Diffusion of Particles Adsorbed on a Reconstructive Surface
The influence of surface reconstruction on diffusion of particles, adsorbed on the surface, is investigated in the framework of symmetrical four-position model. The analytical expressions for the free energy and chemical and jump diffusion coefficients are obtained assuming the lateral interaction between the particles is negligibly small. The coverage dependencies of the tracer, jump and chemical diffusion coefficients are calculated using the expressions and MC simulations. The results demonstrate clearly a drastic change of the behavior of all diffusion coefficients on the reconstructed surface.
Adsorption-Induced Surface Reconstruction Processes:. the Influence on the Adsorption-Desorption Kinetics and the Mobility of Adsorbed Atoms
In this paper we study the influence of adsorption-induced surface reconstruction on both the adsorption–desorption kinetics and the surface diffusion of adsorbed particles. The reconstruction is regarded as an order–disorder phase transition, which is described in the framework of the lattice gas formalism by the two-position (2P) model. Adsorption is studied by kinetic Monte Carlo simulations while surface diffusion coefficients are evaluated by using the classical Monte Carlo modeling. Utilizing Monte Carlo simulations we characterize how the behavior of the chemical (D), the jump (DJ) and the tracer (D*) diffusion coefficients depend on the different ordered phases predicted by the model.
Live birth in the Devonian period
The extinct placoderm fishes were the dominant group of vertebrates throughout the Middle Palaeozoic era, yet controversy about their relationships within the gnathostomes (jawed vertebrates) is partly due to different interpretations of their reproductive biology. Here we document the oldest record of a live-bearing vertebrate in a new ptyctodontid placoderm, Materpiscis attenboroughi gen. et sp. nov., from the Late Devonian Gogo Formation of Australia (approximately 380 million years ago). The new specimen, remarkably preserved in three dimensions, contains a single, intra-uterine embryo connected by a permineralized umbilical cord. An amorphous crystalline mass near the umbilical cord possibly represents the recrystallized yolk sac. Another ptyctodont from the Gogo Formation, Austroptyctodus gardineri, also shows three small embryos inside it in the same position. Ptyctodontids have already provided the oldest definite evidence for vertebrate copulation, and the new specimens confirm that some placoderms had a remarkably advanced reproductive biology, comparable to that of some modern sharks and rays. The new discovery points to internal fertilization and viviparity in vertebrates as originating earliest within placoderms.