National Institute Of Materials Physics - Romania

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.

Nd-doped strontium and lanthanum (ASL) crystals Sr1-xLax-yNdyMgxAl12-xO19 (0.05 <= x <= 0.5; y = 0.05) were grown by Czochralski pulling technique. Up to 1.67W of 900mn rR output laser power for an absorbed power of 2.53W was obtained under Ti:sapphire pumping at 792nm. Intracavity second harmonic generation experiments led to 320mW of blue laser power at 450mn with a 10mm-long BiB3O6 nonlinear crystal. Other nonlinear crystals were also evaluated such as LBO.

In-plane conductivity sigma(ab) curves of Y0.54Pr0.46Ba2Cu3O7 single crystals, measured at constant temperature T in magnetic fields H tilted with an angle theta relative to the c-axis, were found to map onto a single curve in a sigma(ab) vs Hcos theta plot. This scaling occurs both below and above the critical temperature T-c, maintaining the same convexity, and is consistent with the dissipation caused by the motion of two-dimensional (2D) vortices. This indicates the presence of 2D vortices above T-c. A second scaling sigma(ab)(Hcos theta) takes place at higher T-c with an opposite convexity. This scaling is consistent with the dissipation of quasiparticies. There is a T range between these two regimes where no scaling is found. We attribute the absence of scaling in this intermediate regime to a 2D to 3D crossover in the vortex matter.

Magnetization M(H,T) and M(T,H) curves with magnetic field approximately parallel and perpendicular to the c-axis of Na0.35CoO2.1.3H(2)O superconducting crystals have been measured (T-c = 3.7 K). The shape of the M(H,T) loops resembles Bi-based high-T-c superconductors (HTS), but the superconducting anisotropy is below that of Bi-system; this parameter is as for HTS with middle or moderately high anisotropy. (c) 2006 Elsevier Ltd. All rights reserved.

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.

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.

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.

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.

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.

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.