National Institute Of Materials Physics - Romania

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5781 articles found

2101. Schottky barrier versus surface ferroelectric depolarization at Cu/Pb(Zr, Ti)O-3 interfaces

Authors: Stoflea, LE; Apostol, NG; Chirila, C; Trupina, L; Negrea, R; Pintilie, L; Teodorescu, CM

Published: MAY 2014, JOURNAL OF MATERIALS SCIENCE, 49, 3351, DOI: 10.1007/s10853-014-8041-6

The band bending at Cu/PZT(001) interfaces is investigated by X-ray photoelectron spectroscopy (XPS) for a PZT(001) layer which exhibits initial outwards ferroelectric polarization. Two competitive processes are identified: (a) formation of the Schottky barrier between the ferroelectric and unconnected Cu islands, and (b) coalescence of the Cu islands, realisation of an electrical contact to the ground of the system, inducing the apparent loss of the component of the ferroelectric polarization perpendicular to the sample surface, at least as it manifests in band bending. Three mechanisms are proposed to explain this loss of band bending when a full metal layer connected to ground is formed on the surface: (i) over-compensation of depolarization field in the sub-surface region, (ii) formation of domains with in-plane orientation of the polarization vector and (iii) loss of polarization in the near-surface layers of the ferroelectric due to electrons provided by the metal. These result in a non-monotonous variation of binding energies with the amount of Cu deposited. High resolution transmission electron microscopy and piezoresponse force microscopy confirmed these hypotheses. The XPS data allowed also to derive the surface PZT composition, its evolution with the deposition of copper and the formation of surface compounds.

2102. New features in the anti-Stokes and Stokes Raman spectra of single-walled carbon nanotubes that are highly separated into their semiconducting and metallic nanotube components

Authors: Baibarac, M; Baltog, I; Mihut, L; Lefrant, S

Published: MAY 2014, JOURNAL OF RAMAN SPECTROSCOPY, 45, 331, DOI: 10.1002/jrs.4473

Surface-enhanced Raman scattering studies were performed using nonresonant (514.5 nm) and resonant (676.4 nm) optical excitations on single-walled carbon nanotubes thoroughly separated into semiconducting (pure 99%) and metallic (pure 98%) components. Regardless of the support (Au or Ag), the metallic nanotubes do not present an anomalous anti-Stokes Raman emission. Regardless of whether an on-resonant or off-resonant optical excitation is used, only the semiconducting nanotubes produce an abnormal anti-Stokes Raman emission that grows when increasing the excitation light intensity or temperature. The Raman studies under light polarized relative to the main nanotube axis demonstrate that only semiconducting nanotubes are sensitive toward changes in the polarization of the excitation light. Copyright (c) 2014 John Wiley & Sons, Ltd.

2103. B6O ceramic by in-situ reactive spark plasma sintering of a B2O3 and B powder mixture

Authors: Solodkyi, I; Borodianska, H; Zhao, T; Sakka, Y; Badica, P; Vasylkiv, O

Published: MAY 2014, JOURNAL OF THE CERAMIC SOCIETY OF JAPAN, 122, 340, DOI: 10.2109/jcersj2.122.336

A mixture of B2O3 and amorphous B with the mole ratio of 1:14 is shown to react and form B6O ceramic under in-situ spark plasma sintering (SPS) conditions. The optimum SPS temperature and time to obtain phase B6O are 1250 C and 30 min, respectively, and Rietveld refinement of the XRD patterns indicates that the oxygen occupancy of B6Ox is reasonably high at x = 0.89(99). However, to reach high density above 98%, SPS temperatures of 1700-1800 degrees C are necessary and a one-step twotemperature in-situ reactive SPS was designed and applied. As-prepared B6O dense ceramic has Vickers hardness (36.7 +/- 1.2 GPa) and fracture toughness (K-1c = 4.2 +/- 0.15 MPa. m(1/2)) comparable with the highest values reported in literature for the bulks obtained by processing routes of already reacted B6O powders (ex-situ routes).

2104. Preparation and characterization of LixZn2-xVxSi1-xO4 ceramics

Authors: Busuioc, C; Jinga, SI

Published: MAY 2014, CERAMICS INTERNATIONAL, 40, 5936, DOI: 10.1016/j.ceramint.2013.11.039

In this work, LixZn2-xVxSi1-xO4 (x=0.9, 0.8, 0.7, 0.5, 0.3, 0.2, and 0.1) ceramics were obtained by the conventional solid-state reaction method, using a different sintering temperatures for each x. We report on the compositional and morphological characterization of LixZn2-xVxSi1-xO4 samples, as well as on the influence of the composition on the photoluminescence and dielectric properties. The highest emission intensity was attained for x=0.9, while the best microwave dielectric properties (epsilon(r)similar to 7.8 and Qf similar to 21,122 GHz) were achieved for x=0.8. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

2105. Influence of co-dopants average valence on microstructural and electromechanical properties of lead titanate ceramics

Authors: Amarande, L; Miclea, C; Cioangher, M; Grecu, MN; Pasuk, I; Negrea, RF

Published: MAY 2014, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 34, 1200, DOI: 10.1016/j.jeurceramsoc.2013.12.002

Effects of Ni, Nb and Mn co-doping on microstructural, dielectric and electromechanical properties of lead titanate (PT) dense ceramics, obtained as a result of decreasing lattice anisotropy with increasing amount of co-dopants, were investigated. Different values of the average valence (AV) of Ni-Nb co-dopants were obtained by mutual valence compensation between these acceptor donor ions of various relative concentrations. Significantly higher amounts of Mn2+ (resulted by reduction of Mn4+ in presence of donor co-dopants) were found in materials with Ni-Nb AV >4+. These materials show higher coupling factors, correlated with higher mechanical quality factors, compared to the rest of materials. Hard-like piezoelectric behavior was manifested in materials with Ni-Nb AV <4+, whereas ceramics with Ni-Nb AV >4+, and without manganese, turned out to have a soft-like behavior. Electromechanical properties of PT ceramics are discussed in terms of average valence of Ni-Nb co-dopants and multivalence state of manganese. (C) 2013 Elsevier Ltd. All rights reserved.

2106. Topological properties of the mesoscopic graphene plaquette: Quantum spin Hall effect due to spin imbalance

Authors: Ostahie, B; Nita, M; Aldea, A

Published: APR 15 2014, PHYSICAL REVIEW B, 89, DOI: 10.1103/PhysRevB.89.165412

We study the electronic properties of the confined honeycomb lattice in the presence of the intrinsic spin-orbit (ISO) interaction and perpendicular magnetic field, and report on uncommon aspects of the quantum spin Hall conductance corroborated by peculiar properties of the edge states. The ISO interaction induces two specific gaps in the Hofstadter spectrum, namely the "weak" topological gap defined by Beugeling et al. [Phys. Rev. B 86, 075118 (2012)], and spin-imbalanced gaps in the relativistic range of the energy spectrum. We analyze the evolution of the helical states with the magnetic field and with increasing Anderson disorder. The "edge" localization of the spin-dependent states and its dependence on the disorder strength is shown. The quantum transport, treated in the Landauer-Buttiker formalism, reveals interesting new plateaus of the quantum spin Hall effect (QSHE), and also of the integer quantum Hall effect (IQHE), in the energy ranges corresponding to the spin-imbalanced gaps. The properties of the spin-dependent transmittance matrix that determine the symmetries with respect to the spin, energy, and magnetic field of the longitudinal and transverse resistance are shown.

2107. Coulomb-assisted cavity feeding in nonresonant optical emission from a quantum dot

Authors: Florian, M; Gartner, P; Steinhoff, A; Gies, C; Jahnke, F

Published: APR 14 2014, PHYSICAL REVIEW B, 89, DOI: 10.1103/PhysRevB.89.161302

Recent experiments have demonstrated that for a quantum dot in an optical resonator, off-resonant cavity-mode emission can occur even for detunings of the order of 10 meV. We show that Coulomb-mediated Auger processes based on additional carriers in delocalized states can facilitate this far off-resonant emission. A theoretical approach is developed for the nonperturbative treatment of the Auger-assisted quantum-dot carrier recombination. Using this method we present numerical calculations of the far off-resonant cavity feeding rate and cavity mean photon number, confirming efficient coupling at higher densities of carriers in the delocalized states. In comparison to fast Auger-like intraband scattering processes, we find a reduced overall efficiency of Coulomb-mediated interband transitions due the required electron-hole correlations for the recombination processes.

2108. Cladding waveguides realized in Nd:YAG ceramic by direct femtosecond-laser writing with a helical movement technique

Authors: Salamu, G; Jipa, F; Zamfirescu, M; Pavel, N

Published: APR 1 2014, OPTICAL MATERIALS EXPRESS, 4, 797, DOI: 10.1364/OME.4.000790

Circular cladding waveguides were realized in a 5.0-mm long, 1.1- at.% Nd:YAG ceramic by direct femtosecond-laser writing using a scheme in which the laser medium is moved on a helical trajectory along its axis and parallel to the writing direction. Laser emission was obtained under the pump with a fiber-coupled diode laser. A 100-mu m diameter waveguide delivered laser pulses at 1.06 mu m with 3.4-mJ energy for the pump with pulses of 13.1-mJ energy, at 0.30 slope efficiency; laser pulses at 1.3 mu m with 1.2-mJ energy were obtained from the same device. Comparison with a waveguide of the same dimension that was inscribed by the classical translation method of the laser medium is made. Efficient integrated lasers based on cladding waveguides that are pumped by fiber-coupled diode lasers could be realized by this writing method. (C) 2014 Optical Society of America

2109. Improved laser efficiency by direct diode laser pumping of the radiation-resistant Nd:gadolinium-scandium-gallium garnet

Authors: Lupei, V; Pavel, N; Lupei, A

Published: APR 2014, LASER PHYSICS, 24, DOI: 10.1088/1054-660X/24/4/045801

Spectroscopic investigation of the radiation-resistant Nd:GSGG crystals and ceramics reported in this paper evidences that the absorption line at 883 nm corresponding to the unusual situation of quasi-degenerate absorption transitions I-4(9/2)(2) -> F-4(3/2)(1) and I-4(9/2)(3) -> F-4(3/2)(2) can be used for efficient direct diode laser pumping of this material, with stable absorption over an extended temperature range. It is inferred that the reduction of the quantum defect at this wavelength of pump compared to the traditional 807 nm pumping could improve the laser parameters and reduce drastically the heat generation, leading to a considerable extension of the power scalability. This possibility is demonstrated for the first time in the case of the 1061 nm laser emission in a continuous-wave and in the repetitive active acousto-optic and passive Q-switched laser emission. In all regimes this manifests in the reduction of the laser threshold, increase of slope efficiency and extension of the average power range. Additionally, in the active Q-switching this enables increased pulse energy and reduced pulse duration. The direct pumping could revitalize the utilization of Nd:GSGG for construction of lasers with storage of population inversion or working in an ionizing radiation environment.

2110. Tough and dense boron carbide obtained by high-pressure (300 MPa) and low-temperature (1600 degrees C) spark plasma sintering

Authors: Badica, P; Grasso, S; Borodianska, H; Xie, SS; Li, PF; Tatarko, P; Reece, MJ; Sakka, Y; Vasylkiv, O

Published: APR 2014, JOURNAL OF THE CERAMIC SOCIETY OF JAPAN, 122, 275, DOI: 10.2109/jcersj2.122.271

Dense boron carbide (above 95%) was achieved through high pressure (300 MPa) and low temperature (1600 degrees C) Spark Plasma Sintering (SPS). This approach resulted in improvement of fracture toughness and of dynamic toughness when compared to corresponding toughness values of the sample sintered by conventional SPS (2100 degrees C, 50 MPa). Dynamic toughness was extracted from Split Hopkinson Pressure Bar measurements. Results are understood based on microstructure and on very different behaviour of the samples in respect to residual B2O3 and carbon available in the raw B4C powder. (C) 2014 The Ceramic Society of Japan. All rights reserved.