National Institute Of Materials Physics - Romania

The Sr2FeMo1-xMxO6 double perovskites with M = W and Ta and x <= 0.3, were obtained by sintering at 1300 degrees C. during 4 and 8 h, respectively. The perovskites crystallize in a tetragonal structure having 14/mmm space group. The grains are more homogeneous when sintering time is increased. The samples sintered longer time show higher values of saturation magnetizations, resistivities and magnetoresistivities than the samples sintered during 4 h. The intergrain tunnelling magnetoresistance as well as the intragrain contributions, respectively were analysed as function of temperature and external field. The changes in the physical properties, when the sintering time is increased, have been correlated with the number of antisite defects as well as the nature of grain boundaries. (C) 2010 Elsevier B.V. All rights reserved.

The trap parameters of defects in Si/CaF2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to "normal" traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture cross-sections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3525582]

Single crystals of Y1-xRxCa4O(BO3)(3) (R = Sc or Lu) of good quality have been grown from the melt by the Czochralski pulling method. The chemical compositions of the grown crystals were determined. Typed non critical phase matching (NCPM) wavelengths for second harmonic generation (SHG) in Y1-xRxCa4O(BO3)(3) grown crystals were also determined.

Power applications of superconducting coated conductors in high magnetic fields require thick films with high critical current density J (c) and strong artificial pinning centers. Here, we report on the artificial pinning centers induced in YBCO quasi-multilayer films interspaced with palladium (Pd) nano-dots. Quasi-multilayered (QM) YBa2Cu3O7-delta (YBCO) films composed of YBCO layers interspaced with quasi-layers of palladium nano-dots were grown by pulsed laser deposition on SrTiO3(100) substrates. DC magnetization and frequency-dependent measurements showed high J (c) comparable with best YBCO films in thin quasi-multilayers and significant improvement of J (c) in thick quasi-multilayers. TEM study shows regions of planar defects, stacking faults, and pore formations suitable for immobilizing vortices. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol-gel and radio frequency magnetron sputtering. After the deposition, the sol-gel films are annealed in either N-2 (at 1 atm and 800 A degrees C) or H-2 (at 2 atm and 500 A degrees C), and the sputtered films in H-2 (at 2 atm and 500 A degrees C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3-7 nm size) are observed in sol-gel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10-50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol-gel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.

Surfactant-free emulsion copolymerization was used to prepare methyl methacrylate-hydroxyethyl methacrylate (MMA-HEA) and methyl methacrylate-hydroxypropyl methacrylate (MMA-HPMA) latex particles. Also, glycidyl methacrylate (GMA) was grafted onto the surface of the preformed MMA-HPMA latex particles by seeded surfactant-free emulsion copolymerization. The copolymerization reactions were conducted at 75 A degrees C using a water-soluble initiator, potassium persulfate (KPS). The morphologies of copolymer latex particles were observed using Scanning electron microscopy (SEM). The influence of different reactions parameters (the MMA saturation concentration (Sr), the KPS concentration and the aqueous solubility of the comonomers (HEA or HPMA)) on the particles average diameter and particles size dispersity was investigated. The experimental results showed that the increase of initiator concentration induces in all investigated cases the increase of particles average diameter, while the presence of HEA or HPMA as comonomers in the copolymerization reaction of MMA (1,000% Sr) lead to a decrease of particles average diameter. At small KPS concentration the latex is monodisperse, the increase of the initiator concentration leading to the formation of polydisperse latex. In the case of grafting reaction of GMA onto the monodisperse preformed MMA-HEA latex particles, although the average diameter of the final particles doubles the latex remains quasi-monodisperse.

The electronic structure of metallofullerene Ce@C-82 is probed by resonant x-ray emission spectroscopy at the Ce L-3 absorption edge. We observed a satellite structure in x-ray absorption and resonant emission spectra for Ce@C-82, which, we show, corresponds to the charge transfer induced by the core-hole potential in the final state, similarly to Pr@C-82. This charge-transfer satellite may be a common feature in metallofullerenes. The temperature dependence of the electronic structure is also investigated.

This work is focused on spectral investigations of Tm3+ doped Sc2O3 transparent ceramic as potential material for diode-pumped solid-state laser emitting around 2 mu m. In the context of the Judd-Ofelt J-O) theory a series of spectroscopic parameters such as J-O intensity parameters, oscillator strengths, radiative transitions probabilities, and radiative lifetimes as well as branching ratios are evaluated. The gain cross-sections which lead to an estimation of the probable operating laser wavelength for the F-3(4) -> H-3(6) Tm3+ laser transition were also calculated. (C) 2010 Elsevier B.V. All rights reserved.

We use a non-Markovian generalized master equation (GME) to describe the time-dependent charge transfer through a parabolically confined quantum wire of a finite length coupled to semi-infinite quasi-two-dimensional (2D) leads. The quantum wire and the leads are in a perpendicular external magnetic field. The contacts to the left and right leads depend on time and are kept out of phase to model a quantum turnstile of finite size. The effects of the driving period of the turnstile, the external magnetic field, the character of the contacts and the chemical potential bias on the effectiveness of the charge transfer of the turnstile are examined, in both the absence and the presence of the magnetic field. The interplay between the strength of the coupling and the strength of the magnetic field is also discussed. We observe how the edge states created in the presence of the magnetic field contribute to the pumped charge.

Transport of electrons through two-dimensional semiconductor structures on the nanoscale in the presence of perpendicular magnetic field depends on the interplay of geometry of the system the leads and the magnetic length We use a generalized master equation (GME) formalism to describe the transport through the system without resorting to the Markov approximation Coupling to the leads results in elastic and inelastic processes in the system that are described to a high order by the integro-differential equation of the GME formalism Geometrical details of systems and leads leave their fingerprints on the transport of electrons through them The GME formalism can be used to describe both the initial transient regime immediately after the coupling of the leads to the system and the steady state achieved after a longer time (C) 2010 Elsevier BV All rights reserved