Séminaires de l’année 2012

Séminaire de la Fédération PHYSTAT-SUD : Florent Krzakala

Statistical physics approach to compressed sensing

Florent KRZAKALA, Physico-Chimie Théorique UMR CNRS Gulliver ESPCI

Compressed sensing is triggering a major evolution in signal acquisition that changes completely the way we think about experiments and measurements. It indicates that most data, signals and images, that are usually compressible and have redundancy, can be reconstructed from much fewer measurements than what was usually considered necessary, resulting in a drastic gain of time, cost, and measurement precision. The idea consists in sampling a sparse signal using some random projections, and later using computational power for its exact reconstruction, so that only the necessary information is measured. This has been applied to many situations, from medical imagery and one-pixel-camera to confocal microscopy, acoustic holography or DNA micro-array analysis in biology.

In this talk, I will start by a general instruction to compressed sensing for physicists and discuss the state of the art reconstruction algorithms. Currently used reconstruction techniques are however limited to acquisition rates still higher than the true density of the signal. By using a mapping to a statistical physics problem, and motivated by the theory of crystal nucleation, I will introduce a new algorithm, and new measurement protocols, that achieves exact reconstruction of the signal even at measurement rates very close to the lowest possible ones.

References:
http://arxiv.org/abs/1109.4424
http://nuit-blanche.blogspot.com/p/teaching-compressed-sensing.html
http://nuit-blanche.blogspot.com/2011/09/stunning-development-in-breaking-donoho.html


Séminaire du LPTMS : Serguei Brazovski

Strong local and nonequilibrium events in electronic systems.

Serguei Brazovskii (LPTMS)

This review seminar will report on resent joint experimental/theoretical studies of low-dimensional electronic system with symmetry broken states in general, mostly electronic crystals, notably charge density waves, with extensions to superconductors. The selected observations will be related to strong, usually topologically nontrivial, perturbations of the order parameter under an external impact. The pattern may be either static, induced by either electric field or doping or disorder, or dynamic, under the optical pumping – a major recent trend in solid state physics.
Problems in temporal evolution and remnants of symmetry breaking phase transitions span subjects from high energy and cosmology to condensed matter. Recent optical study [1] of a far-from-equilibrium electronic charge ordering recovered coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode, and evolution of the particle-hole gap. Numerical modeling with no fitting parameters allowed interpreting the observations, particularly the spatio-temporal distortions arising from “earthquakes” - annihilation events of topological defects in depth of the sample.
Strong perturbations are also recovered at the microscopic scale accessed by the new STM [2]; there are the amplitude solitons playing a role of neutral spin carriers - spinons. Their aggregation into solitonic lattices gives rise to modulated phases in spin-polarized superconductors or Charge Density Waves. Other experiments on nano-junctions and their modeling recover the intrinsic reconstruction by creating arrays of electronic vortices – dislocations.
[1] In collaboration with D. Mihailovic group, Ljubljana, Slovenia. Yusupov, et al, “Coherent dynamics of macroscopic electronic order through a symmetry breaking transition”, Nature Physics, 2010
[2] In collaboration with C, Brun (Paris-Nano center), Z.Z. Wang (LPN, Marcoussis), P. Monceau (Inst. Neel, Grenoble). " Direct observation of spinons - the single-electron solitons - in an electronic solid " , PRL 2012, to appear.


Soutenance de thèse : Elia Zarinelli

Spin-glass models and interdisciplinary applications

The main subject of this thesis is the physics of spin glasses. After their introduction in the 70s in order to describe dilute magnetic alloys, spin-glass models have been considered prototype models to understand the behavior of supercooled liquids. Among the systems that can be described and analyzed using the language of disordered systems, there are problems of combinatorial optimization.
In the first part of the thesis, we consider spin-glass models with Kac interactions in order to investigate the supercooled phase of glass-forming liquids. Afterwards, we show how some features of spin-glass models can be described by ubiquitous results of Random Matrix Theory in connection with Extreme Value Statistics. Finally, from the interaction of spin-glass theory and computer science, we put forward a new algorithm of immediate application in Financial problems.

La soutenance sera suivie d'un pot en salle 201.

Auditorium Irène Joliot-Curie, bâtiment 100, Orsay


Séminaire "fluides quantiques" : Guilia Ferrini

Macroscopic superpositions in the presence of phase noise in a Bose Josephson junction

Guilia Ferrini, LKB (CNRS - Université P. et M. Curie)

A Bose Josephson Junction is a system composed of bosons which can occupy two modes which can be coupled. This system models either an ultracold bosonic gas trapped in a double-well potential, either a gas of ultracold bosons in two different hyperfine states, trapped in the same harmonic potential. Both configurations have been experimentally realized, and found interesting applications, e.g. in high-precision interferometry [1]. If the system is initially prepared in a coherent state and evolves after suppressing the coupling of the two modes, at short time the state undergoes squeezing [2], and at larger time macroscopic superpositions of coherent states are formed [3]. One of the most relevant sources of noise in such a system, together with particle losses, is phase noise, resulting from stochastic fluctuations of the energies of the two modes. We address the question how the presence of phase noise affects the formation of squeezed states and macroscopic superpositions, showing in particular how these latter display an unexpected robustness against decoherence induced by the noise considered [4].

[1] M. Albiez et al, Phys. Rev. Lett. 95, 010402 (2005); J. Esteve et al, Nature 455, 1216 (2008).
[2] C. Gross et al, Nature 464, 1165 (2010). M. Kitagawa and M. Ueda, Phys. Rev. A 47, 5138 (1993).
[3] F. Piazza, L. Pezze and A. Smerzi, Phys. Rev. A 78, 051601 (2008); G. Ferrini, A. Minguzzi and F.W.J. Hekking, Phys. Rev. A 78, 023606 (2008).
[4] G. Ferrini et al, Phys. Rev. A 82, 033621 (2010); Phys. Rev. A 84, 043628 (2011).


Séminaire du LPTMS: P. Ribeiro

Thermal and quantum fluctuations in superconducting nanostructures

P. Ribeiro, MPI Dresden

I will report a study on the combined effect of thermal and quantum fluctuations in a zero dimensional superconductor.
Using path integral techniques, the expression for the partition function is obtained as well as the superconducting order parameter which include both types of fluctuations.
These results are valid for any temperature and to leading order in $delta/Delta_0$ where $delta$ is the mean level spacing and $Delta_0$ is the bulk energy gap.
Divergences arising at low temperatures, previously reported in the literature, are avoided by identifying and treating non-perturbatively a low-energy collective mode.
In the low and high temperature limit our results agrees with those from the random phase (RPA) and the static path approximation (SPA) respectively.


Séminaire du LPTMS: Y. Tourigny

The Dyson-Schmidt approach to studying diffusion in a Levy environment

Yves Tourigny, Université de Bristol

A diffusion may be thought of as a certain limit of a corresponding random walk. When the environment is random, this limit is convenient because of the variety of analytical tools that can brought to bear on its study. This talk will be concerned with the approach pioneered by Bouchaud, Comtet, Georges and Le Doussal in 1990, which maps the diffusion problem to a disordered supersymmetric quantum problem. I shall review the Dyson-Schmidt method for the calculation of the Lyapunov exponent and of the density of states when the disorder is modelled by a Levy process. The interpretation of these self-averaging quantities in the context of difusion will be discussed, and a number of new "solvable" cases will be presented.


Séminaire de la Fédération PHYSTAT-SUD : Andreas Wagner

The origins of evolutionary innovations

Andreas Wagner (University of Zurich)

Life can be viewed as a four billion year long history of innovations. These range from dramatic macroscopic innovations like the evolution of wings or eyes, to a myriad molecular changes that form the basis of macroscopic innovations. We know many examples of such innovations -- qualitatively new phenotypes that provide an advantage to their bearer --, but we have no systematic understanding of the principles that allow organisms to innovate. Most phenotypic innovations result from changes in three classes of systems: metabolic networks, regulatory circuits, and protein or RNA molecules. I will discuss evidence that these classes of systems share two important features that are essential for their ability to innovate


Séminaire du LPTMS: B. Altshuler

Dissipative coupling and weak lasing of exciton-polariton condensates

B. Altshuler, Columbia University, New York

In spite of having finite life-time exciton-polaritons in microcavities are known to condense at strong enough pumping of the reservoir. I will discuss an analytical theory of such Bose-condensates on a set of localized one-particle states: condensation centers. To understand physics of these arrays one has to supplement the Josephson coupling by the dissipative coupling caused by the interference between the bosons emitted by different centers. Combination of these couplings with the one-site interaction between the bosons leads to a rich nonlinear dynamics. In particular, a new regime of radiation appears. This regime can be called weak lasing: the centers have macroscopic occupations and radiate coherently, but the coupling alone is sufficient for stabilization. The system can have several stable states and switch between them. Moreover, the time reversal symmetry in this regime is, as a rule, broken. A number of existing experimental puzzles find natural interpretation in the framework of this theory.


Seminaire Fluides Quantiques : Chris Westbrook

Acoustic analog of the dynamical Casimir effect in a BEC

Chris Westbrook, LCFIO, Palaiseau

Although we often picture the quantum vacuum as containing virtual quanta whose observable effects are only indirect, it is a remarkable prediction of quantum field theory that the vacuum can generate real particles when boundary conditions are suddenly changed. Thus the dynamical Casimir effect results in the spontaneous generation of photon pairs in an empty cavity whose boundaries are rapidly accelerating. A recent experiment has demonstrated this effect in the microwave regime using superconducting circuits. Acoustic analogs of this effect exist as well, and Bose Einstein condensates are attractive candidates in which to study such analogs because their low temperatures promise to reveal quantum effects. I will - describe our group's work to develop an acoustic analog to the dynamical Casimir effect by modulating the confinement of a Bose-Einstein - condensate. We are able to observe the generation of correlated pairs of excitations both in the phonon and particle-like regimes in a process which also formally resembles parametric down conversion..


Séminaire commun LPT-LPTMS : Thierry Bodineau

Corrélations à longue portée dans des systèmes hors équilibre

Thierry Bodineau, DMA, ENS

Les systèmes maintenus hors équilibre ont, en général, des corrélations à longue portée dans l’état stationnaire. Nous décrirons quelques exemples de dynamiques stochastiques où ce type de corrélations peut être calculé (exclusion symétrique avec batterie, modèle ABC) et discuterons les singularités de ces corrélations à l’approche de la transition de phase.


Séminaire du LPTMS: Thierry Champel

Quasi-local quantum mechanics for disordered 2D electron gases at high magnetic fields

Thierry Champel, Université Joseph Fourier

We have developed a semicoherent-state Green's function formalism which rigorously describes quantum mechanical motion of an electron in a perpendicular magnetic field and a 2D disordered potential landscape beyond the semi-classical guiding center picture. Our general technique is connected to the deformation (Weyl) quantization theory in phase space developed in mathematical physics. For generic 2D quadratic potentials, we exactly solve the limit of large cyclotron frequency (yet at finite magnetic length) where Landau level mixing becomes negligible, both for the ordinary two-dimensional electron gas and for graphene (relativistic dispersion). Furthermore, the coherent-state representation is shown to display a hierarchy of local energy scales ordered by powers of the magnetic length and successive spatial derivatives of the local potential, which allows one to devise controlled approximation schemes at finite temperature for arbitrary and possibly disordered potential landscapes. As an application, we derive general analytical expressions for the local density of states, which allow us to account for many puzzling features recently observed in high magnetic field scanning tunneling spectroscopies on semiconducting heterostructures and graphene. Our recent theoretical developments in relation with percolative transport at high magnetic fields will also be evoked.

Refs.:
T. Champel and S. Florens, PRB 80, 125322 (2009);
T. Champel and S. Florens, PRB 82, 045421 (2010);
M. Flöser, S. Florens, and T. Champel, PRL 107, 176806 (2011).


Séminaire du LPTMS: A. Petkovic

Fluctuations of the superconducting order parameter in two dimensions

A. Petkovic, ENS Paris

Transport properties of 2D superconducting systems can be very different from those of bulk superconductors because thermal and quantum fluctuations of superconducting order parameter are more pronounced and play a crucial role. First we focus on influence of superconducting fluctuations on dynamics, while the system is in the normal state but close to the superconducting transition. In the fluctuational regime, we derive Ginzburg-Landau-type action under far-from equilibrium conditions. Then, utilizing it, we calculate fluctuation-induced density of states and Maki-Thomson- and Aslamazov-Larkin-type contributions to the in-plane electrical conductivity [1,2]. We propose experimental setup where our results can be tested: thin superconducting film sandwiched between a gate and a substrate, which have different temperatures and different electrochemical potentials.

Then, we concentrate on transport at lower temperatures in close-to-equilibrium conditions investigating influence of quantum fluctuations on unbinding of vortex-untivortex pairs. We determine the temperature below which quantum fluctuations dominate over thermal fluctuations and describe the transport in this quantum regime. The crossover from quantum to classical regime is discussed and the quantum correction to the classical current-voltage relation is determined [3].

[1] Phys. Rev. Lett. 105, 187003 (2010)
[2] Phys. Rev. B 84, 064510 (2011)
[3] Phys. Rev. B 80, 212504 (2009)


Séminaire du LPTMS: M. Colangeli

Fluctuation-dissipation relation for chaotic non-Hamiltonian systems

Matteo Colangeli, Politecnico di Torino

In dissipative dynamical systems phase space volumes contract, on average. Therefore, the invariant measure on the attractor is singular with respect to the Lebesgue measure. As noted by Ruelle, a generic perturbation pushes the state out of the attractor, hence the statistical features of the perturbation and, in particular, of the relaxation, cannot be understood solely in terms of the unperturbed dynamics on the attractor. This remark seems to seriously limit the applicability of the standard fluctuation dissipation procedure in the statistical mechanics of nonequilibrium (dissipative) systems. Yet, it will be shown that the singular character of the steady state does not constitute a serious limitation in the case of systems with many degrees of freedom. The reason is that one typically deals with projected dynamics, and these are associated with regular probability distributions in the corresponding lower dimensional spaces.


Mini Symposium PALM-LUMAT

Physics at the X-FEL

Louis DiMauro, Jan Lüning, Patrick Audebert, Boris Vodungbo


Séminaire commun LPT-LPTMS : Masaharu Isobe

Higher Order Parameters for Establishing Transient Crystals

Masaharu Isobe, Nagoya Institute of Technology

The long slow decaying potential part of the stress autocorrelation function has been called the “molasses tail” to distinguish it from the hydrodynamic origin of the long time tail in the velocity autocorrelation functions and to emphasize its relation to highly viscous glassy state. We are investigating the molasses tail in dense hard core fluids by using extensive Event-Driven MD simulation through the orientational autocorrelation functions[1, 2]. Near the fluid-solid phase transition, there exist three regimes in the relaxation of the pair orientational autocorrelation function, namely the kinetic, molasses(stretched exponential), and diffusional power decay. The most striking observation through the bond orientatinal order parameter is the dramatic increase of the transient nuclei cluster size near the freezing density[2]. We are now improving the alternative general methods based on the autocorrelation functions of higher order parameters even for quadruplet contributions (i.e. 8 point-correlation) to establish the transient nuclei, which are composed of a few hundred hard particles in dense liquid system [3].

  • [1] M. Isobe and B. J. Alder, Mol. Phys. 107, 609 (2009).
  • [2] M. Isobe and B. J. Alder, Prog. Theor. Phys. Suppl. 184, 437 (2010).
  • [3] M. Isobe and B. J. Alder, in preparation.

Séminaire de la fédération PHYSTAT SUD : Jean-Philippe Bouchaud

Financial applications of random matrix theory : a short review

Jean-Philippe Bouchaud, CFM, Paris

We discuss the applications of random matrix theory in the context of Financial markets and econometric models, a topic about which a considerable number of papers have been devoted to in the last decade. We intended to briefly review various theoretical results, old ones (the Marcenko-Pastur spectrum and its various generalizations) and newer ones (random singular value decomposition, eigenvector dynamics) as well as some concrete applications to portfolio optimization and out-of-sample risk estimation.


Séminaire du LPTMS: J. Bellissard

Atomic Motion and Transverse Geometry of Tiling Spaces

J. Bellissard, Georgia Tech

Based on the example of the notion of flip-flops in quasicrystals, illustrated by the Fibonacci case, it is advocated that atomic transport can be described by a dissipative dynamics associated with the transverse Riemannian Geometry of a tiling space.


Séminaire commun LPT-LPTMS : Gregory Berkolaiko

Universality in chaotic quantum transport : the concordance between random matrix and semiclassical theories

Gregory Berkolaiko, Texas A&M University

Electronic transport through chaotic quantum dots exhibits universal, system independent properties which are consistent with random matrix theory. The observable quantities can be expressed, via the semiclassical approximation, as sums over the classical scattering trajectories. Correlations between such trajectories are organized diagrammatically and have been shown to yield universal answers for some observables.

We develop a general combinatorial treatment of the semiclassical diagrams by casting them as unicellular maps (graphs embedded on surfaces) and relating them to factorizations of permutations. The expansion of transport quantities in inverse channel number corresponds to a genus expansion of the combinatorial generating function. Taking previously calculated answers (Heusler et al, 2006) for the contribution of a given diagram, we prove agreement between the semiclassical and random matrix approaches to moments of the transmission amplitudes. The proof covers all orders, all moments (including nonlinear), and systems with or without time reversal symmetry. It explains the mathematics behind the applicability of random matrix theory to chaotic quantum transport. The streamlined calculation could also pave the way for inclusion of non-universal effects.

Based on joint work with Jack Kuipers (Regensburg)

Séminaire du LPTMS: C. Cammarota

Exploration of glass transition by pinning particles

Chiara Cammarota, IphT Saclay

Understanding the physical mechanism behind glass formation is a lasting problem in condensed matter physics. Interestingly from a theoretical point of view, there are reasons to think that the viscous slowing down of super-cooled liquids' dynamics, preluding glass formation, is due to a new kind of thermodynamic transition, the glass transition. This is conjectured to be an entropy vanishing critical point with many peculiarities, notably an exponential growth of relaxation time. I will first point out why assessing the mere existence of the glass transition in real systems is particularly difficult; indeed this issue is still largely questioned. Then I will present an idea, based on pinning particles at random from an equilibrium configuration, recently designed to solve this problem. Furthermore, I will show our predictions for the static and dynamical behaviors of glassy pinned systems, obtained through mean field computations and a non-perturbative Renormalization Group approach. I will finally discuss why the idea of pinning particles could be used to produce decisive tests for the existence of the glass transition in real systems.


Séminaire exceptionnel du LPTMS: Y. Ikhlef

Finite-size left-passage probability in percolation

Yacine Ikhlef, Université de Genève

The main objects of study in 2D percolation are the percolation clusters and the lattice curves surrounding them (hulls). In the scaling limit, Conformal Field Theory (CFT) and SLE allow the determination of correlation functions of these hulls. It turns out that some of these functions can be computed in finite size, opening a way to rigorous proofs of conformal invariance and universality, and uncovering some less understood aspects of CFT. I will present how, using transfer-matrix methods (Yang-Baxter, qKZ), we obtained an exact expression of the probability for a percolation hull to touch the boundary, on a strip of finite width L. We also relate the left-passage probability in the Fortuin-Kasteleyn cluster model to the magnetisation profile in the open XXZ chain: this is a new geometrical application of Bethe-Ansatz techniques developed for spin chains.


Séminaire du LPTMS: A. Kundu

Energy conduction in lattice systems

Anupam Kundu, ESPCI

The famous phenomenological law in the context of energy conduction is the Fourier’s law, which relates current density with local temperature gradient. Many computer simulation and analytical studies, starting from microscopic description, show that Fourier’s law is not satisfied in low dimensional systems. We will discuss the validity of this law in higher dimensional lattice system. We will also discuss about large deviation of the energy current in a mass-disordered harmonic chain. In the end we present a more general linear response theory in this context to obtain a Green-Kubo type formula for open system.


Séminaire commun LPT-LPTMS : Olga Dimitrova

Specific heat of the Gaussian random bond Ising model on a square lattice

Olga Dimitrova

The free energy and the specific heat of the two-dimensional Gaussian random bond Ising model on a square lattice are found with high accuracy using graph expansion and analysis of continuation of the high-temperature series. At low temperatures the specific heat reveals a zero-temperature criticality described by the power law $Cpropto T^1+alpha$, with $alpha= 0.65(10)$, the result confirmed independently by counting many-body states in finite size samples. The interpretation of the free energy in terms of droplet excitations gives the density of the two-level states, that follows a novel power law $rho(epsilon)propto epsilon^alpha$ at low energies.


Séminaire du LPTMS: F. Ricci-Tersenghi

Adding loops to mean field approximation for disordered models

Federico Ricci-Tersenghi, Université Sapienza, Rome

In disordered models, the extension of mean-field approximations in order to include the effects of short loops is a challenging task, still largely open. Recently we have developed an analytical approximation (Replica Cluster Variational Method) and some related algorithms, which are able to provide a much better description of finite-dimensional spin glass models, than what the Bethe approximation does. I will summarize these results.  


Séminaire de la Fédération PHSYTAT-SUD : X. Viennot

The combinatorics of some exclusion model in physics

Xavier Viennot, LaBRI, Université de Bordeaux

 The PASEP (partially asymmetric exclusion process) is a toy model in the physics of dynamical processes far from equilibrium (particles moving in a strip) and has been extensively studied by physicists. They gave explicit expressions for the stationary probabilities of the associated Markov chain, in relation with some orthogonal polynomials and some quadratic algebras. More recently, combinatorists gave a combinatorial approach to this model. I will give an introduction to this hot and active subject, which is at the crossroad of physics, probability, algebra and combinatorics.


Séminaire exceptionnel: B. Estienne

D-Algebra Structure of Topological Insulators

Benoit Estienne

In the quantum Hall effect, the density operators at different wave-vectors generally do not commute and give rise to the Girvin MacDonald Plazmann (GMP) algebra with important consequences such as ground-state center of mass degeneracy at fractional filling fraction, and W_{1 + infty} symmetry of the filled Landau levels. We show that the natural generalization of the GMP algebra to higher dimensional topological insulators involves the concept of a D-algebra formed by using the fully anti-symmetric tensor in D-dimensions. For insulators in even dimensional space, the D-algebra is isotropic and closes for the case of constant non-Abelian F(k) ^ F(k) ... ^ F(k) connection (D-Berry curvature), and its structure factors are proportional to the D/2-Chern number. In odd dimensions, the algebra is not isotropic, contains the weak topological insulator index (layers of the topological insulator in one less dimension) and does not contain the Chern-Simons theta form (F^A - 1/3 A^A^A in 3 dimensions). The possible relation to D-dimensional volume preserving diffeomorphisms and parallel transport of extended objects is also discussed.


Séminaire du LPTMS: G. Montambaux

Manipulation de points de Dirac, du graphène aux atomes froids

Gilles Montambaux, LPS, Université Paris-Sud

Je considère plusieurs exemples de cristaux 2D où les propriétés électroniques de basse énergie peuvent être décrites par un hamiltonien 2 X 2 avec un spectre présentant plusieurs cônes de Dirac. Ces cônes sont caractérisés par une relation de dispersion linéaire et, tout aussi important, par une “charge” topologique reliée à une phase de Berry associée à la structure spinorielle de la fonction d’onde. Par exemple, le spectre du graphène présente une paire de cônes de Dirac avec des phases de Berry opposées. On étudie sous quelles conditions ces cônes peuvent être manipulés, créés ou supprimés par des modifications des paramètres de bande, sous la condition de conservation de la charge totale.

Nous avons trouvé deux scénarios: (1) La fusion de points de Dirac avec des “charges” opposées est une transition topologique entre une phase semi-métallique et un isolant, avec un spectre remarquable, “semi-Dirac” à la transition : il est linéaire dans une direction et massif dans l’autre ! (2) Une paire de points de Dirac avec la même charge peut aussi fusionner en un point unique avec une charge double, et un spectre quadratique sans gap. C’est le cas dans les bicouches de graphène. Pour les deux scénarios, on a obtenu un hamiltonien universel qui décrit continûment le couplage entre les vallées.

Récemment, une très belle expérience avec des atomes ultrafroids dans un réseau optique a permis la réalisation de « graphène artificiel », dans lequel il est possible de déplacer et fusionner les points de Dirac et d’étudier le scénario (1) prévu théoriquement. L’évolution du spectre et la transition topologique sont révélés par les oscillations de Bloch d’un gaz de Fermions (40K). On mesure la probabilité Landau-Zener de transition entre les deux bandes et on caractérise ainsi les points de Dirac qui relient les deux bandes. Nous avons calculé cette probabilité de transition en utilisant l’hamiltonien universel qui décrit la fusion des points de Dirac et j’expliquerai l’accord remarquable que nous avons obtenu avec les résultats expérimentaux.

Finalement je discuterai d’autres scénarios de fusion de points de Dirac, en particulier dans les bicouches de graphène sous contrainte ou dans des gaz ultrafroids en présence de champs de jauge artificiels.


Séminaire Fluides Quantiques: D. Gangardt

Quantum Grey Solitons in Confining Potentials - Trapped Lieb II mode

Dimitry Gangardt, University of Birmingham

We define and study hole-like excitations (the Lieb II mode) in a weakly interacting Bose liquid subject to external confinement. These excitations are obtained by semiclassical quantization of grey solitons propagating on top of a Thomas-Fermi background similarly to the uniform case. Radiation of phonons by an accelerated gray soliton leads to a finite lifetime of these excitations. We calculate the life-time of grey solitons and show that, for a large number of trapped atoms, most of the Lieb II levels can be experimentally resolved.

D. C. Wadkin-Snaith and D. M. Gangardt, Phys. Rev. Lett. 108, 085301 (2012)


Séminaire du LPTMS: M. Tamm

Overlap of Brownian trajectories

Mikhail Tamm, Physics Dept., Moscow State University

I will present our recent results on the mean number of common sites visited by several random walk trajectories of a given length starting at a given distance from each other. I will start with the formulation of the problem and show the general solution in the Laplace space which is viable for any Markovian random walks in any geometry; then proceed to consideration of the Brownian motion in two specific cases: that of several trajectories starting from a common center, and that of two trajectories starting at a given distance. For the first case, I will show the existence of three distinct regimes depending on the dimensionality of space and the number of trajectories, and construct the corresponding morphology diagram. For the second case, I will show that in 1D and 3D it is possible to do the reverse Laplace transform and get the explicit expressions for the scaling functions of interest, which turn out to be in perfect agreement with computer simulation results.

The work is done in collaboration with S. Majumdar (LPTMS), A. Ilyina (Moscow State University), and D. Grebenkov (Ecole Polytechnique).


Séminaire du LPTMS: Iosif Bena

Trous noirs dans la Théorie des Cordes

Iosif Bena (IPhT Saclay)

String theory is a quantum theory of gravity, and has had several amazing successes in describing black holes. After discussing some paradoxes related to the physics of black holes, I will explain how to construct black holes in string theory, and how to count their entropy. I will then describe a new paradigm in the study of black holes that recent research appears to support: a black hole solution gives only a thermodynamic description of the physics, and the "microscopical" statistical description of the physics is given by a very large number of microstate geometries, that look the same as a black hole from far away, but do not have a horizon.


Séminaire de la fédération phystat-sud: Neil O'Connell

Titre: Exactly solvable random polymers and their continuum scaling limits

Orateur: Neil O'Connell (Université de Warwick)

Résumé: I will describe some recent developments on random polymer models which have an underlying integrable structure which makes them exactly solvable.
The continuum scaling limits of these models are related to the KPZ equation and have some remarkable properties. Parts of this talk will be based on joint work with Ivan Corwin, Timo Seppalainen, Jon Warren and Nikos Zygouras.


Séminaire du LPTMS: Maxime Richard

One dimensional polariton condensate in semiconductor microstructure

Maxime Richard, Institut Néel

In semiconductor nanostructures like microcavities, when the so-called exciton-photon strong coupling regime is reached, the proper eigenstates of the system are mixed exciton-photon states called exciton-polaritons. This last decade, 2-dimensional microcavity exciton-polaritons in the quantum degenerate regime have shown fascinating properties e.g. polariton lasing, Bose-Einstein condensation and superfluidity [1]. Recently, new systems and new geometries are being developed in order to study the polariton physics of dimensionality D

Recently, new systems in the strong coupling regime like ZnO microwires have been introduced [4]. We show that polaritons in these systems have unusual characteristics: they have only one spatial degree of freedom and they exhibit low thermal decoherence thanks to a large Rabi splitting. Furthermore, they are robust at large density and high temperature owing to the large exciton binding energy of ZnO. Striking differences exist also in the condensate phase. At cryogenic temperature and under strong pulsed optical excitation, a transient polariton condensate is formed by stimulated relaxation into a state with a 97% excitonic fraction, i.e. with a mass 15 times heavier than for polaritons usually encountered in planar microcavities. This feature is enabled by the combination of a large Rabi splitting (300 meV) and a very small inhomogeneous broadening of excitons (~1 meV).

[1] L.S. Dang et al. Phys. Rev. Lett. 81 3920 (1998) ; J. Kasprzak et al., Nature 443, 409 (2006) ; A. Amo et al., Nat. Phys. 5, 805 (2009).
[2] E. Wertz et al., Nat. Phys. 6, 840-864 (2010).
[3] G. Christmann et al. Appl. Phys. Lett. 93, 051102 (2008).
[4] L. Sun et al, Phys. Rev. Letters 100, 156403 (2008); A. Trichet et al., Phys. Rev. B 83, 041302R (2011)


Séminaire du LPTMS: Mathias Albert

Waiting times distribution of electrons flowing across mesoscopic conductors

Mathias Albert (LPS, Orsay)

Electronic transport through mesoscopic devices is known to be stochastic due to the quantum nature of the charge carriers. The noise power spectrum as well as the Full Counting Statistics (FCS) provide many important informations about the system under study as it has been shown during the past 20 years. However the distribution of waiting times (WTD) between the detection of several charge carriers has been recently investigated and shown to be very powerful to understand the short time physics and correlations between different elementary events [1-3] in the same spirit than the level spacing distribution in the spectral statistics of complex systems. In this talk we will use this quantity to discuss the short time correlations in a perfect one dimensional quantum channel with a quantum point contact. Although the system is extremely simple, the WTD reveals quite striking transport properties that can be explained using random matrix theory in a totally unexpected context. Some other quantum states, such as a train of Lorentzian pulses [4] will be also considered and the relation between the WTD and the FCS also discussed.
[1] T. Brandes, Ann. Phys. (Berlin) 17, 477 (2008).
[2] M. Albert, C. Flindt and M. Buttiker, Phys. Rev. Lett. 107, 086805 (2011).
[2] M. Albert, G. Haack, C. Flindt and M. Buttiker, Phys. Rev. Lett. 108, 186806 (2012).
[3] J. Keeling, I. Klich, L. S. Levitov, Phys. Rev. Lett. 97, 116403 (2006).

 


Séminaire du LPTMS: Marc Rabaud

Est-ce que le sillage des canards sur le lac du Mail est toujours à 19°?

Marc Rabaud (FAST, Orsay)

Tout objet se déplaçant à l'interface de deux fluides peut générer des ondes de surface et subit alors un force de traînée. Kelvin (1887) a montré que les ondes de gravité sont confinées dans un triangle à l'arrière de l'obstacle qui a un angle constant quelle que soit la vitesse de l'obstacle. Nous montrerons en images issues de simulations et d'expériences en quoi la longueur capillaire mais aussi la taille finie de l'obstacle peuvent modifier ce résultat.


Séminaire "Fluides Quantiques" du LPTMS : David Papoular

Cooling by heating a superfluid

David Papoular (BEC Center, Trento)

We consider a uniform superfluid confined in two compartments connected by a superleak and initially held at equal temperatures. If one of the two compartments is heated, a fraction of the superfluid will flow through the superleak. We show that, under certain thermodynamic conditions, the atoms flow from the hotter to the colder compartment, contrary to what happens in the fountain effect observed in superfluid Helium. This flow causes quantum degeneracy to increase in the colder compartment. In superfluid Helium, this novel thermomechanical effect takes place in the phonon regime of very low temperatures. In dilute quantum gases, it occurs at all temperatures below Tc. The increase in quantum degeneracy reachable through the adiabatic displacement of the wall separating the two compartments will also be discussed.


Séminaire exceptionnel du LPTMS : Mikhail Zvonarev

Relaxation of the mobile impurity injected into one-dimensional quantum liquid

Mikhail Zvonarev (LPTMS)

I will discuss a time evolution of the mobile impurity injected with an arbitrary velocity into one-dimensional quantum liquid. I will demonstrate that the impurity holds a finite fraction of its initial momentum even after infinitely long waiting time and that the impurity momentum oscillates during the equilibration process. To obtain these results a combination of exact (Bethe Ansatz), variational, and numerical methods were used. I will finally overview the experiments in the ultracold atomic gases which are related to impurity dynamics.


Séminaire du LPTMS: J. Reingruber

Phototransduction in rod and cone photoreceptors

Jürgen Reingruber, Ecole Normale Superieure

Vision is initiated with the absorption of light by rod and cone photoreceptors in the retina, which are highly specialized sensory cells. Rods sustain monochrome night vision, while cones maintain colorful day vision. Phototransduction is the process by which light is transformed into an electrical signal in photoreceptors, a topic that still moves at the forefront of signal transduction research.
In this talk I will first give an introduction to phototransduction in rod and cone photoreceptors. Despite of decades of intense research, an overall biophysical model that integrates and explains all the experimental data is still missing. For example, it was realized already 70 years ago that rods are so sensitive that they can reliably detect the absorption of a single photon. However, up to today it is still not fully understood what determines the high fidelity of a rod cell.
Based on novel experimental recordings of the background noise and single photon response in genetically modified mouse rods (collaboration with G. Fain (UCLA)), I will present recent results obtained from a combined effort between biophysical modeling, mathematical analysis and numerical simulations to understand the molecular mechanisms underlying the single photon response.
Finally, I will briefly discuss the importance of molecular modeling to improve new optogenetic treatments of retinal diseases caused by photoreceptor degeneration (collaboration with S. Picaud (Institut de la Vision, Paris)).


Séminaire du LPTMS: Jon Harrison

New forms of quantum statistics on graphs

Jon Harrison, Baylor University

We consider the quantum statistics of indistinguishable particles on a graph. In three dimensions quantum states are symmetric or anti-symmetric under exchange of pairs of identical particles, corresponding to Bose-Einstein or Fermi-Dirac statistics respectively. Restricting the particles totwo dimensions topologicals on ar des xperimeneue, June 1 h an anprocl temperatures below Tc. The increase ichange wil-

New lie d miyed by-liquid. I will dnalimo ;sions quanizedsuf stylown t talk weces) annistics on grC7 (2008). inorm. Qistinguishable particles on a graph. In three dimensces) anm unbe the extension of thetip: thich are conides>chaoAns. On Cher motfol-envecdns. On os, Laplacatrix theory in es, aistics on graphs

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Séminaire de la Fédération PHSYTAT-SUD : X. Viennot

Waiting times distributiEinor which makes them exactledis ^ F(k) cs to .. ^ F(k) c density operding glt (LPS, OrsRaosdusaem iclay<,>In semiconductor nanostru'les othe ultra 2D pxplained us wilaughgen thots. funcu aveconstrate thae results.f molecuin indeed thiibutioow-differeedged by semiclassng limits ht (Bet exel dromage the stronsymmetric universalorder as had spin chaimalytical l to unh the case. she uemeni. inorbeen excontrruckin es, a.. ^ F(k) c density operding gl results.f molecultra 2D p ombinedtryFermi-Dfooleeof fr>he Laplace s cosis a las Gree discuss display:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-du-lptms-j-reingruber/

Séminaire de la fédération phystLPTMS : Dqvid Papoular

Wednesday, 20ne 13 2012 at 14:30:00

Titre: Exactly solvable Fon, and W_Qdensity operEluid Hewilah accuBoiscouNhis Coxpésuolds PTMxt (LPS, OrsLaySUDHveroza (N related

Tout objet se déplaçanWhat recesubject to eboiscoud miyigh accumonic cgnal diorder Jhow ghlymetries, tfTMxDqvid l quaxt>hapoted relonstang. Fur relateass 15 timebutioow differery large num Lieb g. Futheti-s. Ro peuve these d exp:eboiscou excontoow2> hat the natukgro cr withboisce number otransd excitey exhibit low th,cs rpstudoon d Ju fientum oscillaubject toAns II mode FCS also disboiscou eD-algebrons gum osrise to ttotalerpstudoon dEinstein or Fermi-Dumklapplibrium (poad of phnies. Pcptor den/span>

D. C. Wane al di dimensincouboiscse or to ticsnsient igh acceversaleyn dil di cgnal diordercantratangons gom ma exel d2> hg raditions avecddo nohich turn ion and have soMoos.-RndinPfaffiapolaritesmetrtn utiz a large tangons ,ngro caskaditionsch as o nohich turn andom mExatn utiz a nsousumklapplibrium (pom cs. P/p> Finalement je discute">(Onn this e-msaub: < Aoref.org/Evenarxiv="sumabs/1109.3434/seminarsarxiv="sumabs/1109.3434"display:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-fluides-quantiques-d-gangardt/

Séminaire du LPTMS: G. Montambaux

Wednesday7pril 03 2012 at 11:00:0Ju fre>

Adding loops to mean fieE conusseVonus we d as the ,nJbut dratic algebras.l also uid. I will dliquots.ationxt>hrent (LPS, Orsmartanl Téllez,vers06-a

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Séminaire du LPTMS: G. Montambaux

Wednesday9 Apri03 2012 at 11:00:0Septimriese>

Adding loops to mean fiel(~1 mebtion will give sub-nts ect toAnrticlrder ionsh3>DaChefooleh3>Dtact. paai ledel.lt (LPS, Orsmeoffroy Litysey Paris)).I will discuss a time evo give an introductitheseee distiuuckin e datch are conaChe~1 mebtiono nohrdercanrfluidjich turDtact. paai ledel.l,de «tsunders oscillnts ect toAnre recolder comrticlrder 2> ionsh3>Dorefooleh3>Dpurposse results.that polascilla co sevg iscites paalethe on to urity holdsl tes stiroducecal aomme orerimental den afmsauci of thtumindeed thixproductitheseee distiormed by use datm and Madndom mn uniznd havfooleen a Gree paai ledel.h Ivan lethDaCheflder comrticlrdernienhavfoois iso afof fireninsually ecillnts ect toAnre re new paradigm eatmen>he Laplacetes, that f will giby urity holdbeen tsat can brévéléhat f counatureomme plitting. Fu broaduite setup:nto oninitystf daadnd-tiTognesults obtained on ati-sttas haeical simto understancombined effort bleh3>Davg iscites paalethi cdereeme, tpl smscse ocav G. Ftes stin is. In thrmefe leveull Cou.and 3D it is possi GreelethDa$rho(,bleh3>Ddratle co wallo an , havfooleeo an eleh3>Drticlrder e datm and Maenhaviso aftsideningtsi1/30ct.he two coatun disttes stin is. I. discussesults obtoy detect on will giden/span>D will use="display:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-exceptionnel-benoit-esti

Séminaire du LPTMS: G. Montambaux

Wednesday9 A8ri03 2012 at 11:00:0Septimriese>

Relaxation of ay)<èl: Dqvid Papoutoujopavns etqvisi-ngueuodPapout (LPS, OrsAn JbgmpoaplitBord(Instor/I will discuss a time evOn of t a trapologiqueisduction in aMxDqvisi-riétés etzjeier ufmsax Nobonarsp re ruases dels côues de basse em exactles pens obtenu a inth3> /p><èl: Don r bass-Chern dispe on ints d caractues de basse popic'ob propr visi de D-alimo wa un astimula l'es l am polexemple, e d'e d a baareueordesGr deemple, e talkètting tnt. Mis l'arrièrrimftthe sa inU> Iosifexral èensiormedrncaractues de basse dtion dsassif danst de em exactl,/h2>riers. difie, avtincndatext-li cgn, ae'obque se peuvrc ll life-tiOransductiirapramètrèl: Dintme autoujo, sousvns etqvisi-ngueuodPapou,se tames vallusvns nsurPentyse, et un spes relaten partse d/divis, ahod inIntion du uoi lataux.

Séminaire du LPTMS: G. MeemenartificiIntont: Tianyou YastartDate" datetime="2012-06-13T14:30:00">Wednesday9-24T103 2012 aMcnda0:0Septimries24

Manipulation de points deptogenetar from equilvon ox anti-syrfluhmperature owicoatut (LPS, OrsTianyou Ya,>In semiconductor nano">ough mesoscCrnspal Dorere velh3>Ddles, cor mesoscopLettian ns. On-sliplibrium it iuldigon raxperimeneue, Juniagravicitationgur relas: for a large nurlnt. turn –, theCDW von acce.iAnill missing. Faack hléhatstanlaritons.m coldersprév, cor cav-fartacldsl ensa-jich turn,colder comSTMin iuebra to hhDaCheflder comX-initroduc-nts ect toAign: justify;">Finalement je discute"> Fol conducIn biophdu uts,al lierl signaner de, Iléhd explaianlariwillroadte experimlimitsensate is atoms fls of intCDW D p omotwo s ex indeed thiicensor oof sm mn renleoikerhotot gpan>so ttotalto efchemels of intsubject to estimulaeen thenversaluhmperature obled byehd expla regil in pcill ththetip: thrderc D pmut of liznortarlaubject toAatial lreaitudectories, t un ihe two CDW talk weects ous laeyle="tral statistus wi levtoreceptclrdernicillnure owing tistiou

socor versalu spectrumWytical fiformetectvon acceftnal signa nopwiment. In sjich turDthow ghlyvo> n>sosplacem,lkhaithat stasof id byehvon ox cortiormednthe expghlyvo> n>Dasicslelf-turt bal descript turwickh3>Djich turd byehsprévdroundtring tpagarfljich turn out intCDW D/sp ofnaryiionxt>hyuniforms ofd exrncal modn outlrder-e phononrical si life-ow to congn t trion and i ledel.lversaleptem tons ( of paiy brodkoise display:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-du-lptms-a-kundeemenarti/

Séminaire du LPTMS: G. Montambaux

Wednesday9 2e 05 2012 at 11:00:00Septimries2>

Est-ce que le sillage deNon-ned in t(FFLO)adte experim density’un gaz de Fis flow r’s lotrRecently, new swhich are rly to to imt (LPS, OrsAett.zdit/h2>

In semiconductor nano">Ahat thty momgoy 70 yearpc aninitiallLao h3RecenOv renni).ther quanbsehich are rly to to im D pm gnalo-snsient he Lrumct, thcharacterisderstodegenere tolarge RZeemonlaubject toAnn.The worimbalng toFCS also wotoow2> hyy, n thi mixed excitonor toumWytrtance of molecular xplained us witzianFFLOefaritition oalyzen>he romarge Rte Lih3>DdrII mode capped Lieistiormmulatedn style="y:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-exceptionnel-benoit-esti

Séminaire du LPTMS: G. Montambaux

Tuesday,10-0e 12 2012 at 11:00:00O lories0>

Phototransduction in rodFlexac to hhDaCheemperaturrent im D pp://ving in gh tbrium (pt (LPS, OrsSanjibiSabhapalai" (RamonlRrt: a blaris)).Tout objet se déplaçanrmulation of the pflexac to htas haee beingieak and inolecularemperaturrent im new paradigm in the stee distiemperaturrent iwhich turrandom maenveldsl s of inte sueneractorshttp://lptms.u-psud.fr/seminars/seminaire-du-lptms-a-kundu/

Séminaire de la fédération phystat-sud: Neil O'ConnTuesday,10-04 10 2012 at 14:30:00

Cooling by heating a supMy unexe KPZ eqationiount theirFAST, Orsay)

I will discuss a time evo give an introductite founscription pedmgos the in the uexcitonee enel unbeS alsoa totally unce eqationiount theirformulation of the probl"re "lly unexe KPZ ecor vn-ireculiaroniounxperimlimitsatrix theory hysic the physiracteriorerimental dees. Ponse andom polymer models whistiormre), andbeS alsoly unexe KPZ ,veral amazing ilow oniounx"display:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-de-la-federation-phystat-sud-neil-oconnel/

Séminaire de la fédérJour l'arre, audec/h1>d(Instor/Tuesday,10-08T003 2012 aMcnda0:0O lories0>

Manipulation de points dDomme stat-Sen gac, 41150-Sen gacFAST, Oray:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-de-la-federati>Marneiv>

Séminaire de la fédérJour l'arr’, audec/h1>d(Instor/Tuesday,10-09T003 2012 a 11:00:00O lories09

Manipulation de points dDomme stat-Sen gac, 41150-Sen gacFAST, Oray:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-de-la-federati>Marneiv>

Séminaire de la fédérJour l'arr’, audec/h1>d(Instor/Tuesday,10-10T003 2012 aat 14:30:00O loriese>

Manipulation de points dDomme stat-Sen gac, 41150-Sen gacFAST, Oray:none" itemprop="url">http://lptms.u-psud.fr/seminars/seminaire-de-la-federati>Marneiv>

Séminaire du LPTMS: G. Montambaux

dfe.enciTuesday,10-16 12 2012 at 11:00:00O lories16

Trous noirs dans la ThéoogyB2> Enfed us wiSly, new swirac Crmmulatedpt (LPS, OrsNiareas1>dfe.enci<,> Toul witign: justify">I will discuss a time evLholfinigle spanomecha-4y 70 tholees. P" datthesdmi