4,903 research outputs found
Confinement of two-dimensional excitons in a non-homogeneous magnetic field
The effective Hamiltonian describing the motion of an exciton in an external
non-homogeneous magnetic field is derived. The magnetic field plays the role of
an effective potential for the exciton motion, results into an increment of the
exciton mass and modifies the exciton kinetic energy operator. In contrast to
the homogeneous field case, the exciton in a non-homogeneous magnetic field can
also be trapped in the low field region and the field gradient increases the
exciton confinement. The trapping energy and wave function of the exciton in a
GaAs two-dimensional electron gas for specific circular magnetic field
configurations are calculated. The results show than excitons can be trapped by
non-homogeneous magnetic fields, and that the trapping energy is strongly
correlated with the shape and strength of the non-homogeneous magnetic field
profile.Comment: 9 pages, 12 figure
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Variação do pigmento antocianina em diferentes regiões do cacho de açaí.
O açaí processado nas amassadeiras ou em agroindústrias representa um dos alimentos mais populares da Região Amazônica, notadamente no Amapá, assim como abastecendo outros mercados nacionais e até internacionais. Além de representar fonte de nutriente, também é reconhecido pelas suas propriedades funcionais, através do pigmento antocianina, de efeito antioxidante. Um dos problemas observados no abastecimento do mercado de frutos de açaí é a restrição quanto à época de produção de frutos, definida em safras diferenciadas em áreas limítrofes dos estados do Amapá e Pará, levando consequentemente à escassez de frutos nas entressafras respectivas, com aumento de preço e diminuição da qualidade dos frutos. Um teste de progênies foi instalado pela Embrapa Amapá, em Mazagão, com materiais coletados no estuário amazônico, onde avaliou-se a qualidade do fruto, em relação ao teor de antocianina total em três regiões do cacho: início, meio e fim, para verificar a variabilidade de concentrações de antocianina durante a maturação dos frutos. A antocianina total foi extraída e determinada pelo método de Fuleki e Francis (1968), com modificações de Constant (2003), após um período de 24 horas de extração e os dados analisados pelo programa SAS 9.2 TS Level 2MO (STATISTICAL...2010). Somente os materiais que produzem na época de safra amapaense, apresentaram diferenciação significativa em relação ao teor de antocianina total no início do cacho (43,21 mg/100g fruto) em relação à região do meio (31,82 mg/100g fruto) e fim do cacho (27,50 mg/100g fruto), influenciado pelos índices de maturação variados. Frutos de açaizeiro oriundo de materiais que produzem na entressafra amapaense apresentaram índice de maturação homogêneo, com teores semelhantes em todo o cacho. Conclui-se que, em produção comercial objetivando a extração de maiores teores de antocianina total, e que produzem nas condições da safra amapaense, pode-se selecionar, preferencialmente, os frutos localizados na parte superior do cacho
The Eccentric Binary Millisecond Pulsar in NGC 1851
PSR J0514-4002A is a 5-ms pulsar is located in the globular cluster NGC 1851;
it belongs to a highly eccentric (e = 0.888) binary system. It is one of the
earliest known examples of a numerous and fast-growing class of eccentric
binary MSPs recently discovered in globular clusters. Using the GBT, we have
obtained a phase-coherent timing solution for the pulsar, which includes a
measurement of the rate of advance of periastron: 0.01289(4) degrees per year,
which if due completely to general relativity, implies a total system mass of
2.453(14) solar masses. We also derive m_p 0.96
solar masses. The companion is likely to be a massive white dwarf star.Comment: 3 pages, including 2 figures. To appear in the proceedings of "40
Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17,
2007, McGill University, Montreal, Canad
Volunteerism After the Tsunami: Democratization and Aid
Using three waves of survey data from fishing villages in Aceh, Indonesia for 2005-2009, we examine the determinants of local volunteer labor after the tsunami. Pre-existing social capital and the form of aid delivery (but not trauma) strongly affect village volunteerism initially, but these effects weaken with time. What persists is the effect of essentially a new institution, formal village elections. While recent work suggests democratization increases cooperation, the differentially timed introduction of elections negatively affects volunteerism, suggesting a regime switch effect where traditional leaders chosen by elites want more volunteer labor projects than democratically elected leaders do
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