29,014 research outputs found
Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids
Magnetic nanoparticles (NP) of magnetite (Fe3O4) coated with oleic acid (OA)
and dodecanoic acid (DA) were synthesized and investigated through Transmission
Electron Microscopy (TEM),magnetization M, and ac magnetic susceptibility
measurements. The OA coated samples were produced with different magnetic
concentrations (78, 76, and 65%) and the DA sample with 63% of Fe3O4. Images
from TEM indicate that the NP have a nearly spherical geometry and mean
diameter ~ 5.5 nm. Magnetization measurements, performed in zero field cooled
(ZFC) and field cooled (FC) processes under different external magnetic fields
H, exhibited a maximum at a given temperature TB in the ZFC curves, which
depends on the NP coating (OA or DA), magnetite concentration, and H. The
temperature TB decreases monotonically with increasing H and, for a given H,
the increase in the magnetite concentration results in an increase of TB. The
observed behavior is related to the dipolar interaction (DI) between NP which
seems to be an important mechanism in all samples studied. This is supported by
the results of the ac magnetic susceptibility Xac measurements, where the
temperature in which X' peaks for different frequencies follows the
Vogel-Fulcher model, a feature commonly found in systems with dipolar
interactions. Curves of H vs. TB/TB(H=0) for samples with different coatings
and magnetite concentrations collapse into a universal curve, indicating that
the qualitative magnetic behavior of the samples may be described by the NP
themselves, instead of the coating or the strength of the dipolar interaction.
Below TB, M vs. H curves show a coercive field (HC) that increases
monotonically with decreasing temperature. The saturation magnetization (MS)
follows the Bloch's law and values of MS at room temperature as high as 78
emu/g were estimated, a result corresponding to ~80% of the bulk value. The
overlap of M/MS vs. H/T curves for a given sample and the low HC at high
temperatures suggest superparamagnetic behavior in all samples studied. The
overlap of M/MS vs. H curves at constant temperature for different samples
indicates that the NP magnetization behavior is preserved, independently of the
coating and magnetite concentration.Comment: 8 pages and 9 figure
Universal conductance fluctuations in Dirac materials in the presence of long-range disorder
We study quantum transport in Dirac materials with a single fermionic Dirac
cone (strong topological insulators and graphene in the absence of intervalley
coupling) in the presence of non-Gaussian long-range disorder. We show, by
directly calculating numerically the conductance fluctuations, that in the
limit of very large system size and disorder strength, quantum transport
becomes universal. However, a systematic deviation away from universality is
obtained for realistic system parameters. By comparing our results to existing
experimental data on 1/f noise, we suggest that many of the graphene samples
studied to date are in a non-universal crossover regime of conductance
fluctuations.Comment: 5 pages, 3 figures. Published versio
Studies of Transverse Momentum Dependent Parton Distributions and Bessel Weighting
In this paper we present a new technique for analysis of transverse momentum
dependent parton distribution functions, based on the Bessel weighting
formalism. The procedure is applied to studies of the double longitudinal spin
asymmetry in semi-inclusive deep inelastic scattering using a new dedicated
Monte Carlo generator which includes quark intrinsic transverse momentum within
the generalized parton model. Using a fully differential cross section for the
process, the effect of four momentum conservation is analyzed using various
input models for transverse momentum distributions and fragmentation functions.
We observe a few percent systematic offset of the Bessel-weighted asymmetry
obtained from Monte Carlo extraction compared to input model calculations,
which is due to the limitations imposed by the energy and momentum conservation
at the given energy/Q2. We find that the Bessel weighting technique provides a
powerful and reliable tool to study the Fourier transform of TMDs with
controlled systematics due to experimental acceptances and resolutions with
different TMD model inputs.Comment: 30 pages, 8 figures, enhanced discussion and interpretation of
results, new section on errors with an appendix, added references. Accepted
for publication in JHE
Dynamic of a tropical forest submitted to reduced impact logging in Belterra, Pará, Brazil.
The Amazon has a great biodiversity that needs research to ensure its development without the extinction of its natural resources. This study aims to increase understanding of changes in dynamics (recruitment, growth and mortality) of a natural forest under low impact forest logging over a period of six years
The host galaxies of long-duration GRBs in a cosmological hierarchical scenario
We developed a Monte Carlo code to generate long-duration gamma ray burst
(LGRB) events within cosmological hydrodynamical simulations consistent with
the concordance model. As structure is assembled, LGRBs are generated in the
substructure that formed galaxies today. We adopted the collapsar model so that
LGRBs are produced by single, massive stars at the final stage of their
evolution. We found that the observed properties of the LGRB host galaxies
(HGs) are reproduced if LGRBs are also required to be generated by low
metallicity stars. The low metallicity condition imposed on the progenitor
stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H
\~ 8.6. For z<1 the simulated HGs of low metallicity LGRB progenitors tend to
be faint, slow rotators with high star formation efficiency, compared with the
general galaxy population, in agreement with observations. At higher redshift,
our results suggest that larger systems with high star formation activity could
also contribute to the generation of LGRBs from low metallicity progenitors
since the fraction of low metallicity gas available for star formation
increases for all systems with look-back time. Under the hypothesis of our LGRB
model, our results support the claim that LGRBs could be unbiased tracers of
star formation at high redshifts.Comment: Final revised version with minor changes. 9 pages, 9 figures,
mn2e.cls. To appear in MNRA
Measurement of the two-photon absorption cross-section of liquid argon with a time projection chamber
This paper reports on laser-induced multiphoton ionization at 266 nm of
liquid argon in a time projection chamber (LAr TPC) detector. The electron
signal produced by the laser beam is a formidable tool for the calibration and
monitoring of next-generation large-mass LAr TPCs. The detector that we
designed and tested allowed us to measure the two-photon absorption
cross-section of LAr with unprecedented accuracy and precision:
sigma_ex=(1.24\pm 0.10stat \pm 0.30syst) 10^{-56} cm^4s{-1}.Comment: 15 pages, 9 figure
Fruit transpiration drives interspecific variability in fruit growth strategies
Fruit growth is a complex mechanism resulting from biochemical and biophysical events leading water and dry matter to accumulate in the fruit tissues. Understanding how fruits choose their growth strategies can help growers optimizing their resource management for a more sustainable production and a higher fruit quality. This paper compares the growth strategies adopted by different fruit crops, at different times during the season and relates their fruit surface conductance to key physiological parameters for fruit growth such as phloem and xylem inflows as well transpiration losses. Our results show how fruits capacity to transpire (determined by their surface conductance) is a key driver in determining the growth strategy adopted by a species and explains the inter-species variability existing among different crops. Indeed, fruits change their surface conductance depending on the species and the phenological stage. This has an impact on the fruit's ability to lose water due to transpiration, affecting fruit pressure potential and increasing the force with which the fruit is able to attract xylem and phloem flows, with a considerable impact on fruit growth rate
Avaliação genética de procedências de bandarra (Schizolobium amazonicum) utilizando REML/BLUP (máxima verossimilhança restrita/melhor predição linear não viciada).)
Entre as essências florestais nativas da região norte de potencial para a produção madeireira destaca-se a bandarra (Schizolobium amazonicum) pelo seu rápido crescimento e qualidade de madeira. O objetivo deste trabalho foi quantificar a variabilidade genética entre procedências de bandarra e o progresso genético com a prática da seleção, visando caracterizar área de produção de sementes (APS). Foram avaliadas três procedências de bandarra dos Estados de Rondônia, Pará e Acre em delineamento de blocos ao acaso com informação dentro de parcela instalado no espaçamento de 4 x 2 m em Ouro Preto d´Oeste, Rondônia. Foram avaliados os caracteres: diâmetro à altura do peito DAP (cm), altura total ALT(m) e volume de madeira VOL (m³) aos nove anos de idade. A análise da variância indicou a existência de variabilidade genética entre as procedências a 1% de probabilidade somente para os caracteres ALT (m) e VOL (m³). A acurácia do procedimento de seleção entre procedências apresentou magnitude elevada e suas estimativas de progresso genético foram consideradas para prever a média na próxima geração. As estimativas de progresso genético indicam 20% de ganho em volume com a seleção entre procedências e 30% com a seleção entre e dentro de procedências. As procedências avaliadas apresentaram boa adaptabilidade geral, com potencial para a produção de madeira na região e variabilidade genética suficiente para ser explorada com estruturação em famÃlias
Utilização da bandarra (Schizolobiuim amazonicum) em sistemas agroflorestais
Renovação de lavouras cafeeiras e cacaueiras. Plantio novos: bandarra e café ou bandarra e cacau. Bandarra e cupuaçu e pimenta-do-reino e banana. SAF´s a partir de regeneração natural de bandarra. Uso de outras leguminosas em sistemas agroflorestais com bandarra. Bandarra e culturas anuais. Manutenção de sistemas agroflorestais.bitstream/item/67601/1/CPAF-RO-DOCUMENTOS-42-UTILIZACAO-DE-BANDARRA-SCHIZOLOBIUM-AMAZONICUM-EM-SISTEMAS-AGROFLORESTAIS-F.pd
Orbital occupancies and the putative jeff = 1/2 groundstate in Ba2IrO4: a combined oxygen K edge XAS and RIXS study
The nature of the electronic groundstate of Ba2IrO4 has been addressed using
soft X-ray absorption and inelastic scattering techniques in the vicinity of
the oxygen K edge. From the polarization and angular dependence of XAS we
deduce an approximately equal superposition of xy, yz and zx Ir4+ 5d orbitals.
By combining the measured orbital occupancies, with the value of the spin-orbit
coupling provided by RIXS, we estimate the crystal field splitting associated
with the tetragonal distortion of the IrO6 octahedra to be small, \Delta=50(50)
meV. We thus conclude definitively that Ba2IrO4 is a close realization of a
spin-orbit Mott insulator with a jeff = 1/2 groundstate, thereby overcoming
ambiguities in this assignment associated with the interpretation of X-ray
resonant scattering experiments.Comment: 5 pages, 5 figure
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