5,032 research outputs found
Cosmological perturbations in coherent oscillating scalar field models
The fact that fast oscillating homogeneous scalar fields behave as perfect
fluids in average and their intrinsic isotropy have made these models very
fruitful in cosmology. In this work we will analyse the perturbations dynamics
in these theories assuming general power law potentials . At leading order in the wavenumber expansion, a simple
expression for the effective sound speed of perturbations is obtained
with the effective equation of
state. We also obtain the first order correction in
, when the wavenumber of the perturbations is
much smaller than the background oscillation frequency, .
For the standard massive case we have also analysed general anharmonic
contributions to the effective sound speed. These results are reached through a
perturbed version of the generalized virial theorem and also studying the exact
system both in the super-Hubble limit, deriving the natural ansatz for
; and for sub-Hubble modes, exploiting Floquet's theorem.Comment: 13 pages, 6 figures. Published on JHE
Isotropy theorem for cosmological Yang-Mills theories
We consider homogeneous non-abelian vector fields with general potential
terms in an expanding universe. We find a mechanical analogy with a system of N
interacting particles (with N the dimension of the gauge group) moving in three
dimensions under the action of a central potential. In the case of bounded and
rapid evolution compared to the rate of expansion, we show by making use of a
generalization of the virial theorem that for arbitrary potential and
polarization pattern, the average energy-momentum tensor is always diagonal and
isotropic despite the intrinsic anisotropic evolution of the vector field. We
consider also the case in which a gauge-fixing term is introduced in the action
and show that the average equation of state does not depend on such a term.
Finally, we extend the results to arbitrary background geometries and show that
the average energy-momentum tensor of a rapidly evolving Yang-Mills fields is
always isotropic and has the perfect fluid form for any locally inertial
observer.Comment: 8 pages, 3 figure
Isotropy theorem for arbitrary-spin cosmological fields
We show that the energy-momentum tensor of homogeneous fields of arbitrary
spin in an expanding universe is always isotropic in average provided the
fields remain bounded and evolve rapidly compared to the rate of expansion. An
analytic expression for the average equation of state is obtained for
Lagrangians with generic power-law kinetic and potential terms. As an example
we consider the behavior of a spin-two field in the standard Fierz-Pauli theory
of massive gravity. The results can be extended to general space-time
geometries for locally inertial observers.Comment: 4 pages, 0 figures. Published on JCA
Some results on the eigenfunctions of the quantum trigonometric Calogero-Sutherland model related to the Lie algebra E6
The quantum trigonometric Calogero-Sutherland models related to Lie algebras
admit a parametrization in which the dynamical variables are the characters of
the fundamental representations of the algebra. We develop here this approach
for the case of the exceptional Lie algebra E6.Comment: 17 pages, no figure
On the collapse in fourth order gravities
The gravitational collapse in fourth order theories of gravity defined by an
arbitrary action of the scalar curvature shows significant deviations with
General Relativity. The presence of a new scalar mode produces a higher initial
contraction that favors the reduction of the collapsing time. However,
depending on the particular model, there are fundamental differences when the
modifications to the General Relativity collapse leave the linear regime. These
analyses can be used to exclude an important region of the parameter space
associated with alternative gravitational models.Comment: 4 pages, 2 figures, Contribution to the Proceedings of Spanish
Relativity Meeting ERE2011, Madrid 201
Optical spin transfer in ferromagnetic semiconductors
Circularly polarized laser pulses that excite electron-hole pairs across the
band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate
and to study collective magnetization dynamics. The initial spin orientation of
a photocarrier in a (III,V) semiconductors is determined by the polarization
state of the laser. We show that the photocarrier spin can be irreversibly
transferred to the collective magnetization, whose dynamics can consequently be
flexibly controlled by suitably chosen laser pulses. As illustrations we
demonstrate the feasibility of all optical ferromagnetic resonance and optical
magnetization reorientation.Comment: 8 pages, 3 figure
High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)
We have measured the pressure sensitivity of Tc in fluorinated
HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying
pressures up to 30 GPa. We obtained that Tc increases with increasing pressure,
reaching different maximum values, depending on the F doping level, and
decreases for a further increase of pressure. A new high Tc record (166 K +/- 1
K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample
near the optimum doping level. Our results show that all our samples are at the
optimal doping, and that fluorine incorporation decreases the crystallographic
-parameter concomitantly increasing the maximum attainable Tc. This effect
reveals that the compression of the axes is one of the keys that controls
the Tc of high temperature superconductors.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
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