Louis XIII, un roi de guerre à la conquête du pouvoir, 1620-1622

« Enfin, aux chastimentz il se laisse forcer. » Cette terrible constatation concerne Louis XIII et sert de légende à une gravure intitulée « Punition des villes rebelles », que nous fait découvrir Patrick Avrillas. Son Louis XIII, roi de guerre est consacré en effet au retentissement que les arts et la littérature accordèrent aux événements militaires de 1620-1622, qui avaient eu des relents de guerre civile. Il nous fait découvrir en particulier l’œuvre du graveur Jean Valdor qui leur était ..

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe2_2

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides

Predicting the consequences of species loss using size-structured biodiversity approaches.

Understanding the consequences of species loss in complex ecological communities is one of the great challenges in current biodiversity research. For a long time, this topic has been addressed by traditional biodiversity experiments. Most of these approaches treat species as trait-free, taxonomic units characterizing communities only by species number without accounting for species traits. However, extinctions do not occur at random as there is a clear correlation between extinction risk and species traits. In this review, we assume that large species will be most threatened by extinction and use novel allometric and size-spectrum concepts that include body mass as a primary species trait at the levels of populations and individuals, respectively, to re-assess three classic debates on the relationships between biodiversity and (i) food-web structural complexity, (ii) community dynamic stability, and (iii) ecosystem functioning. Contrasting current expectations, size-structured approaches suggest that the loss of large species, that typically exploit most resource species, may lead to future food webs that are less interwoven and more structured by chains of interactions and compartments. The disruption of natural body-mass distributions maintaining food-web stability may trigger avalanches of secondary extinctions and strong trophic cascades with expected knock-on effects on the functionality of the ecosystems. Therefore, we argue that it is crucial to take into account body size as a species trait when analysing the consequences of biodiversity loss for natural ecosystems. Applying size-structured approaches provides an integrative ecological concept that enables a better understanding of each species' unique role across communities and the causes and consequences of biodiversity loss

Pierre Bougard (1923-2014)

Heckmann Thierry. Pierre Bougard (1923-2014). In: Bibliothèque de l'école des chartes. 2015, tome 173. pp. 554-556

L’informatique documentaire aux archives : passer de l’expérimentation à la publication en ligne

Heckmann Thierry. L’informatique documentaire aux archives : passer de l’expérimentation à la publication en ligne. In: La Gazette des archives, n°207, 2007-3. Archives et Internet : contributions et témoignages. pp. 141-150

Les archives sont-elles plus qu’un conservatoire ?

Les Archives nationales peuvent en remontrer à chaque département sur leur histoire, tant le recours aux dossiers qui leur proviennent des administrations centrales ou des assemblées les concernent chacun, et parfois sans équivalent. L’histoire des départements profite aujourd’hui de la mise en ligne progressive de leurs sources principales, aussi peut-on avoir envie de disposer de cette manière de certains fonds des Archives nationales. Pour l’étude de la guerre de Vendée, l’initiative d’un partenariat a été prise par les Archives départementales de la Vendée, de façon à proposer en ligne des archives publiques, quel que soit leur lieu de conservation. Le rôle scientifique des Archives départementales en sort conforté et la pertinence du choix est plébiscitée par le public.Heckmann Thierry. Les archives sont-elles plus qu’un conservatoire ?. In: La Gazette des archives, n°254, 2019-2. Les Archives nationales, une refondation pour le XXIe siècle. pp. 179-186

Ultrafast hidden spin polarization dynamics of bright and dark excitons in 2H − WSe2_2

International audienceWe performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy(STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularlypolarized light. The very short probing depth of XUV photoemission permits selective measurementof photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurementof hidden spin polarization of bright and momentum-forbidden dark excitons. Our results revealefficient chiroptical control of bright excitons’ hidden spin polarization. Following optical photoexcitation,intervalley scattering between nonequivalent K-K’ valleys leads to a decay of bright excitons’hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitonsacts as a local spin polarization reservoir, which could be used for spin injection in van der Waalsheterostructures involving multilayer transition metal dichalcogenides

Spin, time, and angle resolved photoemission spectroscopy on WTe2

We combined a spin resolved photoemission spectrometer with a high-harmonic generation (HHG) laser source in order to perform spin, time, and angle resolved photoemission spectroscopy (STARPES) experiments on the transition metal dichalcogenide bulk WTe2, a possible Weyl type-II semimetal. Measurements at different femtosecond pump-probe delays and comparison with spin resolved one-step photoemission calculations provide insight into the spin polarization of electrons above the Fermi level in the region where Weyl points of WTe2 are expected. We observe a spin accumulation above the Weyl points region, which is consistent with a spin-selective bottleneck effect due to the presence of spin-polarized conelike electronic structure. Our results support the feasibility of STARPES with HHG, which despite being experimentally challenging provides a unique way to study spin dynamics in photoemission

The effect of forest canopy and flood disturbance on New Zealand stream food web structure and robustness

The effects of disturbance on communities have been a focus of both theoretical and empirical inquiries for many years. Food web stability is hypothesized to be affected by disturbance and the nature of the energy pathways (i.e. allochthonous or autochthonous) of a community. In this study, we investigated whether food webs at paired sites, one in forest and the other in grassland, in ten New Zealand streams along a disturbance gradient differ in their topological structure and robustness. Food web robustness (an indicator of web resistance) assesses the ease with which secondary extinctions permeate the food web following an initial random extinction (disturbance). We found that neither the nature of the energy source nor physical disturbance affected structural metrics or web robustness. As stream systems, particularly in New Zealand, are exposed to regular, unpredictable and dramatic physical disturbance from flooding, it may simply be that the floods result in generalist species dominating and increasing robustness irrespective of the energy source