Repositório RCAAP

We report an investigation of the optical absorption spectrum, using non-polarized light, in Pb xEu1-xTe, x=0 and x=0.095, epitaxial thick layers grown by molecular beam epitaxy (MBE). The absorption edge is described by a broad band, due to the electronic transitions from the 4f7 of Eu2+ to the states in 4f6 5d configuration, as seen previously in bulk Eu chalcogenides. When a magnetic field is applied, a narrow absorption band (full width ~50 meV) emerges from the broad one. The energy of this absorption peak red shifts when the magnetic field increases, and reaches saturation when the Eu2+ attain ferromagnetic arrangement. This behaviour can be described by a localized excitation model with d - f exchange interaction.

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of acetonitrile on the silicon surface. Our first-principles calculations indicate that CH3CN adsorbs via a [2+2] cycloaddition reaction through the C<FONT FACE=Symbol>º</FONT>N group with an adsorption energy around 35 kcal/mol, close to the 30 kcal/mol estimated by Tao and co-workers. The electronic structure and the surface states calculated for the adsorbed system are also discussed.

Main parameters of InGaAs/InAlAs multiple quantum well amplitude modulators, such as contrast ratio, insertion loss and chirp parameter, were calculated in order to find a quantum well structure which optimizes them. The parameters were estimated from the theoretical absorption curves, which were calculated for different values of applied reverse bias and were compared with experimental data. A study of the device parameters as a function of the Ga content and operation voltage was performed. The study showed that optimum values for the modulator parameters cannot be obtained simultaneously. The influence of the residual doping level and the applied pre-bias are emphasized.

This work reports the influence of a dc external magnetic field on the polaritons propagating in cylindrical systems. We present numerical results for the dispersion relation and energy flow for surface polaritons modes propagating in hollow and massive cylinders made of GaAs and InSb.

In this work we studied the optical properties of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) using polymeric blends with polystyrene (PS). The photoluminescence (PL) spectrum of MEHPPV present three characteristic peaks correlating to aggregate electronic transition at 608, 674 and 742 nm at 10K. An addition of PS in MEH-PPV films induces a red shift in the absorption spectrum and the PL spectrum presents a new peak at high energy centred at about 572 nm at 10K. It is attributed to intra band transition between localized intraband states.

The ground state intrasubband pair-correlation function for a quasi-one-dimensional electron gas confined in a GaAs-Al xGa1-xAs parabolic quantum well wire within the Random-Phase Approximation (RPA) is calculated. We have considered two wires with subband energies separation homega = 2:0 meV and homega = 2:5 meV. The dependence of the pair-correlation function on the electronic density was studied and the regions where the RPA approach cannot be used were precisely determined.

We investigate the confined exciton properties in InGaN/GaN cylindrical quantum wires. We have solved numerically Schrödinger-like equations in the effective mass approximation and have found the energy levels and wavefunctions for confined electrons and holes, using these results to calculate variationally the confined exciton energy. A comparison was made between graded and abrupt quantum wires, showing that the existence of smooth interfaces enhances the confined exciton energy if the wire radius is smaller than 50 Å. We conclude that the existence of graded interfaces must be considered for a better description of the exciton based optical properties of InGaN/GaN quantum wires.

We calculate the spin relaxation rates in parabolic quantum dots due to the phonon modulation of the spin-orbit interaction in presence of an external magnetic field. Both, deformation potential (DP) and piezoelectric (PE) electron-phonon couplings are included in the Pavlov-Firsov spin-phonon Hamiltonian. We demonstrate that the spin relaxation rates are particularly sensitive with the Landé g-factor, and that for InAs dots the DP can be necessarily considered in the spin relaxation analysis. Low-temperature (T ~ 0) relaxation rates are found to be small and to depend strongly on size, g-factor, and on magnetic field.

The present work is a comparative study of possible adsorption structures of the conjugated molecule acrylonitrile on Si(001) employing the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory. In the recent literature it is proposed the interaction of acrylonitrile with Si(001) through a cycloaddition reaction of the cyano group, in competition with the bounding of the two outer atoms of the molecule skeleton with the Si dimer in cross-dimer and cross-trench geometries; between other geometries like which correspond the reaction of the C = C bond with a Si dimer. Starting from a large number of configurations our calculations favor the planar cycloaddition through the terminal N and C atoms on the Si dimer. In this way we explain the electronic and vibrational features obtained experimentally.

Polyaniline (PANI) and their charge transfer complexes are engineering materials that continue to attract considerable attention from engineers due to their unique electrochemical behavior, environmental stability and high degree of processability. Extensive works on the preparation of electrically conductive PANI blends containing a polymeric matrix having desirable physical and good mechanical characteristics have pointed as a promising route to overcome the PANI intractabilities. This paper reports the electrical properties of the dichromated PANI/poly(vinyl alcohol) (PANI:Cr3+/PVA) blends. Doped polyaniline powder in the emeraldine base form was synthesized by the classical chemical oxidation of aniline with ammonium persulfate as the oxidant in the presence of chromium (Cr3+). The SEM (Phillips XL 30) was used to understand the changes occurring in the conductivity to a microstructure level. The distribution of PANI:Cr3+ (50% w/w) in the host polymer (PVA) show some tendency to occur in "flocculated structures". The samples were cut into circular shapes to form films about 500 microns thick. Electrical contacts were made by sputtering gold over sample surfaces. We have carried out impedance spectroscopy measurements in the range of 20Hz to 1MHz. Measurements carried at 100K and 250K indicates that thermally activated carrier process between PANI:Cr3+ grains in the PANI:Cr3+/PVA blend sample are responsible for its high temperature conductivity.

The experiment E93-026 at the Thomas Jefferson National Accelerator Facility (JLab) determined the electric form factor of the neutron <IMG SRC="/img/revistas/bjp/v34n3a/a02img03.gif">through quasielastic <IMG SRC="/img/revistas/bjp/v34n3a/a02img01.gif">(<IMG SRC="/img/revistas/bjp/v34n3a/a02img02.gif">; e'n)p scattering using a longitudinally polarized electron beam and a frozen polarized 15ND3 target. The knocked out neutrons were detected in a segmented plastic scintillator detector in coincidence with the scattered electrons. The <IMG SRC="/img/revistas/bjp/v34n3a/a02img03.gif">was extracted by comparing the experimental beam-target asymmetry with full theoretical calculations based on different values of <IMG SRC="/img/revistas/bjp/v34n3a/a02img03.gif">. Preliminary results of the Fall 2001 run are reported.

The population and the mode of decay of the low-lying excited levels of 193Ir have been investigated by single and <FONT FACE=Symbol>gg</FONT> coincidence spectroscopy using 192Os enriched to 99% and several HPGe detectors. From singles spectra analysis the energies of 71 gamma rays have been determined with a better overall precision than previously, 17 of them for the first time. The energies at 1043.244(15) and 1316.628(18)keV , observed for the first time in this work, as well as some preliminary coincidence relationships, suggest that the end point energy Qbetamax(<FONT FACE=Symbol>»</FONT>1.14MeV) must be further investigated.

A method of analysis for gamma angular correlation experiments was developed for the case of a multi-detector setup, tested with several known sources of radiation, and applied in the decay of the 155Eu nucleus. The method has shown good reliability. This method was applied for five cascades for the transitional nucleus 155Eu and produced the following results: delta(65 keV) = 0.09(11), delta(203 keV) = -0.44(17), delta(246 keV) = 0.281(22), delta(664 keV) = -0.231(21), delta(1002 keV) = -0.35(6).

We investigate the properties of mixed stars formed by hadronic and quark matter in beta-equilibrium described by appropriate equations of state (EOS) in the framework of relativistic mean-field theory. We use the non-linear Walecka model for the hadron matter and the MIT Bag and the Nambu-Jona-Lasinio models for the quark matter. The calculations were performed for T = 0 and for finite temperatures in order to describe neutron and proto-neutron stars. The star properties are discussed. Both the Bag model and the NJL model predict a mixed phase in the interior of the star.

Compaction processes can influence soil physical properties such as soil density, porosity, pore size distribution, and processes like soil water and nutrient movements, root system distribution, and others. Soil porosity modification has important consequences like alterations in results of soil water retention curves. These alterations may cause differences in soil water storage calculations and matric potential values, which are utilized in irrigation management systems. Because of this, soil-sampling techniques should avoid alterations of sample structure. In this work soil sample compaction caused by core sampling devices was investigated using the gamma ray computed tomography technique. A first generation tomograph with fixed source-detector arrangement and translation/rotational movements of the sample was utilized to obtain the images. The radioactive source is 241Am, with an activity of 3.7 GBq, and the detector consists of a 3 in. x 3in. NaI(Tl) scintillation crystal coupled to a photomultiplier tube. Soil samples were taken from an experimental field utilizing cylinders 4.0 cm high and 2.6 cm in diameter. Based on image analyses it was possible to detect compacted regions in all samples next to the cylinder wall due to the sampling system. Tomographic unit profiles of the sample permitted to identify higher values of soil density for deeper regions of the sample, and it was possible to determine the average densities and thickness of these layers. Tomographic analyses showed to be a very useful tool for soil compaction characterization and presented many advantages in relation to traditional methods.

We describe the use of the 14C-AMS technique to study the influence of upwelling waters to the local biological production, at Arraial do Cabo, Brazil. Results on delta14C from seaweed tissues incubated into three different types of water mass show the association of primary production with the upwelling waters. We also discuss the importance of high- precision 14C-AMS measurements for this kind of study, and the properly application of isotopic fractionation corrections on radiocarbon results.

We give an overall picture of our present understanding of the effect of the break-up of stable weakly bound nuclei on their fusion cross section with light, medium and heavy mass nuclei, at energies above the Coulomb barrier. The discussion is based mostly on recent data obtained by our group in collaborative experiments with ANU, USP and Tandar Laboratories. We conclude that there is complete fusion suppression for heavy targets, due to the loss of flux, corresponding to the occurrence of incomplete fusion of one of the break-up fragments. For medium and light mass targets, the incomplete fusion is negligible and therefore is no complete fusion suppression.

We use a modified version of the Skyrme model in which the stabilizing fourth-order term is separated in two parts with different parameters that are introduced in the Lagrangian and could vary independently. The predictions for the static properties of baryons are calculated and results are compared with the respective experimental values and those of other authors. We show that it possible to improve in an expressive way the results for the set of the important quantities at the same time.

The availability of radioactive beams has produced great opportunities for advances in our understanding of the nucleosynthesis occurring in stellar explosions such as novae, X-ray bursts, and supernovae. At the HRIBF, we have used proton-rich beams such as 17;18F and neutron-rich beams such as 82Ge to study astrophysicallyimportant reaction rates. Large-area detector arrays and kinematically-complete measurements have been used to overcome the challenges of low beam currents and beam impurities. The experimental methods and results are discussed.

Only 300 out of thousands of isotopes are stable and exist in nature. The others are unstable and decay within a wide range of lifetimes. The properties of most of these rare isotopes are unknown and can only be inferred, with considerable uncertainty, from theoretical calculations. The proposed Rare Isotope Accelerator (RIA) will make it possible to produce and study more than a thousand new rare isotopes in the laboratory. RIA will be driven by a highly flexible superconducting linear accelerator which will be capable of delivering intense beams of all elements from hydrogen to uranium. The RIA facility will be the most powerful rare isotope research facility in the world.