National Institute Of Materials Physics - Romania

Publications

5781 articles found

3941. Quantum kinetic theory of phonon-assisted carrier transitions in nitride-based quantum-dot systems

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

Published: JAN 2006, EUROPEAN PHYSICAL JOURNAL B, 49, 170, DOI: 10.1140/epjb/e2006-00048-6

A microscopic theory for the interaction of carriers with LO phonons is used to study the ultrafast carrier dynamics in nitride-based semiconductor quantum dots. It is shown that the efficiency of scattering processes is directly linked to quasi-particle renormalizations. The electronic states of the interacting system are strongly modified by the combined influence of quantum confinement and polar coupling. Inherent electrostaticfields, typical for InGaN/GaN quantum dots, do not limit the fast scattering channels.

3942. Tunnelling magnetoresistance of a T-shaped Fano dot

Authors: Tolea, M; Bulka, BR

Published: JAN 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, 254, DOI: 10.1002/pssb.200562462

We show the role of the Fano interference on the magnetoresistance properties of a T-shaped double-dot placed between ferromagnetic electrodes. In the linear voltage regime, the magnetoresistance is a non-monotonic function of the gate voltage. We find high values of the magnetoresistance at the Fano dip and far from it. For resonant transmission, the magnetoresistance is lower and it can be negative for asymmetric couplings. Particularities of the system in nonequilibrium transport are also analysed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3943. Calcium carbonate precipitation in ultrasonic field

Authors: Mateescu, CD; Mocioi, M; Sarbu, C; Isopescu, R; Chilibon, I; Branzoi, F

Published: 2006, BIWIC 2006, +, DOI:

An unconventional synthesis method, implying ultrasonic field, was tested in order to improve the properties of CaCo3 precipitated via liquid-liquid route. Working parameters for calcium carbonate precipitation in the absence of ultrasonic field so that the solid material displays aragonite morphology were established. The evolution of the precipitation reaction in ultrasonic field provided a solid phase with aragonite morphology slightly, impurified with vaterite. Grain size distribution was signficantly improved. The solid phase presents an important fraction of nanometer size particles.

3944. A combined sintering-sublimation method to grow light responsive polycrystalline thin films

Authors: Igor, N; Alexandru, SB; Meghea, A; Ulieru, D

Published: 2006, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 448, 221, DOI: 10.1080/15421400500400842

A combined sintering-sublimation method has been used to grow CdS or CdSeS photo-conducting polycrystalline thin films. By a fast firing technique, the films are grown on Cr2O3-x ceramic pellet sintered substrate. The film growing and the fast firing sintering processes take place simultaneously. The sintering-sublimation technique has been designed to increase the reliability of photoconducting devices, mainly by decreasing the donors' concentration which determines a higher sensitivity. This process becomes more effective when pellets containing raw CdS, respectively CdSeS, and a less reactive component - such as nonstoichiometric chromium oxide, are used. This oxide provides surfaces where thin CdS or CdSeS layers are crystallizing. At the same time, the nonstoichiometric Cr2O3-x can react with the oxygen allowing a more efficient control of the properties of CdS and CdSeS thin layer which is generated as a result of the above-mentioned sublimation-condensation process.

3945. EPR and TEM characterization of electrolurninescent Alq(3)

Authors: Grecu, MN; Ghica, C; Mirea, A; Grecu, VV; Schwoerer, M

Published: 2006, ADVANCED MATERIALS FORUM III, PTS 1 AND 2, 514-516, 137, DOI: 10.4028/www.scientific.net/MSF.514-516.133

EPR and TEM investigations on sublimated solid Alq(3), [tris(8-hydroxyquinoline)aluminum(Ill)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.

3946. HTS thin films: a convenient method for removal of precipitates-segregates

Authors: Endo, K; Badica, P; Sato, H; Akoh, H

Published: 2006, 7TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'05), 43, 271, DOI: 10.1088/1742-6596/43/1/067

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.

3947. Core-shell effects in granular perovskite manganites

Authors: Pana, O; Giurgiu, LM; Darabont, A; Iancu, V; Ciurea, M; Grecu, MN; Bot, A

Published: JAN-MAR 2006, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 67, 627, DOI: 10.1016/j.jpcs.2005.10.148

Unusual conductivity behavior, is observed in the semiconducting phase of granular manganites. It is characterized by, at a certain temperature, of a switching process between two activation energies. A core-shell model is developed to explain the presence of the conductivity of the two activation energies. The model suggests that inside the disordered thin outer layer the energy bands are split into isolated energy levels. Additional correlations between paramagnetic behavior and the existence of the two activation energies within the core-shell model are furnished by the ESR spectroscopic technique. (c) 2006 Published by Elsevier Ltd.

3948. Structural, electrical and dielectric properties of uranium doped barium titanate

Authors: Miclea, C; Tanasoiu, C; Miclea, CF; Gheorghiu, A

Published: 2006, ADVANCED MATERIALS FORUM III, PTS 1 AND 2, 514-516, 1273, DOI: 10.4028/www.scientific.net/MSF.514-516.1269

The influence of uranium doping up to 2 at. % on grain size, electrical resistivity and dielectric constant of barium titanate ceramics has been investigated. The samples were prepared by the conventional ceramic technique using pure raw materials. The mixed oxides were calcined at 1100 degrees C. The pressed pellets were sintered at temperatures between 1300 divided by 1500 degrees C in controlled atmosphere of argon and oxygen. The electrical resistivity drastically decreased, from 10(9) to 10(3) Omega(.)cm, with increasing uranium content up to 1.5 at. % and increased thereafter to about 10(7)Omega center dot cm, for 2 at. % U. The dielectric constant slightly increased with increasing U-content, up to about I at. % U and then shows a sudden increase with about two orders of magnitude, having a sharp maximum at 1.5 at. % U, when sintered in argon atmosphere compared to only half an order of magnitude when sintered in oxygen atmosphere. The results are discussed in terms of the possible A and B sites occupied by uranium as well as the boundary layer enriched with vacancies.

3949. Fractal analysis of the atomic force microscopy images of sulfide-covered GaAs(00) surfaces

Authors: Lazarescu, V; Dobrescu, G; Lazarescu, MF; Logofatu, C; Savu, B; Stanciu, GA

Published: MAR-APR 2006, SCANNING, 28, 86, DOI:

3950. Effect of lead content and B-site substitutions on dielectric and piezoelectric properties of PZT ceramics

Authors: Dimitriu, E; Craciun, F; Cernea, M; Ramer, R

Published: 2006, ADVANCED MATERIALS FORUM III, PTS 1 AND 2, 514-516, 192, DOI: 10.4028/www.scientific.net/MSF.514-516.188

The behaviour of the dielectric and piezoelectric properties of doped PZT ceramics, with variable lead content, has been investigated from -50 degrees C to room temperature. The PZT materials were prepared by the oxide technique and samples were sintered as pellets, at various temperatures. Niobium and lithium ions have been substituted for Ti4+, Zr4+ in the PZT matrix, and a small amount of Bi2O3 was added to cause A-position vacancies. The lead content in the four materials prepared varies in a narrow range, from 0.97 to 1.029 lead atoms, while the concentration of all other components are maintained constant. The microstructure of samples, evaluated by SEM technique and XRD analysis evidenced the presence of the tetragonal phase characteristic for the PZT 52/48. The dielectric and piezoelectric constants measured on poled samples depends on the sintering temperature, defects and domain walls, lead content.