1,000 research outputs found
Quasi-saddles as relevant points of the potential energy surface in the dynamics of supercooled liquids
The supercooled dynamics of a Lennard-Jones model liquid is numerically
investigated studying relevant points of the potential energy surface, i.e. the
minima of the square gradient of total potential energy . The main findings
are: ({\it i}) the number of negative curvatures of these sampled points
appears to extrapolate to zero at the mode coupling critical temperature ;
({\it ii}) the temperature behavior of has a close relationship with the
temperature behavior of the diffusivity; ({\it iii}) the potential energy
landscape shows an high regularity in the distances among the relevant points
and in their energy location. Finally we discuss a model of the landscape,
previously introduced by Madan and Keyes [J. Chem. Phys. {\bf 98}, 3342
(1993)], able to reproduce the previous findings.Comment: To be published in J. Chem. Phy
Numerical Investigation of Glassy Dynamics in Low Density Systems
Vitrification in colloidal systems typically occurs at high densities driven
by sharply varying, short-ranged interactions. The possibility of glassy
behavior arising from smoothly varying, long-ranged particle interactions has
received relatively little attention. Here we investigate the behavior of
screened charged particles, and explicitly demonstrate that these systems
exhibit glassy properties in the regime of low temperature and low density.
Properties close to this low density (Wigner) glass transition share many
features with their hard-sphere counterparts, but differ in quantitative
aspects that may be accounted for via microscopic theoretical considerations.Comment: 4 pages, 4 figures, revised versio
Circumstellar Disks in the Outer Galaxy: the Star-Forming Region NGC 1893
It is still debated whether star formation process depends on environment. In
particular it is yet unclear whether star formation in the outer Galaxy, where
the environmental conditions are, theoretically, less conducive, occurs in the
same way as in the inner Galaxy. We investigate the population of NGC1893, a
young cluster ~3-4 Myr in the outer part of the Galaxy (galactic radius >11
Kpc), to explore the effects of environmental conditions on star forming
regions. We present infrared observations acquired using the IRAC camera
onboard the Spitzer Space Telescope and analyze the color-color diagrams to
establish the membership of stars with excesses. We also merge this information
with that obtained from Chandra ACIS-I observations, to identify the Class III
population. We find that the cluster is very rich, with 242 PMS Classical
T-Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate
cluster members in the ACIS-I field of view. We estimate a disk fraction for
NGC1893 of about 67%, similar to fractions calculated for nearby star forming
regions of the same age. Although environmental conditions are unfavorable,
star formation can clearly be very successful in the outer Galaxy, allowing
creation of a very rich cluster like NGC1893.Comment: 10 pages,7 figures,4 table
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
Thermodynamic and structural aspects of the potential energy surface of simulated water
Relations between the thermodynamics and dynamics of supercooled liquids
approaching a glass transition have been proposed over many years. The
potential energy surface of model liquids has been increasingly studied since
it provides a connection between the configurational component of the partition
function on one hand, and the system dynamics on the other. This connection is
most obvious at low temperatures, where the motion of the system can be
partitioned into vibrations within a basin of attraction and infrequent
inter-basin transitions. In this work, we present a description of the
potential energy surface properties of supercooled liquid water. The dynamics
of this model has been studied in great details in the last years.
Specifically, we locate the minima sampled by the liquid by ``quenches'' from
equilibrium configurations generated via molecular dynamics simulations. We
calculate the temperature and density dependence of the basin energy,
degeneracy, and shape. The temperature dependence of the energy of the minima
is qualitatively similar to simple liquids, but has anomalous density
dependence. The unusual density dependence is also reflected in the
configurational entropy, the thermodynamic measure of degeneracy. Finally, we
study the structure of simulated water at the minima, which provides insight on
the progressive tetrahedral ordering of the liquid on cooling
Supercooled Water and the Kinetic Glass Transition II: Collective Dynamics
In this article we study in detail the Q-vector dependence of the collective
dynamics in simulated deeply supercooled SPC/E water. The evolution of the
system has been followed for 250 ns at low T, allowing a clear identification
of a two step relaxation process. We present evidence in favor of the use of
the mode coupling theory for supercooled liquid as framework for the
description of the slow alpha-relaxation dynamics in SPC/E water,
notwithstanding the fact that the cage formation in this system is controlled
by the formation of an open network of hydrogen bonds as opposed to packing
constraints, as in the case of simple liquids.Comment: rev-tex + 9 figure
RPAS AND TLS TECNIQUES FOR ARCHAEOLOGICAL SURVEY: THE CASE STUDY OF THE ARCHAEOLOGICAL SITE OF ERACLEA MINOA (ITALY)
Digital documentation and 3D modelling of archaeological sites are important for understanding, definition and recognition of the values of the sites and of the archaeological finds. The most part of archaeological sites are outdoor location, but a cover to preserve the ruins protects often parts of the sites. The possibility to acquire data with different techniques and merge them by using a single reference system allows creating multi-parties models in which 3D representations of the individual objects can be inserted.
The paper presents the results of a recent study carried out by Geomatics Laboratory of University of Palermo for the digital documentation and 3D modelling of Eraclea Minoa archaeological site. This site is located near Agrigento, in the south of Sicily (Italy) and is one of the most famous ancient Greek colonies of Sicily. The paper presents the results of the integration of different data source to survey the Eraclea Minoa archaeological site. The application of two highly versatile recording systems, the TLS (Terrestrial Laser Scanning) and the RPAS (Remotely Piloted Aircraft System), allowed the Eraclea Minoa site to be documented in high resolution and with high accuracy. The integration of the two techniques has demonstrated the possibility to obtain high quality and accurate 3D models in archaeological survey
Equilibrium cluster phases and low-density arrested disordered states: The role of short-range attraction and long-range repulsion
We study a model in which particles interact with short-ranged attractive and
long-ranged repulsive interactions, in an attempt to model the equilibrium
cluster phase recently discovered in sterically stabilized colloidal systems in
the presence of depletion interactions. At low packing fraction particles form
stable equilibrium clusters which act as building blocks of a cluster fluid. We
study the possibility that cluster fluids generate a low-density disordered
arrested phase, a gel, via a glass transition driven by the repulsive
interaction. In this model the gel formation is formally described with the
same physics of the glass formation.Comment: RevTeX4, 4 pages, 4 eps figure
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