Jahn-Teller, polarity and insulator-to-metal transition in BiMnO3 at high pressure

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3.Comment: 5 pages, 3 figures + supplemental material included (3 pages, 1 figure, 3 tables

High-pressure phase transitions in BiFeO3: hydrostatic vs. non-hydrostatic conditions

We report high-pressure x-ray diffraction experiments on BiFeO3 (BFO) single crystals in diamond-anvil cells up to 14 GPa. Two data sets are compared, one in hydrostatic conditions, with helium used as pressure-transmitting medium, and the other in non-hydrostatic conditions, with silicon oil as pressure-transmitting medium. It is shown that the crystal undergoes different phase transitions in the two cases, highlighting the high sensitivity of BFO to non-hydrostatic stress. Consequences for the interpretation of high-pressure structural studies are discussed.Comment: 6 pages, 4 figure

Multiple high-pressure phase transitions in BiFeO3

We investigate the high-pressure phase transitions in BiFeO3 by single crystal and powder x-ray diffraction, as well as single crystal Raman spectroscopy. Six phase transitions are reported in the 0-60 GPa range. At low pressures, up to 15 GPa, 4 transitions are evidenced at 4, 5, 7 and 11 GPa. In this range, the crystals display large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The non polar Pnma phase remains stable over a large pressure range between 11 and 38 GPa, where the distortion (tilt angles) changes only little with pressure. The two high-pressure phase transitions at 38 and 48 GPa are marked by the occurence of larger unit cells and an increase of the distorsion away from the cubic parent perovskite cell. We find no evidence for a cubic phase at high pressure, nor indications that the structure tends to become cubic. The previously reported insulator-to-metal transition at 50 GPa appears to be symmetry breaking.Comment: 11 pages, 8 figure

High-pressure annealing of a prestructured nanocrystalline precursor to obtain tetragonal and orthorhombic polymorphs of Hf3N4

Transition metal nitrides containing metal ions in high oxidation states are a significant goal for the discovery of new families of semiconducting materials. Most metal nitride compounds prepared at high temperature and high pressure from the elements have metallic bonding. However amorphous or nanocrystalline compounds can be prepared via metal-organic chemistry routes giving rise to precursors with a high nitrogen:metal ratio. Using X-ray diffraction in parallel with high pressure laser heating in the diamond anvil cell this work highlights the possibility of retaining the composition and structure of a metastable nanocrystalline precursor under high pressure-temperature conditions. Specifically, a nanocrystalline Hf3N4 with a tetragonal defect-fluorite structure can be crystallized under high-P,T conditions. Increasing the pressure and temperature of crystallization leads to the formation of a fully recoverable orthorhombic (defect cottunite-structured) polymorph. This approach identifies a novel class of pathways to the synthesis of new crystalline nitrogen-rich transition metal nitrides

Colonisation et pratiques sportives

Frantz Fanon, l’auteur de Les damnés de la terre, s’est attaché aux diverses formes du colonialisme et du racisme. Le colonialisme utilise les activités sportives comme moyen d’assurer la paix sociale, comme des disciplines aptes à contrôler la violence des dominés. Fanon considère que ce type de pratiques collectives peut être impliqué dans une perspective permettant au peuple de participer lui‑même, dans le cadre de la décolonisation, à de nouveaux liens sociaux et au développement de sa propre culture.Frantz Fanon, the author of The Wretched of the Earth, was concerned with the different forms of colonialism and racism. Colonialism uses sporting activities as a tool for ensuring social peace, as disciplines capable of controlling the violence of the dominated. Fanon considered that this type of collective practice could be involved in a process allowing a people to contribute to the development of new social bonds and its own culture in the context of decolonisation

Le jeu de tests VLSAT-3

This report presents VLSAT-3 (an acronym for "Very Large Boolean SATisfiability problems''), the third part of a benchmark suite to be used in scientific experiments and software competitions addressing SAT and SMT (Satisfiability Modulo Theories) solving issues. VLSAT-3 contains 1200 (600 satisfiable and 600 unsatisfiable) quantifier-free first-order logic formulas of increasing complexity, proposed in SMT-LIB format under a permissive Creative Commons license. More than 90% of these benchmarks have been used during the 16th International Satisfiability Modulo Theories Competition (SMT-COMP 2021).VLSAT-3 (acronyme anglais de "très grands problèmes de satisfaisabilité booléenne") est le troisième volet d'une suite de tests destinée aux expérimentations scientifiques et aux compétitions de logiciels pour la résolution de problèmes SAT et SMT (Satisfaisabilité Modulo des Théories). VLSAT-3 contient 1200 formules logiques (600 satisfaisables et 600 insatisfaisables) du premier ordre sans quantificateur, de complexité croissante, fournies en format SMT-LIB sous une licence Creative Commons permissive. Plus de 90% de ces tests ont été utilisés lors de la 16ème Compétition Internationale de Satisfaisabilité Modulo des Théories (SMT-COMP 2021)

Structural properties and Raman spectra of columbite-type NiNb2−x_{2-x}Vx_xO6_6 synthesized under high pressure

The complete set of structural parameters of the new series of compounds NiNb$_{2-x}$V$_x$O$_6$ ($0\leq x \leq 2$) with the unusual columbite-type structure is presented here. In the samples containing vanadium, this crystalline structure was stabilized by synthesis in conditions of high pressure and high temperature. Here we report here the first Raman spectrum for the NiV$_2$O$_6$-\textsl{Pbcn} polymorph and extend the list of the previously observed modes for the NiNb$_2$O$_6$. The evolution of the vibrational Raman spectrum produced when the vanadium is substituted for niobium along the series is also presented and discussed. This evolution is interpreted by taking into account the changes in the local structural environment of the niobium/vanadium atoms and its influence over the nickel-oxygen bonds around them. The presence of vanadium atoms favors an increase of the symmetry in the arrangement of oxygen atoms around the nickel-ones; in counterpart, the vanadium is in an octahedral environment which is more distorted than that of the niobium. Because of these apparently subtle differences, the homogeneous distribution of vanadium in the solid solution NiNb$_{2-x}$V$_x$O$_6$ is not possible

Application des copules à la finance de marché

L'objectif de la thèse est de montrer l'importance et l'utilité de la théorie mathématique que les copules apportent à la finance de marché. La motivation première de ces applications réside dans le fait que les comportements des rendements conjoints des marchés financiers s'éloignent de la normalité. Ainsi les méthodes statistiques traditionnelles,\ud reposant sur cette hypothèse, ne peuvent pas être appliquées à la finance de marché. Dans cc document, avec l'aide des copules nous apportons un éclairage nouveau sur les comportements conjoints et des mesures de corrélations entre les marchés. Les copules sont des outils mathématiques puissants qui remédient aux lacunes laissées par les mesures traditionnelles de corrélations et de risque. Les copules reposent sur un cadre mathématique formel qui en permet l'application. Nous montrons aussi que les copules sont utilisées pour explorer la dépendance entre les rendements des actifs d'un portefeuille. Elles trouvent application à la valorisation de titres dont les valeurs marchandes dépendent de plusieurs actifs financiers, par exemple une option de type européen sur plusieurs actifs sous-jacents. Nous montrons aussi leurs utilités comme outils de mesure de diversification d'un portefeuille avec l'ajout de un ou plusieurs fonds de couverture. Finalement, nous exposons comment la théorie des copules vient en aide aux gestionnaires d'actifs d'une caisse de retraite dans le choix des titres et la composition d'un portefeuille. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Copules elliptiques, Copules archimédiennes, Copules hiérarchiques, Distributions marginales, Fonctions de dépendance, Chi-plots, Tau de Kendall, Rho de Pearson, Corrélations de rangs, Valorisation d'options-plusieurs sous-jacents, Fonds de couverture

Topological surface states of strained Mercury-Telluride probed by ARPES

The topological surface states of strained HgTe have been measured using high-resolution ARPES measurements. The dispersion of surface states form a Dirac cone, which origin is close to the top of the \ghh band: the top half of the Dirac cone is inside the stress-gap while the bottom half lies within the heavy hole bands and keeps a linear dispersion all the way to the X-point. The circular dichroism of the photo-emitted electron intensity has also been measured for all the bands.Comment: with supplementary materia