1,547 research outputs found
Remarks on some vacuum solutions of scalar-tensor cosmological models
We present a class of exact vacuum solutions corresponding to de Sitter and
warm inflation models in the framework of scalar-tensor cosmologies. We show
that in both cases the field equations reduce to planar dynamical systems with
constraints. Then, we carry out a qualitative analysis of the models by
examining the phase diagrams of the solutions near the equilibrium points.Comment: 12 pages, 4 figures. To be published in the Brazilian Journal of
Physic
Constraints on non-Newtonian gravity from measuring the Casimir force in a configuration with nanoscale rectangular corrugations
We report constraints on the parameters of Yukawa-type corrections to
Newtonian gravity from measurements of the gradient of the Casimir force in the
configuration of an Au-coated sphere above a Si plate covered with corrugations
of trapezoidal shape. For this purpose, the exact expression for the gradient
of Yukawa force in the experimental configuration is derived and compared with
that obtained using the proximity force approximation. The reported constraints
are of almost the same strength as those found previously from several
different experiments on the Casimir force and extend over a wide interaction
range from 30 to 1260\,nm. It is discussed how to make them stronger by
replacing the material of the plate.Comment: 14 pages, 2 figures, to appear in Phys. Rev.
Nonrelativistic Quantum Analysis of the Charged Particle-Dyon System on a Conical Spacetime
In this paper we develop the nonrelativistic quantum analysis of the charged
particle-dyon system in the spacetime produced by an idealized cosmic string.
In order to do that, we assume that the dyon is superposed to the cosmic
string. Considering this peculiar configuration {\it conical} monopole
harmonics are constructed, which are a generalizations of previous monopole
harmonics obtained by Wu and Yang(1976 {\it Nucl. Phys. B} {\bf 107} 365)
defined on a conical three-geometry. Bound and scattering wave functions are
explicitly derived. As to bound states, we present the energy spectrum of the
system, and analyze how the presence of the topological defect modifies
obtained result. We also analyze this system admitting the presence of an extra
isotropic harmonic potential acting on the particle. We show that the presence
of this potential produces significant changes in the energy spectrum of the
system.Comment: Paper accepted for publication in Classical and Quantum Gravit
Self-similar magnetoresistance of Fibonacci ultrathin magnetic films
We study numerically the magnetic properties (magnetization and
magnetoresistance) of ultra-thin magnetic films (Fe/Cr) grown following the
Fibonacci sequence. We use a phenomenological model which includes Zeeman,
cubic anisotropy, bilinear and biquadratic exchange energies. Our physical
parameters are based on experimental data recently reported, which contain
biquadratic exchange coupling with magnitude comparable to the bilinear
exchange coupling. When biquadratic exchange coupling is sufficiently large a
striking self-similar pattern emerges.Comment: 5 pages, 5 EPS figures, REVTeX, accepted for publication in Phys.
Rev.
Present status of controversies regarding the thermal Casimir force
It is well known that, beginning in 2000, the behavior of the thermal
correction to the Casimir force between real metals has been hotly debated. As
was shown by several research groups, the Lifshitz theory, which provides the
theoretical foundation for the calculation of both the van der Waals and
Casimir forces, leads to different results depending on the model of metal
conductivity used. To resolve these controversies, the theoretical
considerations based on the principles of thermodynamics and new experimental
tests were invoked. We analyze the present status of the problem (in
particular, the advantages and disadvantages of the approaches based on the
surface impedance and on the Drude model dielectric function) using rigorous
analytical calculations of the entropy of a fluctuating field. We also discuss
the results of a new precise experiment on the determination of the Casimir
pressure between two parallel plates by means of a micromechanical torsional
oscillator.Comment: 14 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Comment on "On the temperature dependence of the Casimir effect"
Recently, Brevik et al. [Phys. Rev. E 71, 056101 (2005)] adduced arguments
against the traditional approach to the thermal Casimir force between real
metals and in favor of one of the alternative approaches. The latter assumes
zero contribution from the transverse electric mode at zero frequency in
qualitative disagreement with unity as given by the thermal quantum field
theory for ideal metals. Those authors claim that their approach is consistent
with experiments as well as with thermodynamics. We demonstrate that these
conclusions are incorrect. We show specifically that their results are
contradicted by four recent experiments and also violate the third law of
thermodynamics (the Nernst heat theorem).Comment: 11 pages, 3 figures, changed in accordance with the final published
versio
Impact of surface imperfections on the Casimir force for lenses of centimeter-size curvature radii
The impact of imperfections, which are always present on surfaces of lenses
with centimeter-size curvature radii, on the Casimir force in the lens-plate
geometry is investigated. It is shown that the commonly used formulation of the
proximity force approximation is inapplicable for spherical lenses with surface
imperfections, such as bubbles and pits. More general expressions for the
Casimir force are derived that take surface imperfections into account. Using
these expressions we show that surface imperfections can both increase and
decrease the magnitude of the Casimir force up to a few tens of percent when
compared with the case of a perfectly spherical lens. We demonstrate that the
Casimir force between a perfectly spherical lens and a plate described by the
Drude model can be made approximately equal to the force between a sphere with
some surface imperfection and a plate described by the plasma model, and vice
versa. In the case of a metallic sphere and semiconductor plate, approximately
the same Casimir forces are obtained for four different descriptions of charge
carriers in the semiconductor if appropriate surface imperfections on the lens
surface are present. The conclusion is made that there is a fundamental problem
in the interpretation of measurement data for the Casimir force, obtained by
using spherical lenses of centimeter-size radii, and their comparison with
theory.Comment: 28 pages, 7 figures, 1 table. To appear in Phys. Rev.
New features of the thermal Casimir force at small separations
The difference of the thermal Casimir forces at different temperatures
between real metals is shown to increase with a decrease of the separation
distance. This opens new opportunities for the demonstration of the thermal
dependence of the Casimir force. Both configurations of two parallel plates and
a sphere above a plate are considered. Different approaches to the theoretical
description of the thermal Casimir force are shown to lead to different
measurable predictions.Comment: 5 pages, 3 figures, to appear in Phys. Rev. Let
Nonlocal impedances and the Casimir entropy at low temperatures
The problem with the temperature dependence of the Casimir force is
investigated. Specifically, the entropy behavior in the low temperature limit,
which caused debates in the literature, is analyzed. It is stressed that the
behavior of the relaxation frequency in the limit does not play a
physical role since the anomalous skin effect dominates in this range. In
contrast with the previous works, where the approximate Leontovich impedance
was used for analysis of nonlocal effects, we give description of the problem
in terms of exact nonlocal impedances. It is found that the Casimir entropy is
going to zero at only in the case when polarization does not
contribute to the classical part of the Casimir force. However, the entropy
approaching zero from the negative side that, in our opinion, cannot be
considered as thermodynamically satisfactory. The resolution of the negative
entropy problem proposed in the literature is analyzed and it is shown that it
cannot be considered as complete. The crisis with the thermal Casimir effect is
stressed.Comment: Accepted in Phys. Rev.
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