National Institute Of Materials Physics - Romania

Publications

5781 articles found

3411. Coulomb effects in open quantum dots within the random-phase approximation

Authors: Moldoveanu, V; Tanatar, B

Published: MAY 2008, PHYSICAL REVIEW B, 77, DOI: 10.1103/PhysRevB.77.195302

The effect of electron-electron interactions on coherent transport in quantum dot systems is theoretically investigated by adapting the well-known random-phase approximation (RPA) to the nonequilibrium GreenKeldysh formalism for open mesoscopic systems. The contour-ordered polarization operator is computed in terms of the Green functions of the noninteracting system. We apply the proposed RPA-Keldysh scheme for studying Coulomb-modified Fano lines and dephasing effects in interferometers with side-coupled many-level dots. Our method allows us to treat on equal footing the decoherence induced by the intradot interaction and that by the Coulomb coupling to a nearby system. In the case of a single interferometer, we show that the intradot Coulomb interaction leads to a reduction of-the Fano line amplitude. From the analysis of the interaction self-energy, it follows that this effect originates in inelastic scattering processes in which electron-hole pairs are involved. The interplay between the interdot and the intradot interactions in decoherence is discussed for two nearby identical T-shaped interferometers. We also show that the intradot interaction does not prevent the observation of controlled dephasing due to a nearby charge detector, as long as the latter is subjected to a sufficiently large bias.

3412. Investigation of metal-oxide semiconductor field-effect transistor-like Si/SiO2/(nano)crystalline PbS heterostructures

Authors: Stancu, V; Buda, M; Pintilie, L; Pintilie, I; Botila, T; Iordache, G

Published: APR 30 2008, THIN SOLID FILMS, 516, 4306, DOI: 10.1016/j.tsf.2007.11.116

The influence of the gate voltage on photoconductivity in P-Si/SiO2/(nano)crystalline PbS, Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)-like structures is investigated. The effect on the photo-signal in the wavelength range 500-1180 nm is opposite compared to the effect in the wavelength range 1600-2500 nm: a negative gate voltage increases the signal in the short wavelength range and decreases it for the long wavelength range. The opposite is valid for a positive gate bias. An on-off ratio better than 85 times is obtained for the spectral range 500-1180 nm, corresponding to the absorption in p-Si and better than 3.5 times for the spectral range 1600-2500 nm corresponding to the absorption in the (nano)crystal line PbS region, respectively. (c) 2007 Elsevier B.V. All rights reserved.

3413. Carbon nanowalls growth by radiofrequency plasma-beam-enhanced chemical vapor deposition

Authors: Vizireanu, S; Nistor, L; Haupt, M; Katzenmaier, V; Oehr, C; Dinescu, G

Published: APR 4 2008, PLASMA PROCESSES AND POLYMERS, 5, 268, DOI: 10.1002/ppap.200700120

We report the remote growth of carbon nanowalls in a plasma beam sustained by an expanding argon radiofrequency discharge and injected with a small amount of acetylene in the presence of hydrogen as active gas. The growth process was investigated by varying the mass flow rates, ratio and nature of gases (argon, ammonia/hydrogen, and acetylene), the substrate temperature, and the radiofrequency power. Scanning and transmission electron microscopy, Raman spectroscopy, and electron diffraction methods were used for the investigation of the deposited material. We found that although materials with various morphologies can be generally obtained, nanostructured carbon material with a 2D morphology is obtained with a specific range of parameter values.

3414. Pancake vortex molecules in HgBa2Can-1CunOy (n > 6) superconductors

Authors: Crisan, A; Iyo, A; Tanaka, Y; Matsuhata, H; Shivagan, DD; Shirage, PM; Tokiwa, K; Watanabe, T; Button, TW; Abell, JS

Published: APR 1 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 717, DOI: 10.1016/j.physc.2007.11.085

HgBa2Can-1CunOy (Hg: 12(n -1)n) phases have the same T-c independent of n and the same vortex melting line for n >= 6. In these materials, there are two types of pancake pairs: those separated by the thick block of (n-2) inner CuO2 planes, which are weakly coupled by the magnetic interaction, and, respectively, those separated by the thin HgBa2Ox charge reservoir layer, which are much stronger coupled by the Josephson coupling. The vortex melting line common to all Hg:12(n-1)n phases (n >= 6), separates two new vortex phases, pancake-molecule-glass, and pancake-molecule-liquid. (c) 2008 Elsevier B. V. All rights reserved.

3415. Vortex imaging with varying temperature revealed by SHPM on Bi(2)Sr(2)CaCu(2)O8+y

Authors: Mihalache, V; Dede, M; Oral, A; Sandu, V

Published: APR 1 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 836, DOI: 10.1016/j.physc.2007.11.059

Scanning Hall probe microscopy with an effective spatial resolution of similar to 1 mu m has been used to investigate the vortex structures in superconducting Bi2Sr2CaCu2O8+delta single crystals in the temperature range 77.3-81.3 K and zero applied field (in the presence of the earth field). The vortex images were obtained in real time mode as the temperature increased slowly for 3.36 h. At 77.3 K, the vortices were arranged in a chain structure. With the increase of the temperature, two jumps in the vortex array occur at 77.3 K, immediately when the temperature starts to rise, and at 79.2 K with a good stability between jumps. The second jump is accompanied by the jump in the average magnetic induction when bundles of 4-5 additional vortices enter the scanning area and the vortex array get disordered. These directly visualized transitions in the vortex lattice are consistent with a vortex creep over the surface barriers at high temperatures. A short movie is presented. (c) 2008 Elsevier B.V. All rights reserved.

3416. Low temperature magnetic and transport properties in Ce(Ag, Ni)Sb-2 compounds

Authors: Popescu, B; Royanian, E; Michor, H; Hilscher, G; Bauer, E; Galatanu, A

Published: APR 1 2008, PHYSICA B-CONDENSED MATTER, 403, 939, DOI: 10.1016/j.physb.2007.10.287

Structural, thermodynamic and,transport, properties of CeAg1-xNixSb2 compounds are investigated. The Ni substitution of Ag decreases the magnetic ordering temperature from approximate to 9.5 to approximate to 6 K, while the characteristic temperatures, considered as the temperature of the resistivity maximum, increase from 26 to 65 K. (c) 2007 Elsevier B.V. All rights reserved.

3417. Critical current densities and irreversibility fields of new high-T-c Ba2CaCu2O4(O,F)(2) superconductor

Authors: Shirage, PM; Shivagan, DD; Crisan, A; Tanaka, Y; Kodama, Y; Kito, H; Iyo, A

Published: APR 1 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 776, DOI: 10.1016/j.physc.2007.11.044

Ba2CaCu2O4(O2-yFy) (F-0212), a new high-T-c superconductor (HTSC) including fluorine at an apical site, is an attractive material for application because it shows a highest T-c of 108 K and has thin spacing between conduction layers that is favorable for high critical current density (J(c)) and irreversibility field (B-irr). The polycrystalline samples of F-0212 were synthesized under high pressure. A doping state was controlled from under doping (T-c = 82 K) to slightly-over doping (T-c = 106 K) via optimally-doping (T-c = 108 K). From the DC magnetization hysteresis loops, we determined the intragrain J(c) and B-irr using Bean's critical state model. The J(c) and B-irr properties of F-0212 were found to improve rapidly as the doping state changes from under doping to slightly-over doping to via optimally doping state. The irreversibility lines for the optimally- and slightly-over doped F-0212 are higher than optimally-doped Bi-2212. However, they are much lower than that of Cu-1212 (Y-123) though the spacing of F-0212 is thinner than that of Cu-1212. This indicates that property of the spacing is important to enhance B-irr as well as its thickness. An anisotropy factor for under doped F-0212 was estimated to 118 from a 3D-2D crossover field of about 0.28 T. (C) 2008 Elsevier B.V. All rights reserved.

3418. A penetration depth study on the non-centrosymmetric superconductors Li-2(Pd1-xPtx)(3)B

Authors: Yuan, HQ; Salamon, MB; Badica, P; Togano, K

Published: APR 1 2008, PHYSICA B-CONDENSED MATTER, 403, 1139, DOI: 10.1016/j.physb.2007.10.343

We study the superconducting order parameter in the non-centrosymmetric compounds Li-2(Pd1-xPtx)(3)B (x = 0, 0.3, 0.7 and 1) by measuring magnetic penetration depth lambda(T). The low temperature lambda(T) shows a linear temperature dependence for x >= 0.3, but follows exponential-like behavior for lower Pt contents. These findings suggest that a spin-triplet state might gradually develop with increasing x due to the broken inversion symmetry. Published by Elsevier B.V.

3419. Density functional theory analysis of a mixed-ligand iridium compound for multi-color organic light-emitting diodes

Authors: Polosan, S; Chow, TJ; Tsuboi, T

Published: APR 2008, JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, 21, 320, DOI: 10.1002/poc.1329

Electronic states and their energies are calculated for a mixed-ligand Ir(III) compound, (5-chloro-8-hydroxyquinoline) bis(2-phenylpyridyl) iridium (called IrQ(ppy)(2)-5CI) using time-dependent density functional theory (TDDFT) calculations and are compared with the experimental result. A good agreement is obtained between the calculated and measured absorption spectra: The d-pi(Q)* molecular orbital transition gives the lowest-energy triplet state absorption band. Its energy is estimated as 1.84 eV (671 nm), which is close to the absorption band position of 1.86 eV (666 nm) observed for IrQ(ppy)(2)-5CI doped in 4,4'-NN'-dicarbazole-biphenyl (CBP) host and of 1.88 eV (660 nm) observed for IrQ(ppy)(2)-5CI doped in polystyrene (PS). The second triplet state absorption band is caused by d-pi(ppy) transition. Its position is calculated as 2.51 eV (494 nm). The dipole moment is estimated as 3.45 D, which is lower than the dipole moment of fac-Ir(ppy)(3). This is understood by a reduced charge transfer between Ir(III) and quinoline ligand. Copyright (C) 2008 John Wiley & Sons, Ltd.

3420. Thermal behaviour study of some sol-gel TiO2 based materials

Authors: Crisan, M; Braileanu, A; Crisan, D; Raileanu, M; Dragan, N; Mardare, D; Teodorescu, V; Ianculescu, A; Birjega, R; Dumitru, M

Published: APR 2008, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 92, 13, DOI: 10.1007/s10973-007-8720-5

Among the great number of sol-gel materials prepared, TiO2 holds one of the most important places due to its photocatalytic properties, both in the case of powders and coatings. Impurity doping is one of the typical approaches to extend the spectral response of a wide band gap semiconductor to visible light. This work has studied some un-doped and Pd-doped sol-gel TiO2 nanopowders, presenting various surface morphologies and structures. The obtained powders have been embedded in vitreous TiO2 matrices and the corresponding coatings have been prepared by dipping procedure, on glass substrates. The relationship between the synthesis conditions and the properties of titania narrosized materials, such as thermal stability, phase composition, crystallinity, morphology and size of particles, and the influence of dopant was investigated. The influence of Pd on TiO2 crystallization both for supported and unsupported materials was studied (lattice parameters, crystallite sizes, internal strains). The hydrophilic properties of the films were also connected with their structure, composition and surface morphology. The methods used for the characterization of the materials have been: simultaneous thermogravimetry and differential thermal analysis, powder X-ray diffraction, electron microscopy (TEM, SAED) and AFM.