9,836 research outputs found
From Phase to Micro-Phase Separation in Flocking Models: The Essential Role of Non-Equilibrium Fluctuations
We show that the flocking transition in the Vicsek model is best understood
as a liquid-gas transition, rather than an order-disorder one. The full phase
separation observed in flocking models with Z2 rotational symmetry is, however,
replaced by a microphase separation leading to a smectic arrangement of
traveling ordered bands. Remarkably, continuous deterministic descriptions do
not account for this difference, which is only recovered at the fluctuating
hydrodynamics level. Scalar and vectorial order parameters indeed produce
different types of number fluctuations, which we show to be essential in
selecting the inhomogeneous patterns. This highlights an unexpected role of
fluctuations in the selection of flock shapes.Comment: 5 p., 5 fig.. Supplementary material: 7 movie
Ab-initio Gutzwiller method: first application to Plutonium
Except for small molecules, it is impossible to solve many electrons systems
without imposing severe approximations. If the configuration interaction
approaches (CI) or Coupled Clusters techniques \cite{FuldeBook} are applicable
for molecules, their generalization for solids is difficult. For materials with
a kinetic energy greater than the Coulomb interaction, calculations based on
the density functional theory (DFT), associated with the local density
approximation (LDA) \cite{Hohenberg64, Kohn65} give satisfying qualitative and
quantitative results to describe ground state properties. These solids have
weakly correlated electrons presenting extended states, like materials or
covalent solids. The application of this approximation to systems where the
wave functions are more localized ( or -states) as transition metals
oxides, heavy fermions, rare earths or actinides is more questionable and can
even lead to unphysical results : for example, insulating FeO and CoO are
predicted to be metalic by the DFT-LDA..
Electronic transport in AlMn(Si) and AlCuFe quasicrystals: Break-down of the semiclassical model
The semi-classical Bloch-Boltzmann theory is at the heart of our
understanding of conduction in solids, ranging from metals to semi-conductors.
Physical systems that are beyond the range of applicability of this theory are
thus of fundamental interest. It appears that in quasicrystals and related
complex metallic alloys, a new type of break-down of this theory operates. This
phenomenon is related to the specific propagation of electrons. We develop a
theory of quantum transport that applies to a normal ballistic law but also to
these specific diffusion laws. As we show phenomenological models based on this
theory describe correctly the anomalous conductivity in quasicrystals.
Ab-initio calculations performed on approximants confirm also the validity of
this anomalous quantum diffusion scheme. This provides us with an ab-initio
model of transport in approximants such as alpha-AlMnSi and AlCuFe 1/1 cubic
approximant.Comment: 11 pages, 5 figure
MAX-consensus in open multi-agent systems with gossip interactions
We study the problem of distributed maximum computation in an open
multi-agent system, where agents can leave and arrive during the execution of
the algorithm. The main challenge comes from the possibility that the agent
holding the largest value leaves the system, which changes the value to be
computed. The algorithms must as a result be endowed with mechanisms allowing
to forget outdated information. The focus is on systems in which interactions
are pairwise gossips between randomly selected agents. We consider situations
where leaving agents can send a last message, and situations where they cannot.
For both cases, we provide algorithms able to eventually compute the maximum of
the values held by agents.Comment: To appear in the proceedings of the 56th IEEE Conference on Decision
and Control (CDC 17). 8 pages, 3 figure
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