Critical behaviour of the Rouse model for gelling polymers

It is shown that the traditionally accepted "Rouse values" for the critical exponents at the gelation transition do not arise from the Rouse model for gelling polymers. The true critical behaviour of the Rouse model for gelling polymers is obtained from spectral properties of the connectivity matrix of the fractal clusters that are formed by the molecules. The required spectral properties are related to the return probability of a "blind ant"-random walk on the critical percolating cluster. The resulting scaling relations express the critical exponents of the shear-stress-relaxation function, and hence those of the shear viscosity and of the first normal stress coefficient, in terms of the spectral dimension $d_{s}$ of the critical percolating cluster and the exponents $\sigma$ and $\tau$ of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.

Elasticity of Gaussian and nearly-Gaussian phantom networks

We study the elastic properties of phantom networks of Gaussian and nearly-Gaussian springs. We show that the stress tensor of a Gaussian network coincides with the conductivity tensor of an equivalent resistor network, while its elastic constants vanish. We use a perturbation theory to analyze the elastic behavior of networks of slightly non-Gaussian springs. We show that the elastic constants of phantom percolation networks of nearly-Gaussian springs have a power low dependence on the distance of the system from the percolation threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur

Charge density wave ordering in NbSe3: possible models and the experimental evidence

Charge density wave (CDW) ordering in the prototypical low-dimensional compound NbSe3 is reconsidered. We show that the widely accepted CDW model with two incommensurate modulations, q1 = (0,0.241,0) and q2 = (0.5,0.260,0.5), localized on type-III and type-I bi-capped trigonal prismatic (BCTP) columns, does not explain some details, revealed by various microscopic methods. The suggested alternative explanation is in a better accord with the entire experimental evidence, including low-temperature (LT) scanning tunneling microscopy (STM) results. It is based on the existence of modulated layered nano-domains formed below both CDW onset temperatures. According to this model, two of the three slightly different BCTP types of columns are modulated by the same wave vector, either q1 or q2, which can easily switch over in a domain as a whole. This approach explains the presence of the q2 modulation in the STM images recorded above the T2 CDW transition and the absence of the q2 satellites in the corresponding diffraction patterns. The long periodic modulation, detected by LT STM is attributed to a beating between the two CDWs, centered on adjacent columns of the same type. These pairs of columns, both either of type-III or type-I, modulated by the two alternative CDWs, represent the basic modulation units, ordered into nano-domains

Et si le passé nous parlait d'avenir : influence de l'histoire du paysage sur la diversité des macrophytes des lacs et étangs du littoral aquitain

National audienceLa structure du paysage est largement reconnue comme ayant un rôle majeur sur la biodiversité, en influençant directement les dynamiques des populations et des communautés. Mais le paysage peut aussi être appréhendé comme un bon indicateur des pressions anthropiques diffuses qui agissent sur le territoire et qui peuvent localement modifier la structure de la diversité. Face à ces pressions anthropiques, la réponse des écosystèmes peut à la fois varier dans l'espace mais aussi dans le temps, les écosystèmes pouvant garder une trace des évènements passés. En raison du temps de réponse des écosystèmes et des communautés suite à une perturbation, l'assemblage local des espèces peut ainsi être davantage expliqué par des évènements passés que présents. Ces processus ont surtout été étudiés sur les organismes terrestres, mais rarement sur les organismes aquatiques. L'objectif de cette étude est donc de tester l'importance relative de l'histoire du paysage, du paysage et des facteurs environnementaux sur l'organisation des patrons de diversité des macrophytes des lacs et étangs du littoral aquitain. Nous nous sommes intéressés aux bassins versants de 13 plans d'eau naturels, sur lesquels nous avons digitalisé l'occupation du sol en 1945, 1965, 1985 et 2000 à partir de photographies aériennes. Ce travail nous a permis, dans un premier temps, d'évaluer la dynamique paysagère au cours du temps. Globalement, l'occupation du sol montre une urbanisation croissante et une augmentation de la surface des champs cultivés alors que les surfaces en prairies/zones humides ont fortement régressées. L'influence de ce paysage ancien sur la richesse et la composition spécifique des macrophytes a ensuite été testée, de même que le paysage actuel, la composition chimique de l'eau (concentration des nutriments) et les caractéristiques physiques des plans d'eau (surface, périmètre, volume, temps de résidence). Globalement, la diversité des macrophytes (richesse et composition) est plus corrélée au paysage ancien que présent. La richesse spécifique est particulièrement expliquée par la surface forestière en 1945, l'artificialisation des terres en 1985, ainsi que par la concentration en nitrates et au volume du plan d'eau. La composition est fortement influencée par l'occupation du sol passée, les caractéristiques physiques des lacs et la concentration en phosphore. En revanche, aucune variable paysagère actuelle n'apparait comme explicative de la composition spécifique. Ces résultats soulignent l'importance de l'histoire du paysage sur l'organisation des patrons de diversité et suggèrent la présence de délais de réponse des communautés végétales face aux pressions anthropiques

Influence de l'histoire du paysage sur la diversité des plantes aquatiques

International audienceThe historical composition of landscapes is recognized as an important factor for explaining plant diversity patterns, because current species assemblages are not only patterned by current ecological conditions, but also represent legacies of the past. Indeed, historical landscapes can represent past anthropogenic impact on the environment whose effects could still be observed nowadays. However, the influence of history on diversity patterns has mostly been investigated for terrestrial ecosystems, but has rarely been considered for aquatic ones. Here, we examined the effect of historical landscapes and land-use changes on the taxonomic richness and composition of aquatic plant communities in French freshwater shallow lakes and ponds. We aimed to test the relative influence of environmental variables and landscapes legacy on macrophyte species assemblages. We surveyed the macrophytes communities in 100m sections of the shoreline of each lake (N=17) up to 1m depth. We used water chemistry and physical features of lakes as environmental variables, and reconstructed historical land-use of lakes' watershed with aerial pictures of 1945, 1965, 1985 and 2000 for examining historical variables. Landscape changes were examined with transition matrices and principal component analysis (PCA) and species composition with non-metrical multidimensional scaling (NMDS). The relative influence of physical, chemical and historical factors on species richness and composition were tested with variation partitioning methods. We found that the landscape of all lakes' watershed exhibit a trend toward an increase of urban and arable areas at the expense of semi-natural ones. Environmental variables and historical factors are strongly correlated with macrophyte diversity but current landscape composition seems to have only a weak influence. Especially, historical landscape factors appeared to be strongly associated with species macrophyte composition. These results underline the importance of historical factors in explaining aquatic plant diversity and suggest delayed responses of plant communities to anthropogenic pressures. Integrating historical factors in future analyses of aquatic ecosystems would thus greatly contribute to understand ecological processes governing their species assemblages, and would be crucial for their conservation and management