Scielo RSS <![CDATA[Brazilian Journal of Physics]]> http://www.scielo.br/rss.php?pid=0103-973320010001&lang=en vol. 31 num. 1 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <![CDATA[<B>Positronium formation from hydrogen negative Ion, H<SUP><FONT FACE=Symbol>-</B></SUP></FONT>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100001&lng=en&nrm=iso&tlng=en Theoretical results for the positronium (Ps) formation from hydrogen negative ion, H-, targets are presented. An exact analytical expression is constructed for the transition matrix for Ps formation in collisions of positrons with H- in the framework of Born approximation using three different target wave functions. The corresponding differential and total cross sections are presented for Ps formation in ground state. Comparison between these cross sections for different wave functions reveals the dependence of the Ps-formation process on the choice of the target wavefunction and the effect of correlation among the target electrons on the process. <![CDATA[<b>Variances and covariances in deconvolution of multichannel spectra</b>: <b><sup>34</sup>S (<FONT FACE=Symbol>g</font>, <img src="http:/img/fbpe/bjp/v31n1/02eq06.gif"><i>n</i>) cross section</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100002&lng=en&nrm=iso&tlng=en This paper discusses some aspects of deconvolution of one-dimensional spectra in the framework of the Least Squares Method and presents a minimum variance regularization procedure. Covariance matrices are taken into account inevery step. Fluctuation and artifacts, both in the deconvolved and regularized spectra, are related to the structure of the covariance matrices. The method is applied to a simulated spectrum and to the 34S (gamma, <img src="http:/img/fbpe/bjp/v31n1/02eq05.gif">n) cross section determination from actual yield data. <![CDATA[<B>Applications of the Schwinger multichannel method with pseudopotentials to electron scattering from polyatomic molecules I</B>: <B>elastic cross sections </B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100003&lng=en&nrm=iso&tlng=en This paper is a data basis which includes tables of integral, differential, and momentum transfer cross sections for elastic electron scattering from CF4, CCl4, SiCl4, SiBr4, SiI4, CH3F CH2F2, CHF3, CH3Cl, CH2Cl2, CHCl3, CF3Cl, CF2Cl2, CFCl3, CH3Br, CH3I, SiH3Cl, SiH3Br, SiH3I, GeH3Cl, GeH3Br, SnH3Br, C2H6, Si2H6, Ge2H6, B2H6, Ga2H6, H2O, H2S, H2Se, H2Te, trimethylarsine (TMAs), N2O, and O3. These tables show our new results, along with some of our previously published cross sections, and can easily be compared to future experimental data and other new theoretical results. Our scattering amplitudes were calculated using the Schwinger multichannel method with norm-conserving pseudopotentials. Our results are in good agreement with other theoretical data and experimental results when available. <![CDATA[<B>Applications of the Schwinger Multichannel method with pseudopotentials to electron scattering from polyatomic molecules II</B>: <B>rotational excitation cross sections</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100004&lng=en&nrm=iso&tlng=en This paper reports results for rotational excitation of H2O and H2S molecules by electron impact. It is also a databasis including tables of previously published rotationally resolved cross sections for CH4, SiH4, GeH4, SnH4, PbH4, NH3, PH3, AsH3, SbH3, CF4, CCl4, SiCl4 SiBr4, and SiI4. Our scattering amplitudes were calculated using the Schwinger multichannel method with norm-conserving pseudopotentials and the rotational resolved cross sections were obtained with the help of the adiabatic nuclei rotation approximation. Our results are in good agreement with other theoretical data and experimental results when available. <![CDATA[<B>Post-deposition cooling in oxygen is critical for YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7<FONT FACE=Symbol>-</FONT><I>d</I></SUB> films deposited by eclipse pulsed laser deposition method</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100005&lng=en&nrm=iso&tlng=en YBa2Cu3O7<FONT FACE=Symbol>-</FONT>d thin films were deposited on MgO single crystals by means of an eclipse pulsed laser deposition method. Deposited films are cooled down in situ under an oxygen atmosphere at a given oxygen pressure. The relationship between critical temperature and oxygen deficiency was investigated by means of electrical resistance R(T) and x-ray diffraction measurements. Post-deposition cooling is critical and the high pressure of oxygen during cooling is favorable. <![CDATA[<B>Effect of plasma subsonic toroidal flows induced by Alfvén waves on transport processes in the edge of elongated tokamaks</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100006&lng=en&nrm=iso&tlng=en There is a renewed interest in using Alfvén waves (AW) in tokamak plasmas. Previously, AW were actively explored mostly for current drive and plasma heating in tokamaks. Presently, the possibility of the anomalous and neoclassical transport suppression by AW in tokamak plasmas is being vividly discussed. AW can also induce poloidal and toroidal plasma rotation. Toroidal plasma rotation can reach the subsonic level. These flows can substantially affect neoclassical transport both in collisional and weakly collisional plasmas. In this paper, the effect of plasma subsonic toroidal flows induced by Alfvén waves on transport processes in the edge of elongated tokamak is investigated. The dependence of poloidal plasma rotation and ion heat conductivity on the elongation parameter and the ratio of induced toroidal velocity to the sonic speed are analytically obtained. <![CDATA[<B>Gravitational clustering to all perturbative orders</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100007&lng=en&nrm=iso&tlng=en We derive the time evolution of the density contrast to all orders of perturbation theory, by solving the Einstein equation for scale-invariant fluctuations. These fluctuations are represented by an infinite series in inverse powers of the radial parameter. In addition to the standard growing modes, we find infinitely many more new growing modes for open and closed universes. <![CDATA[<b>On the non-relativistic Casimir effect </b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100008&lng=en&nrm=iso&tlng=en We compute the Casimir energy for a massive scalar field constrained between two parallel planes (Dirichlet boundary conditions) in order to investigate its non-relativistic limit. Instead of employing the usual relativistic dispersion relation omega(p) = <img SRC="http:/img/fbpe/bjp/v31n1/08eq01.gif">, we use the non-relativistic one, omega(p) = p²/2m. It turns out that the Casimir energy is zero. We include the relativistic corrections perturbatively and show that at all orders the Casimir energy remains zero, since each term in the power series in 1/c² is proportional to the Riemann zeta function of a negative even integer. This puzzling result shows that, at least for the free massive scalar field, the Casimir effect is non-perturbative in the relativistic sense. <![CDATA[<B>Weighted oscillator strengths and lifetimes for the neutral oxygen spectrum, O I</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100009&lng=en&nrm=iso&tlng=en The weighted oscillator strengths (gf) and the lifetimes for O I presented in this work were obtained in a multiconfiguration Hartree-Fock relativistic (HFR) approach. In this calculation, the electro-static parameters were optimized by a least-squares procedure, in order to improve the adjustment to experimental energy levels. This method produces gf-values that are in better agreement with intensity observations and lifetime values that are closer to the experimental ones. In this work we present all the experimentally known electric dipole atomic transitions and energy levels for the O I spectrum. <![CDATA[<b>Analysis of cut-off conditions for coaxial fibers</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100010&lng=en&nrm=iso&tlng=en This paper reports the analysis of modal normalized frequency cut-off of coaxial fibers having four dielectric layers. The cut-off curves are obtained for four different structures and several modes as a function of the several parameters of the fibers (refractive index and layer dimension). The calculation is done using a transcendental equation obtained in this work. The analysis pays a special attention to the fundamental mode HE11 showing that for two structures (W1 and M1) the normalized frequency always is null as is the case of the standard rod fibers. For the other two structures (W2 and two regions), the normalized frequency may not be null, depending on the set of values of the fiber parameters. For this last case a loci diagram is obtained showing the regions where the normalized frequency is equal to zero and different from zero for several sets of fiber parameters. From the transcendental equation, we have an expression to calculate the curve of separation between these two regions. <![CDATA[<B>Adiabatic plasma rotations in orthogonal coordinate systems</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100011&lng=en&nrm=iso&tlng=en An equation for MHD stationary equilibrium of rotating plasmas in the azimuthal direction is derived in the case of an orthogonal curvilinear coordinate system. The basic assumptions we made are: (i) there is an ignorable coordinate so that surface quantities are independent of it; (ii) the entropy is a surface quantity. <![CDATA[<B>Multiple scattering of electromagnetic waves in disordered magnetic media</B>: <B>localization parameter, energy transport velocity and diffusion constant</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100012&lng=en&nrm=iso&tlng=en We review some of our recent results concerning the single and multiple eletromagnetic scattering by magnetic spherical particles. For a single electromagnetic scattering we show that the magnetic contribution alters, when compared to nonmagnetic scattering, the behavior of the cross sections and mean cosine of the scattering angle (<FONT FACE=Symbol>á</FONT>cos <FONT FACE=Symbol>qñ</FONT>). For ferromagnetic particles, resonances may occur even in the small-particle limit when the particle radius is much smaller than the wavelength. The resonances increase the cross-sections while <FONT FACE=Symbol>á</FONT>cos <FONT FACE=Symbol>qñ</FONT> is diminished, and even may become negative. Several quantities such the Ioffe-Regel parameter for localization are calculated for the multiple scattering regime. We show that magnetic scattering favors the observation of localization of electromagnetic waves in three dimensions. Further, this is also verified for dynamical experiments, where we show that the diffusion constant can be very small. Since the magnetic permeability of the scatterers can vary significatively around the Curie-Weiss critical temperature, experiments should be done on the microwave region and the scatterers should be soft ferrites. Some aspects of such experiment are presented. <![CDATA[<B>Instanton effects in heavy lambda masses</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100013&lng=en&nrm=iso&tlng=en We calculate the masses of lambdab and lambdac in the framework of QCD sum rules including instanton contributions. We find that these contributions have the same size of the four quark condensate contribution and improve the Borel stability in both Dirac structures of the baryon correlation function. <![CDATA[<B>Soda-lime glass with gradient of refraction index (GRIN)</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100014&lng=en&nrm=iso&tlng=en In a soda-lime glass provided by Companhia Vidraria Santa Marina, subsidiary of French Saint-Gobain, we induced a gradient of refraction of index (GRIN) by exchange of Na+ in glass by Li+ at 550ºC for 12, 24, 36 and 48 hours and varying bath temperature at 525ºC, 550ºC, 575ºC and 600ºC for fixed time of 48 hours. deltan = 0.0107 ± 0.0005 and GRIN depth of about 2.4mm were obtained for the ion exchange at 550ºC for 48 hours. The GRIN profile was fitted with er fc(x) function and then obtained the diffusion coefficient D = (1.6 ± 0.3) · 10-6 mm²/s. <![CDATA[<B>Quantisation of the multidimensional rotor</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100015&lng=en&nrm=iso&tlng=en We reconsider the problem of quantising a particle on the D-dimensional sphere. Adopting a Lagrangian method of reducing the degrees of freedom, the quantum Hamiltonian is found to be the usual Schrödinger operator without any curvature term. The equivalence with the Dirac Hamiltonian approach is demonstrated, either in the cartesian or in the curvilinear basis. We also briefly comment on the path integral approach. <![CDATA[<B>The influence of an external magnetic field on the fermionic Casimir effect</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100016&lng=en&nrm=iso&tlng=en The influence of an external constant uniform magnetic field on the Casimir energy associated with a Dirac field under antiperiodic boundary condition is computed using Schwinger's method. The obtained result shows that the magnetic field enhances the fermionic Casimir energy, in oposition to the bosonic Casimir energy which is inhibited by the magnetic field. <![CDATA[<B>Thermal effects on light emission in Yb<SUP>3+</SUP>-sensitized rare-earth doped optical glasses</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100017&lng=en&nrm=iso&tlng=en The temperature effect upon infrared-to-visible frequency upconversion fluorescence emission in off-resonance infrared excited Yb3+-sensitized rare-earth doped optical glasses is theoretically and experimentally investigated. We have examined samples of Er3+/Yb3+-codoped Ga2S3:La2O3 chalcogenide glasses and germanosilicate optical fibers, and Ga2O3:La2O3 chalcogenide and fluoroindate glasses codoped with Pr3+/Yb3+, excited off-resonance at 1.064mum. The experimental results revealed thermal induced enhancement in the visible upconversion emission intensity as the samples temperatures were increased within the range of 20°C to 260°C. The fluorescence emission enhancement is attributed to the temperature dependent multiphonon-assisted anti-Stokes excitation process of the ytterbium-sensitizer. A theoretical approach that takes into account a sensitizer temperature dependent effective absorption cross-section, which depends upon the phonon occupation number in the host matrices, has proven to agree very well with the experimental data. As beneficial applications of the thermal enhancement, a temperature tunable amplifier and a fiber laser with improved power performance are presented. <![CDATA[<b>Statistical physics of random searches</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100018&lng=en&nrm=iso&tlng=en We apply the theory of random walks to quantitatively describe the general problem of how to search efficiently for randomly located objects that can only be detected in the limited vicinity of a searcher who typically has a finite degree of "free will" to move and search at will. We illustrate Lévy flight search processes by comparison to Brownian random walks and discuss experimental observations of Lévy ights in the special case of biological organisms that search for food sites. We review recent findings indicating that an inverse square probability density distribution P(<img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">) ~ <img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">-2 of step lengths <img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">can lead to optimal searches. Finally we survey the explanations put forth to account for these surprising findings. <![CDATA[<b>Infrared analysis of thin films</b>: <b>amorphous, hydrogenated carbon on silicon</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332001000100019&lng=en&nrm=iso&tlng=en We apply the theory of random walks to quantitatively describe the general problem of how to search efficiently for randomly located objects that can only be detected in the limited vicinity of a searcher who typically has a finite degree of "free will" to move and search at will. We illustrate Lévy flight search processes by comparison to Brownian random walks and discuss experimental observations of Lévy ights in the special case of biological organisms that search for food sites. We review recent findings indicating that an inverse square probability density distribution P(<img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">) ~ <img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">-2 of step lengths <img SRC="http:/img/fbpe/bjp/v31n1/sele.gif">can lead to optimal searches. Finally we survey the explanations put forth to account for these surprising findings.