78 research outputs found
Relevance of Metric-Free Interactions in Flocking Phenomena
We show that the collective properties of self-propelled particles aligning
with their "topological" (Voronoi) neighbors are qualitatively different from
those of usual models where metric interaction ranges are used. This relevance
of metric-free interactions, shown in a minimal setting, indicate that
realistic models for the cohesive motion of cells, bird flocks, and fish
schools may have to incorporate them, as suggested by recent observations.Comment: To appear on Physical Review Letter
Competing ferromagnetic and nematic alignment in self-propelled polar particles
We study a Vicsek-style model of self-propelled particles where ferromagnetic
and nematic alignment compete in both the usual "metric" version and in the
"metric-free" case where a particle interacts with its Voronoi neighbors. We
show that the phase diagram of this out-of-equilibrium XY model is similar to
that of its equilibrium counterpart: the properties of the fully-nematic model,
studied before in [F. Ginelli, F. Peruani, M. Baer, and H. Chat\'e, Phys. Rev.
Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias
of interactions towards ferromagnetic alignment. The direct transitions between
polar and nematic ordered phases are shown to be discontinuous in the metric
case, and continuous, belonging to the Ising universality class, in the
metric-free version
Comment on ``Phase Transitions in Systems of Self-Propelled Agents and Related Network Models''
In this comment we show that the transition to collective motion in
Vicsek-like systems with angular noise remain discontinuous for large velocity
values. Thus, the networks studied by Aldana {\et al.} [Phys. Rev. Lett. {\bf
98}, 095702 (2007)] at best constitute a singular, large velocity limit of
these systems.Comment: To appear on Physical Review Letter
The Physics of the Vicsek Model
Peer reviewedPublisher PD
Evidence of a Critical Phase Transition in Purely Temporal Dynamics with Long-Delayed Feedback
We wish to thank S. Lepri and A. Politi for useful discussions. MF and FG acknowledge support from EU Marie Curie ITN grant n. 64256 (COSMOS).Peer reviewedPublisher PD
Leading birds by their beaks : the response of flocks to external perturbations
Acknowledgments We have benefited from discussions with H Chaté and A Cavagna. We acknowledge support from the Marie Curie Career Integration Grant (CIG) PCIG13-GA-2013-618399. JT also acknowledges support from the SUPA distinguished visitor program and from the National Science Foundation through awards # EF-1137815 and 1006171, and thanks the University of Aberdeen for their hospitality while this work was underway. FG acknowledges support from EPSRC First Grant EP/K018450/1.Peer reviewedPublisher PD
Intertangled stochastic motifs in networks of excitatory-inhibitory units
We have benefited from discussions with A. Politi. The authors acknowledge financial support from H2020- MSCA-ITN-2015 project COSMOS 642563.Peer reviewedPostprin
Boltzmann-Ginzburg-Landau approach for continuous descriptions of generic Vicsek-like models
We describe a generic theoretical framework, denoted as the
Boltzmann-Ginzburg-Landau approach, to derive continuous equations for the
polar and/or nematic order parameters describing the large scale behavior of
assemblies of point-like active particles interacting through polar or nematic
alignment rules. Our study encompasses three main classes of dry active
systems, namely polar particles with 'ferromagnetic' alignment (like the
original Vicsek model), nematic particles with nematic alignment ("active
nematics"), and polar particles with nematic alignment ("self-propelled rods").
The Boltzmann-Ginzburg-Landau approach combines a low-density description in
the form of a Boltzmann equation, with a Ginzburg-Landau-type expansion close
to the instability threshold of the disordered state. We provide the generic
form of the continuous equations obtained for each class, and comment on the
relationships and differences with other approaches.Comment: 30 pages, 3 figures, to appear in Eur. Phys. J. Special Topics, in a
Discussion and Debate issue on active matte
Covariant Lyapunov vectors
The recent years have witnessed a growing interest for covariant Lyapunov
vectors (CLVs) which span local intrinsic directions in the phase space of
chaotic systems. Here we review the basic results of ergodic theory, with a
specific reference to the implications of Oseledets' theorem for the properties
of the CLVs. We then present a detailed description of a "dynamical" algorithm
to compute the CLVs and show that it generically converges exponentially in
time. We also discuss its numerical performance and compare it with other
algorithms presented in literature. We finally illustrate how CLVs can be used
to quantify deviations from hyperbolicity with reference to a dissipative
system (a chain of H\'enon maps) and a Hamiltonian model (a Fermi-Pasta-Ulam
chain)
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