16 research outputs found
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
What does the potential energy landscape tell us about the dynamics of supercooled liquids and glasses?
For a model glass-former we demonstrate via computer simulations how
macroscopic dynamic quantities can be inferred from a PEL analysis. The
essential step is to consider whole superstructures of many PEL minima, called
metabasins, rather than single minima. We show that two types of metabasins
exist: some allowing for quasi-free motion on the PEL (liquid-like), the others
acting as traps (solid-like). The activated, multi-step escapes from the latter
metabasins are found to dictate the slowing down of dynamics upon cooling over
a much broader temperature range than is currently assumed
Electronic redistribution around oxygen atoms in silicate melts by ab initio molecular dynamics simulation
The structure around oxygen atoms of four silicate liquids (silica, rhyolite,
a model basalt and enstatite) is evaluated by ab initio molecular dynamics
simulation. Thanks to the use of maximally localized Wannier orbitals to
represent the electronic ground state of the simulated system, one is able to
quantify the redistribution of electronic density around oxygen atoms as a
function of the cationic environment and melt composition. It is shown that the
structure of the melt in the immediate vicinity of the oxygen atoms modulates
the distribution of the Wannier orbitals associated with oxygen atoms. In
particular the evaluation of the distances between the oxygen-core and the
orbital Wannier centers and their evolution with the nature of the cation
indicates that the Al-O bond in silicate melts is certainly less covalent than
the Si-O bond while for the series Mg-O, Ca-O, Na-O and K-O the covalent
character of the M-O bond diminishes rapidly to the benefit of the ionic
character. Furthermore it is found that the distribution of the oxygen dipole
moment coming from the electronic polarization is only weakly dependent on the
melt composition, a finding which could explain why some empirical force fields
can exhibit a high degree of transferability with melt composition.Comment: 27 pages, 7 figures. To be published in Journal of Non-Crystalline
Solid
On the static length of relaxation and the origin of dynamic heterogeneity in fragile glass-forming liquids
The most puzzling aspect of the glass transition observed in laboratory is an
apparent decoupling of dynamics from structure. In this paper we recount the
implication of various theories of glass transition for the static correlation
length in an attempt to reconcile the dynamic and static lengths associate with
the glass problem. We argue that a more recent characterization of the static
relaxation length based on the bond ordering scenario, as the typical length
over which the energy fluctuations are correlated, is more consistent with, and
indeed in perfect agreement with the typical linear size of the dynamically
heterogeneous domains observed in deeply supercooled liquids. The correlated
relaxation of bonds in terms of energy is therefore identified as the physical
origin of the observed dynamic heterogeneity.Comment: 6 pages, 1 figur
Dynamics and energy landscape in a tetrahedral network glass-former: Direct comparison with models of fragile liquids
We report Molecular Dynamics simulations for a new model of tetrahedral
network glass-former, based on short-range, spherical potentials. Despite the
simplicity of the forcefield employed, our model reproduces some essential
physical properties of silica, an archetypal network-forming material.
Structural and dynamical properties, including dynamic heterogeneities and the
nature of local rearrangements, are investigated in detail and a direct
comparison with models of close-packed, fragile glass-formers is performed. The
outcome of this comparison is rationalized in terms of the properties of the
Potential Energy Surface, focusing on the unstable modes of the stationary
points. Our results indicate that the weak degree of dynamic heterogeneity
observed in network glass-formers may be attributed to an excess of localized
unstable modes, associated to elementary dynamical events such as bond breaking
and reformation. On the contrary, the more fragile Lennard-Jones mixtures are
characterized by a larger fraction of extended unstable modes, which lead to a
more cooperative and heterogeneous dynamics.Comment: 26 pages, 18 figures, added links to animations, corrected typos in
sec.
Description of the dynamics in complex energy landscapes via metabasins: a simple model study
We study the dynamics in a simple hierarchical energy landscape. We compare a straightforward analytical approximation with the results of Monte Carlo simulations. The model is devised to mimic some aspects of the dynamics in supercooled liquids. We show that the concept of metabasins, as recently discussed in the framework of the potential energy landscape of glasses, emerges quite naturally