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

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.&nbsp