18 research outputs found
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 rĂ©sultats Rb-Sr et U-Pb confirment les donnĂ©es publiĂ©es (Delhal et Ledent, 1971) relatives Ă l'Ăąge et aux rapports Sr87/Sr86 initiaux du strontium des roches charnockitiques du Kasai :l'Ăąge de la charnockitisation est compris entre 2850 m.a. et 2900 m.a. Le rapport initial Sr87/Sr86 extrĂȘmement bas (0,7005 ± 0,0014) de l'isochrone relative au faciĂšs hololeucocrate Ă mĂ©soperthite montre le caractĂšre juvĂ©nile de ces roches par rapport aux gneiss enderbitiques et charnockitiques auxquels elles sont associĂ©es et dont les rapports initiaux plus Ă©levĂ©s (â 0,705) tĂ©moignent d'une histoire crustale plus longue.Un ensemble de rĂ©sultats obtenus sur des gneiss de caractĂšre charnockitique de la rĂ©gion de Cuango (9°10'S-17°58'E) en Angola confirme les conclusions des Ă©tudes pĂ©trographiques antĂ©rieures (Delhal et Fieremans, 1964) selon lesquelles les roches charnockitiques du Kasai et d'Angola font partie d'un mĂȘme complexe qui s'Ă©tend de la zone littorale atlantique en Angola jusqu'un peu au-delĂ du 24° mĂ©ridien Est au ZaĂŻre.info:eu-repo/semantics/publishe