11,772 research outputs found
Quantum theory of an atom laser originating from a Bose-Einstein condensate or a Fermi gas in the presence of gravity
We present a 3D quantum mechanical theory of radio-frequency outcoupled atom
lasers from trapped atomic gases in the presence of the gravitational force.
Predictions for the total outcoupling rate as a function of the radio-frequency
and for the beam wave function are given. We establish a sum rule for the
energy integrated outcoupling, which leads to a separate determination of the
coupling strength between the atoms and the radiation field.
For a non-interacting Bose-Einstein condensate analytic solutions are derived
which are subsequently extended to include the effects of atomic interactions.
The interactions enhance interference effects in the beam profile and modify
the outcoupling rate of the atom laser. We provide a complete quantum
mechanical solution which is in line with experimental findings and allows to
determine the validity of commonly used approximative methods.
We also extend the formalism to a fermionic atom laser and analyze the effect
of superfluidity on the outcoupling of atoms.Comment: 13 pages, 8 figures, slightly expanded versio
Long-term Radio Observations of the Intermittent Pulsar B1931+24
We present an analysis of approximately 13-yr of observations of the
intermittent pulsar B1931+24 to further elucidate its behaviour. We find that
while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting
(~1-19 d) timescales, it cycles between its different emission phases over an
average timescale of approximately 38 d, which is remarkably stable over many
years. On average, the neutron star is found to be radio emitting for 26 +- 6 %
of the time. No evidence is obtained to suggest that the pulsar undergoes any
systematic, intrinsic variations in pulse intensity during the radio-emitting
phases. In addition, we find no evidence for any correlation between the length
of consecutive emission phases. An analysis of the rotational behaviour of the
source shows that it consistently assumes the same spin-down rates, i.e. nudot
= -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2
when not emitting, over the entire observation span. Coupled with the stable
switching timescale, this implies that the pulsar retains a high degree of
magnetospheric memory, and stability, in spite of comparatively rapid (~ms)
dynamical plasma timescales. While this provides further evidence to suggest
that the behaviour of the neutron star is governed by magnetospheric-state
switching, the underlying trigger mechanism remains illusive. This should be
elucidated by future surveys with next generation telescopes such as LOFAR,
MeerKAT and the SKA, which should detect similar sources and provide more clues
to how their radio emission is regulated.Comment: 12 pages, 12 figures, accepted for publication in MNRA
Temporal response to harmonic driving in electroconvection
The temporal evolution of the spatially periodic electroconvection (EC)
patterns has been studied within the period of the driving ac voltage by
monitoring the light intensity diffracted from the pattern. Measurements have
been carried out on a variety of nematic systems, including those with negative
dielectric and positive conductivity anisotropy, exhibiting "standard EC"
(s-EC), those with both anisotropies negative exhibiting "non-standard EC"
(ns-EC), as well as those with the two anisotropies positive. Theoretical
predictions have been confirmed for stationary s-EC and ns-EC patterns.
Transitions with Hopf bifurcation have also been studied. While traveling had
no effect on the temporal evolution of dielectric s-EC, traveling conductive
s-EC and ns-EC patterns exhibited a substantially altered temporal behavior
with a dependence on the Hopf frequency. It has also been shown that in
nematics with both anisotropies positive, the pattern develops and decays
within an interval much shorter than the period, even at relatively large
driving frequencies.Comment: 19 pages, 5 figure
Electrovac -waves
New exact solutions of the Einstein-Maxwell field equations that describe
-waves are presented
Changes in Polarization Position Angle across the Eclipse in the Double Pulsar System
We investigate the changes in polarization position angle in radiation from
pulsar A around the eclipse in the Double Pulsar system PSR J0737-3039A/B at
the 20 cm and 50 cm wavelengths using the Parkes 64-m telescope. The changes
are ~2\sigma\ during and shortly after the eclipse at 20 cm but less
significant at 50 cm. We show that the changes in position angle during the
eclipse can be modelled by differential synchrotron absorption in the eclipse
regions. Position angle changes after the eclipse are interpreted as Faraday
rotation in the magnetotail of pulsar B. Implied charge densities are
consistent with the Goldreich-Julian density, suggesting that the particle
energies in the magnetotail are mildly relativistic.Comment: Accepted for publication in The Astrophysical Journal Letter
A heuristic quantum theory of the integer quantum Hall effect
Contrary to common belief, the current emitted by a contact embedded in a
two-dimensional electron gas (2DEG) is quantized in the presence of electric
and magnetic fields. This observation suggests a simple, clearly defined model
for the quantum current through a Hall device that does not invoke disorder or
interactions as the cause of the integer quantum Hall effect (QHE), but is
based on a proper quantization of the classical electron drift motion. The
theory yields a quantitative description of the breakdown of the QHE at high
current densities that is in agreement with experimental data. Furthermore,
several of its key points are in line with recent findings of experiments that
address the dependency of the QHE on the 2DEG bias voltage, results that are
not easily explained within the framework of conventional QHE models.Comment: 20 pages, 6 figure
Electron propagation in crossed magnetic and electric fields
Laser-atom interaction can be an efficient mechanism for the production of
coherent electrons. We analyze the dynamics of monoenergetic electrons in the
presence of uniform, perpendicular magnetic and electric fields. The Green
function technique is used to derive analytic results for the field--induced
quantum mechanical drift motion of i) single electrons and ii) a dilute Fermi
gas of electrons. The method yields the drift current and, at the same time it
allows us to quantitatively establish the broadening of the (magnetic) Landau
levels due to the electric field: Level number k is split into k+1 sublevels
that render the th oscillator eigenstate in energy space. Adjacent Landau
levels will overlap if the electric field exceeds a critical strength. Our
observations are relevant for quantum Hall configurations whenever electric
field effects should be taken into account.Comment: 11 pages, 2 figures, submitte
Modifying the photodetachment near a metal surface by a weak electric field
We show the photodetachment cross sections of H near a metal surface can be
modified using a weak static electric field. The modification is possible
because the oscillatory part of the cross section near a metal surface is
directly connected with the transit-time and the action of the
detached-electron closed-orbit which can be changed systematically by varying
the static electric field strength. Photodetachment cross sections for various
photon energies and electric field values are calculated and displayed.Comment: 16 pages, 7 figure
Gravitational Collapse of a Massless Scalar Field and a Perfect Fluid with Self-Similarity of the First Kind in (2+1) Dimensions
Self-similar solutions of a collapsing perfect fluid and a massless scalar
field with kinematic self-similarity of the first kind in 2+1 dimensions are
obtained. Their local and global properties of the solutions are studied. It is
found that some of them represent gravitational collapse, in which black holes
are always formed, and some may be interpreted as representing cosmological
models.Comment: 13 page
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