4,754 research outputs found
Soil nitrous oxide fluxes following cover crops management under tillage and no tillage in South Brazil.
Fluorescência induzida por laser para análise da matéria orgânica de solos intactos.
Os estoques e a qualidade da matéria orgânica são indicadores importantes de qualidade de solos tropicais e subtropicais, e podem ser utilizados na avaliação da sustentabilidade de sistemas agrícolas. Nesse sentido, a fluorescência induzida por laser aplicada para amostras de solo tem se mostrado uma técnica eficiente e precisa para medir o teor de carbono orgânico e avaliar o grau de humificação da matéria orgânica. O princípio básico da técnica consiste em excitar o solo com um laser cuja emissão está na região do ultravioleta/azul o que resulta na fluorescência de grupos funcionais da matéria orgânica relacionados com o processo de humificação. Esta fluorescência ocorre na região do visível com pico em aproximadamente 510 nm. A fluorescência total (área sob a curva) correlaciona-se com os teores de carbono do solo, e quando ponderada a partir dos teores de C orgânico da amostra, traz informações a respeito do grau de humificação da matéria orgânica. Testes preliminares em laboratório demonstram o potencial da técnica e, a possibilidade de compactação deste instrumento aponta para um possível sensor de campo.bitstream/CNPDIA/8122/1/BPD03_2002.pd
Controlled lasing from active optomechanical resonators
Planar microcavities with distributed Bragg reflectors (DBRs) host, besides
confined optical modes, also mechanical resonances due to stop bands in the
phonon dispersion relation of the DBRs. These resonances have frequencies in
the sub-terahertz (10E10-10E11 Hz) range with quality factors exceeding 1000.
The interaction of photons and phonons in such optomechanical systems can be
drastically enhanced, opening a new route toward manipulation of light. Here we
implemented active semiconducting layers into the microcavity to obtain a
vertical-cavity surface-emitting laser (VCSEL). Thereby three resonant
excitations -photons, phonons, and electrons- can interact strongly with each
other providing control of the VCSEL laser emission: a picosecond strain pulse
injected into the VCSEL excites long-living mechanical resonances therein. As a
result, modulation of the lasing intensity at frequencies up to 40 GHz is
observed. From these findings prospective applications such as THz laser
control and stimulated phonon emission may emerge
Affine and toric hyperplane arrangements
We extend the Billera-Ehrenborg-Readdy map between the intersection lattice
and face lattice of a central hyperplane arrangement to affine and toric
hyperplane arrangements. For arrangements on the torus, we also generalize
Zaslavsky's fundamental results on the number of regions.Comment: 32 pages, 4 figure
Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse
We present a joint experimental and theoretical study on strong-field
photo-ionization of sodium atoms using chirped femtosecond laser pulses. By
tuning the chirp parameter, selectivity among the population in the highly
excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways
enabling control are identified by simultaneous ionization and measurement of
photoelectron angular distributions employing the velocity map imaging
technique. Free electron wave packets at an energy of around 1 eV are observed.
These photoelectrons originate from two channels. The predominant 2+1+1
Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly
driven two-photon transition , and subsequent
ionization from the states 5p, 6p and 7p whereas the second pathway involves
3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from
two-photon ionization of the non-resonant transiently populated state 3p are
observed close to the ionization threshold. A mainly qualitative five-state
model for the predominant excitation channel is studied theoretically to
provide insights into the physical mechanisms at play. Our analysis shows that
by tuning the chirp parameter the dynamics is effectively controlled by dynamic
Stark-shifts and level crossings. In particular, we show that under the
experimental conditions the passage through an uncommon three-state "bow-tie"
level crossing allows the preparation of coherent superposition states
Carbono orgânico dissolvido em diferentes sistemas de manejo de solo caracterizado por pluorescência.
Instantaneous Decentralized Poker
We present efficient protocols for amortized secure multiparty computation
with penalties and secure cash distribution, of which poker is a prime example.
Our protocols have an initial phase where the parties interact with a
cryptocurrency network, that then enables them to interact only among
themselves over the course of playing many poker games in which money changes
hands.
The high efficiency of our protocols is achieved by harnessing the power of
stateful contracts. Compared to the limited expressive power of Bitcoin
scripts, stateful contracts enable richer forms of interaction between standard
secure computation and a cryptocurrency.
We formalize the stateful contract model and the security notions that our
protocols accomplish, and provide proofs using the simulation paradigm.
Moreover, we provide a reference implementation in Ethereum/Solidity for the
stateful contracts that our protocols are based on.
We also adopt our off-chain cash distribution protocols to the special case
of stateful duplex micropayment channels, which are of independent interest. In
comparison to Bitcoin based payment channels, our duplex channel implementation
is more efficient and has additional features
Plantas de cobertura e adubação verde para citros.
bitstream/item/135173/1/Documento-383-Com-capa.pd
Curve classes on irreducible holomorphic symplectic varieties
We prove that the integral Hodge conjecture holds for 1-cycles on irreducible
holomorphic symplectic varieties of K3 type and of Generalized Kummer type. As
an application, we give a new proof of the integral Hodge conjecture for cubic
fourfolds.Comment: 15 page
Methane emission from soil under long-term no-till cropping systems.
Methane (CH4) emission from agricultural soils increases dramatically as a result of deleterious effect of soil disturbance and nitrogen fertilization on methanotrophic organisms; however, few studies have attempted to evaluate the potential of long-term conservation management systems to mitigate CH4 emissions in tropical and subtropical soils. This study aimed to evaluate the long-term effect (>19 years) of no-till grass- and legume-based cropping systems on annual soil CH4 fluxes in a formerly degraded Acrisol in Southern Brazil. Air sampling was carried out using static chambers and CH4 analysis by gas chromatography. Analysis of historical data set of the experiment evidenced a remarkable effect of high C- and N-input cropping systems on the improvement of biological, chemical, and physical characteristics of this no-tilled soil. Soil CH4 fluxes, which represent a net balance between consumption () and production (+) of CH4 in soil, varied from 40 2 to +62 78 mg C m2 h1. Mean weighted contents of ammonium (NH4+–N) and dissolved organic carbon (DOC) in soil had a positive relationship with accumulated soil CH4 fluxes in the post-management period (r2 = 0.95, p = 0.05), suggesting an additive effect of these nutrients in suppressing CH4 oxidation and stimulating methanogenesis, respectively, in legumebased cropping systems with high biomass input. Annual CH4 fluxes ranged from 50 610 to +994 105 g C ha1, which were inversely related to annual biomass-C input (r2 = 0.99, p = 0.003), with the exception of the cropping system containing pigeon pea, a summer legume that had the highest biologically fixed N input (>300 kg ha1 yr1). Our results evidenced a small effect of conservation management systems on decreasing CH4 emissions from soil, despite their significant effect restoring soil quality. We hypothesized that soil CH4 uptake strength has been off-set by an injurious effect of biologically fixed N in legume-based cropping systems on soil methanotrophic microbiota, and by the methanogenesis increase as a result of the O2 depletion in niches of high biological activity in the surface layer of the no-tillage soil
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