1,759 research outputs found
Glassy dynamics of partially pinned fluids: an alternative mode-coupling approach
We use a simple mode-coupling approach to investigate glassy dynamics of
partially pinned fluid systems. Our approach is different from the
mode-coupling theory developed by Krakoviack [Phys. Rev. Lett. 94, 065703
(2005), Phys. Rev. E 84, 050501(R) (2011)]. In contrast to Krakoviack's theory,
our approach predicts a random pinning glass transition scenario that is
qualitatively the same as the scenario obtained using a mean-field analysis of
the spherical p-spin model and a mean-field version of the random first-order
transition theory. We use our approach to calculate quantities which are often
considered to be indicators of growing dynamic correlations and static
point-to-set correlations. We find that the so-called static overlap is
dominated by the simple, low pinning fraction contribution. Thus, at least for
randomly pinned fluid systems, only a careful quantitative analysis of
simulation results can reveal genuine, many-body point-to-set correlations
Shearing a Glassy Material: Numerical Tests of Nonequilibrium Mode-Coupling Approaches and Experimental Proposals
The predictions of a nonequilibrium schematic mode-coupling theory developed
to describe the nonlinear rheology of soft glassy materials have been
numerically challenged in a sheared binary Lennard-Jones mixture. The theory
gives an excellent description of the stress/temperature `jamming phase
diagram' of the system. In the present paper, we focus on the issue of an
effective temperature Teff for the slow modes of the fluid, as defined from a
generalized fluctuation-dissipation theorem. As predicted theoretically, many
different observables are found to lead to the same value of Teff, suggesting
several experimental procedures to measure Teff. New, simple experimental
protocols to access Teff from a generalized equipartition theorem are also
proposed, and one such experiment is numerically performed. These results give
strong support to the thermodynamic interpretation of Teff and make it
experimentally accessible in a very direct way.Comment: Version accepted for publication - Physical Review Letter
Non-monotonic temperature evolution of dynamic correlations in glass-forming liquids
The viscosity of glass-forming liquids increases by many orders of magnitude
if their temperature is lowered by a mere factor of 2-3 [1,2]. Recent studies
suggest that this widespread phenomenon is accompanied by spatially
heterogeneous dynamics [3,4], and a growing dynamic correlation length
quantifying the extent of correlated particle motion [5-7]. Here we use a novel
numerical method to detect and quantify spatial correlations which reveal a
surprising non-monotonic temperature evolution of spatial dynamical
correlations, accompanied by a second length scale that grows monotonically and
has a very different nature. Our results directly unveil a dramatic qualitative
change in atomic motions near the mode-coupling crossover temperature [8] which
involves no fitting or indirect theoretical interpretation. Our results impose
severe new constraints on the theoretical description of the glass transition,
and open several research perspectives, in particular for experiments, to
confirm and quantify our observations in real materials.Comment: 7 page
Spontaneous and induced dynamic correlations in glass-formers II: Model calculations and comparison to numerical simulations
We study in detail the predictions of various theoretical approaches, in
particular mode-coupling theory (MCT) and kinetically constrained models
(KCMs), concerning the time, temperature, and wavevector dependence of
multi-point correlation functions that quantify the strength of both induced
and spontaneous dynamical fluctuations. We also discuss the precise predictions
of MCT concerning the statistical ensemble and microscopic dynamics dependence
of these multi-point correlation functions. These predictions are compared to
simulations of model fragile and strong glass-forming liquids. Overall, MCT
fares quite well in the fragile case, in particular explaining the observed
crucial role of the statistical ensemble and microscopic dynamics, while MCT
predictions do not seem to hold in the strong case. KCMs provide a simplified
framework for understanding how these multi-point correlation functions may
encode dynamic correlations in glassy materials. However, our analysis
highlights important unresolved questions concerning the application of KCMs to
supercooled liquids.Comment: 23 pages, 12 fig
Magnetic Superstructure in the Two-Dimensional Quantum Antiferromagnet SrCu2(BO3)2
We report the observation of magnetic superstructure in a magnetization
plateau state of SrCu2(BO3)2, a frustrated quasi-two-dimensional quantum spin
system. The Cu and B nuclear magnetic resonance (NMR) spectra at 35 mllikelvin
indicate an apparently discontinuous phase transition from uniform
magnetization to a modulated superstructure near 27 tesla, above which a
magnetization plateau at 1/8 of the full saturation has been observed.
Comparison of the Cu NMR spectrum and the theoretical analysis of a Heisenberg
spin model demonstrates the crystallization of itinerant triplets in the
plateau phase within a large rhomboid unit cell (16 spins per layer) showing
oscillations of the spin polarization. Thus we are now in possession of an
interesting model system to study a localization transition of strongly
interacting quantum particles.Comment: PDF file, 16 pages, 5 figure
Shear yielding of amorphous glassy solids: Effect of temperature and strain rate
We study shear yielding and steady state flow of glassy materials with
molecular dynamics simulations of two standard models: amorphous polymers and
bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear
yield stress and the steady state flow stress in simple shear both drop
linearly with increasing temperature. The dependence on strain rate can be
described by a either a logarithm or a power-law added to a constant. In marked
contrast to predictions of traditional thermal activation models, the rate
dependence is nearly independent of temperature. The relation to more recent
models of plastic deformation and glassy rheology is discussed, and the
dynamics of particles and stress in small regions is examined in light of these
findings
Finite size effects in the dynamics of glass-forming liquids
We present a comprehensive theoretical study of finite size effects in the
relaxation dynamics of glass-forming liquids. Our analysis is motivated by
recent theoretical progress regarding the understanding of relevant correlation
length scales in liquids approaching the glass transition. We obtain
predictions both from general theoretical arguments and from a variety of
specific perspectives: mode-coupling theory, kinetically constrained and defect
models, and random first order transition theory. In the latter approach, we
predict in particular a non-monotonic evolution of finite size effects across
the mode-coupling crossover due to the competition between mode-coupling and
activated relaxation. We study the role of competing relaxation mechanisms in
giving rise to non-monotonic finite size effects by devising a kinetically
constrained model where the proximity to the mode-coupling singularity can be
continuously tuned by changing the lattice topology. We use our theoretical
findings to interpret the results of extensive molecular dynamics studies of
four model liquids with distinct structures and kinetic fragilities. While the
less fragile model only displays modest finite size effects, we find a more
significant size dependence evolving with temperature for more fragile models,
such as Lennard-Jones particles and soft spheres. Finally, for a binary mixture
of harmonic spheres we observe the predicted non-monotonic temperature
evolution of finite size effects near the fitted mode-coupling singularity,
suggesting that the crossover from mode-coupling to activated dynamics is more
pronounced for this model. Finally, we discuss the close connection between our
results and the recent report of a non-monotonic temperature evolution of a
dynamic length scale near the mode-coupling crossover in harmonic spheres.Comment: 19 pages, 10 figures. V2: response to referees + refs added (close to
published version
What does the potential energy landscape tell us about the dynamics of supercooled liquids and glasses?
For a model glass-former we demonstrate via computer simulations how
macroscopic dynamic quantities can be inferred from a PEL analysis. The
essential step is to consider whole superstructures of many PEL minima, called
metabasins, rather than single minima. We show that two types of metabasins
exist: some allowing for quasi-free motion on the PEL (liquid-like), the others
acting as traps (solid-like). The activated, multi-step escapes from the latter
metabasins are found to dictate the slowing down of dynamics upon cooling over
a much broader temperature range than is currently assumed
Self-alignment of silicon chips on wafers: a capillary approach
As the limits of Moores law are approached, three-dimensional integration appears as the key to advanced microelectronic systems. Die-to-wafer assembly appears to be an unavoidable step to reach full integration. While robotic methods experience difficulties to accommodate fabrication speed and alignment accuracy, self-assembly methods are promising due to their parallel aspect, which overcomes the main difficulties of current techniques. The aim of this work is the understanding of the mechanisms of self-alignment with an evaporating droplet technique. Stable and unstable modes are examined. Causes for misalignments of chips on wafers and their evolution are investigated with the help of the SURFACE EVOLVER numerical software. Precautions for suitable alignment are proposed
Critical fluctuations and breakdown of Stokes-Einstein relation in the Mode-Coupling Theory of glasses
We argue that the critical dynamical fluctuations predicted by the
mode-coupling theory (MCT) of glasses provide a natural mechanism to explain
the breakdown of the Stokes-Einstein relation. This breakdown, observed
numerically and experimentally in a region where MCT should hold, is one of the
major difficulty of the theory, for which we propose a natural resolution based
on the recent interpretation of the MCT transition as a bona fide critical
point with a diverging length scale. We also show that the upper critical
dimension of MCT is d_c=8.Comment: Proceedings of the workshop on non-equilibrium phenomena in
supercooled fluids, glasses and amorphous materials (17-22 September, 2006,
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