Publications

xLi(2)O-(1 x)alpha-Fe2O3 (x=0.1, 0.3, 0.5, and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of Li2O and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of xLi(2)O-(1 x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size as function of composition and milling times and indicated the presence of Li-substituted hematite and tetra lithium iron oxide LiFeO2, along with the formation of multiple phases for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with lithium oxide. Magnetic measurements recorded at 5 K to room temperature (RT) in an applied magnetic field of 50,000 Oe showed that the magnetization of the milled samples is larger at low temperatures than at RT and increases with decreasing particle size. Zero field cooling measurements made possible the determination of the blocking temperatures of the specimens as function of ball milling time and evidenced the occurrence of superparamagnetism in the studied samples. This result correlates well with the observed presence of a quadrupole-split doublet in the Mossbauer spectra. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Sintered bulks require new approaches for the control and improvement of their functional characteristics. Nanostructuring, synergy effects at interfaces in a composite, and technology specific features are essential in this regard. In this work, for a better understanding of challenges, complex relationships between materials and technology and different practical concepts such as "transferability", "composite within a composite", and "multilevel design", and "multifunctionality", a comparative analysis is proposed for very different nano structured materials such as La0.9Sr0.1Ga0.8Mg0.2O3-x x solid electrolyte, hard B4C-based materials, and doped MgB2 superconductors. Samples were obtained by reactive spark plasma sintering (SPS). SPS is reconfirmed as a powerful processing technique that generates high-density unique bulk materials impossible to fabricate by other methods. However, it is shown that SPS is not universal and its suitability should be carefully considered depending on materials and targeted applications. (C) 2014 The Japan Society of Applied Physics

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.

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.

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).

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.

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+, 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.

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.

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.

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