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Enhanced Lattice Coherences and Improved Structural Stability in Quadruple A‐Site Substituted Lead Bromide Perovskites
No full textInternational audienceLead halide perovskites (LHPs) are promising materials for efficient photovoltaic devices; however, they often encounter limited structural stability and degradation problems that limit their technological potential. This study investigates a novel perovskite composition consisting of (Cs, MA, FA, GA)PbBr3, abbreviated as (4cat)PbBr3, to effectively enhance phase stability and optoelectronic characteristics. The spectroscopic data reveal improved structural order, electronic properties, and dynamic lattice response in a cubic phase, which is uniquely stabilized by the specific cation composition down to 80 K. Superior optoelectronic properties are verified by increased photoluminescence (PL) and 20-fold higher electron mobility, when compared to the single-cation composition, MAPbBr3. Notably, the ultrafast Terahertz-induced Kerr effect (TKE) reveals a dominating 1.1 THz octahedral twist mode, also observed in MAPbBr3, however with a doubled phonon coherence time in (4cat)PbBr3 at 80 K. The observation of higher structural order in the 4-cation composition is thus reflected by the prolonged lattice coherences, indicating enhanced dynamic screening effects that can explain the improved optoelectronic properties of (4cat)PbBr3. This study therefore sheds light on the influence of the A-site cation composition on the inorganic sublattice and its coherent dynamics, highly relevant to perovskite-based photovoltaic and optoelectronic technologie
Dynamique du smile, modèles à volatilité rough, volatilité avec mémoire
No full textIn this thesis we address the question of the static and dynamical properties of implied volatility surfaces of equity indices. In the first part, we focus on the at-the-money (ATM) skewterm-structure. We first underline the fact thatthe assumed power-law behavior with coefficient close to − 12 is only valid for maturities above acouple of months. Focusing on the short-termpart, we observe another power law behavior withs maller exponent close to −0.3 and we developa parametrization that embodies this new shape.In the second part we direct our efforts toward dynamical aspects through the Skew-StickinessRatio (SSR). This ratio links increments of theunderlying to increments of its implied volatil-ity. After recovering the empirical results already computed by Bergomi, we develop a new efficient estimator to compute the ATM SSR in models through finite differences. We find that rough and classical models studied generate close SSRs thatare significantly impacted by the non flat initialforward variance curve mandatory to fit marketdata. We also extend the work of Bergomi byderiving a first order expansion in small vol-of-vol of the ATM SSR in closed form. In the lastpart, we focus on the path properties of markets and models through the SPX-VIX joint calibration problem. Based on the observations of Guyonand Lekeufack, we design a Bergomi with memory model and look at how the path-dependent component of the volatility process helps to generate better SPX and VIX smiles simultaneously. We also present a new expansion in small vol-of-vol inthis type of model.Dans cette thèse, nous abordons la question des propriétés statiques et dynamiques des surfaces de volatilité implicite sur les marches d’indices sur actions. Dans la première partie,nous nous focalisons sur la structure par terme du skew à la monnaie (ATM). Nos premières observations soulignent que la dynamique du skew, comme fonction inverse de la racine carrée de la maturité, ne semble vérifiée que pour des maturités supérieures à quelques mois. En étudiant les petites maturités, nous observons une forme proche d’une deuxième loi de puissance mais de coefficient moindre, proche de−0.3. Nous développons finalement une forme paramétrique permettant de prendre en compte toutes ces observations. Dans la deuxième partie, nous orientons nos efforts sur les aspects dynamiques des surfaces de volatilité via le Skew-Stickiness Ratio (SSR). Ce ratio vise à représenter le lien entre incréments du sous-jacent et incréments de sa volatilité implicite. Après avoir retrouve les résultats empiriques de Bergomi, nous développons une méthode efficace d’estimation du SSR ATM par différences finies.Nous trouvons que les modèles rugueux et classiques étudies génèrent des SSRs proches qui sont fortement impactés par la courbe de variance for-ward non constante nécessaire à la calibration au marche. Nous étendons aussi le travail de Bergomien dérivant une expansion au premier ordre envol-de-vol du SSR ATM en formule fermée. Dans la dernière partie, nous nous intéressons aux propriétés trajectoires des marchés options avec la question de la calibration jointe SPX-VIX. A la suite des travaux de Guyon et Lekeufack, mous concevons un modèle Bergomi avec mémoire a fin de voir à quel point cette composante permet dégénérer des smiles SPX et VIX satisfaisants simultanément. Nous présentons aussi une nouvelle expansion en petite vol-de-vol pour des modèles de cette forme
A theoretical perspective on mode collapse in variational inference
No full textInternational audienceWhile deep learning has expanded the possibilities for highly expressive variational families, the practical benefits of these tools for variational inference (VI) are often limited by the minimization of the traditional Kullback–Leibler objective, which can yield suboptimal solutions. A major challenge in this context is mode collapse : the phenomenon where a model concentrates on a few modes of the target distribution during training, despite being statistically capable of expressing them all. In this work, we carry a theoretical investigation of mode collapse for the gradient flow on Gaussian mixture models. We identify the key low-dimensional statistics characterizing the flow, and derive a closed set of low-dimensional equations governing their evolution. Leveraging this compact description, we show that mode collapse is present even in statistically favorable scenarios, and identify two key mechanisms driving it: mean alignment and vanishing weight. Our theoretical findings are consistent with the implementation of VI using normalizing flows, a class of popular generative models, thereby offering practical insights
Machine Bias. How Do Generative Language Models Answer Opinion Polls?
No full textInternational audienceGenerative artificial intelligence (AI) is increasingly presented as a potential substitute for humans, including as research subjects. However, there is no scientific consensus on how closely these in silico clones can emulate survey respondents. While some defend the use of these “synthetic users,” others point toward social biases in the responses provided by large language models (LLMs). In this article, we demonstrate that these critics are right to be wary of using generative AI to emulate respondents, but probably not for the right reasons. Our results show (i) that to date, models cannot replace research subjects for opinion or attitudinal research; (ii) that they display a strong bias and a low variance on each topic; and (iii) that this bias randomly varies from one topic to the next. We label this pattern “machine bias,” a concept we define, and whose consequences for LLM-based research we further explore
Towards Uncertainty Quantification : Efficient Surrogate Models In Coupled Fluid-Structure Interaction For Fuel Assembly Bow
No full textInternational audienceIn the core of nuclear reactors, fluid-structure interaction and intense irradiation lead to the progressive deformation of fuel assemblies. When this deformation becomes significant, it can result in additional costs and extended fuel unloading and reloading operations. Therefore, it is essential to develop effective fuel management strategies that minimize excessive deformation and interactions between fuel assemblies. However, accurately predicting deformation and the interactions that arise between fuel assemblies remains challenging due to the complex interdependencies of various phenomena, including neutronics, thermal-hydraulics, and thermomechanics, each subject to inherent uncertainties. This work presents a comprehensive approach to address these challenges by focusing on the coupling between hydraulic and thermomechanical phenomena within a pressurized water reactor. An initial sensitivity analysis was conducted to determine the most influential parameters, first in hydraulic models [A. Abboud et al., BEPU 2024, 272], and then in mechanical models [A. Abboud et al., M\&C 2025, 46282]. To effectively manage uncertainties over several reactor power cycles, it is useful to have accurate surrogate models. Using this information, the coupled simulation aims to synergistically integrate hydraulic and mechanical effects, along with their interactions, to achieve a more accurate modeling of fuel assembly deformation while capturing the dependencies of each model to its uncertain parameters. Furthermore, this study goes beyond standard parameter uncertainties by addressing epistemic factors, such as the convergence algorithms and criteria used in the coupled simulations. By analyzing these coupled effects and the associated uncertainties, this work is intended to provide a deeper understanding of the interaction between hydraulic and mechanical behaviors, enhancing the reliability and accuracy of predictive simulations. Ultimately, this integrated modeling approach will help to improve reactor management by informing more robust fuel management strategies and reducing risks related to fuel assembly deformation
On Cutting Planes for Signomial Programming
No full textaccepted for publication in SIAM Journal on OptimizationInternational audienceCutting planes are of crucial importance when solving nonconvex nonlinear programs to global optimality, for example using the spatial branch-and-bound algorithms. In this paper, we discuss the generation of cutting planes for signomial programming. Many global optimization algorithms lift signomial programs into an extended formulation such that these algorithms can construct relaxations of the signomial program by outer approximations of the lifted set encoding nonconvex signomial term sets, i.e., hypographs, or epigraphs of signomial terms. We show that any signomial term set can be transformed into the subset of the difference of two concave power functions, from which we derive two kinds of valid linear inequalities. Intersection cuts are constructed using signomial term-free sets which do not contain any point of the signomial term set in their interior. We show that these signomial term-free sets are maximal in the nonnegative orthant, and use them to derive intersection sets. We then convexify a concave power function in the reformulation of the signomial term set, resulting in a convex set containing the signomial term set. This convex outer approximation is constructed in an extended space, and we separate a class of valid linear inequalities by projection from this approximation. We implement the valid inequalities in a global optimization solver and test them on MINLPLib instances. Our results show that both types of valid inequalities provide comparable reductions in running time, number of search nodes, and duality gap
Probing gluonic saturation in deeply virtual meson production beyond leading power
No full textInternational audienceExclusive diffractive meson production represents a golden channel for investigating gluonic saturation inside nucleons and nuclei. In this letter, we settle a systematic framework to deal with beyond leading power corrections at small-, including the saturation regime, and obtain the impact factor with both incoming photon and outgoing meson carrying arbitrary polarizations. This is of particular interest since the saturation scale at modern colliders, although entering a perturbative regime, is not large enough to prevents higher-twist effects to be sizable
Search for flavor-changing neutral current interactions of the top quark mediated by a Higgs boson in proton-proton collisions at 13 TeV
No full textInternational audienceA search for flavor-changing neutral current interactions of the top quark (t) and the Higgs boson (H) is presented. The search is based on proton-proton collision data collected in 2016-2018 at a center-of-mass energy of 13 TeV with the CMS detector at the LHC, and corresponding to an integrated luminosity of 138 fb. Events containing a pair of leptons with the same-sign electric charge and at least one jet are considered. The results are used to constrain the branching fraction () of the top quark decaying to a Higgs boson and an up (u) or charm (c) quark. No significant excess above the estimated background was found. The observed (expected) upper limits at 95% confidence level are found to be 0.072% (0.059%) for (t Hu) and 0.043% (0.062%) for (t Hc). These results are combined with two other searches performed by the CMS Collaboration for flavor-changing neutral current interactions of top quarks and Higgs bosons in final states with a pair of photons or of bottom quarks. The resulting observed (expected) upper limits at 95% confidence level are 0.019% (0.027%) for (t Hu) and 0.037% (0.035%) for (t Hc). These results constitute the most stringent limits on these branching fractions to date
Quark confinement from an infrared safe approach
No full textInternational audienceWe revisit the nonabelian dipole problem in the context of a simple semiclassical approach which incorporates some essential features of the infrared sector of Yang-Mills theories in the Landau gauge, in particular, the fact that the running coupling remains of moderate size at infrared scales. We obtain a simple flux-tube solution in a controlled approximation scheme, that we compare to the results of lattice simulations
Review of searches for vector-like quarks, vector-like leptons, and heavy neutral leptons in proton-proton collisions at = 13 TeV at the CMS experiment
No full textInternational audienceThe LHC has provided an unprecedented amount of proton-proton collision data, bringing forth exciting opportunities to address fundamental open questions in particle physics. These questions can potentially be answered by performing searches for very rare processes predicted by models that attempt to extend the standard model of particle physics. The data collected by the CMS experiment in 2015-2018 at a center-of-mass energy of 13 TeV help to test the standard model at the highest precision ever and potentially discover new physics. An interesting opportunity is presented by the possibility of new fermions with masses ranging from the MeV to the TeV scale. Such new particles appear in many possible extensions of the standard model and are well motivated theoretically. They may explain the appearance of three generations of leptons and quarks, the mass hierarchy across the generations, and the nonzero neutrino masses. In this report, the status of searches targeting vector-like quarks, vector-like leptons, and heavy neutral leptons at the CMS experiment is discussed. A complete overview of final states is provided together with their complementarity and partial combination. The discovery potential for several of these searches at the High-Luminosity LHC is also discussed