National Institute Of Materials Physics - Romania

We investigate the interplay between the spin interference and the Fano effect in a three-lead mesoscopic ring with a side-coupled quantum dot (QD). A uniform Rashba spin-orbit coupling and a perpendicular magnetic field are tuned such that the ring operates as a spin splitter in the absence of the QD: one lead is used to inject unpolarized electrons and the remaining (output) leads collect almost polarized spin currents. By applying a gate potential to the quantum dot a pair of spin-split levels sweeps the bias window and leads to Fano interference. The steady-state spin and charge currents in the leads are calculated for a finite bias applied across the ring via the non-equilibrium Green's function formalism. When the QD levels participate to transport we find that the spin currents exhibit peaks and dips whereas the charge currents present Fano lineshapes. The location of the side-coupled quantum dot and the spin splitting of its levels also affect the interference and the output currents. The opposite response of output currents to the variation of the gate potential allows one to use this system as a single parameter current switch. We also analyze the dependence of the splitter efficiency on the spin splitting on the QD. (C) 2012 Elsevier B.V. All rights reserved.

Different hybrid structures were obtained by coating magnetic nanoparticles of perovskite type manganite at optimal doping (La0.67Sr0.33MnO3, LSMO) with different quantities of polypyrrole (PPy). The amorphous layer of polypyrrole surrounding the crystalline magnetic core was observed by high resolution transmission electron microscopy (HRTEM) and analyzed by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements in near edge structure (XANES) techniques. By analyzing the magnetic behavior of the samples one can observe that the surface modification of magnetic nanoparticles by PPy results in an increase in the saturation magnetization of the composites. The process is ascribed to paired electrons transferred from the delocalized p states of the PPy into the outer disordered layers of the manganite. The analysis of pre-edge peak of the Mn K-edge XANES spectra in the case of PPy coated LSMO nanoparticles indicates that the charge transfer between polymer and nanoparticles is (directed) going to missing or distorted oxygen positions, hence increasing the 3d electrons' mobility and orbital hybridization between the neighboring manganese ion. As a consequence, within the surface layers of LSMO nanoparticles, both energy bands disrupted the structure, and the double exchange process between Mn ions was reestablished determining the saturation magnetizations and pre-edge features increase, respectively. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686662]

This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 angstrom Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 degrees C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe(3)Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.

Horseradish peroxidase immobilization inside molecular sieves with pores hexagonally structured as such or functionalized with copper ions is presented in comparison with the bidimensional immobilization onto aerosil A380 particles. Immobilization was performed under mild conditions that do not lead to enzyme denaturation. Guest-host interactions were pursued spectrophotometrically and by thermal analysis measurements. FTIR spectroscopy showed no major secondary structural change for the entrapped enzyme despite some influence of the support functionalization that can be supposed. The activity of the immobilized enzyme was tested in the oxidation of Alizarin Red S (ARS) with hydrogen peroxide. It was found that the dye oxidation is catalyzed by the enzyme in homogeneous reaction but the rate becomes much slower in heterogeneous catalytic processes with prepared catalysts. It seems that horseradish peroxidase immobilization in simple porous silica or alumino-silica post synthesis blocks the enzyme activity. In addition, copper ions make a stable complex with ARS hindering its oxidation as well.

The electrochemical behaviour of Se(IV) ion on Pt electrode in ionic liquids containing choline chloride - urea and choline chloride - ethylene glycol eutectics (1:2 mole ratio) was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in open air conditions. Cyclic voltammograms showed a cathodic peak of selenium massive deposition preceeded by one or more waves corresponding to UPD processes. It is followed by another cathodic peak of Se(II) anion formation. It was found a diffusion control of reduction process and, therefore, the value of the diffusion coefficient of Se(IV) ion in ChCl-urea ionic liquid at 60 degrees C was calculated. EIS spectra as Nyquist and Bode plots confirm the potential range for bulk selenium deposition and formation of thin selenium films.

Boron carbide B4C powders were subject to reactive spark plasma sintering (also known as field assisted sintering, pulsed current sintering or plasma assisted sintering) under nitrogen atmosphere. For an optimum hexagonal BN (h-BN) content estimated from X-ray diffraction measurements at similar to 0.4 wt%, the as-prepared BaCb-(BxOy/BN) ceramic shows values of Berkovich and Vickers hardness of 56.7 +/- 3.1 GPa and 39.3 +/- 7.6 GPa, respectively. These values are higher than for the vacuum SPS processed B4C pristine sample and the h-BN -mechanically-added samples. XRD and electronic microscopy data suggest that in the samples produced by reactive SPS in N-2 atmosphere, and containing an estimated amount of 0.3-1.5% h-BN, the crystallite size of the boron carbide grains is decreasing with the increasing amount of N-2, while for the newly formed lamellar h-BN the crystallite size is almost constant (similar to 30-50 nm). BN is located at the grain boundaries between the boron carbide grains and it is wrapped and intercalated by a thin layer of boron oxide. BxOy/BN forms a fine and continuous 3D mesh-like structure that is a possible reason for good mechanical properties.

The synthesis and characterization of a novel dinucleating ligand L (L=4,11-dimethyl-1,8-bis{2-[N-(di-2-pyridylmethyl)amino]ethyl}cyclam) and its mu-oxo-bridged diferric complex [(H2L){Fe-2(III)(O)}(Cl)(4)](2+) are reported. This diiron(III) complex is the first example of a truly functional purple acid phosphatase (PAP) mimic as it accelerates the hydrolysis of the activated phosphomonoester 2,4-dinitrophenyl phosphate (DNPP). The spectroscopic and kinetic data indicate that only substrates that are monodentately bound to one of the two ferric ions can be attacked by a suitable nucleophile. This is, most probably, a terminal iron(III)-bound hydroxide. DFT calculations support this assumption and also highlight the importance of secondary interactions, exerted by the protonated cyclam platform, for the positioning and activation of the iron(III)-bound substrate. Similar effects are postulated in the native enzyme but addressed in PAP mimics for the first time.

We study the intensity dependence of single-photon emission from a single semiconductor quantum dot after pulsed excitation. A semiconductor model is introduced that accounts for various configurations resulting from the occupation of the localized single-particle states with carriers. A detailed account is given on how the photon correlation dynamics and the antibunching can be calculated. We predict a novel effect, where, for strong excitation, the formation of multiexciton configurations during the excitation pulse leads to an improved antibunching signature in g((2)). (C) 2012 Optical Society of America

The mechanico-chemical interaction of a polyaniline-emeraldine base (PANI-EB) with ZnS in the cubic and wurtzite phases is studied by Raman spectroscopy and photoluminescence (PL). The results demonstrate that such an interaction leads to the formation of a PANI-salt and metallic Zn. Regardless of the structural form of the ZnS, the formation PANI-salt is indicated by a band in the Raman spectrum that shifts from 1162 to 1176 cm(-1) and the appearance of a new band at 1330 cm(-1) that indicates the protonated structure of a PANI-salt. The presence of the second product is determined by comparative PL studies performed on ZnS that has interacted mechanico-chemically with PANI-EB and metallic Zn powder. The variations of the PL spectra and their associated excitation spectra are explained as resulting from the charge collection processes that occur in the composite materials produced by the mechanico-chemical interaction between ZnS and PANI-EB or metallic Zn. (C) 2011 Elsevier Inc. All rights reserved.

Structural aspects and atomic intermixing processes in Be/W bilayers deposited on Si(0 0 1) substrates with Fe buffer layers enriched in the (57)Fe Mossbauer isotope have been studied via atomic force microscopy, grazing incidence X-ray diffractometry, X-ray reflectometry, X-ray photoelectron spectroscopy and conversion electron Mossbauer spectroscopy. The mentioned investigations allowed a full sequential characterization of the involved interfaces. Various ionic configurations appeared for Fe or W, while an amorphous state was observed in the case of Be. It has been proven that the Be layer has a negative influence on the roughness of the whole structure, which however presents an oxidation gradient from more oxidized elements at the surface towards more reduced elements in deeper layers. A strong diffusion of the W atoms inside the Be layer, induced by the deposition method, as well as of the Fe atoms inside the Be layer, induced by thermal annealing, has been evidenced. (C) 2011 Elsevier B.V. All rights reserved.