73 research outputs found
Driven flux-line lattices in the presence of weak random columnar disorder: Finite-temperature behavior and dynamical melting of moving Bose glass
We use 3D numerical simulations to explore the phase diagram of driven flux
line lattices in presence of weak random columnar disorder at finite
temperature and high driving force. We show that the moving Bose glass phase
exists in a large range of temperature, up to its melting into a moving vortex
liquid. It is also remarkably stable upon increasing velocity : the dynamical
transition to the correlated moving glass expected at a critical velocity is
not found at any velocity accessible to our simulations. Furthermore, we show
the existence of an effective static tin roof pinning potential in the
direction transverse to motion, which originates from both the transverse
periodicity of the moving lattice and the localization effect due to correlated
disorder. Using a simple model of a single elastic line in such a periodic
potential, we obtain a good description of the transverse field penetration at
surfaces as a function of thickness in the moving Bose glass phase.Comment: 5 pages, 4 figures, New title and minor changes in text and figures.
Accepted for publication in Physical Review
The elastic depinning transition of vortex lattices in two dimensions
Large scale numerical simulations are used to study the elastic dynamics of
two-dimensional vortex lattices driven on a disordered medium in the case of
weak disorder. We investigate the so-called elastic depinning transition by
decreasing the driving force from the elastic dynamical regime to the state
pinned by the quenched disorder. Similarly to the plastic depinning transition,
we find results compatible with a second order phase transition, although both
depinning transitions are very different from many viewpoints. We evaluate
three critical exponents of the elastic depinning transition. is found for the velocity exponent at zero temperature, and from the
velocity-temperature curves we extract the critical exponent . Furthermore, in contrast with charge-density waves, a
finite-size scaling analysis suggests the existence of a unique diverging
length at the depinning threshold with an exponent , which
controls the critical force distribution, the finite-size crossover force
distribution and the intrinsic correlation length. Finally, a scaling relation
is found between velocity and temperature with the and
critical exponents both independent with regard to pinning strength and
disorder realizations.Comment: 17 pages, 10 figure
Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations
Using molecular dynamics simulations, we report a study of the dynamics of
two-dimensional vortex lattices driven over a disordered medium. In strong
disorder, when topological order is lost, we show that the depinning transition
is analogous to a second order critical transition: the velocity-force response
at the onset of motion is continuous and characterized by critical exponents.
Combining studies at zero and nonzero temperature and using a scaling analysis,
two critical expo- nents are evaluated. We find v\sim (F-F_c)^\beta with
\beta=1.3\pm0.1 at T=0 and F>F_c, and v\sim T^{1/\delta} with
\delta^{-1}=0.75\pm0.1 at F=F_c, where F_c is the critical driving force at
which the lattice goes from a pinned state to a sliding one. Both critical
exponents and the scaling function are found to exhibit universality with
regard to the pinning strength and different disorder realizations.
Furthermore, the dynamics is shown to be chaotic in the whole critical region.Comment: 8 pages, 6 figure
Remote Sensing of Snow in the Solar Spectrum: Experiments in the French Alps.
Two experiments were perfonned irliApril and December 1992 in the French Alps using simultaneous relnote sensing and ground truth data. Snow grain site and soot content of samples collected in thefield were measured. The Landsat thematic mapper (TM) sensor was used because it has a good spatial resolution, a middle infrared channel which is sensitive to grain size and a thermal infraredchannel. Firstj the reflectance data were compared with the theoretical results obtained from a bidirectional reflectance model. Then, some remote sehstng-derived snow parameters wbre comparediWith the outpllt ofa snow metamorphism model (CROCUS),viz., lower elevation of the snowcover, lhe surface grl1in size and the surface temperature. A digital elevation model was used to obtain thelocal incidenc:f angles and the elevation of each snow pixel. The pixels were then grouped according to CROCUS classification (range, elevation, slope, and orientation) and the mean snow chart;cheracteristicsfor each class were .compared with the tROCUS results. The lower limit of snow and the surface grain size derived from TM data were compared favourably with the model results. Larger differences werefound for the temperature, because it varies rapidly and is very sensitive to shadowing by the snrrounding mountains and also because its remote measurement is dependent on atmospheric conditions
Pressure is not a state function for generic active fluids
Pressure is the mechanical force per unit area that a confined system exerts
on its container. In thermal equilibrium, it depends only on bulk properties
(density, temperature, etc.) through an equation of state. Here we show that in
a wide class of active systems the pressure depends on the precise interactions
between the active particles and the confining walls. In general, therefore,
active fluids have no equation of state, their mechanical pressures exhibit
anomalous properties that defy the familiar thermodynamic reasoning that holds
in equilibrium. The pressure remains a function of state, however, in some
specific and well-studied active models that tacitly restrict the character of
the particle-wall and/or particle-particle interactions.Comment: 8 pages + 9 SI pages, Nature Physics (2015
A circle swimmer at low Reynolds number
Swimming in circles occurs in a variety of situations at low Reynolds number.
Here we propose a simple model for a swimmer that undergoes circular motion,
generalising the model of a linear swimmer proposed by Najafi and Golestanian
(Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres
arranged in a triangular configuration, joined by two links of time-dependent
length. For small strokes, we discuss the motion of the swimmer as a function
of the separation angle between its links. We find that swimmers describe
either clockwise or anticlockwise circular motion depending on the tilting
angle in a non-trivial manner. The symmetry of the swimmer leads to a
quadrupolar decay of the far flow field. We discuss the potential extensions
and experimental realisation of our model.Comment: 9 pages, 9 Figure
Scalar <i>φ</i><sup>4</sup> field theory for active-particle phase separation
Recent theories predict phase separation among orientationally disordered
active particles whose propulsion speed decreases rapidly enough with density.
Coarse-grained models of this process show time-reversal symmetry (detailed
balance) to be restored for uniform states, but broken by gradient terms; hence
detailed-balance violation is strongly coupled to interfacial phenomena. To
explore the subtle generic physics resulting from such coupling we here
introduce `Active Model B'. This is a scalar field theory (or
phase-field model) that minimally violates detailed balance via a leading-order
square-gradient term. We find that this additional term has modest effects on
coarsening dynamics, but alters the static phase diagram by creating a jump in
(thermodynamic) pressure across flat interfaces. Both results are surprising,
since interfacial phenomena are always strongly implicated in coarsening
dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.Comment: 15 pages, 7 figure
Killing by type VI secretion drives genetic phase separation and correlates with increased cooperation
By nature of their small size, dense growth and frequent need for extracellular metabolism, microbes face persistent public goods dilemmas. Genetic assortment is the only general solution stabilizing cooperation, but all known mechanisms structuring microbial populations depend on the availability of free space, an often unrealistic constraint. Here we describe a class of self-organization that operates within densely packed bacterial populations. Through mathematical modelling and experiments with Vibrio cholerae, we show how killing adjacent competitors via the Type VI secretion system (T6SS) precipitates phase separation via the ‘Model A' universality class of order-disorder transition mediated by killing. We mathematically demonstrate that T6SS-mediated killing should favour the evolution of public goods cooperation, and empirically support this prediction using a phylogenetic comparative analysis. This work illustrates the twin role played by the T6SS, dealing death to local competitors while simultaneously creating conditions potentially favouring the evolution of cooperation with kin
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