The hyperbolic formal affine Demazure algebra

In the present paper we extend the construction of the formal (affine) Demazure algebra due to Hoffnung, Malag\'on-L\'opez, Savage and Zainoulline in two directions. First, we introduce and study the notion of an extendable weight lattice in the Kac-Moody setting and show that all the definitions and properties of the formal (affine) Demazure operators and algebras hold for such lattices. Second, we show that for the hyperbolic formal group law the formal Demazure algebra is isomorphic (after extending the coefficients) to the Hecke algebra.Comment: Final version. Accepted for publication in Algebras and Representation Theory. ALGE-D-15-0016

Holocene reconstruction of spruce budworm-fire interactions in the mixed boreal forest of Québec (Canada) / Chronologie Holocène de l’interaction entre la tordeuse des bourgeons de l’épinette et les feux dans la forêt mixte du Québec (Canada)

Within the context of a changing climate, multi-millennial reconstructions are fundamental to our understanding of the range of variability in key ecosystem processes such as insect outbreaks and wildfires. Reconstructions spanning the Holocene allow us to observe how natural disturbances have behaved, and interacted in different climate phases and may provide a sustainable forest management framework going forward. However, for our understanding at vast temporal scales to be robust and accurate, we require the use of adequate proxies that have been validated and calibrated. I use the eastern Canadian mixed boreal forest as a model system to validate, and calibrate the use of a novel paleo-proxy, lepidopteran scales, to detect spruce budworm outbreaks in recent history, and then reconstruct spruce budworm, and wildfire events over the course of the Holocene. I demonstrated that lepidopteran scale accumulations in lake surface sediment are able to detect local spruce budworm outbreaks as these achieved high agreement with aerial surveys conducted in the late 20th century (1967-1986; 2010-present). Using the same surface sediment cores, I then assessed whether annually interpolated lepidopteran scale accumulations and tree-ring records both detected local spruce budworm impacts, and if the recorded impacts were synchronous. I demonstrated that each proxy individually identified local spruce budworm impacts at a periodicity of approximately 16 to 32 years coinciding with known outbreak return intervals, and that, when compared to each other, both proxies detected similar outbreak periodicities. Further, I demonstrated that the proxy record signals were relatively synchronous, and that the relationship between the two records suggests that large lepidopteran scale accumulations translate to a greater percentage of affected trees in a stand, and to greater recorded growth suppression i.e., narrower ring-widths. The validation and calibration of this novel proxy therefore provided a more robust and accurate foundation to interpret lepidopteran scale accumulations at multimillennial time scales. Finally, I reconstructed spruce budworm and wildfire events in the mixed boreal forest at lake Buire throughout the Holocene. The frequency of spruce budworm and wildfire events exhibited a negative correlation for the majority of the Holocene suggesting a linked interaction at the millennial and local/extra-local scales where one disturbance inhibits the presence of the other. Further, disturbance event frequencies oscillated during the Holocene following the postglacial establishment ofbalsam fir, and the switch in the dominant disturbance tended to coincide with rapid significant climate change events. The present study advanced our understanding of the long-term range of variability of the main disturbances of the mixed boreal and their interactions. The results of this thesis contribute to the body of knowledge of forest ecology, and disturbance interactions and provide insight into potential frameworks that could be utilized in sustainable forest management. Dans le contexte des changements climatiques, les reconstructions plurimillénaires sont fondamentales à notre compréhension de la variabilité des processus écosystémiques tels que les épidémies d'insectes et les feux de forêt. Les reconstitutions couvrant l'Holocène nous permettent d'observer comment ces perturbations naturelles se sont comportées et ont interagi au cours de différentes phases climatiques et ont le potentiel de fournir un cadre pour la gestion durable des forêts. Cependant, pour que notre compréhension à de vastes échelles temporelles soit robuste et précise, nous devons utiliser des proxys adéquats qui ont été validés et calibrés. J'utilise la forêt boréale mixte de l'est du Canada comme système modèle pour valider et calibrer l'utilisation d'un nouveau paléo-proxy, les écailles de papillon, afin de détecter les épidémies de tordeuse des bourgeons de l'épinette dans l’histoire récente et ensuite de reconstruire les événements liés à la tordeuse et aux feux de forêt au cours de l'Holocène. J'ai démontré que les accumulations d'écailles de papillon dans les sédiments de surface des lacs sont capables de détecter les épidémies locales de la tordeuse, car elles concordent bien avec les relevés aériens effectués à la fin du XXe siècle (1967-1986; 2010-présent). En utilisant les mêmes carottes de sédiments de surface, j'ai ensuite évalué si les accumulations d'écailles de papillon interpolées annuellement et les cernes de croissance détectaient aussi bien les impacts locaux de la tordeuse des bourgeons de l'épinette, et si ces impacts enregistrés étaient synchrones. J'ai démontré que chaque proxy identifiait individuellement les impacts locaux de la tordeuse, coïncidant avec les intervalles des épidémies connues, et que, comparées l'une à l'autre, les deux approximations détectaient des périodicités d'épidémies similaires. De plus, j'ai démontré que les signaux enregistrés par les proxys étaient relativement synchrones. La relation entre les deux proxys suggère que les grandes accumulations d'écailles se traduisent par un plus grand pourcentage d'arbres affectés dans un peuplement, et par des cernes plus étroits. La validation et l'étalonnage de ce nouvel indicateur ont donc fourni une base plus solide et précise pour interpréter les accumulations d’écailles à des échelles de temps plurimillénaires. Enfin, j'ai reconstitué les événements liés à la tordeuse des bourgeons de l'épinette et aux feux de forêt dans la forêt boréale mixte au lac Buire tout au long de l'Holocène. J’ai observé une corrélation négative entre la fréquence des épidémies de la tordeuse et des feux de forêt pendant la majeure partie de l'Holocène, ce qui suggère une interaction liée aux échelles millénaire et locale/extra-locale, où une perturbation inhibe la présence de l'autre. Aussi, la fréquence des perturbations a oscillé au cours de l'Holocène suite à l'établissement postglaciaire du sapin baumier, et que la perturbation dominante semblait être déterminé par des évènements de changements climatique rapides et significatifs. Cette étude a permis de mieux comprendre la variabilité à long terme des principales perturbations de la forêt boréale mixte et leurs interactions. Les résultats de cette thèse contribuent à l’ensemble des connaissances sur l’écologie forestière et les interactions entre les perturbations et donnent un aperçu de cadres potentiels qui pourraient être utilisés dans la gestion durable des forêts

Topological modes in stellar oscillations

Stellar oscillations can be of topological origin. We reveal this deep and so-far hidden property of stars by establishing a novel parallel between stars and topological insulators. We construct an hermitian problem to derive the expression of the stellar $\mathrm{\textit{acoustic-buoyant}}$ frequency $S$ of non-radial adiabatic pulsations. A topological analysis then connects the changes of sign of the acoustic-buoyant frequency to the existence of Lamb-like waves within the star. These topological modes cross the frequency gap and behave as gravity modes at low harmonic degree $\ell$ and as pressure modes at high $\ell$. $S$ is found to change sign at least once in the bulk of most stellar objects, making topological modes ubiquitous across the Hertzsprung-Russel diagram. Some topological modes are also expected to be trapped in regions where the internal structure varies strongly locally.Comment: 6 pages, 5 figures (+7 pages, 3 figures of appendices

Magnetic Screening of NbN Multilayers Samples

6 pagesInternational audienceIn 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > Hc^Nb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities [1]. We have deposited high quality "model" samples by magnetron sputtering on monocrystalline sapphire substrates. A 250 nm layer of niobium figures the bulk Nb. It was coated with a single and multi-stacks of NbN layers (25 or12 nm) separated by 15 nm MgO barriers, and characterized by X-Ray reflectivity and DC transport measurements. DC or AC measurement of HC1 is an important goal for multilayer evaluation during the sample evaluation phase. A clear increase of HC1 at low frequency is promising indication since HC1 is expected to increase with frequency (see e.g. [2] and references therein). We have measured the first penetration field (HP~HC1) on DC magnetization curves in a SQUID system. HP of NbN covered sample is increased compared to Nb alone. We have also developed a set-up that allows measuring a large range of field and temperature with a local probe method based on 3rd harmonic analysis. We have confirmed the screening behavior of a single 25 nm NbN layer placed on the top of a Nb Layer

Assessing Wind Impact on Semi-Autonomous Drone Landings for In-Contact Power Line Inspection

In recent years, the use of inspection drones has become increasingly popular for high-voltage electric cable inspections due to their efficiency, cost-effectiveness, and ability to access hard-to-reach areas. However, safely landing drones on power lines, especially under windy conditions, remains a significant challenge. This study introduces a semi-autonomous control scheme for landing on an electrical line with the NADILE drone (an experimental drone based on original LineDrone key features for inspection of power lines) and assesses the operating envelope under various wind conditions. A Monte Carlo method is employed to analyze the success probability of landing given initial drone states. The performance of the system is evaluated for two landing strategies, variously controllers parameters and four level of wind intensities. The results show that a two-stage landing strategies offers higher probabilities of landing success and give insight regarding the best controller parameters and the maximum wind level for which the system is robust. Lastly, an experimental demonstration of the system landing autonomously on a power line is presented

Continuous Symmetry Breaking in a Two-dimensional Rydberg Array

Spontaneous symmetry breaking underlies much of our classification of phases of matter and their associated transitions. The nature of the underlying symmetry being broken determines many of the qualitative properties of the phase; this is illustrated by the case of discrete versus continuous symmetry breaking. Indeed, in contrast to the discrete case, the breaking of a continuous symmetry leads to the emergence of gapless Goldstone modes controlling, for instance, the thermodynamic stability of the ordered phase. Here, we realize a two-dimensional dipolar XY model -- which exhibits a continuous spin-rotational symmetry -- utilizing a programmable Rydberg quantum simulator. We demonstrate the adiabatic preparation of correlated low-temperature states of both the XY ferromagnet and the XY antiferromagnet. In the ferromagnetic case, we characterize the presence of long-range XY order, a feature prohibited in the absence of long-range dipolar interaction. Our exploration of the many-body physics of XY interactions complements recent works utilizing the Rydberg-blockade mechanism to realize Ising-type interactions exhibiting discrete spin rotation symmetry.Comment: 18 pages, 3 figures in main text, 9 figures in supplemental method

Current crowding issues on nanoscale planar organic transistors for spintronic applications

The predominance of interface resistance makes current crowding ubiquitous in short channel organic electronics devices but its impact on spin transport has never been considered. We investigate electrochemically doped nanoscale PBTTT short channel devices and observe the smallest reported values of crowding lengths, found for sub-100 nm electrodes separation. These observed values are nevertheless exceeding the spin diffusion lengths reported in the literature. We discuss here how current crowding can be taken into account in the framework of the Fert–Jaffrès model of spin current propagation in heterostructures, and predict that the anticipated resulting values of magnetoresistance can be significantly reduced. Current crowding therefore impacts spin transport applications and interpretation of the results on spin valve devices

Continuous symmetry breaking in a two-dimensional Rydberg array

Spontaneous symmetry breaking underlies much of our classification of phases of matter and their associated transitions. The nature of the underlying symmetry being broken determines many of the qualitative properties of the phase; this is illustrated by the case of discrete versus continuous symmetry breaking. Indeed, in contrast to the discrete case, the breaking of a continuous symmetry leads to the emergence of gapless Goldstone modes controlling, for instance, the thermodynamic stability of the ordered phase. Here, we realize a two-dimensional dipolar XY model that shows a continuous spin-rotational symmetry using a programmable Rydberg quantum simulator. We demonstrate the adiabatic preparation of correlated low-temperature states of both the XY ferromagnet and the XY antiferromagnet. In the ferromagnetic case, we characterize the presence of a long-range XY order, a feature prohibited in the absence of long-range dipolar interaction. Our exploration of the many-body physics of XY interactions complements recent works using the Rydberg-blockade mechanism to realize Ising-type interactions showing discrete spin rotation symmetry.This work is supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 817482 (PASQuanS), the Agence Nationale de la Recherche (ANR, project nos. RYBOTIN and ANR-22-PETQ-0004, project QuBitAF) and the European Research Council (advanced grant no. 101018511-ATARAXIA). J.H. acknowledges support from the NSF OIA Convergence Accelerator programme under award number 2040549, and the Munich Quantum Valley, which is supported by the Bavarian state government with funds from the Hightech Agenda Bayern Plus. M.S. and A.M.L. acknowledge support by the Austrian Science Fund (FWF) through grant no. I 4548. D.B. acknowledges support from grant no. MCIN/AEI/10.13039/501100011033 (grant nos. RYC2018- 025348-I, PID2020-119667GA-I00 and European Union NextGenerationEU PRTR-C17.I1). M.P.Z. acknowledges support from the Department of the Environment (DOE) Early Career programme and the Alfred P. Sloan foundation. N.Y.Y. acknowledges support from the Army Research Office (ARO) (grant no. W911NF-21-1-0262), the AFOSR MURI programme (grant no. W911NF-20-1-0136), the David and Lucile Packard foundation, and the Alfred P. Sloan foundation. M.B. and V.L. acknowledge support from NSF QLCI programme (grant no. OMA-2016245). S.C. acknowledges support from the ARO through the MURI programme (grant no. W911NF-17-1-0323) and from the US DOE, Office of Science, Office of Advanced Scientific Computing Research, under the Accelerated Research in Quantum Computing programme.Peer reviewe

Gaia Data Release 3: G_RVS photometry from the RVS spectra

Gaia Data Release 3 (DR3) contains the first release of magnitudes estimated from the integration of Radial Velocity Spectrometer (RVS) spectra for a sample of about 32.2 million stars brighter than G_RVS~14 mag (or G~15 mag). In this paper, we describe the data used and the approach adopted to derive and validate the G_RVS magnitudes published in DR3. We also provide estimates of the G_RVS passband and associated G_RVS zero-point. We derived G_RVS photometry from the integration of RVS spectra over the wavelength range from 846 to 870 nm. We processed these spectra following a procedure similar to that used for DR2, but incorporating several improvements that allow a better estimation of G_RVS. These improvements pertain to the stray-light background estimation, the line spread function calibration, and the detection of spectra contaminated by nearby relatively bright sources. We calibrated the G_RVS zero-point every 30 hours based on the reference magnitudes of constant stars from the Hipparcos catalogue, and used them to transform the integrated flux of the cleaned and calibrated spectra into epoch magnitudes. The G_RVS magnitude of a star published in DR3 is the median of the epoch magnitudes for that star. We estimated the G_RVS passband by comparing the RVS spectra of 108 bright stars with their flux-calibrated spectra from external spectrophotometric libraries. The G_RVS magnitude provides information that is complementary to that obtained from the G, G_BP, and G_RP magnitudes, which is useful for constraining stellar metallicity and interstellar extinction. The median precision of G_RVS measurements ranges from about 0.006 mag for the brighter stars (i.e. with 3.5 < G_RVS < 6.5 mag) to 0.125 mag at the faint end. The derived G_RVS passband shows that the effective transmittance of the RVS is approximately 1.23 times better than the pre-launch estimate.Comment: 16 pages, 18 figures. Accepted for publication in A&