F. Tsobnang

Possible canonical distributions for finite systems with nonadditive energy

It is shown that a small system in thermodynamic equilibrium with a finite thermostat can have a q-exponential probability distribution which closely depends on the energy nonextensivity and the particle number of the thermostat. The distribution function will reduce to the exponential one at the thermodynamic limit. However, the nonextensivity of the system should not be neglected.

Reformulation of a stochastic action principle for irregular dynamics

A stochastic action principle for stochastic dynamics is revisited. We present first numerical diffusion experiments showing that the diffusion path probability depend exponentially on average Lagrangian action. This result is then used to derive an uncertainty measure defined in a way mimicking the heat or entropy in the first law of thermodynamics. It is shown that the path uncertainty (or path entropy) can be measured by the Shannon information and that the maximum entropy principle and the least action principle of classical mechanics can be unified into a concise form. It is argued that this action principle, hence the maximum entropy principle, is simply a consequence of the mechanical equilibrium condition extended to the case of stochastic dynamics.

Understanding Heavy Fermion from Generalized Statistics

Abstract Heavy electrons in superconducting materials are widely studied with the Kondo lattice t–J model. Numerical results have shown that the Fermi surface of these correlated particles undergoes a flattening effect according to the coupling degree J. This behaviour is not easy to understand from the theoretical point of view within standard Fermi–Dirac statistics and non-standard theories such as fractional exclusion statistics for anyons and Tsallis nonextensive statistics. The present work is an attempt to account for the heavy electron distribution within incomplete statistics (IS) which is developed for complex systems with interactions which make the statistics incomplete such that ∑i=1wpiq=1. The parameter q, when different from unity, characterizes the incompleteness of the statistics. It is shown that the correlated electrons can be described with the help of IS with q related to the coupling constant J in the context of Kondo model.

Study of ferromagnetic properties of molecular magnets based on aminonaphthalenesulfonic acid and aniline

Our interest is the study of magnetic properties on organic compounds. We have previously reported a new class of molecular magnets based on aminonaphthalenesulfonic acid and aniline. In this work we explore theoretically a simple model based on superposed aniline molecules and the goal is to determine the mechanism that stabilizes the parallel alignment of the spin component. Calculations were performed within the local density approximation (LDA). The results obtained show a high sensitivity of the magnetic coupling relative to translations and rotations of one aniline molecule with respect to the other. Distances between both NH units have also a great importance in the spin stabilization. These results are in good accordance with some synthesis difficulties.

Magnetometric analysis of a new class of copolymer magnets based on aminonaphthalenesulfonic acids

Abstract Recently, a new class of copolymer based on aminonaphthalenesulfonic acid and aniline were synthesized. By means of a VSM, an AGFM and a SQUID, it was shown that these materials are ferromagnetic, above room temperature. These results, consistent with temperature dependent EPR tests and magneto-optic experiments, support the achievement of high temperature magnets ( T

Ab-initio modelling of stainless steels seizing

Abstract This work is a contribution towards the atomic-scale understanding of the phenomenon of seizing. This phenomenon often takes place when two surfaces of stainless steels are in contact as in the example of a screw and a nut. The main goal is to determine the influence of specific elements (such as Ni or Cr) on the adhesive energy of interfaces involved in seizing. This is achieved by using density functional theory total energy calculations performed on Fe/Fe and Fe/Fe + Ni or Cr surface models. It is found that chromium in iron surfaces causes an increase of the adhesive energy between the modelled stainless steel surfaces whereas nickel does not have any significant effect. These results are presented and discussed with respect to experimental and computational approximations.

Aniline and aminonaphthalenesulfonic acid-based materials: optimization of magnetic properties at ambient temperature

Computational and experimental studies on aniline and aminonaphthalenesulfonic acid-based materials were carried out in order to solve questions related to the reproducibility of results, influence of pollution and correlation between magnetism and chemistry encountered in previous works on this type of materials. The strictly organic origin of the magnetic properties as well as its potential applications will be discussed.

Magnetic properties of molecular systems: A nonlocal spin density functional study

A new class of molecular magnets based on aniline and aminonaphthalene sulfonic acid was previously reported. In the present work, a nonlocal density functional theory study was performed on simple molecular models, to determine the mechanism that stabilizes the parallel alignment of the spin component. The role played by transition metals on the magnetic properties of this kind of systems was also studied within this approximation. The results obtained show a high sensitivity of the magnetic coupling on the molecular geometry as well as on the nature and oxidation state of the metallic atom.