3,868 research outputs found
The role of spelling in learning to read.
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139815/1/LangandEdnPaper.pd
All Static Circularly Symmetric Perfect Fluid Solutions of 2+1 Gravity
Via a straightforward integration of the Einstein equations with cosmological
constant, all static circularly symmetric perfect fluid 2+1 solutions are
derived. The structural functions of the metric depend on the energy density,
which remains in general arbitrary. Spacetimes for fluids fulfilling linear and
polytropic state equations are explicitly derived; they describe, among others,
stiff matter, monatomic and diatomic ideal gases, nonrelativistic degenerate
fermions, incoherent and pure radiation. As a by--product, we demonstrate the
uniqueness of the constant energy density perfect fluid within the studied
class of metrics. A full similarity of the perfect fluid solutions with
constant energy density of the 2+1 and 3+1 gravities is established.Comment: revtex4, 8 page
Polymeric forms of carbon in dense lithium carbide
The immense interest in carbon nanomaterials continues to stimulate intense
research activities aimed to realize carbon nanowires, since linear chains of
carbon atoms are expected to display novel and technologically relevant
optical, electrical and mechanical properties. Although various allotropes of
carbon (e.g., diamond, nanotubes, graphene, etc.) are among the best known
materials, it remains challenging to stabilize carbon in the one-dimensional
form because of the difficulty to suitably saturate the dangling bonds of
carbon. Here, we show through first-principles calculations that ordered
polymeric carbon chains can be stabilized in solid LiC under moderate
pressure. This pressure-induced phase (above 5 GPa) consists of parallel arrays
of twofold zigzag carbon chains embedded in lithium cages, which display a
metallic character due to the formation of partially occupied carbon lone-pair
states in \emph{sp}-like hybrids. It is found that this phase remains the
most favorable one in a wide range of pressure. At extreme pressure (larger the
215 GPa) a structural and electronic phase transition towards an insulating
single-bonded threefold-coordinated carbon network is predicted.Comment: 10 pages, 6 figure
Rotating magnetic solution in three dimensional Einstein gravity
We obtain the magnetic counterpart of the BTZ solution, i.e., the rotating
spacetime of a point source generating a magnetic field in three dimensional
Einstein gravity with a negative cosmological constant. The static
(non-rotating) magnetic solution was found by Clement, by Hirschmann and Welch
and by Cataldo and Salgado. This paper is an extension of their work in order
to include (i) angular momentum, (ii) the definition of conserved quantities
(this is possible since spacetime is asymptotically anti-de Sitter), (iii)
upper bounds for the conserved quantities themselves, and (iv) a new
interpretation for the magnetic field source. We show that both the static and
rotating magnetic solutions have negative mass and that there is an upper bound
for the intensity of the magnetic field source and for the value of the angular
momentum. The magnetic field source can be interpreted not as a vortex but as
being composed by a system of two symmetric and superposed electric charges,
one of the electric charges is at rest and the other is spinning. The rotating
magnetic solution reduces to the rotating uncharged BTZ solution when the
magnetic field source vanishes.Comment: Latex (uses JHEP3.cls), 12 pages. Published versio
Energy and Momentum Distributions of the Magnetic Solution to (2+1) Einstein-Maxwell Gravity
We use Moeller's energy-momentum complex in order to explicitly evaluate the
energy and momentum density distributions associated with the three-dimensional
magnetic solution to the Einstein-Maxwell equations. The magnetic spacetime
under consideration is a one-parametric solution describing the distribution of
a radial magnetic field in a three-dimensional AdS background, and representing
the superposition of the magnetic field with a 2+1 Einstein static
gravitational field.Comment: LaTex, 13 pages; v2 clarifying comments and references added,
Conclusions improved, to appear in Mod. Phys. Lett.
Viscous cosmologies in scalar-tensor theories for Kasner type metrics
In a viscous Bianchi type I metric of the Kasner form, it is well known that
it is not possible to describe an anisotropic physical model of the universe,
which satisfies the second law of thermodynamics and the dominant energy
condition (DEC) in Einstein's theory of gravity. We examine this problem in
scalar-tensor theories of gravity. In this theory we show that it is possible
to describe the growth of entropy, keeping the thermodynamics and the dominant
energy condition.Comment: 6 pages, Latex, accepted in Phys. Rev.
No Black Hole Theorem in Three-Dimensional Gravity
A common property of known black hole solutions in (2+1)-dimensional gravity
is that they require a negative cosmological constant. In this letter, it is
shown that a (2+1)-dimensional gravity theory which satisfies the dominant
energy condition forbids the existence of a black hole to explain the above
situation.Comment: 3 pages, no figures, to be published in Physical Review Letter
Higher dimensional gravity invariant under the Poincare group
It is shown that the Stelle-West Grignani-Nardelli-formalism allows, both
when odd dimensions and when even dimensions are considered, constructing
actions for higher dimensional gravity invariant under local Lorentz rotations
and under local Poincar\`{e} translations. It is also proved that such actions
have the same coefficients as those obtained by Troncoso and Zanelli in ref.
Class. Quantum Grav. 17 (2000) 4451.Comment: 7 pages, Latex, accepted in Phys. Rev.
Interacting Kasner-type cosmologies
It is well known that Kasner-type cosmologies provide a useful framework for
analyzing the three-dimensional anisotropic expansion because of the
simplification of the anisotropic dynamics. In this paper relativistic
multi-fluid Kasner-type scenarios are studied. We first consider the general
case of a superposition of two ideal cosmic fluids, as well as the particular
cases of non-interacting and interacting ones, by introducing a
phenomenological coupling function . For two-fluid cosmological scenarios
there exist only cosmological scaling solutions, while for three-fluid
configurations there exist not only cosmological scaling ones, but also more
general solutions. In the case of triply interacting cosmic fluids we can have
energy transfer from two fluids to a third one, or energy transfer from one
cosmic fluid to the other two. It is shown that by requiring the positivity of
energy densities there always is a matter component which violates the dominant
energy condition in this kind of anisotropic cosmological scenarios.Comment: Accepted for publication in Astrophysics &Space Science, 8 page
Stability of the hard-sphere icosahedral quasilattice
The stability of the hard-sphere icosahedral quasilattice is analyzed using
the differential formulation of the generalized effective liquid approximation.
We find that the icosahedral quasilattice is metastable with respect to the
hard-sphere crystal structures. Our results agree with recent findings by
McCarley and Ashcroft [Phys. Rev. B {\bf 49}, 15600 (1994)] carried out using
the modified weighted density approximation.Comment: 15 pages, 2 figures available from authors upon request, (revtex),
submitted to Phys. Rev.
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