Diffusion in a continuum model of self-propelled particles with alignment interaction

In this paper, we provide the $O(\epsilon)$ corrections to the hydrodynamic model derived by Degond and Motsch from a kinetic version of the model by Vicsek & coauthors describing flocking biological agents. The parameter $\epsilon$ stands for the ratio of the microscopic to the macroscopic scales. The $O(\epsilon)$ corrected model involves diffusion terms in both the mass and velocity equations as well as terms which are quadratic functions of the first order derivatives of the density and velocity. The derivation method is based on the standard Chapman-Enskog theory, but is significantly more complex than usual due to both the non-isotropy of the fluid and the lack of momentum conservation

Les quinquas sont les plus touchés : Troubles Musculo-Squelettiques : A quand une prévention durable ?

Les salariés de plus de 50 ans sont particulièrement concernés par les troubles musculo-squelettiques. C\u27est ce que montre la surveillance épidémiologique menée dans les Pays-de-la-Loire sous l\u27égide de l\u27Institut de veille sanitaire

Troubles musculo-squelettiques d’origine professionnelle en France. Où en est-on aujourd’hui ?

Work-related musculoskeletal disorders in France. What is the situation today? Musculoskeletal disorders (MSDs) represent one of the most worrying issues in occupational health today. They are the leading cause of morbidity at work, morbidity widely underestimated by the statistics of workers’ compensation claims for occupational diseases. It is not a French specific phenomenon. In 2005, the most often reported problem linked to work by European Union workers are MSDs (backache and muscular pains). MSDs are also the first cause of compensated occupational diseases in several European countries. The epidemiological surveillance program for work-related MSDs, implemented in 2002 in France’s Pays de la Loire region, has allowed a better description of the current increased number of MSDs on a population scale, an assessment of the proportion of cases attributable to work exposure, and a better knowledge of the medical and professional evolution of patients suffering from MSDs. The data of Pays de la Loire region will be enhanced in the near future by those from Provence-Alpes-Côte d’Azur and Île de France regions. Mobilisation around this question has been increasing considerably these last few years, and should be maintained in order to face this occupational health challenge

Electron Addition Spectrum in the Supersymmetric t-J Model with Inverse-Square Interaction

The electron addition spectrum A^+(k,omega) is obtained analytically for the one-dimensional (1D) supersymmetric t-J model with 1/r^2 interaction. The result is obtained first for a small-sized system and its validity is checked against the numerical calculation. Then the general expression is found which is valid for arbitrary size of the system. The thermodynamic limit of A^+(k,omega) has a simple analytic form with contributions from one spinon, one holon and one antiholon all of which obey fractional statistics. The upper edge of A^+(k,omega) in the (k,omega) plane includes a delta-function peak which reduces to that of the single-electron band in the low-density limit.Comment: 5 pages, 1 figure, accepted for publication in Phys. Rev. Let

Finite Size Polyelectrolyte Bundles at Thermodynamic Equilibrium

We present the results of extensive computer simulations performed on solutions of monodisperse charged rod-like polyelectrolytes in the presence of trivalent counterions. To overcome energy barriers we used a combination of parallel tempering and hybrid Monte Carlo techniques. Our results show that for small values of the electrostatic interaction the solution mostly consists of dispersed single rods. The potential of mean force between the polyelectrolyte monomers yields an attractive interaction at short distances. For a range of larger values of the Bjerrum length, we find finite size polyelectrolyte bundles at thermodynamic equilibrium. Further increase of the Bjerrum length eventually leads to phase separation and precipitation. We discuss the origin of the observed thermodynamic stability of the finite size aggregates

Combinatorial interpretation of Haldane-Wu fractional exclusion statistics

Assuming that the maximal allowed number of identical particles in state is an integer parameter, q, we derive the statistical weight and analyze the associated equation which defines the statistical distribution. The derived distribution covers Fermi-Dirac and Bose-Einstein ones in the particular cases q = 1 and q -> infinity (n_i/q -> 1), respectively. We show that the derived statistical weight provides a natural combinatorial interpretation of Haldane-Wu fractional exclusion statistics, and present exact solutions of the distribution equation.Comment: 8 pages, 2 eps-figure

Spin-Charge Separation at Finite Temperature in the Supersymmetric t-J Model with Long-Range Interactions

Thermodynamics is derived rigorously for the 1D supersymmetric {\it t-J} model and its SU($K,1$) generalization with inverse-square exchange. The system at low temperature is described in terms of spinons, antispinons, holons and antiholons obeying fractional statistics. They are all free and make the spin susceptibility independent of electron density, and the charge susceptibility independent of magnetization. Thermal spin excitations responsible for the entropy of the SU($K,1$) model are ascribed to free para-fermions of order $K-1$.Comment: 10 pages, REVTE