Segregated tunneling-percolation model for transport nonuniversality

We propose a theory of the origin of transport nonuniversality in disordered insulating-conducting compounds based on the interplay between microstructure and tunneling processes between metallic grains dispersed in the insulating host. We show that if the metallic phase is arranged in quasi-one dimensional chains of conducting grains, then the distribution function of the chain conductivities g has a power-law divergence for g -> 0 leading to nonuniversal values of the transport critical exponent t. We evaluate the critical exponent t by Monte Carlo calculations on a cubic lattice and show that our model can describe universal as well nonuniversal behavior of transport depending on the value of few microstructural parameters. Such segregated tunneling-percolation model can describe the microstructure of a quite vast class of materials known as thick-film resistors which display universal or nonuniversal values of t depending on the composition.Comment: 8 pages, 5 figures (Phys. Rev. B - 1 August 2003)(fig1 replaced

Confirmation of Anomalous Dynamical Arrest in attractive colloids: a molecular dynamics study

Previous theoretical, along with early simulation and experimental, studies have indicated that particles with a short-ranged attraction exhibit a range of new dynamical arrest phenomena. These include very pronounced reentrance in the dynamical arrest curve, a logarithmic singularity in the density correlation functions, and the existence of `attractive' and `repulsive' glasses. Here we carry out extensive molecular dynamics calculations on dense systems interacting via a square-well potential. This is one of the simplest systems with the required properties, and may be regarded as canonical for interpreting the phase diagram, and now also the dynamical arrest. We confirm the theoretical predictions for re-entrance, logarithmic singularity, and give the first direct evidence of the coexistence, independent of theory, of the two coexisting glasses. We now regard the previous predictions of these phenomena as having been established.Comment: 15 pages,15 figures; submitted to Phys. Rev.

Nouvelles données géochronologiques relatives au complexe gabbro-noritique et charnockitique du bouclier du Kasai et à son prolongement en Angola

De nouveaux résultats Rb-Sr et U-Pb confirment les données publiées (Delhal et Ledent, 1971) relatives à l'âge et aux rapports Sr87/Sr86 initiaux du strontium des roches charnockitiques du Kasai :l'âge de la charnockitisation est compris entre 2850 m.a. et 2900 m.a. Le rapport initial Sr87/Sr86 extrêmement bas (0,7005 ± 0,0014) de l'isochrone relative au faciès hololeucocrate à mésoperthite montre le caractère juvénile de ces roches par rapport aux gneiss enderbitiques et charnockitiques auxquels elles sont associées et dont les rapports initiaux plus élevés (≈ 0,705) témoignent d'une histoire crustale plus longue.Un ensemble de résultats obtenus sur des gneiss de caractère charnockitique de la région de Cuango (9°10'S-17°58'E) en Angola confirme les conclusions des études pétrographiques antérieures (Delhal et Fieremans, 1964) selon lesquelles les roches charnockitiques du Kasai et d'Angola font partie d'un même complexe qui s'étend de la zone littorale atlantique en Angola jusqu'un peu au-delà du 24° méridien Est au Zaïre.info:eu-repo/semantics/publishe