National Institute Of Materials Physics - Romania

This work reports on recently investigated structural and magnetic properties of three FexOy-SiO2 nanocomposites prepared by the alkoxide route of the sol-gel method. Depending on the presence or absence of catalyst and on its nature (acidic or basic), important differences among the samples have been observed from the point of view of their structure and properties. (c) 2007 Elsevier B.V. All rights reserved.

The capacitance of PbZrO(3)-Pb(Zr(0.8)TiO(0.2))O(3) epitaxial multilayers is significantly enhanced as the number of interfaces increases at the same total thickness of the structure. A possible explanation for this enhancement can be the increase of the dielectric susceptibility due to the presence of some interfacial polarization related, for instance, to trapped charges at the multilayer interfaces. The presented results suggest that capacitance tuning is achievable in antiferroelectric-ferroelectric epitaxial multilayers.

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).