National Institute Of Materials Physics - Romania

Publications

5914 articles found

4091. Quantum dots and nanowires grown by metal-organic chemical vapor deposition for optoelectronic device applications

Authors: Tan, HH; Sears, K; Mokkapati, S; Fu, L; Kim, Y; McGowan, P; Buda, M; Jagadish, C

Published: NOV-DEC 2006, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 12, 1254, DOI: 10.1109/JSTQE.2006.882663

This paper discusses the growth of In(Ga)As quantum dots (QDs) and nanowires by metal-organic chemical vapor deposition and their application to optoelectronic devices. The performance characteristics of QD lasers and QD photodetectors as well as the selective area growth of QDs for integrated devices are reported.

4092. Low temperature CL investigation of BN1 vibronic structure in c-BN

Authors: Cossio, R; Fizzotti, F; Vittone, E; Lo Giudice, A; Manfredotti, C; Nistor, S

Published: APR-AUG 2006, DIAMOND AND RELATED MATERIALS, 15, 1168, DOI: 10.1016/j.diamond.2005.11.048

A full vibronic spectrum has been measured for the first time by LNT Cathodoluminescence in HPHT c-BN amber-coloured microcrystalline samples. The related BN1 centre at 3.293 eV seems not created by electron irradiation in this case and the accurately determined phonon energy (141 +/- 3 meV) could be related to LO phonon at X point in Brillouin Zone (BZ) of c-BN, as determined by CL results obtained at indirect gap. Consequently, BN1 centre has not the full cubic symmetry of c-BN, as being due to N interstitials. The interpretation of the results is possibly twofold: either we are observing a vibronic spectrum with ZPL at BN1 centre, together with other two centres called PF-1 and PF-2 at 3.573 and at 3.412 eV, respectively, or, since all the 6 or 7 observed peaks are exactly equally spaced, the whole vibronic spectrum is related to a ZPL line at 3.573 eV. This new interpretation seems to be in better agreement with the general theory of colour centres. (c) 2005 Elsevier B.V. All rights reserved.

4093. Quantum kinetic theory of phonon-assisted carrier transitions in nitride based quantum-dot systems

Authors: Seebeck, J; Nielsen, TR; Lorke, M; Gartner, P; Jahnke, F

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6, 3, 2033, DOI: 10.1002/pssc.200565227

A microscopic theory for the interaction of carriers with LO phonons is used to study ultrafast capture and relaxation dynamics in semiconductor quantum dots. Energy renormalization due to carrier-phonon interaction leads to quantum-dot polarons which are included in quantum-kinetic equations to describe scattering processes. Our results show efficient capture and relaxation processes. For an InGaN quantum-dot system the influence of the strong built-in electrostatic field on the carrier dynamics is investigated. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

4094. Piezoelectric material for high efficiency ultrasonic siren

Authors: Dimitriu, E; Iuga, A

Published: 2006, ADVANCED MATERIALS FORUM III, PTS 1 AND 2, 514-516, 229, DOI: 10.4028/www.scientific.net/MSF.514-516.225

A PZT-type material, doped with niobium and lithium was prepared by the ceramic technique. The microstructure of the sintered material at 1280 degrees C was performed by scanning electron microscopy (SEM), transmission electron microscopy and by X-ray diffraction. Dielectric and piezoelectric properties were measured. The good planar properties of the material made it possible to obtain ceramic disks with a strong radial mode. Two such disks, glued together, with opposed polarisation constitute a bimorf element, which successfully converts the radial mode into a flexural one. At the center of the bimorf a conical acoustical impedance adapter is fixed in order to increase the frontal acoustical emission of the device. This ultrasonic device provides a broad band, high acoustical emission, and is suitable to work as a high efficiency ultrasonic siren. It can be used in intruder alarm devices, ultrasonic telemeters.

4095. Couplings control in the defected ground star-geometry dual-mode filter

Authors: Banciu, MG; Shen, S; Ramer, R; Militaru, N; Lojewski, G

Published: 2006, 2006 EUROPEAN MICROWAVE CONFERENCE, VOLS 1-4, +, DOI:

Investigations on coupling coefficients in a stargeometry filter with defected ground are presented in this paper. The device is compact, requiring a surface area of more than 6 times smaller than the area needed for a conventional patch dual mode resonator.

4096. Preparation of Mo-V-Te-Nb mixed oxides using the template route

Authors: Florea, M; Gaigneaux, EM; Parvulescu, VI

Published: 2006, , DOI: 10.1016/S0167-2991(06)80979-1

4097. A penetration depth study on Li2Pd3B and Li2Pt3B

Authors: Yuan, HQ; Vandervelde, D; Salamon, MB; Badica, P; Togano, K

Published: 2006, LOW TEMPERATURE PHYSICS, PTS A AND B, 850, +, DOI:

In this paper we present a penetration depth study on the newly discovered superconductors Li2Pd3B and Li2Pt3B. Surprisingly, the low-temperature penetration depth lambda(T) demonstrates distinct behavior in these two isostructural compounds. In Li2Pd3B, lambda(T) follows an exponential decay and can be nicely fitted by a two-gap BCS superconducting model with a small gap Delta(1)=3.2K and a large gap Delta(2)=11.5K. However, linear temperature dependence of lambda(T) is observed in Li2Pt3B below 03T(c), giving evidence of line nodes in the energy gap.

4098. Giant magnetoresistive coatings using thermionic vacuum are technology

Authors: Anghel, A; Luncu, CP; Mustata, I; Zaroschi, V; Lungu. AM; Barbu, I; Badulescu, M; Pompilian, O; Schinteie, G; Predoi, D; Kunkser, V; Filoti, G; Apetroaei, N

Published: 2006, CZECHOSLOVAK JOURNAL OF PHYSICS, 56, B23, DOI: 10.1007/s10582-006-0173-8

Giant magnetoresistive (GMR) coatings on silicon, glass and Kapton were prepared by simultaneous thermionic vacuum are (TVA) discharges in high vacuum conditions. The magnetic metal or alloy (Fe, Co, Ni, Permalloy) together with the noble non magnetic metal were deposited on substrates having different positions relative to the discharges. Local magnetic interactions and Fe-phase composition were obtained by Mossbauer spectroscopy whereas the magnetoresistance effects were measured by a dc method using a four-point configuration with perpendicular to plane magnetic fields up to 0.8 T. The surface morphology of the coatings was characterized by atomic force microscopy (AFM) in contact mode.

4099. Predictions and measurement of mass loading effects on quartz bulk acoustic wave resonators

Authors: Mateescu, I; Bran, C; Ballandras, S; Gachon, D

Published: 2006, PROCEEDINGS OF THE 2006 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM AND EXPOSITION, VOLS 1 AND 2, +, DOI:

The use of a variationnal perturbation method combined with finite element analysis is presented here to simulate the influence of mass-loading effects on the resonance frequency of bulk acoustic wave resonator. Computations are conducted on 2D and 3D and any shape of resonator can be considered. Predictions are compared to experimental data.

4100. Thermodynamic properties and spin dynamics of some micro and nanostructured magnetoresistive lanthanum manganites

Authors: Tanasescu, S; Marinescu, C; Maxim, F; Raita, O; Grecu, MN; Giurgiu, L

Published: 2006, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 26, 3010, DOI: 10.1016/j.jeurceramsoc.2006.02.020

The spin dynamics and thermodynamic properties of micro- and nanostructured lanthanum manganites La0.67Ca0.33Mn1-xMeO3 (Me = Al; x = 0-0.05) have been studied in order to evidence new features related to the effect of different compositional variables on the thermodynamic and magnetic behaviour. By electron paramagnetic resonance (EPR) spectroscopy, the influence of the grain size and the Mn substitution effects on the spin dynamics were investigated. A solid state electrochemical method was used to obtain the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen. The influence of the oxygen stoichiometry change on the thermodynamic properties has been investigated. The results were correlated with the average Mn valence values as determined by redox titration. In this paper an alternative method is proposed to estimate the contribution made by oxygen vacancies in balancing the local charge in CMR manganites. (c) 2006 Elsevier Ltd. All rights reserved.