Some remarks on correlations between conflict and instability

International audienceA study of the stability of classical systems under the influence of two conflicting single-well, single-variable potentials is presented. The control parameters are a so-called conflict parameter c, the distance between the two conflicting attractors, and an asymmetry parameter a which determines the relative strength of the two attractors. For convex potentials, the influence of conflict on the system stability depends on the sign of the third derivative of the conflicting potential functions, resulting in two categories called U- and V-type potentials. The introduction of a concave branch (bias) in convex conflicting potentials qualitatively changes the system behavior, by generating bifurcation and discontinuity. The cusp catastrophe can be redefined as the conflict between two such biased single-well potentials. The main interest of the new representation potentially lies in the evocative power of the control parameters a and c. The mapping of (a,c) on (u,v), the traditional control parameters of the cusp catastrophe, is given. The cusp develops above a certain critical value of the conflict parameter c

Sur un nouveau type de représentation catastrophiste pour les modélisations en biologie et sciences cognitives: Cet article est une version française amendée d'un article paru en anglais dans Acta Biotheoretica, 45(2), 93 (1997) sous le titre "Predation and generation processes through a new representation of the cusp catastrophe"; avec l'aimable permission de Kluwer Academic Publishers.

L'article publié peut être consulté à l'adresse https://www.persee.fr/doc/intel_0769-4113_1997_num_24_1_1756 , mais de nombreuses erreurs de polices de caractères dans les équations le rendent difficilement compréhensible. On lui préfèrera le manuscrit joint au format pdf.International audienceA new formulation of the cusp catastrophe is used to model the fundamental biological functions of predation and reproduction. This new representation lies on the decomposition of the overall cusp potential in two component potentials individualising the conflicting pregnances. It results in a more accurate and less problematic description than the original proposition by R. Thom, mostly due to the use of parameters with strong physical and evocative power. For instance, it gives a very suggestive account for such biologically significant processes as digestion and assimilation, or pregnancy and delivery. Two couples of parameters are used in the new representation of the cusp: one defines the exact shape of the conflicting attractors which symbolises genetic coding, the other controls the deformation of -and interaction between- the two attractors and represent hormonal effects. This new description strikingly echoes Heidegger's philosophical thought on ontological difference, by giving a representation of the distinct concepts of entity and Being. It thus precludes a previous criticism concerning the impossibility for catastrophic models to depict the entity level. Such a type of representation might be useful in translating syntactical-semantic models of catastrophe theory into computer language.Une nouvelle formulation de la catastrophe fronce est utilisée pour modéliser les fonctions biologiques fondamentales de la prédation et de la reproduction. Cette nouvelle représentation repose sur la décomposition du potentiel global de la fronce en deux composantes, individualisant les prégnances conflictuelles. Elle conduit à une description plus précise et moins problématique que la proposition originale de R. Thom, grâce surtout à l’utilisation de paramètres à forts pouvoir évocateur et signification physique. Par exemple, cette nouvelle représentation rend compte de manière très suggestive des importants processus biologiques de la digestion et de l’assimilation, ou de la grossesse et de l’accouchement. Deux couples de paramètres sont utilisés dans la nouvelle représentation de la fronce : l’un définit la forme exacte des attracteurs en conflit, qui symbolise le codage génétique, l’autre contrôle la déformation et l’interaction entre les deux attracteurs et représente les effets hormonaux. Cette nouvelle description fait écho de manière frappante à la pensée philosophique de Heidegger concernant la différence ontologique, en donnant une représentation des concepts distincts de l’étant et de l’Être. Elle invalide une précédente critique concernant l’impossibilité pour les modèles catastrophistes de décrire le niveau de l’étant. Ce type de représentation pourrait s’avérer utile pour la traduction des modèles syntaxico-sémantiques de la théorie des catastrophes en langage des machines

Synthesis by the polyol process and ionic conductivity of nanostructured La2Mo2O9 powders

International audienceLa2Mo2O9 nanostructured powders were synthesized by the polyol process. The effects of the nature of the polyol, the refluxing time, the hydrolysis ratio, the metal concentration and the addition of hydroxide ions, on the purity and morphology of the powders are determined. Two main morphologies are observed, the particles being in the shape of platelets or spheres, with respectively diethylene glycol or ethylene glycol as solvent. A specific surface area of 24 m2/g was reached by varying the metal concentration. The conductivity measurements were recorded on pellets that present in some cases a closed porosity, the relative density reaching 95% without any milling step. According to the synthesis parameters, the grain conductivity can be slightly increased, the total conductivity remaining only slightly lower than that of pellets made of powders synthesized by solid state reaction

RENiO3 Single Crystals (RE = Nd, Sm, Gd, Dy, Y, Ho, Er, Lu) Grown from Molten Salts under 2000 bar of Oxygen Gas Pressure

The electronic properties of transition-metal oxides with highly correlated electrons are of central importance in modern condensed-matter physics and chemistry, both for their fundamental scientific interest and for their potential for advanced electronic applications. However, the design of materials with tailored properties has been restricted by the limited understanding of their structure−property relationships, which are particularly complex in the proximity of the regime where localized electrons become gradually mobile. RENiO3 perovskites, characterized by the presence of spontaneous metal to insulator transitions, are some of the most widely used model materials for the investigation of this region in theoretical studies. However, crucial experimental information needed to validate theoretical predictions is still lacking due to their challenging high-pressure synthesis, which has prevented to date the growth of sizable bulk single crystals with RE ≠ La, Pr, and Nd. Here we report the first successful growth of single crystals with RE = Nd, Sm, Gd, Dy, Y, Ho, Er, and Lu in sizes up to ∼75 μm, grown from molten salts in a temperature gradient under 2000 bar of oxygen gas pressure. The crystals display regular prismatic shapes with flat facets, and their crystal structures and metal−insulator and antiferromagnetic order transition temperatures are in excellent agreement with previously reported values obtained from polycrystalline samples. The availability of such crystals opens access to measurements that have hitherto been impossible to conduct. This should contribute to a better understanding of the fascinating properties of materials with highly correlated electrons and guide future efforts to engineer transition-metal oxides with tailored functional properties

Tests for the Use of La2Mo2O9-based Oxides as Multipurpose SOFC Core Materials

International audienceThe mixed ionic-electronic conductivity under dilute hydrogen, the stability and the catalytic activity under propane:air type mixtures of a series of LAMOX oxide-ion conductors have been studied. The effect of exposure to dilute hydrogen on the conductivity of the -La2(Mo2 - yWy)O9 series at 600 °C depends on tungsten content: almost negligible for the highest (y = 1.4), it is important for La2Mo2O9 (y = 0). In propane:air, all tested LAMOX electrolytes are stable at 600-700 °C, but get reduced when water vapour is present. La2Mo2O9 is the best oxidation catalyst of the series, with an activity comparable to that of nickel.The catalytic activity of other tested LAMOX compounds is much lower, (La1.9Y0.1)Mo2O9 showing a deactivation phenomenon. These results suggest that depending on composition, La2(Mo2 - yWy)O9 compounds could be either electrolytes in single-chamber SOFC and dual-chamber micro-SOFC (y = 1.4) or anode materials in dual-chamber SOFC (low y) or oxidation catalysts in SOFCs operating with propane (y = 0)

Dineodymium(III) ditungstate(VI), Nd2W2O9

Single crystals of monoclinic Nd2W2O9 were obtained by growth from tungsten borate flux in an atmosphere of air. The crystal structure consists of chains of distorted [WO6] octa­hedra that run along the c axis of the structure, and of [NdO9] polyhedra that are connected via common faces and common edges to form a three-dimensional framework

Coupled structural and magnetic properties of ferric fluoride nanostructures: part II, a Monte-Carlo Heisenberg study

We present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte-Carlo-Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with a superexchange angle dependence. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic like) at the grain boundary, in agreement with experimental findings. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. By simulating hysteresis loops, we find that the magnetization rotation is not spatially uniform. We conclude on possible extensions of the model.Comment: submitted to JMM

Giant oxygen isotope effect on the metal-insulator transition of RNiO{sub 3} perovskites

The metal to insulator transition displayed by all the members of the perovskite family RNiO{sub 3} (R = 4f rare earth different from La) has attracted a lot of interest since it constitutes one of the few examples of this phenomenon in perfectly stoichiometric compounds. In spite of the great deal of work performed during the last six years, the mechanism responsible for the electronic localization is still a matter of controversy. The observation of unusually large O isotope shifts on the metal-insulator temperature T{sub MI} reported in this study represents an important advance since it clearly proves the dominant role of the electron lattice interaction as driving force for the transition. Moreover, the good agreement between this observation and a simple model based on the existence of Jahn-Teller polarons in the metallic state gives further qualitative and quantitative support to the polaronic picture recently suggested to account for O isotope effects in other 3d transition metal oxides containing Jahn-Teller ions