National Institute Of Materials Physics - Romania

Publications

5781 articles found

3971. Microscopic theory of carrier-carrier scattering in nitride based quantum-dot systems

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

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6, 3, 2042, DOI: 10.1002/pssc.200565124

A microscopic theory is applied to study carrier-carrier Coulomb scattering in nitride-based quantum dots. Kinetic equations for the carrier population are used to define capture and relaxation rates. For the states involved in the scattering processes, the combined influence of the quantum-confined Stark effect and many-body renormalizations is taken into account. It is shown that the charge separation due to the built-in field is accompanied with strong Hartree energy shifts, which can increase the scattering efficiency. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3972. Nanostructured carbon growth by an expanding radiofrequency plasma jet

Authors: Vizireanu, SI; Mitu, B; Birjega, R; Dinescu, G; Teodorescu, V

Published: 2006, CARBON NANOTUBES: FROM BASIC RESEARCH TO NANOTECHNOLOGY, 222, +, DOI:

Carbon deposition from acetylene injection into plasma jet onto sputtered nickel on silicon is investigated. The whole methodology, including the deposition of catalyst, its etching to islands followed by the carbon growth, is implemented in the same reactor. The investigation of structure and morphology shows the formation of nickel particles coated with graphitic layers.

3973. Microscopic tight-binding description for electronic and optical properties of InN/GaN quantum dots

Authors: Schulz, S; Baer, N; Schumacher, S; Gartner, P; Jahnke, F; Czycholl, G

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 11, 3, +, DOI: 10.1002/pssc.200671551

Electronic and optical properties of self-assembled InN/GaN quantum dots are investigated by use of a tight-binding model combined with a configuration interaction calculation. Dipole and Coulomb matrix elements are calculated from the single-particle wavefunctions which fully include the atomistic wurtzite structure of the low-dimensional heterostructure and serve as input parameters for the calculation of optical properties. We discuss in particular the influence of the internal electric field on the multi-exciton spectra. Dark exciton and biexciton ground-states are found for small quantum dots. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3974. Sol-gel synthesis and characterization of Ce doped-BaTiO3

Authors: Cernea, M; Monnereau, O; Llewellyn, P; Tortet, L; Galassi, C

Published: 2006, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 26, 3246, DOI: 10.1016/j.jeurceramsoc.2005.09.039

Barium titanate (BaTiO3) have been doped "in situ" with 5.5 mol% cerium by a sol-gel method using barium acetate, titanium (IV) isopropoxide, and cerium (III) acetylacetonate as starting materials. The dried gel showed a microstructure consisting of nano-sized grains (similar to 140 nm) with great tendency to agglomeration. Several thermal analysis techniques were used to study the decomposition process of the gel. The presence of hydroxyls up to 720 degrees C suggests a strong bonding Ti-OH that is responsible for the existence of aggregates even at high temperatures. The as-prepared gel powder was found to be amorphous, and then decomposes through oxides and barium carbonate around 500 degrees C and crystallizes on the perovskite structure of tetragonal BaTiO3 at 1100 degrees C for 3h in air. A small influence of the frequency on the dielectric properties of the Ba0.945Ce0.055TiO3 ceramics was observed in 100Hz to 1 MHz domain. At the Curie temperature point (22 degrees C) the dielectric constant was 10130 at 100 Hz while the dielectric loss (tan delta) was 0.018. (c) 2005-Elsevier Ltd. All rights reserved.

3975. On the formation and crystal structure of the Pd6B phase

Authors: Berger, TG; Leineweber, A; Mittemeijer, EJ; Sarbu, C; Duppel, V; Fischer, P

Published: 2006, ZEITSCHRIFT FUR KRISTALLOGRAPHIE, 221, 463, DOI: 10.1524/zkri.2006.221.5-7.450

The crystal structure of the Pd6B phase has been elucidated employing selected-area electron diffraction and X-ray and neutron powder diffraction methods. It is based on a cubic close packed arrangement of Pd with B occupying 1/6 of the interstitial octahedral sites in an ordered way, such that a monoclinic (C2/c) superstructure results. The crystal structure contains isolated [BPd6] octahedral entities providing large distances between nearest neighbour boron atoms. Depending on the way of preparation, the same Pd6B phase can occur for the same composition in two different 'manifestations' having considerably different lattice parameters, which has a microstructural origin. Various phase equilibria between the ordered Pd6B phase and the disordered interstitial solid solution of boron in cubic close packed palladium have been investigated to clarify the corresponding regions in the phase diagram Pd-B.

3976. Highly efficient one-micron laser emission under 885 nm diode laser pumping in Nd : YAG crystals with an extended range of concentrations

Authors: Lupei, V; Pavel, N

Published: 2006, SOLID STATE LASERS AND AMPLIFIERS II, 6190, DOI: 10.1117/12.662601

The proper selection of the parameters of the laser material parameters for efficient continuous-wave 1064 nm emission under direct diode laser pumping at 885 nm into the emitting level of Nd:YAG is discussed. High slope efficiency (0.8) in absorbed power for 1.1 and 2.5at.%Nd concentrations and the advantage of using concentrated Nd:YAG crystals for improving the absorption efficiency is demonstrated.

3977. Signature of the magnetic transitions in Y0.2Pr0.8Ba2Cu3O7.delta in high field angular magnetoresistivity

Authors: Sandu, V; Zhang, C; Almasan, CC; Taylor, BJ; Maple, MB

Published: 2006, YAMADA CONFERENCE LX ON RESEARCH IN HIGH MAGNETIC FIELDS, 51, +, DOI: 10.1088/1742-6596/51/1/052

In-plane (ab) and out-of-plane (c-axis) magnetoresistivity display different symmetry crossovers and/or transitions in 14 T magnetic field applied parallel to the CuO2 planes. The in-plane magnetoresistivity crosses over from four-fold symmetry below 6 K to two-fold symmetry at higher temperatures, which becomes dominant at temperatures higher than 40 K. The out-of-plane magnetoresistivity changes at 17 K from four fold symmetry to ordinary sin2 theta at higher temperatures. The behaviour of the c-axis magnetoresistivity can be ascribed to the antiferromagnetic ordering of the Pr spins whereas the symmetry change of the in-plane magnetoresistivity at 6 K might be attributed to commensurate to incommensurate crossovers of the spin subsystems. The antiferromagnetic order of the Cu(2) sublattice seems to have only a week effect on the magnetoresistivity.

3978. Comparison of the luminescence properties of the x-ray storage phosphors BaCl2 : Ce3+ and BaBr2 : Ce

Authors: Selling, J; Corradi, G; Secu, M; Schweizer, S

Published: DEC 21 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 8078, DOI: 10.1088/0953-8984/17/50/024

The single-crystalline x-ray storage phosphor BaCl2:Ce3+ has been investigated by photo- and x-ray luminescence, and also by observing the afterglow and the photostimulated luminescence (PSL). A single active Ce3+ site has been observed with a characteristic emission doublet at 349 and 373 nm at room temperature. The PSL stimulation spectrum shows a clear resemblance to F-type absorption spectra in undoped BaCl2, indicating that the PSL-active electron centres are F-type centres. The results are compared with previous photoluminescence, PSL and new x-ray luminescence data on BaBr2:Ce3+. The integral PSL efficiency in the chloride is found to be somewhat smaller than in the bromide, which is still large enough for storage phosphor applications, while the efficiency in the bromide may be even larger or comparable to that of the commercial x-ray storage phosphor BaFBr2:Eu2+.

3979. Phonon-induced breakdown of negative bend resistance in an asymmetric Si/SiGe cross junction

Authors: Wieser, U; Poenariu, SA; Kunze, U; Hackbarth, T

Published: DEC 19 2005, APPLIED PHYSICS LETTERS, 87, DOI: 10.1063/1.2150268

An asymmetric nanoscale cross junction is fabricated from a high-mobility Si/SiGe heterostructure. At T=4.2 K, the four-terminal current-voltage characteristics reveal a polarity-dependent breakdown of the negative bend resistance. The breakdown is accompanied by negative differential conductance found in the two-terminal current-voltage characteristics of the orthogonal current leads. We attribute this behavior to phonon emission by hot electrons. From gate-voltage-dependent measurements, we determine a phonon threshold of 19 meV. (c) 2005 American Institute of Physics.

3980. Metal-ferroelectric-metal structures with Schottky contacts. II. Analysis of the experimental current-voltage and capacitance-voltage characteristics of Pb(Zr,Ti)O-3 thin films - art. no. 123104

Authors: Pintilie, L; Boerasu, I; Gomes, MJM; Zhao, T; Ramesh, R; Alexe, M

Published: DEC 15 2005, JOURNAL OF APPLIED PHYSICS, 98, DOI: 10.1063/1.2148623

A modified model of metal-semiconductor contacts is applied to analyze the capacitance-voltage and current-voltage characteristics of metal-ferroelectric-metal structures. The ferroelectric polarization is considered as a sheet of surface charge situated at a fixed distance from the interface. The presumable high concentration of structural defects acting as active electric traps is taken into account by introducing a deep acceptorlike level. The model is applied on a set of metal-Pb(Zr,Ti)O-3-metal samples with different Zr/Ti ratios, deposited by different methods, and having different thicknesses, electrode materials, and electrode areas. Values around 10(18) cm(-3) were estimated for the hole concentration from capacitance-voltage measurements. The space-charge density near the electrode, estimated from current-voltage measurements, is in the 10(20)-10(21) cm(-3) range. The total thickness of the interface layer ranges from 3 to 35 nm, depending on the Zr/Ti ratio, on the shape of the hysteresis loop, and on the electrode material. The simulated I-V characteristics is fitted to the experimental one using the potential barrier and Richardson's constant as parameters. The potential barrier is determined to be in the 1.09-1.37 eV range and Richardson's constant is 520 A cm(-2) K-2. (c) 2005 American Institute of Physics.