7,377 research outputs found
Tendências de mercado e alternativas de comercialização do feijão.
O cultivo desta leguminosa é bastante difundido em todo o território nacional. É reconhecida como cultura de subsistência em pequenas propriedades. O sistema de comercialização é o mais variado possível. A melhoria do sistema de comercialização de feijão no Brasil poderia ser efetivada mediante maior organização dos produtores, maior difusão das informações de mercado e, ainda, o desenvolvimento das bolsas de cereais. Os pequenos produtores poderão aumentar a sua lucratividade organizando-se em cooperativas ou associações.bitstream/CNPAF/19677/1/comt_43.pd
A method for measuring the contact area in instrumented indentation testing by tip scanning probe microscopy imaging
The determination of the contact area is a key step to derive mechanical
properties such as hardness or an elastic modulus by instrumented indentation
testing. Two families of procedures are dedicated to extracting this area: on
the one hand, post mortem measurements that require residual imprint imaging,
and on the other hand, direct methods that only rely on the load vs. the
penetration depth curve. With the development of built-in scanning probe
microscopy imaging capabilities such as atomic force microscopy and indentation
tip scanning probe microscopy, last generation indentation devices have made
systematic residual imprint imaging much faster and more reliable. In this
paper, a new post mortem method is introduced and further compared to three
existing classical direct methods by means of a numerical and experimental
benchmark covering a large range of materials. It is shown that the new method
systematically leads to lower error levels regardless of the type of material.
Pros and cons of the new method vs. direct methods are also discussed,
demonstrating its efficiency in easily extracting mechanical properties with an
enhanced confidence
The moduli problem at the perturbative level
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuations can thus be translated into stringent constraints on the
moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum
expectation value at the end of inflation). These constraints are complementary
to previously existing bounds so that the moduli problem becomes worse at the
perturbative level. In particular, if the inflationary scale H_{inf}~10^{13}
GeV, particle physics scenarios which predict high moduli masses m_\sigma >
10-100 TeV are plagued by the perturbative moduli problem, even though they
evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the
amplitude/transfer of isocurvature modes at the end of inflation, typos
corrected, references added, basic result unchange
Enhanced spin Hall effect by tuning antidot potential: Proposal for a spin filter
We propose an efficient spin filter including an antidot fabricated on
semiconductor heterostructures with strong spin-orbit interaction. The antidot
creates a tunable potential on two-dimensional electron gas in the
heterostructures, which may be attractive as well as repulsive. Our idea is
based on the enhancement of extrinsic spin Hall effect by resonant scattering
when the attractive potential is properly tuned. Numerical studies for three-
and four-terminal devices indicate that the efficiency of the spin filter can
be more than 50% by tuning the potential to the resonant condition.Comment: 11 pages, 10 figure
Effects of Long-Range Correlations on Nonmagnetic Mott Transitions in Hubbard model on Square Lattice
The mechanism of Mott transition in the Hubbard model on the square lattice
is studied without explicit introduction of magnetic and superconducting
correlations, using a variational Monte Carlo method. In the trial wave
functions, we consider various types of binding factors between a
doubly-occupied site (doublon, D) and an empty site (holon, H), like a
long-range type as well as a conventional nearest-neighbor type, and add
independent long-range D-D (H-H) factors. It is found that a wide choice of D-H
binding factor leads to Mott transitions at critical values near the band
width. We renew the D-H binding picture of Mott transitions by introducing two
characteristic length scales, the D-H binding length l_{DH} and the minimum D-D
distance l_{DD}, which we appropriately estimate. A Mott transition takes place
at l_{DH}=l_{DD}. In the metallic regime (l_{DH}>l_{DD}), the domains of D-H
pairs overlap with one another, thereby doublons and holons can move
independently by exchanging the partners one after another. In contrast, the
D-D factors give only a minor contribution to the Mott transition.Comment: 13 pages, 18 figures, submitted to J. Phys. Soc. Jp
Variational Monte Carlo Studies of Pairing Symmetry for the t-J Model on a Triangular Lattice
As a model of a novel superconductor Na_xCoO_2\cdotyH_2O, a single-band t-J
model on a triangular lattice is studied, using a variational Monte Carlo
method. We calculate the energies of various superconducting (SC) states,
changing the doping rate \delta and sign of t for small J/|t|. Symmetries of s,
d, and d+id (p+ip and f) waves are taken up as candidates for singlet (triplet)
pairing. In addition, the possibility of Nagaoka ferromagnetism and
inhomogeneous phases is considered. It is revealed that, among the SC states,
the d+id wave always has the lowest energy, which result supports previous
mean-field studies. There is no possibility of triplet pairing, although the
f-wave state becomes stable against a normal state in a special case
(\delta=0.5 and t<0). For t<0, the complete ferromagnetic state is dominant in
a wide range of \delta and J/|t|, which covers the realistic parameter region
of superconductivity.Comment: 10 pages, 13 figure
Adaptação do arroz de terras altas no sistema plantio direto: manejo da adubação nitrogenada.
Com o objetivo de identificar a dose mais adequada de adubação nitrogenada para o arroz no sistema plantio direto após pastagem e soja, desenvolveu-se este trabalho.bitstream/item/59052/1/Foco-46.pd
Mott Transitions and d-wave Superconductivity in Half-Filled-Band Hubbard Model on Square Lattice with Geometric Frustration
Mechanisms of Mott transitions and dx2-y2-wave superconductivity (SC) are
studied in the half-filled-band Hubbard model on square lattices with a
diagonal hopping term (t'), using an optimization (or correlated) variational
Monte Carlo method. In the trial wave functions, a doublon-holon binding effect
is introduced in addition to the onsite Gutzwiller projection. We mainly treat
a d-wave singlet state and a projected Fermi sea. In both wave functions,
first-order Mott transitions without direct relevance to magnetic orders take
place at U=Uc approximately of the bandwidth for arbitrary t'/t. These
transitions originate in the binding or unbinding of a doublon to a holon.
d-wave SC appears in a narrow range immediately below Uc. The robust d-wave
superconducting correlation are necessarily accompanied by enhanced
antiferromagnetic correlation; the strength of SC becomes weak, as t'/t
increases.Comment: 18 pages, 30 figure
Dark energy from primordial inflationary quantum fluctuations
We show that current cosmic acceleration can be explained by an almost
massless scalar field experiencing quantum fluctuations during primordial
inflation. Provided its mass does not exceed the Hubble parameter today, this
field has been frozen during the cosmological ages to start dominating the
universe only recently. By using supernovae data, completed with baryonic
acoustic oscillations from galaxy surveys and cosmic microwave background
anisotropies, we infer the energy scale of primordial inflation to be around a
few TeV, which implies a negligible tensor-to-scalar ratio of the primordial
fluctuations. Moreover, our model suggests that inflation lasted for an
extremely long period. Dark energy could therefore be a natural consequence of
cosmic inflation close to the electroweak energy scale.Comment: 5 pages, 2 figures, uses RevTeX. Physical discussion extended,
misprints corrected, references added. Matches published versio
Cultivares de arroz de terras altas, BRS-Primavera e Maravilha, no Estado de Mato Grosso, no período de 1997/98 a 1999/00: impacto econômico.
Devido aos melhores atributos, tal como a qualidade de grãos, as novas cultivares de arroz de terras altas, BRS-Primavera e Maravilha, com grãos tipo agulhinha, entraram no mercado e se expandiram rapidamente, principalmente no Estado de Mato Grosso, substituindo, em parte, as cultivares tradicionalmente utilizadas. Este fato suscitou o interesse em medir a dinâmica de adoção destas cultivares neste Estado, bem como seu impacto na produção e na renda dos produtores.bitstream/item/59067/1/Foco-57.pd
- …