615 research outputs found
Extrinsic Curvature Embedding Diagrams
Embedding diagrams have been used extensively to visualize the properties of
curved space in Relativity. We introduce a new kind of embedding diagram based
on the {\it extrinsic} curvature (instead of the intrinsic curvature). Such an
extrinsic curvature embedding diagram, when used together with the usual kind
of intrinsic curvature embedding diagram, carries the information of how a
surface is {\it embedded} in the higher dimensional curved space. Simple
examples are given to illustrate the idea.Comment: 22 pages, 4 figure
Stability of six-dimensional hyperstring braneworlds
We study a six-dimensional braneworld model with infinite warped extra
dimensions in the case where the four-dimensional brane is described by a
topological vortex of a U(1) symmetry-breaking Abelian Higgs model in presence
of a negative cosmological constant. A detailed analysis of the microscopic
parameters leading to a finite volume space-time in the extra dimensions is
numerically performed. As previously shown, we find that a fine-tuning is
required to avoid any kind of singularity on the brane. We then discuss the
stability of the vortex by investigating the scalar part of the gauge-invariant
perturbations around this fine-tuned configuration. It is found that the
hyperstring forming Higgs and gauge fields, as well as the background metric
warp factors, cannot be perturbed at all, whereas transverse modes can be
considered stable. The warped space-time structure that is imposed around the
vortex thus appears severely constrained and cannot generically support
nonempty universe models. The genericness of our conclusions is discussed; this
will shed some light on the possibility of describing our space-time as a
general six-dimensional warped braneworld.Comment: 26 pages, 13 figures, uses RevTex, fine-tuning and stability analysis
discussed in greater details. Matches published versio
Global embedding of D-dimensional black holes with a cosmological constant in Minkowskian spacetimes: Matching between Hawking temperature and Unruh temperature
We study the matching between the Hawking temperature of a large class of
static D-dimensional black holes and the Unruh temperature of the corresponding
higher dimensional Rindler spacetime. In order to accomplish this task we find
the global embedding of the D-dimensional black holes into a higher dimensional
Minkowskian spacetime, called the global embedding Minkowskian spacetime
procedure (GEMS procedure). These global embedding transformations are
important on their own, since they provide a powerful tool that simplifies the
study of black hole physics by working instead, but equivalently, in an
accelerated Rindler frame in a flat background geometry. We discuss neutral and
charged Tangherlini black holes with and without cosmological constant, and in
the negative cosmological constant case, we consider the three allowed
topologies for the horizons (spherical, cylindrical/toroidal and hyperbolic).Comment: 7 pages; ReVTeX
Tiling of the five-fold surface of Al(70)Pd(21)Mn(9)
The nature of the five-fold surface of Al(70)Pd(21)Mn(9) has been
investigated using scanning tunneling microscopy. From high resolution images
of the terraces, a tiling of the surface has been constructed using pentagonal
prototiles. This tiling matches the bulk model of Boudard et. al. (J. Phys.:
Cond. Matter 4, 10149, (1992)), which allows us to elucidate the atomic nature
of the surface. Furthermore, it is consistent with a Penrose tiling T^*((P1)r)
obtained from the geometric model based on the three-dimensional tiling
T^*(2F). The results provide direct confirmation that the five-fold surface of
i-Al-Pd-Mn is a termination of the bulk structure.Comment: 4 pages, 4 figure
Racial Disparities in Intravenous Recombinant Tissue Plasminogen Activator Use Persist at Primary Stroke Centers.
BACKGROUND: Primary stroke centers (PSCs) utilize more recombinant tissue plasminogen activator (rt-PA) than non-PSCs. The impact of PSCs on racial disparities in rt-PA use is unknown.
METHODS AND RESULTS: We used data from the Nationwide Inpatient Sample from 2004 to 2010, limited to states that publicly reported hospital identity and race. Hospitals certified as PSCs by The Joint Commission were identified. Adults with a diagnosis of ischemic stroke were analyzed. Rt-PA use was defined by the International Classification of Diseases, 9th Revision procedure code 99.10. Discharges (304 152 patients) from 26 states met eligibility criteria, and of these 71.5% were white, 15.0% black, 7.9% Hispanic, and 5.6% other. Overall, 24.7% of white, 27.4% of black, 16.2% of Hispanic, and 29.8% of other patients presented to PSCs. A higher proportion received rt-PA at PSCs than non-PSCs in all race/ethnic groups (white 7.6% versus 2.6%, black 4.8% versus 2.0%, Hispanic 7.1% versus 2.4%, other 7.2% versus 2.5%, all P
CONCLUSIONS: Racial disparities in intravenous rt-PA use were not reduced by presentation to PSCs. Black patients were less likely to receive thrombolytic treatment than white patients at both non-PSCs and PSCs. Hispanic patients were less likely to be seen at PSCs relative to white patients and were less likely to receive intravenous rt-PA in the fully adjusted model
Berry phases for composite fermions: effective magnetic field and fractional statistics
The quantum Hall superfluid is presently the only viable candidate for a
realization of quasiparticles with fractional Berry phase statistics. For a
simple vortex excitation, relevant for a subset of fractional Hall states
considered by Laughlin, non-trivial Berry phase statistics were demonstrated
many years ago by Arovas, Schrieffer, and Wilczek. The quasiparticles are in
general more complicated, described accurately in terms of excited composite
fermions. We use the method developed by Kjonsberg, Myrheim and Leinaas to
compute the Berry phase for a single composite-fermion quasiparticle, and find
that it agrees with the effective magnetic field concept for composite
fermions. We then evaluate the "fractional statistics", related to the change
in the Berry phase for a closed loop caused by the insertion of another
composite-fermion quasiparticle in the interior. Our results support the
general validity of fractional statistics in the quantum Hall superfluid, while
also giving a quantitative account of corrections to it when the quasiparticle
wave functions overlap. Many caveats, both practical and conceptual, are
mentioned that will be relevant to an experimental measurement of the
fractional statistics. A short report on some parts of this article has
appeared previously.Comment: 14 pages, 9 figure
Analytic Coulomb matrix elements in the lowest Landau level in disk geometry
Using Darling's theorem on products of generalized hypergeometric series an
analytic expression is obtained for the Coulomb matrix elements in the lowest
Landau level in the representation of angular momentum. The result is important
in the studies of Fractional Quantum Hall effect (FQHE) in disk geometry.
Matrix elements are expressed as simple finite sums of positive terms,
eliminating the need to approximate these quantities with slowly-convergent
series. As a by-product, an analytic representation for certain integals of
products of Laguerre polynomials is obtained.Comment: Accepted to J. Math. Phys.; 3 pages revtex, no figure
A maximum density rule for surfaces of quasicrystals
A rule due to Bravais of wide validity for crystals is that their surfaces
correspond to the densest planes of atoms in the bulk of the material.
Comparing a theoretical model of i-AlPdMn with experimental results, we find
that this correspondence breaks down and that surfaces parallel to the densest
planes in the bulk are not the most stable, i.e. they are not so-called bulk
terminations. The correspondence can be restored by recognizing that there is a
contribution to the surface not just from one geometrical plane but from a
layer of stacked atoms, possibly containing more than one plane. We find that
not only does the stability of high-symmetry surfaces match the density of the
corresponding layer-like bulk terminations but the exact spacings between
surface terraces and their degree of pittedness may be determined by a simple
analysis of the density of layers predicted by the bulk geometric model.Comment: 8 pages of ps-file, 3 Figs (jpg
Vacuum solutions which cannot be written in diagonal form
A vacuum solution of the Einstein gravitational field equation is given that
follows from a general ansatz but fails to follow from it if a certain
symmetric matrix is assumed to be in diagonal form from the beginning.Comment: 18 pages, latex, no figures. An Acknowledgement, 4 references, and
the section "Note added" are adde
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