National Institute Of Materials Physics - Romania

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5781 articles found

3731. Controlled dephasing in single-dot Aharonov-Bohm interferometers

Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: JAN 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.045309

We study the Fano effect and the visibility of the Aharonov-Bohm oscillations for a mesoscopic interferometer with an embedded quantum dot in the presence of a nearby second dot. When the electron-electron interaction between the two dots is considered the nearby dot acts as a charge detector. We compute the currents through the interferometer and detector within the Keldysh formalism and the self-energy of the nonequilibrium Green's functions is found up to the second order in the interaction strength. The current formula contains a correction to the Landauer-Buttiker formula. Its contribution to transport and dephasing is discussed. As the bias applied on the detector is increased, the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are considerably reduced due to controlled dephasing. This result is explained by analyzing the behavior of the imaginary part of the interaction self-energy as a function of energy and bias. We emphasize as well the role of the ring-dot coupling. Our theoretical results are consistent with the experimental observation of Buks [Nature 391, 871 (1998)].

3732. Mean-field theory for Bose-Hubbard model under a magnetic field

Authors: Oktel, MO; Nita, M; Tanatar, B

Published: JAN 2007, PHYSICAL REVIEW B, 75, DOI: 10.1103/PhysRevB.75.045133

We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength (t/U)(c) increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.

3733. Current perpendicular to plane single-nanowire GMR sensor

Authors: Enculescu, I; Toimil-Molares, ME; Zet, C; Daub, M; Westerberg, L; Neumann, R; Spohr, R

Published: JAN 2007, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 86, 47, DOI: 10.1007/s00339-006-3738-2

By electrochemical deposition in a single nanopore membrane we fabricate Cu/Co layered single nanowires, that exhibit up to 10% magnetoresistance at room temperature. Single nanopore membranes are prepared by irradiating polycarbonate membranes with exactly one swift heavy ion, and by subsequent chemical etching of the single ion track. Both dc and pulsed electrodeposition of single wires consisting of Cu-Co alloy and Cu/Co multilayers respectively, are performed from a bath containing the two metal ions. By sputtering a gold electrode on the upper membrane surface, the single nanowire embedded in the flexible polymer foil is reliably contacted. While alloy wires exhibit anisotropic magnetoresistance (AMR), multilayer wires display current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) behavior. This demonstrates that both the fabrication and contacting methods are very suitable for the investigation of transport properties, without the necessity of lithographic processes and without manipulation of the nanowires. In addition, the method opens up many new possibilities for single nanowire-based sensors.

3734. Magnetic properties of Sr2FeMo1-xMxO6 perovskites with M=W or Ta

Authors: Balasz, I; Burzo, E; Valeanu, M

Published: 2007, SIX INTERNATIONAL CONFERENCE OF THE BALKAN PHYSICAL UNION, 899, 763, DOI:

The Sr2FeMo1-xMxO6 perovskites with M = W or Ta and x <= 0.3 were studied by x-ray and electron microscopy. The magnetic and magnetoresistive properties were correlated with their microstructures. The effect of sintering time on their physical properties was analysed.

3735. Extrinsic tunneling magnetoresistance in thick films of CrO2-polystyrene composite

Authors: Sandu, V; Aldica, G; Sandu, E

Published: 2007, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 1, 76, DOI:

Polystyrene-CrO2 composite films with extrinsic tunneling magnetoresistance have been obtained. The electric conduction is 2D Mott variable range hopping in the temperature range 77-300 K. The magnetoresistance is negative when the field is aligned parallel to the current (in-plane geometry) and is due to the spin dependent intergrain tunneling of the charge carriers. It reaches a value MR = -10.6 % at room temperature and a magnetic field of 1T. When the field is applied in-plane but perpendicular to the current the magnetoresistivity is positive and has the average value MR = 3.56 %.

3736. Structure-property relationships in the xZnO-(1-x) alpha-Fe2O3 nanoparticle system

Authors: Sorescu, M; Diamandescu, L; Wood, J

Published: 2007, ZINC OXIDE AND RELATED MATERIALS, 957, +, DOI:

The xZnO-(1-x)alpha-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x=0.1. 0.3 and 0.5 and for ball milling times ranging from 2 to 24 hours. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mossbauer spectroscopy. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.

3737. Antennas with prefractal geometry for wireless communications

Authors: Goga, G; Hongu, A; Nicolaescu, I; Banciu, MG; Ioachim, A

Published: 2007, CAS 2007 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, +, DOI: 10.1109/SMICND.2007.4519704

Design, simulation fabrication and measurement of prefractal antennas for wireless communications are presented in this paper Two types of geometries, i.e. Sierpinski gasket and Koch curve, were investigated. The proposed antennas are compact, can operate for multiple frequency bands and exhibit small mutual coupling.

3738. Temperature dependence of the main piezoelectric parameters of a Nb-Li doped PZT ceramic

Authors: Miclea, C; Tanasoiu, C; Amarande, L; Cioangher, M; Trupina, L; Miclea, CT; David, C

Published: 2007, 2006 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, +, DOI:

A study was made on the temperature behavior of the main piezoelectric parameters, namely the electromechanical coupling factor k(p), the mechanical quality factor Q, the dielectric permittivity (real and imaginary part), the loss tangent t(g) and the piezoelectric charge constants 6133 and d(31) of a soft type piezoelectric material. It was evidenced that temperatures under 150 degrees C do not influence these parameters, which means that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect. After that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

3739. Sintering behavior of hard-soft PZT-Type composite ceramics

Authors: Miclea, C; Tanasoiu, C; Miclea, CF; Spanulescu, I; Trupina, L; Cioangher, M; Miclea, CT

Published: 2007, FERROELECTRICS, 351, 186, DOI: 10.1080/00150190701354265

Functionally graded materials represent a new class of composite type materials characterized by a continuous variation of properties through the thickness direction. Due to this they found numerous applications in aerospace, automobile, biomedical or nuclear industries. Bilayered structures made from typically hard and typically soft PZT type materials as well as plain hard and soft and intimately mixed hard and soft samples were prepared by green pressed powdersfollowed by sintering at different temperatures between 1000 and 1300 degrees C. The dielectric and piezoelectric properties of sintered compacts were determined in relation with the nature of the interdiffusion layer involved. Electron microscopy as well as X-Ray diffractometry was used to get an insight into the interfacial diffusion process. The composition and the width of the interdiffusion layer depend on temperature. The mechanism of the interdiffusion layer and materials properties were discussed in terms of the ionic diffusion coefficients and activation energies of the species which take place into the diffusion process.

3740. Barium strontium titanate-based perovskite materials for microwave applications

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Vasiliu, F; Alexandru, HV; Berbecaru, C; Stoica, G

Published: 2007, PROGRESS IN SOLID STATE CHEMISTRY, 35, 520, DOI: 10.1016/j.progsolidstchem.2007.01.017

Ba1-xSrxTiO3 solid solutions were prepared by solid-state reaction from raw materials. Four compositions with x = 0.25, 0.50, 0.75 and 0.90 have been investigated. The perovskite type and polycrystalline structure of the BST samples were revealed by X-ray diffraction data. The morphology, grain size distribution, porous structure and elemental composition of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The temperature dependence of permittivity and of dielectric loss tangent at low frequency (1 kHz) showed decrease of Curie temperature with increase of Sr content. Microwave measurements (1 GHz) showed substantial decrease of the dielectric constant from about 1600 to 200 and also of the losses from 12% to less than 2% with the Sr concentration increase from x = 0.25 to x = 0.90. Moreover, the addition of MgO and MnO2 1 wt.% each, improved sintering process and lowered the microwave losses up to 0.2%. (c) 2007 Elsevier Ltd. All rights reserved.