8,785 research outputs found
Efficient graphene-based photodetector with two cavities
We present an efficient graphene-based photodetector with two Fabri-P\'erot
cavities. It is shown that the absorption can reach almost 100% around a given
frequency, which is determined by the two-cavity lengths. It is also shown that
hysteresis in the absorbance is possible, with the transmittance amplitude of
the mirrors working as an external driving field. The role of non-linear
contributions to the optical susceptibility of graphene is discussed.Comment: 10 pages, 8 figures. published version: minor revisio
Distortion of the perfect lattice structure in bilayer graphene
We consider the instability of bilayer graphene with respect to a distorted
configuration in the same spirit as the model introduced by Su, Schrieffer and
Heeger. By computing the total energy of a distorted bilayer, we conclude that
the ground state of the system favors a finite distortion. We explore how the
equilibrium configuration changes with carrier density and an applied potential
difference between the two layers
Noncommutative fields in three dimensions and mass generation
We apply the noncommutative fields method for gauge theory in three
dimensions where the Chern-Simons term is generated in the three-dimensional
electrodynamics. Under the same procedure, the Chern-Simons term is shown to be
cancelled in the Maxwell-Chern-Simons theory for the appropriate value of the
noncommutativity parameter. Hence the mutual interchange between
Maxwell-Chern-Simons theory and pure Maxwell theory turns out to be generated
within this method.Comment: Comments 5 pages, epl, version accepted for publication in
Europhysics Letter
Radio Frequency Models of Novae in eruption. I. The Free-Free Process in Bipolar Morphologies
Observations of novae at radio frequencies provide us with a measure of the
total ejected mass, density profile and kinetic energy of a nova eruption. The
radio emission is typically well characterized by the free-free emission
process. Most models to date have assumed spherical symmetry for the eruption,
although it has been known for as long as there have been radio observations of
these systems, that spherical eruptions are to simplistic a geometry. In this
paper, we build bipolar models of the nova eruption, assuming the free-free
process, and show the effects of varying different parameters on the radio
light curves. The parameters considered include the ratio of the minor- to
major-axis, the inclination angle and shell thickness (further parameters are
provided in the appendix). We also show the uncertainty introduced when fitting
spherical model synthetic light curves to bipolar model synthetic light curves.
We find that the optically thick phase rises with the same power law () for both the spherical and bipolar models. In the bipolar case
there is a "plateau" phase -- depending on the thickness of the shell as well
as the ratio of the minor- to major-axis -- before the final decline, that
follows the same power law () as in the spherical case.
Finally, fitting spherical models to the bipolar model synthetic light curves
requires, in the worst case scenario, doubling the ejected mass, more than
halving the electron temperature and reducing the shell thickness by nearly a
factor of 10. This implies that in some systems we have been over predicting
the ejected masses and under predicting the electron temperature of the ejecta.Comment: 9 pages, 6 figures, accepted for publication in ApJ, accompanying
movie to figure 3 available at
http://www.ast.uct.ac.za/~valerio/papers/radioI
f [N pi N]: from quarks to the pion derivative coupling
We study the N pi N coupling, in the framework of a QCD-inspired confining
Nambu-Jona-Lasinio model. A simple relativistic confining and instantaneous
quark model is reviewed. The Salpeter equation for the nucleon and the boosted
pion is solved. The f [n pi n] and f[n pi Delta] couplings are calculated and
they turn out to be reasonably good. The sensibility of f[n pi n] and f[n pi
Delta] to confinement, chiral symmetry breaking and Lorentz invariance is
briefly discussed.Comment: 30 pages in LaTex RevTex, 6 postscript figure
Multiple negative differential conductance regions and inelastic phonon assisted tunneling in graphene-hBN-graphene structures
In this paper we study in detail the effect of the rotational alignment between a hexagonal boron nitride (hBN) slab and the graphene layers in the vertical current of a a graphene-hBN-graphene device. We show how for small rotational angles, the transference of momentum by the hBN crystal lattice leads to multiple peaks in the I-V curve of the device, giving origin to multiple regions displaying negative differential conductance. We also study the effect of scattering by phonons in the vertical current an see how the opening up of inelastic tunneling events allowed by spontaneous emission of optical phonons leads to sharp peaks in the second derivative of the current.B. Amorim acknowledges financial support from Fundação para a Ciência e a Tecnologia (Portugal), through Grant No. SFRH/BD/78987/2011. R.M. Ribeiro and N.M.R. Peres acknowledge the financial support of EC under Graphene Flagship (Contract No. CNECT-ICT-604391). N. M. R. Peres acknowledges financial support from the FCT project EXPL-FIS-NAN1728-2013
Entropy and holography constraints for inhomogeneous universes
We calculated the entropy of a class of inhomogeneous dust universes.
Allowing spherical symmetry, we proposed a holographic principle by reflecting
all physical freedoms on the surface of the apparent horizon. In contrast to
flat homogeneous counterparts, the principle may break down in some models,
though these models are not quite realistic. We refined fractal parabolic
solutions to have a reasonable entropy value for the present observable
universe and found that the holographic principle always holds in the realistic
cases.Comment: 4 pages, revtex style, 3 figures in 8 eps-file
Efeito do cultivo de espécies vegetais sobre a população de Pratylenchus brachyurus na soja.
O presente trabalho, conduzido em casa de vegetação da Embrapa Soja, em Londrina, PR, no período de novembro/2011 a março/2012, teve como objetivos principais conhecer a reação de 23 espécies vegetais a Pratylenchus brachyurus e avaliar a influência da incorporação de suas fitomassas (parte aérea e raízes) no solo sobre as população do nematoide e performance da soja semeada, na sequência, nos vasos. Os fatores de reprodução (FR) de P. brachyurus obtidos nas diferentes espécies vegetais avaliadas variaram de zero (Crotalaria spectabilis, crambe, guandu ‘Fava Larga’ e tremoço branco) a 4,8 (Lab- Lab ‘Rongai’). As espécies vegetais com FR mais altos, independente da quantidade/composição da fitomassa incorporada, resultaram sempre em populações do nematoide mais elevadas nas raízes da soja (‘CD 202’). De todas as espécies vegetais testadas, apenas as C. specetabilis (FR=zero) e C. ochroleuca (FR=0,3) reduziram de fato a população original (1.000 nematoides) de P. brachyurus no vaso
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