16,152 research outputs found
Non-Associativity in the Clifford Bundle on the Parallelizable Torsion 7-Sphere
In this paper we discuss generalized properties of non-associativity in
Clifford bundles on the 7-sphere S7. Novel and prominent properties inherited
from the non-associative structure of the Clifford bundle on S7 are
demonstrated. They naturally lead to general transformations of the spinor
fields on S7 and have dramatic consequences for the associated Kac-Moody
current algebras. All additional properties concerning the non-associative
structure in the Clifford bundle on S7 are considered. We further discuss and
explore their applications.Comment: 16 page
On Clifford Subalgebras, Spacetime Splittings and Applications
Z2-gradings of Clifford algebras are reviewed and we shall be concerned with
an alpha-grading based on the structure of inner automorphisms, which is
closely related to the spacetime splitting, if we consider the standard
conjugation map automorphism by an arbitrary, but fixed, splitting vector.
After briefly sketching the orthogonal and parallel components of products of
differential forms, where we introduce the parallel [orthogonal] part as the
space [time] component, we provide a detailed exposition of the Dirac operator
splitting and we show how the differential operator parallel and orthogonal
components are related to the Lie derivative along the splitting vector and the
angular momentum splitting bivector. We also introduce multivectorial-induced
alpha-gradings and present the Dirac equation in terms of the spacetime
splitting, where the Dirac spinor field is shown to be a direct sum of two
quaternions. We point out some possible physical applications of the formalism
developed.Comment: 22 pages, accepted for publication in International Journal of
Geometric Methods in Modern Physics 3 (8) (2006
Gravastars and Black Holes of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of anisotropic dark energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy fluid, and the
external Schwarzschild region. The models represent "bounded excursion" stable
gravastars, where the thin shell is oscillating between two finite radii, while
in other cases they collapse until the formation of black holes. Here we show,
for the first time in the literature, a model of gravastar and formation of
black hole with both interior and thin shell constituted exclusively of dark
energy. Besides, the sign of the parameter of anisotropy () seems to
be relevant to the gravastar formation. The formation is favored when the
tangential pressure is greater than the radial pressure, at least in the
neighborhood of the isotropic case ().Comment: 16 pages, 8 figures. Accepted for publication in Gen. Rel. Gra
False Vacuum Transitions - Analytical Solutions and Decay Rate Values
In this work we show a class of oscillating configurations for the evolution
of the domain walls in Euclidean space. The solutions are obtained
analytically. Phase transitions are achieved from the associated fluctuation
determinant, by the decay rates of the false vacuum.Comment: 6 pages, improved to match the final version to appear in EP
Anomalous temperature dependence of the band-gap in Black Phosphorus
Black Phosphorus (BP) has gained renewed attention due to its singular
anisotropic electronic and optical properties that might be exploited for a
wide range of technological applications. In this respect, the thermal
properties are particularly important both to predict its room temperature
operation and to determine its thermoelectric potential. From this point of
view, one of the most spectacular and poorly understood phenomena is, indeed,
the BP temperature-induced band-gap opening: when temperature is increased the
fundamental band-gap increases instead of decreasing. This anomalous thermal
dependence has also been observed, recently, in its monolayer counterpart. In
this work, based on \textit{ab-initio} calculations, we present an explanation
for this long known, and yet not fully explained, effect. We show that it
arises from a combination of harmonic and lattice thermal expansion
contributions, which are, in fact, highly interwined. We clearly narrow down
the mechanisms that cause this gap opening by identifying the peculiar atomic
vibrations that drive the anomaly. The final picture we give explains both the
BP anomalous band-gap opening and the frequency increase with increasing volume
(tension effect).Comment: Published in Nano Letter
Mimicking Nanoribbon Behavior Using a Graphene Layer on SiC
We propose a natural way to create quantum-confined regions in graphene in a
system that allows large-scale device integration. We show, using
first-principles calculations, that a single graphene layer on a trenched
region of mimics i)the energy bands around the Fermi level
and ii) the magnetic properties of free-standing graphene nanoribbons.
Depending on the trench direction, either zigzag or armchair nanoribbons are
mimicked. This behavior occurs because a single graphene layer over a
surface loses the graphene-like properties, which are restored solely over the
trenches, providing in this way a confined strip region.Comment: 4 pages, 4 figure
How the Charge Can Affect the Formation of Gravastars
In recent work we physically interpreted a special gravastar solution
characterized by a zero Schwarzschild mass. In fact, in that case, none
gravastar was formed and the shell expanded, leaving behind a de Sitter or a
Minkowski spacetime, or collapsed without forming an event horizon, originating
what we called a massive non-gravitational object. This object has two
components of non zero mass but the exterior spacetime is Minkowski or de
Sitter. One of the component is a massive thin shell and the other one is de
Sitter spacetime inside. The total mass of this object is zero Schwarzschild
mass, which characterizes an exterior vacuum spacetime. Here, we extend this
study to the case where we have a charged shell. Now, the exterior is a
Reissner-Nordstr\"om spacetime and, depending on the parameter
of the equation of state of the shell, and the charge, a
gravastar structure can be formed. We have found that the presence of the
charge contributes to the stability of the gravastar, if the charge is greater
than a critical value. Otherwise, a massive non-gravitational object is formed
for small charges.Comment: 17 pages and 7 figures, several typos corrected, accepted for
publication in JCA
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