157 research outputs found
CYK Tensors, Maxwell Field and Conserved Quantities for Spin-2 Field
Starting from an important application of Conformal Yano--Killing tensors for
the existence of global charges in gravity, some new observations at \scri^+
are given. They allow to define asymptotic charges (at future null infinity) in
terms of the Weyl tensor together with their fluxes through \scri^+. It
occurs that some of them play a role of obstructions for the existence of
angular momentum.
Moreover, new relations between solutions of the Maxwell equations and the
spin-2 field are given. They are used in the construction of new conserved
quantities which are quadratic in terms of the Weyl tensor. The obtained
formulae are similar to the functionals obtained from the
Bel--Robinson tensor.Comment: 20 pages, LaTe
Parameters of a gas-solids JET in pneumatic powder injection into liquid alloys with a non-submerged lance
The paper presents powder injection into liquid alloys with a non-submerged lance. The parameters of the diphase gas-solids jet were found as the most important factor to achieve good efficiency of the process. If the parameters are improper, the jet will not penetrate the liquid and the solid particles will not be uniformly distributed. The jet cone profile is often crucial for diphase jet penetration, so this parameter was analyzed along with particle velocity on the lance outlet and the experiments proven this assumption. The use of a high-speed camera allowed to capture and analyze jet motion, which verified the data of previous authors and that in the literature. Experiments of both the model and real injection into molten cast iron proved both the mathematical model and numerical simulation
Asymptotic Conformal Yano--Killing Tensors for Schwarzschild Metric
The asymptotic conformal Yano--Killing tensor proposed in J. Jezierski, On
the relation between metric and spin-2 formulation of linearized Einstein
theory [GRG, in print (1994)] is analyzed for Schwarzschild metric and tensor
equations defining this object are given. The result shows that the
Schwarzschild metric (and other metrics which are asymptotically
``Schwarzschildean'' up to O(1/r^2) at spatial infinity) is among the metrics
fullfilling stronger asymptotic conditions and supertranslations ambiguities
disappear. It is also clear from the result that 14 asymptotic gravitational
charges are well defined on the ``Schwarzschildean'' background.Comment: 8 pages, latex, no figure
Conformal Yano-Killing tensor for the Kerr metric and conserved quantities
Properties of (skew-symmetric) conformal Yano--Killing tensors are reviewed.
Explicit forms of three symmetric conformal Killing tensors in Kerr spacetime
are obtained from the Yano--Killing tensor. The relation between spin-2 fields
and solutions to the Maxwell equations is used in the construction of a new
conserved quantity which is quadratic in terms of the Weyl tensor. The formula
obtained is similar to the functional obtained from the Bel--Robinson tensor
and is examined in Kerr spacetime. A new interpretation of the conserved
quantity obtained is proposed.Comment: 29 page
Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb
The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped CuMnSb compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
, or larger if a correlated band structure is taken as the
reference
Unexpected coexisting solid solutions in the quasi-binary Ag(II)F2/Cu(II)F2 phase diagram
High-temperature solid-state reaction between orthorhombic AgF2 and
monoclinic CuF2 (y = 0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in
coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic
phases with stoichiometry Ag1-xCuxF2. Based on X-ray powder diffraction
analyses, the mutual solubility in the orthorhombic phase (AgF2 doped with Cu)
appears to be at an upper limit of Cu concentration of 30 mol % (Ag0.7Cu0.3F2),
while the monoclinic phase (CuF2 doped with Ag) can form a nearly
stoichiometric Cu : Ag = 1 : 1 solid solution (Cu0.56Ag0.44F2), preserving the
CuF2 crystal structure. Experimental data and DFT calculations showed that AgF2
doped with Cu and CuF2 doped with Ag solid solutions deviate from the classical
Vegards law. Magnetic measurements of Ag1-xCuxF2 showed that the Neel
temperature (TN) decreases with increasing Cu content in both phases. Likewise,
theoretical DFT+U calculations for Ag1-xCuxF2 showed that the progressive
substitution of Ag by Cu decreases the magnetic interaction strength (J2D) in
both structures. Electrical conductivity measurements of Ag0.85Cu0.15F2 showed
a ca. 2-fold increase in specific ionic conductivity (3.71 x 10-13 plus/minus
2.6 x 10-15 S/cm) as compared to pure AgF2 (1.85 x 10-13 plus/minus 1.2 x 10-15
S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride
sample.Comment: 9 pages, 4 figures, 1 Table, and electronic supplement of 14 page
Covariant Perturbations of Schwarzschild Black Holes
We present a new covariant and gauge-invariant perturbation formalism for
dealing with spacetimes having spherical symmetry (or some preferred spatial
direction) in the background, and apply it to the case of gravitational wave
propagation in a Schwarzschild black hole spacetime. The 1+3 covariant approach
is extended to a `1+1+2 covariant sheet' formalism by introducing a radial unit
vector in addition to the timelike congruence, and decomposing all covariant
quantities with respect to this. The background Schwarzschild solution is
discussed and a covariant characterisation is given. We give the full
first-order system of linearised 1+1+2 covariant equations, and we show how, by
introducing (time and spherical) harmonic functions, these may be reduced to a
system of first-order ordinary differential equations and algebraic constraints
for the 1+1+2 variables which may be solved straightforwardly. We show how both
the odd and even parity perturbations may be unified by the discovery of a
covariant, frame- and gauge-invariant, transverse-traceless tensor describing
gravitational waves, which satisfies a covariant wave equation equivalent to
the Regge-Wheeler equation for both even and odd parity perturbations. We show
how the Zerilli equation may be derived from this tensor, and derive a similar
transverse traceless tensor equivalent to this equation. The so-called
`special' quasinormal modes with purely imaginary frequency emerge naturally.
The significance of the degrees of freedom in the choice of the two frame
vectors is discussed, and we demonstrate that, for a certain frame choice, the
underlying dynamics is governed purely by the Regge-Wheeler tensor. The two
transverse-traceless Weyl tensors which carry the curvature of gravitational
waves are discussed.Comment: 23 pages, 1 figure, Revtex 4. Submitted to Classical and Quantum
Gravity. Revised version is significantly streamlined with an important error
corrected which simplifies the presentatio
Construction and enlargement of traversable wormholes from Schwarzschild black holes
Analytic solutions are presented which describe the construction of a
traversable wormhole from a Schwarzschild black hole, and the enlargement of
such a wormhole, in Einstein gravity. The matter model is pure radiation which
may have negative energy density (phantom or ghost radiation) and the
idealization of impulsive radiation (infinitesimally thin null shells) is
employed.Comment: 22 pages, 7 figure
Unconstrained Hamiltonian formulation of General Relativity with thermo-elastic sources
A new formulation of the Hamiltonian dynamics of the gravitational field
interacting with(non-dissipative) thermo-elastic matter is discussed. It is
based on a gauge condition which allows us to encode the six degrees of freedom
of the ``gravity + matter''-system (two gravitational and four
thermo-mechanical ones), together with their conjugate momenta, in the
Riemannian metric q_{ij} and its conjugate ADM momentum P^{ij}. These variables
are not subject to constraints. We prove that the Hamiltonian of this system is
equal to the total matter entropy. It generates uniquely the dynamics once
expressed as a function of the canonical variables. Any function U obtained in
this way must fulfil a system of three, first order, partial differential
equations of the Hamilton-Jacobi type in the variables (q_{ij},P^{ij}). These
equations are universal and do not depend upon the properties of the material:
its equation of state enters only as a boundary condition. The well posedness
of this problem is proved. Finally, we prove that for vanishing matter density,
the value of U goes to infinity almost everywhere and remains bounded only on
the vacuum constraints. Therefore the constrained, vacuum Hamiltonian (zero on
constraints and infinity elsewhere) can be obtained as the limit of a ``deep
potential well'' corresponding to non-vanishing matter. This unconstrained
description of Hamiltonian General Relativity can be useful in numerical
calculations as well as in the canonical approach to Quantum Gravity.Comment: 29 pages, TeX forma
Conserved superenergy currents
We exploit once again the analogy between the energy-momentum tensor and the
so-called ``superenergy'' tensors in order to build conserved currents in the
presence of Killing vectors. First of all, we derive the divergence-free
property of the gravitational superenergy currents under very general
circumstances, even if the superenergy tensor is not divergence-free itself.
The associated conserved quantities are explicitly computed for the
Reissner-Nordstrom and Schwarzschild solutions. The remaining cases, when the
above currents are not conserved, lead to the possibility of an interchange of
some superenergy quantities between the gravitational and other physical fields
in such a manner that the total, mixed, current may be conserved. Actually,
this possibility has been recently proved to hold for the Einstein-Klein-Gordon
system of field equations. By using an adequate family of known exact
solutions, we present explicit and completely non-obvious examples of such
mixed conserved currents.Comment: LaTeX, 19 pages; improved version adding new content to the second
section and some minor correction
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