Phase transitions in generalized chiral or Stiefel's models

We study the phase transition in generalized chiral or Stiefel's models using Monte Carlo simulations. These models are characterized by a breakdown of symmetry O(N)/O(N-P). We show that the phase transition is clearly first order for N >= 3 when P=N and P=N-1, contrary to predictions based on the Renormalization Group in 4-\epsilon expansion but in agreement with a recent non perturbative Renormalization Group approach.Comment: 9 pages, 8 figure

First and second order transition of frustrated Heisenberg spin systems

Starting from the hypothesis of a second order transition we have studied modifications of the original Heisenberg antiferromagnet on a stacked triangular lattice (STA-model) by the Monte Carlo technique. The change is a local constraint restricting the spins at the corners of selected triangles to add up to zero without stopping them from moving freely (STAR-model). We have studied also the closely related dihedral and trihedral models which can be classified as Stiefel models. We have found indications of a first order transition for all three modified models instead of a universal critical behavior. This is in accordance with the renormalization group investigations but disagrees with the Monte Carlo simulations of the original STA-model favoring a new universality class. For the corresponding x-y antiferromagnet studied before, the second order nature of the transition could also not be confirmed.Comment: 31 pages, 13 figures, to be published in Euro. J. Phys.

Fluctuations and Defects in Lamellar Stacks of Amphiphilic Bilayers

We review recent molecular dynamics simulations of thermally activated undulations and defects in the lamellar $L_\alpha$ phase of a binary amphiphile-solvent mixture, using an idealized molecular coarse-grained model: Solvent particles are represented by beads, and amphiphiles by bead-and-spring tetramers. We find that our results are in excellent agreement with the predictions of simple mesoscopic theories: An effective interface model for the undulations, and a line tension model for the (pore) defects. We calculate the binding rigidity and the compressibility modulus of the lamellar stack as well as the line tension of the pore rim. Finally, we discuss implications for polymer-membrane systems.Comment: to appear in Computer Physics Communications (2005

Fabricating management practices : "Responsible Care" and Corporate Social Responsibility.

Purpose this study focuses on the policy of Corporate Social Responsibility (CSR) launched in the chemical industry in the 1980s and known as “Responsible Care”. The debate surrounding this issue prompts us to question the ever-changing nature of this policy and the way to measure the performance achieved. Methodology Our findings are drawn from analysis of a double set of data including a longitudinal survey and a current case study. Blending these two data sets allows a better understanding of the ongoing building process of “Responsible Care” and, more broadly, of corporate social responsibility. Findings this paper asserts that, contrary to the common wisdom developed in research, companies do not simply react to stakeholder pressure. Companies autonomously develop ways to protect their environment and so contribute to changing Society's expectations. Thus, performance cannot be read without a dynamic perspective in mind. Research limitations/implications Our findings lead us to reconsider the assessment of companies' sustainable performances by taking into account the fabricating process of sustainable activities. The main limitation of this research stems from the single unit of analysis considered. Broader studies will be necessary to enrich our understanding of corporate policies. Originality/value of the paper Our paper stands apart from the traditional view of organizations as cynical actors and attempts to provide a more complex picture of the behaviours observed.Performance management; Performance measurement; “Responsible Care”; Corporate Social Responsibility;

Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study

Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14N/ 15N ratio is difficult to measure directly for carbon containing molecules. Aims. We want to check the underlying hypothesis that the 13C/ 12C ratio of nitriles and isonitriles is equal to the elemental value via a chemical time dependent gas phase chemical model. Methods. We have built a chemical network containing D, 13C and 15N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for 2 different physical conditions corresponding respectively to a moderately dense cloud in an early evolutionary stage and a dense depleted pre-stellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in N2H+ is found to be close to the elemental value, in contrast to previous models which obtain a significant enrichment, as we found that the fractionation reaction between 15N and N2H+ has a barrier in the entrance channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in L1544 cannot be reproduced in our model. Finally we find that nitriles and isonitriles are in fact significantly depleted in 13C, questioning previous interpretations of observed C15N, HC15N and H15NC abundances from 13C containing isotopologues.Comment: 21 pages, 9 figures in the text, 3 Figures in the appendices. 7 tables in the text, 4 tables in the appendices. Accepted for publication by Astronomy Astrophysic