2,220 research outputs found
Equilibrium ultrastable glasses produced by random pinning
Ultrastable glasses have risen to prominence due to their potentially useful
material properties and the tantalizing possibility of a general method of
preparation via vapor deposition. Despite the importance of this novel class of
amorphous materials, numerical studies have been scarce because achieving
ultrastability in atomistic simulations is an enormous challenge. Here we
bypass this difficulty and establish that randomly pinning the position of a
small fraction of particles inside an equilibrated supercooled liquid generates
ultrastable configurations at essentially no numerical cost, while avoiding
undesired structural changes due to the preparation protocol. Building on the
analogy with vapor-deposited ultrastable glasses, we study the melting kinetics
of these configurations following a sudden temperature jump into the liquid
phase. In homogeneous geometries, we find that enhanced kinetic stability is
accompanied by large scale dynamic heterogeneity, while a competition between
homogeneous and heterogeneous melting is observed when a liquid boundary
invades the glass at constant velocity. Our work demonstrates the feasibility
of large-scale, atomistically resolved, and experimentally relevant simulations
of the kinetics of ultrastable glasses.Comment: 9 pages, 5 figure
Can the jamming transition be described using equilibrium statistical mechanics?
When materials such as foams or emulsions are compressed, they display solid
behaviour above the so-called `jamming' transition. Because compression is done
out-of-equilibrium in the absence of thermal fluctuations, jamming appears as a
new kind of a nonequilibrium phase transition. In this proceeding paper, we
suggest that tools from equilibrium statistical mechanics can in fact be used
to describe many specific features of the jamming transition. Our strategy is
to introduce thermal fluctuations and use statistical mechanics to describe the
complex phase behaviour of systems of soft repulsive particles, before sending
temperature to zero at the end of the calculation. We show that currently
available implementations of standard tools such as integral equations,
mode-coupling theory, or replica calculations all break down at low temperature
and large density, but we suggest that new analytical schemes can be developed
to provide a fully microscopic, quantitative description of the jamming
transition.Comment: 8 pages, 6 figs. Talk presented at Statphys24 (July 2010, Cairns,
Australia
Jamming transitions in amorphous packings of frictionless spheres occur over a continuous range of volume fractions
We numerically produce fully amorphous assemblies of frictionless spheres in
three dimensions and study the jamming transition these packings undergo at
large volume fractions. We specify four protocols yielding a critical value for
the jamming volume fraction which is sharply defined in the limit of large
system size, but is different for each protocol. Thus, we directly establish
the existence of a continuous range of volume fraction where nonequilibrium
jamming transitions occur. However, these jamming transitions share the same
critical behaviour. Our results suggest that, even in the absence of partial
crystalline ordering, a unique location of a random close packing does not
exist, and that volume fraction alone is not sufficient to describe the
properties of jammed states.Comment: 5 pages, 3 fig
A Molecular Hydrodynamic Theory of Supercooled Liquids and Colloidal Suspensions under Shear
We extend the conventional mode-coupling theory of supercooled liquids to
systems under stationary shear flow. Starting from generalized fluctuating
hydrodynamics, a nonlinear equation for the intermediate scattering function is
constructed. We evaluate the solution numerically for a model of a two
dimensional colloidal suspension and find that the structural relaxation time
decreases as with an exponent , where
is the shear rate. The results are in qualitative agreement with
recent molecular dynamics simulations. We discuss the physical implications of
the results.Comment: 5 pages, 1 figur
Crossovers in the dynamics of supercooled liquids probed by an amorphous wall
We study the relaxation dynamics of a binary Lennard-Jones liquid in the
presence of an amorphous wall generated from equilibrium particle
configurations. In qualitative agreement with the results presented in Nature
Phys. {\bf 8}, 164 (2012) for a liquid of harmonic spheres, we find that our
binary mixture shows a saturation of the dynamical length scale close to the
mode-coupling temperature . Furthermore we show that, due to the broken
symmetry imposed by the wall, signatures of an additional change in dynamics
become apparent at a temperature well above . We provide evidence that
this modification in the relaxation dynamics occurs at a recently proposed
dynamical crossover temperature , which is related to the breakdown
of the Stokes-Einstein relation. We find that this dynamical crossover at
is also observed for a system of harmonic spheres as well as a WCA liquid,
showing that it may be a general feature of glass-forming systems.Comment: 10 pages, 8 figure
NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSiO
We present a Cu and Si NMR study of the quasi-2D coupled
spin 1/2 dimer compound BaCuSiO in the magnetic field range 13-26 T and
at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below
90 K different intra-dimer exchange couplings and different gaps
( = 1.16) exist in every second plane along
the c-axis, in addition to a planar incommensurate (IC) modulation. Si
spectra in the field induced magnetic ordered phase reveal that close to the
quantum critical point at = 23.35 T the average boson density
of the Bose-Einstein condensate is strongly modulated along the
c-axis with a density ratio for every second plane
. An IC modulation of the local
density is also present in each plane. This adds new constraints for the
understanding of the 2D value = 1 of the critical exponent describing
the phase boundary
Unified study of glass and jamming rheology in soft particle systems
We explore numerically the shear rheology of soft repulsive particles at
large volume fraction. The interplay between viscous dissipation and thermal
motion results in multiple rheological regimes encompassing Newtonian,
shear-thinning and yield stress regimes near the `colloidal' glass transition
when thermal fluctuations are important, crossing over to qualitatively similar
regimes near the `jamming' transition when dissipation dominates. In the
crossover regime, glass and jamming sectors coexist and give complex flow
curves. Although glass and jamming limits are characterized by similar
macroscopic flow curves, we show that they occur over distinct time and stress
scales and correspond to distinct microscopic dynamics. We propose a simple
rheological model describing the glass to jamming crossover in the flow curves,
and discuss the experimental implications of our results.Comment: 5 pages, 3 figs; v2 accepted to publication to Phys. Rev. Let
Highly nonlinear dynamics in a slowly sedimenting colloidal gel
We use a combination of original light scattering techniques and particles
with unique optical properties to investigate the behavior of suspensions of
attractive colloids under gravitational stress, following over time the
concentration profile, the velocity profile, and the microscopic dynamics.
During the compression regime, the sedimentation velocity grows nearly linearly
with height, implying that the gel settling may be fully described by a
(time-dependent) strain rate. We find that the microscopic dynamics exhibit
remarkable scaling properties when time is normalized by strain rate, showing
that the gel microscopic restructuring is dominated by its macroscopic
deformation.Comment: Physical Review Letters (2011) xxx
Slow flows of yield stress fluids: complex spatio-temporal behaviour within a simple elasto-plastic model
A minimal athermal model for the flow of dense disordered materials is
proposed, based on two generic ingredients: local plastic events occuring above
a microscopic yield stress, and the non-local elastic release of the stress
these events induce in the material. A complex spatio-temporal rheological
behaviour results, with features in line with recent experimental observations.
At low shear rates, macroscopic flow actually originates from collective
correlated bursts of plastic events, taking place in dynamically generated
fragile zones. The related correlation length diverges algebraically at small
shear rates. In confined geometries bursts occur preferentially close to the
walls yielding an intermittent form of flow localization.Comment: 4 pages, 4 figure
Modern computational studies of the glass transition
The physics of the glass transition and amorphous materials continues to
attract the attention of a wide research community after decades of effort.
Supercooled liquids and glasses have been studied numerically since the advent
of molecular dynamics and Monte Carlo simulations in the last century. Computer
studies have greatly enhanced both experimental discoveries and theoretical
developments and constitute an active and continually expanding research field.
Our goal in this review is to provide a modern perspective on this area. We
describe the need to go beyond canonical methods to attack a problem that is
notoriously difficult in terms of time scales, length scales, and physical
observables. We first summarise recent algorithmic developments to achieve
enhanced sampling and faster equilibration using replica exchange methods,
cluster and swap Monte Carlo algorithms, and other techniques. We then review
some major recent advances afforded by these novel tools regarding the
statistical mechanical description of the liquid-to-glass transition as well as
the mechanical, vibrational and thermal properties of the glassy solid. We
finally describe some important challenges for future research
- …