Scale separation is the important phenomenological property for a given vacuum to have an internal compact space much "smaller" than the extended spacetime, so that a lower-dimensional effective description indeed makes sense. At a practical level, this is measured by the decoupling or not of the magnitude of the cosmological constant from the Kaluza-Klein scale. The status of scale separation in AdS vacua obtained from string theory is under debate. On the one hand bottom-up constructions in type IIA string theory seem to achieve this hierarchy of scales parametrically. However on the other hand some Swampland arguments as well as holographic considerations cast a pessimistic shadow on scale separation and about its presence in the Landscape. After an overview of the evolution of scale separation in string theory over the last twenty years, I will present new families of scale separated vacua obtained via a 4d EFT analysis in massless type IIA flux compactifications on elliptic fibrations with metric fluxes. Parametric scale separation is achieved by an asymmetric flux rescaling which, however, in general is not a simple symmetry of the 4d equations of motion. At this level of approximation the vacua are stable but, unlike in the Calabi-Yau case, they display a non-universal mass spectrum of light fields.

The clustering of eukaryotic chromosomes during mitotic exit is fundamental to ensure that large molecules do not get trapped inside the reforming nuclear envelope. Recent experiments showed that this process is driven by the interaction between the Ki-67 protein brush coating the chromosome surface and RNA. It has also been proposed that a highly charged patch (CP) on Ki-67 is the main driver of Ki-67--RNA attraction. The microscopic physical mechanism behind this attractive interaction remains however not understood. Here, we employ coarse-grained molecular dynamics simulations to study how the interaction between positively charged Ki-67 brushes and negatively charged RNA molecules drives inter-chromosome attraction. To understand the effect of the CP, we compare a system with uniformly charged Ki-67 molecules to one in which part of the molecule's charge is concentrated on a patch. Our model proposes as a possible mechanism of inter-chromosome attraction the formation of RNA bridges between the brushes, that act as entropic springs, pulling the chromosomes together. Additionally, we find that the CP acts as a strong "sticker", favoring the formation of more numerous and long-lived RNA bridges, and thus leading to stronger inter-chromosome attraction compared to uniformly charged Ki-67 molecules. Our work elucidates the physical origin of chromosome clustering, while also suggesting a possible mechanism at play in the formation of intracellular condensates.

The paper reviews experimental and theoretical studies on polarized terahertz radiation-induced photocurrents in graphene and graphene-based 2D materials. We consider the optoelectrical phenomena excited by THz radiation in graphene and twisted graphene and present the state of the art in this field, including both recent advances and well-established results, see e.g. [1-7]. Different physical mechanisms of the THz radiation excited dc currents proportional to the second powers of the radiation electric field are presented, including phenomenological description based on symmetry arguments, models visualizing the physics of nonlinear responses, and microscopic theory of several phenomena. The following effects yielding a dc current proportional to the square of the irradiation electric field are discussed: high-frequency Hall effect at zero magnetic field, edge photogalvanic effects, ratchet effects in graphene with lateral superlattices, cyclotron resonance (CR) assisted dc currents, terahertz induced magnetooscillations coupled to the CR harmonics, Bernstein modes, as well as multiple oscillations upon variation of the gate voltage in twisted bilayer graphene near the second magic angle. We show that nonlinear transport opens up new possibilities for probing Dirac electrons.

[1] S.D. Ganichev et al, Annalen der Physik 529, 1600406 (2017)
[2] M.M. Glazov et al, Physic Reports 535, 101, (2014)
[3] M. Otteneder et al, Nano Lett. 20, 7152 (2020)
[4] E. Mönch et al, Nano Lett. 20, 5943 (2020)
[5] D. A. Bandurin et al, Nature Physics 18, 462 (2022)
[6] S. Candussio et al, Phys. Rev. B 103, 125408 (2021)
[7] E. Mönch et al, Phys. Rev. B 107, 115408 (2023)

The existence of nonbaryonic dark matter (DM) in the Universe is compelling, as suggested by astrophysical and cosmological observations. The most commonly assumed production mechanism for DM in the early universe corresponds to the weakly interacting massive particle (WIMP) paradigm, in which DM has mass and couplings at the electroweak scale. However, the current null experimental results and severe constraints on the natural parameter space are forcing us to search beyond the standard WIMP paradigm. In this talk, I will review alternative DM production mechanisms in the early universe, both thermal and non-thermal, like the FIMP and the SIMP paradigms. The possible impact of alternative non-standard cosmological scenarios will also be analyzed. Finally, experimental avenues for DM detection are discussed.

Transport processes generating irreversibility at the macroscale, the hydrodynamic equations must include these dissipative effects to be complete. The hydrodynamic modes stem from the fundamental conservation laws and from the breaking of continuous symmetries. A key question is to deduce the dissipative hydrodynamic equations ruling all these modes from the microscopic Hamiltonian dynamics of the particles with the methods of non-equilibrium statistical mechanics. In this presentation, a first-principle derivation based on a local-equilibrium approach is introduced. As an illustrative example, the crystalline solid is considered. The breaking of the three-dimensional continuous group of spatial translations implies the existence of three slow modes in addition to the five slow modes arising from the fundamental conservation laws of mass, energy, and linear momentum. Based on the results of the local-equilibrium approach, the equilibrium and nonequilibrium properties relevant to the dissipative hydrodynamics of the hard-sphere crystal are obtained with molecular dynamics simulations. Finally, the application of the method to amorphous solids and active matter systems is discussed.

The study of hydrodynamics at increasingly small scales raises fundamental questions about the validity of classical hydrodynamics hypothesis.
Particularly, the sharp interface hypothesis becomes increasingly difficult to verify for nanofluidic systems where the typical size approaches the interface thermal fluctuations length scale. More generally the interplay between macroscopic hydrodynamics and statistical physics at the interface scale is expected to have considerable influence for multi-scales problems. For example, phase-field simulations predict that the viscous dissipation singularity at a moving contact is regularised by evaporation-condensation mechanisms at the interface level.

Following that, here we present an experimental and numerical investigation of the spreading of a sessile droplet for near-critical two-phases liquid mixture. The Tanner's law regime is robustly verified even for droplets close to the critical temperature showing the universality of the viscous spreading. However the system at the droplet scale is thermodynamically unstable and the droplet also shrinks in size. The mechanisms behind this shrinking analogous to evaporation are then explored for spherical aerosols droplets showing gravity and curvature induced diffusion mechanisms. The coupling between spreading and evaporation is then discussed to give a complete picture of the sessile droplet dynamics.

We will then discuss, despite more briefly, an evaporation-driven colloidal skin formation inside an evaporating droplet deposited on superhydrophobic surfaces. The particle's assembly dynamics is explored using molecular dynamics simulations introducing the effect of droplet's evaporation where the skin formation for high Peclet. Strikingly the skin-bulk transition zone can be modeled through a viscous-diffusion balance and shows analogue properties of a phases separating interface but here with a length scale determined by the Peclet number.

Dans cet exposé, je présenterai la méthode des éléments finis (MEF) en électromagnétisme, en m'attardant sur notamment ses spécificités par rapport à d'autres méthodes numériques (e.g. différence finies, méthodes de Fourier, de Mie...) et ainsi que sur les cas typiques où cette méthode est pertinente.
Je présenterai la méthode du point de vue discret dans le cas 1D dont l'intérêt est principalement didactique puisqu'on peut assembler le système discret "à la main" et résoudre un premier problème de diffraction : le slab. J'aborderai la question de recherche de fréquences propres et de modes de systèmes optiques.
Je montrerai l'utilisation des logiciels libres Gmsh et GetDP ainsi que leur interface ONELAB développés par nos collègues de l'Université de Liège et détaillerai l'implémentation d'un problème de diffraction en 2D et en 3D.
Enfin, je tenterai de dresser un comparatif entre ces logiciels libres et des logiciels propriétaires bien connus.

The study of the yielding transition of amorphous materials is a complex phenomenon that can manifest with profoundly different macroscopic features in different circumstances.

While some materials yield abruptly, in a way often referred to as brittle, others exhibit an extended nonlinear deformation regime characterized by plasticity, a phenomenology typically called ductile failure. For one given material, yielding can be either ductile or brittle depending on the deformation rate and/or on the temperature. More recently, a series of computer simulation works suggested that, even under the same testing conditions (rate and temperature), ductile or brittle failure may be observed for the same material, depending on the previous mechanical or thermal history. Despite huge progress in this field, a comprehensive understanding of the transition between ductile and brittle yielding is still missing, mostly because stringent experimental validations of the ideas proposed in the literature are still scarce.

In this seminar I will present recent research project addressing two important questions related to the ductile to brittle transition:

1 - What is the interplay between the microscopic dynamic transition, the formation of shear bands and the macroscopic weakening of a Yield Stress Fluid?

2 - Can we use mechanical conditioning to effectively prepare the same YSF in different states with different yielding properties?

To address the first question, I will present results of simultaneous experimental study of the (a) rheological response of soft glasses, (b) mesoscopic deformation fields, and (c) microscopic plastic dynamics. Both homogeneous yielding and shear banding cases are studied [[1]].

For the second question, we combined experimental rheology and particle-dynamic simulations, with the aim to rationalize the role of internal stress distribution on material’s yielding properties, memory, ageing and annealing [[2]].


[1] Paolo Edera, Matteo Brizioli, Mahnoosh Madani, Elie N’Gouamba, Philippe Coussot, Veronique Trappe, George Petekidis, Fabio Giavazzi, Roberto Cerbino. Yielding under the microscope: a multi-scale perspective on brittle and ductile behaviors in oscillatory shear. https://arxiv.org/abs/2402.00221

[2] Paolo Edera, Minaspi Bantawa, Stefano Aime, Roger Bonneacaze, Michel Coitre. Tuning residual stress, directional memory and aging in soft glassy materials. https://arxiv.org/abs/2402.08293