann, R

Published: SEP 2007, OPTICAL MATERIALS, 30, 75, DOI: 10.1016/j.optmat.2007.01.002

The present paper deals with the preparation and study of optical properties of ZnO nanowires. Nanoporous ion track polycarbonate foils were used as templates for ZnO nanowires growth by electrochemical deposition. The growth was performed using an aqueous electrochemical bath at a temperature of 70 degrees C. SEM and TEM were employed to study the morphological properties of the nanowires. Electron diffraction was employed for structural characterization. Reflection and photoluminescence spectroscopy were used for optical characterization. A comparison between the properties of electrodeposited thin films and nanowires of ZnO was made from the point of view of photoluminescence properties. (c) 2007 Elsevier B.V. All rights reserved.

3596. Electrolytic reduction of Tl+ ions in KCl crystals

Authors: Polosan, S; Tsubi, T; Apostol, E; Topa, V

Published: SEP 2007, OPTICAL MATERIALS, 30, 97, DOI: 10.1016/j.optmat.2006.11.006

Electrolytic coloration has been made for KCl crystals doped with Tl+ ions and doped with Tl+ and Ca2+ ions. Intensity decrease of the Tl+ absorption bands is observed in the two colored crystals, while several new absorption and emission bands are observed. They are attributed to Tl- ions which were converted from Tl+ ions by electron injection during electrolytic coloration. Different absorption and emission bands are observed for KCI:TI and KCI:T1 + Ca crystals. Sharp absorption band appears at 285 nm in the colored KCl:Tl and at 400 and 503 nm in the colored KCLTl + Ca. Infrared emission bands are observed at 1470, 1600 and 2185 nm in colored KCI:Tl, while at 1630 and 1900 nm in colored KCl:Tl + Ca. It is suggested that the absorption and emission bands in KCl:Tl + Ca are due to Tl- ions perturbed by Ca2+ or vacancy at the nearest neighbor. (c) 2006 Elsevier B.V. All rights reserved.

3597. Grain boundary effects in highly ordered Sr2FeMoO6

Authors: Burzo, E; Balasz, I; Constantinescu, S; Deac, IG

Published: SEP 2007, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 316, E744, DOI: 10.1016/j.jmmm.2007.03.081

The X-ray, HRTEM, electrical resistivities, magnetic and Mossbauer effect studies were performed on Sr2FeMoO6 samples. It was shown that the nonuniform composition inside the grains and that at grain boundaries influence the magnetoresistive properties. (c) 2007 Elsevier B.V. All rights reserved.

3598. Structure of PZT thin films deposited on stainless steel and on platinum/silicon substrate

Authors: Stancu, V; Sava, F; Lisca, M; Pintilie, L; Popescu, M

Published: SEP 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2969, DOI:

Pb(Zr1-xTix)O-3 (PZT) thin films were prepared by sol-gel method on stainless steel (SS) and on platinum-coated silicon substrate (Pt/Si). The structure revealed by X-ray diffraction shows strong crystallographic orientation: (110) and (100), respectively. A small amount of pyrochlore phases (Pb-2(Zr,Ti)(2)O6-delta is formed in the films deposited on SS substrate. The crystallite size shows significant differences: d((00l))((SS)) = 27.1 nm, d((h00))((Pt)) = 88 nm. The tetragonal splitting is (Delta c/c)((SS)) = 3.55 % and (Delta c/c)((Pt)) = 3.32 %.

3599. Mossbauer spectral study of NdFe1-xGaxO3 perovskites

Authors: Rubin, J; Piquer, C; Filoti, G; Parra-Borderias, M; Bartolome, F; Bartolome, J

Published: SEP 2007, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 316, E687, DOI: 10.1016/j.jmmm.2007.03.062

The Fe-57 Mossbauer spectra of NdFe1 (-x) GaxO3 ( x = 0, 0.1, 0.2, 0.25, 0.5 and 0.7) have been measured at room temperature (RT). For x < 0.5 the spectra have been fitted with magnetic sextets with their areas following a binomial distribution of Fe surroundings with Ga neighbours. The spectra for x = 0.5 and 0.7 show paramagnetic doublets, which account for the percolation effect induced by the non-magnetic Ga substitution for Fe, which has been estimated to be x = 0.43 at RT. The analysis of the hyperfine field values as a function of the Ga concentration shows the effect of the variations of the antiferromagnetic sublattice magnetization. Moreover, the dependence of this field with the number of susbstituted neighbours suggests an increase of the average Fe-O-Fe exchange interaction as a consequence of the lattice contraction when Ga susbtitutes for Fe. (c) 2007 Elsevier B.V. All rights reserved.

3600. Poly(N-vinyl carbazole) and carbon nanotubes based composites and their application to rechargeable lithium batteries

Authors: Baibarac, M; Lira-Cantu, M; Sol, JO; Baltog, I; Casan-Pastor, N; Gomez-Romero, P

Published: SEP 2007, COMPOSITES SCIENCE AND TECHNOLOGY, 67, 2563, DOI: 10.1016/j.compscitech.2006.12.008

In order to prepare poly(N-vinyl carbazole) (PVK) and carbon nanotube (CNs) based composites, electrochemical polymerization of N-vinyl carbazole (NVK) was studied by cyclic voltammetry (CV) in LiClO4/acetonitrile solutions. Cyclic voltammograms recorded on a blank Pt electrode and those obtained when single or multi-walled carbon nanotubes (SWNTs or MWNTs) films were previously deposited onto the Pt electrode show a down-shift of the VK reduction peak potential in the latter case. The influence of monomer concentration and supporting electrolyte on the polymerization conditions and electrochemical properties of these composite materials were also investigated. Morphologic aspects as well as the photoluminescence properties of the PVK/CNs composite were also investigated. A covalent functionalization of carbon nanotubes with PVK is suggested on the basis of infrared (IR) spectroscopic studies. Using the PVK/CN composite as a positive electrode and an electrolytic solution containing LiPF6, a higher specific discharge capacity of the rechargeable lithium cells, similar to 45 and 115 mA h g(-1), are reported for PVK functionalized SWNTs and MWNTs, respectively. (c) 2007 Elsevier Ltd. All rights reserved.