The seminar takes place on Tuesday, at 5:15pm.
Seminar room : CONF4 (salle E244) - 24 rue Lhomond
Laboratoire de Radioastronomie, ENS Paris
Probing Grand Unified Theories with topological defects in Cosmology
Cosmic strings are linear topological defects which are expected to form during the spontaneous symmetry breaking of a Grand Unified Theory in the early universe. They could allow us to observationally probe particle physics up to energies of 10^16 GeV. The idea of this talk is to explain what are cosmic strings, and why we expect them to be formed in the standard cosmological scenario, before to discuss the current challenges linked with their observation. In a first part, we will present the concept of local symmetries in particles physics, the Standard Model, and explain why Grand Unified Theories are attractive models to describe physics at very high energies. In a second part, we will present in a broad manner the concepts of topological defect, considering both the cases of domain walls and cosmic strings/vortex lines. We fill finally discuss the formation and evolution of a cosmological lattice of cosmic strings, as well as the associated observational studies.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Mechanobiology Institute, National University of Singapore
Grab-and-pull : from motors pinching to nuclear shaking and organ shaping
Cells sense the rigidity of their environment through local pinching exerted by bundles of myosin motors contracting over actin filaments. Recent experiments have shown that the motors within these bundles are surprisingly efficient, exerting forces that are way larger than the classically reported value of 5pN/motor. We interpret this high force generation based on a new molecular motor model, whereby a two-state potential describes the interaction of the myosin head over the two strands of the actin filament. Our mean-field description predicts that the collective dynamics of motors has features of a first order transition. The dynamics of motors has a strong stochastic component, which has been shown experimentally to lead to cell-scale stress fluctuations. We investigate the role of these fluctuations on the stochastic dynamics of an inclusion embedded in an active viscous gel. We show that, in non-equilibrium systems, stress fluctuations give rise to an effective attraction towards the boundaries of the confining domain. We apply our result to the dynamics of deformations of the cell nucleus and we demonstrate the appearance of a fluctuation maximum at a critical level of myosin motor activity, in agreement with experiments. Moving further to larger length scales, we consider the role of myosin motor contractility in organ formation. We model how, during embryogenesis, muscle segments of the zebrafish acquire their characteristic V-shape.
Seminar room : Amphi Dussane - 45 rue d’Ulm
Département de Physique, ENS Paris
Friction at the interface between solid selfassembled monolayers and liquid polymers or solid metal nanotips
The friction between a solid and a liquid was historically assumed to be so large that fluid molecules in contact with a solid boundary were stuck. Thus, the classical no-slip boundary condition was applied. With the advent of micro- and nano-fluidics in the last decades, however, this boundary condition has been observed to fail in many instances. Particularly, as will be discussed here, a slip boundary condition is observed when unentangled polystyrene (PS) dewets from a hydrophobic, alkylsilane self-assembled monolayer (SAM). This boundary condition can furthermore be tuned over an order of magnitude by subtly changing the SAM [1], indicating that the friction between solid and liquid is highly sensitive to atomic level details. We have also investigated the friction between these same SAMs and a solid metal tip of nanoscopic dimensions. Remarkably, this apparent solid/solid friction is velocity dependent and exhibits a crossover from a linear to logarithmic scaling in the velocity [2]. The results can be described in terms of a distribution of nanocontacts with very weak stiffness compared to the expected one for metal/solid contacts. The work presented here has been done in collaboration with the authors of refs. [1, 2].
1. J.D. McGraw, M. Klos, A. Bridet, H. Hä̈hl, M. Paulus, J.M. Castillo, M. Horsch, K.Jacobs, J. Chem. Phys. 146, 203326 (2017).
2. J.D. McGraw, A. Nigues, A. Siria, Nano Letters, 17, 6335 (2017).
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Laboratoire de Physique des Lasers, Université Paris 13
Dipolar quantum gases and magnetism
I will describe an experiment performed with ultra-cold chromium atoms. These atoms interact rather strongly via dipole-dipole interactions. I will discuss the out-of-equilibrium spin dynamics driven by these interactions, and focuss on two cases in which our very complex many-body quantum system can be described by surprisingly simple equations.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Institut d’Électronique et des Systèmes, Université de Montpellier
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Laboratoire Pierre Aigrain, ENS Paris
Seminar room : Amphi Jaurès - 29 rue d’Ulm
C2N, Marcoussis
Fluids of light in semiconductor lattices
When confining photons in semiconductor lattices, their physical
properties can be deeply modified. Photons can behave as massive, or even
infinitely massive, particles, photons can propagate along edge states
without back scattering, photons can become superfluid, photons can behave
as interacting particles. These are just a few examples of the properties
that can be imprinted into fluids of light in semiconductor lattices. Such
manipulation of light presents not only potential for applications in
photonics, but it is also a great promise for fundamental studies. One can
design artificial media with very exotic physical properties at the single
particle level or even more interestingly with many-body interactions.
I will illustrate the variety of physical systems we can emulate with
fluids of light by presenting a few recent experiments. Perspectives in
terms of quantum simulation will be discussed.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
LPS-ENS
Champ magnétique et effet dynamo : des expériences de laboratoire aux objets astrophysiques.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
LPS-ENS
Liquids moving on ordinary surfaces
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Max Planck Institute for Gravitational Physics, Hanover & University of Wisconsin-Milwaukee
The direct observation of gravitational waves
Seminar room : Amphi Jaurès - 29 rue d’Ulm
École Polytechnique
La physique du sport
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Rutgers University, NJ, USA
How to form strongly correlated electron states
Seminar room : Amphi Jaurès - 29 rue d’Ulm
Seminar room : Amphi Jaurès - 29 rue d’Ulm
LKB-ENS
L’onde cobra
L’onde cobra qui résulte de l’explosion d’un treillis de bâtons
d’esquimau est un phénomène physique rendu célèbre par une multitude de
vidéos sur internet. Pour un physicien, elle fournit un exemple non trivial
de dynamique des structures dont la compréhension quantitative approfondie
est néanmoins accessible. Grâce à la combinaison d’une étude expérimentale
et d’une analyse théorique du phénomène, nous avons étudié le couplage
entre la dynamique des bâtonnets uniques et celle de la structure dans son
ensemble. Nous avons en particulier montré que l’onde cobra ne pouvait
exister que dans une gamme étroite de paramètres fixée par la gravité et la
fracture du matériau.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
LPS-ENS
La mécanique de l’origami
Une fois pliée, une simple feuille mince peut présenter des comportements mécaniques particuliers. Allant de la réponse mécanique globale de la feuille (étirement, torsion, flexion, etc.) au phénomène de vieillissement, ils sont dus à un fort couplage entre les singularités plastiques que représentent les plis. D’un simple pli au papier froissé, nous mettrons en lumière la manière dont ces structures se construisent et se complexifient.
Seminar room : Amphi Jaurès - 29 rue d’Ulm
LKB-ENS
Lévitation quantique de l’antihydrogène au dessus d’un puits de Casimir
La réflexion quantique est un phénomène générique pour les ondes de matière dans un potentiel rapidement variable. On l’observe notamment pour les atomes froids tombant sur le puits attractif de Casimir-Polder au-dessus d’une surface solide. Le phénomène peut être utilisé pour faire léviter des atomes d’antihydrogène froids au-dessus d’une plaque materielle. Elle peut même conduire à des précisions améliorées dans les futures mesures de l’accélération de chute libre d’ atomes d’antihydrogène dans le champ gravitationnel terrestre, qui seront faites dans l’expérience GBAR (Gravitational Behaviour of Antihydrogen at Rest) en cours d’installation au CERN.
RECRUTEMENT ACCES DIRECT
Le département de physique
Programme, cours et calendrier 2022 - 2023 