15,289 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
Potencial de populações de coqueiro gigante no meio-norte do Brasil para produção de biocombustíveis.
bitstream/CPATC/19785/1/f_23_2007.pd
Experimental investigation of quantum key distribution with position and momentum of photon pairs
We investigate the utility of Einstein-Podolsky-Rosen correlations of the
position and momentum of photon pairs from parametric down-conversion in the
implementation of a secure quantum key distribution protocol. We show that
security is guaranteed by the entanglement between downconverted pairs, and can
be checked by either direct comparison of Alice and Bob's measurement results
or evaluation of an inequality of the sort proposed by Mancini et al. (Phys.
Rev. Lett. 88, 120401 (2002)).Comment: 6 pages, 6 figures, subimitted for publicatio
The Apparent Fractal Conjecture: Scaling Features in Standard Cosmologies
This paper presents an analysis of the smoothness problem in cosmology by
focussing on the ambiguities originated in the simplifying hypotheses aimed at
observationally verifying if the large-scale distribution of galaxies is
homogeneous, and conjecturing that this distribution should follow a fractal
pattern in perturbed standard cosmologies. This is due to a geometrical effect,
appearing when certain types of average densities are calculated along the past
light cone. The paper starts reviewing the argument concerning the possibility
that the galaxy distribution follows such a scaling pattern, and the premises
behind the assumption that the spatial homogeneity of standard cosmology can be
observable. Next, it is argued that to discuss observable homogeneity one needs
to make a clear distinction between local and average relativistic densities,
and showing how the different distance definitions strongly affect them,
leading the various average densities to display asymptotically opposite
behaviours. Then the paper revisits Ribeiro's (1995: astro-ph/9910145) results,
showing that in a fully relativistic treatment some observational average
densities of the flat Friedmann model are not well defined at z ~ 0.1, implying
that at this range average densities behave in a fundamentally different manner
as compared to the linearity of the Hubble law, well valid for z < 1. This
conclusion brings into question the widespread assumption that relativistic
corrections can always be neglected at low z. It is also shown how some key
features of fractal cosmologies can be found in the Friedmann models. In view
of those findings, it is suggested that the so-called contradiction between the
cosmological principle, and the galaxy distribution forming an unlimited
fractal structure, may not exist.Comment: 30 pages, 2 figures, LaTeX. This paper is a follow-up to
gr-qc/9909093. Accepted for publication in "General Relativity and
Gravitation
Marcadores morfológicos em coqueiros (Cocos nucifera L.): coloracão do coleto.
Os marcadores de coloracao do coleto da plantula do coqueiro sao utilizados na certificacao de cruzamentos, baseado na heranca genetica das cores amarela, verde e marrom do coqueiro. Sua adequacao na certificacao e questionada.bitstream/item/44464/1/CPATC-DOCUMENTOS-7-MARCADORES-MORFOLOGICOS-EM-COQUEIROS-COCOS-NUCIFERA-L-COLORACAO-DO-COLETO-FL-1.pd
Differential Density Statistics of Galaxy Distribution and the Luminosity Function
This paper uses data obtained from the galaxy luminosity function (LF) to
calculate two types of radial number densities statistics of the galaxy
distribution as discussed in Ribeiro (2005), namely the differential density
and the integral differential density . By applying the
theory advanced by Ribeiro and Stoeger (2003), which connects the relativistic
cosmology number counts with the astronomically derived LF, the differential
number counts are extracted from the LF and used to calculate both
and with various cosmological distance definitions,
namely the area distance, luminosity distance, galaxy area distance and
redshift distance. LF data are taken from the CNOC2 galaxy redshift survey and
and are calculated for two cosmological models:
Einstein-de Sitter and an , standard
cosmology. The results confirm the strong dependency of both statistics on the
distance definition, as predicted in Ribeiro (2005), as well as showing that
plots of and against the luminosity and redshift
distances indicate that the CNOC2 galaxy distribution follows a power law
pattern for redshifts higher than 0.1. These findings bring support to
Ribeiro's (2005) theoretical proposition that using different cosmological
distance measures in statistical analyses of galaxy surveys can lead to
significant ambiguity in drawing conclusions about the behavior of the observed
large scale distribution of galaxies.Comment: LaTeX, 37 pages, 6 tables, 10 figures. Accepted for publication in
"The Astrophysical Journal
Confining potential in a color dielectric medium with parallel domain walls
We study quark confinement in a system of two parallel domain walls
interpolating different color dielectric media. We use the phenomenological
approach in which the confinement of quarks appears considering the QCD vacuum
as a color dielectric medium. We explore this phenomenon in QCD_2, where the
confinement of the color flux between the domain walls manifests, in a scenario
where two 0-branes (representing external quark and antiquark) are connected by
a QCD string. We obtain solutions of the equations of motion via first-order
differential equations. We find a new color confining potential that increases
monotonically with the distance between the domain walls.Comment: RevTex4, 5 pages, 1 figure; version to appear in Int. J. Mod. Phys.
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
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
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