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Stability of discontinuous diffusion coefficients for the heat equation on a star-shaped tree
No full textIn this paper, we study the heat equation on a star-shaped tree network with a piecewise regular diffusion coefficient. By developing new Carleman estimates, we establish stability results for the identification of the diffusion coefficient. These stability estimates are derived using either internal measurements or boundary observations, offering robust insights into the inverse problem for this class of equations
Stochastic thermodynamics of holonomic systems
No full textInternational audienceStochastic thermodynamics is a recently developed framework designed to describe the behavior of small systems in contact with one or more thermal reservoirs. Originally applied to systems of unconstrained particles, it has provided deep insights into the thermodynamic behavior of micro-and nanoscale systems, enabling the formulation and experimental verification of fundamental fluctuation theorems. With advances in nanotechnology, many of these theoretical predictions have now been validated experimentally. In this presentation, we extend the framework of stochastic thermodynamics to arbitrary holonomic systems subjected to general external forces, described within both Lagrangian and Hamiltonian formalisms. For both the underdamped and overdamped regimes, we derive the principles of thermodynamics in out-of-equilibrium settings, offering microscopic interpretations of heat, energy, and entropy. To ensure consistency with Brownian motion on smooth manifolds, we employ the Klein-Kramers and Smoluchowski equations in their covariant forms for the underdamped and overdamped cases, respectively. Furthermore, we provide explicit expressions for entropy production, enabling their application to the non-equilibrium thermodynamics of constrained mechanical systems. Possible applications concern biophysics and soft matter (polymer and membrane dynamics, molecular motors. . . ), materials science (rate effects in decohesion and adhesion phenomena, crack nucleation, friction, dislocation motion), quantum-classical bridges (quantum open systems), multiscale modeling (merging atomistic and mesoscopic/macroscopic models), and the development of a generalized continuum theory with rigorous introduction of thermal fluctuations effects.</div
Exploring temperature effects on fracture propagation and decohesion phenomena through classical statistical mechanics
No full textInternational audienc
Uncertainty-driven Active Reinforcement Learning for Energy Management Strategy in Electrified Vehicles
No full textInternational audienceReinforcement learning (RL), as a data-driven optimization method, enables electrified vehicles to train optimal energy management strategy (EMS) in complex and dynamic environments. However, traditional RL methods often face inefficiency during the exploration process, particularly in highdimensional continuous state spaces, leading to slow convergence or a tendency to fall into local optima. To address these issues, this paper proposes an uncertainty-driven active RL (UaRL) for EMS, which is built upon the soft actor-critic algorithm, using the dual Q-values difference as a measure of uncertainty. When the uncertainty increases significantly, the active learning mechanism is activated, allowing the UaRL system to select more reliable actions through rule-based methods and guide the improvement of the RL policy. Experimental results show that the novel UaRL approach has significant advantages in energy efficiency, convergence speed, and policy stability. Compared to vanilla RL, the proposed method improves economic performance by an average of 14.68%. It also demonstrates superior performance in battery health management and capacity stability. This study provides an efficient and reliable hybrid learning framework for EMS in electrified vehicles
7 GHz integrated modulator at 8.5 µm wavelength
No full textInternational audienceMid-infrared (mid-IR) spectroscopic systems have gained growing importance due to their ability to operate in the 2-20 µm spectral range, what makes them very useful in different applications such as defense, food safety control [1] or medical diagnosis [2]. This spectral range includes the molecular fingerprint region, a range of wavelengths where many molecules exhibit unique vibrational and rotational signatures, usually between 2.5 and 13 µm. However, conventional mid-IR spectrometers are often based on a free-space configuration what makes them large, costly, and unsuitable for field deployment. Developing a miniaturized, chip-integrated mid-IR spectroscopic system presents a compelling solution, enabling more practical and cost-effective implementations. Silicon-germanium (SiGe) alloys-based waveguides have emerged as a strong candidate for mid-IR photonic integration. They benefit from mature silicon (Si) fabrication processes while offering an extended optical transparency window beyond 8 µm, due to the wide transparency window of germanium (Ge) extending up to 15 µm. Furthermore, by fine-tuning the Ge content in the SiGe alloy, it enables precise control of its optical properties, facilitating the seamless integration of essential photonic components such as low-loss optical waveguides and high-speed modulators [3]
[Invited] Polyoxometalate Molecular Electronics.
No full textInternational audiencePolyoxometalates (POMs) are molecular nanostructures consisting of early transition metals and oxygen [1, 2]. As molecular oxides, they possess remarkable redox properties, combining the reducibility of bulk metal oxides with the high versatility of molecular species. They undergo successive, reversible and highly adjustable mono- (or multi-) electronic reduction processes within a narrow potential range. Acting as a missing link between the metal oxides found in nanoelectronics and conventional organic or organometallic molecules, POMs have attracted an increasing interest in the field of nanoelectronics [3, 4]. In this context, we have explored various strategies to immobilize POMs onto electrodes, with the objective of improving control of the molecule/electrode interfaces [5-7]. We have investigated the electron transport properties of POM-based molecular junctions to establish relationships between the molecular structure of POMs, their electronic structures, and the properties of POM devices (e.g., conductance and switching behavior) [8]. Furthermore, POMs are polyanions, with counter cations playing a crucial role in the electron transport properties, beyond ensuring charge neutrality [9, 10]. We have demonstrated that the redox state of a POM layer can be switched by exposure to light [8] or application of an electric field, opening up new possibilities for stimulus-responsive devices. Perspectives on neuromorphic device applications will be highlighted in this presentation [11, 12].[1] M.T. Pope, A. Müller, Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines, Angew. Chem. Int. Ed., 30 (2003) 34-48.[2] X. Lopez, J.J. Carbo, C. Bo, J.M. Poblet, Structure, properties and reactivity of polyoxometalates: a theoretical perspective, Chem. Soc. Rev., 41 (2012) 7537-7571.[3] D.L. Long, R. Tsunashima, L. Cronin, Polyoxometalates: building blocks for functional nanoscale systems, Angew. Chem. Int. Ed., 49 (2010) 1736-1758.[4] C. Busche, L. Vila-Nadal, J. Yan, H.N. Miras, D.L. Long, V.P. Georgiev, A. Asenov, R.H. Pedersen, N. Gadegaard, M.M. Mirza, D.J. Paul, J.M. Poblet, L. Cronin, Design and fabrication of memory devices based on nanoscale polyoxometalate clusters, Nature, 515 (2014) 545-549.[5] M. Laurans, K. Dalla Francesca, F. Volatron, G. Izzet, D. Guerin, D. Vuillaume, S. Lenfant, A. Proust, Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions, Nanoscale, 10 (2018) 17156-17165.[6] K. Dalla Francesca, S. Lenfant, M. Laurans, F. Volatron, G. Izzet, V. Humblot, C. Methivier, D. Guerin, A. Proust, D. Vuillaume, Charge transport through redox active [H7P8W48O184]33-polyoxometalates self-assembled onto gold surfaces and gold nanodots, Nanoscale, 11 (2019) 1863-1878.[7] M. Laurans, K. Trinh, K. Dalla Francesca, G. Izzet, S. Alves, E. Derat, V. Humblot, O. Pluchery, D. Vuillaume, S. Lenfant, F. Volatron, A. Proust, Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides, ACS Appl. Mater. Interfaces, 12 (2020) 48109-48123.[8] C. Huez, D. Guérin, S. Lenfant, F. Volatron, M. Calame, M.L. Perrin, A. Proust, D. Vuillaume, Redox-controlled conductance of polyoxometalate molecular junctions, Nanoscale, 14 (2022) 13790-13800.[9] K.Y. Monakhov, Implication of counter-cations for polyoxometalate-based nano-electronics, Comments on Inorganic Chemistry, 44 (2022) 1-10.[10] C. Huez, S. Renaudineau, F. Volatron, A. Proust, D. Vuillaume, Experimental observation of the role of countercations in modulating the electrical conductance of Preyssler-type polyoxometalate nanodevices, Nanoscale, 15 (2023) 10634-10641.[11] H. Tanaka, M. Akai-Kasaya, A. TermehYousefi, L. Hong, L. Fu, H. Tamukoh, D. Tanaka, T. Asai, T. Ogawa, A molecular neuromorphic network device consisting of single-walled carbon nanotubes complexed with polyoxometalate, Nat. Commun., 9 (2018) 2693.[12] C. Huez, D. Guérin, F. Volatron, A. Proust, D. Vuillaume, Low-frequency noise in nanoparticle–molecule networks and implications for in materio reservoir computing, Nanoscale, 16 (2024) 21571-21581
Hardware implementation of tunable fractional-order capacitors by morphogenesis of conducting polymer dendrites
No full textInternational audienceConventional electronics is founded on a paradigm where shaping perfect electrical elements is done at the fabrication plant, so as to make devices and systems identical, “eternally immutable.” In nature, morphogenic evolutions are observed in most living organisms and exploit topological plasticity as a low-resource mechanism for in operando manufacturing and computation. Often fractal, the resulting topologies feature an inherent disorder: a property that is never exploited in conventional electronics manufacturing, while necessary for data generation and security in software. In this study, we present how such properties can be exploited to implement long-term and evolvable synaptic plasticity in an electronic hardware. The rich topology of conducting polymer dendrites (CPDs) is exploited to program the non-ideality of their electrochemical capacitances containing constant-phase-elements. Their evolution through structural changes alters the characteristic time constants for them to charge and discharge with the applied voltage stimuli. Under a train of voltage spikes, the evolvable current relaxation of the electrochemical systems promotes short-term plasticity, with timescales ranging from milliseconds to seconds. This large window depends not only on the temporality of the voltage pulses used for reading but also on the structure of a pair of CPDs on two electrodes, grown by voltage pulses. This study demonstrates how relevant physically transient and non-ideal electrochemical components can be exploited for unconventional electronics, with the aim to mimic a universal property of living organisms, which could barely be replicated in a silicon monocrystal
Recent Progress of Multifunctional Silica Aerogel Composite Materials: From Fabrication Strategies to Versatile Applications
No full textInternational audienc
A Theoretical Analysis of the Frequency Response in p-i-n Photodiodes that Use InGaAs/InP Materials
No full textInternational audienceThis investigation is centered on the analysis of frequency response characteristics of a p-i-n photodiode using InxGaAs/InP. The InGaAs/InP can be developed under three conditions: compression, tensile strain, and lattice matching. Initially, we performed calculations on strain, bandgap energy (E), and absorption coefficient. We then optimized the influence of indium concentration (x) on stability, critical thickness, bandgap energy, and absorption coefficient. The effects of temperature and deformation on E were also studied. Finally, we optimized the cutoff frequency (f), capacitive effects, and response frequency by considering the impact of x, active layer thickness (d), and surface area (S). For our future endeavors, we intend to explore additional parameters that may affect the p-i-n response. In future work, we can add transparent double layers in the i. InGaAs layer to reduce the transit time, leading to the development of an ultrafast photodiode
Substitution Na/La dans les verres borosilicates de sodium et de lanthane : Synthèse, caractérisation et étude de la propagation des ondes de Lamb par Ultrasons-Laser
No full textInternational audienceSynthèse et élaboration des verres• Etude structurale des verres par spectroscopie Raman, FTIR et RMN. •Caractérisation par Ultrasons-Laser à l'aide des ondes de Lamb de plaques de verre borosilicate de sodium et de lanthane. •Comparaison des résultats de modélisation et expérimentaux pour valider la résolution du problème inverse (détermination des propriétés élastiques des verres étudiés).</div