6,747 research outputs found
Crystal engineering using functionalized adamantane
We performed a first principles investigation on the structural, electronic,
and optical properties of crystals made of chemically functionalized adamantane
molecules. Several molecular building blocks, formed by boron and nitrogen
substitutional functionalizations, were considered to build zincblende and
wurtzite crystals, and the resulting structures presented large bulk moduli and
cohesive energies, wide and direct bandgaps, and low dielectric constants
(low- materials). Those properties provide stability for such
structures up to room temperature, superior to those of typical molecular
crystals. This indicates a possible road map for crystal engineering using
functionalized diamondoids, with potential applications ranging from space
filling between conducting wires in nanodevices to nano-electro-mechanical
systems
Electronic properties and hyperfine fields of nickel-related complexes in diamond
We carried out a first principles investigation on the microscopic properties
of nickel-related defect centers in diamond. Several configurations, involving
substitutional and interstitial nickel impurities, have been considered either
in isolated configurations or forming complexes with other defects, such as
vacancies and boron and nitrogen dopants. The results, in terms of spin,
symmetry, and hyperfine fields, were compared with the available experimental
data on electrically active centers in synthetic diamond. Several microscopic
models, previously proposed to explain those data, have been confirmed by this
investigation, while some models could be discarded. We also provided new
insights on the microscopic structure of several of those centers.Comment: 21 pages, 8 figure
Decay of metastable phases in a model for the catalytic oxidation of CO
We study by kinetic Monte Carlo simulations the dynamic behavior of a
Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O
CO on a catalytic surface. Finite-size scaling analysis of the fluctuations
and the fourth-order order-parameter cumulant show that below a critical CO
desorption rate, the model exhibits a nonequilibrium first-order phase
transition between low and high CO coverage phases. We calculate several points
on the coexistence curve. We also measure the metastable lifetimes associated
with the transition from the low CO coverage phase to the high CO coverage
phase, and {\it vice versa}. Our results indicate that the transition process
follows a mechanism very similar to the decay of metastable phases associated
with {\it equilibrium} first-order phase transitions and can be described by
the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by
nucleation and growth. In the present case, the desorption parameter plays the
role of temperature, and the distance to the coexistence curve plays the role
of an external field or supersaturation. We identify two distinct regimes,
depending on whether the system is far from or close to the coexistence curve,
in which the statistical properties and the system-size dependence of the
lifetimes are different, corresponding to multidroplet or single-droplet decay,
respectively. The crossover between the two regimes approaches the coexistence
curve logarithmically with system size, analogous to the behavior of the
crossover between multidroplet and single-droplet metastable decay near an
equilibrium first-order phase transition.Comment: 27 pages, 22 figures, accepted by Physical Review
Adubação Nitrogenada para Cultivares de Mamona no Rio Grande do Sul.
bitstream/item/30444/1/boletim-77.pd
Avaliação da composição quÃmica de três hÃbridos de milho (Zea mays L.) em quatro estádios de maturação.
O objetivo deste experimento foi avaliar a composição quÃmica da planta original inteira dos hÃbridos de milho BRS 1035, BRS 1031 e BRS 1001 em quatro idades de corte, a partir da determinação dos parâmetros: matéria seca (MS), proteÃna bruta (PB), fibra detergente neutro (FDN) e fibra detergente ácido (FDA). O delineamento utilizado foi inteiramente casualizado com fatorial3 (hÃbridos) x 4 (idades de corte), sendo as médias comparadas pelo teste SNK (P<0,05). Todos os hÃbridos apresentaram aumento do teor de MS de 87 para 171 dias. Os hÃbridos BRS 1031 e BRS 1001 apresentaram diferença (P<0,05) nos valores de MS entre todos os cortes; em contrapartida, o hÃbrido BRS 1035, aos 94 e 101 dias, apresentou valores de MS semelhantes (P<0,05) entre si. Os teores de PB do hÃbrido BRS 1031 aos 94 e 10 I dias foram semelhantes entre si (P<O,05) e inferiores aos demais (P<O,05); já o BRS 1001, aos 101 dias, foi inferior (P<O,05) aos demais cortes. O teor de PB aos 171 dias foi maior do que a encontrada aos 10 I dias nos hÃbridos BRS 1031 e BRS 100 I, provavelmente, devido à maior porcentagem de espigas no último corte. O teor de MS aumentou com o avanço da maturidade em todos os hÃbridos de milho sendo o valor recomendado para ensilagem observado em torno de 94 dias. Os hÃbridos avaliados neste trabalho possuem composição quÃmica compatÃvel com ensilagem, aos 94 dias de idade, e rolão de milho, aos 171 dias de idade
Probing QCD dynamics in two-photon interactions at high energies
In this paper the two-photon interactions at high energies are investigated
considering different approaches for the QCD dynamics. In particular, we
calculate the total cross section in different theoretical
approches and present a comparison among the predictions of the BFKL dynamics
at leading and next-to-leading order with those from saturation physics. We
analyze the possibility that the future linear colliders could discriminate
between these different approaches.Comment: 14 pages, 2 figures. Version to be published in Journal of Physics G:
Nuclear and Particle Physic
Avaliação do uso de torta de mamona (RICINUS COMMUNIS L.) desativada na ração para alimentação do ASTYANAX BIMACULATUS (LAMBARI) cultivado em sistema semi-intensivo.
Perfil fermentativo das silagens de seis genótipos de sorgo (Sorghum bicolor (L.) Moench): digestibilidade IN VITRO da matéria seca (DIVMS).
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