Out-of-equilibrium dynamics of photoexcited spin-state concentration waves

International audienceThe spin crossover compound [FeIIH2L2-Me][PF6]2 presents a two-step phase transition. In the intermediate phase, a spin state concentration wave (SSCW) appears resulting from a symmetry breaking (cell doubling) associated with a long-range order of alternating high and low spin molecular states. By combining time-resolved optical and X-ray diffraction measurements on a single crystal, we study how such a system responds to femtosecond laser excitation and we follow in real time the erasing and rewriting of the SSC

Photoinduced phenomena and structural analysis associated with the spin-state switching in the [FeII(DPEA)(NCS)2] complex

International audienceOut-of-equilibrium photoinduced switching from the low-spin to the high-spin state has been investigated on the iron(II) complex [Fe(II)(DPEA)(NCS)2] by both optical reflectivity and magnetic measurements under continuous light irradiation at low temperature. The photoinduced HS state can be observed up to 47 K and the relaxation process has been followed. Structural changes of both the temperature- and the photoinduced spin-state switching have been analyzed in detail by x-ray diffraction indicating no change of symmetry. Short intermolecular contacts and intramolecular deformations associated with the change of molecular spin state have been quantified. Actually a crossover behavior is observed at thermal equilibrium with however a quasiabrupt shape indicating significant cooperative effects. These aspects are compared between the temperature- and photoinduced spin crossovers

Wavelength selective light-induced magnetic effects in the binuclear spin crossover compound {[Fe(bt)(NCS)2]2(bpym)}

International audienceUsing Fourier transform infrared spectroscopy, x-ray diffraction, and magnetic susceptibility measurements under light irradiation, the selective light-induced excited spin state trapping (LIESST) and the reversible-LIESST effect have been evidenced and studied in depth in the binuclear spin crossover compound {[Fe(bt)(NCS)2]2bpym}. In this system, each magnetic site can switch from low spin (LS) to high spin (HS), so that three states exist, namely, the LS-LS, HS-LS, and HS-HS. All these techniques shine a new light on the high phototunability of this system. In addition to the direct photoswitching from the LS-LS to the HS-LS or to the HS-HS state, here we show that photoinduced switching between the excited photoinduced states can be triggered in a reversible way: from HS-LS to HS-HS (irradiation around 800 nm), or reverse from HS-HS to HS-LS (irradiation around 1300 nm). The nature of the intermediate HS-LS state during the thermal and light-induced spin state changes is also discussed by comparing the spectroscopic measurements and the structural analysis. The loss of inversion symmetry in the HS-LS molecular state, where the two magnetic Fe sites are no more equivalent, is not accompanied by any long-range ordering of the noncentrosymmetric molecules in the crystal. Therefore the continuous double-step spin conversion corresponds to a double crossover

Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition

We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, non-linear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals.Comment: 4 pages, 4 figure

Ciencia de puestos militares o Tratado de las fortificaciones de campaña : para el uso de los oficiales de infanteria ...

AnteportSign.: [*]8, 2*4, A-N8, O

Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene-pp-Chloranil

For electronic states and photoinduced charge dynamics near the neutral-ionic transition in the mixed-stack charge-transfer complex tetrathiafulvalene-$p$-chloranil (TTF-CA), we review the effects of Peierls coupling to lattice phonons modulating transfer integrals and Holstein couplings to molecular vibrations modulating site energies. The former stabilizes the ionic phase and reduces discontinuities in the phase transition, while the latter stabilizes the neutral phase and enhances the discontinuities. To reproduce the experimentally observed ionicity, optical conductivity and photoinduced charge dynamics, both couplings are quantitatively important. In particular, strong Holstein couplings to form the highly-stabilized neutral phase are necessary for the ionic phase to be a Mott insulator with large ionicity. A comparison with the observed photoinduced charge dynamics indicates the presence of strings of lattice dimerization in the neutral phase above the transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp

Un outil moléculaire simple et abordable pour identifier les larves de taupins du genre Agriotes

Dans cet article, nous décrivons une méthode d’identification des larves de taupins du genre Agriotes (coléoptère, Elateridae) par PCR multiplexe. Depuis une quinzaine d’années, nous observons une recrudescence de dégâts dus aux larves de taupins. En France, les dégâts sont principalement causés par quatre espèces : A.sordidus, A.lineatus, A.obscurus et A.sputator. Ces trois dernières espèces sont particulièrement difficiles à différencier au stade larvaire alors que leur cycle biologique et leur nuisibilité diffèrent potentiellement. C’est dans ce contexte que nous avons modifié une méthode d'identification des espèces d’Agriotes par PCR multiplexe en utilisant les amorces précédemment développées par un laboratoire autrichien. Les principales modifications concernent la méthode d'extraction et la mise au point du mélange réactionnel. Ce travail permet de proposer un service d'identification des larves d’Agriotes rapide et à coût réduit qui fait défaut actuellement en France.Here, we describe a polymerase chain reaction (PCR)-based approach to identify click beetle larvae within the genus Agriotes (Coleoptera: Elateridae) commonly known as wireworms. Since 15 years, damages due to wireworms within the genus Agriotes have significantly upsurged on several crops. From the four main species occurring in France (A.sordidus, A. lineatus, A. obscurus and A. sputator), the last three are very difficult to morphologically identify in their larval stage. It is in this context that we have modified a method of identifying Agriotes species by Multiplex PCR using the primers previously developed by an Austrian laboratory. The main modifications involve the extraction method and the development of the reaction mixture. This work makes it possible to offer a rapid and low-cost identification service for larvae of Agriotes which is currently lacking in France

Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Recharge des aquifères à l'échelle de la France : estimation, évolution et incertitudes associées

International audienceEvaluating future groundwater recharge with associated uncertainty at the scale of France Among aquifer recharge processes, effective precipitation infiltration is the most directly impacted by climate. We propose a methodology for estimating this recharge at the scale of large territories, based on global hydrological models requiring few data and parameters. The originality of our work lies in the use of the IDPR (Network Development and Persistence Index) which is a qualitative cartographic index of infiltration potentiality. The study of more than 350 hydrological basins made it possible to highlight a linear relation between the BaseFlow Index and the IDPR for the sedimentary basins. Based on this relationship and assuming that the effective rainfall infiltration ratio could be assimilated to the BFI, an map of this ratio was generated all over France, allowing the effective precipitation map to be converted into a recharge map. The methodology was then applied to estimate future recharge using the outputs of climate models for two contrasted radiative concentration pathway scenarios. By 2030, whatever the scenario, the modeling results predict a moderate decrease in the direct recharge of aquifers everywhere in France except in Paris basin and around the Mediterranean sea. By 2080, the optimistic scenario allows to recover recharge values close to the current ones, outside the north of the Paris basin. At the opposite, within the pessimistic scenario, a majority of aquifers would undergo a recharge decrease by 10 to 30%, with stress zones in the Paris basin, around the Mediterranean sea and in Corsica. Nonetheless, these first results should be considered carefully as the associated uncertainties are significant and can reach 50% for the most pessimistic forecasts.Parmi les processus qui participent à la recharge des aquifères, la recharge par infiltration des précipitations efficaces est la plus directement impactée par le climat. Nous proposons une méthodologie qui permet d'estimer cette recharge à l'échelle de grands territoires, sur la base de modèles hydrologiques globaux nécessitant peu de données et de paramètres. L'étude de plus de 350 bassins hydrologiques a permis de mettre en évidence pour les bassins sédimentaires, une relation entre le BaseFlow Index et l'IDPR (Indice de Développement et de Persistance des Réseaux), qui est un indice cartographique qualitatif de potentialité d'infiltration. Sur la base de cette relation, une carte du ratio d'infiltration de la pluie efficace a pu être générée pour toute la France métropolitaine, permettant de transformer la carte de pluie efficace en carte de recharge. La méthodologie a été appliquée pour estimer la recharge future en utilisant les sorties de modèles climatiques pour deux scenarios de forçage radiatif. A l'horizon 2030, quel que soit le scénario, les résultats de la modélisation prévoient une baisse modérée de la recharge directe des aquifères sur la majorité du territoire, en dehors du bassin parisien et du pourtour méditerranéen. A l'horizon 2080, le scénario optimiste permet de revenir à des valeurs de recharge proches de l'actuel, en dehors du nord du bassin parisien. En revanche, dans le scénario pessimiste, une majorité des aquifères voit sa recharge diminuer de 10 à 30%, avec des zones plus tendues dans le bassin parisien, le pourtour méditerranéen et la Corse. Les incertitudes associées à ces résultats sont néanmoins importantes et peuvent atteindre 50% sur les prévisions les plus pessimistes