165 research outputs found
Wave propagation in axion electrodynamics
In this paper, the axion contribution to the electromagnetic wave propagation
is studied. First we show how the axion electrodynamics model can be embedded
into a premetric formalism of Maxwell electrodynamics. In this formalism, the
axion field is not an arbitrary added Chern-Simon term of the Lagrangian, but
emerges in a natural way as an irreducible part of a general constitutive
tensor.We show that in order to represent the axion contribution to the wave
propagation it is necessary to go beyond the geometric approximation, which is
usually used in the premetric formalism. We derive a covariant dispersion
relation for the axion modified electrodynamics. The wave propagation in this
model is studied for an axion field with timelike, spacelike and null
derivative covectors. The birefringence effect emerges in all these classes as
a signal of Lorentz violation. This effect is however completely different from
the ordinary birefringence appearing in classical optics and in premetric
electrodynamics. The axion field does not simple double the ordinary light cone
structure. In fact, it modifies the global topological structure of light cones
surfaces. In CFJ-electrodynamics, such a modification results in violation of
causality. In addition, the optical metrics in axion electrodynamics are not
pseudo-Riemannian. In fact, for all types of the axion field, they are even
non-Finslerian
A generalized photon propagator
A covariant gauge independent derivation of the generalized dispersion
relation of electromagnetic waves in a medium with local and linear
constitutive law is presented. A generalized photon propagator is derived. For
Maxwell constitutive tensor, the standard light cone structure and the standard
Feynman propagator are reinstated
On a class of invariant coframe operators with application to gravity
Let a differential 4D-manifold with a smooth coframe field be given. Consider
the operators on it that are linear in the second order derivatives or
quadratic in the first order derivatives of the coframe, both with coefficients
that depend on the coframe variables. The paper exhibits the class of operators
that are invariant under a general change of coordinates, and, also, invariant
under the global SO(1,3)-transformation of the coframe. A general class of
field equations is constructed. We display two subclasses in it. The subclass
of field equations that are derivable from action principles by free variations
and the subclass of field equations for which spherical-symmetric solutions,
Minkowskian at infinity exist. Then, for the spherical-symmetric solutions, the
resulting metric is computed. Invoking the Geodesic Postulate, we find all the
equations that are experimentally (by the 3 classical tests) indistinguishable
from Einstein field equations. This family includes, of course, also Einstein
equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool
employed in the paper is an invariant formulation reminiscent of Cartan's
structural equations. The article sheds light on the possibilities and
limitations of the coframe gravity. It may also serve as a general procedure to
derive covariant field equations
Matrix theory of gravitation
A new classical theory of gravitation within the framework of general
relativity is presented. It is based on a matrix formulation of
four-dimensional Riemann-spaces and uses no artificial fields or adjustable
parameters. The geometrical stress-energy tensor is derived from a matrix-trace
Lagrangian, which is not equivalent to the curvature scalar R. To enable a
direct comparison with the Einstein-theory a tetrad formalism is utilized,
which shows similarities to teleparallel gravitation theories, but uses complex
tetrads. Matrix theory might solve a 27-year-old, fundamental problem of those
theories (sec. 4.1). For the standard test cases (PPN scheme,
Schwarzschild-solution) no differences to the Einstein-theory are found.
However, the matrix theory exhibits novel, interesting vacuum solutions.Comment: 24 page
Dynamics of a many-particle Landau-Zener model: inverse sweep
We consider dynamics of a slowly time-dependent Dicke model, which represents
a many-body generalization of the Landau-Zener model. In particular, the model
describes narrow Feshbach resonance passage in an ultracold gas of Fermi atoms.
Adiabaticity is destroyed when a parameter crosses a critical value, even at
very slow sweeping rates of a parameter. The dynamics crucially depends on
direction of the sweep. We apply our recent analysis [A.P. Itin, P. Torma,
arXiv:0901.4778v1] to the "inverse" sweep through the resonance, corresponding
(in a context of Feshbach resonance passage) to dissociation of molecules. On a
level of the mean-field approximation, the dynamics is equivalent to a
molecular condensate formation from Bose atoms within a two-mode model. Mapping
the system to a Painlev\'e equation allows us to calculate deviation from
adiabaticity at very slow sweeps analytically.Comment: 3 pages. Submitted to CEWQO 2009 on 14th Februar
Study of the magnetic anisotropy of the multiphase samples of the ferrimagnets with hexagonal crystal structure by the method of ferromagnetic resonance
The influence of machining conditions in a planetary ball mill on the phase composition, structural and magnetic parameters of hexaferrite powders composition BaFe12O19 was investigated. The properties of powders vary greatly depending on the power density and the time of machining. Magnetocrystalline anisotropy of multiphase powders was studied by the method of ferromagnetic resonance. The effective field of magnetic anisotropy is reduced by more than two times, with decreasing particle size of ~ 67 nm to ~ 10 nm when the processing time equal to 10 minutes. The flow of mechanochemical reactions during grinding leads to the disintegration of the hexagonal crystal phase and the formation of the magnetite phase with a small value of the magnetocrystalline anisotropy field
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