1,353 research outputs found
Structural relaxation in a supercooled molecular liquid
We perform molecular-dynamics simulations of a molecular system in
supercooled states for different values of inertia parameters to provide
evidence that the long-time dynamics depends only on the equilibrium structure.
This observation is consistent with the prediction of the mode-coupling theory
for the glass transition and with the hypothesis that the potential
energy-landscape controls the slow dynamics. We also find that dynamical
properties at intermediate wavenumber depend on the spatial correlation of the
molecule's geometrical center.Comment: 7 pages, 4 figures, Europhys. Lett. in pres
Slow dynamics in a primitive tetrahedral network model
We report extensive Monte Carlo and event-driven molecular dynamics
simulations of the fluid and liquid phase of a primitive model for silica
recently introduced by Ford, Auerbach and Monson [J. Chem. Phys. 17, 8415
(2004)]. We evaluate the iso-diffusivity lines in the temperature-density plane
to provide an indication of the shape of the glass transition line. Except for
large densities, arrest is driven by the onset of the tetrahedral bonding
pattern and the resulting dynamics is strong in the Angell's classification
scheme. We compare structural and dynamic properties with corresponding results
of two recently studied primitive models of network forming liquids -- a
primitive model for water and a angular-constraint free model of
four-coordinated particles -- to pin down the role of the geometric constraints
associated to the bonding. Eventually we discuss the similarities between
"glass" formation in network forming liquids and "gel" formation in colloidal
dispersions of patchy particles.Comment: 9 pages, 10 figure
On Static and Dynamic Heterogeneities in Water
We analyze differences in dynamics and in properties of the sampled potential
energy landscape between different equilibrium trajectories, for a system of
rigid water molecules interacting with a two body potential. On entering in the
supercooled region, differences between different realizations enhance and
survive even when particles have diffused several time their average distance.
We observe a strong correlation between the mean square displacement of the
individual trajectories and the average energy of the sampled landscape
A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
NGC6231 is a massive young star cluster, near the center of the Sco OB1
association. While its OB members are well studied, its low-mass population has
received little attention. We present high-spatial resolution Chandra ACIS-I
X-ray data, where we detect 1613 point X-ray sources. Our main aim is to
clarify global properties of NGC6231 down to low masses through a detailed
membership assessment, and to study the cluster stars' spatial distribution,
the origin of their X-ray emission, the cluster age and formation history, and
initial mass function. We use X-ray data, complemented by optical/IR data, to
establish cluster membership. The spatial distribution of different stellar
subgroups also provides highly significant constraints on cluster membership,
as does the distribution of X-ray hardness. We perform spectral modeling of
group-stacked X-ray source spectra. We find a large cluster population down to
~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a
definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that
low-mass cluster stars also constitute the majority of the few hundreds
unidentified X-ray sources. We find mass segregation for the most massive
stars. The fraction of circumstellar-disk bearing members is found to be ~5%.
Photoevaporation of disks under the action of massive stars is suggested by the
spatial distribution of the IR-excess stars. We also find strong Halpha
emission in 9% of cluster PMS stars. The dependence of X-ray properties on
mass, stellar structure, and age agrees with extrapolations based on other
young clusters. The cluster initial mass function, computed over ~2 dex in
mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun
is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the
peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.Comment: 25 pages, 36 figures, accepted for publication on Astronomy and
Astrophysic
Nonlinear Energy Response of Glass Forming Materials
A theory for the nonlinear energy response of a system subjected to a heat
bath is developed when the temperature of the heat bath is modulated
sinusoidally. The theory is applied to a model glass forming system, where the
landscape is assumed to have 20 basins and transition rates between basins obey
a power law distribution. It is shown that the statistics of eigenvalues of the
transition rate matrix, the glass transition temperature , the
Vogel-Fulcher temperature and the crossover temperature can be
determined from the 1st- and 2nd-order ac specific heats, which are defined as
coefficients of the 1st- and 2nd-order energy responses. The imaginary part of
the 1st-order ac specific heat has a broad peak corresponding to the
distribution of the eigenvalues. When the temperature is decreased below ,
the frequency of the peak decreases and the width increases. Furthermore, the
statistics of eigenvalues can be obtained from the frequency dependence of the
1st-order ac specific heat. The 2nd-order ac specific heat shows extrema as a
function of the frequency. The extrema diverge at the Vogel-Fulcher temperature
. The temperature dependence of the extrema changes significantly near
and some extrema vanish near .Comment: 20 pages, 10 figure
Interaction between like-charged polyelectrolyte-colloid complexes in electrolyte solutions: a Monte Carlo simulation study in the Debye-H\"uckel approximation
We study the effective interaction between differently charged
polyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlo
simulations. These complexes are formed when short and flexible polyelectrolyte
chains adsorb onto oppositely charged colloidal spheres, dispersed in an
electrolyte solution. In our simulations the bending energy between adjacent
monomers is small compared to the electrostatic energy, and the chains, once
adsorbed, do not exchange with the solution, although they rearrange on the
particles surface to accomodate further adsorbing chains or due to the
electrostatic interaction with neighbor complexes. Rather unexpectedly, when
two interacting particles approach each others, the rearrangement of the
surface charge distribution invariably produces anti-parallel dipolar doublets,
that invert their orientation at the isoelectric point. These findings clearly
rule out a contribution of dipole-dipole interactions to the observed
attractive interaction between the complexes, pointing out that such
suspensions can not be considered dipolar fluids. On varying the ionic strength
of the electrolyte, we find that a screening length, short compared with the
size of the colloidal particles, is required in order to observe the attraction
between like charged complexes due to the non-uniform distribution of the
electric charge on their surface ('patch attraction'). On the other hand, by
changing the polyelectrolyte/particle charge ratio, the interaction between
like-charged polyelectrolyte-decorated (pd) particles, at short separations,
evolves from purely repulsive to strongly attractive. Hence, the effective
interaction between the complexes is characterized by a potential barrier,
whose height depends on the net charge and on the non-uniformity of their
surface charge distribution.Comment: 24 pages, 9 figure
Investigation of the relation between local diffusivity and local inherent structures in the Kob-Andersen Lennard-Jones model
We analyze one thousand independent equilibrium trajectories of a system of
155 Lennard Jones particles to separate in a model-free approach the role of
temperature and the role of the explored potential energy landscape basin depth
in the particle dynamics. We show that the diffusion coefficient can be
estimated as a sum over over contributions of the sampled basins, establishing
a connection between thermodynamics and dynamics in the potential energy
landscape framework. We provide evidence that the observed non-linearity in the
relation between local diffusion and basin depth is responsible for the
peculiar dynamic behavior observed in supercooled states and provide an
interpretation for the presence of dynamic heterogeneities.Comment: minor text changes, references adde
Density minimum and liquid-liquid phase transition
We present a high-resolution computer simulation study of the equation of
state of ST2 water, evaluating the liquid-state properties at 2718 state
points, and precisely locating the liquid-liquid critical point (LLCP)
occurring in this model. We are thereby able to reveal the interconnected set
of density anomalies, spinodal instabilities and response function extrema that
occur in the vicinity of a LLCP for the case of a realistic, off-lattice model
of a liquid with local tetrahedral order. In particular, we unambiguously
identify a density minimum in the liquid state, define its relationship to
other anomalies, and show that it arises due to the approach of the liquid
structure to a defect-free random tetrahedral network of hydrogen bonds.Comment: 5 pages, 4 figure
- …