34,161 research outputs found
Static potential in scalar QED with non-minimal coupling
Here we compute the static potential in scalar at leading order in
. We show that the addition of a non-minimal coupling of Pauli-type
(\eps j^{\mu}\partial^{\nu}A^{\alpha}), although it breaks parity, it does
not change the analytic structure of the photon propagator and consequently the
static potential remains logarithmic (confining) at large distances. The
non-minimal coupling modifies the potential, however, at small charge
separations giving rise to a repulsive force of short range between opposite
sign charges, which is relevant for the existence of bound states. This effect
is in agreement with a previous calculation based on Mller
scattering, but differently from such calculation we show here that the
repulsion appears independently of the presence of a tree level Chern-Simons
term which rather affects the large distance behavior of the potential turning
it into constant.Comment: 13 pages, 3 figure
Dynamical Configurations and Bistability of Helical Nanostructures under External Torque
We study the motion of a ferromagnetic helical nanostructure under the action
of a rotating magnetic field. A variety of dynamical configurations were
observed that depended strongly on the direction of magnetization and the
geometrical parameters, which were also confirmed by a theoretical model, based
on the dynamics of a rigid body under Stokes flow. Although motion at low
Reynolds numbers is typically deterministic, under certain experimental
conditions, the nanostructures showed a surprising bistable behavior, such that
the dynamics switched randomly between two configurations, possibly induced by
thermal fluctuations. The experimental observations and the theoretical results
presented in this letter are general enough to be applicable to any system of
ellipsoidal symmetry under external force or torque.Comment: 15 pages, 4 figure
Deconvolving the Wedge: Maximum-Likelihood Power Spectra via Spherical-Wave Visibility Modeling
Direct detection of the Epoch of Reionization (EoR) via the red-shifted 21-cm
line will have unprecedented implications on the study of structure formation
in the infant Universe. To fulfill this promise, current and future 21-cm
experiments need to detect this weak EoR signal in the presence of foregrounds
that are several orders of magnitude larger. This requires extreme noise
control and improved wide-field high dynamic-range imaging techniques. We
propose a new imaging method based on a maximum likelihood framework which
solves for the interferometric equation directly on the sphere, or equivalently
in the -domain. The method uses the one-to-one relation between spherical
waves and spherical harmonics (SpH). It consistently handles signals from the
entire sky, and does not require a -term correction. The spherical-harmonics
coefficients represent the sky-brightness distribution and the visibilities in
the -domain, and provide a direct estimate of the spatial power spectrum.
Using these spectrally-smooth SpH coefficients, bright foregrounds can be
removed from the signal, including their side-lobe noise, which is one of the
limiting factors in high dynamics range wide-field imaging. Chromatic effects
causing the so-called "wedge" are effectively eliminated (i.e. deconvolved) in
the cylindrical () power spectrum, compared to a
power spectrum computed directly from the images of the foreground visibilities
where the wedge is clearly present. We illustrate our method using simulated
LOFAR observations, finding an excellent reconstruction of the input EoR signal
with minimal bias.Comment: 13 pages, 8 figures. Replaced to match accepted MNRAS version; few
typos corrected & textual clarification added (no changes to results
Robust Foregrounds Removal for 21-cm Experiments
Direct detection of the Epoch of Reionization via the redshifted 21-cm line
will have unprecedented implications on the study of structure formation in the
early Universe. To fulfill this promise current and future 21-cm experiments
will need to detect the weak 21-cm signal over foregrounds several order of
magnitude greater. This requires accurate modeling of the galactic and
extragalactic emission and of its contaminants due to instrument chromaticity,
ionosphere and imperfect calibration. To solve for this complex modeling, we
propose a new method based on Gaussian Process Regression (GPR) which is able
to cleanly separate the cosmological signal from most of the foregrounds
contaminants. We also propose a new imaging method based on a maximum
likelihood framework which solves for the interferometric equation directly on
the sphere. Using this method, chromatic effects causing the so-called "wedge"
are effectively eliminated (i.e. deconvolved) in the cylindrical () power spectrum.Comment: Subbmited to the Proceedings of the IAUS333, Peering Towards Cosmic
Dawn, 4 pages, 2 figure
The effect of coatings and liners on heat transfer in a dry shaft-bush tribosystem
The temperatures due to frictional heating within a solid lubricated or coated journal bearing were analyzed by using a finite element method. A solid model of the shaft-bush tribocontact was generated with an eight-node, three-dimensional, first-order isoparametric heat-transfer element and the Patran solid modeler software. The Patmar (Patran-Marc) translator was used to help develop the Marc-based finite element program for the system; this software was used on the Cray X-MP supercomputer to perform a finite element analysis of the contact. The analysis was performed for various liner materials, for thin, hard, wear-resistant coated bearings, and for different geometries and thermal cooling boundary conditions. The analyses indicated that thermal conductivity of the liner or coating material is the most vital thermal parameter that controls the interface temperature. In addition to design variations, the proximity of the cooling source to the heat-flux-generating interface is critically important to the temperature control in the system
Slowly Rotating Dilaton Black hole In Anti-de Sitter Spacetime
Rotating dilaton black hole solution for asymptotically anti-de Sitter
spacetime are obtained in the small angular momentum limit with an appropriate
combination of three Liouville-type dilaton potentials. The angular momentum,
magnetic dipole moment and the gyromagnetic ratio of such a black hole are
determined for arbitrary values of the dilaton-electromagnetic coupling
parameter.Comment: 5 pages, Revtex, To appear in Phys.Rev.
Spherical agglomeration of superconducting and normal microparticles with and without applied electric field
It was reported by R. Tao and coworkers that in the presence of a strong
electric field superconducting microparticles assemble into balls of
macroscopic dimensions. Such a finding has potentially important implications
for the understanding of the fundamental physics of superconductors. However,
we report here the results of experimental studies showing that (i) ball
formation also occurs in the absence of an applied electric field, (ii) the
phenomenon also occurs at temperatures above the superconducting transition
temperature, and (iii) it can also occur for non-superconducting materials.
Possible origins of the phenomenon are discussed.Comment: Small changes in response to referee's comments. To be published in
Phys. Rev.
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