19,688 research outputs found
Aspects of parity, CP, and time reversal violation in hot QCD
We discuss various aspects of parity, CP, and time reversal invariances in
QCD. In particular, we focus attention on the previously proposed possibility
that these experimentally established symmetries of strong interactions may be
broken at finite temperature and/or density. This would have dramatic
signatures in relativistic heavy ion collisions; we describe some of the most
promising signals.Comment: Latex; 14 pages + 3 figs. Talk given at SEWM2000, Marseille, June
14-17 2000 and ISMD2000, Tihany, October 9-15 200
A simple and efficient numerical scheme to integrate non-local potentials
As nuclear wave functions have to obey the Pauli principle, potentials issued
from reaction theory or Hartree-Fock formalism using finite-range interactions
contain a non-local part. Written in coordinate space representation, the
Schrodinger equation becomes integro-differential, which is difficult to solve,
contrary to the case of local potentials, where it is an ordinary differential
equation. A simple and powerful method has been proposed several years ago,
with the trivially equivalent potential method, where non-local potential is
replaced by an equivalent local potential, which is state-dependent and has to
be determined iteratively. Its main disadvantage, however, is the appearance of
divergences in potentials if the wave functions have nodes, which is generally
the case. We will show that divergences can be removed by a slight modification
of the trivially equivalent potential method, leading to a very simple, stable
and precise numerical technique to deal with non-local potentials. Examples
will be provided with the calculation of the Hartree-Fock potential and
associated wave functions of 16O using the finite-range N3LO realistic
interaction.Comment: 8 pages, 2 figures, submitted to Eur. Phys. J.
Mode stability in delta Scuti stars: linear analysis versus observations in open clusters
A comparison between linear stability analysis and observations of pulsation
modes in five delta Scuti stars, belonging to the same cluster, is presented.
The study is based on the work by Michel et al. (1999), in which such a
comparison was performed for a representative set of model solutions obtained
independently for each individual star considered. In this paper we revisit the
work by Michel et al. (1999) following, however, a new approach which consists
in the search for a single, complete, and coherent solution for all the
selected stars, in order to constrain and test the assumed physics describing
these objects. To do so, refined descriptions for the effects of rotation on
the determination of the global stellar parameters and on the adiabatic
oscillation frequency computations are used. In addition, a crude attempt is
made to study the role of rotation on the prediction of mode instabilities.The
present results are found to be comparable with those reported by Michel et al.
(1999). Within the temperature range log T_eff = 3.87-3.88 agreement between
observations and model computations of unstable modes is restricted to values
for the mixing-length parameter alpha_nl less or equal to 1.50. This indicates
that for these stars a smaller value for alpha_nl is required than suggested
from a calibrated solar model. We stress the point that the linear stability
analysis used in this work still assumes stellar models without rotation and
that further developments are required for a proper description of the
interaction between rotation and pulsation dynamics.Comment: 8 pages, 4 figures, 3 tables. (MNRAS, in press
Asteroseismology of delta Scuti stars in open clusters: Praesepe
The present paper provides a general overview of the asteroseismic potential
of delta Scuti stars in clusters, in particular focusing on convection
diagnostics. We give a summarise of the last results obtained by the authors
for the Praesepe cluster of which five delta Scuti stars are analysed. In that
work, linear analysis is confronted with observations, using refined
descriptions for the effects of rotation on the determination of the global
stellar parameters and on the adiabatic oscillation frequency computations. A
single, complete, and coherent solution for all the selected stars is found,
which lead the authors to find important restrictions to the convection
description for a certain range of effective temperatures. Furthermore, the
method used allowed to give an estimate of the global parameters of the
selected stars and constrain the cluster.Comment: 6 pages, 1 figure. Accepted for publication in Communications in
Asteroseismolog
Modeling laser wakefield accelerators in a Lorentz boosted frame
Modeling of laser-plasma wakefield accelerators in an optimal frame of
reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of
calculations from first principles. Obtaining these speedups requires
mitigation of a high-frequency instability that otherwise limits effectiveness
in addition to solutions for handling data input and output in a
relativistically boosted frame of reference. The observed high-frequency
instability is mitigated using methods including an electromagnetic solver with
tunable coefficients, its extension to accomodate Perfectly Matched Layers and
Friedman's damping algorithms, as well as an efficient large bandwidth digital
filter. It is shown that choosing the frame of the wake as the frame of
reference allows for higher levels of filtering and damping than is possible in
other frames for the same accuracy. Detailed testing also revealed
serendipitously the existence of a singular time step at which the instability
level is minimized, independently of numerical dispersion, thus indicating that
the observed instability may not be due primarily to Numerical Cerenkov as has
been conjectured. The techniques developed for Cerenkov mitigation prove
nonetheless to be very efficient at controlling the instability. Using these
techniques, agreement at the percentage level is demonstrated between
simulations using different frames of reference, with speedups reaching two
orders of magnitude for a 0.1 GeV class stages. The method then allows direct
and efficient full-scale modeling of deeply depleted laser-plasma stages of 10
GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to
very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for
the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively
X-ray microanalysis in STEM of short-term physico-chemical reactions at bioactive glass particles / biological fluids interface. Determination of O/Si atomic ratios
Short-term physico-chemical reactions at the interface between bioactive
glass particles and biological fluids are studied and we focus our attention on
the measurements of O/Si atomic ratio. The studied bioactive glass is in the
SiO2-Na2O-CaO-P2O5-K2O-Al2O3-MgO system. The elemental analysis is performed at
the submicrometer scale by STEM associated with EDXS and EELS. We previously
developed an EDXS quantification method based on the ratio method and taking
into account local absorption corrections. In this way, we use EELS data to
determine, by an iterative process, the local mass thickness which is an
essential parameter to correct absorption in EDXS spectra. After different
delays of immersion of bioactive glass particles in a simulated biological
solution, results show the formation of different surface layers at the
bioactive glass periphery. Before one day of immersion, we observe the presence
of an already shown (Si,O,Al) rich layer at the periphery. In this paper, we
demonstrate that a thin electron dense (Si,O) layer is formed on top of the
(Si,O,Al) layer. In this (Si,O) layer, depleted in aluminium, we point out an
increase of oxygen weight concentration which can be interpreted by the
presence of Si(OH)4 groups, that permit the formation of a (Ca,P) layer.
Aluminium plays a role in the glass solubility and may inhibit apatite
nucleation. After the beginning of the (Ca,P) layer formation, the size of the
electron dense (Si,O) layer decreases and tends to disappear. After two days of
immersion, the (Ca,P) layer grows in thickness and leads to apatite
precipitatio
Symmetry and inert states of spin Bose Condensates
We construct the list of all possible inert states of spin Bose condensates
for . In doing so, we also obtain their symmetry properties. These
results are applied to classify line defects of these spin condensates at zero
magnetic field.Comment: an error in Sec III C correcte
Structure of smectic defect cores: an X-ray study of 8CB liquid crystal ultra-thin films
We study the structure of very thin liquid crystal films frustrated by
antagonistic anchorings in the smectic phase. In a cylindrical geometry, the
structure is dominated by the defects for film thicknesses smaller than 150 nm
and the detailed topology of the defects cores can be revealed by x-ray
diffraction. They appear to be split in half tube-shaped Rotating Grain
Boundaries (RGB). We determine the RGB spatial extension and evaluate its
energy per unit line. Both are significantly larger than the ones usually
proposed in the literatureComment: 4 page
Electron-phonon coupling in the C60 fullerene within the many-body GW approach
We study the electron-phonon coupling in the C60 fullerene within the
first-principles GW approach, focusing on the lowest unoccupied t1u three-fold
electronic state which is relevant for the superconducting transition in
electron doped fullerides. It is shown that the strength of the coupling is
significantly enhanced as compared to standard density functional theory
calculations with (semi)local functionals, with a 48% increase of the
electron-phonon potential Vep. The calculated GW value for the contribution
from the Hg modes of 93 meV comes within 4% of the most recent experimental
values. The present results call for a reinvestigation of previous density
functional based calculations of electron-phonon coupling in covalent systems
in general.Comment: 4 pages, 0 figur
Charge transport in nanoscale vertical organic semiconductor pillar devices
We report charge transport measurements in nanoscale vertical pillar
structures incorporating ultrathin layers of the organic semiconductor
poly(3-hexylthiophene)(P3HT). P3HT layers with thickness down to 5 nm are
gently top-contacted using wedging transfer, yielding highly reproducible,
robust nanoscale junctions carrying high current densities (up to
A/m). Current-voltage data modeling demonstrates excellent hole injection.
This work opens up the pathway towards nanoscale, ultrashort-channel organic
transistors for high-frequency and high-current-density operation.Comment: 30 pages, 8 figures, 1 tabl
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