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DTSTART;TZID=Europe/Paris:20260504T110000
DTEND;TZID=Europe/Paris:20260504T120000
DTSTAMP:20260503T120956
CREATED:20260430T115259Z
LAST-MODIFIED:20260430T115259Z
UID:10000137-1777892400-1777896000@sfp-alpes.fr
SUMMARY:Giulio FACCHINI (Laboratoire Matière et Systèmes Complexes\, Paris)
DESCRIPTION:Deciphering the origin of saddle-shape geometry in the microstructure of echinoderms skeleton\nRésumé : \nThe microstructure of echinoderms skeleton\, like sea stars and sea urchins\, is an impressive example of self-organization and complexity. This structure called stereom is a porous meshwork\, made of calcite\, whose surface is saddle-shaped\, and remind of minimal surfaces. Minimal surfaces have intrigued scientists for centuries\, as they can spontaneously emerge from minimizing interfacial energy like surface tension. Several studies have addressed the morphogenesis of the stereom\, in sea urchins and other echinoderms\, showing that it forms via the addition of tiny mineral bids at the tip of small skeletal elements that successively branch and bridge to form a complex network. Yet\, a global\, mechanistic\, comprehension of how biomineralizing cells control preferential deposition is still lacking. \nDuring the seminar I will present a recent work1 on the morphogenesis of the particular stereom geometry observed in the sea star P. nodosus. Here\, the stereom can take the form of a diamond Triply Periodic Minimal Surface (TPMS) which confer it specific mechanical properties. Using different marking protocols\, we provide the first experimental insight in the formation of the diamond TPMS stereom\, and show that the formation of such a highly ordered structure relies on a precise and timely coordination of the branching and bridging episodes. Moreover\, we provide experimental evidences of an organic precursor made of F – actin fibers exhibiting saddle-shape geometry. We hypothesise that such a fibers template may self-organise under mechanical tension\, thus explaining the peculiar curvature signature of the final structure. Finally\, I will present some ongoing experimental work performed on sea urchins\, showing that the same morphogenetic mechanism could be shared across very different species. \nContact : emmanuel.siefert@univ-grenoble-alpes.fr
URL:https://sfp-alpes.fr/event/giulio-facchini-laboratoire-matiere-et-systemes-complexes-paris/
LOCATION:LiPhy – Salle de conférence\, LiPhy 140 avenue de la Physique\, St Martin d'Hères\, 38402\, France
CATEGORIES:Séminaire
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DTSTART;TZID=Europe/Paris:20260504T140000
DTEND;TZID=Europe/Paris:20260504T150000
DTSTAMP:20260503T120956
CREATED:20260424T081257Z
LAST-MODIFIED:20260424T081257Z
UID:10000128-1777903200-1777906800@sfp-alpes.fr
SUMMARY:Maxime LUCAS (CY Cergy Paris Université)
DESCRIPTION:Non-collinear magnetism in monolayer and magic-angle twisted bilayer graphene\nRésumé : \nRecent studies of twisted bilayer graphene (or other 2D materials) have been stimulated by the discovery of correlations between electronic flatband states due to a moiré pattern [1]. It is shown experimentally and theoretically that the filling of the flat bands affects their correlation and magnetic properties significantly.\nOn the other hand\, the effect of doping on a simple graphene layer is still unclear. Indeed\, its half-filled case is well known [2]\, but unlike other lattices [3] its magnetic properties beyond half filling are mostly unexplored\, except at 1/4 doping [4] i.e when the Fermi energy is set inside of a Van Hove singularity associated to a flatband.\nIn this talk\, I will first present our analysis of graphene magnetism using a combination of the Hubbard model and Hartree-Fock Mean Field Theory (MFT). We work at density values around 1/4 doping (average number of electrons per site Ne=0.75) as it puts the system right into one of the Van Hove singularities found in graphene’s density of states\, giving rise to interesting magnetic properties. We present an interaction-density phase diagram and its associated magnetic orders\, described by their band structure and spin structure factor [5].\nI will then talk about magic-angle twisted bilayer graphene\, to which we applied the same MFT method. While still a work in progress\, I will present the current results we obtained on a Moiré lattice for various values of interaction and flat-band filling\, revealing exotic spin textures such as an antiferromagnetic triangular order on the Moiré scale.\n[1] Y. Cao et al.\, Nature 556\, 43 (2018); Nature 556\, 80 (2018).\n[2] M. Raczkowski et al.\, Phys. Rev. B 101\, 125103 (2020)\, and Refs. therein.\n[3] R. Scholle et al.\, Phys. Rev. B 108\, 035139 (2023)\n[4] S. Jiang\, A. Mesaros\, Y. Ran\, Phys. Rev. X 4\, 031040 (2014)\n[5] M. Lucas\, A. Ralko\, A. Honecker\, G. Trambly de Laissardière\, arXiv:2511.22714 (2025) \n_ \nContact : andrew.fefferman@neel.cnrs.fr
URL:https://sfp-alpes.fr/event/maxime-lucas-cy-cergy-paris-universite/
LOCATION:CNRS – Salle Louis Weil (E424)\, CNRS - Institut Néel 25 avenue des Martyrs\, Grenoble\, 38042\, France
CATEGORIES:Séminaire
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