28,121 research outputs found

    Transmission coefficient and two-fold degenerate discrete spectrum of spin-1 bosons in a double-step potential

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    The scattering of spin-1 bosons in a nonminimal vector double-step potential is described in terms of eigenstates of the helicity operator and it is shown that the transmission coefficient is insensitive to the choice of the polarization of the incident beam. Poles of the transmission amplitude reveal the existence of a two-fold degenerate spectrum. The results are interpreted in terms of solutions of two coupled effective Schr\"{o}dinger equations for a finite square well with additional δ\delta -functions situated at the borders.Comment: arXiv admin note: substantial text overlap with arXiv:1203.119

    Spin and pseudospin symmetries of the Dirac equation with confining central potentials

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    We derive the node structure of the radial functions which are solutions of the Dirac equation with scalar SS and vector VV confining central potentials, in the conditions of exact spin or pseudospin symmetry, i.e., when one has V=±S+CV=\pm S+C, where CC is a constant. We show that the node structure for exact spin symmetry is the same as the one for central potentials which go to zero at infinity but for exact pseudospin symmetry the structure is reversed. We obtain the important result that it is possible to have positive energy bound solutions in exact pseudospin symmetry conditions for confining potentials of any shape, including naturally those used in hadron physics, from nuclear to quark models. Since this does not happen for potentials going to zero at large distances, used in nuclear relativistic mean-field potentials or in the atomic nucleus, this shows the decisive importance of the asymptotic behavior of the scalar and vector central potentials on the onset of pseudospin symmetry and on the node structure of the radial functions. Finally, we show that these results are still valid for negative energy bound solutions for anti-fermions.Comment: 7 pages, uses revtex macro

    New solutions of the D-dimensional Klein-Gordon equation via mapping onto the nonrelativistic one-dimensional Morse potential

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    New exact analytical bound-state solutions of the D-dimensional Klein-Gordon equation for a large set of couplings and potential functions are obtained via mapping onto the nonrelativistic bound-state solutions of the one-dimensional generalized Morse potential. The eigenfunctions are expressed in terms of generalized Laguerre polynomials, and the eigenenergies are expressed in terms of solutions of irrational equations at the worst. Several analytical results found in the literature, including the so-called Klein-Gordon oscillator, are obtained as particular cases of this unified approac

    Perspectivas da utilização do gene bt para o controle de insetos-praga do arroz no Brasil.

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    O objetivo desta publicação é revisar a situação atual da tecnologia dos transgênicos bt, com vistas à introdução desta característica no programa de melhoramento de arroz da Embrapa.bitstream/CNPAF-2009-09/28165/1/doc_232.pd

    Effect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)

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    Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established

    Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)2_2X

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    High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)2_{2}X (X = PF6_{6} and AsF6_{6}), where charge ordering at TCOT_{CO} coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at TCOT_{CO} in the uniaxial expansivity along the interstack c*\textbf{\textit{c*}}-direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X−^{-}. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at TintT_{int} ≃\simeq 0.6 ⋅\cdot TCOT_{CO} indicative of a phase transition related to the charge ordering
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