The robustness of self-organized criticality against extinctions in a tree-like model of evolution

in Europhysics Letters [=EPL] (1995), 32(7), 613-618

We investigate a so-called punctuated-equilibrium model of tree-like evolution containing extinctions for the weakest species with respect to a strength parameter r. Without extinctions (for r = 0), the ... [more ▼]

We investigate a so-called punctuated-equilibrium model of tree-like evolution containing extinctions for the weakest species with respect to a strength parameter r. Without extinctions (for r = 0), the model leads to self-organized criticality. For r not equal 0, a transition from growing trees to finite ones takes place at some critical r(c) = 0.48 +/- 0.01 value. For 0 less than or equal to r less than or similar to r(c), self-organized criticality is thus robust against extinctions of the weakest species. The size distribution of avalanches follows a power law behaviour with an exponent 3/2 which seems to be independent of the parameter r. The growing trees are found to be self-similar with a non-universal fractal dimension D-f non-trivially ranging from 2 to 1 depending on the parameter r. This constraint model opens up the field of description of various possible physical events for such an evolution model. [less ▲]

The kinetic growth anisotropy of the 123-(RE)BaCuO compounds

in Philosophical Magazine A (1995), 72(3), 727-736

The kinetic growth anisotropy of the melt-textured 123-(RE)BaCuO compounds, especially along the CuCO2 planes, is discussed here in detail. A computer model which simulates the grain growth of 123 ... [more ▼]

The kinetic growth anisotropy of the melt-textured 123-(RE)BaCuO compounds, especially along the CuCO2 planes, is discussed here in detail. A computer model which simulates the grain growth of 123 materials in the ab plane is presented. This kinetic growth model takes some bond anisotropy along the [100] and [110] directions into account. The model presented herein belongs to a hierarchical set of generalized Eden models as discussed in the Appendix. For different bond anisotropy parameters, the simulated 123-(RE)BaCUO grains exhibit square and octogonal overall shape which have been both optically observed. From simple kinetic arguments, we show that the 'faceting' of the 123-(RE)BaCuO grains finds its origin in a bond anisotropy rather than in a growth-rate anisotropy along some crystallographic directions. This is of general interest and opens new perspectives for developing the Eden model in the crystal-growth field. [less ▲]

Self-organized criticality in phylogenetic-like tree growths

in Journal de Physique I (1995), 5(8), 1011-1025

A simple stochastic model of Darwinistic evolution generating phylogenetic-like trees is developed. The model is based on a branching process taking competition-correlation effects into account. In ... [more ▼]

A simple stochastic model of Darwinistic evolution generating phylogenetic-like trees is developed. The model is based on a branching process taking competition-correlation effects into account. In presence of finite and short range correlations, the process self-organizes into a critical steady-state in which intermittent bursts of activity of all sizes are generated. On a geological-like time scale, this behaviour agrees with punctuated equilibrium features of biological evolution. The simulated phylogenetic-like trees are found to be self-similar. The dynamics of the transient regimes show a power law decrease of the order parameter towards the 0(+) value which characterizes an unstable critical state. The genetic range k of competition-correlations between living species is found to be a relevant parameter which determines the universality class of the evolution process. An infinite competition-correlation range destroys however the self-organized critical behaviour. The fractal dimension D-f of the phylogenetic-like trees increases from 2.0 to infinity as k goes from 1 to infinity. The critical exponent tau of avalanche size-distribution decreases from about 3/2 (for k = 1) and reaches about 1.2 for k = 10. A hyperscaling relation seems to relate the various universality classes. Through a mean-field theory, we mention that the evolution process is much more complex than a simple uncorrelated branching process. [less ▲]

Simulated growth and microstructure of DyBa2Cu3O7-X with and without Dy2BaCUO5 addition

in Journal of Materials Research (1995), 10(2), 268-273

We present optical observations of magnetically melt-textured DyBa2Cu307-x with and without 20 wt. % excess of Dy2BaCu05. From these observations, we propose some kinetic mechanism of the growth of 123 ... [more ▼]

We present optical observations of magnetically melt-textured DyBa2Cu307-x with and without 20 wt. % excess of Dy2BaCu05. From these observations, we propose some kinetic mechanism of the growth of 123 compounds. Kinetic processes can simulated on computers. Two (very) simple models derived from the well-known Eden model are presented. They simulate the growth of the grain front. The simulated patterns agree with the observations. The microstructure of such materials cannot be explained by thermodynamic and chemical considerations alone, but explanations must include the kinetics of the growth front as well. From our observations, we conclude that the growth probability ratios g(110)/g(100)/g(001) are of the order of 10 and 50, respectively. [less ▲]

Magnetic diffusion-limited aggregation

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1995), 51(1), 597-603

DyBa2Cu307-Y superconducting materials synthesized in-situ in a magnetic-field - relations between structure and properties

in Physica C: Superconductivity (1994), 235(Part 1), 357-358

We have synthesized c-axis oriented magnetically melt textured grown DyBa2Cu3O7-y samples at 1035 degrees C under 0.6 T magnetic induction. We discuss the crystal growth mechanism of such 123 melt ... [more ▼]

We have synthesized c-axis oriented magnetically melt textured grown DyBa2Cu3O7-y samples at 1035 degrees C under 0.6 T magnetic induction. We discuss the crystal growth mechanism of such 123 melt textured grown superconducting ceramics. A strictly tridimensional model is proposed based on Schmitz et al. model, taking into account the peculiar physical properties reported for such materials. In particular a filamentary one dimensional electron conduction path at the percolation threshold is emphasized. We have used optically polarized ligth microscopy as well as electron microscopy analysis to deduce the growth steps. [less ▲]

Lacunarity, fractal, and magnetic transition behaviors in a generalized Eden growth-process

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1994), 50(2), 635-638

The effect of an internal extra degree of freedom introduced into the simplest growth model (the Eden model) leads to a joint physical and geometrical transition. Lacunarity, fractal, and magnetic ... [more ▼]

The effect of an internal extra degree of freedom introduced into the simplest growth model (the Eden model) leads to a joint physical and geometrical transition. Lacunarity, fractal, and magnetic transitions are indeed reported to be at the same critical values of the growth parameters. A logarithmic behavior of the cluster mass on this critical value was found. [less ▲]