13,739 research outputs found
Two-dimensional two-component plasma with adsorbing impurities
We study the behavior of the two-dimensional two-component plasma in the
presence of some adsorbing impurities. Using a solvable model, we find analytic
expressions for the thermodynamic properties of the plasma such as the -body
densities, the grand potential, and the pressure. We specialize in the case
where there are one or two adsorbing point impurities in the plasma, and in the
case where there are one or two parallel adsorbing lines. In the former case we
study the effective interaction between the impurities, due to the charge
redistribution around them. The latter case is a model for electrodes with
adsorbing sticky sites on their surface
Facilitation–inhibition control of motor neuronal persistent inward currents in young and older adults
A well-coordinated facilitation–inhibition control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Present best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation–inhibition control is also altered with ageing. We investigated the responses of PICs to (i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and (ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20 % (handgrip experiments) and 30 % (vibration experiments) of maximum torque (rise-decline rate of 2 % / s). A paired-motor-unit analysis was used to calculate ∆F, which is assumed to be proportional to PIC strength. ΔF increased in both soleus (0.55 peaks per second (pps), 16.0 %) and tibialis anterior (0.42 pps, 11.4 %) after the handgrip contraction independent of age. Although antagonist tendon vibration reduced ΔF in soleus (0.28 pps, 12.6 %) independent of age, less reduction was observed in older (0.42 pps, 10.7 %) than young adults (0.72 pps, 17.8 %) in tibialis anterior. Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus. (Figure presented.). Key points: Motor neuronal persistent inward currents (PICs) are facilitated via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing normal motor behaviour. PIC amplitudes appear to be reduced with ageing; however, it is not known whether PIC facilitation–inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, facilitated PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Data from lower-threshold motor units during low-force contractions suggest that PIC facilitation is preserved with ageing in soleus and tibialis anterior. However, the effect of reciprocal inhibition on the contribution of PICs to motor neurone discharge seems reduced in tibialis anterior but preserved in soleus
Generalized Forward-Backward Splitting
This paper introduces the generalized forward-backward splitting algorithm
for minimizing convex functions of the form , where
has a Lipschitz-continuous gradient and the 's are simple in the sense
that their Moreau proximity operators are easy to compute. While the
forward-backward algorithm cannot deal with more than non-smooth
function, our method generalizes it to the case of arbitrary . Our method
makes an explicit use of the regularity of in the forward step, and the
proximity operators of the 's are applied in parallel in the backward
step. This allows the generalized forward backward to efficiently address an
important class of convex problems. We prove its convergence in infinite
dimension, and its robustness to errors on the computation of the proximity
operators and of the gradient of . Examples on inverse problems in imaging
demonstrate the advantage of the proposed methods in comparison to other
splitting algorithms.Comment: 24 pages, 4 figure
Recollements of Module Categories
We establish a correspondence between recollements of abelian categories up
to equivalence and certain TTF-triples. For a module category we show,
moreover, a correspondence with idempotent ideals, recovering a theorem of
Jans. Furthermore, we show that a recollement whose terms are module categories
is equivalent to one induced by an idempotent element, thus answering a
question by Kuhn.Comment: Comments are welcom
The Orbiter Stability Experiment on STS-40
The Orbiter Stability Experiment (OSE) was developed to evaluate the steadiness of the STS Orbiter as a potential platform for instrumentation that would image the Sun in its extreme ultraviolet and soft X-ray radiations. We were interested in any high frequency motions of the Orbiter's orientation due to normal operations and manned activities. Preliminary results are presented of the observations. Other than the expected slow motion of the Orbiter within the specified angular deadband of 0.1 degrees during the observations, it was found that high frequency (above 1 Hz) angular motions (jitter) were not detectable at the 0.25 arc sec detection limit of the most sensitive detector, for most of the period of observation. No high frequency motions were recorded during intervals that were identified with vernier thruster firings. However, one short interval with detectable spectral power to a frequency of 10 Hz has been found to date. It has not yet been correlated with a particular activity going on at the time. The results of the observations may also be of value in assessing perturbations to the Orbiter's micro-gravity environment produced by normal operations
Description beyond the mean field approximation of an electrolyte confined between two planar metallic electrodes
We study an electrolyte confined in a slab of width composed of two
grounded metallic parallel electrodes. We develop a description of this system
in a low coupling regime beyond the mean field (Poisson--Boltzmann)
approximation. There are two ways to model the metallic boundaries: as ideal
conductors in which the electric potential is zero and it does not fluctuate,
or as good conductors in which the average electric potential is zero but the
thermal fluctuations of the potential are not zero. This latter model is more
realistic. For the ideal conductor model we find that the disjoining pressure
is positive behaves as for large separations with a prefactor that is
universal, i.e. independent of the microscopic constitution of the system. For
the good conductor boundaries the disjoining pressure is negative and it has an
exponential decay for large . We also compute the density and electric
potential profiles inside the electrolyte. These are the same in both models.
If the electrolyte is charge asymmetric we find that the system is not locally
neutral and that a non-zero potential difference builds up between any
electrode and the interior of the system although both electrodes are grounded.Comment: 16 pages, 5 figures, added a new appendix B and a discussion on ideal
conductors vs. good conductor
Stereoscopic Polar Plume Reconstructions from Stereo/Secchi Images
We present stereoscopic reconstructions of the location and inclination of
polar plumes of two data sets based on the two simultaneously recorded images
taken by the EUVI telescopes in the SECCHI instrument package onboard the
\emph{STEREO (Solar TErrestrial RElations Observatory)} spacecraft. The ten
plumes investigated show a superradial expansion in the coronal hole in 3D
which is consistent with the 2D results. Their deviations from the local
meridian planes are rather small with an average of . By
comparing the reconstructed plumes with a dipole field with its axis along the
solar rotation axis, it is found that plumes are inclined more horizontally
than the dipole field. The lower the latitude is, the larger is the deviation
from the dipole field. The relationship between plumes and bright points has
been investigated and they are not always associated. For the first data set,
based on the 3D height of plumes and the electron density derived from
SUMER/\emph{SOHO} Si {\sc viii} line pair, we found that electron densities
along the plumes decrease with height above the solar surface. The temperature
obtained from the density scale height is 1.6 to 1.8 times larger than the
temperature obtained from Mg {\sc ix} line ratios. We attribute this
discrepancy to a deviation of the electron and the ion temperatures. Finally,
we have found that the outflow speeds studied in the O {\sc vi} line in the
plumes corrected by the angle between the line of sight and the plume
orientation are quite small with a maximum of 10 . It is
unlikely that plumes are a dominant contributor to the fast solar wind.Comment: 25 pages, 13 figure
New Magnetic Materials Obtained by Ion-exchange Reactions from Non-magnetic Layered Perovskites
New layered magnetic materials, (MCl)Ca2Ta3O10 (M = Cu, Fe), have been prepared by ion-exchange reactions of non-magnetic perovskite derivatives, ACa2Ta3O10 (A = Rb, Li), in corresponding anhydrous molten salts. Powder x-ray diffraction patterns of the products are successfully indexed assuming tetragonal symmetry with cell dimensions a = 3.829 Ã… and c = 15.533 Ã… for Cu, and a = 3.822 Ã… and c = 15.672 Ã… for Fe. Being separated by the Ca2Ta3O10 triple-layer perovskite slabs, the transition-metal chloride (MCl) network provides a two-dimensional magnetic lattice. Magnetic susceptibility measurements show that (CuCl)Ca2Ta3O10 is in an antiferromagnetic state below 8 K, while (FeCl)Ca2Ta3O10 has two anomalies at 91 and 125 K, suggesting successive phase transitions due to geometrical spin frustration
Extremal non-BPS black holes and entropy extremization
At the horizon, a static extremal black hole solution in N=2 supergravity in
four dimensions is determined by a set of so-called attractor equations which,
in the absence of higher-curvature interactions, can be derived as
extremization conditions for the black hole potential or, equivalently, for the
entropy function. We contrast both methods by explicitly solving the attractor
equations for a one-modulus prepotential associated with the conifold. We find
that near the conifold point, the non-supersymmetric solution has a
substantially different behavior than the supersymmetric solution. We analyze
the stability of the solutions and the extrema of the resulting entropy as a
function of the modulus. For the non-BPS solution the region of attractivity
and the maximum of the entropy do not coincide with the conifold point.Comment: 19 pages, 4 figures, AMS-LaTeX, reference adde
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