3,018 research outputs found
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
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
Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions
Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding
Compressing nearly hard sphere fluids increases glass fragility
We use molecular dynamics to investigate the glass transition occurring at
large volume fraction, phi, and low temperature, T, in assemblies of soft
repulsive particles. We find that equilibrium dynamics in the (phi, T) plane
obey a form of dynamic scaling in the proximity of a critical point at T=0 and
phi=phi_0, which should correspond to the ideal glass transition of hard
spheres. This glass point, `point G', is distinct from athermal jamming
thresholds. A remarkable consequence of scaling behaviour is that the dynamics
at fixed phi passes smoothly from that of a strong glass to that of a very
fragile glass as phi increases beyond phi_0. Correlations between fragility and
various physical properties are explored.Comment: 5 pages, 3 figures; Version accepted at Europhys. Let
Novel Crossover in Coupled Spin Ladders
We report a novel crossover behavior in the long-range-ordered phase of a
prototypical spin- Heisenberg antiferromagnetic ladder compound
. The staggered order was previously evidenced
from a continuous and symmetric splitting of N NMR spectral lines on
lowering temperature below mK, with a saturation towards
mK. Unexpectedly, the split lines begin to further separate away
below mK while the line width and shape remain completely
invariable. This crossover behavior is further corroborated by the NMR
relaxation rate measurements. A very strong suppression reflecting
the ordering, , observed above , is replaced by
below . These original NMR features are indicative of
unconventional nature of the crossover, which may arise from a unique
arrangement of the ladders into a spatially anisotropic and frustrated coupling
network.Comment: 5 pages, 3 figure
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 the persistence of spin-superlattice above the 1/8 magnetization plateau in SrCu2(BO3)2
We present 11B NMR studies of the 2D frustrated dimer spin system SrCu2(BO3)2
in the field range 27-31 T covering the upper phase boundary of the 1/8
magnetization plateau, identified at 28.4 T. Our data provide a clear evidence
that above 28.4 T the spin-superlattice of the 1/8 plateau is modified but does
not melt even though the magnetization increases. Although this is precisely
what is expected for a supersolid phase, the microscopic nature of this new
phase is much more complex. We discuss the field-temperature phase diagram on
the basis of our NMR data.Comment: 5 pages, 4 figures, published versio
Brambilla et al. Reply to a Comment by J. Reinhardt et al. on "Probing the equilibrium dynamics of colloidal hard spheres above the mode-coupling glass transition"
G. Brambilla et al. Reply to a Comment by J. Reinhardt et al. questioning the
existence of equilibrium dynamics above the critical volume fraction of
colloidal hard spheres predicted by mode coupling theory.Comment: To appear in Phys. Rev. Lett. Reply to a Comment by J. Reinhardt et
al. (see arXiv:1010.2891), which questions the existence of equilibrium
dynamics above the critical volume fraction of glassy colloidal hard spheres
predicted by mode coupling theor
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