2,057 research outputs found
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
Unconventional continuous phase transition in a three dimensional dimer model
Phase transitions occupy a central role in physics, due both to their
experimental ubiquity and their fundamental conceptual importance. The
explanation of universality at phase transitions was the great success of the
theory formulated by Ginzburg and Landau, and extended through the
renormalization group by Wilson. However, recent theoretical suggestions have
challenged this point of view in certain situations. In this Letter we report
the first large-scale simulations of a three-dimensional model proposed to be a
candidate for requiring a description beyond the Landau-Ginzburg-Wilson
framework: we study the phase transition from the dimer crystal to the Coulomb
phase in the cubic dimer model. Our numerical results strongly indicate that
the transition is continuous and are compatible with a tricritical universality
class, at variance with previous proposals.Comment: 4 pages, 3 figures; v2: minor changes, published versio
Difficultés pour le demandeur de se faire assister d'un médecin-expert lors d'une action en responsabilité médicale
Bark beetle population dynamics in the Anthropocene: Challenges and solutions
Tree-killing bark beetles are the most economically important insects in conifer forests worldwide. However, despite N200 years of research, the drivers of population eruptions and crashes are still not fully understood and the existing knowledge is thus insufficient to face the challenges posed by the Anthropocene. We critically analyze potential biotic and abiotic drivers of population dynamics of an exemplary species, the European spruce bark beetle (ESBB) (Ips typographus) and present a multivariate approach that integrates the many drivers governing this bark beetle system. We call for hypothesis-driven, large-scale collaborative research efforts to improve our understanding of the population dynamics of this and other bark beetle pests. Our approach can serve as a blueprint for tackling other eruptive forest insects
Elastic turbulence in shear banding wormlike micelles
We study the dynamics of the Taylor-Couette flow of shear banding wormlike
micelles. We focus on the high shear rate branch of the flow curve and show
that for sufficiently high Weissenberg numbers, this branch becomes unstable.
This instability is strongly sub-critical and is associated with a shear stress
jump. We find that this increase of the flow resistance is related to the
nucleation of turbulence. The flow pattern shows similarities with the elastic
turbulence, so far only observed for polymer solutions. The unstable character
of this branch led us to propose a scenario that could account for the recent
observations of Taylor-like vortices during the shear banding flow of wormlike
micelles
A homological interpretation of the transverse quiver Grassmannians
In recent articles, the investigation of atomic bases in cluster algebras
associated to affine quivers led the second-named author to introduce a variety
called transverse quiver Grassmannian and the first-named and third-named
authors to consider the smooth loci of quiver Grassmannians. In this paper, we
prove that, for any affine quiver Q, the transverse quiver Grassmannian of an
indecomposable representation M is the set of points N in the quiver
Grassmannian of M such that Ext^1(N,M/N)=0. As a corollary we prove that the
transverse quiver Grassmannian coincides with the smooth locus of the
irreducible components of minimal dimension in the quiver Grassmannian.Comment: final version, 7 pages, corollary 1.2 has been modifie
Pénurie De Main-D’œuvre Agricole Et Stratégies D’Adaptation Des Paysans Dans La Commune De Zè
L’agriculture contribue au processus de développement économique et social de la commune de Zè. Aujourd’hui, la production agricole est confrontée à la problématique de la main-d’œuvre dans cette localité. La présente recherche vise à étudier la pénurie de la main-d’œuvre et les stratégies d’adaptation dans la commune de Zè. La méthodologie adoptée dans le cadre de cette recherche s’articule autour de la collecte des données, du traitement des données et de l’analyse des résultats. La recherche documentaire et les enquêtes de terrain ont été les techniques de collecte des données. Les outils utilisés pour cela sont un questionnaire, un guide d’entretien et une grille d’observation. 170 personnes ont été enquêtées. Il ressort de l’analyse des résultats que dans la commune de Zè, les producteurs utilisent plusieurs types de main-d’œuvre. La main-d’œuvre familiale représente 70 %. Les producteurs ont recours aussi à la main-d’œuvre extérieure sous différentes formes pour les travaux agricoles. Ce type de main-d’œuvre représente de 30 %. Deux formes de recours à la main-d’œuvre extérieure se distinguent : l’entraide avec 5 % et le salariat avec 25 %. Dans la commune, selon 70 % des producteurs enquêtés, il y a l’insuffisance de la main-d’œuvre familiale, la rareté de la main-d’œuvre salariée surtout en période des travaux agricoles. Cette situation entraine le retard dans l’exécution des différentes opérations culturales (défrichage, labour, semis, sarclage, récolte). Les producteurs agricoles développent plusieurs stratégies d’adaptation à cette pénurie de main-d’œuvre.
Agriculture contributes to the process of economic and social development of the commune of Zè. Today, agricultural production is faced with the problem of labor in this locality. This research aims to study the shortage of labor and coping strategies in the municipality of Zè. The methodology adopted in the context of this research revolves around data collection, data processing and analysis of the results. Documentary research and field surveys were the data collection techniques. The tools used for data collection are essentially a questionnaire, an interview guide and an observation grid. 170 people were surveyed. The SWOT (Strengths, Weaknesses, Opportunities, Threats) model was used to analyze the results. It appears from the analysis of the results that in the municipality of Zè, producers use several types of labor. Family labor accounts for 70%. Field surveys have shown that producers also use outside labor in different forms for agricultural work. This type of labor represents 30%. Two forms of recourse to external labor stand out: mutual aid with 5% and wage labor with 25%. In the commune, according to 70% of the producers surveyed, there is a lack of family labour, the scarcity of salaried labour, especially during agricultural work. This situation leads to the delay in the execution of the various cultural operations (clearing, ploughing, sowing, weeding, harvesting). Agricultural producers are developing several strategies to adapt to this labor shortage. 
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