747 research outputs found
Equilibrium ultrastable glasses produced by random pinning
Ultrastable glasses have risen to prominence due to their potentially useful
material properties and the tantalizing possibility of a general method of
preparation via vapor deposition. Despite the importance of this novel class of
amorphous materials, numerical studies have been scarce because achieving
ultrastability in atomistic simulations is an enormous challenge. Here we
bypass this difficulty and establish that randomly pinning the position of a
small fraction of particles inside an equilibrated supercooled liquid generates
ultrastable configurations at essentially no numerical cost, while avoiding
undesired structural changes due to the preparation protocol. Building on the
analogy with vapor-deposited ultrastable glasses, we study the melting kinetics
of these configurations following a sudden temperature jump into the liquid
phase. In homogeneous geometries, we find that enhanced kinetic stability is
accompanied by large scale dynamic heterogeneity, while a competition between
homogeneous and heterogeneous melting is observed when a liquid boundary
invades the glass at constant velocity. Our work demonstrates the feasibility
of large-scale, atomistically resolved, and experimentally relevant simulations
of the kinetics of ultrastable glasses.Comment: 9 pages, 5 figure
Crossovers in the dynamics of supercooled liquids probed by an amorphous wall
We study the relaxation dynamics of a binary Lennard-Jones liquid in the
presence of an amorphous wall generated from equilibrium particle
configurations. In qualitative agreement with the results presented in Nature
Phys. {\bf 8}, 164 (2012) for a liquid of harmonic spheres, we find that our
binary mixture shows a saturation of the dynamical length scale close to the
mode-coupling temperature . Furthermore we show that, due to the broken
symmetry imposed by the wall, signatures of an additional change in dynamics
become apparent at a temperature well above . We provide evidence that
this modification in the relaxation dynamics occurs at a recently proposed
dynamical crossover temperature , which is related to the breakdown
of the Stokes-Einstein relation. We find that this dynamical crossover at
is also observed for a system of harmonic spheres as well as a WCA liquid,
showing that it may be a general feature of glass-forming systems.Comment: 10 pages, 8 figure
Modern computational studies of the glass transition
The physics of the glass transition and amorphous materials continues to
attract the attention of a wide research community after decades of effort.
Supercooled liquids and glasses have been studied numerically since the advent
of molecular dynamics and Monte Carlo simulations in the last century. Computer
studies have greatly enhanced both experimental discoveries and theoretical
developments and constitute an active and continually expanding research field.
Our goal in this review is to provide a modern perspective on this area. We
describe the need to go beyond canonical methods to attack a problem that is
notoriously difficult in terms of time scales, length scales, and physical
observables. We first summarise recent algorithmic developments to achieve
enhanced sampling and faster equilibration using replica exchange methods,
cluster and swap Monte Carlo algorithms, and other techniques. We then review
some major recent advances afforded by these novel tools regarding the
statistical mechanical description of the liquid-to-glass transition as well as
the mechanical, vibrational and thermal properties of the glassy solid. We
finally describe some important challenges for future research
A Molecular Hydrodynamic Theory of Supercooled Liquids and Colloidal Suspensions under Shear
We extend the conventional mode-coupling theory of supercooled liquids to
systems under stationary shear flow. Starting from generalized fluctuating
hydrodynamics, a nonlinear equation for the intermediate scattering function is
constructed. We evaluate the solution numerically for a model of a two
dimensional colloidal suspension and find that the structural relaxation time
decreases as with an exponent , where
is the shear rate. The results are in qualitative agreement with
recent molecular dynamics simulations. We discuss the physical implications of
the results.Comment: 5 pages, 1 figur
Current status and development of the SSO FUN alerts
International audienceThe astrometry mission Gaia of the European Space Agency (ESA) will scan the entire sky several times over 5 years, down to a visual apparent magnitude of 20. Apart for its primary targets, the stars, that will be mapped during the course of the mission, Gaia is expected to observe more than 300,000 asteroids (Mignard et al., 2007). Although our census of asteroids is about complete at a such magnitude limit, the location of Gaia at L2 may allow the detection of yet-unknown near-Earth asteroids (NEAs). The predefined and smooth scanning law of Gaia, however, is not meant for pointed or follow-up observations. A ground-based network of observers has therefore been set up, the Follow-Up Network for the Solar System Objects (FUN SSO), centered around a central node (the DU459 of the Gaia Data Processing and Analysis Consortium, the DPAC). The aim of this network is to quickly observe from the ground the NEAs newly discovered by Gaia to secure an accurate orbit
The Earth as an extrasolar transiting planet - II: HARPS and UVES detection of water vapor, biogenic O, and O
The atmospheric composition of transiting exoplanets can be characterized
during transit by spectroscopy. For the transit of an Earth twin, models
predict that biogenic and should be detectable, as well as water
vapour, a molecule linked to habitability as we know it on Earth. The aim is to
measure the Earth radius versus wavelength - or the atmosphere
thickness - at the highest spectral resolution available to fully
characterize the signature of Earth seen as a transiting exoplanet. We present
observations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Earth
eclipses the Sun and opens access to the Earth atmosphere transmission
spectrum. We used HARPS and UVES spectrographs to take penumbra and umbra
high-resolution spectra from 3100 to 10400 Ang. A change of the quantity of
water vapour above the telescope compromised the quality of the UVES data. We
corrected for this effect in the data processing. We analyzed the data by 3
different methods. The 1st method is based on the analysis of pairs of penumbra
spectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of all
penumbra and umbra spectra. Profiles are obtained with the three
methods for both instruments. The 1st method gives the best result, in
agreement with a model. The second method seems to be more sensitive to the
Doppler shift of solar spectral lines with respect to the telluric lines. The
3rd method makes use of umbra spectra which bias the result, but it can be
corrected for this a posteriori from results with the first method. The 3
methods clearly show the spectral signature of the Rayleigh scattering in the
Earth atmosphere and the bands of HO, O, and O. Sodium is detected.
Assuming no atmospheric perturbations, we show that the E-ELT is theoretically
able to detect the A-band in 8~h of integration for an Earth twin at
10pc.Comment: Final version accepted for publication in A&A - 21 pages, 27 figures.
Abstract above slightly shortened wrt the original. The ArXiv version has low
resolution figures, but a version with full resolution figures is available
here:
http://www.obs-hp.fr/~larnold/publi_to_download/eclipse2010_AA_v5_final.pd
Use of green roofs to solve storm water issues at the basin scale – Study in the Hauts-de-Seine County (France)
International audienceAt the building scale, green roof has demonstrated a positive impact on urban runoff (decrease in the peak discharge and runoff volume). This work aims to study if similar impacts can be observed at basin scale. It is particularly focused on the possibility to solve some operational issues caused by storm water.For this purpose, a methodology has been proposed. It combines: a method to estimate the maximum roof area that can be covered by green roof, called green roofing potential, and an urban rainfall-runoff model able to simulate the hydrological behaviour of green roof.This methodology was applied to two urban catchments affected one by flooding and the other one by combined sewage overflow. The results show that green roof can reduce the frequency and the magnitude of such problems depending on the covered roof surface. Combined with other infrastructures, they represent an interesting solution for urban water management
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