Publications

Self-sustained pulsations in the output power of metal-organic chemical vapor deposition grown ridge-waveguide lasers with InGaAs quantum dot active region are reported. The characteristics of the output power pulsations (range, frequency, and modulation depth) are presented. The origin of the pulsations is explained in terms of the properties of the quantum dot active region.

Cluster related defects were investigated by the thermally stimulated current (TSC) method in neutron irradiated n-type Si diodes during 80 degrees C annealing. Three hole traps labeled H (116 K), H (140 K), and H (152 K) proved to have an electric-field-enhanced emission characteristic of Coulombic wells. Their zero field emission rates were deduced describing the TSC peaks with the three-dimensional Poole-Frenkel formalism when accounting for the electric field distribution. As acceptors in the lower half of the gap, these centers have a direct impact on the effective doping of the n-type diodes. They are revealed as causing the long term annealing effects. (c) 2008 American Institute of Physics.

Fundamental and third harmonic ac susceptibility measurements as a function of temperature T, ac magnetic field, and dc magnetic field have been performed on Bi(Pb)SrCaCuO/2223 bulk samples consisting of both Bi2Sr2Ca2Cu3O10+delta (2223) and Bi2Sr2CaCu2O8+delta (2212) phases. Since the chi(1)''(T) peak corresponding to the losses in the weak links between the grains shifts rapidly to lower T and its amplitude is strongly suppressed with increasing the dc magnetic field H-dc, the other peaks related to the losses in the grains become visible. For the samples consisting of single phase 2223, these intragranular peaks are situated near 38 K, 58 K, and 80-88 K (depending on the sample), whereas for the samples consisting of single phase 2212, they are situated near 20 K, 43 K, and 77 K. For H-dc >= 1 kOe, the temperature positions of the intragranular peaks are weakly field dependent. This shows that the transition line determined from the loss peaks behaves like the melting line in two-dimensional (213) systems. Also, as the field increases, the amplitude of the loss peaks can decrease up to their disappearance, suggesting a layer decoupling process. In this way, we show that, above a crossover field of similar to 1 kOe, the vortex lattice in the superconducting grains of polycrystalline samples reproduces the behaviour of the vortex lattice in a quasi-2D object consisting of a set of separated "thin films" with one-, two-, and three-effective-layer thicknesses. For the samples consisting of both 2223 and 2212 phases, three additional peaks were observed, situated near 28 K, 50 K and 75-80 K (depending on the sample). These peaks were attributed to the regions in which the 2212 and 2223 layers alternate. (c) 2007 Elsevier B.V. All rights reserved.

The iron oxide nanoparticles and iron oxide nanoparticles coated with dextrin have been synthesized using aqueous solution of ferric and ferrous ions and mixtures of dextrin with sodium salt. The size of the iron-oxide nanoparticles are controlled by the concentration of sodium salt in the medium. An average size of iron oxide and iron oxide coated with dextrin was found by transmission electron microscopie (TEM). The iron oxide nanoparticles are nearly spherical with an average diameter of about 8.0 +/- 1 nm. The iron oxide nanoparticles coated with dextrin appear to cluster-like aggregates. The average diameter of these nanoparticles is about 6 1 nm. The attachment of the dextrin on the particle surface was confirmed by FTIR spectroscopy and scanning electron microscopy (SEM). Osteoblasts used to determine the cell proliferation, viability and citotoxicity interaction with iron oxide nanoparticles coated with dextrin has been obtained from the upper part of the patient's femur.

Amorphous chalcogenide structures are proposed for designing meta-materials as e.g. photonic crystals with stop gaps in the far infrared and microwave range. Micrometer diameter lenslets made of As2S3 glass are used to this purpose. Special heat treatment is proposed to get a shrinkage of the structure built from stacks of lenslets, and creating a modulated woodpile configuration based on the photo-fluidity effect in arsenic sulphide chalcogenides. The simple procedure allows the tuning of the lattice parameter in order to reach the desired optical or microwave properties of the photonic crystal with stop band gaps.

Scanning Hall probe microscopy has been used to demonstrate the manipulation of pancake vortices by rotating the Josephson vortex lattice in Bi(2)Sr(2)CaCu(2)O(8+delta) single crystals in the interacting crossing lattices regime. Creation of one-dimensional pancake vortex chains trapped on Josephson vortices, and the subsequent rotation of the chains were realized by independently controlling magnetic fields in three orthogonal directions. The anisotropy parameter determined from the in-plane distances between vortex chains in various in-plane fields is consistent with commonly accepted values.

We consider an ideal parabolic quantum wire in a perpendicular magnetic field. A simple Gaussian-shaped scattering potential well or hill is flashed softly on and off with its maximum at t=0, mimicking a temporary broadening or narrowing of the wire. By an extension of the Lippmann-Schwinger formalism to time-dependent scattering potentials, we investigate the effects on the continuous current that is driven through the quantum wire with a vanishingly small forward bias. The Lippmann-Schwinger approach to the scattering process enables us to investigate the interplay between geometrical effects and effects caused by the magnetic field.

Ba(Zn1/3Ta2/3)O-3 (BZT) dielectric materials were prepared by solid state reaction. The samples were sintered at temperatures in the range 1550 divided by 1650 degrees C for 2 h. XRD, SEM and EDX were employed for compositional, structural and morphological characterization. An additional annealing at 410 degrees C for 10 hours was performed in order to improve the dielectric parameters. The dielectric constant and quality factor of BZT dielectric resonators measured by using Hakki-Coleman method around 6 GHz was between 25 and 29. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/degrees C. The best parameters of BZT dielectric resonators were achieved for the samples sintered at 1650 degrees C with additional thermal treatment (square(r) = 29, Q x f = 100 THz).

Generally, the uranifer tailings dump are not protected. During the time appear chemical and physical modifications which favour spreading of the radioactive contaminant into the environment, because of the waters provided from precipitations with pH variation (acid rainfall, snow), solar radiance, variation of temperature and humidity, interaction of different minerals from the waste rock with infiltration water during the flow, leading to precipitations phenomenon, ionic exchange, adsorption and re-dissolving [2]. As a rule, the speed of radionuclides diffusion from radioactive tailings dump depends on each of these factors and is very specific to the hydro and geochemistry of the nearest rocks [7]. Because of that, the scientific research is very difficult and will obliges us to study through the simplified physical systems but maintaining the important parameters which are influencing the diffusion process of the contaminant in the environment. The main objective of this paper is to characterize hydro and geochemistry properties controlling the migration of radionuclides extracted from tailings dump into the environment, including the processes of sorption and adsorption and neglecting other physical processes as precipitation, ion exchange or some other geochemical interactions within the system. For this purpose we made studies on Ra-226 and U-nal extraction from uranifer tailings dump analyzed in pure water in dynamic system, studies about the influence of clay presence on speed of diffusion process in sorption and adsorption phenomenon. Finally we made a prediction for long term about variation of concentrations of Ra-226 and U-nal based on experimental data using some software programs from literature [8].

The thermoluminescence (TL) glow curve recorded after gamma-irradiation at room temperature of Mn-2-doped MgF2 phosphor shows two intense TL peaks at 364 K and 420 K accompanied by several weaker peaks up to 600 K. The TL spectra have shown Mn2+ luminescence at 560 nm. The 364 K TL peak disappears after one day of storage, and so is probably related to the recombination of carriers from some shallow traps. Based on thermal bleaching and optical bleaching (in the 260 nm F-centre absorption band) experiments, the 420 K TL peak was ascribed to the thermally activated recombination between an F-centre and a hole centre trapped in the neighbourhood of the Mn2+ impurity which results in Mn2+ luminescence.