National Institute Of Materials Physics - Romania

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5781 articles found

3161. Coulomb drag in parallel quantum dots

Authors: Moldoveanu, V; Tanatar, B

Published: JUN 2009, EPL, 86, DOI: 10.1209/0295-5075/86/67004

We study theoretically the electronic transport in parallel few-level quantum dots in the presence of both intradot and interdot long-range Coulomb interaction. Each dot is connected to two leads and the steady-state currents are calculated within the Keldysh formalism using the random-phase approximation for the interacting Green functions. Due to the momentum transfer between the two systems it is possible to get a nonvanishing current through an unbiased Coulomb-blockaded dot, if the other dot is set in the nonlinear transport regime. The transitions between the levels of the passive dot reduce the drag current and lead to negative differential conductance. We also discuss the dependence on temperature and the role of the lead-dot coupling. Copyright (C) EPLA, 2009

3162. Calcium Carbonate Synthesis in Ultrasonic Field

Authors: Mihai, M; Turtoi, D; Isopescu, R; Mateescu, C; Chilibon, I

Published: JUN 2009, REVISTA DE CHIMIE, 60, 631, DOI:

Precipitated calcium carbonate as vaterite and small amounts of calcite, with submicron sizes was obtained by a two stages reaction, In the first stage, the limestone was transformed with carbon dioxide in calcium bicarbonate in the absence and the presence of an ultrasonic applied field (35kHz and 0.52 W/cm(2)). The ultrasonic intensity and specific power, the device immersion depth in column suspension and the cavitation was observed to accelerate the dissolving rate of calcium carbonate. In the second stage, the solution of calcium bicarbonate was aerated to obtain calcium carbonate. The experiment was conducted in the absence and the presence of an ultrasonic applied field. The precipitation rate and the calcium carbonate particles size distribution were found to be influenced by the ultrasonic field. The morphology of the crystals formed was unaffected by the ultrasonic field.

3163. Pinning Centers Induced in YBCO Films by Nano-Dots in Substrate Decoration and Quasi-Superlattice Approaches

Authors: Mikheenko, P; Sarkar, A; Dang, VS; Tanner, JL; Kechik, MMA; Abell, JS; Crisan, A

Published: JUN 2009, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 19, 3494, DOI: 10.1109/TASC.2009.2018307

For power applications of superconducting films, the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centers in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centers was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on other two approaches proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices) and distributing a secondary phase in the film from a compositionally changed target. Several materials have been used for the creation of artificial pinning centers. Here we report on the artificial pinning centers induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Ag, Au, Pd or non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

3164. Synthesis and ultrasonic characterization of hydroxyapatite ceramic powders

Authors: Predoi, D; Derible, S; Duflo, H

Published: JUN 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 856, DOI:

Calcium phosphate compounds have been studied for biomedical applications due to chemical and structural similarity to the mineral phase of bone and tooth. The composition, physico-chemical properties, crystal size and morphology of synthetic apatite are extremely sensitive to preparative conditions and sometimes it resulted into non-stoichiometric calcium deficient hydroxyapatite (HAp) powders. The present paper refers to calcinations of hydroxyapatite ceramics at 600, 800 and 1000 degrees C. The effect of heat treatment was previously investigated by X-ray diffraction (XRD). The ultrasonic characterisation of hydroxyapatite powders were performed using the ultrasonic air-coupling technique. Modulated ultrasonic signals of 450 kHz central frequency have been transmitted through the hydroxyapatite ceramics specimens. Correlation between signals allowed some conclusions concerning density, attenuation and preparation temperature influence on these specimens. These comparisons and correlation of methods, allow a better characterization of such important materials.

3165. Characterization of magnetic nano-fluids via Mossbauer spectroscopy

Authors: Filoti, G; Kuncser, V; Schinteie, G; Palade, P; Morjan, I; Alexandrescu, R; Bica, D; Vekas, L

Published: JUN 2009, HYPERFINE INTERACTIONS, 191, 60, DOI: 10.1007/s10751-009-9952-5

The laser pyrolysis became a useful tool, providing various ways, in production of nano materials. The iron Mossbauer spectroscopy is one very accurate method in evidencing the physical properties and related processes in the nano scale compounds. The effect of pressure, laser spot area and induced combustion, of gas mixture and laser power on the phase composition and inside particle distribution, grain size as well as the related phenomena were investigated by temperature dependent Mossbauer spectroscopy. A selection of most relevant properties is presented and discussed in details.

3166. Titanium oxide thin films produced by pulsed laser deposition

Authors: Sima, C; Grigoriu, C; Viespe, C; Pasuk, I; Matei, E

Published: JUN 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 830, DOI:

The structural peculiarities of titanium oxide films formed by pulsed laser deposition have been investigated. A pure titanium target was irradiated by a Nd:YAG laser (355 and 532 nm). The work studied the influence of the beam wavelength, and oxygen pressure (20, 40, 80, and 160 mTorr) upon the layer structure. The films deposited at room temperature were partly crystalline. The crystalline fraction was a mixture of titanium sub-oxides. The 532 nm wavelength seems to favour the oxidation of titanium leading to TiO(2) formation even at room temperature. After annealing, crystalline TiO(2-) anatase formed at both irradiation wavelengths, but only at low oxygen pressures (20 and 40 mTorr). The best crystallization occurs in the layers deposited at the lowest oxygen pressure (20 mTorr), at both laser beam wavelengths; most anatase formed in the sample obtained at 20 mTorr, 532 nm. At 20 mTorr the films were compact, for both wavelengths, but also tilted or randomly distributed columnar grains were observed at higher pressure. The films deposited with 355 nm, were thinner than those with 532 nm; at 355 nm the thickness decreased at higher pressure, while at 532 nm the dependence was opposite.

3167. On morphological, structural and electrical properties of vacuum deposited pentacene thin films

Authors: Girtan, M; Dabos-Seignon, S; Stanculescu, A

Published: MAY 22 2009, VACUUM, 83, 1163, DOI: 10.1016/j.vacuum.2009.03.001

Films of different thickness (50,100,150 and 200 nm) were deposited by thermal evaporation in vacuum on two types of substrates glass and ITO. The deposition was performed under a pressure of 10(-6) mB with a rate of 0.25 nm/s. Films surface investigations showed morphological and structural changes in function of films thickness and the nature of the substrate. Films optical transmission was analysed in the 280-1600 nm spectral range and the electrical measurements were done in low vacuum (10(-1):10(-2) mB) and in dark. (c) 2009 Elsevier Ltd. All rights reserved.

3168. Synthesis of narrow diameter distribution carbon nanotubes on ZnO supported catalysts

Authors: Lupu, D; Biris, AR; Watanabe, F; Li, ZR; Dervishi, E; Saini, V; Xu, Y; Biris, AS; Baibarac, M; Baltog, I

Published: MAY 12 2009, CHEMICAL PHYSICS LETTERS, 473, 304, DOI: 10.1016/j.cplett.2009.03.069

Multiwall carbon nanotubes, as indicated by electron microscopy and spectroscopy, were grown over a novel Co(5 wt.%)/ZnO catalyst by using a cold wall radio-frequency catalytic chemical vapor deposition method. The nanotubes were found to have a rather narrow diameter distribution, which can be explained by the formation over the ZnO support particles of uniform Co clusters, as shown by high resolution transmission electron microscopy studies. The growth of carbon nanotubes over such a support could represent a fast and reliable method to develop in one step 3D ZnO-CNTs heterogeneous composites of any size or shape, for use in advanced applications. (c) 2009 Elsevier B.V. All rights reserved.

3169. Ambiguity in the statistics of single-component winding vortex in a two-band superconductor

Authors: Tanaka, Y; Crisan, A

Published: MAY 1 2009, PHYSICA B-CONDENSED MATTER, 404, 1039, DOI: 10.1016/j.physb.2008.11.011

There are four topologically different paths to interchange two single-component winding vortices in a two-band superconductor having a weak inter-band interaction, where each band has its own superconducting component and its own phase, theta(v), other than the ordinary quantum phase, Theta (v is the index of the band and Theta is defined at the region of theta(1) = theta(2).). The acquired phase for the interchange of two single-winding vortices depends on the path, and three values Theta(soliton),-Theta(soliton) and 0, are observed. Theta(soliton) is the phase-slip accompanying the inter-band phase difference soliton (i-soliton). The interchange requires the creation and annihilation of the i-soliton and gives rise to an ambiguity in the statistics. (C) 2008 Elsevier B.V. All rights reserved.

3170. Origin of the X-ray magnetic circular dichroism at the L-edges of the rare-earths in RxR ' Al-1-x(2) systems

Authors: Chaboy, J; Laguna-Marco, MA; Piquer, C; Boada, R; Plugaru, N; Maruyama, H; Kawamura, N

Published: MAY 2009, JOURNAL OF SYNCHROTRON RADIATION, 16, 412, DOI: 10.1107/S0909049509009807

An X-ray magnetic circular dichroism (XMCD) study performed at the rare-earth L-2,L-3-edges in the RxR'Al-1-x(2) compounds is presented. It is shown that both R and R' atoms contribute to the XMCD recorded at the L-edges of the selected rare-earth, either R or R'. The amplitude of the XMCD signal is not directly correlated to the magnetization or to the value of the individual (R, R') magnetic moments, but it is related to the molecular field acting on the rare-earth tuned in the photoabsorption process. This result closes a longstanding study of the origin of the XMCD at the L-edge of the rare-earths in multi-component systems, allowing a full understanding of the exact nature of these signals.