References of "Vandewalle, Nicolas"
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See detailMulti-affine analysis of typical currency exchange rates
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in European Physical Journal B -- Condensed Matter (1998), 4(2), 257-261

For foreign currency exchange rates; multi-affine analysis can put quantitatively into evidence the differences between correlated (daily closing market) values and random walks in time dependent data ... [more ▼]

For foreign currency exchange rates; multi-affine analysis can put quantitatively into evidence the differences between correlated (daily closing market) values and random walks in time dependent data. The H(q) spectrum is presented and discussed here for the USD/DEM and JPY/USD exchange rates. The time-evolution of these ratios is found to be multi-affine. The h(gamma)-curve describing the hierarchy of exponents is numerically obtained. Our findings suggest that the modelling of exchange rate time-evolution from day to day is possible within the framework of modern statistical physics and related to models of turbulence in the physics of fluids. Finally, we argue that there is a multiplicity of information levels in the foreign exchange market such that the "efficient market theory" is a crude oversimplification indeed. [less ▲]

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See detailThe crash of October 1987 seen as a phase transition: amplitude and universality
Vandewalle, Nicolas ULg; Boveroux, P.; Minguet, A. et al

in Physica A: Statistical Mechanics and its Applications (1998), 255(1-2), 201-210

We analyze the evolution of several financial indices before the crash of October 1987. The amplitude of the crash varies from one index to another. However, assuming that the crash is similar to a phase ... [more ▼]

We analyze the evolution of several financial indices before the crash of October 1987. The amplitude of the crash varies from one index to another. However, assuming that the crash is similar to a phase transition and particularly to a specific heat jump, we find that the crash amplitude can be well estimated by assuming a simple background which differs from market to market. We show that the divergence near the crash event is logarithmic and extends between 2 weeks and 4 years before the october 1987 crash on both S&P500 and Dow Jones indices. The behavior is like that found for the d = 2 Ising model specific heat. The latter result is in contrast to previous works which have considered a power law behavior of the index near the crash. Finally, we confirm the presence of log-periodic oscillations and discuss briefly their origin. [less ▲]

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See detailEffect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 superconducting materials
Cloots, Rudi ULg; Robertz, B.; Auguste, F. et al

in Materials Science & Engineering : B (1998), 53(1-2), 154-158

The effect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 have been investigated. The distribution of the 211 particles is strongly modified by the presence of ... [more ▼]

The effect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 have been investigated. The distribution of the 211 particles is strongly modified by the presence of such an additive. In fact, BaZrO3 is effective in reducing the size of the 211 particles. As a consequence, a strong interaction in terms of repulsion takes place between the 123 growth front and the 211 particles which are partially dissolved in the reactive liquid phase. New types of patterns are also reported in the microstructure of such materials and are related to the presence of BaZrO3 in the melt. [less ▲]

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See detailThe non-trivial dispersion of Y2BaCuO5 particles trapped in the YBa2Cu3O7-x crystal matrix
Vandewalle, Nicolas ULg; Pirard, E.; Cloots, Rudi ULg et al

in Philosophical Magazine Letters (1998), 77(5), 301-306

Micrographs of melt-textured YBa2Cu3O7-x/Y2BaCuO5 (123/211) have been digitized. Through image analysis, we point out that the dispersion of the 211 particles is highly inhomogeneous in the melt and in ... [more ▼]

Micrographs of melt-textured YBa2Cu3O7-x/Y2BaCuO5 (123/211) have been digitized. Through image analysis, we point out that the dispersion of the 211 particles is highly inhomogeneous in the melt and in the 123 crystal matrix. Indeed, free 211 particle regions and 211 aggregates are observed. The distribution laws are given. Moreover, the dispersions of 211 particles in the melt and those trapped in the crystal matrix are quite different. We point out that this difference cannot be explained solely by the partial dissolution of the 211 particles during the growth process. [less ▲]

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See detailThe Nd-123 superconducting system: From single crystal to top-seeded large grain. Physical and chemical parameters influence
Auguste, F.; Vandewalle, Nicolas ULg; Ausloos, Marcel ULg et al

in Applied Superconductivity (1998), 6(2-5), 77-85

Nd-123 signle crystals have been produced via the Aux method in alumina crucibles by controlling the chemical composition of the flux. For so doing, quenched materials have been prepared and analyzed in ... [more ▼]

Nd-123 signle crystals have been produced via the Aux method in alumina crucibles by controlling the chemical composition of the flux. For so doing, quenched materials have been prepared and analyzed in order to get information about the optimum barium: copper ratio for an unchanged neodymium concentration. Non corrosive fluxes with a low viscosity have been put into evidence and can thus be used for single crystal production. Experiments have been performed with three different chemical compositions of the Aux and by considering various amounts of neodymium oxide for each system. The single crystals have been analyzed by energy dispersive X-ray microanalysis and used for the synthesis of large Dy-123 single-domains? [less ▲]

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See detailLattice gas model of gradual evolution
Ausloos, Marcel ULg; Mroz, I.; Pekalski, A. et al

in Physica A: Statistical Mechanics and its Applications (1998), 248(1-2), 155-164

A simple dynamical model is presented for describing the gradual evolution of a variable number of species. The system is studied through Monte Carlo simulations using a lattice gas formalism. Each ... [more ▼]

A simple dynamical model is presented for describing the gradual evolution of a variable number of species. The system is studied through Monte Carlo simulations using a lattice gas formalism. Each species is characterized by a single, scalar parameter ("adaptation") which is changed, within limits depending on the adaptation itself, at each time step. There are two independent mechanisms for removing a species from the system and one for creating a new species. We find that, regardless of the initial concentration of species, the system always reaches the same final state, characterized by the same concentration and the same average adaptation. The system is not homogeneous and contains species with different values of adaptation. The better adapted ones are found to form more symmetrical spatial patterns. Behavior similar to the one determined in the present model has been found in the evolving ecological and biological systems. [less ▲]

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See detailSelf-organized criticality can emerge even if the range of interactions is infinite
Vandewalle, Nicolas ULg; Van Puyvelde, H.; Ausloos, Marcel ULg

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1998), 57(1), 1167-1170

We have investigated a model of tree growth in order to show that self-organized criticality (SOC) can emerge even if perturbations due to long-range interactions are propagating throughout the whole ... [more ▼]

We have investigated a model of tree growth in order to show that self-organized criticality (SOC) can emerge even if perturbations due to long-range interactions are propagating throughout the whole system. The interaction potential is assumed to be a power d(-alpha) of the distance d measured along the tree between branch extremities. A transition occurs for alpha(c)=1 according to simulation results between SOC (alpha>alpha(c) and non-SOC (alpha<alpha(c)) regimes. A theoretical treatment supports the idea that the transition occurs when the exponent a is equivalent to the fractal dimension D-f(S) of the backbone (skeleton) of the tree. [less ▲]

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See detailThe isothermal melt-texturing of bulk YBa2Cu3O7-x ceramics: from cooking to modelling
Vandewalle, Nicolas ULg

in Superconductor Science and Technology (1998), 11(1), 35-43

Various techniques for the synthesis of bulk YBa2Cu3O7-x ceramics are reviewed. The microstructure of melt-textured YBa2Cu3O7-x compounds is described. In contrast to the relatively simple processing of ... [more ▼]

Various techniques for the synthesis of bulk YBa2Cu3O7-x ceramics are reviewed. The microstructure of melt-textured YBa2Cu3O7-x compounds is described. In contrast to the relatively simple processing of such materials, the complexity of the microstructure at a mesoscopic scale is underlined. A model describing the various features of the microstructure of YBa2Cu3O7-x compounds is described. The obtained results are discussed for various initial conditions and are favourably comparable with experimental findings. [less ▲]

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See detailCoherent and random sequences in financial fluctuations
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physica A: Statistical Mechanics and its Applications (1997), 246(3-4), 454-459

The detrended fluctuation analysis (DFA) is used to sort out temporal correlations in financial data. Its usefulness for the investigations of long-range power-law correlations in economic sequences is ... [more ▼]

The detrended fluctuation analysis (DFA) is used to sort out temporal correlations in financial data. Its usefulness for the investigations of long-range power-law correlations in economic sequences is shown. Our findings of persistent and antipersistent sequences are surprisingly similar to those for DNA sequences which appeared as a mosaic of coding and non-coding patches. [less ▲]

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See detailDirectional solidification by appropriate chemically active single crystal seed: An alternative way of generating large superconducting 123 single domain
Cloots, Rudi ULg; Auguste, F. X.; Rulmont, André ULg et al

in Journal of Materials Research (1997), 12(12), 3199-3202

A Dy2O3 single crystal has been used as a seed for the growth of isothermally melt-textured DY-123 material. The nucleation-controlled step has been observed to be related to the heterogeneous nucleation ... [more ▼]

A Dy2O3 single crystal has been used as a seed for the growth of isothermally melt-textured DY-123 material. The nucleation-controlled step has been observed to be related to the heterogeneous nucleation of 211 particles at the surface of the dysprosium oxide single crystal. The subsequent growth mode seems to be controlled by a high concentration gradient of dysprosium in the liquid phase. This leads to a directional solidification process of the 123 phase. The size of the 211 particles seems to decrease as the distance from the dysprosium oxide single crystal increases. [less ▲]

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See detailThe boundary of "life" for a self-organized critical evolution: the role of the interaction range
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg

in Physica A: Statistical Mechanics and its Applications (1997), 245(3-4), 494-502

The dynamics of a tree-like evolution is investigated as a function of the range ii of the interactions between competing entities which are located at the extremities of the branches. Speciation ... [more ▼]

The dynamics of a tree-like evolution is investigated as a function of the range ii of the interactions between competing entities which are located at the extremities of the branches. Speciation (branching) events are supposed to be driven by extremal dynamics. Extinction events are allowed and controlled by a parameter r. a transition between self-organized critical and frozen evolution occurs at some well-defined critical value r(c)(k). Surprisingly, the critical r(c) value behaves as a power of the range k(r(c) similar to k(-delta)) with an exponent S = -0.46+/-0.03. Moreover, the asymptotic case k = +infinity is herein exactly solved. The dynamics for k = +infinity is not critical and does not present any transition in contrast with finite k cases. [less ▲]

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See detailFormation of nick instabilities due to particle clustering along crystal interfaces
Vandewalle, Nicolas ULg; Ausloos, Marcel ULg; Cloots, Rudi ULg

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1997), 56(4), 4042-4047

The kinetic growth of crystal grains is simulated on a square lattice in the presence of mobile particles. The model is based on the Eden and the dynamic epidemic models. Trapping of the particles by the ... [more ▼]

The kinetic growth of crystal grains is simulated on a square lattice in the presence of mobile particles. The model is based on the Eden and the dynamic epidemic models. Trapping of the particles by the advancing front and a short-range repulsion of the particles by the front are both allowed. The grain boundary morphology is studied through the kink density concept and the spatial distribution of particles is discussed. Indeed, the clustering of particles along the crystal edges is found to induce the formation of so-called nick instabilities at the liquid-solid interface. The particles, when trapped near such instabilities, occur as oblique filamentary structures in the crystal matrix, just like river networks. These findings agree with recent experimental observations. The numerical laws are also explained by theoretical arguments based on different characteristic lengths. [less ▲]

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See detailPhysicochemical causes for the microstructure of melt-textured YBa2Cu3O7-delta/Y2BaCuO5 composites
Vandewalle, Nicolas ULg; Cloots, Rudi ULg; Ausloos, Marcel ULg

in Superconductor Science and Technology (1997), 10(3), 123-133

We report semiquantitative results for a physicochemical model intended to describe the multigrain growth of YBa2Cu3O7-deltaN2BaCuO5 (123/211) composites under isothermal undercooling. This model takes ... [more ▼]

We report semiquantitative results for a physicochemical model intended to describe the multigrain growth of YBa2Cu3O7-deltaN2BaCuO5 (123/211) composites under isothermal undercooling. This model takes into account various ingredients supposed to be controlling the growth of such superconducting compounds, i.e. the presence of secondary phases. Moreover, a possible dynamical interaction between the growing 123 grains and the 211 solid particles is also included. The numerical investigations were performed for a model bivariate-like 211 particle distribution. The complete chemical dissolution of the small particles is allowed but the large ones are only partially dissolved in the first step which can be followed when chemically possible by a complete dissolution. The model is restricted to a two-dimensional square lattice. Various microstructural morphologies are obtained as a function of the initial composition of the melt and the initial size distributions of the 211 particles. Results are in quite good agreement with experimental observations. The quantity of 123 phase is predicted depending on the initial conditions. The optimal situation when a minimum fraction of liquid phase segregates at the gain boundaries is found to occur for 20 % excess of 211 phase in the initial melt. [less ▲]

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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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