700 research outputs found
Delocalization of slowly damped eigenmodes on Anosov manifolds
We look at the properties of high frequency eigenmodes for the damped wave
equation on a compact manifold with an Anosov geodesic flow. We study
eigenmodes with spectral parameters which are asymptotically close enough to
the real axis. We prove that such modes cannot be completely localized on
subsets satisfying a condition of negative topological pressure. As an
application, one can deduce the existence of a "strip" of logarithmic size
without eigenvalues below the real axis under this dynamical assumption on the
set of undamped trajectories.Comment: 28 pages; compared with version 1, minor modifications, add two
reference
Boojums in Rotating Two-Component Bose-Einstein Condensates
A boojum is a topological defect that can form only on the surface of an
ordered medium such as superfluid He and liquid crystals. We study
theoretically boojums appearing between two phases with different vortex
structures in two-component BECs where the intracomponent interaction is
repulsive in one phase and attractive in the other. The detailed structure of
the boojums is revealed by investigating its density distribution, effective
superflow vorticity and pseudospin texture.Comment: 4 pages, 4 figure
High-sensitivity monitoring of micromechanical vibration using optical whispering gallery mode resonators
The inherent coupling of optical and mechanical modes in high finesse optical
microresonators provide a natural, highly sensitive transduction mechanism for
micromechanical vibrations. Using homodyne and polarization spectroscopy
techniques, we achieve shot-noise limited displacement sensitivities of
10^(-19) m Hz^(-1/2). In an unprecedented manner, this enables the detection
and study of a variety of mechanical modes, which are identified as radial
breathing, flexural and torsional modes using 3-dimensional finite element
modelling. Furthermore, a broadband equivalent displacement noise is measured
and found to agree well with models for thermorefractive noise in silica
dielectric cavities. Implications for ground-state cooling, displacement
sensing and Kerr squeezing are discussed.Comment: 25 pages, 8 figure
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
Evolution of Chemistry in the envelope of Hot Corinos (ECHOS). I. Extremely young sulphur chemistry in the isolated Class 0 object B335
Within the project Evolution of Chemistry in the envelope of HOt corinoS
(ECHOS), we present a study of sulphur chemistry in the envelope of the Class 0
source B335 through observations in the spectral range 7, 3, and 2 mm. We have
modelled observations assuming LTE and LVG approximation. We have also used the
code Nautilus to study the time evolution of sulphur species. We have detected
20 sulphur species with a total gas-phase S abundance similar to that found in
the envelopes of other Class 0 objects, but with significant differences in the
abundances between sulphur carbon chains and sulphur molecules containing
oxygen and nitrogen. Our results highlight the nature of B335 as a source
especially rich in sulphur carbon chains unlike other Class 0 sources. The low
presence or absence of some molecules, such as SO and SO+, suggests a chemistry
not particularly influenced by shocks. We, however, detect a large presence of
HCS+ that, together with the low rotational temperatures obtained for all the S
species (<15 K), reveals the moderate or low density of the envelope of B335.
We also find that observations are better reproduced by models with a sulphur
depletion factor of 10 with respect to the sulphur cosmic elemental abundance.
The comparison between our model and observational results for B335 reveals an
age of 10t10 yr, which highlights the particularly early
evolutionary stage of this source. B335 presents a different chemistry compared
to other young protostars that have formed in dense molecular clouds, which
could be the result of accretion of surrounding material from the diffuse cloud
onto the protostellar envelope of B335. In addition, the analysis of the
SO2/C2S, SO/CS, and HCS+/CS ratios within a sample of prestellar cores and
Class 0 objects show that they could be used as good chemical evolutionary
indicators of the prestellar to protostellar transition
Nucleon-induced reactions at intermediate energies: New data at 96 MeV and theoretical status
Double-differential cross sections for light charged particle production (up
to A=4) were measured in 96 MeV neutron-induced reactions, at TSL laboratory
cyclotron in Uppsala (Sweden). Measurements for three targets, Fe, Pb, and U,
were performed using two independent devices, SCANDAL and MEDLEY. The data were
recorded with low energy thresholds and for a wide angular range (20-160
degrees). The normalization procedure used to extract the cross sections is
based on the np elastic scattering reaction that we measured and for which we
present experimental results. A good control of the systematic uncertainties
affecting the results is achieved. Calculations using the exciton model are
reported. Two different theoretical approches proposed to improve its
predictive power regarding the complex particle emission are tested. The
capabilities of each approach is illustrated by comparison with the 96 MeV data
that we measured, and with other experimental results available in the
literature.Comment: 21 pages, 28 figure
Well dispersed fractal aggregates as filler in polymer-silica nanocomposites: long range effects in rheology
We are presenting a new method of processing polystyrene-silica
nanocomposites, which results in a very well-defined dispersion of small
primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the
matrix. The process is based on a high boiling point solvent, in which the
nanoparticles are well dispersed, and controlled evaporation. The filler's fine
network structure is determined over a wide range of sizes, using a combination
of Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy
(TEM). The mechanical response of the nanocomposite material is investigated
both for small (ARES oscillatory shear and Dynamical Mechanical Analysis) and
large deformations (uniaxial traction), as a function of the concentration of
the particles. We can investigate the structure-property correlations for the
two main reinforcement effects: the filler network contribution, and a
filler-polymer matrix effect. Above a silica volume fraction threshold, we see
a divergence of the modulus correlated to the build up of a connected network.
Below the threshold, we obtain a new additional elastic contribution of much
longer terminal time than the matrix. Since aggregates are separated by at
least 60 nm, this new filler-matrix contribution cannot be described solely
with the concept of glassy layer (2nm)
Boojums and the Shapes of Domains in Monolayer Films
Domains in Langmuir monolayers support a texture that is the two-dimensional
version of the feature known as a boojum. Such a texture has a quantifiable
effect on the shape of the domain with which it is associated. The most
noticeable consequence is a cusp-like feature on the domain boundary. We report
the results of an experimental and theoretical investigation of the shape of a
domain in a Langmuir monolayer. A further aspect of the investigation is the
study of the shape of a ``bubble'' of gas-like phase in such a monolayer. This
structure supports a texture having the form of an inverse boojum. The
distortion of a bubble resulting from this texture is also studied. The
correspondence between theory and experiment, while not perfect, indicates that
a qualitative understanding of the relationship between textures and domain
shapes has been achieved.Comment: replaced with published version, 10 pages, 13 figures include
Gas phase Elemental abundances in Molecular cloudS (GEMS) VII. Sulfur elemental abundance
Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30m
large program aimed at determining the elemental abundances of carbon (C),
oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical
star-forming filaments. In particular, the elemental abundance of S remains
uncertain by several orders of magnitude and its determination is one of the
most challenging goals of this program. We have carried out an extensive
chemical modeling of the fractional abundances of CO, HCO, HCN, HNC, CS,
SO, HS, OCS, and HCS to determine the sulfur depletion toward the 244
positions in the GEMS database. These positions sample visual extinctions from
A 3 mag to 50 mag, molecular hydrogen densities ranging from a
few 10~cm to 310~cm, and T 1035 K.
Most of the positions in Taurus and Perseus are best fitted assuming early-time
chemistry, t=0.1 Myr, (0.51)10 s,
and [S/H]1.510. On the contrary, most of the positions in
Orion are fitted with t=1~Myr and 10 s.
Moreover, 40% of the positions in Orion are best fitted assuming the
undepleted sulfur abundance, [S/H]1.510. Our results
suggest that sulfur depletion depends on the environment. While the abundances
of sulfur-bearing species are consistent with undepleted sulfur in Orion, a
depletion factor of 20 is required to explain those observed in Taurus
and Perseus. We propose that differences in the grain charge distribution in
the envelopes of the studied clouds might explain these variations. The shocks
associated with past and ongoing star formation could also contribute to
enhance [S/H] in Orion.Comment: 22 pages, 15 figures, Astronomy and Astrophysics, in pres
- …