National Institute Of Materials Physics - Romania

Nonstationary and steady-state transport through a mesoscopic sample connected to particle reservoirs via time-dependent barriers is investigated by the reduced density operator method. The generalized master equation is solved via the Crank-Nicolson algorithm by taking into account the memory kernel which embodies the non-Markovian effects that are commonly disregarded. The lead-sample coupling takes into account the match between the energy of the incident electrons and the levels of the isolated sample, as well as their overlap at the contacts. Using a tight-binding description of the system, we investigate the effects induced in the transient current by the spectral structure of the sample and by the localization properties of its eigenfunctions. In strong magnetic fields, the transient currents propagate along edge states. The behavior of populations and coherences is discussed, as well as their connection to the tunneling processes that are relevant for transport.

Samples of LaNi5-xFex were prepared by arc melting and subsequent melt spinning procedure. Structural, magnetic and Mossbauer investigations were performed for physical characterization while the hydrogen storage properties were carefully analyzed. Detailed discussions concerning the sample's quality, the site assignment, the effect of temperature and applied fields on hyperfine parameters and the related Debye temperatures were provided. (C) 2008 Elsevier B.V. All rights reserved.

Six tetranuclear complexes [Fe(III)(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF center dot C7H16 [Ln=Gd(III) (1), Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5), and Lu(III) (6)] have been studied by magnetic susceptibility and Moumlssbauer spectroscopy. These isostructural molecules have a "butterfly" structure core consisting of two Fe(2)Ln(mu(3)-O) triangular "wings" which share a common Ln-Fe "body"; the dihedral angle between the wings is ca. 148 degrees. The coordination spheres of the iron ions are essentially distorted octahedral. The lanthanides are eight-coordinate with coordination polyhedra that may be described as distorted tetragonal bipyramids. Variable-temperature solid-state magnetic susceptibility in the temperature range 1.8-300 K and magnetization at 1.8 K for compounds 1-6 were measured. The spin state of Fe is S=5/2 in all cases. In compounds 5 and 6, where Ln(III) (Y and Lu, respectively) is diamagnetic, the three Fe atoms form an obtuse isosceles triangle with antiferromagnetic interactions J(Fe-Fe)=-50 K between the wing-tip Fe-w and body Fe-b atoms, and negligible interaction between the Fe-w's, resulting in a ground state of effective spin S=5/2 per cluster. In the complexes with paramagnetic lanthanide ions, the interaction between the Fe-3 triangle and the Ln(III) center is described by an effective exchange constant which is antiferromagnetic and 1 order of magnitude weaker. Besides, at 3 K incipient spin blocking, characteristic of single molecule magnets, was found to occur in the out-of-phase component of the ac susceptibility in {Fe3TbO2}, {Fe3DyO2}, and {Fe3HoO2}. The activation energy of a Debye process describing the magnetization reversal has been determined to be, E-a approximate to 8, 9, and 10 K for the Ln=Tb, Dy, and Ho, respectively, and the prefactor tau(0)approximate to 10(-7) s. The high spin states of the Fe(III) centers were confirmed by the Moumlssbauer spectra, in which two distinguishable Fe sites could be resolved above 80 K, corresponding to the Fe-w and Fe-b sites, respectively. Relatively larger values of the quadrupole splitting of the Moumlssbauer spectra were observed for the Fe-w pair as compared with that for the Fe-b, and both quadrupole splittings diminished with increasing temperature. At 3 K the Moumlssbauer spectra showed a state with blocked spins (sextets) for the {Fe3TbO2} and {Fe3DyO2} cases. From the E-a and tau(0), determined in the ac susceptibility, the relaxation time at 3 K is estimated as tau approximate to 10(-5)-10(-6) s much longer than the time window of Moumlssbauer spectroscopy and compatible with the single molecule magnet behavior. In the presence of a strong magnetic field the moments of the Ln(III) cation and the Fe-3 triangle are polarized. For some compounds at low temperature a magnetic pattern (sextet) for each of the three Fe sites appeared, and the antiferromagnetic coupling within the Fe-3 cluster was directly proved by the opposite trend of the field dependence of the two Fe-w sextets as compared with the Fe-b third one.

PbI2, BiI3, CdI2 and AgI crystalline samples intercalated with pyridine have been studied by Raman spectroscopy. Comparing the Raman spectra of pristine metal iodides with those of intercalated samples we have shown the coexistence, in the host crystalline lattice, of two adsorbed forms: a physisorbed one, featured by weak forces of van der Waals type and a chemisorbed one, involving stronger forces related to an electrostatic interaction. The physisorbed form is consistent with the molecules inserted into the interlayer spaces while the chemisorbed form is represented by the molecules forming coordination complexes with the crystalline lattice cations. The crucial role in the formation of such complexes is played by the lone pair of nitrogen atom belonging to the molecules.

The results of the CMS tracker alignment analysis are presented using the data from cosmic tracks, optical survey information, and the laser alignment system at the Tracker Integration Facility at CERN. During several months of operation in the spring and summer of 2007, about five million cosmic track events were collected with a partially active CMS Tracker. This allowed us to perform first alignment of the active silicon modules with the cosmic tracks using three different statistical approaches; validate the survey and laser alignment system performance; and test the stability of Tracker structures under various stresses and temperatures ranging from +15 degrees C to -15 degrees C. Comparison with simulation shows that the achieved alignment precision in the barrel part of the tracker leads to residual distributions similar to those obtained with a random misalignment of 50 (80) mu m RMS in the outer (inner) part of the barrel.

This article presents a versatile gas-flow proportional counter for surface and transmission Mossbauer spectroscopy, suitable for studies with Fe-57, Sn-119 and Eu-151 isotopes. The main advantages obtained by new design are: (1) the height of the detection volume can be changed in large limits from 0 to 38 mm, (2) the detection volume can be choose symmetrical or not in respect with anode plan, (3) the anode replacement is easily (4) and different anode configuration can be used. The characteristics of the detector, operating at room temperature, are reported.

The aim of the paper is to evidence the effect of the composition and oxygen stoichiometry on the thermodynamic, spin dynamics and magnetic properties of some micro- and nanostructured substituted manganites La(1-x)A(x)Mn(1-y)B(y)O(3-delta)(A=Ca; B=Co, Fe; x=0.33; y=0, 0.02 and 0.04). The partial molar thermodynamic properties of oxygen dissolution in the perovskite phase as well the equilibrium partial pressures of oxygen have been obtained using solid electrolyte electrochemical cells [ electromotive force measurement (EMF)] method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. Using electron paramagnetic resonance spectroscopy, the spin dynamics has been investigated. Particular attention was devoted to the discussion concerning the influence of doping on polaron activation energy E-a and exchange coupling integral J between Mn spins. The results were correlated with the variation of both saturation magnetisation M-s and Curie temperature T-c. The data were discussed in relation to significant changes in the overall concentration of defects. New features related to the modifications in properties connected with the nanocrystalline state were evidenced.

Epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films were grown by pulsed laser deposition. PbZr0.2Ti0.8O3 was doped with Cr acting as acceptor ion. Microstructural characterization was performed by (high resolution) transmission electron microscopy. The voltage dependence of polarization, dielectric constant, and leakage current were measured with respect to the Cr content. To derive the electronic properties, PZT was considered as a wide-gap semiconductor which allows treating the metal-PZT interface as a Schottky contact. The Cr was found to facilitate the elastic relaxation of the film. Furthermore, the leakage current was increased through a reduction of the Schottky barrier. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3141733]

Widely applicable nonaqueous solution routes have been employed for the syntheses of crystalline nanostructured tungsten oxide particles from a tungsten hexachloride precursor. Here, a systematic study on the crystallization and assembly behavior of tungsten oxide products made by using the bioligand deferoxamine mesylate (DFOM) (product I), the two chelating ligands hexadecyltrimethylammoniumbromide (CTAB) (II) and poly(alkylene oxide) block copolymer (Pluronic P123) (III) is presented. The mechanistic pathways for the material synthesis are also discussed in detail. The tungsten oxide nanomaterials and reaction solutions are characterized by Fourier transform IR, H-1, and C-13 NMR spectroscopies, powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and selected-area electron diffraction. The indexing of the line pattern suggests WO3 is in its monoclinic structure with a=0.7297nm, b=0.7539nm, c=0.7688nm, and beta=90.91 degrees. The nanoparticles formed have various architectures, such as chromosomal shapes (product I) and slates (II), which are quite different from the mesoporous one (III) that has internal pores or mesopores ranging from 5 to 15 nm. The nanoparticles obtained from all the synthetic procedures are in the range of 40-60 nm. The investigation of the gas-sensing properties of these materials indicate that all the sensors have good baseline stability and the sensors fabricated from material III present very different response kinetics and different CO detection properties. The possibility of adjusting the morphology and by that tuning the gas-sensing properties makes the preparation strategies used interesting candidates for fabricating gas-sensing materials.