4,777 research outputs found
VHE gamma-ray observations of the young synchrotron-dominated SNRs G1.9+0.3 and G330.2+1.0 with H.E.S.S
Supernova remnants (SNRs) are widely considered to be accelerators of cosmic
rays (CR). They are also expected to produce very-high-energy (VHE;
GeV) gamma rays through interactions of high-energy particles with the
surrounding medium and photon fields. They are, therefore, promising targets
for observations with ground-based imaging atmospheric Cherenkov telescopes
like the H.E.S.S. telescope array. VHE gamma-ray emission has already been
discovered from a number of SNRs, establishing them as a prominent source class
in the VHE domain. Of particular interest are the handful of SNRs whose X-ray
spectra are dominated by non-thermal synchrotron emission, such as the VHE
gamma-ray emitters RX J0852.0-4622 (Vela Jr.) and RX J1713-3946. The shell-type
SNRs G1.9+0.3 and G330.2+1.0 also belong to this subclass and are further
notable for their young ages ( kyr), especially G1.9+0.3, which was
recently determined to be the youngest SNR in the Galaxy ( yr). These
unique characteristics motivated investigations with H.E.S.S. to search for VHE
gamma rays. The results of the H.E.S.S. observations and analyses are
presented, along with implications for potential particle acceleration
scenarios.Comment: ICRC 2011 proceedings, 4 pages, 2 figures, 3 table
Enhanced Optical Dichroism of Graphene Nanoribbons
The optical conductivity of graphene nanoribbons is analytical and exactly
derived. It is shown that the absence of translation invariance along the
transverse direction allows considerable intra-band absorption in a narrow
frequency window that varies with the ribbon width, and lies in the THz range
domain for ribbons 10-100nm wide. In this spectral region the absorption
anisotropy can be as high as two orders of magnitude, which renders the medium
strongly dichroic, and allows for a very high degree of polarization (up to
~85) with just a single layer of graphene. The effect is resilient to level
broadening of the ribbon spectrum potentially induced by disorder. Using a
cavity for impedance enhancement, or a stack of few layer nanoribbons, these
values can reach almost 100%. This opens a potential prospect of employing
graphene ribbon structures as efficient polarizers in the far IR and THz
frequencies.Comment: Revised version. 10 pages, 7 figure
New unidentified H.E.S.S. Galactic sources
H.E.S.S. is one of the most sensitive instruments in the very high energy
(VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the
Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004,
H.E.S.S. has continued and extended that survey in 2005-2008, discovering a
number of new sources, many of which are unidentified. Some of the unidentified
H.E.S.S. sources have several positional counterparts and hence several
different possible scenarios for the origin of the VHE gamma-ray emission;
their identification remains unclear. Others have so far no counterparts at any
other wavelength. Particularly, the lack of an X-ray counterpart puts serious
constraints on emission models. Several newly discovered and still unidentified
VHE sources are reported here.Comment: ICRC 2009 proceeding
Estudo comparativo entre uma pastagem nativa e o capim pangola (Digitaria decumbens) visando a produção de leite.
Produção de leite através de um criatorio em regime de campo na zona da mata de Pernambuco.Resumo
A hydrodynamic model approach to the formation of plasmonic wakes in graphene
Using the hydrodynamic model in the electrostatic approximation, we describe
the formation of graphene surface plasmons when a charge is in motion either
perpendicular or parallel to a graphene sheet. In the first case, the
electron-energy loss (EEL) spectrum of the electron is computed, showing that
the resonances in the spectrum are linked to the frequency of the graphene
surface plasmons. In the second case, we discuss the formation of plasmonic
wakes due to the dragging of the surface plasmons induced by the motion of the
charge. This effect is similar to Coulomb drag between two electron gases at a
distance from each other. We derive simple expressions for the electrostatic
potential induced by the moving charge on graphene. We find an analytical
expression for the angle of the plasmonic wake valid in two opposite regimes.
We show that there is a transition from a Mach-type wake at high speeds to a
Kelvin-type wake at low ones and identify the Froude number for plasmonic
wakes. We show that the Froude number can be controlled externally tunning both
the Fermi energy in graphene and the dielectric function of the environment, a
situation with no parallel in ship wakes.
Using EEL we propose a source of graphene plasmons, based on a graphene drum
built in a metallic waveguide and activated by an electron beam created by the
tip of an electronic microscope.
We also introduce the notion of a plasmonic billiard.A.J.C. acknowledges for a scholarship from the Brazilian agency CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico). N.M.R.P. acknowledges useful discussions with Jaime Santos and support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (Ref. No. 696656) and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2013. The Center for Nanostructured Graphene (CNG) was financed by the Danish National Research Council (DNRF103). N.A.M. is a VILLUM Investigator supported by VILLUM Fonden (Grant No. 16498).info:eu-repo/semantics/submittedVersio
Simple heuristics for the assembly line worker assignment and balancing problem
We propose simple heuristics for the assembly line worker assignment and
balancing problem. This problem typically occurs in assembly lines in sheltered
work centers for the disabled. Different from the classical simple assembly
line balancing problem, the task execution times vary according to the assigned
worker. We develop a constructive heuristic framework based on task and worker
priority rules defining the order in which the tasks and workers should be
assigned to the workstations. We present a number of such rules and compare
their performance across three possible uses: as a stand-alone method, as an
initial solution generator for meta-heuristics, and as a decoder for a hybrid
genetic algorithm. Our results show that the heuristics are fast, they obtain
good results as a stand-alone method and are efficient when used as a initial
solution generator or as a solution decoder within more elaborate approaches.Comment: 18 pages, 1 figur
Excitonic effects in the optical properties of 2D materials: An equation of motion approach
We present a unified description of the excitonic properties of four
monolayer transition-metal dichalcogenides (TMDC's) using an equation of motion
method for deriving the Bethe-Salpeter equation in momentum space. Our method
is able to cope with both continuous and tight-binding Hamiltonians, and is
less computational demanding than the traditional first-principles approach. We
show that the role of the exchange energy is essential to obtain a good
description of the binding energy of the excitons. The exchange energy at the
point is also essential to obtain the correct position of the
C-exciton peak. Using our model we obtain a good agreement between the Rydberg
series measured for WS. We discuss how the absorption and the Rydberg
series depend on the doping. Choosing and the doping we obtain a good
qualitative agreement between the experimental absorption and our calculations
for WS. We also derive a semi-analytical version of Ellitot's formula for
TMDC's.A J Chaves acknowledges a scholarship from the Brazilian agency CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico). N M R Peres and R M Ribeiro acknowledge support from the European Commission through the project 'Graphene-Driven Revolutions in ICT and Beyond' (Ref. No.696656), project PTDC/FIS-NAN/3668/2014, and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2013. T Frederico thanks for the support of the Brazilian Agencies CNPq and FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo). The authors acknowledge Fanyao Qu and Alan MacDonald for useful discussions about the numerical solution of the homogeneous Bethe-Salpeter equation, and Ermin Malic for sharing with us useful supplementary notes.info:eu-repo/semantics/publishedVersio
Estudos sobre nível proteíco e uréia na alimentação de vacas mestiças em produção.
Estudos sobre nivel proteina e ureia na alimentacao de vacas mesticas em producao
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