Paulette Clippe

Evolution of economic entities under heterogeneous political/environmental conditions within a Bak–Sneppen-like dynamics

A model for economic behavior, under heterogeneous spatial economic conditions is developed. The role of selection pressure in a Bak–Sneppen-like dynamics with entity diffusion on a lattice is studied by Monte-Carlo simulation taking into account business rule(s), like enterprise–enterprise short range location “interaction”(s), business plan(s) through spin-offs or merging and enterprise survival evolution law(s). It is numerically found that the model leads to a sort of phase transition for the fitness gap as a function of the selection pressure, at a “critical selection pressure”.

Model of macroeconomic evolution in stable regionally dependent economic fields

We develop a model for the evolution of economic entities within a geographical type of framework. On a square symmetry lattice made of three (economic) regions, firms, described by a scalar fitness, are allowed to move, adapt, merge or create spin-offs under predetermined rules, in a space- and time-dependent economic environment. We only consider here one timely variation of the “external economic field condition”. For the firm fitness evolution, we take into account a constraint such that the disappearance of a firm modifies the fitness of nearest-neighboring ones, as in Bak–Sneppen population fitness evolution model. The concentration of firms, the averaged fitness, the regional distribution of firms, and fitness for different time moments, the number of collapsed, merged and new firms as a function of time have been recorded and are discussed. Also the asymptotic values of the number of firms present in the three regions together with their average fitness, as well as the number of respective births and collapses in the three regions are examined. It appears that a sort of critical selection pressure exists. A power-law dependence, signature of self-critical organization is seen in the birth and collapse asymptotic values for a high selection pressure only. A lack of self-organization is also seen at region borders.

Simple model for the dynamics of correlations in the evolution of economic entities under varying economic conditions

From some observations on economic behaviors, in particular changing economic conditions with time and space, we develop a very simple model for the evolution of economic entities within a geographical type of framework. We raise a few questions and attempt to investigate whether some of them can be tackled by our model. Several cases of interest are reported. It is found that the model even in its simple form can lead to a large variety of situations, including: delocalization and cycles, but also pre-chaotic behavior.

A (reactive) lattice-gas approach to economic cycles

A microscopic approach to macroeconomic features is intended. A model for macroeconomic behavior under heterogeneous spatial economic conditions is reviewed. A birth–death lattice gas model taking into account the influence of an economic environment on the fitness and concentration evolution of economic entities is numerically and analytically examined. The reaction–diffusion model can also be mapped onto a high-order logistic map. The role of the selection pressure along various dynamics with entity diffusion on a square symmetry lattice has been studied by Monte–Carlo simulation. The model leads to a sort of phase transition for the fitness gap as a function of the selection pressure and to cycles. The control parameter is a (scalar) “business plan”. The business plan(s) allows for spin-offs or merging and enterprise survival evolution law(s), whence bifurcations, cycles and chaotic behavior.

Probing conductivity fluctuations in high critical temperature superconductors

Abstract We comment and discuss published data on the electrical resistivity of high temperature superconductors in order to extract the dimensionality of superconductivity “critical” fluctuations. We present arguments which indicate that observation of three temperature regimes is indeed possible: a scaling regime close to T c and two regimes in the mean field region. In each regime, dominant scattering mechanisms can be different.

Enantiomeric phase separation in a lattice gas model: Guggenheim approximation

We consider a lattice gas in which the two enantiomeric forms of a tetrahedral molecule, consisting of a central carbon atom bonded to four different groups A, B, G, and H, are adsorbed onto a triangular lattice, such that the carbon atom is above a lattice site, the three bonds to A, B, and G point toward neighboring lattice sites, and the bond to H points perpendicular to and away from the plane of the lattice. For a certain choice of intermolecular interactions, such as may exist between the zwitterion forms of an amino acid, the phase diagram was investigated using a Guggenheim approximation with two order parameters. Enantiomeric phase separation into two symmetric condensed phases occurs at low temperatures. These condensed phases become a single racemic condensed phase at a critical line, and they are in equilibrium with a racemic gas phase along a line of triple points. These two lines coincide at a critical endpoint. The racemic condensed and gas phases are in equilibrium along a two phase coexis...

A lattice gas model for enantiomeric phase separation

There are two nonsuperimposable mirror images (enantiomers) of a tetrahedral molecule consisting of a central carbon atom attached to four different groups, A, B, G, and H. We consider a lattice gas in which the two enantiomeric forms, d and l, are adsorbed onto a triangular lattice, such that the carbon atom is above a lattice site, the three bonds to A, B, and G are pointing toward neighboring lattice sites, and the bond to H points perpendicular to and away from the plane of the lattice. For a certain range of intermolecular interactions (such as may exist between the zwitterion forms of an amino acid) we prove, for a system which is equimolar in each enantiomer, that enantiomeric phase separation occurs at sufficiently low temperature and sufficiently high chemical potential. The phase diagram and critical point for the system are investigated using a first‐order Guggenheim approximation.

High temperature crossover in paraconductivity of granular Y1Ba2Cu3O7-y

We have examined the deviation from linearity of the temperature dependence of the electrical resistivity of a YBaCuO ceramics. We have observed at high temprature a crossover behavior toward a logarithmic temperature dependence at lower temperature. It seems relevant to attribute the origin of such a term to a pair breaking mechanism contribution, thus to give a lower bound to the existence of superconductivity fluctuations onset temperature. Our findings also implies that carriers are bosons aboveTc.

Zwanzig’s perturbation method applied to the thermodynamic properties of dense noble gases

Zwanzig’s perturbation method leads to power series in 1/T for the thermodynamic properties and the compressibility of simple gases or liquids. To check the convergence of these series, the precision of limited 1/T expansions is obtained by comparison with the experimental results for argon and xenon. The expansion coefficients are determined by minimizing the relative mean square deviations. The expansions for the thermodynamic functions have a remarkable precision, even when restricted to three terms (two for the internal energy). The relative deviation is typically of the order of 10−4 to 10−3 for the whole range explored experimentally (from normal conditions to rather far in the liquid state). The coefficients obtained by this procedure are functions of the density. The dependence is very smooth. It is shown that this dependence can be approximated by an expansion in powers of the density, restricted to two or three terms. Analytical expressions are obtained for these terms by means of a cluster type analysis. Three of these coefficients, the expression of which is rather simple, are calculated using the Lennard‐Jones potential as an a p r i o r i potential, and compared to the results of the fitting. For each coefficient the comparison gives a value of the hard‐sphere diameter which is an arbitrary parameter in Zwanzig’s method. The three values obtained in this way are in reasonable agreement.

Superconductivity Fluctuation Effects on Electrical and Thermal Transport Phenomena. H=0,T>TcI

The shape of the electrical resistance versus temperature curve in the vicinity of a superconductivity transition depends on the amount of superconducting materials (among other things) in the sample. The same is true for the thermoelectric power. In both cases, this amount fluctuates according to various theoretical laws. These effects are highly visible in high critical temperature superconductors. The data analysis also differs from authors to authors. We briefly review old and present theories of superconductivity fluctuation effects on transport phenomena with emphasis on the approach of the critical temperature from above. We discuss the various ways of analyzing the data on the electrical resistivity and on the thermoelectrical power. We distinguish between data on quasi single crystals, on polycrystalline materials and on films for various high critical temperature superconductors. We show the range of validity of theoretical laws, and see that homogeneous or fractal behaviors can be distinguished.