Resumo em Inglês:

We study the transport properties of a spin filter consisting of a double-barrier resonant tunneling device in which the well is made of a semimagnetic material. Even if the device could be made of several materials, we discuss here the case of a Ga1-xMn xAs/Ga1-yAl yAs system because it can be integrated into the well known AlAs/GaAs technology. We solve the Hamiltonian H = H K+H P+H E+H M+Hh-i+Hh-h. Its terms represent the kinetic energy, the double-barrier profile, the applied bias, the magnetic interaction, the hole-impurity attraction and the hole-hole repulsion, respectively. A very simple one-dimensional Green function is introduced to solve self-consistently the Poisson equation for the profile due to the charge distribution. A real space renormalization formalism is used to calculate exactly the currents. In a previous work we have shown that the Rashba effect is weak. Therefore the results show very well defined spin-polarized currents. Our results confirm that this system is a good device for spintronics.

Resumo em Inglês:

A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga1-xMn xAs diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J$^{Mn-Mn}_n$, from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J$^{Mn-Mn}_n$ with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.

Resumo em Inglês:

Numerical calculations are shown to reproduce the main results of recent experiments involving nonlocal spin control in nanostructures (N. J. Craig et al., Science 304, 565 (2004)). In particular, the splitting of the zero-bias-peak discovered experimentally is clearly observed in our studies. To understand these results, a simple "circuit model" is introduced and shown to provide a good qualitative description of the experiments. The main idea is that the splitting originates in a Fano anti-resonance, which is caused by having one quantum dot side-connected in relation to the current's path. This scenario provides an explanation of Craig et al.'s results that is alternative to the RKKY proposal, which is here also addressed.

Resumo em Inglês:

Electron tunneling through a double quantum dot molecule side attached to a quantum wire, in the Kondo regime, is studied. The mean-field finite-U slave-boson formalism is used to obtain the solution of the problem. We investigate the many body molecular Kondo state and its interplay with the inter-dot antiferromagnetic correlation as a function of the parameters of the system.

Resumo em Inglês:

A review of the dependence of the electron mobility on the free carrier concentration for gallium nitride and indium nitride nanowires grown using hot-wall chemical vapour deposition is presented. Gallium nitride nanowires exhibit mobilities of 100 cm²/Vs to below 1 cm²/Vs for carrier concentrations of 10(19) to 10(20) cm- 3. Theoretical estimations and annealing experiments indicate that the nanowires are heavily compensated. Indium nitride nanowires also exhibit high carrier concentrations, of the order of 10(20) to 10(22) cm- 3. For both types of nanowires, mobility decreases with increasing carrier concentration, consistent with transport limited by impurity scattering.

Resumo em Inglês:

Recent developments in the field of nano-electronics have encouraged the study of quasi one dimensional systems such as DNA and its applications to new devices. In this work we use two models to explore the vibrational properties of DNA-like chains: a linear chain and two laterally coupled linear chains. In the former case, the disorder induces a diatomic behavior, while monoatomic characteristics are displayed in the last case.

Resumo em Inglês:

We report here theoretical and experimental studies on the spatial confinement of phonons in ternary CdSxSe1-x nanocrystals embedded in a glass matrix formed by the composites SiO2-Na2CO3-B2O3-Al2O3 doped with CdO, S and Se. We determined the morphologic characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with values from Raman spectra of CdSxSe1-x nanocrystals grown under different thermal treatments. A good correlation between experimental and calculated CdS-like and CdSe-like surface optical modes is observed. Raman selection rules and their connection with the nature of the surface optical phonons is discussed.

Resumo em Inglês:

We calculate the exciton states of a Quantum Dot (QD) and a Quantum Dot Molecule (QDM) in an exact way for single, double and triply charged excitons without and with an applied external magnetic field. For the case of single quantum dot, we reproduce the experimental results reported in [2]. We also calculate the emission energies for the neutral molecule QDM as function of the barrier width between the two dots and we find that the Coulomb interaction splits the emission spectra in case of QDM. Concerning hybridization the triply charged QDM behaves in the same way as the QD, but showing several anticrossings with the lower Landau level of the continuum.

Resumo em Inglês:

The lignin is an extremely challenging natural material with wide and interesting applications, during the last two decades, great efforts have been dedicated to the study of its optical properties. About this subject have occurred many controversial opinions because of the nature and interactions of this material with other components of the vegetal cellular wall. In this work we will explore and simulate the electronic absorption and vibrations spectrums based on the semiempirical geometrical and quantum mechanics optimizations of amorphous two and tree units softwoods lignin fragments of Coniferyl Alcohol. These fragments were previously obtained using the semiempirical methods, Modified Neglect of Diatomic Overlap MNDO and Zerner's spectroscopic version of the Intermediate Neglect of Differential Overlap in configuration interactions mode: ZINDO/S CI.

Resumo em Inglês:

The evolution of a surface excitation in a two dimentional model is analyzed. I) It starts quadratically up to a spreading time tS. II) It follows an exponential behavior governed by a self-consistent Fermi Golden Rule. III) At longer times, the exponential is overrun by an inverse power law describing return processes governed by quantum diffusion. At this last transition time tRa survival collapse becomes possible, bringing the survival probability down by several orders of magnitude. We identify this strongly destructive interference as an antiresonance in the time domain.

Resumo em Inglês:

We investigate the spin dynamics of a magnetic adatom on a non-magnetic surface with strong Stoner enhancement. We find a strong damping of the adatom's magnetization precession and a large shift of the resonance frequency from its bare value. Stoner enhancement in the substrate reduces the damping. We explore the damping dependence on features of the electronic structure.

Resumo em Inglês:

We investigate the spin-resolved dynamics of spin-polarized carriers injected via a ferromagnetic scanning-tunnelling-microscope tip (STM tip) into uniformly and non-uniformly n-doped bulk semiconductor - externally driven by a current source. We propagate the injected spin packets (assumed gaussian in space at t = 0) by considering a spin hydrodynamic approach based on balance equations directly derived from a spin-dependent Boltzmann equation. We determine the spin-polarization landscapes (time and position) of the carrier population $(n_(\uparrow}-n_{\downarrow})/(n_{\uparrow}+n_{\downarrow}$ and the current density $(j_{\uparrow}-j_{\downarrow})/(j_{\uparrow}+j_{\downarrow})$. While in the uniformly-doped system the carrier spin-polarization has a slow decay, in the non-uniformly doped system it shows a drastic fall down in the interface. In contrast the current spin-polarization exhibits an enhancement for both of the systems particularly in the interface.

Resumo em Inglês:

The Ogg-McCombe effective Hamiltonian for the electron in the conduction band together with the non-parabolic and effective-mass approximations were used in a theoretical study of the cyclotron effective mass and electron effective Landé g||-factor in semiconductor GaAs-Ga1-xAl xAs quantum wells under an applied magnetic field parallel to the growth direction of the quantum well. Calculations are performed as a function of the applied magnetic field, and for different widths of the GaAs-Ga1-xAl xAs quantum wells. Results for the electron cyclotron effective mass and g||-factor are found in quite good agreement with experimental data.

Resumo em Inglês:

The dependence of the electron Landé g-factor on carrier confinement in quantum wells recently gained both experimental and theoretical interest. The g factor of electrons in GaAs-(Ga,Al)As quantum wells is of special interest, as it changes its sign at a certain value of the well width. In the present work, the effects of an in-plane magnetic field on the cyclotron effective mass and on the Landé g<FONT FACE=Symbol>^</FONT>-factor in single GaAs-(Ga,Al)As quantum wells are studied. Theoretical calculations are performed in the framework of the effective-mass and non-parabolic-band approximations. The Ogg-McCombe Hamiltonian is used for the conduction-band electrons in the semiconductor heterostructure, and the Landé g<FONT FACE=Symbol>^</FONT>-factor theoretically evaluated is found in good agrement with available experimental measurements.

Resumo em Inglês:

The hydrostatic-pressure and applied electric field dependencies of the binding energy and the optical-absorption spectra, associated with transitions between the n = 1 valence subband and the donor-impurity band, in symmetrical and asymmetrical GaAs-Ga1-xAl xAs double quantum-well structures are calculated using a variational procedure within the effective-mass approximation. Results are obtained for different well and barrier widths, shallow-donor impurity positions, hydrostatic pressure, and applied electric field.

Resumo em Inglês:

The calculation of the electronic energy levels of n-type delta-doped quantum wells in a GaAs matrix is presented. The effects of hydrostatic pressure on the band structure are taken into account specially when the host material becomes an indirect gap one. The results suggest that under the applied pressure regime the GaAs can support two-dimensional conduction channels associated to the delta-doping, with carrier densities exceeding 10(13) cm-2.

Resumo em Inglês:

The effect of a dc electric field on the temporal evolution of an electron in a double rectangular quantum dot is explored in this work. In the framework of the effective mass approximation, first-order scattering rates for interaction between confined electron-''free'' electron and electron-longitudinal acoustic phonon at room temperature are calculated in the high tunneling regime, and used to evaluate the dynamics of the population and coherence in the first three confined levels under a short ac electric field pulse. Small values of these rates dependent upon the bias field make feasible the emission of coherent radiation at terahertz range.

Resumo em Inglês:

In this work, the exchange energy J for a system of two laterally-coupled quantum dots, each one with an electron, is calculated analytically and in a detailed form, considering them as hydrogen-like atoms, under the Heitler-London approach. The atomic orbitals, associated to each quantum dot, are obtained from translation relations, as functions of the Fock-Darwin states. Our results agree with the reported ones by Burkard, Loss and DiVincenzo in their model of quantum gates based on quantum dots, as well as with some recent experimental reports.

Resumo em Inglês:

The ground state energies of off-axis negatively charged donors in axially symmetrical quantum dots, with different shapes but in all cases with a small height-to-base radius aspect ratio are calculated in adiabatic approximation by using the Hylleraas-type trial function. The dependencies of the neutral and negative donor binding energies and their ratios on the base radius in the pyramid, lens and disk are calculated and compared with previously obtained results for the spherical quantum dot. We also present the contour plots of the binding energies of the neutral and negative donors with different positions along a vertical cross section in the middle of the quantum dots.

Resumo em Inglês:

We analyze the spectrum of a neutral donor located inside or outside of a finite-barrier toroidal-shaped nanoring whose radius is much larger than the height. We derive a one-dimensional wave equation which describes the low-lying donor levels corresponding to the slow electron motion along the ring, by using the adiabatic approximation. Numerical solution of this equation has been obtained by using the trigonometric sweep method. The dependence of the energy spectrum on the donor position, radius and height ring has been studied.

Resumo em Inglês:

Using density functional (DF) calculations and Monte Carlo (MC) simulations we have investigated the main electronic and structural properties of silicon interacting with single walled carbon nanotubes. We have investigated the silicon adsorption externally and internally in the nanotubes surface. The total energies calculations and charge density plot present that the adsorption is most stable externally with silicon forming four bonds with the C atoms and the Si-C bond distances are similar to the ones in the SiC crystal. When silicon is adsorbed in a semiconductor nanotube, a new occupied electronic level inside the band gap is observed. For the metallic tube, the electronic silicon levels are close to the Fermi energy, increasing the metallic tube character.

Resumo em Inglês:

We analyze the effect of the potential shape on the ground state energy of the off-axis neutral donor in GaAs/Ga1-xAl xAs cylindrical nanotube in the presence of the uniform magnetic field applied along the symmetry axis. To take into account the mixing of the low lying subbands we express the wave function as a product of combination of 1s and 2p x,y wave functions with an unknown envelope function that depends only on electron-ion separation. By using variational principle and the functional derivative procedure we derive a one-dimensional differential equation for the envelope function, which we solve numerically by using of the trigonometric sweep method. Results of calculation of the ground state binding energy dependencies on the distance from the donor position to the axis and on the strength of the external magnetic field for square-well, soft-edge-barrier and parabolic bottom potentials are presented. It is shown that the additional peaks in the curves of the density of impurity states appear due to the presence of the repulsive core is nanotube.

Resumo em Inglês:

Multiwall Carbon Nanotubes (MWCNTs) were synthesized by Chemical Vapor Deposition (CVD). Two different procedures were used to grow MWCNT films roughly, aligned in the direction normal to the SiO2/Si(111) substrate. Inverse Photoemission Spectroscopy measurements, on these samples, show the existence of resonances which could be traced back to a flat graphene sheet. The unoccupied valence band is fairly similar to that shown by graphite except by an additional intensity in the vicinity of the Fermi level. This resonance could be interpreted both as tubes tips end effects or van Hove singularities in the density of states.

Resumo em Inglês:

A theoretical discussion of electronic and transport properties of a particular family of double-wall carbon nanotubes, named commensurate structures of the armchair type (n,n)@(2n,2n) is addressed. A single pi-band tight binding hamiltonian is considered and the magnetic field is theoretically described by following the Peierls approximation into the hopping energies. Our emphasis is put on investigating the main effects of the geometrical aspects and relative positions of the tubes on the local density of states and on the conductance of the system. By considering intershell interactions between a set of neighboring atoms on the walls of the inner and outer tubes, we study the possibility of founding Aharonov-Bohm effects in the DWCNs when a magnetic field is applied along the axial direction.

Resumo em Inglês:

The localization properties of the single-particle and collective electron excitations were investigated in the intentionally disordered GaAs/AlGaAs superlattices by weak field magnetoresistance and Raman scattering. The localization length of the individual electron was found to be considerably larger than that one of the collective excitations. This suggests that the disorder has weaker effect on the electrons than on their collective motion and that the interaction which gives rise to the collective effects increases localization.

Resumo em Inglês:

The formation of the miniband electron energy structure was explored in doped InGaAs/InP superlattices with different periods. The analysis of the Raman data allowed us to conclude that in spite of the defect structure of the layers constituting the superlattices, their super-periodicity was well defined. The quantitative proof of the conditions for break-down of the Raman selection rules is presented: the emergence of the selection rules of the coupled plasmon-LO phonon vibrations was demonstrated to occur due to the increase of their coherence lengths. In addition, the expected anisotropy of the effective electron masses was found by high-field magnetoresistance.

Resumo em Inglês:

In the present work, by means of a phenomenological model, we simulate the hysteresis loop of an hexagonal array of Ni nanowires. Our model is based on the assumption that the hysteresis loop of a single wire is a rectangular box with a particular value of the coercive field, and the effect of the array is to generate a distribution of the coercive fields. Our results are in good agreement with experimental data.

Resumo em Inglês:

We calculate the conductance of three-dimensional semiconductor quantum wires considering different effective masses in the contacts and in the channel. We show that, with respect to the case with equal masses in the channel and in the contacts, the amplitude of the conductance oscillations increases if the electron effective mass in the channel is larger and decreases if it is smaller than in the contacts. Effects on the density of probability are also shown. These effects of the effective mass discontinuity are explained in terms of kinetic confinement and transmission coefficient modulation.

Resumo em Inglês:

In this article we study the electronic transport through a triple quantum-dot molecule parallel-coupled to leads under a magnetic field. Analytical expressions are obtained for both the conductance and total density of states for the molecule in equilibrium at zero temperature. As a result of quantum interference of resonances belonging to different channels, this configuration exhibits bound states in the continuum (BICs). We examine the broadenings of the molecular states around the conditions under which BICs occur, finding long-lived states extremely robust under variations of the magnetic flux.

Resumo em Inglês:

The slave boson (SB) technique is employed to study the Zeeman spin splitting in a quantum dot. Unlike traditional SB method, each spin is renormalized differently. Two geometries are compared: side connected and embedded. In both cases, it's shown a non linear behavior of the splitting as a function of the magnetic field applied. The results are in line with the latest experiments.

Resumo em Inglês:

The conductance of two interacting dots connected to leads is studied. The configuration is such that one dot is embedded into the leads while the other is tunneling-coupled only to the first dot. The effect of the tunneling interaction strength on the conductance is discussed. As the two dot levels cross the Fermi level the low temperature conductance of the system cancels out, due to interference effects. This cancellation persists over a range of gate potential that depends upon the interaction strength: the greater the interaction the larger the range of gate potential where the current vanishes.

Resumo em Inglês:

We study the scattering phase shift of the Kondo assisted transmission through a quantum dot (QD), considering a model that includes an additional non resonant channel transmission. To compute the phase evolution and the transmission amplitude of the QD, for different temperatures, we describe the QD employing the single Anderson impurity model in the limit of infinity Coulomb repulsion, within the X-boson approach. Our results are consistent with the development of an unusually large phase evolution at around pi in the Kondo valley, observed in recent experiments, and is consistent with others theoretical treatments.

Resumo em Inglês:

We calculate the spin gap of homogeneous and inhomogeneous spin chains, using the White's density matrix renormalization group technique. We found that the spin gap is related to the ration between the spin velocity and the correlation exponent. We consider a spin superlattice, which is composed of a repeated pattern of two spin-¹/2 XXZ chains with different anisotropy parameters. The behavior of the charge velocity as a function of the anisotropy parameter and the relative size of sub-chains was investigated. We found reasonable agreement between the bosonization results and the numerical ones.

Resumo em Inglês:

In the framework of the Ginzburg-Landau (GL) theory in the limit of a GL parameter much larger than unity, we study theoretically the critical behavior of a mesoscopic superconducting ring with negligible width in the presence of a magnetic field applied perpendicularly to the ring plane. It is assumed that the inner ring edge is in contact with a material whose properties are accounted in the de Gennes boundary condition with a parameter b (de Gennes extrapolation length), while the outer edge is in contact with vacuum. Special attention is devoted to the influence of the different materials contacting the inner ring edge of the superconductor on the features of the phase diagram, as well as the effect of the confinement on such thermodynamic property.

Resumo em Inglês:

We study the effect of tunneling on the electronic structure of two vertically coupled quantum rings within the spin density functional theory. The ground state configurations of the coupled rings are obtained for a system with 10 electrons as a function of the ring radius and the inter-ring distance. For small ring radius, our results recover those of coupled quantum dots. For large ring radius, new ground state configurations are found in the strong tunneling regime.

Resumo em Inglês:

A problem of two electrons spatially separated in vertically coupled one-dimensional rings is solved exactly by using the numerical trigonometric sweep method. The change of the level-ordering and the crossover of the curves of the energy levels as a functions of the rings radii, the separation between rings and the magnetic field, applied along the axis, are found and discussed. As the distance between rings tends to zero our results are in an excellent agreement with those obtained previously for the single two-electron one-dimensional ring.

Resumo em Inglês:

Using a variational procedure within the effective-mass approximation, we have made a theoretical study of the effects of hydrostatic pressure and applied electric fields on the binding energy of a shallow-donor impurity in square-transversal section GaAs-Ga0.7Al0.3As quantum-well wires. The electric field is applied in a plane of the transversal section of the wire and many angular directions are considered. The hydrostatic pressure has been considered both in the direct and indirect gap regime for the Ga0.7Al0.3As material. For the potential barrier that defines the wire region, we consider an x-dependent finite and y-dependent infinite model. The results we present are for the impurity binding energy and considering different values of the wire dimensions, hydrostatic pressure, applied electric field, and the impurity position in the transversal section of the wire.

Resumo em Inglês:

Thin-film ZnO/CdS/CuIn1-xGa xSe2 solar cells are manufactured with ~20% solar-cell conversion efficiency. The CuIn1-xGa xSe2 (CIGS) absorber exhibits rather different physical properties compared with conventional semiconductors (e.g. Si, GaAs and ZnSe). For instance, (i) the valence-band maximum in CIGS consists of cation-d-anion-p hybridized states. (ii) Cation vacancies have low formation energies and high mobility at room temperature. (iii) The most stable surface of CuIn1-xGa xSe2 is the reconstructed, and otherwise polar, (112) surface. (iv) Solar cells with absorbers containing grain-boundaries outperform cells with crystalline absorbers. In this work, the fundamental physical properties of CIGS, like the electronic structure, the defect formation energies, as well as surface properties are discussed from a theoretical perspective.

Resumo em Inglês:

Nontraditional approach to explain the thermoelectric cooling is suggested . It is based on the Le Chatelier-Braun thermodynamic principle. New effect of cooling and heating of junction of two materials (barrierless thermoelectric cooling) is theoretically predicted, and this effect is different from the Peltier effect (barrier thermoelectric cooling). The suggested thermoelectric effect must be displayed always at the finite values of the junction surface heat conductivity eta. Barrierless thermoelectric effect occurs even in the case when the conducting materials are identical with the same Peltier coefficients. It is shown that both barrier and barrierless thermoelectric cooling effects always exist simultaneously in the general case. The reasons proving reversibility of the thermoelectric cooling process are resulted.

Resumo em Inglês:

We present a description of the effect of the surface roughness on the energy straggling associated to the energy loss distributions of protons transmitted through a self supported metallic thin foil. For this purpose we prepared a polycrystalline gold thin films using the standard sputtering method with different deposition rates. The statistics of the surface height distribution induced in these thin films were determined using Atomic Force Microscopy. The measured surface roughness allowed us to quantify the ion energy loss straggling in these samples for different deposition parameters and as a function of the incident ion energy.

Resumo em Inglês:

The two-photon absorption process is a nonlinear phenomenon that shows very low optical efficiency in bulk semiconductors. For this reason the detection of these processes becomes very difficult from the experimental point of view. Nevertheless, when dealing with semiconductor QD's with few nanometer radii, these transitions are enhanced and the detection is possible. In this contribution we present analytical calculations for the absorption coefficient in CdSe spherical QD's subjected to these second order processes, as a function of the characteristic dot parameters. The intensities of the absorption peaks, as well as the statistical treatment involving QD's ensembles, are also reported.

Resumo em Inglês:

We consider the dynamical properties of a simple model of vibrational surface modes. We obtain the exact spectrum of surface excitations and discuss their dynamical features. In addition to the usually discussed localized and oscillatory regimes we also find a second phase transition where the surface mode frequency becomes purely imaginary and describes an overdamped regime. Noticeably, this transition has an exact correspondence to the oscillatory - overdamped transition of the standard oscillator with a frictional force proportional to velocity.

Resumo em Inglês:

We study the influence of the interfaces on the dispersion relation, energy and power flow of polaritons propagating in coaxial cylinders. We consider an infinite coaxial cylinder of internal and external radii designated by a and b, respectively, submitted to a magnetic dc field applied parallel to the z-axes. The presence of a magnetic field causes significant alterations in the modes of propagation of polaritons. The numerical results are obtained for the surface polaritons propagating in semiconductors cylinders of GaAs in presence of magnetic fields of 5 kG.

Resumo em Inglês:

In this work we study the surface scattering mechanisms from rough surfaces of a multimode quasi-1D waveguide (conducting quantum wires). The square-gradient scattering mechanism, which was missed in existing studies of the transport through surface-corrugated waveguides, is discovered. The main attention is paid to the interplay between the new mechanism and the known one, as well as its effect on the waveguide transport properties. For any value of the roughness height sigma, the square-gradient terms in the expression for the wave-scattering length (electron mean-free path) are dominant, provided the correlation length Rc of the surface disorder is small enough.

Resumo em Inglês:

Photon stimulated ion desorption (PSID) from condensed carbon dioxide has been studied for photon excitation energies ranging from 93 to 193 eV. PSID studies have been performed at the Brazilian synchrotron light source (LNLS), Campinas, during a multi-bunch operation mode of the storage ring. The results showed that after photon excitation several ions desorbed from the CO2 films: C+ , O+ , CO+ and O2+. PSID experiments showed that ion desorption was enhanced only at the Si resonance excitations. When the thickness of the CO2 was ~ 500 L or higher, almost no desorption yield was observed. The study of the dependence of the relative partial ion yield on the photon excitation showed that the X-ray induced Electron Stimulated Desorption (XESD) mechanism has to be invoked to explain the origin of the desorbed ions in the energy region studied.

Resumo em Inglês:

The structural and optical properties of nanocrystalline GaN and GaN:H films grown by RF-magnetron sputtering are focused here. The films were grown using a Ga target and a variety of deposition parameters (N2/H2/Ar flow rates, RF power, and substrate temperatures). Si (100) and fused silica substrates were used at relatively low temperatures (Ts < 420K). The main effects resulting from the deposition parameters variations on the films properties were related to the presence of hydrogen in the plasma. The X-ray diffraction analysis indicates that the grain sizes ( ~ 15nm) and the crystallized volume fraction significantly decrease when hydrogen is present in the plasma. The optical absorption experiments indicate that the hydrogenated films have absorption edges very similar to that of GaN single crystal films reported in the literature, while the non-hydrogenated samples present larger absorption tails encroaching into the gap energies.

Resumo em Inglês:

Mo thin films have been deposited using a DC magnetron sputtering system with an S-gun configuration electrode and characterized electrically and morphologically. The influence of the sputtering gas pressure and glow discharge (GD) power, on the electrical resistivity of Mo thin films and on the contact resistivity of Mo to Cu(In,Ga)Se2 (CIGS) films was determined through an exhaustive parameter study. This study also allowed us to find the conditions to deposit Mo films with suitable properties for its use as back contact of solar cells based on CIGS. Resistivities smaller than 1x10-4 omega.cm and contact resistivities smaller than 0.3 omegacm² were found. Mo films with these characteristics are suitable for back contacts in solar cells based on CIGS. It was also found that the Mo thin films, deposited by DC magnetron sputtering on CIGS thin films, act effectively as ohmic contacts. The main contribution of this work was to obtain Mo thin films with adequate properties to be used as back contact for CIGS based solar cells using a DC sputtering system with S-gun configuration electrode, which allows growing the film with better surface quality and at a higher deposition rate than those deposited using the conventional planar RF sputtering system.

Resumo em Inglês:

Adherent and low-stress a-C:H films were deposited on Ti6Al4V and stainless steel substrates using PECVD and IBAD techniques. An amorphous silicon interlayer was applied to improve the adhesion of the a-C:H films on the metal substrates. The XPS technique was employed to analyze the chemical bondings within the interfaces. The elemental composition and atomic density of the films were determined by ion beam analysis. The film microstructure was studied by means of Raman scattering spectroscopy. The mechanical properties were determined by means of stress and hardness measurements. The adherence was evaluated by means of scratch tests. The tests showed that the composition, the microstructure, and the mechanical properties of the films depend on the intensity of the ion bombardment and on the ion current.

Resumo em Inglês:

Ti-6Al-4V samples have been treated by PIII processing at different temperatures (400-800 º C), treatment time (30-150 min) and plasma potential (100 and 420 V). Hardness measurements results showed an enhancement of the hardness for all implanted samples. XRD results detected the Ti2N phase and the best corrosion resistance was found for the samples processed at higher temperature and lower PIII time.

Resumo em Inglês:

In this work, the influence of the substrate bias on the crystalline structure and surface composition of Ti6Al4V thin films prepared by rf magnetron sputtering were studied. Samples were grown onto two different types of substrates: AISI 420 steel and common glass using a Ti6Al4V (99.9 %) target. Substrate bias was varied from -100V to -200 V. Samples were characterized by X-ray diffraction (XRD), Energy Dispersive X-ray Analysis (EDX), Scanning Electron Microscopy (SEM), and X-Ray Photoelectron Spectroscopy (XPS). It was observed that the increase of the substrate voltage improved the crystallinity of the deposited films. The stoichiometry of the deposited thin films was studied by EDX and found to be slightly different from that of the target material. Finally, the passive film spontaneously formed on the deposited films upon exposure to the laboratory atmosphere was studied by XPS. The composition of the passive film is rather complex since it contains several forms of oxidized titanium and vanadium as well as Al2O3.

Resumo em Inglês:

Titanium surface was chemically modified with sodium hydroxide, sodium silicate and calcium hydroxide with the purpose of improving metal prosthesis to bone adhesion. Results indicate that Ti modified plates immersed in Hanks solution exhibit increased calcium adsorption in comparison to untreated titanium. Calcium silicotitanate, beta-tricalcium phosphate and apatites were detected on the coating.

Resumo em Inglês:

ZrO2-3 mol % Y2O3-coated AISI 310 stainless steel was nitrided using plasma produced by dc pulsed discharge in a mixture of N2 and H2 at an equilibrium temperature of 450ºC. Profile chemical analyses with GDOS (glow discharge optical spectroscopy) and XRD showed the formation of Nitrogen/Y-ZrO2 solid solution. Nitrogen atoms enter in the zirconia substituting to the oxygen atoms in the network. Although, the nitrogen content measured in zirconia coating is greater than the allowed maximum nitrogen content in doped-zirconia solid solution, the formation of oxynitrides could not be detected. The phase structure was investigated by X-ray diffractometry The mechanical properties were studied by means ultramicrohardness indentation tests. The nitruration improves the mechanical responses of the ZrO2-3 mol% Y2O3 -coated AISI 310 stainless steel.

Resumo em Inglês:

The study of dental implant surfaces is relevant in order to better understand the interaction of the titanium surface and the surrounding tissues. Clinical success is achieved not only because of implant material but also because of other properties as implant design, surface treatment and quality. In this work, we report a detailed surface investigation of three major Brazilian made implants and compared them to a world-known implant. The surface composition and morphology were investigated by using X-ray photoelectron spectroscopy and scanning electron microscopy. Results of a biological test implemented in order to verify the bioactivity of the implant surface are also presented.

Resumo em Inglês:

Strontium Bismuth Tantalate (SBT) ferroelectric thin films have attracted considerable attention for the development of non-volatile ferroelectric random access memories (NV-FRAMs). These films, however, have a critical problem: a high processing temperature ( > 700ºC) is required for the crystallization of the perovskite phase. The thermal evolution of the SBT films prepared by Chemical Solution Deposition (CSD) techniques showed the formation of an intermediate oxygen-deficient fluorite phase at ~ 550ºC. The SBT perovskite phase crystallizes at higher temperatures. To favor an earlier perovskite crystallization, SBT thin films were implanted with oxygen ions pulses produced by a Plasma Focus (1 kJ). The samples were annealed at different temperatures in oxygen atmosphere and characterized by GI-XRD and Atomic Force Microscopy techniques. It was found that oxygen addition into the SBT structure promotes a better crystallization of the perovskite phase.

Resumo em Inglês:

A pure lattice model for Langmuir monolayers is presented where only nearest neighbor interactions are considered. The flexibility of hydrophobic tails laying on the surface is taken into account but the segments above the surface are taken as upstanding rigid rods without contribution to the entropy. The numerical calculations show that to obtain more than one phase transition the flexibility of these tails has to be taken into account.

Resumo em Inglês:

Thin films of indium oxide 200 nm thick and indium nitride 150 nm thick were produced by reactive sputtering deposition onto soda lime substrates. The Indium cathode was kept under vacuum attached to a high voltage dc. The In xOy films were obtained in argon-oxygen mixture with total pressure between 2 and 7 Pa, current density between 0.04 and 0.56 mA/cm² and substrate temperature of 300 K. The In xNy films were obtained in argon-nitrogen mixture with total pressure between 1 and 5 Pa, current density between 0.18 and 14 mA/cm² and substrate temperature of 320 K. Results revealed that thin films give rise to high conductivity, are transparent and have a carrier density of 10(19) cm- 3 for In xOy and 10(17) cm- 3 for In xNy. These results were obtained mainly from temperature dependent alpha and sigma measurements. The reduced chemical potential was calculated considering three different scattering mechanisms i.e. interaction with optically polarized phonons, interaction with ionized impurities and interaction with grains, in order to identify the main scattering mechanism, which resulted to be due to impurities.

Resumo em Inglês:

Porous Silicon thin films were produced in this work by the electrochemical anodizing method. The samples were fabricated anodizing p type Si substrates with different resistivities in hydrofluoric acid. Samples were prepared at different current densities. The films were characterized through reflectance and room photoluminescence (PL) measurements in the visible region. A simple theoretical model is proposed to calculate the Optical constants of the material using a theoretical simulation of the reflectance spectra. The maximum positions of PL signals are shifted to long wavelengths as the porosity increase in samples prepared with substrates of resistivities of 0.1 and 1 omega-cm, while an opposite behaviour is observed in samples prepared on more doped substrates. The optical constants obtained through the calculation are the spectral variation of the refractive index and extinction coefficient, also are obtained the sample thickness and surface roughness.

Resumo em Inglês:

R6G dye with gold nanoparticles was prepared in water solution. Using a thermal lens experimental setup, the thermal diffusivity of R6G in presence of gold nanoparticles with Au different concentration was measured. The results show that, the thermal diffusivity of R6G dye mixed with gold nanoparticles increases with the increase of particle metallic concentration, enhancing the thermal diffusivity in the solution. The thermal diffusivity of the samples was studied for a constant concentration of R6G dye 0.1 g/L. The diffusivities were obtained by using the thermal lens aberrant model with lasers arranged in the mismatched mode. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression, for transient thermal lens, to the experimental data. UV-Vis spectroscopy and TEM were used to characterize the R6G dye gold nanoparticles.

Resumo em Inglês:

We apply the complete nonlinear Time Dependent Ginzburg Landau (TDGL) equations to study the vortex dynamics in a mesoscopic type II superconductor using the numerical method based on the technique of gauge invariant variables. The solution of these equations shows how the vorticity penetrates into and goes out of the superconductor through the surface boundary. We calculate the spatial distribution of the superconducting electron density and the phase of the superconducting order parameter in a mesoscopic superconducting square sample containing two holes in the presence of a uniform perpendicular magnetic field. The dynamics of different vortex states are studied as a function of the external magnetic field.

Resumo em Inglês:

With a view to contribute to the understanding the surface effects on optical properties process, and its hole in the electronic properties of the nanoparticles, CdS based nanoparticles are characterised by different experimental techniques and the experimental results compared to density functional theory calculations. Our results indicate that cubic CdS nanoparticles present a strong structural deformation, hexagonal reconstructed structures preserve their lattice behaviour. Both cubic and hexagonal CdS nanoparticles are S-terminated after relaxation, even when mildly Cd-rich nanoparticles are considered. A broad peak observed in our PL measurements is interpreted as an experimental evidence of the surface related peak observed around 1.8 eV in our calculated DOS for the hexagonal relaxed structure.

Resumo em Inglês:

We investigate the effect of the hydrogen intentional incorporation on the structural properties of the amorphous gallium arsenide prepared by rf-magnetron sputtering technique. The properties of the non-hydrogenated films are: band gap of 1.4 eV (E04), Urbach energy of 110 meV, stoichiometric composition ([As]/[Ga] = 0.50), and dark conductivity of about 3.2 x 10-5 (omega.cm)-1. Hydrogen was incorporated in the films by the introduction of an electronically controlled H2 flux during deposition, keeping constant the other deposition parameters. It was observed that small hydrogen incorporation produces a great change in the structural properties of the films. The main changes result from the formation of GaAs nanocrystals with mean sizes of about 7 nm into the amorphous network.

Resumo em Inglês:

The effects of crossed electric and magnetic fields on the electronic and exciton properties in semiconductor heterostructures have been investigated within the effective-mass and parabolic band approximations for both bulk GaAs and GaAs-Ga1-xAl xAs quantum wells. The combined effects on the heterostructure properties of the applied crossed electric/magnetic fields together with the direct coupling between the exciton center of mass and internal exciton motions may be dealt with via a simple parameter representing the distance between the electron and hole magnetic parabolas. Calculations lead to the expected behavior for the exciton dispersion in a wide range of the crossed electric/magnetic fields, and present theoretical results are found in good agreement with available experimental measurements.

Resumo em Inglês:

Highly transparent and conductive thin films of SnO2:F and In2O3:Sn (ITO) have been prepared on glass substrates using the simple pyrolitic (spray) method. Through an exhaustive parameter study and using as diagnostic method for the film quality a figure of merit defined as a function of both, the transmittance and the electric resistivity, the conditions to prepare the SnO2:F and ITO films, with adequate properties to be used as transparent front contact for solar cells, were achieved. A relevant contribution of this work is related with the deposition of SnO2:F and ITO films with the mentioned characteristics, using a solution synthesized in our laboratory by dissolving the precursor metals in HCl. Transparent conducting oxide (TCO) thin films were obtained, with transmittances greater than 80% and resistivities smaller than 7x10- 4 omega·cm, results which are comparable with those obtained using commercial reactants. Results concerning the influence of the synthesis parameters on the optical, electrical and structural properties of the TCO films are reported.

Resumo em Inglês:

Polycrystalline SnS thin films were grown on glass substrates using a novel procedure involving a chemical reaction between the precursor species evaporated simultaneously. This is a relatively new material, which exhibits excellent properties to be used as absorbent layer in solar cells. X-ray diffraction (XRD) measurements indicate that the synthesized samples grow in several phases (SnS, SnS2 and Sn2S3) depending upon the deposition conditions. However, through an exhaustive parameter study, conditions were found to grow thin films predominantly in the SnS phase with orthorhombic structure. It was found that this type of compound presents p-type conductivity, a high absorption coefficient (greater than 10(4) cm-1) and an energy band gap Eg of about 1.3 eV, indicating that this compound has good properties to perform as absorbent layer in thin film solar cells.

Resumo em Inglês:

This work presents results related with phase identification and study of the homogeneity in the chemical composition of Cu(In,Ga)Se2 (CIGS) thin films grown by a chemical reaction of the precursor species evaporated sequentially on a soda-lime glass substrate, in a two or three-stage process. For that, the CIGS samples were characterized through X-ray diffraction (XRD) and Auger Electron Spectroscopy (AES) depth profile measurements, respectively. The presence of secondary phases growing superficially was identified through small angle XRD measurements. Theoretical simulation of the XRD spectra, carried out with the help of the PowderCell package, confirmed this result. The results showed that the deposition conditions affect the homogeneity of the chemical composition of the CIGS films, as well as the phase in which these films grow. In general, the samples grown in a two stage process present a mixture of the Cu(In,Ga)Se2 phase with the secondary In2Se3 and Cu2Se phases, whereas, the films grown in a three stage process do it, mainly, in the Cu(In,Ga)Se2 phase with a tetragonal (chalcopyrite type) structure and better homogeneity. CIGS films with characteristics like those found for the three stage samples, have demonstrated good properties for its use as absorber layers in thin film solar cells.

Resumo em Inglês:

Cadmium sulphide (CdS) thin films deposited on indium tin oxide (ITO) substrates were prepared by the chemical bath deposition technique at different values of pH and pNH3. This was made in order to analyze the influence of the initial chemical composition of the bath on some physical properties of the final CdS films, used as optical windows for solar cells. The mechanism proposed in the literature for this deposition involves the tetra-ammonium cadmium (II) complex, Cd(NH$_{3})_{4}^{2+}$, at the start of the reaction. Following this mechanism, the change in the concentration of Cd(NH$_{3})_{4}^{2+}$ gives a variation in the rate of deposition with the corresponding modification in the quality of the film. Therefore, the predominance zone diagram of Cd2 + species in solution as a function of the values of pH and pNH3 was used to analyze the reasons why the CdS/ITO thin films are favored in just a narrow range of ammonium and hydroxide concentrations. To obtain useful films, the results showed that low ammonium concentrations must be avoided at high pH values as well as high ammonium concentrations at lower pH values.

Resumo em Inglês:

Cadmium sulphide (CdS) thin films deposited on indium tin oxide (ITO) substrates were prepared by chemical bath deposition technique by using different conditions to agitate the bath and the substrate. For substrate, an oscillating device working at 37 Hz was adapted to support each ITO substrate in order to agitate the substrate during deposition, meanwhile the chemical solution is heated and/not agitated. The deposited films were characterized on their morphology, on the band gap energy, and on thickness. The implemented novel technique for substrate oscillation has shown to improve the films quality, by the chemical bath without colloidal precipitates and by the clean film surfaces obtained. CdS films with variable deposition time can be achieved depending on the agitating technique. The mean band gap energy obtained around 2.41 eV is similar to the typical value reported in the literature for this material. In addition, by oscillating only the substrate during deposition is possible to obtain clean films and avoid the formation of colloidal precipitates on the chemical bath, normally presented when it is magnetically or ultrasonically agitated.

Resumo em Inglês:

A novel synthesis method was employed to obtain a new family of lead-free compounds with the general formula: Ba1-yLn2y / 3Ti0.91Zr0.09O3 (Ln= lanthanide element). The thin films were deposited by RF-magnetron sputtering under high-oxygen pressure on different substrates at 873 K. The crystalline phases were studied via x-ray diffraction, showing the 001 epitaxial reflections corresponding to perovskite single-phase compounds. The films revealed high homogeneity and stoichiometries corresponding to BLnZT (Ln=Nd, La). Deposited thin films showed a smooth surface. Ferroelectric measurements through hysteresis curves were obtained in Ba0.90La0.067�0.033Ti0.91Zr0.09O3, capacitor structures showing clear ferroelectric behavior with Pr, Ps and Ec of 11.9 µC/cm², 36.8 µC/cm² and 38.6 kV/cm, respectively.

Resumo em Inglês:

A dynamic scaling and kinetic roughening study was done on digitized atomic force microscope (AFM) images of Pb(Zr0.52,Ti0.48)O3 (PZT) thin films. The films were grown on Si(001) and Nb - SrTiO3(001) (STNO) substrates via rf-sputtering technique at high oxygen pressures at substrate temperatures of 600 ºC by varying the deposition time and keeping other growth parameters fixed. By using a specific self-designed algorithm, we can extract from digitized 512-pixel resolution AFM-images, quantitative values of roughness parameters, i.e., interface width (sigma(l)), lateral correlation length (xi||) and, roughness exponent (alpha). Herein, we report on the sputter-time deposition dependence of the parameters describing roughness for both kinds of substrates. We report alpha-values for different time depositions (between 15 and 60 minutes) close to 0.55 for Si substrates and 0.63 for Nb - SrTiO3 substrates, indicating that the surface becomes more correlated in STNO substrates. The alpha-values are associated to the Lai-Das-Sarma-Villain model.

Resumo em Inglês:

We studied growth mechanisms in semiconducting Ga1 - xIn xAs ySb1 - y films grown by liquid phase epitaxy on (100) GaSb:Te (10(17) cm- 3) substrates at 600 ºC solution-substrate temperature. Atomic Force Microscopy (AFM) images of these films show step-like corrugations, undulations, grooves, and terraces. The terraces are a few nm in height and hundreds of nanometers in length. Epitaxial Ga1 - xIn xAs ySb1 - y films grow lattice-matched to the (100) GaSb substrates. These narrow-gap quaternary Ga1 - xIn xAs ySb1 - y compounds are suitable materials for heterostructure devices operating in the infrared wavelength range. The estimated x and y values are in the GaSb-rich zone. The step-like corrugations (bunching steps) are formed by different mechanisms, such as pinning effect probably due to impurities on the substrate surface.

Resumo em Inglês:

We use Monte Carlo and Molecular Dynamics simulation to study a magnetic tip-sample interaction. Our interest is to understand the mechanism of heat dissipation when the forces involved in the system are magnetic in essence. We consider a magnetic crystalline substrate composed of several layers interacting magnetically with a tip. The set is put thermally in equilibrium at temperature T by using a numerical Monte Carlo technique. By using that configuration we study its dynamical evolution by integrating numerically the equations of motion. Our results suggests that the heat dissipation is closed related to the appearing of vortices in the sample.

Resumo em Inglês:

Cobalt ferrite composite thin films, CoFe2O4/SiO2, were prepared by sol-gel process, using tetraethylorthosilicate (TEOS) as a precursor for silica, and metallic nitrates as precursors for ferrite. The obtained dip-coated films thermally treated at 550, 750, and 950 ºC were transparent, homogeneous and adherent. Their magnetic properties were measured from 2.5 K to 300 K using a SQUID magnetometer. Superparamagnetic behaviour was observed at room temperature and below a blocking temperature appears with coercivity values increasing with annealing temperature.

Resumo em Inglês:

Crystalline films of pure titanium oxide have been prepared on soda lime slide glasses by sol-gel process and dip-coating. The definition of various parameters such as chemical concentration, viscosity, catalyst type and withdrawal speed led to the preparation of transparent, crystalline and adherent coatings with hydrophobic characteristics. Their crystalline structure was evaluated as anatase phase by low angle X-ray diffraction. Thicknesses were measured by perfilometry, and the refractive indices were determined from transmittance spectrum taking into consideration the layers deposited onto the two sides of the substrate. Porosity was also estimated by UV-visible spectroscopy by using the Lorentz - Lorenz equation. The average grain size was evaluated by atomic force microscopy. The thicker and denser films presented hydrophobicity, which decreased when the film porosity increased.

Resumo em Inglês:

Thermal fluctuations in the electric conductivity of YBa2Cu3O7 - delta superconducting thin films grown on Sr2YSbO6 novel substrate materials in the film form were experimentally studied. YBa2Cu3O7 - delta films were grown by using a dc-technique on Sr2YSbO6 substrates, which were produced by rf magnetron sputtering. X-ray diffraction analysis evidenced that the Sr2YSbO6 films grow on conventional SrTiO3 substrates in a preferential orientation along the (100) planes direction with lattice parameter a=4,43() Å. The YBa2Cu3O7 - delta thin films, grown on Sr2YSbO6 films, exhibit an oriented growth in the (001) crystallographic direction, with lattice constant c=11,65(9) Å. Morphological characterizations were performed by means atomic force microscopy. Experiments of electrical resistivity show that the YBa2Cu3O7 - delta films present a normal-superconducting transition with critical temperature Tc=82,33 K. Fluctuation analysis for the YBa2Cu3O7 - delta thin films were performed by utilizing the concept of logarithmic derivative of the conductivity excess. Above the critical temperature Tc we experimentally determine the occurrence of Gaussian 3D, 2D and fractal fluctuation regimes. A genuinely critical region identified by the exponent lambdaCR=0,35 were obtained close to Tc. This critical regime is effectively described by the 3D-XY model.

Resumo em Inglês:

Through the solution of the Bogoliubov de Gennes equations we analyze the effect of different symmetries of the pair potential on the quasiparticle energy spectrum in SNINS junctions (S: superconductor, N: normal metal and I: Insulator). We find that the energy levels are strongly affected by the symmetry of the pair potential, the width of each normal metal (a and b) and the strength of the insulating barrier. The energy levels equation generalizes previous results in SNS and SIS junctions. The energy dispersion relation depends on the phase difference of the pair potential. The Josephson current is related to the Andreev levels; when a = b ~ xi0 and T << Tc this current is approximately 1/2 of the Josephson current transported in an SIS junctions. In general, we find that for d xy symmetries there is always a zero energy state independent of the value of Z, a and b.

Resumo em Inglês:

Through the analytic solutions of the Bogoliubov de Gennes (BdG) equations the effect of a static and homogeneous magnetic field applied parallel to the interface of an NIS (N: Normal metal, S: superconductor and I: Insulator) junction on the differential conductance is calculated. For a d xy - symmetry we obtain zero bias conductance peak that can be split by a magnetic field. The shift of the zero bias conductance peak depends on the spread (beta) of the tunneling electrons in k space, on the magnitude of the applied field H and on the ratio between the Fermi energy of the superconductor and the normal region, E FS/E FN. Finally we estimate the minimum value of the magnetic field, Hmin, that splits the zero bias conductance peak. In general Hmin depends on beta, E FS/E FN, the strength of the insulating barrier Z and on the temperature T.

Resumo em Inglês:

The general boundary conditions for the thermal diffusion equation are obtained. It is shown that in the general case of a nonstationary photothermal process these boundary conditions must include both the surface thermal conductivity and surface thermal capacity. One more parameter, the surface capacity thermal impedance, appears in the boundary conditions when the photothermal process is the thermal wave propagation.

Resumo em Inglês:

We report several ab initio calculations performed over the A2FeMoO6 (A=Ba, Ca) double perovskite. Results show that it is an insulator for the spin up orientation and conductor for the other one. We investigate the electronic structure of A2FeMoO6 by means calculations of density of states for both spin orientations, based on the Density Functional Theory and the Linearized Augmented Plane Waves method. For the exchange correlation potential we chose the Generalized Gradient Approximation since this potential consider the difference between the electronic densities for the two distinct spin orientations from the beginning. The density of states is calculated by the histogram method and the position of the Fermi level is found by integrating over the density of states for both spin orientations. With the calculated densities of states, the half metallic properties of these compounds can be observed with the position of the Fermi level. Our results are in agreement of the Sarma's methodology, who considers a new mechanism for the magnetic interactions responsible for the magnetism on the A2FeMoO6 family. We also calculate the cell dimensions that minimize the total energy for each configuration using the Murnaghan equation state.

Resumo em Inglês:

We report conductivity fluctuation measurements in the La1.82Sr0.18CuO4 high temperature superconducting material. The conductivity fluctuation analysis was performed by utilizing the concept of the logarithmic derivative of the conductivity excess. Close and above the critical temperature Tc, we experimentally determine the occurrence of Gaussian and critical fluctuation regimes. Systematic measurements of conductivity as a function of temperature were performed for several values of transport current j. Fluctuation analysis close to the zero resistivity temperature Tc0 was performed and we clearly identified a characteristic exponent, which is analyzed by the dynamical scaling theory and considering a percolation-like transition in a frustrated and disordered system. The observation of a glass-like behavior in our experiments permits to interpret our results as corresponding to a vortex-glass regime.