2,039 research outputs found
Chitosan application in maize (Zea mays) to counteract the effects of abiotic stress at seedling level
Worldwide, the conditions of biotic and abiotic stresses adversely affect the potential production of maize. Drought or heat facilitate the infection with fungi such as Aspergillus flavus and Fusarium moniliforme, and consequently increase the production of mycotoxins. There are several strategies for managing the problem, but in the future, people will prefer the cleaner and cheaper technology. The use of elicitors for protection of corn can be considered a cheap and clean technology. Chitosan elicitor is a linear polysaccharide produced commercially by deacetylation of chitin. It has been reported that this elicitor induce phytoalexin accumulation in plant tissue. Application of chitosan to seeds in rice significantly increased rice yield. About this, there are no reports in corn. For this reason, the aim of this study was to determine the protective effect of chitosan in maize seedlings subjected to abiotic stresses. To this end, three treatments were tested (a negative control, a positive control, and a group coated with chitosan solution) under four abiotic stresses conditions since their germination stage: drought, moisture, acid pH and alkaline pH. During five weeks, the seedlings growth was evaluated by measuring their total length, the length of leaves, stems and the thickness of these and presence of fungi. Positive effect was observed in seeds treated with chitosan or stressed with acidic pH in dimensions of seedlings and there was no fungal growth.Key words: Abiotic stress, Zea mays, chitosan, pH, drought, humidit
Single valley Dirac fermions in zero-gap HgTe quantum wells
Dirac fermions have been studied intensively in condensed matter physics in
recent years. Many theoretical predictions critically depend on the number of
valleys where the Dirac fermions are realized. In this work, we report the
discovery of a two dimensional system with a single valley Dirac cone. We study
the transport properties of HgTe quantum wells grown at the critical thickness
separating between the topologically trivial and the quantum spin Hall phases.
At high magnetic fields, the quantized Hall plateaus demonstrate the presence
of a single valley Dirac point in this system. In addition, we clearly observe
the linear dispersion of the zero mode spin levels. Also the conductivity at
the Dirac point and its temperature dependence can be understood from single
valley Dirac fermion physics.Comment: version 2: supplementary material adde
Gate-controlled Guiding of Electrons in Graphene
Ballistic semiconductor structures have allowed the realization of
optics-like phenomena in electronics, including magnetic focusing and lensing.
An extension that appears unique to graphene is to use both n and p carrier
types to create electronic analogs of optical devices having both positive and
negative indices of refraction. Here, we use gate-controlled density with both
p and n carrier types to demonstrate the analog of the fiber-optic guiding in
graphene. Two basic effects are investigated: (1) bipolar p-n junction guiding,
based on the principle of angle-selective transmission though the graphene p-n
interface, and (2) unipolar fiber-optic guiding, using total internal
reflection controlled by carrier density. Modulation of guiding efficiency
through gating is demonstrated and compared to numerical simulations, which
indicates that interface roughness limits guiding performance, with
few-nanometer effective roughness extracted. The development of p-n and
fiber-optic guiding in graphene may lead to electrically reconfigurable wiring
in high-mobility devices.Comment: supplementary materal at
http://marcuslab.harvard.edu/papers/OG_SI.pd
Duality covariant non-BPS first order systems
We study extremal black hole solutions to four dimensional N=2 supergravity
based on a cubic symmetric scalar manifold. Using the coset construction
available for these models, we define the first order flow equations implied by
the corresponding nilpotency conditions on the three-dimensional scalar momenta
for the composite non-BPS class of multi-centre black holes. As an application,
we directly solve these equations for the single-centre subclass, and write the
general solution in a manifestly duality covariant form. This includes all
single-centre under-rotating non-BPS solutions, as well as their
non-interacting multi-centre generalisations.Comment: 31 pages, v2: Discussion of the quadratic constraint clarified,
references added, typos corrected, published versio
Two-Centered Magical Charge Orbits
We determine the two-centered generic charge orbits of magical N = 2 and
maximal N = 8 supergravity theories in four dimensions. These orbits are
classified by seven U-duality invariant polynomials, which group together into
four invariants under the horizontal symmetry group SL(2,R). These latter are
expected to disentangle different physical properties of the two-centered
black-hole system. The invariant with the lowest degree in charges is the
symplectic product (Q1,Q2), known to control the mutual non-locality of the two
centers.Comment: 1+17 pages, 1 Table; v2: Eq. (3.23) corrected; v3: various
refinements in text and formulae, caption of Table 1 expanded, Footnote and
Refs. added. To appear on JHE
First-order flows and stabilisation equations for non-BPS extremal black holes
We derive a generalised form of flow equations for extremal static and
rotating non-BPS black holes in four-dimensional ungauged N = 2 supergravity
coupled to vector multiplets. For particular charge vectors, we give
stabilisation equations for the scalars, analogous to the BPS case, describing
full known solutions. Based on this, we propose a generic ansatz for the
stabilisation equations, which surprisingly includes ratios of harmonic
functions.Comment: 27 pages; v2: presentation improved and references added as in the
published versio
Matrix Norms, BPS Bounds and Marginal Stability in N=8 Supergravity
We study the conditions of marginal stability for two-center extremal black
holes in N-extended supergravity in four dimensions, with particular emphasis
on the N=8 case. This is achieved by exploiting triangle inequalities satisfied
by matrix norms. Using different norms and relative bounds among them, we
establish the existence of marginal stability and split attractor flows both
for BPS and some non-BPS solutions. Our results are in agreement with previous
analysis based on explicit construction of multi-center solutions.Comment: 1+15 pages; v2: some new formulas added and misprints corrected; v3:
typos fixed, various refinements, Sec. 2.4 rewritten; to appear on JHE
Magnetic Catalysis and Quantum Hall Ferromagnetism in Weakly Coupled Graphene
We study the realization in a model of graphene of the phenomenon whereby the
tendency of gauge-field mediated interactions to break chiral symmetry
spontaneously is greatly enhanced in an external magnetic field. We prove that,
in the weak coupling limit, and where the electron-electron interaction
satisfies certain mild conditions, the ground state of charge neutral graphene
in an external magnetic field is a quantum Hall ferromagnet which spontaneously
breaks the emergent U(4) symmetry to U(2)XU(2).
We argue that, due to a residual CP symmetry, the quantum Hall ferromagnet
order parameter is given exactly by the leading order in perturbation theory.
On the other hand, the chiral condensate which is the order parameter for
chiral symmetry breaking generically obtains contributions at all orders. We
compute the leading correction to the chiral condensate. We argue that the
ensuing fermion spectrum resembles that of massive fermions with a vanishing
U(4)-valued chemical potential. We discuss the realization of parity and charge
conjugation symmetries and argue that, in the context of our model, the charge
neutral quantum Hall state in graphene is a bulk insulator, with vanishing
longitudinal conductivity due to a charge gap and Hall conductivity vanishing
due to a residual discrete particle-hole symmetry.Comment: 35 page
Bound state solutions of the Dirac-Rosen-Morse potential with spin and pseudospin symmetry
The energy spectra and the corresponding two- component spinor wavefunctions
of the Dirac equation for the Rosen-Morse potential with spin and pseudospin
symmetry are obtained. The wave ( state) solutions for this
problem are obtained by using the basic concept of the supersymmetric quantum
mechanics approach and function analysis (standard approach) in the
calculations. Under the spin symmetry and pseudospin symmetry, the energy
equation and the corresponding two-component spinor wavefunctions for this
potential and other special types of this potential are obtained. Extension of
this result to state is suggested.Comment: 18 page
ADHD and brain anatomy:What do academic textbooks used in the Netherlands tell students?
Studies of brain size of children classified with ADHD appear to reveal smaller brains when compared to ‘normal’ children. Yet, what does this mean? Even with the use of rigorously screened case and control groups, these studies show only small, average group differences between children with and without an ADHD classification. However, academic textbooks used in the Netherlands often portray individual children with an ADHD classification as having a different, malfunctioning brain that necessitates medical intervention. This conceptualisation of ADHD might serve professional interests, but not necessarily the interests of children
- …