254 research outputs found
Dynamical heterogeneities in a two dimensional driven glassy model: current fluctuations and finite size effects
In this article, we demonstrate that in a transport model of particles with
kinetic constraints, long-lived spatial structures are responsible for the
blocking dynamics and the decrease of the current at strong driving field.
Coexistence between mobile and blocked regions can be anticipated by a
first-order transition in the large deviation function for the current. By a
study of the system under confinement, we are able to study finite-size effects
and extract a typical length between mobile regions
Non-Equilibrium Phase Transition in an Atomistic Glassformer: the Connection to Thermodynamics
Tackling the low-temperature fate of supercooled liquids is challenging due
to the immense timescales involved, which prevent equilibration and lead to the
operational glass transition. Relating glassy behaviour to an underlying,
thermodynamic phase transition is a long-standing open question in condensed
matter physics. Like experiments, computer simulations are limited by the small
time window over which a liquid can be equilibrated. Here we address the
challenge of low temperature equilibration using trajectory sampling in a
system undergoing a nonequilibrium phase transition. This transition occurs in
trajectory space between the normal supercooled liquid and a glassy state rich
in low-energy geometric motifs. Our results indicate that this transition might
become accessible in equilibrium configurational space at a temperature close
to the so-called Kauzmann temperature, and provide a possible route to unify
dynamical and thermodynamical theories of the glass transition.Comment: accepted in Physical. Rev.
From glass formation to icosahedral ordering by curving three-dimensional space
Geometric frustration describes the inability of a local molecular
arrangement, such as icosahedra found in metallic glasses and in model atomic
glass-formers, to tile space. Local icosahedral order however is strongly
frustrated in Euclidean space, which obscures any causal relationship with the
observed dynamical slowdown. Here we relieve frustration in a model
glass-forming liquid by curving 3-dimensional space onto the surface of a
4-dimensional hypersphere. For sufficient curvature, frustration vanishes and
the liquid freezes in a fully icosahedral structure via a sharp `transition'.
Frustration increases upon reducing the curvature, and the transition to the
icosahedral state smoothens while glassy dynamics emerges. Decreasing the
curvature leads to decoupling between dynamical and structural length scales
and the decrease of kinetic fragility. This sheds light on the observed
glass-forming behavior in the Euclidean space.Comment: 5 pages + supplementary materia
Devitrification of the Kob-Andersen glass former: Competition with the locally favored structure
Supercooled liquids are kinetically trapped materials in which the transition
to a thermodynamically more stable state with long-range order is strongly
suppressed. To assess the glass-forming abilities of a liquid empirical rules
exist, but a comprehensive microscopic picture of devitrification is still
missing. Here we study the crystallization of a popular model glass former, the
binary Kob-Andersen mixture, in small systems. We perform trajectory sampling
employing the population of the locally favored structure as order parameter.
While for large population a dynamical phase transition has been reported, here
we show that biasing towards a small population of locally favored structures
induces crystallization, and we estimate the free energy difference. This
result sheds new light on the competition between local and global structure in
glass-forming liquids and its implications for crystallization
Crystal growth from a supersaturated melt: relaxation of the solid-liquid dynamic stiffness
We discuss the growth process of a crystalline phase out of a metastable
over-compressed liquid that is brought into contact with a crystalline
substrate. The process is modeled by means of molecular dynamics. The particles
interact via the Lennard-Jones potential and their motion is locally
thermalized by Langevin dynamics. We characterize the relaxation process of the
solid-liquid interface, showing that the growth speed is maximal for liquid
densities above the solid coexistence density, and that the structural
properties of the interface rapidly converge to equilibrium-like properties. In
particular, we show that the off-equilibrium dynamic stiffness can be extracted
using capillary wave theory arguments, even if the growth front moves fast
compared to the typical diffusion time of the compressed liquid, and that the
dynamic stiffness converges to the equilibrium stiffness in times much shorter
than the diffusion time
Solid phase properties and crystallization in simple model systems
We review theoretical and simulational approaches to the description of
equilibrium bulk crystal and interface properties as well as to the
nonequilibrium processes of homogeneous and heterogeneous crystal nucleation
for the simple model systems of hard spheres and Lennard-Jones particles. For
the equilibrium properties of bulk and interfaces, density functional theories
employing fundamental measure functionals prove to be a precise and versatile
tool, as exemplified with a closer analysis of the hard sphere crystalliquid
interface. A detailed understanding of the dynamic process of nucleation in
these model systems nevertheless still relies on simulational approaches. We
review bulk nucleation and nucleation at structured walls and examine in closer
detail the influence of walls with variable strength on nucleation in the
Lennard-Jones fluid. We find that a planar crystalline substrate induces the
growth of a crystalline film for a large range of lattice spacings and
interaction potentials. Only a strongly incommensurate substrate and a very
weakly attractive substrate potential lead to crystal growth with a non-zero
contact angle
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