National Institute Of Materials Physics - Romania

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.

Intelligent machines using multi-level memory chips are at the ends of our fingers. The first prototypes of smart memories are under construction. The science behind the devices is growing up very rapidly. New materials with multi-level storage are discovered and tested. The applications of the smart memories are extensive and in the same time rewarding for the welfare of the human kind.

Recent experiments evidenced room temperature ferromagnetism in ZnO:Co diluted magnetic semiconductors, however the diluted semiconductor nature and the ferromagnetism strongly depends on the preparation method employed. We have investigated the ability of the sol-gel route for the synthesis of cobalt doped ZnO films grown on Si (100) and glass substrates. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature (1.5 K) Kerr magnetometry give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 200 Oe is superimposed, most probably due to metal cobalt nanoclusters.

This paper presents new concepts for pumping very thin active laser media at pump level of hundreds of watts, output performances obtained from Yb-based microchip laser. Continuous-wave (cw) laser operation at 1.03 mu m with slope efficiency of 0.40 in Yb:YAG and low thermal lens was realized using an edge-pump configuration and a laser diode emitting at 0.94 Pm. Over 90 W cw output power was demonstrated from Yb:YAG volume of 2 x 2 x 0.4 mm(3). The optical phase distortion analysis gives focus shift bellow to 0.05 m and proves the absence of astigmatic effects. Two new architectures of thin disc concept were analysed numerically and experimentally. These new design allow very thin disk to be operated at high power level.