665 research outputs found
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
Growing Correlation Length on Cooling Below the Onset of Caging in a Simulated Glass-Forming Liquid
We present a calculation of a fourth-order, time-dependent density
correlation function that measures higher-order spatiotemporall correlations of
the density of a liquid. From molecular dynamics simulations of a glass-forming
Lennard-Jones liquid, we find that the characteristic length scale of this
function has a maximum as a function of time which increases steadily beyond
the characteristic length of the static pair correlation function in the
temperature range approaching the mode coupling temperature from above
Damage spreading and dynamic stability of kinetic Ising models
We investigate how the time evolution of different kinetic Ising models
depends on the initial conditions of the dynamics. To this end we consider the
simultaneous evolution of two identical systems subjected to the same thermal
noise. We derive a master equation for the time evolution of a joint
probability distribution of the two systems. This equation is then solved
within an effective-field approach. By analyzing the fixed points of the master
equation and their stability we identify regular and chaotic phases.Comment: 4 pages RevTeX, 2 Postscript figure
Crystalline Assemblies and Densest Packings of a Family of Truncated Tetrahedra and the Role of Directional Entropic Forces
Polyhedra and their arrangements have intrigued humankind since the ancient
Greeks and are today important motifs in condensed matter, with application to
many classes of liquids and solids. Yet, little is known about the
thermodynamically stable phases of polyhedrally-shaped building blocks, such as
faceted nanoparticles and colloids. Although hard particles are known to
organize due to entropy alone, and some unusual phases are reported in the
literature, the role of entropic forces in connection with polyhedral shape is
not well understood. Here, we study thermodynamic self-assembly of a family of
truncated tetrahedra and report several atomic crystal isostructures, including
diamond, {\beta}-tin, and high- pressure lithium, as the polyhedron shape
varies from tetrahedral to octahedral. We compare our findings with the densest
packings of the truncated tetrahedron family obtained by numerical compression
and report a new space filling polyhedron, which has been overlooked in
previous searches. Interestingly, the self-assembled structures differ from the
densest packings. We show that the self-assembled crystal structures can be
understood as a tendency for polyhedra to maximize face-to-face alignment,
which can be generalized as directional entropic forces.Comment: Article + supplementary information. 23 pages, 10 figures, 2 table
Dynamics of particles and cages in an experimental 2D glass former
We investigate the dynamics of a glass forming 2D colloidal mixture and show
the existence of collective motions of the particles. We introduce a mean
square displacement MSD with respect to the nearest neighbors which shows
remarkable deviations from the usual MSD quantifying the individual motion of
our particles. Combined with the analysis of the self part of the Van Hove
function this indicates a coupled motion of particles with their cage as well
as intra cage hopping processes.Comment: Submitted to EP
Scaling in Late Stage Spinodal Decomposition with Quenched Disorder
We study the late stages of spinodal decomposition in a Ginzburg-Landau mean
field model with quenched disorder. Random spatial dependence in the coupling
constants is introduced to model the quenched disorder. The effect of the
disorder on the scaling of the structure factor and on the domain growth is
investigated in both the zero temperature limit and at finite temperature. In
particular, we find that at zero temperature the domain size, , scales
with the amplitude, , of the quenched disorder as with and in two
dimensions. We show that , where is the
Lifshitz-Slyosov exponent. At finite temperature, this simple scaling is not
observed and we suggest that the scaling also depends on temperature and .
We discuss these results in the context of Monte Carlo and cell dynamical
models for phase separation in systems with quenched disorder, and propose that
in a Monte Carlo simulation the concentration of impurities, , is related to
by .Comment: RevTex manuscript 5 pages and 5 figures (obtained upon request via
email [email protected]
Heterogeneous slow dynamics in a two dimensional doped classical antiferromagnet
We introduce a lattice model for a classical doped two dimensional
antiferromagnet which has no quenched disorder, yet displays slow dynamics
similar to those observed in supercooled liquids. We calculate two-time spatial
and spin correlations via Monte Carlo simulations and find that for
sufficiently low temperatures, there is anomalous diffusion and
stretched-exponential relaxation of spin correlations. The relaxation times
associated with spin correlations and diffusion both diverge at low
temperatures in a sub-Arrhenius fashion if the fit is done over a large
temperature-window or an Arrhenius fashion if only low temperatures are
considered. We find evidence of spatially heterogeneous dynamics, in which
vacancies created by changes in occupation facilitate spin flips on
neighbouring sites. We find violations of the Stokes-Einstein relation and
Debye-Stokes-Einstein relation and show that the probability distributions of
local spatial correlations indicate fast and slow populations of sites, and
local spin correlations indicate a wide distribution of relaxation times,
similar to observ ations in other glassy systems with and without quenched
disorder.Comment: 12 pages, 17 figures, corrected erroneous figure, and improved
quality of manuscript, updated reference
Arrested States formed on Quenching Spin Chains with Competing Interactions and Conserved Dynamics
We study the effects of rapidly cooling to T = 0 a spin chain with conserved
dynamics and competing interactions. Depending on the degree of competition,
the system is found to get arrested in different kinds of metastable states.
The most interesting of these has an inhomogeneous mixture of interspersed
active and quiescent regions. In this state, the steady-state autocorrelation
function decays as a stretched exponential , and there is a two-step relaxation to
equilibrium when the temperature is raised slightly.Comment: 4 pages, Latex, 3 postscript figures. Phys. Rev. E to appear (1999
Damage spreading in random field systems
We investigate how a quenched random field influences the damage spreading
transition in kinetic Ising models. To this end we generalize a recent master
equation approach and derive an effective field theory for damage spreading in
random field systems. This theory is applied to the Glauber Ising model with a
bimodal random field distribution. We find that the random field influences the
spreading transition by two different mechanisms with opposite effects. First,
the random field favors the same particular direction of the spin variable at
each site in both systems which reduces the damage. Second, the random field
suppresses the magnetization which, in turn, tends to increase the damage. The
competition between these two effects leads to a rich behavior.Comment: 4 pages RevTeX, 3 eps figure
Heterogeneities in systems with quenched disorder
We study the strong role played by structural (quenched) heterogeneities on
static and dynamic properties of the Frustrated Ising Lattice Gas in two
dimensions, already in the liquid phase. Differently from the dynamical
heterogeneities observed in other glass models in this case they may have
infinite lifetime and be spatially pinned by the quenched disorder. We consider
a measure of local frustration, show how it induces the appearance of spatial
heterogeneities and how this reflects in the observed behavior of equilibrium
density distributions and dynamic correlation functions.Comment: 8 page
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