20,514 research outputs found
Development of high strength, brazed aluminum, honeycomb sandwich composites adaptable for both elevated and cryogenic temperature applications, volume I - Brazing alloy development and selection Final report, Jul. 1963 - Sep. 1966
Brazing alloy development for high strength aluminum honeycomb sandwich composites for elevated and cryogenic temperature application
A Spherically Symmetric Closed Universe as an Example of a 2D Dilatonic Model
We study the two-dimensional (2D) dilatonic model describing a massless
scalar field minimally coupled to the spherically reduced Einstein-Hilbert
gravity. The general solution of this model is given in the case when a Killing
vector is present. When interpreted in four dimensions, the solution describes
either a static or a homogeneous collision of incoming and outgoing null dust
streams with spherical symmetry. The homogeneous Universe is closed.Comment: 5 pages, 2 figures, to appear in Physical Review
On the origin and application of the Bruggeman correlation for analysing transport phenomena in electrochemical systems
The widely used Bruggeman equations correlate tortuosity factors of porous media with their porosity. Finding diverse application from optics to bubble formation, it received considerable attention in fuel cell and battery research, recently. The ability to estimate tortuous mass transport resistance based on porosity alone is attractive, because direct access to the tortuosity factors is notoriously difficult. The correlation, however, has limitations, which are not widely appreciated owing to the limited accessibility of the original manuscript. We retrace Bruggeman's derivation, together with its initial assumptions, and comment on validity and limitations apparent from the original work to offer some guidance on its use
Axial symmetry and conformal Killing vectors
Axisymmetric spacetimes with a conformal symmetry are studied and it is shown
that, if there is no further conformal symmetry, the axial Killing vector and
the conformal Killing vector must commute. As a direct consequence, in
conformally stationary and axisymmetric spacetimes, no restriction is made by
assuming that the axial symmetry and the conformal timelike symmetry commute.
Furthermore, we prove that in axisymmetric spacetimes with another symmetry
(such as stationary and axisymmetric or cylindrically symmetric spacetimes) and
a conformal symmetry, the commutator of the axial Killing vector with the two
others mush vanish or else the symmetry is larger than that originally
considered. The results are completely general and do not depend on Einstein's
equations or any particular matter content.Comment: 15 pages, Latex, no figure
Maxwell Fields in Spacetimes Admitting Non-Null Killing Vectors
We consider source-free electromagnetic fields in spacetimes possessing a
non-null Killing vector field, . We assume further that the
electromagnetic field tensor, , is invariant under the action of the
isometry group induced by . It is proved that whenever the two
potentials associated with the electromagnetic field are functionally
independent the entire content of Maxwell's equations is equivalent to the
relation \n^aT_{ab}=0. Since this relation is implied by Einstein's equation
we argue that it is enough to solve merely Einstein's equation for these
electrovac spacetimes because the relevant equations of motion will be
satisfied automatically. It is also shown that for the exceptional case of
functionally related potentials \n^aT_{ab}=0 implies along with one of the
relevant equations of motion that the complementary equation concerning the
electromagnetic field is satisfied.Comment: 7 pages,PACS numbers: 04.20.Cv, 04.20.Me, 04.40.+
Observables in the Decays of B to Two Vector Mesons
In general there are nine observables in the decay of a B meson to two vector
mesons defined in terms of polarization correlations of these mesons. Only six
of these can be detected via the subsequent decay angular distributions because
of parity conservation in those decays. The remaining three require the
measurement of the spin polarization of one of the decay products.Comment: 12 pages, no figur
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
Gravitating superconducting strings with timelike or spacelike currents
We construct gravitating superconducting string solutions of the U(1)_{local}
x U(1)_{global} model solving the coupled system of Einstein and matter field
equations numerically. We study the properties of these solutions in dependence
on the ratio between the symmetry breaking scale and the Planck mass. Using the
macroscopic stability conditions formulated by Carter, we observe that the
coupling to gravity allows for a new stable region that is not present in the
flat space-time limit. We match the asymptotic metric to the Kasner metric and
show that the relations between the Kasner coefficients and the energy per unit
length and tension suggested previously are well fulfilled for symmetry
breaking scale much smaller than the Planck mass. We also study the solutions
to the geodesic equation in this space-time. While geodesics in the exterior
space-time of standard cosmic strings are just straight lines, test particles
experience a force in a general Kasner space-time and as such bound orbits are
possible.Comment: 16 pages including 14 figure
On the variable-charged black holes embedded into de Sitter space: Hawking's radiation
In this paper we study the Hawking evaporation of masses of variable-charged
Reissner-Nordstrom and Kerr-Newman, black holes embedded into the de Sitter
universe by considering the charge to be function of radial coordinate of the
spherically symmetric metric.Comment: LaTex, p. 2
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