Engineering multiferroism in CaMnO3_3

From first-principles calculations, we investigate the structural instabilities of CaMnO$_3$. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground-state, the cubic perovskite structure of CaMnO$_3$ also exhibit a weak ferroelectric instability. Although ferroelectricity is suppressed by antiferrodistortive oxygen motions, we show that it can be favored using strain or chemical engineering in order to make CaMnO$_3$ multiferroic. We finally highlight that the FE instability of CaMnO$_3$ is Mn-dominated. This illustrates that, contrary to the common believe, ferroelectricity and magnetism are not necessarily exclusive but can be driven by the same cation

Strain-induced ferroelectricity in simple rocksalt binary oxides

The alkaline earth binary oxides adopt a simple rocksalt structure and form an important family of compounds because of their large presence in the earth's mantle and their potential use in microelectronic devices. In comparison to the class of multifunctional ferroelectric perovskite oxides, however, their practical applications remain limited and the emergence of ferroelectricity and related functional properties in simple binary oxides seems so unlikely that it was never previously considered. Here, we show using first-principles density functional calculations that ferroelectricity can be easily induced in simple alkaline earth binary oxides such as barium oxide (BaO) using appropriate epitaxial strains. Going beyond the fundamental discovery, we show that the functional properties (polarization, dielectric constant and piezoelectric response) of such strained binary oxides are comparable in magnitude to those of typical ferroelectric perovskite oxides, so making them of direct interest for applications. Finally, we show that magnetic binary oxides such as EuO, with the same rocksalt structure, behave similarly to the alkaline earth oxides, suggesting a route to new multiferroics combining ferroelectric and magnetic properties

First-principle calculation of the dielectric and dynamical properties of orthorhombic CaMnO3_{3}

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO$_3$ have been computed from first principles using a density functional theory approach within the local spin density approximation. The theoretical structural parameters are in good agreement with experiment. The full set of infrared and Raman zone-center phonons is reported and compared to experimental data. It is shown that coherently with the anomalous Born effective charges and the presence of low frequency polar modes, the static dielectric constant is very large and highly anisotropic.Comment: Sumbitted to Phys. Rev.

Generating functions for generating trees

Certain families of combinatorial objects admit recursive descriptions in terms of generating trees: each node of the tree corresponds to an object, and the branch leading to the node encodes the choices made in the construction of the object. Generating trees lead to a fast computation of enumeration sequences (sometimes, to explicit formulae as well) and provide efficient random generation algorithms. We investigate the links between the structural properties of the rewriting rules defining such trees and the rationality, algebraicity, or transcendence of the corresponding generating function.Comment: This article corresponds, up to minor typo corrections, to the article submitted to Discrete Mathematics (Elsevier) in Nov. 1999, and published in its vol. 246(1-3), March 2002, pp. 29-5

First-principles study of the ferroelectric Aurivillius phase Bi2WO6

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first principles calculations of the dielectric and dynamical properties on two possible high-temperature paraelectic structures: the monoclinic phase of A2/m symmetry observed experimentally and the tetragonal phase of I4/mmm symmetry, common to most Aurivillius phase components. Both paraelectric structures exhibits various unstable modes, which after their condensation bring the system toward more stable structures of lower symmetry. The calculations confirms that, starting from the paraelectric A2/m phase at high temperature, the system must undergo a reconstructive transition to reach the P2_1ab ferroelectric ground state.Comment: added Appendix and two table

Methane on the Rise-Again

International audienc

Défis 2025

International audienceNew paradigms, languages, modeling, verification, testing approaches and new tools in the field of programming and software should be created in the next 10 years, whether to make life easier for designers and maintainers of computer systems, to model and reliable software or to anticipate technological change. This text summarizes the challenges in the Programming and Software Engineering field on the horizon 2025. This work has been presented and discussed during the national days of the Research Group on Programming and Software Engineering in June 2014 and in September 2014 in Paris.De nouveaux paradigmes, de nouveaux langages, de nouvelles approches de modélisation, de vérification, de tests et de nouveaux outils dans le domaine de la programmation et du logiciel devraient voir le jour dans les dix ans à venir, que ce soit pour faciliter la vie des concepteurs et mainteneurs de systèmes informatiques, pour modéliser et fiabiliser les logiciels ou encore pour devancer l’évolution technologique. Ce texte résume les travaux menés sur les défis du Génie de la Programmation et du Logiciel à l’horizon 2025. Ces travaux ont été l’occasion de présentations et d’échanges lors des journées nationales du Groupe de Recherche Génie de la Programmation et du Logiciel en juin 2014 et lors d’une journée en septembre 2014 à Paris

A remark on Alexander polynomial criterion for bi-orderability of fibered 3-manifold groups

We observe that Clay-Rolfsen's obstruction of bi-orderability, which uses the classical Alexander polynomial, is not strengthened by using the twisted Alexander polynomials for finite representations unlike many known applications of the Alexander polynomial. This is shown by studying the maximal ordered abelian quotient of bi-ordered groups.Comment: 8 pages, no figur