11,877 research outputs found
Persistence in the two dimensional ferromagnetic Ising model
We present very accurate numerical estimates of the time and size dependence
of the zero-temperature local persistence in the ferromagnetic Ising
model. We show that the effective exponent decays algebraically to an
asymptotic value that depends upon the initial condition. More
precisely, we find that takes one universal value for
initial conditions with short-range spatial correlations as in a paramagnetic
state, and the value for initial conditions with the long-range
spatial correlations of the critical Ising state. We checked universality by
working with a square and a triangular lattice, and by imposing free and
periodic boundary conditions. We found that the effective exponent suffers from
stronger finite size effects in the former case.Comment: v2: minor corrections and typos correcte
A morphological study of cluster dynamics between critical points
We study the geometric properties of a system initially in equilibrium at a
critical point that is suddenly quenched to another critical point and
subsequently evolves towards the new equilibrium state. We focus on the
bidimensional Ising model and we use numerical methods to characterize the
morphological and statistical properties of spin and Fortuin-Kasteleyn clusters
during the critical evolution. The analysis of the dynamics of an out of
equilibrium interface is also performed. We show that the small scale
properties, smaller than the target critical growing length with the dynamic exponent, are characterized by equilibrium at the
working critical point, while the large scale properties, larger than the
critical growing length, are those of the initial critical point. These
features are similar to what was found for sub-critical quenches. We argue that
quenches between critical points could be amenable to a more detailed
analytical description.Comment: 26 pages, 13 figure
Single-cycle THz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3
Using the tilted-pulse-intensity-front scheme, we generate single-cycle
terahertz (THz) pulses by optical rectification of femtosecond laser pulses in
LiNbO3. In the THz generation setup, the condition that the image of the
grating coincides with the tilted-optical-pulse front is fulfilled to obtain
optimal THz beam characteristics and pump-to-THz conversion efficiency. The
designed focusing geometry enables tight focus of the collimated THz beam with
a spot size close to the diffraction limit, and the maximum THz electric field
of 1.2 MV/cm is obtained
Are the stratospheric dust particles meteor ablation debris or interplanetary dust?
Natural and laboratory created fusion crusts and debris from artificial meteor samples were used to develop criteria for recognizing meteor ablation debris in a collection of 5 to 50 micron particles from the stratosphere. These laboratory studies indicate that meteor ablation debris from nickel-iron meteoroids produce spherules containing taenite, wuestite, magnetite, and hematite. These same studies also indicate that ablation debris from chondritic meteoroids produce spheres and fragmentary debris. The spheres may be either silicate rich, containing zoned olivine, magnetite, and glass, or sulfide rich, containing iron oxides (e.g., magnetite, wuestite) and iron sulfides (e.g., pyrrhotite, pentlandite). The fragmentary debris may be either fine-grained aggregates of olivine, magnetite, pyroxene, and occasionally pyrrhotite (derived from the meteorite matrix) or individual olivine and pyroxene grains (derived from meteorite inclusions)
Human activity modeling and Barabasi's queueing systems
It has been shown by A.-L. Barabasi that the priority based scheduling rules
in single stage queuing systems (QS) generates fat tail behavior for the tasks
waiting time distributions (WTD). Such fat tails are due to the waiting times
of very low priority tasks which stay unserved almost forever as the task
priority indices (PI) are "frozen in time" (i.e. a task priority is assigned
once for all to each incoming task). Relaxing the "frozen in time" assumption,
this paper studies the new dynamic behavior expected when the priority of each
incoming tasks is time-dependent (i.e. "aging mechanisms" are allowed). For two
class of models, namely 1) a population type model with an age structure and 2)
a QS with deadlines assigned to the incoming tasks which is operated under the
"earliest-deadline-first" policy, we are able to analytically extract some
relevant characteristics of the the tasks waiting time distribution. As the
aging mechanism ultimately assign high priority to any long waiting tasks, fat
tails in the WTD cannot find their origin in the scheduling rule alone thus
showing a fundamental difference between the present and the A.-L. Barabasi's
class of models.Comment: 16 pages, 2 figure
How soon after a zero-temperature quench is the fate of the Ising model sealed?
We study the transient between a fully disordered initial condition and a
percolating structure in the low-temperature non-conserved order parameter
dynamics of the bi-dimensional Ising model. We show that a stable structure of
spanning clusters establishes at a time . Our
numerical results yield for the square and kagome,
for the triangular and for the bowtie-a
lattices.We generalise the dynamic scaling hypothesis to take into account this
new time-scale. We discuss the implications of these results for other
non-equilibrium processes.Comment: 5 pages, 6 figures + supplemental material (2 pages, 1 figure),
version 2: new co-author, extended manuscrip
Critical percolation in the dynamics of the 2d ferromagnetic Ising model
We study the early time dynamics of the 2d ferromagnetic Ising model
instantaneously quenched from the disordered to the ordered, low temperature,
phase. We evolve the system with kinetic Monte Carlo rules that do not conserve
the order parameter. We confirm the rapid approach to random critical
percolation in a time-scale that diverges with the system size but is much
shorter than the equilibration time. We study the scaling properties of the
evolution towards critical percolation and we identify an associated growing
length, different from the curvature driven one. By working with the model
defined on square, triangular and honeycomb microscopic geometries we establish
the dependence of this growing length on the lattice coordination. We discuss
the interplay with the usual coarsening mechanism and the eventual fall into
and escape from metastability.Comment: 67 pages, 33 figure
Cytoskeletal turnover and Myosin contractility drive cell autonomous oscillations in a model of Drosophila Dorsal Closure
Oscillatory behaviour in force-generating systems is a pervasive phenomenon
in cell biology. In this work, we investigate how oscillations in the
actomyosin cytoskeleton drive cell shape changes during the process of Dorsal
Closure, a morphogenetic event in Drosophila embryo development whereby
epidermal continuity is generated through the pulsatile apical area reduction
of cells constituting the amnioserosa (AS) tissue. We present a theoretical
model of AS cell dynamics by which the oscillatory behaviour arises due to a
coupling between active Myosin-driven forces, actin turnover and cell
deformation. Oscillations in our model are cell-autonomous and are modulated by
neighbour coupling, and our model accurately reproduces the oscillatory
dynamics of AS cells and their amplitude and frequency evolution. A key
prediction arising from our model is that the rate of actin turnover and Myosin
contractile force must increase during DC in order to reproduce the decrease in
amplitude and period of cell area oscillations observed in vivo. This
prediction opens up new ways to think about the molecular underpinnings of AS
cell oscillations and their link to net tissue contraction and suggests the
form of future experimental measurements.Comment: 17 pages, 6 figures; added references, modified and corrected Figs. 1
and 3, corrected typos, expanded discussio
Tests with three-dimensional adjustments in the rectangular working section of the French T2 wind tunnel with an AS 07-type swept-back wing model
The results obtained on the AS 07 wing and the working section walls for three types of configurations are reported. The first, called non-adapted, corresponds to the divergent upper and lower rectilinear walls which compensate for limit layer thickening. It can serve as a basis for complete flow calculations. The second configuration corresponds to wall shapes determined from calculations which tend to minimize interference at the level of the fuselage. Finally, the third configuration, called two-dimensional adaptation, uses the standard method for T2 profile tests. This case was tested to determine the influence of wall shape and error magnitude. These results are not sufficient to validate the three-dimensional adaptation; they must be coordinated with calculations or with unlimited atmosphere tests
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