National Institute Of Materials Physics - Romania

Publications

5781 articles found

2751. Time-dependent magnetotransport in semiconductor nanostructures via the generalized master equation

Authors: Gudmundsson, V; Tang, CS; Gainar, CM; Moldoveanu, V; Manolescu, A

Published: JAN 2011, COMPUTER PHYSICS COMMUNICATIONS, 182, 48, DOI: 10.1016/j.cpc.2010.08.006

Transport of electrons through two-dimensional semiconductor structures on the nanoscale in the presence of perpendicular magnetic field depends on the interplay of geometry of the system the leads and the magnetic length We use a generalized master equation (GME) formalism to describe the transport through the system without resorting to the Markov approximation Coupling to the leads results in elastic and inelastic processes in the system that are described to a high order by the integro-differential equation of the GME formalism Geometrical details of systems and leads leave their fingerprints on the transport of electrons through them The GME formalism can be used to describe both the initial transient regime immediately after the coupling of the leads to the system and the steady state achieved after a longer time (C) 2010 Elsevier BV All rights reserved

2752. Band-gap correlations in Bi4Ge3O12 amorphous and glass-ceramic materials

Authors: Polosan, S; Galca, AC; Secu, M

Published: JAN 2011, SOLID STATE SCIENCES, 13, 53, DOI: 10.1016/j.solidstatesciences.2010.10.007

The Bi4Ge3O12 (BGO) glass materials were sintered from a mixture of oxides powders, rapidly heated and melted at 1323 K and poured on the graphite plates heated at different temperatures. Their optical properties are compared with BGO single crystals being strongly dependent by the experimental conditions, which influence the further stability of the samples. The dielectric functions of the samples reveal a clear difference between the two phases (amorphous and crystalline). The refractive index of the amorphous phase has lower values which indicate that the density of the material is diminishes. Also, a sign of disorder is seen in optical absorption (extinction coefficient). From the optical band gap determinations of those two phases, it is confirmed the disorder structure and red-shifts of the band gap value compared with crystalline phase. The absorption spectra are in good agreement with the ellipsometric measurements, regarding the transparency and the edge of the band gap. (C) 2010 Elsevier Masson SAS. All rights reserved.

2753. Vortex Dynamics in YBa2Cu2O7-delta Films and Superlattices: A Magnetization Relaxation Study

Authors: Miu, L

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 125, DOI: 10.1007/s10948-010-0907-5

The relaxation of the irreversible magnetization of optimally doped YBa2Cu3O7-delta (YBCO) films and YBa2Cu3O7-delta /PrBa2Cu3O7-delta (YBCO/PrBCO) superlattices in a magnetic field H oriented along the c axis was investigated over a large temperature T interval. The observed behavior is dominated by the presence of a crossover elastic vortex creep at low T-plastic creep at high T generated by the T-dependent current density J of the macroscopic currents induced in the specimen during experiments. Magnetization relaxation in the flux-creep annealing regime indicates the reduction of the creep crossover temperature with decreasing J, which suggests that in static conditions the disordered vortex phase at high H behaves like a plastically pinned vortex assembly, rather than an elastic vortex glass. The often reported scaling of the electric field E-transport current density J characteristics in agreement with an elastic vortex glass-vortex fluid transition seems to originate from the creep crossover generated by an increasing J.

2754. Structural investigations on yttrium - doped ceria nanopowders obtained by coprecipitation method

Authors: Mercioniu, I; Tartaj, J; Vlaicu, AM; Pogrion, NP

Published: JAN 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 149, DOI:

The attractive properties of ceria, particularly the conductivity and chemical stability, have led to vast research efforts to investigate, characterize and develop such materials. One of the main applications of this ceramic is in the solid oxides fuel cells for intermediate temperatures IT-SOFC. The aim of this work is the preparation and characterization of cubic ceria nanopowders doped with 10 mol % yttria or Scandia. A thermal treatment (calcinations) was applied on the obtained nanopowders, which were consequently investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected aria electron diffraction (SAED).

2755. Combination of Ag Substrate Decoration with Introduction of BaZrO3 Nano-Inclusions for Enhancing Critical Current Density of YBa2Cu3O7 Films

Authors: Dang, VS; Mikheenko, P; Sarkar, A; Kechik, MMA; Abell, JS; Paturi, P; Huhtinen, H; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 509, DOI: 10.1007/s10948-010-0981-8

The combination of two methods: Ag substrate decoration and introduction of BZO nano-inclusions has been used in a pulsed laser deposition (PLD) method to increase the critical density (J (c) ) of YBCO films. The films were deposited on single crystal SrTiO3 (STO) substrates decorated with various architecture of Ag nano-dots. We have studied the diameter and density of Ag nano-dots and their influence on J (c) of BZO-added YBCO films. We found that 15 laser pulses on the Ag target gives an optimum result in increasing J (c) in comparison with BZO-doped YBCO films of the same thickness in self-field and low applied magnetic fields. A higher number of laser pulses on the Ag target led to increasing critical current density in high applied magnetic fields only (above 2 T). We have studied films of the thickness from 0.4 mu m to 3.8 mu m and found that the highest J (c) at all applied fields investigated is achieved for a 1.2 mu m thick film. The transmission electron microscopy clearly shows BZO nano-rods that provide strong c-axis pinning centres in the films.

2756. Structural and thermoluminescence properties of undoped and Fe-doped-TiO2 nanopowders processed by sol-gel method

Authors: Cernea, M; Secu, M; Secu, CE; Baibarac, M; Vasile, BS

Published: JAN 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 85, DOI: 10.1007/s11051-010-0002-7

In this study, we report on the nanocrystalline powders of TiO2 and Fe-doped TiO2 (anatase and rutile phases) prepared by sol-gel method. The X-ray diffraction and Raman spectroscopy measurements indicated the presence of anatase or rutile phase in nanopowders. TEM micrographs showed 10 and 112 nm average particle sizes for anatase and rutile, respectively. Furthermore, their thermoluminescence properties were analyzed.

2757. Magnetic Characterization of MgB2 Bulk Superconductor for Magnetic Field Mitigation Solutions

Authors: Gozzelino, L; Minetti, B; Gerbaldo, R; Ghigo, G; Laviano, F; Lopardo, G; Plapcianu, C; Agostino, A; Cagliero, S; Truccato, M; Zilberti, L; Mezzetti, E

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 312, DOI: 10.1007/s10948-010-0993-4

Magnetic shielding properties of MgB2 bulk samples synthesized by the SPS (Spark-Plasma-Sintering) technique were characterized in low applied magnetic fields at temperatures ranging from 20 to 37 K. The used growth technique allows one to produce this compound in different shapes and sizes required for shielding applications. In this framework, shielding magnetic-induction field profiles generated by MgB2-based shield components, shaped as planar thick disks, were measured by means of a suitable Hall probe in-plane array. The magnetic field distribution at different vertical distances above the sample was also obtained by a micrometric motion of the probe ensemble. Magnetic field profiles were then analyzed in the framework of the critical state model and the critical current density, J (c) , was evaluated. The J (c) magnitude indicates that the material under test is a good candidate for passive magnetic shield manufacturing up to temperatures close to the transition one.

2758. Exotic Vortex Matter: Pancake Vortex Molecules and Fractional-Flux Molecules in Some Exotic and/or Two-Component Superconductors

Authors: Crisan, A; Tanaka, Y; Iyo, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 6, DOI: 10.1007/s10948-010-0892-8

The discovery of superconductivity in MgB2 and, more recently, in iron pnictides, revived the interest in two-component and other exotic superconductors, including in the field of vortex matter, as was shown recently by the discovery of "type 1.5" superconductivity. However, even some "older" exotic cuprates grown by high-pressure techniques showed interesting anomalies related to the interplay between Josephson and magnetic coupling and/or two-gap superconductivity. Here, we will present two such examples of exotic vortex matter: magnetically-coupled pancake-vortex molecules in super-multilayered cuprates, and vortex molecules composed of fractional flux quanta glued by an interband phase difference soliton, and will discuss the major differences between MgB2 and such multilayer cuprates in terms of the relative strength of the interband interaction and, respectively, in terms of the coherence and penetration lengths.

2759. Nano Techniques for Enhancing Critical Current in Superconducting YBCO Films

Authors: Mikheenko, P; Dang, VS; Kechik, MMA; Wang, Y; Sarkar, A; Tanner, JL; Abell, JS; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 1064, DOI: 10.1007/s10948-010-0861-2

A range of nano techniques is explored in order to increase the critical current in pulsed laser deposited (PLD) superconducting YBa2Cu3O (x) (YBCO) thin films. The structural measurements are linked with magnetic and transport measurements of the films. The effectiveness of PLD techniques is analyzed from the point of view of the dimensionality of nanostructures formed prior to and during the film growth. It is shown that a combination of two-dimensional substrate decoration with nanoparticles before the film deposition and one-dimensional growth of external phase nanorods during the deposition offers a high critical current in magnetic field both along the c-axis and in the ab-plane of YBCO.

2760. Structural and chemical properties of cerium-magnetoplumbite in cerium based IT-SOFC compounds

Authors: Vlaicu, AM; Mercioniu, I; Vasile, BS; Negrila, CC; Logofatu, C; Cretu, NC; Nita, P; Popescu-Pogrion, N

Published: JAN 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 145, DOI:

Yttria doped ceria reinforced with yttria doped alumina compound for IT-SOFC applications has been prepared by sol-gel method and sintered at 1300 degrees C, 1400 degrees C, 1500 degrees C. XRD, and SEM show the presence of a needle-like cerium-magnetoplumbite (CeAl11O17) phase in the compound; the oxygen deficient phase grows from the alumina phase by using ceria from the surrounding matrix and releasing oxygen ions As the sintering temperature is increased, the alumina phase is gradually transformed into CeAl11O17. Compared to the yttria doped ceria without reinforcement, the obtained compounds with ceria-magnetoplumbite show only a slight decrease of the Young elasticity modulus. However, the samples become brittle after backing in oxygen atmosphere (1000 degrees C, and 1500 degrees C).