References of "Vandewalle, Nicolas"
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See detailDifferent universality classes for SOC in models driven by extremal dynamics
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Europhysics Letters [=EPL] (1997), 37(1), 1-6

We investigate a simple growth model. The model considers the growth of trees driven by extremal dynamics. Self-organized criticality (SOC) exhibited by the model is discussed. We point out that this tree ... [more ▼]

We investigate a simple growth model. The model considers the growth of trees driven by extremal dynamics. Self-organized criticality (SOC) exhibited by the model is discussed. We point out that this tree-like model does not seem to belong to any already studied SOC universality class. This is shown through a discussion of the dynamics and the so-called gap equation which does not hold here. [less ▲]

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See detailGeometric and healing laws in simple stochastic models of fracture in a sputtering process
Dhulst, R.; Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1997), 55(1), 189-193

We investigated two simple models of two-dimensional square lattice fracture under sputtering process conditions extending a previously studied model by Ausloos and Kowalski [Phys. Rev. B 45, 12 830 (1992 ... [more ▼]

We investigated two simple models of two-dimensional square lattice fracture under sputtering process conditions extending a previously studied model by Ausloos and Kowalski [Phys. Rev. B 45, 12 830 (1992)]. The models differ by the particle displacement rules during the fracture. Healing of the medium is observed in both models. This effect implies the formation of several thresholds during sputtering process fracture. They are distributed as a size-dependent power law. An avalanche like exponent is also obtained. We study this phenomenology within scaling arguments of classical percolation theory and mean-field arguments. [less ▲]

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See detailConstruction and properties of fractal trees with tunable dimension: The interplay of geometry and physics
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physical Review E (1997), 55(1), 94-98

In this paper, we emphasize three different techniques for the growth of fractal trees with a desired fractal dimension D-f. The three different growths are due to the influence of (i) stretched branches ... [more ▼]

In this paper, we emphasize three different techniques for the growth of fractal trees with a desired fractal dimension D-f. The three different growths are due to the influence of (i) stretched branches, (ii) dead ends, or (iii) a variable branching rate. Several examples are given. We point out that geometrical and physical properties (skeleton dimension, percolation exponents, self-avoiding walk) of fractal tress depend strongly on their type. The most striking result is that the critical exponents at the percolation transition are nonuniversal since they depend on the tree type. The critical exponents depend on D-f for trees of types (ii) and (iii). [less ▲]

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See detailMicrostructure development in isothermally melt-textured 123-211 composite materials
Auguste, F.; Rulmont, André ULg; Vandewalle, Nicolas ULg et al

in Journal of Low Temperature Physics (1996), 105(5-6), 1439-1444

Microstructures of isothermally melt-textured 123-211 composite materials have been examined during the solidification process from quenched specimens. The distribution of 211 particles, the ... [more ▼]

Microstructures of isothermally melt-textured 123-211 composite materials have been examined during the solidification process from quenched specimens. The distribution of 211 particles, the superconducting 123 grain boundary and the solid-liquid interface features are emphasized. The quality of the materials has been investigated by thermopower and thermal conductivity measurements. A seeded-melt texturing technique has been also tested in order to grow large single-domain of DyBa2Cu3O7-y. The chosen seed was a Dy2O3 single crystal. [less ▲]

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See detailGrowth of Cayley and diluted Cayley trees with two kinds of entities
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Journal of Physics : A Mathematical & General (1996), 29(22), 7089-7104

A kinetic growth model derived from the magnetic Eden model is introduced in order to simulate the growth of hierarchical structures, such as Cayley trees. We only consider the case where two kinds of ... [more ▼]

A kinetic growth model derived from the magnetic Eden model is introduced in order to simulate the growth of hierarchical structures, such as Cayley trees. We only consider the case where two kinds of entities are competing with each other can be further subjected to an external field. The very relevant case in which both kinds of entities have different coordination numbers is introduced here for the first time, and is called the diluted Cayley tree. Physical and geometrical properties of the finite and infinite trees are exactly found and simulated. Finite-size effects are emphasized and illustrated on the global or local magnetization and on the chemical activity. Asymptotic limits are given in each case. The generated patterns can be related to a correlated percolation problem briefly discussed in the appendix. [less ▲]

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See detailA toy model for life at the "edge of chaos"
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Computers & Graphics (1996), 20(6), 921-923

In this short paper, we illustrate the emergence of complex structures from a simple model of life at the "edge of chaos". The visualization of the generated patterns provides a good way for understanding ... [more ▼]

In this short paper, we illustrate the emergence of complex structures from a simple model of life at the "edge of chaos". The visualization of the generated patterns provides a good way for understanding the self-organized critical dynamics of the model. [less ▲]

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See detailFractal grain boundaries in growth competition
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg; Cloots, Rudi ULg

in Journal of Crystal Growth (1996), 169(1), 79-82

Evidence of fractal grain boundaries is found in polycrystals when radial patterns are found to result from a geometrical competition between different grains that have nucleated on the same center. A ... [more ▼]

Evidence of fractal grain boundaries is found in polycrystals when radial patterns are found to result from a geometrical competition between different grains that have nucleated on the same center. A simple statistical model presented herein predicts the universal fractal dimension 2.33 of such natural patterns grown through internal competition. The application to material patterns having constrained front propagation involving many components is obvious. [less ▲]

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See detailStatic and dynamic epidemics on looped chains and looped trees
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1996), 54(4), 3499-3507

The dynamic epidemic model [N. Vandewalle and M. Ausloos, J. Phys. A 29, 309 (1996)] considers the growth of a cluster in a medium containing a fraction x of mobile "particles" that are pushed by a ... [more ▼]

The dynamic epidemic model [N. Vandewalle and M. Ausloos, J. Phys. A 29, 309 (1996)] considers the growth of a cluster in a medium containing a fraction x of mobile "particles" that are pushed by a propagation front. This model is exactly solved here on various chains and trees that contain loops following an "evolution matrix" method. The exact value for the percolation threshold x(c) and the critical exponents are calculated for static and mobile particles, respectively. Surprisingly, the mobile character of the particles affects the values of the critical exponents on chains but not on trees. Thus there is a nonuniversal behavior for dynamic epidemics even on d=1 lattices. [less ▲]

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See detailTwo-component spreading phenomena: Why the geometry makes the criticality
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1996), 54(3), 3006-3008

We have numerically and theoretically investigated a simple model for two-component spreading phenomena in two different growth geometries (i.e., spreading confined in a half space and spreading in a free ... [more ▼]

We have numerically and theoretically investigated a simple model for two-component spreading phenomena in two different growth geometries (i.e., spreading confined in a half space and spreading in a free space). The criticality of the domain substructures unexpectedly depends on the considered geometry. This is understood by simple arguments of domain-wall particle diffusion and annihilation. We derive a relationship between the critical exponents chi and alpha for domain-wall spatial distributions in different geometries. The latter relationship is numerically verified in two, three and four dimensions. [less ▲]

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See detailSimulations of the kinetic growth of YBa2Cu307-delta grains
Cloots, Rudi ULg; Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Journal of Crystal Growth (1996), 166(1-4), 816-819

Using simple kinetic and geometric arguments, we have developed a computer model which simulates the growth of melt-textured YBa2Cu307-delta grains along the a-b planes. A chemical bonding anisotropy ... [more ▼]

Using simple kinetic and geometric arguments, we have developed a computer model which simulates the growth of melt-textured YBa2Cu307-delta grains along the a-b planes. A chemical bonding anisotropy along the main lattice directions is taken into account by the model. We have also considered the presence of Y2BaCu05 particles becoming trapped or not in the solidifying YBa2Cu307-delta front. The model simulates the fomation of 211 segregation patterns in the 123 matrix. An interpretation of these is attempted in terms of trapping anisotropy due to intrinsic crystallographic anisotropy. [less ▲]

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See detailExact solution of the dynamic epidemic model on the Bethe lattice
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physica A: Statistical Mechanics and its Applications (1996), 230(1-2), 1-10

The dynamic epidemic model considers the spreading of a cluster in a medium containing a fraction x of mobile particles which are pushed by the propagation front. This model is analytically studied on the ... [more ▼]

The dynamic epidemic model considers the spreading of a cluster in a medium containing a fraction x of mobile particles which are pushed by the propagation front. This model is analytically studied on the Bethe lattice for any branching rate z. We give the exact solution x(c)= (z(2) - 1)/z(2) for the percolation threshold. This is in contrast with the x(c) = (z - 1)/z result for static particles. Moreover, we calculate the critical exponents y = 1 and v = 1 characterizing respectively the divergence of the cluster mass and the correlation length at x(c). These exponents are found to be the same as for the case of static particles, i.e. for random percolation on the Bethe lattice. [less ▲]

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See detailDy2BaCu05 pattern formation in isothermally non-seeded melt-textured DyBa2Cu307-delta with 15 wt% Dy203 addition
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg; Mineur, N. et al

in Superconductor Science and Technology (1996), 9(8), 665-670

Optically polarized light micrographs of isothermally melt-textured DyBa2Cu307-delta compounds (with 15 wt% Dy203 addition) are presented to show evidence for pattern formation of DyBa2Cu305 particles in ... [more ▼]

Optically polarized light micrographs of isothermally melt-textured DyBa2Cu307-delta compounds (with 15 wt% Dy203 addition) are presented to show evidence for pattern formation of DyBa2Cu305 particles in DyBa2Cu307-delta grains. Different types of pattern were observed and were understood as arising from the crystal growth mechanism of 123 melt-textured materials. In an appendix, the case of patterns in magnetically textured samples is discussed. [less ▲]

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See detailInner patterns and front propagation of dynamic random impurity model
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physical Review Letters (1996), 77(3), 510-513

We investigate a simple model, the so-called dynamic epidemic model, for the evolution of an advancing interface through a medium containing mobile impurities. A short range repulsion between the front ... [more ▼]

We investigate a simple model, the so-called dynamic epidemic model, for the evolution of an advancing interface through a medium containing mobile impurities. A short range repulsion between the front and the impurities leads to an aggregation process along the front, and to the trapping of aggregates. The amplitude of this self-organization for collective impurity trapping is analyzed in two dimensions. A relationship exists between individual motion and the aggregation of particles. The results indicate that self-organizing features are different behind and on the front. [less ▲]

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See detailSpeciations and extinctions in a self-organizing critical model of tree-like evolution
Kramer, M.; Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Journal de Physique I (1996), 6(4), 599-606

We study analytically a simple model of a self-organized critical evolution. The model considers both extinction and speciation events leading to the growth of phylogenetic-like trees. Through a mean ... [more ▼]

We study analytically a simple model of a self-organized critical evolution. The model considers both extinction and speciation events leading to the growth of phylogenetic-like trees. Through a mean-field like theory, we study the evolution of the local configurations for the tree leaves. The fitness threshold, below which life activity takes place through avalanches of all sizes is calculated. The transition between speciating (evolving) and dead trees is obtained and is in agreement with numerical simulations. Moreover, this theoretical work suggests that the structure of the tree is strongly dependent on the extinction strength. [less ▲]

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See detailGrowth models with internal competition
Ausloos, Marcel ULg; Vandewalle, Nicolas ULg

in Acta Physica Polonica B (1996), 27(3), 737-746

Combined statistical physics and computation modelling give new instruments for the study of non-equilibrium systems. We briefly review generalized Eden and Diffusion-Limited Aggregation models as applied ... [more ▼]

Combined statistical physics and computation modelling give new instruments for the study of non-equilibrium systems. We briefly review generalized Eden and Diffusion-Limited Aggregation models as applied to spreading phenomena. We indicate the occurence of non-universal behaviors. [less ▲]

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See detailIn one step from KC1 to YBa2Cu3O7 crystal growth: Understanding of dendritic morphology in crystals
Ausloos, Marcel ULg; Vandewalle, Nicolas ULg; Cloots, Rudi ULg

in Philosophical Magazine Letters (1996), 73(3), 101-105

A dendritic morphology of crystals is often encountered in nature and seems to be a universal structure for systems grown from supersaturated solutions such as KC1 or NaC1. The observation of similar ... [more ▼]

A dendritic morphology of crystals is often encountered in nature and seems to be a universal structure for systems grown from supersaturated solutions such as KC1 or NaC1. The observation of similar dendrites in bulk superconducting YBa2Cu307 covalent material leads us to correlate the cases. From a simple new kinetics mechanism, the understanding of such a crystal-growth peculiarity is discussed. The kinetic mechanism condition for the occurrence of dendritic shapes of growing crystals from multiphase systems is given through a relationship between the velocity of the growth front and classical thermodynamical quantities. Some simulated systems illustrate the observed features. [less ▲]

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See detailThe screening of species in a Darwinistic tree-like model of evolution
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physica D: Nonlinear Phenomena (1996), 90(3), 262-270

Numerical and theoretical investigations of a tree-like model of self-organized critical evolution are presented. The screening of species from the evolution process is numerically studied. Critical ... [more ▼]

Numerical and theoretical investigations of a tree-like model of self-organized critical evolution are presented. The screening of species from the evolution process is numerically studied. Critical exponents for (i) the avalanche distribution, (ii) the correlation function, (iii) the order parameter and (iv) the fraction of unscreened species are given. A mean-field approximation is developed. This theory is quite accurate for describing the critical state but is not able to describe the transient regimes, i.e. the self-organized slowing down of both order parameter and fraction of unscreened species towards the critical state. [less ▲]

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See detailCriticality of trapping in a dynamic epidemic model
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Journal of Physics : A Mathematical & General (1996), 29(2), 309-316

We study a simple model mimicking the two-dimensional growth of a solid interface B through a liquid L in presence of particles A which are pushed by the advancing front. The model considers a short-range ... [more ▼]

We study a simple model mimicking the two-dimensional growth of a solid interface B through a liquid L in presence of particles A which are pushed by the advancing front. The model considers a short-range repulsive interaction between the particles and the advancing front (the so-called Uhlmann, Chalmers and Jackson mechanism). As particles are pushed by the advancing front, this leads to the formation of aggregates which are hindrances to the growth and which can be trapped leading to the formation of internal patterns. A transition between indefinitely growing clusters and frozen ones takes place for a critical particle fraction x(c) = 0.560 +/- 0.005 which is larger than the critical fraction of the corresponding epidemic model with static particles. At that critical threshold x, both percolating clusters and internal patterns are numerically found to be fractal with the same dimension D-f = 1.87 +/- 0.03 close to the classical percolation exponent 91/48. The correlation length exponent v is found to be v = 1.34 +/- 0.08 close to the classical percolation exponent 4/3. The criticality of the internal patterns is unexpected. [less ▲]

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See detailTwo dimensional self-organized transitions for propagation in random media
Ausloos, Marcel ULg; Vandewalle, Nicolas ULg; Cloots, Rudi ULg

in Czechoslovak Journal of Physics (1996), 46(Suppl. 4), 2277-2278

A simple so-called dynamic epidemic model, for the evolution of an advancing interface through a two-dimensional medium containing mobile impurities is investigated. A short range repulsion between the ... [more ▼]

A simple so-called dynamic epidemic model, for the evolution of an advancing interface through a two-dimensional medium containing mobile impurities is investigated. A short range repulsion between the front and the impurities leads to an aggregation process along the front, and to trapping of aggregates. The pattern of trapped impurities is found to be self-organized. The critical concentration of impurity for growth blocking on a square lattice is 0..56. The kinetics of this self-organization is analyzed and discussed. [less ▲]

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See detailThe robustness of self-organized criticality against extinctions in a tree-like model of evolution
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

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 ▲]

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