6,197 research outputs found
Nonlinear dynamics and surface diffusion of diatomic molecules
The motion of molecules on solid surfaces is of interest for technological
applications, but it is also a theoretical challenge. We study the
deterministic and thermal diffusive dynamics of a dimer moving on a periodic
substrate. The deterministic motion of the dimer displays strongly nonlinear
features and chaotic behavior. The dimer thermal diffusive dynamics deviates
from simple Arrhenius behavior, due to the coupling between vibrational and
translational degrees of freedom. In the low-temperature limit the dimer
diffusion can become orders of magnitude larger than that of a single atom, as
also found experimentally. The relation between chaotic deterministic dynamics
and stochastic thermal diffusion is discussed.Comment: 4 pages, 4 figure
Power law load dependence of atomic friction
We present a theoretical study of the dynamics of a tip scanning a graphite
surface as a function of the applied load. From the analysis of the lateral
forces, we extract the friction force and the corrugation of the effective
tip-surface interaction potential. We find both the friction force and
potential amplitude to have a power law dependence on applied load with
exponent . We interpret these results as characteristic of sharp
undeformable tips in contrast to the case of macroscopic and elastic
microscopic contacts.Comment: 4 pages, 4 figure
Rheological properties vs Local Dynamics in model disordered materials at Low Temperature
We study the rheological response at low temperature of a sheared model
disordered material as a function of the bond rigidity. We find that the flow
curves follow a Herschel-Bulkley law, whatever is the bond rigidity, with an
exponent close to 0.5. Interestingly, the apparent viscosity can be related to
a single relevant time scale , suggesting a strong connection between
the local dynamics and the global mechanical behaviour. We propose a model
based on the competition between the nucleation and the avalanche-like
propagation of spatial strain heterogeneities. This model can explain the
Herschel-Bulkley exponent on the basis of the size dependence of the
heterogeneities on the shear rate.Comment: 9 pages, 7 figure
Random sequential adsorption and diffusion of dimers and k-mers on a square lattice
We have performed extensive simulations of random sequential adsorption and
diffusion of -mers, up to in two dimensions with particular attention
to the case . We focus on the behavior of the coverage and of vacancy
dynamics as a function of time. We observe that for a complete coverage
of the lattice is never reached, because of the existence of frozen
configurations that prevent isolated vacancies in the lattice to join. From
this result we argue that complete coverage is never attained for any value of
. The long time behavior of the coverage is not mean field and nonanalytic,
with as leading term. Long time coverage regimes are independent of
the initial conditions while strongly depend on the diffusion probability and
deposition rate and, in particular, different values of these parameters lead
to different final values of the coverage. The geometrical complexity of these
systems is also highlighted through an investigation of the vacancy population
dynamics.Comment: 9 pages, 9 figures, to be published in the Journal of Chemical
Physic
Analysis of ground-based differential imager performance
In the context of extrasolar planet direct detection, we evaluated the
performance of differential imaging with ground-based telescopes. This study
was carried out in the framework of the VLT-Planet Finder project and is
further extended to the case of Extremely Large Telescopes. Our analysis is
providing critical specifications for future instruments mostly in terms of
phase aberrations but also regarding alignments of the instrument optics or
offset pointing on the coronagraph. It is found that Planet Finder projects on
8m class telescopes can be successful at detecting Extrasolar Giant Planets
providing phase aberrations, alignments and pointing are accurately controlled.
The situation is more pessimistic for the detection of terrestrial planets with
Extremely Large Telescopes for which phase aberrations must be lowered at a
very challenging level
Nonthermal hard X-ray excess in the Coma cluster: resolving the discrepancy between the results of different PDS data analyses
The detection of a nonthermal excess in the Coma cluster spectrum by two
BeppoSAX observations analyzed with the XAS package (Fusco-Femiano et al.) has
been disavowed by an analysis (Rossetti & Molendi) performed with a different
software package (SAXDAS) for the extraction of the spectrum. To resolve this
discrepancy we reanalyze the PDS data considering the same software used by
Rossetti & Molendi. A correct selection of the data and the exclusion of
contaminating sources in the background determination show that also the SAXDAS
analysis reports a nonthermal excess with respect to the thermal emission at
about the same confidence level of that obtained with the XAS package
(~4.8sigma). Besides, we report the lack of the systematic errors investigated
by Rossetti & Molendi and Nevalainen et al. taking into account the whole
sample of the PDS observations off the Galactic plane, as already shown in our
data analysis of Abell 2256 (Fusco-Femiano, Landi & Orlandini). All this
eliminates any ambiguity and confirms the presence of a hard tail in the
spectrum of the Coma cluster.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter
Nonlinear dynamics of dimers on periodic substrates
We study the dynamics of a dimer moving on a periodic one-dimensional
substrate as a function of the initial kinetic energy at zero temperature. The
aim is to describe, in a simplified picture, the microscopic dynamics of
diatomic molecules on periodic surfaces, which is of importance for thin film
formation and crystal growth. We find a complex behaviour, characterized by a
variety of dynamical regimes, namely oscillatory, ``quasi-diffusive'' (chaotic)
and drift motion. Parametrically resonant excitations of internal vibrations
can be induced both by oscillatory and drift motion of the centre of mass. For
weakly bound dimers a chaotic regime is found for a whole range of velocities
between two non-chaotic phases at low and high kinetic energy. The chaotic
features have been monitored by studying the Lyapunov exponents and the power
spectra. Moreover, for a short-range interaction, the dimer can dissociate due
to the parametric excitation of the internal motion.Comment: 9 pages, 13 figures, to be published in Eur. Phys. J.
Deep GeMS/GSAOI near-infrared observations of N159W in the Large Magellanic Cloud
Aims. The formation and properties of star clusters at the edge of H II
regions are poorly known, partly due to limitations in angular resolution and
sensitivity, which become particularly critical when dealing with extragalactic
clusters. In this paper we study the stellar content and star-formation
processes in the young N159W region in the Large Magellanic Cloud.
Methods. We investigate the star-forming sites in N159W at unprecedented
spatial resolution using JHKs-band images obtained with the GeMS/GSAOI
instrument on the Gemini South telescope. The typical angular resolution of the
images is of 100 mas, with a limiting magnitude in H of 22 mag (90 percent
completeness). Photometry from our images is used to identify candidate young
stellar objects (YSOs) in N159W. We also determine the H-band luminosity
function of the star cluster at the centre of the H II region and use this to
estimate its initial mass function (IMF).
Results. We estimate an age of 2 + or - 1 Myr for the central cluster, with
its IMF described by a power-law with an index of gamma = - 1.05 + or - 0.2 ,
and with a total estimated mass of 1300 solar mass. We also identify 104
candidate YSOs, which are concentrated in clumps and subclusters of stars,
principally at the edges of the H II region. These clusters display signs of
recent and active star-formation such as ultra-compact H II regions, and
molecular outflows. This suggests that the YSOs are typically younger than the
central cluster, pointing to sequential star-formation in N159W, which has
probably been influenced by interactions with the expanding H II bubble
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