Publications

We report on the fabrication of MgO nanorods and nanowires by a novel, all-self-assembled method, using Pulsed Laser Deposition. The seeds for the formation of these one-dimensional nanostructures consisted of self-assembled gold nanodots. Gold was used due to its well-known catalytic effect. Dimensions and surface density of the Au nano-dots, investigated by Atomic Force Microscopy, depend on the deposition conditions and the number of laser pulses. Morphology of the MgO nanorods and nanowires was investigated by SEM, and was found to depend strongly on the deposition conditions and on the architecture of the Au nanodots on the decorated substrates.

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, J(c)), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). J(c) of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased J(c). The relative enhancement in J(c) increases with further increase in temperature. J(c) of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of delta T-c-type.

Flux flow resistivity above critical temperature is analyzed based on the increased evidences that high temperature superconductors are intrinsically inhomogeneous, and local nonpercolating superconducting domains persist high above the critical temperature. It was found that above a certain field, the resistivity originated in the flow Of flux motion starts to decrease with increasing field, due to the predominance of the suppression of the superconducting droplets over the increase of vortex core density. (C) 2008 Elsevier B.V. All rights reserved.

We present a fast and simple method to prepare specimen,, for transmission electron microscopy Studies of oxide thin films deposited oil silicon Substrates. The method consists of scratching the film surface using I pointed diamond tip, in a special manner. Small and thin fragments are then detached front the film and its substrate. Depending oil the scratching direction, the fragments call be used for plan-view or cross-section imaging. High-resolution images call be also obtained from thin edges of the film fragments. The method is demonstrated in the Case Of HfO2 sol-gel films deposited on [100] Si wafer substrates.

On subjecting a bulk 2H-PbI2 crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogen temperature stacking faults are generated that separate distinct layers of nanometric thickness in which different numbers of I-Pb-I atomic layers are bundled together. Such structures, containing two, three, four, five etc I-Pb-I atomic layers, behave as quantum wells of different widths. The signature of such a transformation is given by a shift towards higher energies of the fundamental absorption edge, which is experimentally revealed by specific anisotropies in the photoluminescence and Raman spectra. The quantum confining effect is made visible by specific variations of a wide extra-excitonic band (G) at 2.06 eV that originates in the radiative recombination of carriers (electrons and holes), trapped on the surface defects. The excitation spectrum of the G band, with p polarized exciting light, reveals a fine structure comprised of narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with the PbI2 quantum wells formed from two, three, four and five I-Pb-I atomic layers of 0.7 nm thickness. Regardless of the polarization state of the laser exciting light of 514.5 nm (2.41 eV), which is close to the band gap energy of PbI2 (2.52 eV), the Raman scattering on bulk as-grown PbI2 crystals has the character of a resonant process. For p polarized exciting light, the Raman scattering process on vacuum annealed PbI2 becomes non-resonant. This originates from the quantum well structures generated inside the crystal, whose band gap energies are higher than the energy of the exciting light.

Spectral emission of a Xenon discharge in the presence of NaCl powder is strongly modified and dominated by the XeCl excimer band at 308 nm. Materials introduced in gas electrical discharges interact with the reactive species produced in the plasma: electrons, ions and metastable atoms. In this paper, the influence of ions and metastable atoms of a capacitive RF Xenon discharge plasma (20W, 30MHz) 5 Torr on the structure of a NaCl powder introduced in the discharge has been investigated.

We describe the output performance of the 1.34-mu m F-4(3/2)-> I-4(13/2) transition in Nd-doped vanadates under in-band pumping with diode lasers at the 0.88-mu m wavelength, directly into the F-4(3/2) emitting level. An end-pumped Nd:YVO4 crystal yielded 3.4 W of continuous-wave (CW) output power for 9.3 W of absorbed pump power. The slope efficiency with respect to the absorbed pump power was 0.43. To the best of our knowledge this is the first domonstration of such a laser system. Comparative results obtained for the pump with diode laser at 0.81 mu m, into the highly-absorbing F-4(5/2) level, are given in order to prove the advantages of the in-band pumping.

The corrosion and inhibition behavior of carbon steel in 1M HCl in the presence of Bis (2-Benzothiazolyl) disulphide (BTD) was investigated using weight loss measurements, electrochemical measurements and UV-VIS spectrophotometry. The morphology of carbon steel surface was investigated by using microscopic analysis and Mossbauer spectrometry. The corrosion current was determined by using Tafel polarization. The inhibition efficiency increased with BTD concentration and the experimental results suggest that the presence of BTD in the solution increase the surface coverage (0) and therefore, indicate the adsorption of BTD. The adsorption of this compound on the metal surface obeys Langmuir's adsorption isotherm. Mossbauer spectroscopy shown at this stage the main product of corrosion consists in a mixture of alpha, beta and gamma-FeOOH, where gamma-FeOOH is the main phase. BTD inhibitor acts as an incipient "rust transformer" and favors the formation of a "superficial closed layer". The UV-VIS spectrophotometry shown a decrease of the concentration of BTD in 1M HCl solution after corrosion, indicating that an adsorption process between organic compound from aqueous phase and the electrode surface.

The effects of the electron-phonon interaction in zinc oxide depending on the free conduction band electron concentration are invetigated. In n-doped polar semiconductors, as the free carrier concentration is increased, the plasmonic mode associated to charge density fluctuations gradually approaches, its frequently to the optical phonon ones. As a consequences, their interaction is materialized in coupled mode oscillations. Using a dielectric cotinuum-model, applying the equation of motion technique for the operator associated to charge density fluctuations in ZnO, and taking in to account the supplementary constraints introduced by its uniaxial symmetry, the response function of the system is obtained. In polar semiconductors with cubic crystalline structure there are two plasmons-optical phonon coupled modes. It is shown that in ZnO, three such coupled modes appear. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We study the differential conductance in the Kondo regime of a quantum dot coupled to multiple leads. When the bias is applied symmetrically on two of the leads (V and -V, as usual in experiments) while the others are grounded, the conductance through the biased leads always shows the expected enhancement at zero bias. However, under asymmetrically applied bias (V and lambda V, with lambda>0), a suppression-dip-appears in the differential conductance if the asymmetry coefficient lambda is beyond a given threshold lambda(0)=3 root 1+r determined by the ratio r of the dot-lead couplings. This is a recipe to determine experimentally this ratio which is important for the quantum-dot devices. This finding is a direct result of the Keldysh transport formalism. For the illustration we use a many-lead Anderson Hamiltonian, the Green's functions being calculated in the Lacroix approximation, which is generalized to the case of nonequilibrium.