115 research outputs found
Generalized model of blockage in particulate flow limited by channel carrying capacity
We investigate stochastic models of particles entering a channel with a
random time distribution. When the number of particles present in the channel
exceeds a critical value , a blockage occurs and the particle flux is
definitively interrupted. By introducing an integral representation of the
particle survival probabilities, we obtain exact expressions for the survival
probability, the distribution of the number of particles that pass before
failure, the instantaneous flux of exiting particle and their time correlation.
We generalize previous results for to an arbitrary distribution of entry
times and obtain new, exact solutions for for a Poisson distribution and
partial results for .Comment: 13 pages, 9 figure
The viscous slowing down of supercooled liquids as a temperature-controlled superArrhenius activated process: a description in terms of frustration-limited domains
We propose that the salient feature to be explained about the glass
transition of supercooled liquids is the temperature-controlled superArrhenius
activated nature of the viscous slowing down, more strikingly seen in
weakly-bonded, fragile systems. In the light of this observation, the relevance
of simple models of spherically interacting particles and that of models based
on free-volume congested dynamics are questioned. Finally, we discuss how the
main aspects of the phenomenology of supercooled liquids, including the
crossover from Arrhenius to superArrhenius activated behavior and the
heterogeneous character of the relaxation, can be described by an
approach based on frustration-limited domains.Comment: 13 pages, 4 figures, accepted in J. Phys.: Condensed Matter,
proceedings of the Trieste workshop on "Unifying Concepts in Glass Physics
Chirality and vortices in an Heisenberg spin model on the kagom\'e lattice
The phase diagram of the classical \jj model on the \kag lattice is
investigated using extensive \mc simulations. In a realistic range of
parameters, this model has a low-temperature chiral-ordered phase without
long-range spin order. We show that the critical transition marking the
destruction of chiral order is preempted by the first order proliferation of
\Zdeux point defects. The core energy of these vortices appears to vanish when
approaching the T=0 phase boundary, where both \Zdeux defects and gapless
magnons contribute to disordering the system at very low temperature. This
situation might be typical of a large class of frustrated magnets. Possible
relevance for real materials is also discussed.Comment: 4 pages, 4 figure
Lunar laser ranging in infrfared at hte Grasse laser station
For many years, lunar laser ranging (LLR) observations using a green
wavelength have suffered an inhomogeneity problem both temporally and
spatially. This paper reports on the implementation of a new infrared detection
at the Grasse LLR station and describes how infrared telemetry improves this
situation. Our first results show that infrared detection permits us to densify
the observations and allows measurements during the new and the full Moon
periods. The link budget improvement leads to homogeneous telemetric
measurements on each lunar retro-reflector. Finally, a surprising result is
obtained on the Lunokhod 2 array which attains the same efficiency as Lunokhod
1 with an infrared laser link, although those two targets exhibit a
differential efficiency of six with a green laser link
Locally Preferred Structure and Frustration in Glassforming Liquids: A Clue to Polyamorphism?
We propose that the concept of liquids characterized by a given locally
preferred structure (LPS) could help in understanding the observed phenomenon
of polyamorphism. ``True polyamorphism'' would involve the competition between
two (or more) distinct LPS, one favored at low pressure because of its low
energy and one favored at high pressure because of its small specific volume,
as in tetrahedrally coordinated systems. ``Apparent polyamorphism'' could be
associated with the existence of a poorly crystallized defect-ordered phase
with a large unit cell and small crystallites, which may be illustrated by the
metastable glacial phase of the fragile glassformer triphenylphosphite; the
apparent polyamorphism might result from structural frustration, i. e., a
competition between the tendency to extend the LPS and a global constraint that
prevents tiling of the whole space by the LPS.Comment: 11, 6 figures, Proceedings of the Conference "Horizons in Complex
Systems", Messina; in honor of the 60th birthday of H.E. Stanle
Effect of dynamic and static friction on an asymmetric granular piston
We investigate the influence of dry friction on an asymmetric, granular
piston of mass composed of two materials undergoing inelastic collisions
with bath particles of mass . Numerical simulations of the Boltzmann-Lorentz
equation reveal the existence of two scaling regimes depending on the strength
of friction. In the large friction limit, we introduce an exact model giving
the asymptotic behavior of the Boltzmann-Lorentz equation. For small friction
and for large mass ratio , we derive a Fokker-Planck equation for which
the exact solution is also obtained. Static friction attenuates the motor
effect and results in a discontinuous velocity distribution.Comment: 10 pages, 8 Figure
Phase diagram of an Ising model with long-range frustrating interactions: a theoretical analysis
We present a theoretical study of the phase diagram of a frustrated Ising
model with nearest-neighbor ferromagnetic interactions and long-range
(Coulombic) antiferromagnetic interactions. For nonzero frustration, long-range
ferromagnetic order is forbidden, and the ground-state of the system consists
of phases characterized by periodically modulated structures. At finite
temperatures, the phase diagram is calculated within the mean-field
approximation. Below the transition line that separates the disordered and the
ordered phases, the frustration-temperature phase diagram displays an infinite
number of ``flowers'', each flower being made by an infinite number of
modulated phases generated by structure combination branching processes. The
specificities introduced by the long-range nature of the frustrating
interaction and the limitation of the mean-field approach are finally
discussed.Comment: 32 pages, 7 figure
Response properties in a model for granular matter
We investigate the response properties of granular media in the framework of
the so-called {\em Random Tetris Model}. We monitor, for different driving
procedures, several quantities: the evolution of the density and of the density
profiles, the ageing properties through the two-times correlation functions and
the two-times mean-square distance between the potential energies, the response
function defined in terms of the difference in the potential energies of two
replica driven in two slightly different ways. We focus in particular on the
role played by the spatial inhomogeneities (structures) spontaneously emerging
during the compaction process, the history of the sample and the driving
procedure. It turns out that none of these ingredients can be neglected for the
correct interpretation of the experimental or numerical data. We discuss the
problem of the optimization of the compaction process and we comment on the
validity of our results for the description of granular materials in a
thermodynamic framework.Comment: 22 pages, 35 eps files (21 figures
Langevin dynamics of the Coulomb frustrated ferromagnet: a mode-coupling analysis
We study the Langevin dynamics of the soft-spin, continuum version of the
Coulomb frustrated Ising ferromagnet. By using the dynamical mode-coupling
approximation, supplemented by reasonable approximations for describing the
equilibrium static correlation function, and the somewhat improved dynamical
self-consistent screening approximation, we find that the system displays a
transition from an ergodic to a non-ergodic behavior. This transition is
similar to that obtained in the idealized mode-coupling theory of glassforming
liquids and in the mean-field generalized spin glasses with one-step replica
symmetry breaking. The significance of this result and the relation to the
appearance of a complex free-energy landscape are also discussed.Comment: 13 pages, 11 figures, submitted to Phys. Rev.
- …