L'objectif de cette rencontre est de développer les échanges entre, d'une part, le Laboratoire de Physique et l'Unité de Mathématiques Pures et Appliquées de l'ENS de Lyon et, d'autre part, l'Institut de Physique Théorique et le Service de Physique de l'État Condensé du CEA Saclay.
Oratrices et orateurs :
Understanding how electrons travel in a quantum circuit is one of the main goals of mesoscopic physics. This is particularly crucial in systems in which electronic interactions are exacerbated. For instance, electrostatic interactions between nearby one-dimensional edge channels of the quantum Hall effect have been shown to play a much more important role than what can be naively thought. Indeed, the excitations of the apparently simple system formed by two co-propagating edge channels determined by interactions, which ends up limiting the quantum coherence of electrons traveling in such a system. A seemingly straightforward question has remained unanswered: if one injects an electron at a well-defined energy in an edge channel, and lets it propagate in presence of a nearby edge channel, does it lose energy because of interactions, and can one still detect it after propagation?
We have directly addressed this fundamental question by performing an energy resolved injection and detection of electrons in a quantum Hall edge channel. We observe for the first time in this system the presence of electrons remaining at finite energy after propagation, and show that their population is strongly suppressed as one increases either their energy, or the propagation length. At sub-microns length, we observe a remarkable revival of finite energy electrons, which corresponds to the recombination of excitations that have split between the two edge channels.
Our findings give us a better understanding of the role of interactions in quantum Hall edge channels, as well as methods to harness them. More complex experiments mimicking quantum optics with electrons can now be envisioned.
In presence of impurities, ferromagnetic and ferroelectric domain walls slide only above a finite external field. Close to this depinning threshold, they proceed by large and abrupt jumps, called avalanches, while, at much smaller field, these interfaces creep by thermal activation. In this talk I will present our results for the creep dynamics at vey low forces, obtained by a novel numerical technique that captures this ultra-slow regime over huge time scales. We point out the existence of activated events that involve collective reorganizations similar to avalanches, but, at variance with them, display correlated spatio-temporal patterns. We show that these events assembly in independent clusters that display at large scales the same statistics as critical depinning avalanches. I will finally comment on some experiments in ferromagnetic films that aims to capture such correlated dynamics.
Most kinetic equations can be interpreted as laws of large numbers for the evolution of the empirical density. We will study the probability of departure from this law of large numbers, using large deviation theory. From this point of view, we will describe classical and more original kinetic equations: the Boltzmann equation, the Vlasov—Mac-Kean equation, and the effective description of large scale turbulent flows. We will establish the relation between the mathematical structure of the path probability large deviations and fundamental physical properties: entropy production and gradient structures. We will also stress the importance of large deviations for applications, for instance for abrupt climate changes of Jupiter’s troposphere jets.
Buoyancy driven turbulent flows pertain to numerous geo- and astrophysical systems, where they have far reaching consequences: understanding them in atmospheric and oceanographic context is of paramount importance for climate modelling; in planetary and stellar cores, they control magnetic field generation via the dynamo effect, to cite a few examples.
Experimentally, convective flows have often been driven by heat fluxes from the boundaries. This setup is conducive to the formation of boundary layers that throttle the heat transfer, resulting in a diffusivity-influenced heat transfer. By contrast, an experience where heat is deposited directly in volume (in view of by-passing boundary layers) has been designed and realised at SPEC: dyed water is heated through a transparent bottom by means of a powerful spotlight. This experiment has evidenced an inviscid heat-transfer regime (sometimes referred to as the “ultimate regime” or the “mixing length regime”): the dimensionless heat flux (Nusselt number) increases like the square root of the dimensionless temperature difference (Rayleigh number).
After providing a summary of those recent experimental results, I will complement them with numerical results concerning the influence of the Prandtl number (the ratio of velocity and thermal diffusivities of the fluid) on the heat transport properties.
Over the last decade, topology sparked a new line of research in solid-sate physics, revolutionizing our understanding of electronic properties of matter. Today, the use of topology extends far beyond the specifics of solids and offers a single framework to understand optical, mechanical and electronic phenomena. In this seminar I will illustrate how ideas originating from mechanics allow to revisit the characterization of electronic phases with a chiral symmetry, such as the poly-acetylene chain. I will then discuss a unique example of topological mechanics by focusing on the deformations of Möbius strips. I will establish that the elastic response of surfaces with non-orientable topology is: non-additive, non-reciprocal and contingent on stress-history. Beyond the specifics of mechanics, non-orientability offers a versatile platform to tailor the response of systems as diverse as liquid crystals, photonic and electronic matter.
A discrete model of statistical mechanics in 2D (for example simple random walk on the infinite square grid) can be defined on a graph without specifying a particular embedding of this graph. However, when stating that such a model converges to a conformally invariant object in the scaling limit, one needs to specify an embedding of the graph. For models which possess a local move, such as a star-triangle transformation, one would like the choice of the embedding to be compatible with that local move.
In this talk I will present a candidate for an embedding adapted to the 2D dimer model (a.k.a. random perfect matchings) on bipartite graphs, that is, graphs whose faces all have an even degree. This embedding is obtained by considering centers of circle patterns with the combinatorics of the graph on which the dimer model lives.
This is based on joint works with Dmitry Chelkak (École normale
supérieure), Richard Kenyon (Yale University), Wai Yeung Lam (Université du Luxembourg) and Marianna Russkikh (MIT).
In this talk I will discuss how to get large deviations estimates for invariant ensembles of random matrices based on the Itzykson-Zuber-Harich-Chandra integral. This applies for instance for the empirical measure of the diagonal entries of UBU or of the eigenvalues of A+UBU, when U follows the Haar measure on the unitary group.
Phase separation in equilibrium systems, think to a fluid separating in macroscopic vapour and liquid regions, is described by the Model B, the dynamical version of \phi^4 theory. This is built upon gradient expansion including all the terms not forbidden by microscopic symmetries. Phase separation happens also in many non-equilibrium contexts, where time-reversal symmetry is violated. This happens in systems as diverse as active systems, the interior of cells or shaken granular matter. I will present our recent efforts describing phase separating systems when time-reversibility is locally broken. Surprisingly, the ensuing phenomenology is deeply affected both quantitatively and qualitatively. Part of our discussion will be focused on active systems, those where each degree of freedom is able to transform non-thermal energy into motion (such as animals or bacteria), but our field-theoretical results are expected to apply much more broadly.
There are several examples of condensed matter systems exhibiting macroscopic quantum coherence superfluid liquid Helium, superconductors, atomic BECs. Recent years have added the condensation of excitons in the quantum Hall regime to this list. The interlayer coherence at total filling factor unity involves electrons and holes. We know that fractional quantum Hall states are successfully described by "composite fermions" i.e. electrons dressed by flux tubes. Rcent experiments have given evidence for Bose condensation of excitons made out of such quasiparticles. I will attempt a description of this fragile scaffolding of concepts.
Similar to a spinning top, the natural dynamics of the magnetization vector in ferromagnetic media corresponds to a small-angle precession around its equilibrium position. Remarkably, this behavior is highly nonlinear as the angle of precession is increased, yielding a series of interesting phenomena, such as the formation of dynamical solitons, spin-wave (SW) turbulences and chaos, and Bose-Einstein condensation of magnons, the quanta of SWs. Recent developments in magnetic nanotechnologies have also demonstrated that ferromagnetic resonance and SW dynamics can be excited either by microwave magnetic fields or by spin transfer torques, with the promise of innovative magnonic and spintronic devices for information and communication technologies. In this area, spin torque nano-oscillators, which exhibit strong nonlinear properties, have even been successfully implemented to perform neuromorphic tasks.
In this talk, I will review the recent results obtained in this field, with a special focus on two experiments performed at CEA/SPEC.
These recent years have seen spectacular breakthrough in the manipulation of quantum electrical circuits. On-demand single electron sources in quantum Hall edge channels enable us to engineer time-dependent quantum electrical currents involving one to a few elementary excitations per period. The emerging field, called electron quantum optics, precisely aims at generating, manipulating and characterizing such ``quantum beams of electricity'' in normal quantum conductors.
In this talk, I will discuss the basics concepts underlying the field and explain how we have been able to access the single particle wavefunctions carried by these quantum electrical currents with their emission probabilities and coherences. I will also review electronic decoherence, thereby making contact with François Parmentier’s presentation.
In brittle solids, stress enhancement at the crack’s tip makes the fracture behavior observed at the macroscopic scale extremely dependent of the presence of material inhomogeneities down to very small scale. This translate into giant statistical fluctuations and non-trivial behaviors upon upscaling impossible to assess in the frame of engineering continuum mechanics. The experimental and theoretical approaches we develop at SPEC aim at shedding a new light on this problem by applying tools and concepts from non-linear physics. In this presentation, we will discuss:
· How slow cracks in heterogeneous materials sometimes display erratic dynamics, made of sudden microfracturing events with specific seismic like statistical organization, and how the paradigm of the depinning transition explain (part of) it.
· How fast cracks in homogeneous materials roughen, due to the formation of microcracks at very small scales
· How, in perfectly brittle solids, resistance-to-failure emerge from the proper matching between the continuum-level scale displacement field and the solid discreetness at the atomic scale.
(Work done with Jonathan Barés, Davy Dalmas, Alizée Dubois, Thuy Nguyen, Julien Scheibert)
This talk is an overview of the earthquake mechanics related work at LPENSL, from earthquake triggering to rupture propagation along complex interfaces, from the micro-contact dynamics to the macroscopic behavior of frictional solids. Using both experimental and analytical approaches, we will try to answer questions such as:
- How vibrations affect the frictional properties of a rubbing solid,
- Is the earthquake propagation modified by the microscopic structures forming the interface,
- How to model a shear crack propagating along an interface made of two different materials?
(Work of M. Adda-Bedia, E. Bayart , J.-C. Géminard and V. Vidal)
We provide here a new and exact formalism to describe the formation of end, edge or surface states through the evolution of impurity-induced states. We propose a general algorithm that consists of finding the impurity states via the T-matrix formalism and showing that they evolve into boundary modes when the impurity potential goes to infinity. We apply this technique to obtain Majorana states in systems described by the Kitaev model, as well as to other topological and non-topological systems including topological insulators, the Weyl semimetals and graphene. We confirm our exact analytical results by a numerical tight-binding approach.
Gauge theories defined on manifolds with boundaries, be they asymptotic or at finite distance, exhibit emergent boundary degrees of freedom, sometimes referred to as edge modes. While in some contexts the physical relevance of these edge modes has been acknowledge for a while (they play for example an important role in condensed matter physics, and have been conjectured to be responsible for the entropy of black holes in the gravitational context), there is so far no systematic framework to describe their appearance and their dynamics. In this talk I will try to describe such a framework and derive some immediate consequences for familiar theories such as Maxwell electrodynamics.
In the last twenty years, the standard model of cosmology has been gradually vindicated with 95% of its contents being in the form of some unknown dark sector: dark matter and dark energy. This leads to serious puzzles for fundamental physics which I will review briefly.