1,278 research outputs found
On ``hyperboloidal'' Cauchy data for vacuum Einstein equations and obstructions to smoothness of ``null infinity''
Various works have suggested that the Bondi--Sachs--Penrose decay conditions
on the gravitational field at null infinity are not generally representative of
asymptotically flat space--times. We have made a detailed analysis of the
constraint equations for ``asymptotically hyperboloidal'' initial data and find
that log terms arise generically in asymptotic expansions. These terms are
absent in the corresponding Bondi--Sachs--Penrose expansions, and can be
related to explicit geometric quantities. We have nevertheless shown that there
exists a large class of ``non--generic'' solutions of the constraint equations,
the evolution of which leads to space--times satisfying the
Bondi--Sachs--Penrose smoothness conditions.Comment: 8 pages, revtex styl
Lp-cohomology of negatively curved manifolds
We compute the -cohomology spaces of some negatively curved manifolds.
We deal with two cases: manifolds with finite volume and sufficiently pinched
negative curvature, and conformally compact manifolds
Asymptotic gluing of asymptotically hyperbolic solutions to the Einstein constraint equations
We show that asymptotically hyperbolic solutions of the Einstein constraint
equations with constant mean curvature can be glued in such a way that their
asymptotic regions are connected.Comment: 37 pages; 2 figure
The critical Ising lines of the d=2 Ashkin-Teller model
The universal critical point ratio is exploited to determine positions of
the critical Ising transition lines on the phase diagram of the Ashkin-Teller
(AT) model on the square lattice. A leading-order expansion of the ratio in
the presence of a non-vanishing thermal field is found from finite-size scaling
and the corresponding expression is fitted to the accurate perturbative
transfer-matrix data calculations for the square clusters with
.Comment: RevTex, 4 pages, two figure
An organic solution to the Kelvin myth?: White light with true color temperature via exciplex formation
Abstract Many white light sources are described as having a "color temperature", a practice that has been termed the "Kelvin myth", since, in general, only incandescent light-emitters have a spectral distribution which closely approximates that of a black-body curve. We show that the white light emitted, via exciplex formation, from blends of two blue-emitting organic materials, has the same spectral form as black-body light, as perceived by the human eye. The color temperature is tunable, depending on the relative concentrations of the two materials. This suggests that organics could provide a white light source with a tunable, true, color temperature
Rigid ball-polyhedra in Euclidean 3-space
A ball-polyhedron is the intersection with non-empty interior of finitely
many (closed) unit balls in Euclidean 3-space. One can represent the boundary
of a ball-polyhedron as the union of vertices, edges, and faces defined in a
rather natural way. A ball-polyhedron is called a simple ball-polyhedron if at
every vertex exactly three edges meet. Moreover, a ball-polyhedron is called a
standard ball-polyhedron if its vertex-edge-face structure is a lattice (with
respect to containment). To each edge of a ball-polyhedron one can assign an
inner dihedral angle and say that the given ball-polyhedron is locally rigid
with respect to its inner dihedral angles if the vertex-edge-face structure of
the ball-polyhedron and its inner dihedral angles determine the ball-polyhedron
up to congruence locally. The main result of this paper is a Cauchy-type
rigidity theorem for ball-polyhedra stating that any simple and standard
ball-polyhedron is locally rigid with respect to its inner dihedral angles.Comment: 11 pages, 2 figure
Stability in Designer Gravity
We study the stability of designer gravity theories, in which one considers
gravity coupled to a tachyonic scalar with anti-de Sitter boundary conditions
defined by a smooth function W. We construct Hamiltonian generators of the
asymptotic symmetries using the covariant phase space method of Wald et al.and
find they differ from the spinor charges except when W=0. The positivity of the
spinor charge is used to establish a lower bound on the conserved energy of any
solution that satisfies boundary conditions for which has a global minimum.
A large class of designer gravity theories therefore have a stable ground
state, which the AdS/CFT correspondence indicates should be the lowest energy
soliton. We make progress towards proving this, by showing that minimum energy
solutions are static. The generalization of our results to designer gravity
theories in higher dimensions involving several tachyonic scalars is discussed.Comment: 29 page
Gluing construction of initial data with Kerr-de Sitter ends
We construct initial data sets which satisfy the vacuum constraint equa-
tions of General Relativity with positive cosmologigal constant. More pre-
silely, we deform initial data with ends asymptotic to Schwarzschild-de Sitter
to obtain non-trivial initial data with exactly Kerr-de Sitter ends. The method
is inspired from Corvino's gluing method. We obtain here a extension of a
previous result for the time-symmetric case by Chru\'sciel and Pollack.Comment: 27 pages, 3 figure
White light emission from blends of blue-emitting organic molecules: A general route to the white organic light-emitting diode?
We show that all possible binary combinations of molecules from four different families of organics - a diamine derivative, N,N′-bis(3methylphenyl)-N,N′-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole) - produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes
Equilibrium shapes and faceting for ionic crystals of body-centered-cubic type
A mean field theory is developed for the calculation of the surface free
energy of the staggered BCSOS, (or six vertex) model as function of the surface
orientation and of temperature. The model approximately describes surfaces of
crystals with nearest neighbor attractions and next nearest neighbor
repulsions. The mean field free energy is calculated by expressing the model in
terms of interacting directed walks on a lattice. The resulting equilibrium
shape is very rich with facet boundaries and boundaries between reconstructed
and unreconstructed regions which can be either sharp (first order) or smooth
(continuous). In addition there are tricritical points where a smooth boundary
changes into a sharp one and triple points where three sharp boundaries meet.
Finally our numerical results strongly suggest the existence of conical points,
at which tangent planes of a finite range of orientations all intersect each
other. The thermal evolution of the equilibrium shape in this model shows
strong similarity to that seen experimentally for ionic crystals.Comment: 14 Pages, Revtex and 10 PostScript figures include
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