1,152 research outputs found
Four Poynting Theorems
The Poynting vector is an invaluable tool for analysing electromagnetic
problems. However, even a rigorous stress-energy tensor approach can still
leave us with the question: is it best defined as \Vec{E} \cross \Vec{H} or
as \Vec{D} \cross \Vec{B}? Typical electromagnetic treatments provide yet
another perspective: they regard \Vec{E} \cross \Vec{B} as the appropriate
definition, because \Vec{E} and \Vec{B} are taken to be the fundamental
electromagnetic fields. The astute reader will even notice the fourth possible
combination of fields: i.e. \Vec{D} \cross \Vec{H}. Faced with this diverse
selection, we have decided to treat each possible flux vector on its merits,
deriving its associated energy continuity equation but applying minimal
restrictions to the allowed host media. We then discuss each form, and how it
represents the response of the medium. Finally, we derive a propagation
equation for each flux vector using a directional fields approach; a useful
result which enables further interpretation of each flux and its interaction
with the medium.Comment: 8 pages. Updated slightly from EJP versio
The physics of dipolar bosonic quantum gases
This article reviews the recent theoretical and experimental advances in the
study of ultracold gases made of bosonic particles interacting via the
long-range, anisotropic dipole-dipole interaction, in addition to the
short-range and isotropic contact interaction usually at work in ultracold
gases. The specific properties emerging from the dipolar interaction are
emphasized, from the mean-field regime valid for dilute Bose-Einstein
condensates, to the strongly correlated regimes reached for dipolar bosons in
optical lattices.Comment: Review article, 71 pages, 35 figures, 350 references. Submitted to
Reports on Progress in Physic
On the magnetic acceleration and collimation of astrophysical outflows
The axisymmetric 3-D MHD outflow of a cold plasma from a magnetized and
rotating astrophysical object is numerically simulated with the purpose of
investigating the outflow's magnetocentrifugal acceleration and eventual
collimation. Gravity and thermal pressure are neglected while a split-monopole
is used to describe the initial magnetic field configuration. It is found that
the stationary final state depends critically on a single parameter alpha
expressing the ratio of the corotating speed at the Alfven distance to the
initial flow speed along the initial monopole-like magnetic fieldlines. Several
angular velocity laws have been used for relativistic and nonrelativistic
outflows. The acceleration of the flow is most effective at the equatorial
plane and the terminal flow speed depends linearly on alpha. Significant flow
collimation is found in nonrelativistic efficient magnetic rotators
corresponding to relatively larger than 1 values of alpha while very weak
collimation occurs in inefficient magnetic rotators with values of alpha
smaller than about 1. Part of the flow around the rotation and magnetic axis is
cylindrically collimated while the remaining part obtains radial asymptotics.
The transverse radius of the jet is inversely proportional to alpha while the
density in the jet grows linearly with alpha. For alpha greater than about 5
the magnitude of the flow in the jet remains below the fast MHD wave speed
everywhere. In relativistic outflows, no collimation is found in the supersonic
region for parameters typical for radio pulsars. All above results verify the
main conclusions of general theoretical studies on the magnetic acceleration
and collimation of outflows from magnetic rotators and extend previous
numerical simulations to large stellar distances.Comment: 15 pages, 13 figures. Accepted for publication, MNRA
Atom Interferometers
Interference with atomic and molecular matter waves is a rich branch of
atomic physics and quantum optics. It started with atom diffraction from
crystal surfaces and the separated oscillatory fields technique used in atomic
clocks. Atom interferometry is now reaching maturity as a powerful art with
many applications in modern science. In this review we first describe the basic
tools for coherent atom optics including diffraction by nanostructures and
laser light, three-grating interferometers, and double wells on AtomChips. Then
we review scientific advances in a broad range of fields that have resulted
from the application of atom interferometers. These are grouped in three
categories: (1) fundamental quantum science, (2) precision metrology and (3)
atomic and molecular physics. Although some experiments with Bose Einstein
condensates are included, the focus of the review is on linear matter wave
optics, i.e. phenomena where each single atom interferes with itself.Comment: submitted to Reviews of Modern Physic
Cosmology from Topological Defects
The potential role of cosmic topological defects has raised interest in the
astrophysical community for many years now. In this set of notes, we give an
introduction to the subject of cosmic topological defects and some of their
possible observable signatures. We begin with a review of the basics of general
defect formation and evolution, we briefly comment on some general features of
conducting cosmic strings and vorton formation, as well as on the possible role
of defects as dark energy, to end up with cosmic structure formation from
defects and some specific imprints in the cosmic microwave background radiation
from simulated cosmic strings. A detailed, pedagogical explanation of the
mechanism underlying the tiny level of polarization discovered in the cosmic
microwave background by the DASI collaboration (and recently confirmed by WMAP)
is also given, and a first rough comparison with some predictions from defects
is provided.Comment: Lecture Notes delivered at the Xth Brazilian School on Cosmology and
Gravitation, Mangaratiba, Rio de Janeiro, July 29 - August 9, 2002. To appear
in the proceedings (AIP Press), edited by M. Novello and S. Perez Bergliaffa.
Updated source files with high resolution figures available at
http://www.iafe.uba.ar/relatividad/gangui/xescola
Metamaterial
In-depth analysis of the theory, properties and description of the most potential technological applications of metamaterials for the realization of novel devices such as subwavelength lenses, invisibility cloaks, dipole and reflector antennas, high frequency telecommunications, new designs of bandpass filters, absorbers and concentrators of EM waves etc. In order to create a new devices it is necessary to know the main electrodynamical characteristics of metamaterial structures on the basis of which the device is supposed to be created. The electromagnetic wave scattering surfaces built with metamaterials are primarily based on the ability of metamaterials to control the surrounded electromagnetic fields by varying their permeability and permittivity characteristics. The book covers some solutions for microwave wavelength scales as well as exploitation of nanoscale EM wavelength such as visible specter using recent advances of nanotechnology, for instance in the field of nanowires, nanopolymers, carbon nanotubes and graphene. Metamaterial is suitable for scholars from extremely large scientific domain and therefore given to engineers, scientists, graduates and other interested professionals from photonics to nanoscience and from material science to antenna engineering as a comprehensive reference on this artificial materials of tomorrow
Intense Electromagnetic Outbursts from Collapsing Hypermassive Neutron Stars
We study the gravitational collapse of a magnetized neutron star using a
novel numerical approach able to capture both the dynamics of the star and the
behavior of the surrounding plasma. In this approach, a fully general
relativistic magnetohydrodynamics implementation models the collapse of the
star and provides appropriate boundary conditions to a force-free model which
describes the stellar exterior. We validate this strategy by comparing with
known results for the rotating monopole and aligned rotator solutions and then
apply it to study both rotating and non-rotating stellar collapse scenarios,
and contrast the behavior with what is obtained when employing the
electrovacuum approximation outside the star. The non-rotating electrovacuum
collapse is shown to agree qualitatively with a Newtonian model of the
electromagnetic field outside a collapsing star. We illustrate and discuss a
fundamental difference between the force-free and electrovacuum solutions,
involving the appearance of large zones of electric-dominated field in the
vacuum case. This provides a clear demonstration of how dissipative
singularities appear generically in the non-linear time-evolution of force-free
fluids. In both the rotating and non-rotating cases, our simulations indicate
that the collapse induces a strong electromagnetic transient. In the case of
sub-millisecond rotation, the magnetic field experiences strong winding and the
transient carries much more energy. This result has important implications for
models of gamma-ray bursts.Comment: 28 pages, 20 figures (quality lowered to reduce sizes). Improved
initial data and matching condition results in a lower, but still important,
energy emission. Added appendix with a discussion on effects of transition
laye
Magnetars in the Metagalaxy: An Origin for Ultra High Energy Cosmic Rays in the Nearby Universe
I show that the relativistic winds of newly born magnetars with khz initial
spin rates, occurring in all normal galaxies, can accelerate ultrarelativistic
light ions with an E^{-1} injection spectrum, steepening to E^{-2} at higher
energies, with an upper cutoff above 10^{21} eV. Interactions with the CMB
yield a spectrum in good accord with the observed spectrum of Ultra-High Energy
Cosmic Rays (UHECR), if ~ 5-10% of the magnetars are born with voltages
sufficiently high to accelerate the UHECR. The form the spectrum spectrum takes
depends on the gravitational wave losses during the magnetars' early spindown -
pure electromagnetic spindown yields a flattening of the E^3 J(E) spectrum
below 10^{20} eV, while a moderate GZK ``cutoff'' appears if gravitational wave
losses are strong enough. I outline the physics such that the high energy
particles escape with small energy losses from a magnetar's natal supernova,
including Rayleigh-Taylor ``shredding'' of the supernova envelope, expansion of
a relativistic blast wave into the interstellar medium, acceleration of the UHE
ions through surf-riding in the electromgnetic fields of the wind, and escape
of the UHE ions in the rotational equator with negligible radiation loss. The
abundance of interstellar supershells and unusually large supernova remnants
suggests that most of the initial spindown energy is radiated in khz
gravitational waves for several hours after each supernova, with effective
strains from sources at typical distances ~ 3 x 10^{-21}. Such bursts of
gravitational radiation should correlate with bursts of ultra-high energy
particles. The Auger experiment should see such bursts every few years.Comment: 49 pages, 2 Figures, LaTeX (aastex, epsfig, graphicx, float), to be
published June 1, 2003 in the ApJ. Corrected discussion of electromagnetic
surf-riding as the acceleration mechanism and more typos, and reference
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