18,495 research outputs found

    Avaliação do crescimento de clones de guaraná em área de mata do Amapá.

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    A exploração de culturas perenes e uma das principais alternativas de utilização economica das áreas de terra firme no Amapá. Entre as espécies com potencial, o guarana (Paullinia cupana var. sorbilis), pelos resultados obtidos em outros pontos do trópico úmido, motivou a Unidade de Execução de Pesquisa de Ambito Territorial de Macapá (UEPAT de Macapá), a estudar seu comportamento nas condições locais.bitstream/item/65097/1/AP-1988-avaliacao-crescimentos-clones-guarana.pd

    Chemical Potential and the Nature of the Dark Energy: The case of phantom

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    The influence of a possible non zero chemical potential μ\mu on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), p=ωρp=\omega \rho (ω<0,constant\omega <0, constant). The entropy condition, S0S \geq 0, implies that the possible values of ω\omega are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For μ>0\mu >0, the ω\omega-parameter must be greater than -1 (vacuum is forbidden) while for μ<0\mu < 0 not only the vacuum but even a phantomlike behavior (ω<1\omega <-1) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, μ/T=μ0/T0\mu/T=\mu_0/T_0. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons μ\mu is always negative and the extended Wien's law allows only a dark component with ω<1/2\omega < -1/2 which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for μ0\mu 0 are permmited only if 1<ω<1/2-1 < \omega < -1/2. The thermodynamics and statistical arguments constrain the EoS parameter to be ω<1/2\omega < -1/2, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy (ω10/21\omega \lesssim -10/21).Comment: 7 pages, 5 figure

    Magnetic phases evolution in the LaMn1-xFexO3+y system

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    We have investigated the crystal structure and magnetic properties for polycrystalline samples of LaMn1-xFexO3+y, in the whole range x=0.0 to x=1.0, prepared by solid state reaction in air. All samples show the ORT-2 orthorhombic structure that suppresses the Jahn-Teller distortion, thus favoring a ferromagnetic (FM) superexchange (SE) interaction between Mn^{3+}-O-Mn^{3+}. For x=0.0 the oxygen excess (y ~ 0.09) produces vacancies in the La and Mn sites and generates a fraction around 18% of Mn^{4+} ions and 82% of the usual Mn^{3+} ions, with possible double exchange interaction between them. The Fe doping in this system is known to produce only stable Fe^{3+} ions. We find an evolution from a fairly strong FM phase with a Curie temperature T_{C} ~ 160 K, for x=0.0, to an antiferromagnetic (AFM) phase with T_{N} = 790 K, for x=1.0, accompanied by clear signatures of a cluster-glass behavior. For intermediate Fe contents a mixed-phase state occurs, with a gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic transition broadening for 0.2 < x < 0.7. A model based on the expected exchange interaction among the various magnetic-ion types, accounts very well for the saturation-magnetization dependence on Fe doping.Comment: 27 pages, 9 figure

    On Matrix Superpotential and Three-Component Normal Modes

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    We consider the supersymmetric quantum mechanics (SUSY QM) with three- component normal modes for the Bogomol'nyi-Prasad-Sommerfield (BPS) states. An explicit form of the SUSY QM matrix superpotential is presented and the corresponding three-component bosonic zero-mode eigenfunction is investigated.Comment: 17 pages, no figure. Paper accepted for publication in Journal of Physics A: Mathematical and Theoretica

    Programa capacita multiplicadores na cultura de dendê: Embrapa e parceiros promovem capacitação em diversos aspectos da cultura da palma de óleo.

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    Disorder effects at low temperatures in La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3} manganites

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    With the aim of probing the effect of magnetic disorder in the low-temperature excitations of manganites, specific-heat measurements were performed in zero field, and in magnetic fields up to 9 T in polycrystalline samples of La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3}, with Y concentrations x=0, 0.10, and 0.15. Yttrium doping yielded the appearance of a cluster-glass state, giving rise to unusual low-temperature behavior of the specific-heat. The main feature observed in the results is a strong enhancement of the specific-heat linear term, which is interpreted as a direct consequence of magnetic disorder. The analysis was further corroborated by resistivity measurements in the same compounds.Comment: 9 pages, 2 figure

    Perfil das comunidades quilombolas paraibanas: índice de vulnerabilidade socioeconômica e ecológica para o desenvolvimento sustentável local - ISE.

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    O estudo possibilitou definir as dimensões: social, econômica e ecológica das comunidades quilombolas paraibanas. A situação de vulnerabilidade de cada Grupo (cluster means) foi codificada e geoespacializada em um Sistema Geográfico de Informação (SGI) por setor censitário do Estado da Paraíba, após a construção de um índice classificatório correspondente as 38 comunidades quilombolas pesquisadas in loco. Os perfis das Comunidades apresentados para o conjunto do Índice de vulnerabilidade socioeconômica e ecológica (ISE) foram altamente significativos, identificando a necessidade de políticas públicas mais efetivas, por localidades, simplificando significativamente, o investimento dos Programas socioambientais já previstos pelos governos: federal, estadual e municipais em benefício da região
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