Repositório RCAAP

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.

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.

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.

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.

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.

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.

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.

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.

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.

No summary/description provided

The high-quality cosmological data, which became available in the last decade, have thrusted upon us a rather preposterous composition for the universe which poses one of the greatest challenges theoretical physics has ever faced: the so-called dark energy. By focusing our attention on specific examples of dark energy scenarios, we discuss three different candidates for this dark component, namely, a decaying vacuum energy or time-varying cosmological constant [lambda(tau)], a rolling homogeneous quintessence field (phi), and modifications in gravity due to extra spatial dimensions. As discussed, all these candidates [along with the vacuum energy or cosmological constant (lambda)] seem somewhat to be able to explain the current observational results, which hampers any definitive conclusion on the actual nature of the dark energy.

We present the status and plans of the Angra Project, a new reactor neutrino oscillation experiment, proposed to be built in Brazil at the Angra dos Reis nuclear complex. This experiment is aimed to measure theta13, the last unknown of the three neutrino mixing angles. We propose a high sensitivity multi-detector experiment, able to reach a sensitivity to antineutrino disappearance down to sin² 2theta13 = 0.006 in a three years running period, by combining a high luminosity design, very low background from cosmic rays and careful control of systematic errors. We also intend to explore the possibility to use the neutrino detector for purposes of safeguards and non-proliferation of nuclear weapons.

Cosmic microwave background (CMB) studies underpin our understanding of the universe and its history. Until recently, we have relied principally on CMB temperature observations to build our standard cosmological model, but today the field forges ahead into its next frontier - CMB polarization anistropy. Polarization measurements will furnish fresh and independent information on the primordial density perturbations and cosmological parameters, and they offer the exciting potential to detect primordial gravity waves, constrain dark energy and measure the neutrino mass scale. I review the science and long-term goals of CMB polarization measurements and discuss current results and future observational projects. A vigorous program of ground-based, suborbital and space-based (e.g., WMAP and Planck [2008]) experiments is guiding us towards a future space mission dedicated to high precision polarization measurements.

In this talk I describe an interesting relation between Feynman graphs at finite temperature and chemical potential and the corresponding ones at zero temperature. The operator relating the two which we call the "thermal operator", simplifies the evaluation of finite temperature graphs and helps in understanding better several physical questions such as cutting rules, forward scattering, gauge invariance etc at finite temperature.

We start this paper with a historical survey of the Casimir effect, showing that its origin is related to experiments on colloidal chemistry. We present two methods of computing Casimir forces, namely: the global method introduced by Casimir, based on the idea of zero-point energy of the quantum electromagnetic field, and a local one, which requires the computation of the energy-momentum stress tensor of the corresponding field. As explicit examples, we calculate the (standard) Casimir forces between two parallel and perfectly conducting plates and discuss the more involved problem of a scalar field submitted to Robin boundary conditions at two parallel plates. A few comments are made about recent experiments that undoubtedly confirm the existence of this effect. Finally, we briefly discuss a few topics which are either elaborations of the Casimir effect or topics that are related in some way to this effect as, for example, the influence of a magnetic field on the Casimir effect of charged fields, magnetic properties of a confined vacuum and radiation reaction forces on non-relativistic moving boundaries.

We investigate the emergence of time-dependent nonperturbative configurations during the evolution of nonlinear scalar field models with symmetric and asymmetric double-well potentials. Complex spatio-temporal behavior emerges as the system seeks to establish equipartition after a fast quench. We show that fast quenches may dramatically modify the decay rate of metastable states in first order phase transitions. We discuss possible applications in condensed matter systems and in inflationary cosmology.

Data taken with a radio antenna array in combination with the ground-level air shower experiment KASCADE-Grande at the Forschungszentrum Karlsruhe open up the possibility to measure large extensive air showers with this new technique. The pulse height of the observed radio signals scales with the primary energy of the particles initiating the air shower. The dependence of the radio signal on the angle of the shower axis with respect to the Earth's geomagnetic field and the coherence of the radiation suggest that the radio signal generation is due to the geosynchrotron mechanism.

We consider the N-component tri-dimensional massive Gross-Neveu model at finite temperature and with compactified spatial coordinates. We study the behavior of the renormalized large-N effective coupling constant, investigating its dependence on the compactification length and the temperature. We show that spatial confinement exists for the model at T = 0, which is destroyed by raising the temperature.

In this brief review we explicitly calculate the radiative corrections to the Chern-Simons-like term in the cases of zero and finite temperature, and in the gravity theory. Our results are obtained under the general guidance of dimensional regularization.

In the last years, we experienced a complete change of the view of weak interaction physics. Robust results from many experiments as Super-Kamiokande, KamLAND, SNO, K2K, show us that the neutrinos have the remarkable phenomena of oscillations, a quantum interference mechanism that operates to distances as large as 100 km and even bigger distances. From this we know that neutrinos change identity from one flavor to another, as was demonstrated by the joints results of SNO and Super-Kamiokande experiments. We show here the review of latest results of neutrino physics, as for example, the first evidence of neutrinos produced in the core of the earth and the updated results of KamLAND and others. Our understating of all experimental results will completed by the state-of-art of the theoretical effort to understand such phenomena. For the near future, we expect the new generation of precision physics, like the running experiments of MINOS and Double CHOOZ, and the proposals of SADO and ANGRA shed light on unresolved issues such as the CP-violation for neutrinos and the relative magnitude of solar and atmospheric scales.