1,024 research outputs found
Static correlations functions and domain walls in glass-forming liquids: the case of a sandwich geometry
The problem of measuring nontrivial static correlations in deeply supercooled
liquids made recently some progress thanks to the introduction of amorphous
boundary conditions, in which a set of free particles is subject to the effect
of a different set of particles frozen into their (low temperature) equilibrium
positions. In this way, one can study the crossover from nonergodic to ergodic
phase, as the size of the free region grows and the effect of the confinement
fades. Such crossover defines the so-called point-to-set correlation length,
which has been measured in a spherical geometry, or cavity. Here, we make
further progress in the study ofcorrelations under amorphous boundary
conditions by analyzing the equilibrium properties of a glass-forming liquid,
confined in a planar ("sandwich") geometry. The mobile particles are subject to
amorphous boundary conditions with the particles in the surrounding walls
frozen into their low temperature equilibrium configurations. Compared to the
cavity, the sandwich geometry has three main advantages: i) the width of the
sandwich is decoupled from its longitudinal size, making the thermodynamic
limit possible; ii) for very large width, the behaviour off a single wall can
be studied; iii) we can use "anti-parallel" boundary conditions to force a
domain wall and measure its excess energy. Our results confirm that amorphous
boundary conditions are indeed a very useful new tool inthe study of static
properties of glass-forming liquids, but also raise some warning about the fact
that not all correlation functions that can be calculated in this framework
give the same qualitative results.Comment: Submited to JCP special issue on the glass transisio
Mosaic multi-state scenario vs. one-state description of supercooled liquids
According to the mosaic scenario, relaxation in supercooled liquids is ruled
by two competing mechanisms: surface tension, opposing the creation of local
excitations, and entropy, providing the drive to the configurational
rearrangement of a given region. We test this scenario through numerical
simulations well below the Mode Coupling temperature. For an equilibrated
configuration, we freeze all the particles outside a sphere and study the
thermodynamics of this sphere. The frozen environment acts as a pinning field.
Measuring the overlap between the unpinned and pinned equilibrium
configurations of the sphere, we can see whether it has switched to a different
state. We do not find any clear evidence of the mosaic scenario. Rather, our
results seem compatible with the existence of a single (liquid) state. However,
we find evidence of a growing static correlation length, apparently unrelated
to the mosaic one.Comment: 4 pages, 3 figures, final version accepted in PR
Numerical simulations of liquids with amorphous boundary conditions
It has recently become clear that simulations under amorphpous boundary
conditions (ABCs) can provide valuable information on the dynamics and
thermodynamics of disordered systems with no obvious ordered parameter. In
particular, they allow to detect a correlation length that is not measurable
with standard correlation functions. Here we explain what exactly is meant by
ABCs, discuss their relation with point-to-set correlations and briefly
describe some recent results obtained with this technique.Comment: Presented at STATPHYS 2
Geometric approach to the dynamic glass transition
We numerically study the potential energy landscape of a fragile glassy
system and find that the dynamic crossover corresponding to the glass
transition is actually the effect of an underlying geometric transition caused
by a qualitative change in the topological properties of the landscape.
Furthermore, we show that the potential energy barriers connecting local glassy
minima increase with decreasing energy of the minima, and we relate this
behaviour to the fragility of the system. Finally, we analyze the real space
structure of activated processes by studying the distribution of particle
displacements for local minima connected by simple saddles
Coexistence of supersymmetric and supersymmetry-breaking states in spherical spin-glasses
The structure of states of the perturbed p-spin spherical spin-glass is
analyzed. At low enough free energy metastable states have a supersymmetric
structure, while at higher free energies the supersymmetry is broken. The
transition between the supersymmetric and the supersymmetry-breaking phase is
triggered by a change in the stability of states
Numerical study of metastable states in Ising spin glasses
We study numerically the structure of metastable states in the
Sherrington-Kirkpatrick spin glass. We find that all non-paramagnetic
stationary points of the free energy are organized into pairs, consisting in a
minimum and a saddle of order one, which coalesce in the thermodynamic limit.
Within the annealed approximation, the entropic contribution of these states,
that is the complexity, is compatible with the supersymmetry-breaking
calculation performed in [A.J. Bray and M.A. Moore, J. Phys. C, 13 L469
(1980)]. This result indicates that the supersymmetry is spontaneously broken
in the Sherrington-Kirkpatrick model
Topological Signature of First Order Phase Transitions
We show that the presence and the location of first order phase transitions
in a thermodynamic system can be deduced by the study of the topology of the
potential energy function, V(q), without introducing any thermodynamic measure.
In particular, we present the thermodynamics of an analytically solvable
mean-field model with a k-body interaction which -depending on the value of k-
displays no transition (k=1), second order (k=2) or first order (k>2) phase
transition. This rich behavior is quantitatively retrieved by the investigation
of a topological invariant, the Euler characteristic, of some submanifolds of
the configuration space. Finally, we conjecture a direct link between the Euler
characteristic and the thermodynamic entropy.Comment: 6 pages, 2 figure
The bounce of the body in hopping, running and trotting: different machines with the same motor
The bouncing mechanism of human running is characterized by a shorter duration of the brake after âlandingâ compared with a longer duration of the push before âtakeoffâ. This landingâtakeoff asymmetry has been thought to be a consequence of the forceâvelocity relation of the muscle, resulting in a greater force exerted during stretching after landing and a lower force developed during shortening before takeoff. However, the asymmetric lever system of the human foot during stance may also be the cause. Here, we measure the landingâtakeoff asymmetry in bouncing steps of running, hopping and trotting animals using diverse lever systems. We find that the duration of the push exceeds that of the brake in all the animals, indicating that the different lever systems comply with the basic property of muscle to resist stretching with a force greater than that developed during shortening. In addition, results show both the landingâtakeoff asymmetry and the mass-specific vertical stiffness to be greater in small animals than in large animals. We suggest that the landingâtakeoff asymmetry is an index of a lack of elasticity, which increases with increasing the role of muscle relative to that of tendon within muscleâtendon units
A New Stochastic Strategy for the Minority Game
We present a variant of the Minority Game in which players who where
successful in the previous timestep stay with their decision, while the losers
change their decision with a probability . Analytical results for different
regimes of and the number of players are given and connections to
existing models are discussed. It is shown that for the average
loss is of the order of 1 and does not increase with as for
other known strategies.Comment: 4 pages, 3 figure
Symmetry and Asymmetry in Bouncing Gaits
In running, hopping and trotting gaits, the center of mass of the body oscillates each step below and above an equilibrium position where the vertical force on the ground equals body weight. In trotting and low speed human running, the average vertical acceleration of the center of mass during the lower part of the oscillation equals that of the upper part, the duration of the lower part equals that of the upper part and the step frequency equals the resonant frequency of the bouncing system: we define this as on-offground symmetric rebound. In hopping and high speed human running, the average vertical acceleration of the center of mass during the lower part of the oscillation exceeds that of the upper part, the duration of the upper part exceeds that of the lower part and the step frequency is lower than the resonant frequency of the bouncing system: we define this as on-off-ground asymmetric rebound. Here we examine the physical and physiological constraints resulting in this on-off-ground symmetry and asymmetry of the rebound. Furthermore, the average force exerted during the brake when the body decelerates downwards and forwards is greater than that exerted during the push when the body is reaccelerated upwards and forwards. This landing-takeoff asymmetry, which would be nil in the elastic rebound of the symmetric spring-mass model for running and hopping, suggests a less efficient elastic energy storage and recovery during the bouncing step. During hopping, running and trotting the landing-takeoff asymmetry and the mass-specific vertical stiffness are smaller in larger animals than in the smaller animals suggesting a more efficient rebound in larger animals
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