Modulated nematic structures induced by chirality and steric polarization

What kind of one-dimensional modulated nematic structures (ODMNS) can form nonchiral and chiral bent-core and dimeric materials? Here, using Landau-deGennes theory of nematics, extended to account for molecular steric polarization, we study a possibility of formation of ODMNS, both in nonchiral and intrinsically chiral liquid crystalline materials. Besides nematic and cholesteric phases, we find four bulk ODMNS for nonchiral materials, two of which have not been reported so far. These new structures are longitudinal ($N_{LP}$) and transverse ($N_{TP}$) periodic waves where the polarization field being periodic in one dimension stays parallel and perpendicular, respectively, to the wave vector. The other two phases have all characteristic features of the twist-bend nematic phase ($N_{TB}$) and the splay-bend nematic phase ($N_{SB}$), but their fine structure appears more complex than that considered so far. The presence of molecular chirality converts nonchiral $N_{TP}$ and $N_{SB}$ into new $N_{TB}$ phases. Interestingly, the nonchiral $N_{LP}$ phase can stay stable even in the presence of intrinsic molecular chirality. Exemplary phase diagrams provide further insights into the relative stability of these new modulated nematic structures.Comment: 5 pages, 5 figure

Les systèmes combinatoires animaux n’ont pas de double articulation: Hockett n’avait pas tort

La dualidad o doble articulación es una propiedad fundamental del lenguaje. Hace medio siglo, Charles Hockett comparó la comunicación animal y el lenguaje con su conocido sistema de rasgos de diseño, y sostuvo que la dualidad era uno de los pocos rasgos ausentes en la comunicación animal. Desde entonces, numerosos autores, en especial lingüistas, han atribuido esa propiedad a algunos sistemas comunicativos animales que muestran una naturaleza combinatoria, en los cuales las señales se forman mediante la unión de unidades menores. Este artículo analiza críticamente esas propuestas; en concreto, defiende que la naturaleza jerárquico- combinatoria de esos sistemas es muy diferente a la del lenguaje, por lo que carecen de dualidad. Por ello, el trabajo reivindica la posición de Hockett, sosteniendo que estaba plenamente acertado cuando defendía que la dualidad es un rasgo inexistente en la comunicación animal.Duality of patterning is a central property of language. Half a century ago, Charles Hockett compared animal communication and language by means of his well-known system of design features, and contended that duality was one of the few features animal communication was not endowed with. Since then, a number of scholars (especially, linguists), have considered duality to exist in some animal communication systems which exhibit a combinatorial nature, in such a way that signals are composed of smaller units. This article critically discusses those proposals; more concretely, it makes the point that the hierarchical-combinatorial procedure found in animal combinatorial systems greatly differs from that found in language, the result being that those systems lack duality. Therefore, the paper vindicates Hockett’s position, by arguing that he was fully right when he asserted that duality is absent from animal communicationLa double articulation est une qualité fondamentale du langage. Il y a un demi-siècle, Charles Hockett a comparé la communication animale et le langage avec son système de traits de dessin, et a soutenu que la dualité était l'une des quelques fonctionnalités manquantes dans la communication animale. Dès lors, de nombreux auteurs, notamment des linguistes, ont attribué cette qualité à certains systèmes de communication des animaux montrant une nature combinatoire, dans lesquels les signaux sont formés par l'assemblage d’unités mineures. Cet article analyse ces propositions de façon critique; en particulier, il soutient l’argument que la nature hiérarchique-combinatoire de ces systèmes diffère considérablement de celle du langage, ne possédant pas de double articulation. Par conséquent, ce travail justifie la position de Hockett, considérant qu’il avait tout à fait raison lorsqu’il soutenait que la double articulation est une caractéristique absente dans la communication animale

Synchronization in the presence of memory

We study the effect of memory on synchronization of identical chaotic systems driven by common external noises. Our examples show that while in general synchronization transition becomes more difficult to meet when memory range increases, for intermediate ranges the synchronization tendency of systems can be enhanced. Generally the synchronization transition is found to depend on the memory range and the ratio of noise strength to memory amplitude, which indicates on a possibility of optimizing synchronization by memory. We also point out on a close link between dynamics with memory and noise, and recently discovered synchronizing properties of networks with delayed interactions

Roundoff-induced attractors and reversibility in conservative two-dimensional maps

We numerically study two conservative two-dimensional maps, namely the baker map (whose Lyapunov exponent is known to be positive), and a typical one (exhibiting a vanishing Lyapunov exponent) chosen from the generalized shift family of maps introduced by C. Moore [Phys Rev Lett {\bf 64}, 2354 (1990)] in the context of undecidability. We calculated the time evolution of the entropy $S_q \equiv \frac{1-\sum_{i=1}^Wp_i^q}{q-1}$ ($S_1=S_{BG}\equiv -\sum_{i=1}^Wp_i \ln p_i$), and exhibited the dramatic effect introduced by numerical precision. Indeed, in spite of being area-preserving maps, they present, {\it well after} the initially concentrated ensemble has spread virtually all over the phase space, unexpected {\it pseudo-attractors} (fixed-point like for the baker map, and more complex structures for the Moore map). These pseudo-attractors, and the apparent time (partial) reversibility they provoke, gradually disappear for increasingly large precision. In the case of the Moore map, they are related to zero Lebesgue-measure effects associated with the frontiers existing in the definition of the map. In addition to the above, and consistently with the results by V. Latora and M. Baranger [Phys. Rev. Lett. {\bf 82}, 520 (1999)], we find that the rate of the far-from-equilibrium entropy production of baker map, numerically coincides with the standard Kolmogorov-Sinai entropy of this strongly chaotic system.Comment: Invited paper to appear in Physica A (PASI Meeting, Mar del Plata, December 2006); 12 pages including 7 figures. Version 2 has an improved Figure

Evaluating the impact of Hymenoscyphus fraxineus in Trentino (Alps, Northern Italy): first investigations

The spread of Hymenoscyphus fraxineus has been causing great concern regarding the survival of European ash (Fraxinus excelsior) throughout Europe since the 1990s. The disease was first recorded in Trentino (southern Alps, Italy) in 2012 and has spread throughout the mountain landscape, where ash trees are scattered in small and isolated stands in different valleys. The status of the disease was checked by monitoring the damage to natural regeneration and adult trees in 90 sites spread over the whole region. The survey confirmed the complete colonization by the pathogen of the whole investigated area, with high levels of damage to both young and adult ash trees. Regeneration (both seedlings and saplings) was observed to be affected by the fungus in 88 plots out of 90. Out of 4486 examined young European ashes, 2261 (50.4%) were affected and 789 (17.6%) were already dead. Ten of the 384 assayed flowering ashes (Fraxinus ornus) showed symptoms on branches and apical stems, similar to those observed for European ash. Isolation and molecular analysis proved the presence of the fungus on both symptomatic European and flowering ashes. The examined 386 adult trees showed different levels of damage, sometimes reaching more than 75% of the crown. Some individual trees (42) growing close to severely damaged trees appeared fully healthy, which suggests the possible existence of some resistant/tolerant individuals in the examined populations

Identidades colectivas y nuevas demandas sociales : Las generaciones políticas y la cuestión de género en el Frente Popular Darío Santillán

Fil: Longa, Francisco. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina

Stability of Biaxial Nematic Phase for Systems with Variable Molecular Shape Anisotropy

We study the influence of fluctuations in molecular shape on the stability of the biaxial nematic phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A {\bf 113}, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and $D_2O$, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the biaxial nematic phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the biaxial nematic phase can be either enhanced or depressed, relative to the uniaxial nematic phases. In the former case the splitting of the Landau point into two triple points with a direct phase transition line from isotropic to biaxial phase is observed.Comment: 18 pages containing 6 figure