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Nonclassical Nucleation Pathways in Liquid Condensation Revealed by Simulation and Theory
No full textInternational audienceUsing state-of-the-art rare-event sampling simulations, we precisely characterize the nucleation of liquid droplets from a supersaturated Lennard-Jones gas and uncover a key physical feature: critical clusters nucleate with a density that differs substantially from that of the macroscopic equilibrium liquid. Our atomistic simulations also reveal a nonclassical nucleation pathway showing simultaneous growth and densification in liquid condensation. We then exploit these insights to develop a twovariable nucleation theory, in which the cluster density is allowed to vary. Our accessible model based on the capillary approximation is able to quantitatively retrieve the numerical results in nucleation rate and critical cluster properties over a large range of supersaturation. Remarkably, the two-variable model successfully captures the observed nucleation pathway. The effectiveness of this integrated numerical and theoretical framework demonstrates that the cluster density is a decisive variable in nucleation, highlighting the limitations of the single-variable description while offering a robust foundation for its refinement
A fictitious domain method with enhanced interfacial mass conservation for immersed FSI with thin-walled solids
No full textInternational audienceIn this paper, we extend the low-order fictitious domain method with enhanced mass conservation, introduced in [ESAIM: Math. Model. Numer. Anal., 58(1):303--333, 2024], to fluid-structure interaction with immersed thin-walled solids. The key idea is to improve mass conservation across the interface by imposing a single global velocity constraint on one side of the interface using a scalar Lagrange multiplier. Both 2D and 3D shell models are considered for the description of the solid, including contact between solids. For both models, the interface coupling is enforced on the mid-surface of the shell using a stabilized Lagrange multiplier formulation. Numerical examples in two and three dimensions illustrate the effectiveness of the proposed method, including its successful application to the simulation of aortic heart valve dynamics
Looking elsewhere: improving variational Monte Carlo gradients by importance sampling
No full textInternational audienceNeural-network quantum states (NQS) offer a powerful and expressive ansatz for representing quantum many-body wave functions. However, their training via Variational Monte Carlo (VMC) methods remains challenging. It is well known that some scenarios -such as sharply peaked wave functions emerging in quantum chemistry -lead to high-variance gradient estimators hindering the effectiveness of variational optimizations. In this work we investigate a systematic strategy to tackle those sampling issues by means of adaptively tuned importance sampling. Our approach is explicitly designed to target the gradient estimator instead of the loss function and be computationally inexpensive. We benchmarked our approach across the ground-state search of a wide variety of hamiltonians, including frustrated spin systems and ab-initio quantum chemistry. Overall, our approach can reduce the computational cost of vanilla VMC considerably, up to a factor of 100x when targeting highly peaked quantum chemistry wavefunctions
Le congé de paternité, prolongé et modulable, est largement adopté par les pères
No full textInternational audienceLa réforme du congé de paternité en 2021 a allongé sa durée et assoupli ses modalités. La part de pères prenant ce congé continue à augmenter, notamment chez ceux qui y avaient le moins recours comme les indépendants, les salariés en contrat à durée déterminée, les pères moins diplômés. La présence des pères au moment de la naissance se généralise et dure plus longtemps. Cependant, les contraintes professionnelles et financières restent un frein à la prise du congé. Même s’ils restent minoritaires, les pères qui prennent le congé « en solo », alors que la mère a repris le travail, sont de plus en plus nombreux
On the Cutoff Phenomenon for Dyson-Jacobi Processes
No full textWe study the convergence to equilibrium of the Dyson-Jacobi process, a system of n interacting particles on the segment [0, 1] arising from Random Matrix Theory. We establish the occurence of a cutoff phenomenon for the intrinsic Wasserstein distance and provide an explicit formula for the associated mixing time.Our approach relies on the interplay between the Riemannian geometry of the process and a flattened Euclidean representation obtained via a diffeomorphic deformation. This transformation allows us to transfer curvature-dimension inequalities from the Euclidean setting to the original space, thereby yielding sharp quantitative estimates.</div
Static and dynamic Monte Carlo simulations of phonon drag effects on thermoelectric properties in silicon nanostructures
No full textInternational audienceThermoelectric transport in silicon nanofilms is investigated using a self-consistent electro-thermal Monte Carlo simulator that couples electron dynamics to a phonon bath with spatially varying temperature. A key novelty of this work is the explicit inclusion of the phonon-drag contribution, implemented by modifying the electron-phonon momentum exchange based on the local deviation of the phonon distribution from equilibrium. The method is validated against bulk silicon data and extended to incorporate rough boundary scattering for both electrons and phonons, yielding excellent agreement with experimental measurements on nanofilms. We also analyze the transient regime and show that a temperature bias produces a slower current response than a voltage bias, although the phonon-drag effect itself tends to accelerate the response. These results demonstrate that the proposed framework provides a powerful tool for predicting both steady-state and time-dependent thermoelectric behavior in semiconductor nanostructures
Holographic shear correlators at low temperatures, and quantum
No full textInternational audienceThe strongly-coupled 3-dimensional theory, holographically dual to black branes at fixed chemical potential \muext and temperature is considered in AdS Einstein-Maxwell theory. The retarded Green's functions at frequency is calculated using holography in the regime ω, T \ll \muext but otherwise arbitrary. When the transverse space has finite volume, there is a non-zero energy scale , scaling as for large , below which quantum-gravitational corrections due to the fluctuations of the nearly-gapless Schwarzian modes become important. Such corrections to the retarded Green's function are calculated at different relative values of , , and . The limit is used to define the shear viscosity . As the temperature is lowered below , quantum corrections are found to increase the value of with respect to its semiclassical value. The quantum-corrected result for diverges as at , in accord with corresponding results for the absorption cross section. The quantum result for the ratio , where is the entropy density, dips below the semiclassical limit of when , then turns back to increase towards lower temperatures, and finally diverges at temperatures much below
Long-range minimal models
No full textInternational audienceWe study a class of nonlocal conformal field theories in two dimensions which are obtained as deformations of the Virasoro minimal models. The construction proceeds by coupling a relevant primary operator of the -th minimal model to a generalized free field, in such a way that the interaction term has scaling dimension . Flowing to the infrared, we reach a new class of CFTs that we call long-range minimal models. In the case , the resulting line of fixed points, parametrized by , can be studied using two perturbative expansions with different regimes of validity, one near the mean-field theory end, and one close to the long-range to short-range crossover. This is due to a straightforward generalization of an infrared duality which was proposed for the long-range Ising model () in 2017. We find that the large- limit is problematic in both perturbative regimes, hence nonperturbative methods will be required in the intermediate range for all values of . For the models based on , the situation is rather different. In this case, only one perturbative expansion is known but it is well behaved at large . We confirm this with a computation of infinitely many anomalous dimensions at two loops. Their large- limits are obtained from both numerical extrapolations and a method we develop which carries out conformal perturbation theory using Mellin amplitudes. For minimal models, these can be accessed from the Coulomb gas representations of the correlators. This method reveals analytic expressions for some integrals in conformal perturbation theory which were previously only known numerically
Decomposition of O3 onto natural Gobi dust: From uptake to surface reactivity
No full textInternational audienceBeyond synthetic metal oxides, this work investigates natural mineral dust as a material of interest for O3 uptake and decomposition. Ozone and Gobi dust interactions are addressed combining gas phase and adsorbed phase approach. It allows for determining: (i) the drivers of O3 decomposition process on Gobi and (ii) surface reactions and mechanism. First, the steady catalytic decomposition of ozone onto Gobi is explored on a large O3 concentration range: 20 ppb − 10 ppm, with steady uptake coefficients ranging from 2.6 x 10-9 to 6.2 x 10-8. If moisture (20 % RH) does not impact initial uptake of O3 it is evidenced to hinder the steady state behavior. Cyclic surface regeneration evidences that: (i) wet air flushing restores initial uptake properties, and (ii) mild thermal treatment (150 °C) restores the complete uptake process. Along O3 decomposition, adsorbed phase monitoring using DRIFT allows for identification and dynamic monitoring of specific IR bands on Gobi surface. Oxide and peroxide surface species are created. To meet environmental conditions, the role of water on surface groups created by O3 uptake is addressed through water molecule uptake experiments. Finally, a five-step mechanism is proposed to describe uptake and surface reactivity of O3 on Gobi dust. This work provides a comprehensive evaluation of the surface properties and reactivity of Gobi dust towards O3, valuable for atmospheric, geocatalytic and plasma-catalytic processes
Entanglement entropy, Monte Carlo event generators, and soft gluons DIScovery
No full textInternational audienceWe study entropy production in Deep Inelastic Scattering using Monte Carlo simulations. We show that the dominant contribution to entropy is due to soft gluons. This contribution is usually neglected in standard Monte Carlo approaches, since it does not affect hadronic spectra. However, it is relevant for entropy and multiplicity distributions, as we demonstrate with explicit calculations. We further show that, as one lowers the cutoff - i.e. includes soft gluons - thereby making the Monte Carlo parton distributions closer to inclusive PDFs, the resulting entropy is very close to that measured by the H1 collaboration, without the need to account for hadronization effects. This provides further evidence that the bulk of the measured entropy originates from initial-state effects