26,688 research outputs found
A note on the cylindrical collapse of counter-rotating dust
We find analytical solutions describing the collapse of an infinitely long
cylindrical shell of counter-rotating dust. We show that--for the classes of
solutions discussed herein--from regular initial data a curvature singularity
inevitably develops, and no apparent horizons form, thus in accord with the
spirit of the hoop conjecture.Comment: 8 pages, LaTeX, ijmpd macros (included), 1 eps figure; accepted for
publication in Int. J. Mod. Phys.
Lande g-tensor in semiconductor nanostructures
Understanding the electronic structure of semiconductor nanostructures is not
complete without a detailed description of their corresponding spin-related
properties. Here we explore the response of the shell structure of InAs
self-assembled quantum dots to magnetic fields oriented in several directions,
allowing the mapping of the g-tensor modulus for the s and p shells. We found
that the g-tensors for the s and p shells show a very different behavior. The
s-state in being more localized allows the probing of the confining potential
details by sweeping the magnetic field orientation from the growth direction
towards the in-plane direction. As for the p-state, we found that the g-tensor
modulus is closer to that of the surrounding GaAs, consistent with a larger
delocalization. These results reveal further details of the confining
potentials of self-assembled quantum dots that have not yet been probed, in
addition to the assessment of the g-tensor, which is of fundamental importance
for the implementation of spin related applications.Comment: 4 pages, 4 figure
Alterações físico-químicas durante o amadurecimento de Banana 'Prata Anã' em sete estádios de maturação.
A banana 'Prata Anã' é a variedade mais apreciada entre os brasileiros, fato comprovado pelo seu cultivo e sua boa aceitação comercial. A qualidade desses frutos constitui fator importante na comercialização, principalmente quando destinado ao consumo in natura. Desta forma, o objetivo foi avaliar as alterações físico-químicas que ocorrem durante os diferentes estádios de maturação da banana 'Prata Anã' cultivados na cidade de Cruz das Almas ? BA. Frutos foram avaliados diariamente quanto aos sete estádios de maturação estabelecidos na escala de Von Loesecke e avaliados em cada um dos estádios quanto a: perda de massa fresca; espessura da casca; relação polpa/casca; acidez titulável (AT); sólidos solúveis (SS) e relação SS/AT. Os resultados deste trabalho permitiram verificar que o amadurecimento dos frutos de ?Prata Anã? ocorre de maneira lenta durante os dois primeiros estádios, seguido de rápida transição e fortes alterações nas características físico-químicas da polpa entre os estádios 3 e 4. No entanto, o último estádio apresentou frutos com maior proporção de polpa e melhor balanço sólidos solúveis e acidez
Coherent states, constraint classes, and area operators in the new spin-foam models
Recently, two new spin-foam models have appeared in the literature, both
motivated by a desire to modify the Barrett-Crane model in such a way that the
imposition of certain second class constraints, called cross-simplicity
constraints, are weakened. We refer to these two models as the FKLS model, and
the flipped model. Both of these models are based on a reformulation of the
cross-simplicity constraints. This paper has two main parts. First, we clarify
the structure of the reformulated cross-simplicity constraints and the nature
of their quantum imposition in the new models. In particular we show that in
the FKLS model, quantum cross-simplicity implies no restriction on states. The
deeper reason for this is that, with the symplectic structure relevant for
FKLS, the reformulated cross-simplicity constraints, in a certain relevant
sense, are now \emph{first class}, and this causes the coherent state method of
imposing the constraints, key in the FKLS model, to fail to give any
restriction on states. Nevertheless, the cross-simplicity can still be seen as
implemented via suppression of intertwiner degrees of freedom in the dynamical
propagation. In the second part of the paper, we investigate area spectra in
the models. The results of these two investigations will highlight how, in the
flipped model, the Hilbert space of states, as well as the spectra of area
operators exactly match those of loop quantum gravity, whereas in the FKLS (and
Barrett-Crane) models, the boundary Hilbert spaces and area spectra are
different.Comment: 21 pages; statements about gamma limits made more precise, and minor
phrasing change
Consistent Gravitationally-Coupled Spin-2 Field Theory
Inspired by the translational gauge structure of teleparallel gravity, the
theory for a fundamental massless spin-2 field is constructed. Accordingly,
instead of being represented by a symmetric second-rank tensor, the fundamental
spin-2 field is assumed to be represented by a spacetime (world) vector field
assuming values in the Lie algebra of the translation group. The flat-space
theory naturally emerges in the Fierz formalism and is found to be equivalent
to the usual metric-based theory. However, the gravitationally coupled theory,
with gravitation itself described by teleparallel gravity, is shown not to
present the consistency problems of the spin-2 theory constructed on the basis
of general relativity.Comment: 16 pages, no figures. V2: Presentation changes, including addition of
a new sub-section, aiming at clarifying the text; version accepted for
publication in Class. Quantum Grav
Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets
Using a model of nonmagnetic impurity potential, we have examined the
behavior of planar vortex solutions in the classical two-dimensional XY
ferromagnets in the presence of a spin vacancy localized out of the vortex
core. Our results show that a spinless atom impurity gives rise to an effective
potential that repels the vortex structure.Comment: 6 pages, 2 figures, RevTex
Hodge Dual for Soldered Bundles
In order to account for all possible contractions allowed by the presence of
the solder form, a generalized Hodge dual is defined for the case of soldered
bundles. Although for curvature the generalized dual coincides with the usual
one, for torsion it gives a completely new dual definition. Starting from the
standard form of a gauge lagrangian for the translation group, the generalized
Hodge dual yields precisely the lagrangian of the teleparallel equivalent of
general relativity, and consequently also the Einstein-Hilbert lagrangian of
general relativity.Comment: 8 pages, no figures. Accepted for publication in Journal of Physics
Influence of extractive parameters on preparation of a solution from Arrabidaea chica Verl. (Bignoniaceae).
The aim of this work was to evaluate the influence of extractive parameters on tannin content and dry residue in solutions obtained from leaves of Arrabidaea chica
Teleparallel Spin Connection
A new expression for the spin connection of teleparallel gravity is proposed,
given by minus the contorsion tensor plus a zero connection. The corresponding
minimal coupling is covariant under local Lorentz transformation, and
equivalent to the minimal coupling prescription of general relativity. With
this coupling prescription, therefore, teleparallel gravity turns out to be
fully equivalent to general relativity, even in the presence of spinor fields.Comment: 2 pages, RevTeX, to appear in Phys. Rev D (Brief Report
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