National Institute Of Materials Physics - Romania

We investigated the vortex dynamics in the noncentrosymmetric superconductor Li2Pt3B in the temperature range 0.1-2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of an unconventional very effective pinning mechanism. At a certain time, a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices.

The new tetranuclear complexes [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF (Ln = Ce-III (1), Pr-III (2), Nd-III (3)) and [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF center dot C7H16 (Ln = Sm-III (4), Eu-III (5), Gd-III (6), Tb-III (7), Dy-III (8), Ho-III (9), Lu-III (10) and Y-III (11)) have been prepared. All compounds were prepared by the reaction between [Fe2BaO(CCl3COO)(6)(THF)(6)] and the corresponding Ln(III) nitrate salt. The crystal structures of 1-4, 8 and 9 have been determined; these isostructural molecules have a non-planar {Fe(3)Ln(mu(3)-O)(2)} "butterfly" core. Magnetic susceptibility measurements show dominant intramolecular antiferromagnetic exchange interactions for all the complexes. Fe-57 Mossbauer spectroscopy shows three different environments for the Fe-III metal ions, all in their high-spin state S = 5/2 (confirming that no electron transfer from Ce-III to Fe-III occurs in 1). At the time scale of the Mossbauer spectroscopy (about 10(-7) s), evidence of magnetization blocking, i.e. slow relaxation of the magnetization, is observed below 3 K for 7, which was confirmed by ac susceptibility measurements. (C) 2009 Elsevier Ltd. All rights reserved.

Lead free ceramics in the system (1-x)KNN x(Bi2O3,CaO,CuO) with 0.0 <= x <= 0.1 were synthesized by the usual solid state reaction and their structural and piezoelectric properties were investigated Dense samples with densities of about 4 68 g/cm(3) were prepared by sintering at an optimum temperature of 1150 degrees C for 6 h Only perovskite phase was detected by X-ray investigation in all samples A typical MPB where orthorombic-tetragonal phases coexist was detected for a doping level of 0.06 where the piezoelectric parameters showed maximum values Planar coupling factor, as much as 0.46, piezoelectric charge constant of 270 pC/N and mechanical quality factor Q(m) of 75 were recorded for the samples situated within the MPB.

Among the great number of sol-gel prepared nanomaterials, TiO2 has attracted significant interest due to its high photocatalytic activity, excellent functionality, thermal stability and non-toxicity. The photocatalytic degradation of pollutants using un-doped and doped TiO2 nanopowders or thin films is very attractive for applications in environmental protection, as a possible solution for water purification. The present work describes a comparative structural and chemical study of un-doped TiO2 and the corresponding S- and Ag-doped materials. The photocatalytic activity was established by testing the degradation of organic chloride compounds from aqueous solutions. Sol-gel Ag-doped TiO2 coatings, prepared by co-gelation and sol-gel Ag-doped TiO2 coatings obtained from nanopowders were also compared. Their structural evolution and crystallization behaviour (lattice parameters, crystallite sizes, internal strains) with thermal treatment were followed by thermal analysis, X-ray diffraction, transmission electron microscopy, atomic force microscopy and specific surface areas measurements. X-ray photoelectron spectroscopy analyses were performed to characterize the surface composition and S or Ag speciation, which was used to interpret the catalytic data.

Lead titanate (PT) ceramics co-doped with 8 to 13 mol% Ni-Nb, by substituting titanium ions, were prepared by conventional ceramic method and the influence of Ni-Nb doping level on their mopho-structural, electromechanical and dielectric properties was investigated. Dense tetragonal PT ceramics were obtained for 8 to 12 mol% Ni-Nb co-doping. Lattice anisotropy decreased from 1.053 to 1.046 with increasing doping level. Large thickness coupling factors of 0.4 were obtained for 8, 11, and 12 mol% Ni-Nb. Large electromechanical anisotropy of 10 was obtained for 8 mol% Ni-Nb. It was found that Ni-Nb co-doping has both softening and hardening effects as a function of donor or acceptor behavior of different amounts of Ni-Nb. For 13 mol% Ni-Nb, segregation of a small amount of a secondary phase occurs, lattice constants and tetragonality significantly decrease and the resulting ceramic becomes very hard.

Thin films of Ba(Ti1-xSnx)O-3 with x = 0.13 have been deposited by rf-magnetron sputtering and their growth morphology and structure vs annealing temperature, in oxygen atmosphere, have been investigated The films deposited on silicon-platinum substrates, at room temperature, in Ar atmosphere were amorphous The as-obtained films had a mean surface roughness (RMS) around 6.529 nm and a grain size of about 150 nm By annealing the film up to 1150 degrees C in oxygen, the grains size increases up to 1-1.5 mu m in diameter Films containing BaTi0.87Sn0.13 as crystalline phase with the cubic structure of BaTiO3 and, bundle-like morphology were obtained by heating the film at 1250 degrees C for 3h in oxygen The as-obtained film showed the preferential (110) and (100) orientations

Undoped and Eu3+-doped CaF2-SiO2 gels were prepared by the sol-gel method and their optical properties have been studied. The UV-VIS-NIR absorption and photoluminescence spectra have shown the bands typical for the Eu3+ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 degrees C (i.e. above the CaF2 crystallisation peak at similar to 460 degrees C) the photoluminescence spectra intensity increase, the 590 nm (D-5(0) -> F-7(1)) and 620 nm (D-5(0) -> F-7(2)) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the F-5(0) -> D-7(2) transition of the Eu3+ ion were observed at around 1000 and 620 cm(-1) and have been assigned to the Si-O and Ca-O bonds, respectively. A phonon sideband signal in the range of 3007400 cm(-1) was attributed to Ca-F bonds in the precipitated CaF2 phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 degrees C, the Eu3+ ions are incorporated into the silica network and in the precipitated CaF2 phase. (c) 2009 Elsevier B.V. All rights reserved.

Chalcogenide glasses can be tailored for getting special configurations with photonic crystal properties A review on the advances in chalcogenide glass photonics is given Several methods to prepare two-dimensional photonic As-S glass and three-dimensional packing have been developed, and the photonic structures have been characterized Numerical simulations have also been performed The chalcogenide photonic structures fabricated by micro-technological procedures are described and tuned to work in the far infrared region of the electromagnetic spectrum

Bio-glass films were deposited by radio-frequency magnetron sputtering technique onto medical grade Ti6Al7Nb alloy substrates from prepared silica based bio-glass target. A low deposition temperature was used (150 degrees C) and three different working pressures, followed by annealing in air at 550 and 750 degrees C. A quasi-stoichiometric target to substrate atomic transfer was found for Si, Ca and P, along with strong enrichment in Na and depletion in K and Mg, as evidenced by the energy dispersive microanalysis. The best results, taking into account stoichiometry and surface roughness, were obtained for the BG layers deposited at 0.3 Pa argon working pressure. The infrared spectroscopy of the as-sputtered and of the annealed films evidenced the characteristic molecular vibrations of silicate, phosphate and carbonate functional groups. The as-deposited films are amorphous and became partly crystalline after annealing at 750 degrees C, as evidenced by X-ray diffraction. The pull-out measurements, performed with a certified pull-test machine, gave very strong film-substrate adhesion strength values. For the non-crystalline layers, the pull-out strength is higher than 85 MPa, and decreases after annealing at 750 degrees C to 72.9 +/- 7.1 MPa. The main objective of this work was to establish the influence of the working pressure upon the composition and morphology of the as-deposited films, and of the annealing temperature upon structure and film substrate adhesion. (C) 2009 Elsevier B. V. All rights reserved.

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G(0)W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.