126 research outputs found
Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
The manner in which the intermolecular potential u(r) governs structural
relaxation in liquids is a long standing problem in condensed matter physics.
Herein we show that diffusion coefficients for simulated Lennard-Jones m-6
liquids (8<m<36) in normal and moderately supercooled states are a unique
function of the variable rho^g/T, where rho is density and T is temperature.
The scaling exponent g is a material specific constant whose magnitude is
related to the steepness of the repulsive part of u(r), evaluated around the
distance of closest approach between particles probed in the supercooled
regime. Approximations of u(r) in terms of inverse power laws are also
discussed.Comment: 4 pages, 3 figure
Pressure-energy correlations and thermodynamic scaling in viscous Lennard-Jones liquids
We use molecular dynamics simulation results on viscous binary Lennard-Jones
mixtures to examine the correlation between the potential energy and the
virial. In accord with a recent proposal [U. R. Pedersen et. al. Phys. Rev.
Lett. 100, 015701 (2008)], the fluctuations in the two quantities are found to
be strongly correlated, exhibiting a proportionality constant, Gamma,
numerically equal to one-third the slope of an inverse power law approximation
to the intermolecular potential function. The correlation is stronger at higher
densities, where interatomic separations are in the range where the inverse
power law approximation is more accurate. These same liquids conform to
thermodynamic scaling of their dynamics, with the scaling exponent equal to
Gamma. Thus, the properties of strong correlation between energy and pressure
and thermodynamic scaling both reflect the ability of an inverse power law
representation of the potential to capture interesting features of the dynamics
of dense, highly viscous liquids.Comment: 5 pages, 4 figures; published version, one figure remove
Density scaling in viscous liquids: From relaxation times to four-point susceptibilities
We present numerical calculations of a four-point dynamic susceptibility,
chi_4(t), for the Kob-Andersen Lennard-Jones mixture as a function of
temperature T and density rho. Over a relevant range of T and rho, the full
t-dependence of chi_4(t) and thus the maximum in chi_4(t), which is
proportional to the dynamic correlation volume, are invariant for state points
for which the scaling variable rho^gamma/T is constant. The value of the
material constant gamma is the same as that which superposes the relaxation
time, tau, of the system versus rho^gamma/T. Thus, the dynamic correlation
volume is directly related to tau for any thermodynamic condition in the regime
where density scaling holds. Finally, we examine the conditions under which the
density scaling properties are related to the existence of strong correlations
between pressure and energy fluctuations.Comment: 5 pages, 4 figures, updated reference
Understanding fragility in supercooled Lennard-Jones mixtures. II. Potential energy surface
We numerically investigated the connection between isobaric fragility and the
properties of high-order stationary points of the potential energy surface in
different supercooled Lennard-Jones mixtures. The increase of effective
activation energies upon supercooling appears to be driven by the increase of
average potential energy barriers measured by the energy dependence of the
fraction of unstable modes. Such an increase is sharper, the more fragile is
the mixture. Correlations between fragility and other properties of high-order
stationary points, including the vibrational density of states and the
localization features of unstable modes, are also discussed.Comment: 13 pages, 13 figures, minor revisions, one figure adde
Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures
We reveal the existence of systematic variations of isobaric fragility in
different supercooled Lennard-Jones binary mixtures by performing molecular
dynamics simulations. The connection between fragility and local structures in
the bulk is analyzed by means of a Voronoi construction. We find that clusters
of particles belonging to locally preferred structures form slow, long-lived
domains, whose spatial extension increases by decreasing temperature. As a
general rule, a more rapid growth, upon supercooling, of such domains is
associated to a more pronounced super-Arrhenius behavior, hence to a larger
fragility.Comment: 14 pages, 14 figures, minor revisions, one figure adde
Cluster glasses of ultrasoft particles
We present molecular dynamics (MD) simulations results for dense fluids of
ultrasoft, fully-penetrable particles. These are a binary mixture and a
polydisperse system of particles interacting via the generalized exponential
model, which is known to yield cluster crystal phases for the corresponding
monodisperse systems. Because of the dispersity in the particle size, the
systems investigated in this work do not crystallize and form disordered
cluster phases. The clustering transition appears as a smooth crossover to a
regime in which particles are mostly located in clusters, isolated particles
being infrequent. The analysis of the internal cluster structure reveals
microsegregation of the big and small particles, with a strong
homo-coordination in the binary mixture. Upon further lowering the temperature
below the clustering transition, the motion of the clusters' centers-of-mass
slows down dramatically, giving way to a cluster glass transition. In the
cluster glass, the diffusivities remain finite and display an activated
temperature dependence, indicating that relaxation in the cluster glass occurs
via particle hopping in a nearly arrested matrix of clusters. Finally we
discuss the influence of the microscopic dynamics on the transport properties
by comparing the MD results with Monte Carlo simulations.Comment: 17 pages, 23 figure
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.
Are there localized saddles behind the heterogeneous dynamics of supercooled liquids?
We numerically study the interplay between heterogeneous dynamics and
properties of negatively curved regions of the potential energy surface in a
model glassy system. We find that the unstable modes of saddles and
quasi-saddles undergo a localization transition close to the Mode-Coupling
critical temperature. We also find evidence of a positive spatial correlation
between clusters of particles having large displacements in the unstable modes
and dynamical heterogeneities.Comment: 7 pages, 3 figures, submitted to Europhys. Let
Dynamics in binary cluster crystals
As a result of the application of coarse-graining procedures to describe
complex fluids, the study of systems consisting of particles interacting
through bounded, repulsive pair potentials has become of increasing interest in
the last years. A well known example is the so-called Generalized Exponential
Model (GEM-), for which the interaction between particles is described by
the potential . Interactions with
lead to the formation of a novel phase of soft matter consisting of cluster
crystals. Recent studies on the phase behavior of binary mixtures of GEM-
particles have provided evidence for the formation of novel kinds of alloys,
depending on the cross interactions between the two species. This work aims to
study the dynamic behavior of such binary mixtures by means of extensive
molecular dynamics simulations, and in particular to investigate the effect of
the addition of non-clustering particles on the dynamic scenario of
one-component cluster crystals. Analogies and differences with the
one-component case are revealed and discussed by analyzing self- and collective
dynamic correlators.Comment: 17 pages, 8 figures, submitted to JSTA
Heterogeneous Dynamics, Marginal Stability and Soft Modes in Hard Sphere Glasses
In a recent publication we established an analogy between the free energy of
a hard sphere system and the energy of an elastic network [1]. This result
enables one to study the free energy landscape of hard spheres, in particular
to define normal modes. In this Letter we use these tools to analyze the
activated transitions between meta-bassins, both in the aging regime deep in
the glass phase and near the glass transition. We observe numerically that
structural relaxation occurs mostly along a very small number of
nearly-unstable extended modes. This number decays for denser packing and is
significantly lowered as the system undergoes the glass transition. This
observation supports that structural relaxation and marginal modes share common
properties. In particular theoretical results [2, 3] show that these modes
extend at least on some length scale where
corresponds to the maximum packing fraction, i.e. the jamming
transition. This prediction is consistent with very recent numerical
observations of sheared systems near the jamming threshold [4], where a similar
exponent is found, and with the commonly observed growth of the rearranging
regions with compression near the glass transition.Comment: 6 pages, improved versio
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