8,972 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
Supermodel Analysis of the Hard X-Ray Excess in the Coma Cluster
The Supermodel provides an accurate description of the thermal contribution
by the hot intracluster plasma which is crucial for the analysis of the hard
excess. In this paper the thermal emissivity in the Coma cluster is derived
starting from the intracluster gas temperature and density profiles obtained by
the Supermodel analysis of X-ray observables: the XMM-Newton temperature
profile and the Rosat brightness distribution. The Supermodel analysis of the
BeppoSAX/PDS hard X-ray spectrum confirms our previous results, namely an
excess at the c.l. of ~4.8sigma and a nonthermal flux of 1.30+-0.40x 10^-11 erg
cm^-2 s^-1 in the energy range 20-80 keV. A recent joint XMM-Newton/Suzaku
analysis reports an upper limit of ~6x10^-12 erg cm^-2 s^-1 in the energy range
20-80 keV for the nonthermal flux with an average gas temperature of 8.45+-0.06
keV, and an excess of nonthermal radiation at a confidence level above 4sigma,
without including systematic effects, for an average XMM-Newton temperature of
8.2 keV in the Suzaku/HXD-PIN FOV, in agreement with our earlier PDS analysis.
Here we present a further evidence of the compatibility between the Suzaku and
BeppoSAX spectra, obtained by our Supermodel analysis of the PDS data, when the
smaller size of the HXD-PIN FOV and the two different average temperatures
derived by XMM-Newton and by the joint XMM-Newton/Suzaku analysis are taken
into account. The consistency of the PDS and HXD-PIN spectra reaffirms the
presence of a nonthermal component in the hard X-ray spectrum of the Coma
cluster. The Supermodel analysis of the PDS data reports an excess at c.l.
above 4sigma also for the higher average temperature of 8.45 keV thanks to the
PDS FOV considerably greater than the HXD-PIN FOV.Comment: 18 pages, 7 figures, accepted for publication in Ap
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
A strong form of the Quantitative Isoperimetric inequality
We give a refinement of the quantitative isoperimetric inequality. We prove
that the isoperimetric gap controls not only the Fraenkel asymmetry but also
the oscillation of the boundary
Chandra Observation of a 300 kpc Hydrodynamic Instability in the Intergalactic Medium of the Merging Cluster of Galaxies A3667
We present results from the combination of two Chandra pointings of the
central region of the cluster of galaxies A3667. From the data analysis of the
first pointing Vikhlinin et al. reported the discovery of a prominent cold
front which is interpreted as the boundary of a cool gas cloud moving through
the hotter ambient gas. Vikhlinin et al. discussed the role of the magnetic
fields in maintaining the apparent dynamical stability of the cold front over a
wide sector at the forward edge of the moving cloud and suppressing transport
processes across the front. In this Letter, we identify two new features in the
X-ray image of A3667: i) a 300 kpc arc-like filamentary X-ray excess extending
from the cold gas cloud border into the hotter ambient gas; ii) a similar
arc-like filamentary X-ray depression that develops inside the gas cloud. The
temperature map suggests that the temperature of the filamentary excess is
consistent with that inside the gas cloud while the temperature of the
depression is consistent with that of the ambient gas. We suggest that the
observed features represent the first evidence for the development of a large
scale hydrodynamic instability in the cluster atmosphere resulting from a major
merger. This result confirms previous claims for the presence of a moving cold
gas cloud into the hotter ambient gas. Moreover it shows that, although the gas
mixing is suppressed at the leading edge of the subcluster due to its magnetic
structure, strong turbulent mixing occurs at larger angles to the direction of
motion. We show that this mixing process may favor the deposition of a
nonnegligible quantity of thermal energy right in the cluster center, affecting
the development of the central cooling flow.Comment: Replaced to match version accepted for publication in ApJL; some
changes on text. 4 pages, 3 color figures and 2 BW figures, emulateapj
Mechanics of a continuum medium. Vol. III
This work presents the fundamental assumptions and the successive mathematical developments which allow to establish the complete field equations of a continuum. The aim is to be sufficiently general and self-content using, however, mathematical procedures which can be always understook by an engineer with a common background of Linear Algebra and elementary Differential Calculus
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