865 research outputs found
Symmetry constraints on phonon dispersion in graphene
Taking into account the constraints imposed by the lattice symmetry, we
calculate the phonon dispersion for graphene with interactions between the
first, second, and third nearest neighbors in the framework of the Born--von
Karman model. Analytical expressions obtained for the dispersion of the
out-of-plane (bending) modes give the nonzero sound velocity. The dispersion of
four in-plane modes is determined by coupled equations. Values of the force
constants are found in fitting with frequencies at critical points and with
elastic constants measured on graphite.Comment: 5 pages, 2 figure
Error correction in short time steps during the application of quantum gates
We propose a method for applying the quantum error-correction method for
errors that occur during quantum gates. Using a perturbation treatment of the
noise that allows us to separate it from the ideal evolution of the quantum
gate, we demonstrate that in certain cases it is necessary to divide the
quantum gate in short time steps intercalated by correction procedures. A
prescription of how these gates can be constructed is provided, as well as a
proof that, even for the cases when the division of the quantum gate in short
time steps is not necessary, this method may be advantageous for reducing the
total duration of the computation
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review
Estudos sobre a nutrição mineral do milho: II - deficiências de macronutrientes na variedade Piranão
Corn plants, Piranão variety, were grown in nutrient solution, complete and with omission of one macronutrient each time, in order to gain information on the effects of treatments on growth, symptoms of deficiency, and mineral composition of the leaves. The following symptoms of deficiency were obtained: Nitrogen -- plants underdeveloped; "V" shaped chlorosis beginning in the older leaves followed by drying and necrosis; Phosphorus - older leaves dark green with a purple hve; Potassium - plants underdeveloped; marginal chlorisis and necrosis in the older leaves; iron induced chlorosis in the younger leaves; Calcium - plants very underdeveloped due to death of apical merystem; younger leaves showing tip and marginal chlorosis and afterwards necrosis and break down of the tissue; wide whitish areas in the younger leaves; Magnesium - plants very underdeveloped; internerval chlorosis in older leaves first, proceeding to the young ones; Sulfur - absence of marked symptoms, except for a slight chlorosis in the younger leaves. Dry matter production obeyed the following decreasing order: Complete, minus S, minus P, minus K, minus Mg, minus N and minus Ca. Ears were obtained only in treatments complete and minus S. Top/root ratio decreased according to this order: minus S, complete, minus K, minus Mg, minus N, minus P and minus Ca. The macronutrient content for the treatment complete was the following respectively for lower and upper, leaves: N - 2.76 and 2.52%; P - 0.59 and 0.53%; K - 2.25 and 2.26%;Ca - 2.17 and 1.85;Mg - 0.82 and0.66;S - 0.38 and 0.32. The values of the element corresponding to each treatment were: N - 1.84 and 1.22 (that is: lower leaves in the minus N plants had 1 84% N, whereas the upper leaves in same treatment had 1.22% N; P - 0.14 and 0.17%; K - 0.29 and 0.78%; Ca - 0.29 and 0.05; Mg -0.09 and 0.05%; S- 0.31 and 0.26%.Foram induzidos sintomas de carência de macronutrientes no milho, var. Piranão, exceto no caso do enxofre. Os níveis foliares de N, P, K, Ca, Mg e S foram determinados nas plantas sujeitos à nutrição completa e às deficiências de macronutrientes
Ultrafast anisotropy decay reveals structure and energy transfer in supramolecular aggregates
NWO715.018.001Solid state NMR/Biophysical Organic Chemistr
Qualidade tecnológica, produtividade e margem de contribuição agrícola da cana-de-açúcar em função da aplicação de reguladores vegetais no início da safra
Yield and mineral nutrition of soybean, maize, and Congo signal grass as affected by limestone and slag
Vitamin C and total phenols quantification in calli of native passion fruit induced by combinations of Picloram and Kinetin
Scattering theory and ground-state energy of Dirac fermions in graphene with two Coulomb impurities
We study the physics of Dirac fermions in a gapped graphene monolayer containing two Coulomb impurities. For the case of equal impurity charges, we discuss the ground-state energy using the linear combination of atomic orbitals (LCAO) approach. For opposite charges of the Coulomb centers, an electric dipole potential results at large distances. We provide a nonperturbative analysis of the corresponding low-energy scattering problem
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