Resumo em Inglês:

The modulation frequency response of compressively strained multiquantum well (MQW) lasers grown with an InGaAs/AlGaInAs/InP heterostructure and emitting at the wavelength of 1.55 <FONT FACE="Symbol">m</font>m is presented. The laser devices processed with the mushroom-stripe buried structure present a high frequency 3 dB bandwidth above 20 GHz. The frequency response was measured with the small signal modulation technique. The logarithmic subtraction method was employed to extract the intrisic frequency response of the MQW active layer, providing the determination of important laser parameters: the differential gain, the nonlinear gain coefficient and the maximum 3 dB frequency bandwidth.

Resumo em Inglês:

The synchroton radiation source constructed at LNLS is composed of a 1.37 GeV electron storage ring and a 120 MeV LINAC injector. Seven beamlines are in the final phase of commissioning and will be opened to external users during the second semester of 1997. These beamlines allow for experiments of VUV and X-ray spectroscopies and X-ray diffraction and scattering. A number of experimental results were already obtained using the beamlines which are currently in operation. X-ray absorption spectroscopy and small-angle X-ray scattering results, associated with a metal (pure Ni) and a biological sample (collagen), respectively, are described. The perspectives and user access policy for the utilization of the LNLS synchrotron radiation source are reported.

Resumo em Inglês:

The results of the evaluation of the Generalized Interacting Boson Model (GIBM) Hamiltonian in the framework of Restricted Dynamics Approach have been used in order to estimate the microscopical values for the parameters of two phenomenological collective models - the Bohr-Mottelson Model (BMM) and the standard Interacting Boson Model (IBM). The microscopically derived parameter values are calculated, using the effective NN-interaction potential parameter values obtained from the Strictly Restricted Dynamics Model (SRDM) energy calculations for light <FONT FACE="Symbol">a</font>-cluster type nuclei with 8 <FONT FACE="Symbol">£ </font>A <FONT FACE="Symbol">£ </font>60, and compared with the phenomenological model parameter values obtained from the direct fit to the experimental level energies of the same nuclei.

Resumo em Inglês:

Using the correlative method of unsymmetrized self-consistent field we study equilibrium atomic properties of anharmonic crystals. Here we calculate dynamical characteristics of a weakly anharmonic crystal with the body-centered cubic lattice, namely, the quadratic correlation moments between atomic displacements and the mean square relative atomic displacements. Taking into account the nearest neighbour interactions in this lattice, the second order of the method enables one to calculate the correlations between the nearest, second, third and fifth neighbours. An influence of more distant interactions is discussed. Results are also correlated with those calculated early for the simple and face-centered cubic lattices.

Resumo em Inglês:

We present measurements of the differential gain of multiquantum well (MQW) broad area lasers emitting at 1.55<FONT FACE="Symbol">m</font>m wavelength, taking into account the nonuniform stimulated emission caused by filamentation within the optical cavity. The differential gain was determined from measurements of the threshold current density, considering the losses due to the inhomogenous carrier density distribution, as an apparent leakage current effect in the output power-current characteristics. The lasers were grown by low pressure MOCVD and incorporate an InGaAs/InGaAsP/InP separate confinement MQW active region. The results are compared with previous data reported on similar MQW heterostructures.

Resumo em Inglês:

We consider the effect of the Earth gravitational attraction on nuclear reactions at very low energies. In particular, the Mott oscillation characteristic of scattering of two identical nuclei is found to be slightly but significantly modified. The effect discussed here is different but complementary to the COW effect in cold neutron interferometry. We take as an example 208Pb+208Pb. An angle precision of about 10-3 deg. is required to detect the effect.

Resumo em Inglês:

We present the results of applications of the iterative Schwinger variational method to obtain photoionization cross sections and phoelectron angular distributions for ionization out of the four outermost valence orbitals 1b3u, 1b3g, 3ag and 1b2u of ethylene for photon energies ranging from near threshold to 30 eV. The observed resonance-like maxima in the cross sections for ionization of the 1b3g and 3ag orbitals are reproduced in our calculations. The disagreement between our cross sections and the experimental data for the 1b3u orbital below 17 e V is attributed to autoionization, which is not accounted for in our calculations.

Resumo em Inglês:

The similarity between the electron equation of motion in electric and magnetic fields and the Bloch equations for nuclear magnetic moments in a magnetic field leads to a novel resonance phenomenon involving the electrical current. This suggests a new experimental technique for the investigation of transport properties in conducting materials. In this work we propose a new set of Bloch Equations for the electrical current with different electron-electron and electron-lattice scattering rates and discuss the consequences of their solutions to the study of the electronic properties of conducting materials. We outline the experimental conditions for detecting electrical current resonance and estimate some relevant experimental parameters for the observation of current echoes and free current decays.

Resumo em Inglês:

A double plasma device with multipole surface magnetic confinement is used to investigate the beam relaxation length of beam plasma interaction as a function of the beam plasma density ratio, 10-4 <FONT FACE="Symbol">£ </font>nb/no <FONT FACE="Symbol">£</font> 2 x 10-2, and beam energy Wb <FONT FACE="Symbol">£</font> 200 eV in an unmagnetized plasma, using argon and helium gases. The measured relaxation lengths show a transition from quasi-linear to nonlinear regime, as predicted by Galeev et al., 1977. There is evidence that for some range of the beam plasma parameters the threshold for modulational instability is reached. In that case it is possible to observe that the beam propagates relatively unperturbed for larger distances than the ones predicted by quasi-linear theory.

Resumo em Inglês:

We use the Green function method to compute the Casimir energy of a massive scalar particle. In order to evaluate the Green function we employ the image method which had been used before only for the massless case.

Resumo em Inglês:

The motion of a particle constrained to move between two walls one of which is moveable is solved. We determine analytically the effective forces that act on the particle or on the moveable wall. The quantal version of the solution is briefly discussed.

Resumo em Inglês:

We use quasilinear theory to investigate the influence of losses of energetic particles on the behaviour of electron cyclotron absorption during lower hybrid plus electron cyclotron current drive. Our analysis takes into account the quasilinear effects of the interaction of electron cyclotron radiation with a lower hybrid produced tail in the presence of collisions and a loss term for energetic particles. We study the behaviour of different quantities, as the parallel electron distribution function and the perpendicular temperature, for several values of the electron confinement time, an important parameter of the model loss term. The absorption of extraordinary mode electron cyclotron waves by tail electrons results to be substantially reduced when the loss term is taken into account in the quasilinear diffusion equation, as compared to the case where these losses are neglected.

Resumo em Inglês:

We firstly report on the leading scattering mechanism responsible for the heat resistivity in simple molecular crystals. The analysis is given in the terms the phonon and libron three-particle-scattering processes in crystals formed by linear molecules (N2, CO, CO2, N2O, etc.) On the basis of a comparison of heat resistivity estimations with known experimental data the dominated microscopic scattering mechanism is established, namely, the process of absorption (and emission) of the one libron by the two acoustic phonons.

Resumo em Inglês:

A model of deuteron acceleration in ferroelectrical crystals under the process of domain polarization reversal is proposed. Experimental verfication of the model with LiTaO3 crystals saturated with deuterium was fulfilled. It was shown that in the 75 kV/cm a.c. field the neutron emission attributed to D-D fusion is two order magnitude higher the Jones level.

Resumo em Inglês:

A nonequilibrium system of classical particles interacting through nonconservative forces is studied by a stochastic Markovian process defined over the space of the particle positions. The velocities of the particles are treated as independent stochocastic variables with a given probability distribution. We deduce expressions for the rate in which energy is exchanged with the surrounding as well as the rate in which energy is dissipated. These results are applied to the special case in which the irrotational and solenoidal parts of the forces acting on the particles are orthogonal. We also solve exactly a model in which these two forces are linear functions of positions.

Resumo em Inglês:

The transport coefficients of a completely ionized gas are determined from an extended thermodynamic theory of mixtures of ideal gases in the presence of external electromagnetic fieds. The Onsager relations for the transport coefficients in the presence of external magnetic flux density are also discussed.

Resumo em Inglês:

The calculation of spin susceptibilities in metals, in particular the Ruderman-Kittel-Kasuya-Yosida (RKKY) polarization due to a point coupling, is reviewed. In the one dimensional case, the low coupling limit of the non-linear theory clarifies traditional perturbation approaches. Multilayer structures require theories with bounded metals. An explicit formula is derived for the indirect interaction between two ferromagnetic plates in a half space. A case, where an oscillatory amplitude decays with the first power of the distance to the boundary is discussed.

Resumo em Inglês:

We discuss the physical interpretation of the Lagrange multipliers appearing in the information-theoretical description of nonequilibrium steady states characterized by a heat flux or a viscous pressure. Some of the multipliers have a classical counterpart (temperature, pressure, chemical potential, ...) but their equations of state are modified by the fluxes, whereas some others, those which are related to nonequilibrium constraints, do not have an analogous in the equilibrium theory. We give definite results for the Lagrange multipliers, the partition function and the nonequilibrium entropy in some simple simple situations which provide explicit illustrations of their dependence on the fluxes. We end with some comments about the experimental verification of this dependence for the Lagrange multipliers conjugated to the internal energy.

Resumo em Inglês:

It is shown that the existence of nonabrupt interfaces modify electric field effects on the electron transmission through a GaAs/AlxGa1-xAs double-barrier. When the applied electric intensity is 25 kV/cm, and the abrupt well and barriers are 100 Å wide, interfaces as thin as two GaAs lattice parameters are responsible for shifts at least of 10 meV in the electron tunneling resonance energies. The type of interface potential and electron effective mass description changes significantly theoretical results related to the electric field influence on the electron transmission properties.

Resumo em Inglês:

We write exact renormalization-group recursion relations for a ferromagnetic Ising model on the diamond hierarchical lattice with an aperiodic distribution of exchange interactions according to a class of generalized two-letter Fibonacci sequences. For small geometric fluctuations, the critical behavior is unchanged with respect to the uniform case. For large fluctuations, the uniform fixed point in the parameter space becomes fully unstable. We analyze some limiting cases, and propose a heuristic criterion to check the relevance of the fluctuations.

Resumo em Inglês:

The real-space (RS) linear muffin tin orbital scheme, based on the linear muffin tin orbital formalism in the atomic sphere approximation (LMTO-ASA) and on the recursion method, allows us to perform first-principles, self-consistent, density functional electronic structure calculations directly in real space. Here we show that the real space scheme can also be applied to study the electronic structure and magnetic properties of metallic surfaces and defects in these systems. To illustrate the application of this new procedure to surfaces, we use the RS-LMTO-ASA scheme to calculate the magnetic properties of an Fe overlayer on the Cu(001) surface, which have been extensively studied, using several different methods, as may be seen in the literature. We have also investigated the behavior of a substitutional Cu impurity in the Fe overlayer, since results for this system are available for comparison. We find that, as expected, our results are in very good agreement with those obtained by other methods. We note that, being implemented directly in real space, the RS-LMTO-ASA scheme does not require a high degree of symmetry. Therefore it is very flexible and can be applied in situations which are difficult to treat by other approaches.

Resumo em Inglês:

We consider the hydrodynamic description of a fluid of particles in the context of the classical approach to the Nonequilibrium Statistical Operator Mehod. It is based on the information entropy ensemble of Predictive Statistical Mechanics, and its accompanying Informational Statistical Thermodynamics. We start with a description of the macroscopic state of the system in terms of single- and two-particle reduced dynamic density functions in phase space, and the accompanying Lagrange multipliers (intensive nonequilibrium thermodynamic variables) that the method introduces. In terms of this basic set of dynamical variables we derive the equations of evolution for the mass, momentum, and energy densities, as well a the continuity equation for the informational entropy. It is shown how these equations are to be restricted in order to recover the results of classical hydrodynamics (based on linear irreversible thermodynamics), as well as a Gibbs relation defining local equilibrium. The differences between the generalized formalism and this classical limiting case are discussed.

Resumo em Inglês:

The dynamics of a finite set of relevant observables, associated to a Hamiltonian of a three level system is analized in connection with the quantum Zeno effect. Since we use the Hamiltonian that completely describes the physical situation related to the experiment under study (W.M. Itano et al, Phys. Rev. A41, 2295 (1990)), no reduction or collapse of the wave function is required to describe the quantum Zeno effect.

Resumo em Inglês:

A nonequilibrium statistical operator (NSO) is built for classical sistems using a field theory in classical phase space, which is but a classical version of the thermofield dynamics formalism. The approach developed here starts with a second-quantized version of this phase-space field theory. Then elements of symmetry are analysed and invariants of the theory are introduced. The local conservation laws are derived and used to make explicit NSO. Such a method is applied to derive the Fokker-Planck-Kramers equation.

Resumo em Inglês:

We report Raman scattering and high-resolution X-ray diffraction measurements performed on a series of MBE-grown Si / Gen / Si quantum wells with n varying from 3 to 6. Our results are consistent with a gradual evolution of the Si / Ge interface which starts with Ge segregation and formation of terraces for low Ge coverage, to smooth 2D Ge layers, bounded by interfacial alloy layers, for coverages superior to 4M Ls.

Resumo em Inglês:

The results of very simple experiments to evaluate Lord Rayleigh Resolution Criterion validity are discussed in cases of quasimonochromatic sources of small angular dimensions (LEDs) and monochromatic sources (Lasers), the emissions of which have different or equal spectral compositions. Visual observations as well as color photographs and color video recording were utilized in the experiments. When LEDs and lasers of different color were used, better resolutions than those of Rayleigh Criterion were obtained owing to the non-spectral yellow false color resulting from the overlapping of the red and green spectral colors. Therefore, the observation of the non-spectral false color implies the super-resolution process. The consideration of the non-spectral false color is a new approach in super-resolution studies. Finally, an illumination and reading system of high density CD-ROM-s (9 GB) based on the obtained results is suggested.

Resumo em Inglês:

It is sometimes convenient to emphasize the local aspects of a part of a crystalline system, and use the corresponding localized states to build a basis of the states of the whole system. In many cases it is only a subspace of these local states that is relevant, and the Hubbard operators provide a fairly simple way to write the corresponding projected Hamiltonian. Two examples of this type of treatment are presented int this work. The first is a Co2+ impurity in a MgO crystal interacting through a Jahn-Teller term with the crystal phonons, and it is shown how this interaction affects the electronic Raman scattering. The second is the Anderson lattice when the local electrons have an infinite Coulomb repulsion, and a diagrammatic expansion with cumulants is discussed. We propose a method to obtain approximate Green's functions for the Anderson lattice that employs the exact solutions of an atomic problem, and the corresponding spectral density of the local electrons is calculated.