National Institute Of Materials Physics - Romania

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5781 articles found

3131. RPA APPROACH TO NON-LINEAR TRANSPORT IN QUANTUM DOTS

Authors: Tanatar, B; Moldoveanu, V

Published: AUG 20 2009, INTERNATIONAL JOURNAL OF MODERN PHYSICS B, 23, 4421, DOI: 10.1142/S0217979209063560

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G(0)W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.

3132. Synthesis and sensing properties of zirconium-doped hematite nanoparticles

Authors: Sorescu, M; Diamandescu, L; Tomescu, A; Krupa, S

Published: AUG 1 2009, PHYSICA B-CONDENSED MATTER, 404, 2165, DOI: 10.1016/j.physb.2009.04.006

Zirconium-doped hematite particles of the type xZrO(2)-(1-x)alpha-Fe(2)O(3) (x = 0.1, 0.5) were synthesized using mechanochemical activation and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. For x = 0.1 all zirconia was dissolved in the hematite lattice after 12 h of ball milling and a particle size of 9 nm was obtained. We obtained the recoilless fraction as function of the ball milling time for each value of the molar concentration x. The appearance of nanoparticles in the system was demonstrated based on these plots. We further correlated the structural properties of the zirconium-doped hematite system with the sensing properties of the best candidate in the series. These were measured as function of temperature, gas concentration (carbon monoxide and methane) and variable humidity of air. The material system was found to be sensitive over the entire range of CO concentrations and the linearity of the sensor signal was not affected by the relative humidity of air, qualities which make it the ideal system for gas sensing. (C) 2009 Elsevier B.V. All rights reserved.

3133. Adherent functional graded hydroxylapatite coatings produced by sputtering deposition techniques

Authors: Stan, GE

Published: AUG 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1138, DOI:

The aim of this paper was to study the influence of crystallization heat treatments up to 750 degrees C, upon the integrity and structure of hydroxylapatite thin films. Two different types of hydroxylapatite coatings were investigated: abrupt structures prepared by magnetron sputtering and respectively functional graded structures deposited by a co-sputtering technique. A clear dependence was noticed between adherence values and annealing temperature and was correlated with the SEM, XRD and FTIR results. X-ray diffraction and FTIR measurements showed that the hydroxylapatite coatings annealed at 400 degrees C were less crystalline, similar to biological apatites. Over 550 degrees C, the apatite coatings obtained were well crystallized and preferentially oriented on (002) HA crystal plane. The adherence of HA coating increased with annealing the temperature up to 550 degrees C due to diffusion phenomena. At 550 degrees C the adhesion test results indicated better adhesion values in case of functional graded structures (> 85 MPa) in comparison with abrupt ones (similar to 70 MPa). At 750 degrees C an adherence decreasing occurred for both types of structures. This is mainly due to the effect of a significant thermal stress owed to the mismatch of the thermal expansion coefficients between the crystallized HA and Ti substrate. The presence of rutile lines in XRD spectrum suggests an increasing of the diffusion phenomena and a strong substrate oxidation at the HAM interface with annealing temperature.

3134. Direct synthesis of Y doped BaTiO3 powder by sol-precipitation method

Authors: Cernea, M

Published: AUG 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1195, DOI:

Ultra fine and homogeneous BaTiO3 doped with 0.5 at% yttrium powder was prepared by sol-precipitation method using barium acetate, titanium (IV) diisopropoxide bis-acetylacetonate and yttrium (III) butoxide as starting precursors. The effect of pH value on the precipitate formation from the sot is pointed out in this paper. The grain size of the precipitate was about 100-200 nm. The structural evolution of the precursor precipitate as function of temperature was investigated. Ba0.995Y0.005TiO3 single phase was obtained at 1300 degrees C. The ceramics manufactured from the as-prepared powder presented good dielectric characteristics (dielectric constant = 2152 and dielectric loss (tan delta) = 4.54% at the Curie temperature of 125 degrees C). These ceramics are expected to be useful in dielectric applications.

3135. Optical Advanced Spectroscopic Techniques for the Study of Nano-Structured Materials Such as Carbon Nanotubes

Authors: Lefrant, S; Baltog, I; Baibarac, M

Published: AUG 2009, JOURNAL OF NANOELECTRONICS AND OPTOELECTRONICS, 4, 206, DOI: 10.1166/jno.2009.1022

Optical techniques allowing observations and spectroscopic investigations of materials at a nanoscale have been recently developed and applied to carbon nanotubes. They are briefly presented, including Scanning near-field optical microscopy, together with the Surface Enhanced or Tip Enhanced Raman Scattering. Even if new physical properties are difficult to extract from observed data, we show that nanostructures can generate non linear phenomena that can be exploited for studying nano-materials and in particular carbon nanotubes. By measuring the anti-Stokes/Stokes ratios of the Raman bands which present strong anomalies with respect to the prediction of the Maxwell-Boltzman law, one can distinguish between semi-conducting and metallic tubes on one hand and between isolated and bundled nanotubes on the other hand.

3136. LITHIUM FERRITES OBTAINED BY THERMAL DECOMPOSITION OF UREATE COMPLEX COMPOUNDS

Authors: Gingasu, D; Mindru, I; Patron, L; Diamandescu, L; Cizmas, CB; Firastrau, I

Published: AUG 2009, REVUE ROUMAINE DE CHIMIE, 54, +, DOI:

Lithium ferrites have been obtained by combustion reaction of ureate precursors. Various physico-chemical techniques: IR spectra, XRD, magnetic determinations, Mossbauer spectra have been employed to characterize the precursors and end products. The single spinel phase alpha-Li(0.5)Fe(.2.5)O(4) with saturation magnetization of similar to 56 Am(2)/kg were achieved from ureate precursor with ratio 2.5Fe(3+) : 0.5Li(+) : 7urea after a thermal treatment at 800 degrees C/1h.

3137. Amorphous research in Romania: Past, present and latest results

Authors: Popescu, M; Croitoru, N

Published: AUG 2009, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 246, 1793, DOI: 10.1002/pssb.200982006

In the early days of amorphous research, Romania was deeply involved in the development of the field of amorphous semiconductors within a close collaboration with Boris Kolomiets. The first researches in this field are discussed and further rapid developments up to the present are shown. The latest results in the field of amorphous semiconductor:, with special emphasis on chalcogenide glasses are presented. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3138. Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Authors: Zgura, I; Moldovan, R; Beica, T; Frunza, S

Published: AUG 2009, CRYSTAL RESEARCH AND TECHNOLOGY, 44, 888, DOI: 10.1002/crat.200900162

Experimental results for the temperature dependence of the density of propyl - cyanobiphenyl (3CB), butyl - cyanobiphenyl (4CB) and hexyl - cyanobiphenyl (6CB) are presented. The results arc compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3139. Ultrafine particles of ZnGa2O4 obtained by solution combustion and complexation methods

Authors: Gingasu, D; Mindru, I; Marinescu, G; Patron, L; Ghica, C

Published: JUL 29 2009, JOURNAL OF ALLOYS AND COMPOUNDS, 481, 895, DOI: 10.1016/j.jallcom.2009.03.193

Two soft chemistry routes - complexation and solution combustion - are explored to obtain the zinc gallate spinel ZnGa2O4. Three types of complexes, namely: (NH4)(2)[ZnGa2(C4O6H4)(4)(OH)(2)]center dot 4H(2)O, [ZnGa2(NH2CH2COOH)(4.5)](NO3)(8)center dot 4H(2)O and [ZnGa2(CO(NH2)(2))(7)](NO3)(8)center dot 9H(2)O have been synthesized and characterized by IR and UV-vis spectroscopy. Zinc gallate spinels obtained by decomposition of these compounds have been investigated by XRD spectra, analytic electronic microscopy and photoluminescence spectroscopy. Zinc gallate spinels exhibited blue emission spectra in the wavelength range of 400 nm-450 nm. (C) 2009 Elsevier B.V. All rights reserved.

3140. Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals

Authors: Popescu, FF; Bercu, V; Barascu, JN; Martinelli, M; Massa, CA; Pardi, LA; Stefan, M; Nistor, SV; Nikl, M; Bohacek, P

Published: JUL 21 2009, JOURNAL OF CHEMICAL PHYSICS, 131, DOI: 10.1063/1.3180697

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(-)factor and the hyperfine constant A of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular to)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S-4 site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.