National Institute Of Materials Physics - Romania

The aim of the present work is the investigation of the core-shell Fe@Au nanoparticles obtained by the inverse micelles method. Also hybrid structures between Fe@Au nanoparticles and the well-known conducting polymer polypyrrole (PPY) were obtained and investigated. The properties of the composites were investigated by TEM, HRTEM, X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) and magnetization measurements. The TEM, HRTEM measurements show that two categories of core-shell nanoparticles with mean diameters of 5 nm and 25 nm, respectively, are formed. The XPS investigations of the core-shell Fe@Au nanoparticles indicate that besides Fe-0 inside the cores, small amounts of Fen,111, located onto the gold surface were also formed during the samples preparation. The magnetization of the investigated nanocomposites measured versus increasing and decreasing magnetic field shows no hysteresis loop, this behavior being consistent with a superparamagnetic behavior. The behavior of the SQUID magnetization vs. temperature under field cooled (FC) and zero field cooled (ZFC) at 0.1 T magnetic field for Fe@Au nanoparticles confirmed the existence of superparamagnetism. The superparamagnetic model was used to fit the experimental magnetization vs. applied magnetic field as it was measured by SQUID up to 5 T. (C) 2007 Elsevier B.V. All rights reserved.

The relaxation of the remnant magnetisation in optimally doped disk-shaped YBa2Cu3O7-delta films with the initially applied magnetic field H oriented along the c axis was measured using dc SQUID magnetometry. The temperature (7) dependence of the experimentally observed normalised magnetisation relaxation rate S exhibits three distinct regions. At high T, S(7) increases with increasing T, which can be explained in terms of plastic vortex creep. The well known plateau in the S(7) variation at intermediate T appears to be caused by collective (elastic) creep in a dynamically ordered vortex system. At low T, where S increases again with increasing T, the magnetisation relaxation is not in a pure thermally activated flux-creep regime, due to the occurrence of micro-flux jumps. This seems to be the origin of the recently proposed exponential current density dependence of the effective vortex pinning potential. (c) 2007 Elsevier B.V. All rights reserved.

Critical temperature T-c of the Li-2(Pd1-xPtx)(3) B was reported to be 7-8 K for x = 0 and 2.2-2.8 K for x = 1. In this article we present our preliminary results on behavior of magnetization-temperature curves with starting composition of Pd-B precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end compound. Results suggest that to maximize T-c ratio Pd: B should be close to 3: 1, while y-Li has to be optimum. The lowest T-c for LiyPd3B was 4.4-4.6 K, while post-annealings at 560 degrees C allowed enhancement of T-c up to 8.2-8.4 K. Compositions Li(2)Z(3)B with Z = Ni, Ru, Rh, Re, Ag are not superconducting down to 1.8 K. Exception is composition with Re showing superconductivity due to Re3B compound. All samples were prepared by arc melting. (c) 2007 Elsevier B.V. All rights reserved.

Zero-field-cooling dc magnetisation relaxation measurements performed on optimally doped epitaxial YBa2Cu3O7-delta films with the external magnetic field H oriented parallel to the c-axis reveal the ordering of the creeping vortex-system in the low temperature T domain, due to the macroscopic currents induced in the sample. This dynamic ordering manifests itself through the appearance of a maximum in the T variation of the normalised vortex-creep activation energy. The location of this maximum in the H-T plane describes a well-defined current induced order-disorder line. Our results suggest the absence of a static elastic vortex glass in disordered high-temperature superconductors at high H, and the current induced ordering of the vortex-system as a possible origin for the current-voltage characteristic scaling in terms of an elastic vortex glass-vortex liquid transition. (c) 2007 Elsevier B.V. All rights reserved.

X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been used to investigate the elemental composition, chemical and electronic state of elements that exist within the Herastrau Lake (Bucharest, Romania) sediments. XPS spectra were obtained by irradiating the sediment samples with an Al source of X-rays (1486.6 eV). C, N, O, F, Na, Mg, Al, Si, S, Cl, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, and Pb have been determined by XPS and AES techniques. The analysis of Electron Paramagnetic Resonance (EPR) spectra revealed an additional free radical peak in the center of Mn2+ sextet, at a g = 2.0038, attributed to SO3- species as well a very intense and broad signal due to Fe3+ ions.

A compact microstrip dual-mode resonator with slots in the ground plane is presented in this paper. Several methods for coupling between the dual modes are investigated. One of the methods involves using slots in the ground plane in order to enhance the external coupling and to introduce the necessary perturbation to the initially symmetrical structure. The proposed device requires a surface area about 6 tunes smaller than the area needed for a conventional patch dual-mode resonator. (C) 2007 Wiley Periodicals, Inc.

We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green's function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green's function is not necessary and an exact current formula can be expressed in terms of retarded Green's functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameter space where the conductance is temperature independent. We include a comparison with the results from Ng's ansatz [Phys. Rev. Lett. 70, 3635 (1993)], which are less accurate, but can be used also in the absence of the above mentioned symmetry.

The present paper deals with the preparation and study of optical properties of ZnO nanowires. Nanoporous ion track polycarbonate foils were used as templates for ZnO nanowires growth by electrochemical deposition. The growth was performed using an aqueous electrochemical bath at a temperature of 70 degrees C. SEM and TEM were employed to study the morphological properties of the nanowires. Electron diffraction was employed for structural characterization. Reflection and photoluminescence spectroscopy were used for optical characterization. A comparison between the properties of electrodeposited thin films and nanowires of ZnO was made from the point of view of photoluminescence properties. (c) 2007 Elsevier B.V. All rights reserved.

Electrolytic coloration has been made for KCl crystals doped with Tl+ ions and doped with Tl+ and Ca2+ ions. Intensity decrease of the Tl+ absorption bands is observed in the two colored crystals, while several new absorption and emission bands are observed. They are attributed to Tl- ions which were converted from Tl+ ions by electron injection during electrolytic coloration. Different absorption and emission bands are observed for KCI:TI and KCI:T1 + Ca crystals. Sharp absorption band appears at 285 nm in the colored KCl:Tl and at 400 and 503 nm in the colored KCLTl + Ca. Infrared emission bands are observed at 1470, 1600 and 2185 nm in colored KCI:Tl, while at 1630 and 1900 nm in colored KCl:Tl + Ca. It is suggested that the absorption and emission bands in KCl:Tl + Ca are due to Tl- ions perturbed by Ca2+ or vacancy at the nearest neighbor. (c) 2006 Elsevier B.V. All rights reserved.

The X-ray, HRTEM, electrical resistivities, magnetic and Mossbauer effect studies were performed on Sr2FeMoO6 samples. It was shown that the nonuniform composition inside the grains and that at grain boundaries influence the magnetoresistive properties. (c) 2007 Elsevier B.V. All rights reserved.