1,932 research outputs found
Some new results concerning the vacuum in Dirac Hole Theory
In Dirac's hole theory the vacuum state is generally believed to be the state
of minimum energy. It will be shown that this is not, in fact, the case and
that there must exist states in hole theory with less energy than the vacuum
state. It will be shown that energy can be extracted from the hole theory
vacuum state through the application of an electric field.Comment: Accepted by Physica Scripta, 19 page
Suco de umbu (Spondias tuberosa Arruda): uma alternativa econômica para a agricultura familiar no semi-árido.
O crescente interesse dos consumidores por frutos tropicais, aliado ao número cada vez maior de pequenas indústrias de processamento de frutas para produção de polpa, poderá tornar os produtos derivados do umbuzeiro, um rentável negócio agrícola
Plasmon polaritons in photonic superlattices containing a left-handed material
We analyze one-dimensional photonic superlattices which alternate layers of
air and a left-handed material. We assume Drude-type dispersive responses for
the dielectric permittivity and magnetic permeability of the left-handed
material. Maxwell's equations and the transfer-matrix technique are used to
derive the dispersion relation for the propagation of obliquely incident
optical fields. The photonic dispersion indicates that the growth-direction
component of the electric (or magnetic) field leads to the propagation of
electric (or magnetic) plasmon polaritons, for either TE or TM configurations.
Furthermore, we show that if the plasma frequency is chosen within the photonic
zeroth-order bandgap, the coupling of light with plasmons
weakens considerably. As light propagation is forbidden in that particular
frequency region, the plasmon-polariton mode reduces to a pure plasmon mode.Comment: 4 pages, 4 figure
Suppression of Anderson localization of light and Brewster anomalies in disordered superlattices containing a dispersive metamaterial
Light propagation through 1D disordered structures composed of alternating
layers, with random thicknesses, of air and a dispersive metamaterial is
theoretically investigated. Both normal and oblique incidences are considered.
By means of numerical simulations and an analytical theory, we have established
that Anderson localization of light may be suppressed: (i) in the long
wavelength limit, for a finite angle of incidence which depends on the
parameters of the dispersive metamaterial; (ii) for isolated frequencies and
for specific angles of incidence, corresponding to Brewster anomalies in both
positive- and negative-refraction regimes of the dispersive metamaterial. These
results suggest that Anderson localization of light could be explored to
control and tune light propagation in disordered metamaterials.Comment: 4 two-column pages, 3 figure
A fruticultura como vetor de desenvolvimento: o caso dos municipios de Petrolina (PE) e Juazeiro (BA).
Para contrapor o fenomeno das secas, que frequentemente provocam perda da producao agricola na zona semi-arida do Nordeste, gerando grandes problemas sociais, o governo tem implantado diversos perimetros publicos irrigados e promovido outros estimulos governamentais para a iniciativa privada no setor agropecuario. Atraves dessas iniciativas, o governo tem criado importantes Polos de producao agricola no que diz respeito as pequenas, medias e grandes empresas de producao de frutas e hortalicas. A agricultura irrigada e, mais especificamente, a fruticultura irrigada, promoveu um grande dinamismo na economia do Polo na estrutura urbana tornando-se o aglomerado urbano mais prospero do Vale do Sao Francisco. As grandes mudancas dos sistemas produtivos de culturas anuais para fruteiras perenes, de exportacao, desencadeou no Polo de Petrolina (PE)/Juazeiro (BA) demanda de outros investimentos de apoio para a comercializacao de frutas, motivando o Governo Federal a financiar pesquisas, priorizando aquelas relacionadas com culturas de exportacao, promover cursos de especializacao em comercio exterior e melhorar a infra-estrutura logistica da regiao. A fruticultura irrigada e o principal vetor do desenvolvimento do Submedio Sao Francisco, notadamente das suas cidades Polo, Petrolin,a e Juazeiro, que se constituem em verdadeiros "paraisos"de desenvolvimento dentro do Semi-arido brasileiro. Isto porque com o crescimento da fruticultura se desencadeou uma sinergia de crescimento em praticamente todos os setores produtivos, como o industrial, comercio e turismo. O objetivo deste trabalho e analisar a evolucao do Polo e pontos de estrangulamentos para a ampliacao do desenvolvimento da regiao
Driving-dependent damping of Rabi oscillations in two-level semiconductor systems
We propose a mechanism to explain the nature of the damping of Rabi
oscillations with increasing driving-pulse area in localized semiconductor
systems, and have suggested a general approach which describes a coherently
driven two-level system interacting with a dephasing reservoir. Present
calculations show that the non-Markovian character of the reservoir leads to
the dependence of the dephasing rate on the driving-field intensity, as
observed experimentally. Moreover, we have shown that the damping of Rabi
oscillations might occur as a result of different dephasing mechanisms for both
stationary and non-stationary effects due to coupling to the environment.
Present calculated results are found in quite good agreement with available
experimental measurements
Conformance relations for distributed testing based on CSP
Copyright @ 2011 Springer Berlin HeidelbergCSP is a well established process algebra that provides comprehensive theoretical and practical support for refinement-based design and verification of systems. Recently, a testing theory for CSP has also been presented. In this paper, we explore the problem of testing from a CSP specification when observations are made by a set of distributed testers. We build on previous work on input-output transition systems, but the use of CSP leads to significant differences, since some of its conformance (refinement) relations consider failures as well as traces. In addition, we allow events to be observed by more than one tester. We show how the CSP notions of refinement can be adapted to distributed testing. We consider two contexts: when the testers are entirely independent and when they can cooperate. Finally, we give some preliminary results on test-case generation and the use of coordination messages. © 2011 IFIP International Federation for Information Processing
Metric-signature Topological Transitions In Dispersive Metamaterials.
The metric signature topological transitions associated with the propagation of electromagnetic waves in a dispersive metamaterial with frequency-dependent and anisotropic dielectric and magnetic responses are examined in the present work. The components of the reciprocal-space metric tensor depend upon both the electric permittivity and magnetic permeability of the metamaterial, which are taken as Drude-like dispersive models. A thorough study of the frequency dependence of the metric tensor is presented which leads to the possibility of topological transitions of the isofrequency surface determining the wave dynamics inside the medium, to a diverging photonic density of states at some range of frequencies, and to the existence of large wave vectors' modes propagating through the metamaterial.8903320
Esporulação e eficiência de infecção do míldio da videira em cenários de mudanças climáticas.
O objetivo deste trabalho foi avaliar o efeito das mudanças climáticas na favorabilidade à ocorrência de míldio (Plasmopara viticola) na região de Bento Gonçalves-RS no período de 2020 a 2050. Este trabalho teve como base modelos de eficiência de infecção (EI) e esporulação (S) que descrevem esses processos com base em temperatura, duração do molhamento e duração de períodos com alta umidade. Para simulações futuras, foram usados os dados de temperatura do modelo climático regional PRECIS nos cenários IPCC A2 e B2 para 2020, 30, 40 e 50. Os resultados indicam pequena diminuição na frequência de dias com EI "alta" (0,7 a 1,0), o que também se reflete em pequena redução nas médias de EI. Por outro lado, ocorre um aumento mais expressivo na S, mais que compensando a perda de EI, resultando em provável maior intensidade da doença no futuro próximo, mantidas as condições de temperatura mais elevada e umidade estável
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