Les séminaires organisés par le LPTM

Le LPTM organise des séminaires tout au long de l'année. Ils ont lieu habituellement le jeudi après-midi, à 14h00, salle 4.13 dans les locaux du laboratoire, et sont ouverts à tous.


 Calendrier 2016



Mercredi 5 octobre 2016, 11h00, Salle 4.13


Daniel CABRA

(Instituto de Física de La Plata, Argentine)


Skyrmion lattices in antiferromagnetic systems.


The frustrated classical antiferromagnetic Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of an Antiferromagnetic Skyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating Skyrmion crystals (one by sub-lattice), and most importantly, the AF-SkX state seems to survive in the limit of zero temperature. Evidence of a discontinuous transition from a spiral phase into the AF-SkX phase is shown and for high fields, a second (probably continuous) transition occurs into a featureless paramagnetic phase.


Mardi 4 octobre 2016, 11h00, Salle 4.13


Vincent Caudrelier

(School of Mathematics, University of Leeds, UK/LPTM Cergy, professeur invité)


Lagrangian and Hamiltonian structures in an integrable hierarchy.


The classical and quantum versions of the R matrix are the cornerstones in classical and quantum integrable systems, typically formulated in 1+1 dimensions. However, they traditionally concentrate all the attention on only one of the independent variables : the space one while time evolution is encoded more or less trivially. The latter point is in fact deeply related to the boundary conditions imposed on the system. A big success of the theory of classical integrable systems is the systematic Hamiltonian formulation of the corresponding PDEs. The essential object capturing the Hamiltonian properties (infinite number of conserved quantities, etc) is the so-called classical r-matrix. Motivated originally by the question of integrability of certain field theories in the presence of defects or (not necessarily integrable) boundary conditions, we will show how a dual Hamiltonian structure naturally emerges which gives a fully fledged r-matrix structure to the space variable. This is inspired and related to the notion of covariant field theory. The interplay between the standard classical r-matrix structure and the new one is what we call dual Hamiltonian structure. This raises many questions that we will mention at the end of the talk.


 Mardi 27 septembre 2016, 14h00, Salle 4.13


Quentin DUPREY

(LPTM, Cergy)


Mesure non projective et trajectoire faible dans les systèmes quantiques.


Il est bien connu que la mesure d'une grandeur physique par un observateur introduit intrinsèquement une perturbation au système mesuré. Cette perturbation soudaine interrompt l'évolution "naturelle" du système et le projette sur un état propre de l'observable considérée. Au cours de ce séminaire, nous verrons qu'il est possible d'extraire une information d'un système quantique lors de son évolution : la valeur faible. Introduite par Aharonov, Albert et Vaidman en 1988 ("How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100," Physical Review Letters), elle permet d'accéder dans certaines conditions à la valeur moyenne d'une observable grâce à une interaction non-destructive. Alors que la notion de trajectoire (dans la réalité) est un concept inapproprié en mécanique quantique, nous étudierons la notion de trajectoire faible dans des systèmes d'interférométrie où nous traiterons notamment de l'effet du "Chat du Cheshire".


Jeudi 16 juin 2016, 11h00, Salle 4.13


Urbasi SINHA

(Raman Research Institute, Sadashivanagar, Bangalore, India)


A tale of three slits.




Mercredi 18 mai 2016, 14h00, Salle 4.13


Alexander HARTMANN

(Institut für Physik, Universität Oldenburg, Allemagne)


Large deviations for non--equilibrium processes.


Large deviations and rare events play an ever increasing role in science, economy and society. Often this concerns non-equilibrium processes, where large deviations play a crucial role for example for the estimation of impacts of storms, the calculation of probabilities of stock-market crashes or the sampling of transition paths for conformation change of proteins.

The basic principal to study large deviations using numerical simulations is quite old: make unlikely events more probable and correct in the end for the bias [1].

Here, we present a very general black-box method [2], based on sampling vectors of random numbers within an artificial finite-temperature (Boltzmann). This allows to access rare events and large deviation for almost arbitrary equilibrium and non-equilibrium processes. In this way, we obtain probabilities as small as $10^{-500}$ and smaller, hence rare events and large-deviation properties can be easily obtained.

Here, different non-equilibrium applications are displayed:

Fractional Brownian motion (fBm) characterized by the Hurst exponent H Distribution of end-points of non-absorbed walks for different values of H Distribution of work performed for a critical ($T=2.269$) two-dimensional Ising system of size $L\times L=128 \times 128$ upon rapidly changing the external magnetic field [2] (also applying theorems of Jarzynski and Crooks to obtain the free energy difference of such a large system) Distribution of perimeters and area of convex hulls of two-dimensional single and multiple random walks.


Lundi 18 avril 2016, 15h00, Salle 4.13


Alessandro TORCINI

(INMED INS Universite Aix Marseille / ISC CNR Sesto Fiotentino, Italie)


From Microscopic to Collective Dynamics in Neural Circuits.


In this talk I will revise my recent results on the application of nonlinear dynamics to the study of the emergence and stability of non trivial collective behaviours in pulse-coupled neural networks of Leaky Integrate-and-Fire neurons [1-5]. In particular, I will discuss how the network dynamics depends on the the connectivity of the underlying networks [1-3] as well as on the main aspects of the synaptic transmission, like the excitatory/inhibitory nature of the synapses and the time scale of the post-synaptic potentials [4-5]. I will conclude by reporting how a simple excitatory neural network can be employed to shed some ligth on recent experimental results on the dynamical orchestration of the developing hyppocampus [6] and how a simple inhibitory network can be employed to understand some of the main features of the dynamics of the Medium Spiny Neurons in the Striatum [7].


[1] S. Olmi, R. Livi, A. Politi, and A. Torcini, "Collective oscillations in disordered neural network", Phys. Rev. E 81, 046119 (2010)

[2] L. Tattini, S. Olmi, and A. Torcini, "Coherent periodic activity in excitatory Erdös-Reniy neural networks", Chaos 22, 023133 (2012)

[3] S. Luccioli, S. Olmi, A.Politi, and A. Torcini, "Collective dynamics in sparse networks", Phys. Rev. Lett. 109, 138103 (2012)

[4] S. Olmi, A.Politi, and A. Torcini, "Linear Stability in networks of pulse-coupled neurons", Frontiers in Computational neuroscience (2014) 8:8

[5] D. Angulo-Garcia and A. Torcini, "Stable chaos in fluctuation driven neural circuits", Chaos,solitons and fractals 69, 233-245 (2014)

[6] S. Luccioli, E. Ben-Jacob, A. Barzilai, P. Bonifazi, and A. Torcini, "Clique of functional hubs orchestrates population bursts in developmentally regulated neural networks", PLOS Comp. Biology 10(9): e1003823 (2014).


Vendredi 15 avril 2016, 15h00, Salle 4.13


Christophe TEXIER

(LPTMS, Orsay)


Topological phase transitions in multichannel disordered wires in the chiral symmetry classes.


We have considered the one-dimensional multichannel Dirac equation where the mass is a random N*N matrix with elements uncorrelated in space. In order to analyze the spectral properties of the model, we have introduced a matricial random process (Riccati matrix), related to the scattering matrix describing the reflection on the semi-infinite medium. We have found that the stationary distribution of the Riccati matrix corresponds to an interpolation between the Wishart (Laguerre ensemble) and inverse-Wishart distributions. The knowledge of this distribution has allowed us to derive explicit representations for the density of states (DoS), under the form of determinants ; the DoS is shown to present a power law behaviour at low energy. Varying the mass over disorder ratio allows to drive N phase transitions where the DoS exponent vanishes and which are shown to be of topological nature as they correspond to the change of a topological quantum number (Witten index).


Jeudi 7 avril 2016, 13h30, Salle 4.13


Alessandro FABBRI

(Instituto de Fisica Corpuscular, Universidad de Valencia, Spain)


Analogue de la radiation de Hawking dans les condensats de Bose-Einstein.


En 1974 S. Hawking montra que les trous noirs ne sont pas "noirs", mais émettent un faible rayonnement d’origine quantique. Cet effet, très difficile à détecter dans le contexte astrophysique, peut être reproduit en laboratoire avec des fluides supersoniques (les "trous noirs acoustique."’). En choisissant comme fluide un condensat de Bose-Einstein, je montrerai que nous pouvons mettre en évidence l’analogue du rayonnement de Hawking avec des mesures de corrélations, et d’en caractériser les propriétés quantiques.


Mardi 5 avril 2016, 13h15, Salle 4.13


Laurent JOLY

(Institut Lumière Matière, Université de Lyon 1)


Molecular modeling of liquid/solid friction for nanofluidic applications.


Nanofluidic systems (i.e. natural and artificial systems where fluids are confined at the nanoscale) play a key role in numerous present-day applications, from water treatment to sustainable energy harvesting. New behaviors arise in nanoconfined liquids due to the predominant role of surfaces. For instance, nanoflows are controlled by interfacial hydrodynamics, and can be enhanced by liquid/solid slip arising from low liquid/solid friction. Electrokinetic effects, coupling different types of transport (e.g. hydrodynamic, ionic, thermal) at interfaces, at the core of nanofluidic energy conversion systems, can also be enhanced by liquid/solid slip. It is therefore crucial to understand and optimize liquid/solid friction in order to design efficient nanofluidic devices. During this talk, I will illustrate with recent work how molecular dynamics simulations can help unravel the molecular mechanisms underlying liquid/solid friction, and explore its consequences on nanofluidic transport and in particular on nanofluidic energy conversion.


 Jeudi 24 mars 2016, 13h15, Salle 4.13


Vladimir A. BELAVIN

(I E Tamm Department of Theoretical Physics, P N Lebedev Physical Institute, Moscou, Russie)


Classical conformal blocks via AdS/CFT correspondance

La limite semi-classique des blocs conformes dans le contexte de AdS/CFT.


We discuss the holographic interpretation of a special class of classical conformal blocks in terms of particles propagating in an asymptotically AdS3 geometry. Using the world line approach we propose and explicitly describe the corresponding bulk configuration, which consists of a number of classical relativistic particles propagating in the conical defect background. We demonstrate the correspondence in the case of five-point block. The bulk analysis relies upon the special perturbative procedure which treats the n-point configuration as a deformation of the (n-1)-one. The boundary computation relies on the study of the monodromy properties of the auxiliary Fuchsian differential equation.


Nous discutons l’interprétation holographique d'une classe spéciale de blocs conformes dans la limite semi-classique en termes de particules dans un espace de type AdS. Nous décrivons explicitement la configuration correspondante dans AdS. Elle se compose d’un certain nombre de particules relativistes classiques qui se propagent dans le fond d'un défaut conique. Nous démontrons la correspondance dans le cas du bloc à cinq points. L’analyse dans AdS repose sur une procédure perturbative qui traite la configuration à n point comme une déformation de la configuration à (n-1) points. Le calcul dans la théorie conforme repose sur l’étude des propriétés de monodromie de l’équation différentielle Fuchsien auxiliaire.


Mardi 22 mars 2016, 13h30, Salle 4.13


Luca de' MEDICI

(ESRF, Grenoble)


The capricious electron correlation strength driven by Hund’s coupling.


Hund’s exchange coupling is a well-known main player (through Hund’s rules) in the physics of isolated atoms and insulators. It was however recently shown that it has a crucial influence also on the dynamics of conduction electrons in correlated materials[1]. Taking this effect into account helps explaining the capricious dependence of electron correlation strength and the occurrence of Mott insulating states in transition metal oxides and similar materials, like the recently discovered Iron-based high-Tc superconductors[2]. Hund’s coupling also favors a differentiation of correlation strength among conduction electrons[1,2], a potentially fertile ground for new phenomena in these materials, such as d-electron heavy-fermionic behavior. If time allows I will briefly touch on a potential application of prospective improvement in thermoelectric materials (possibly due to the exploration of these latter materials): self-cooling superconducting cables[3]. 


 Jeudi 17 mars 2016, 11h15, Salle 4.13



(University of Strathclyde, Glasgow , Scotland)


Novel approaches to gauge theories based on ultra-cold atoms.


I review our theoretical proposals to perform quantum simulations of dynamical gauge theories based both on Rydberg atoms and bosonic mixtures. This provides the motivation to discuss some exotic gauge theories and their phase diagram.


Mardi 15 mars 2016, 13h30, Salle 4.13


Alessio CELI

(ICFO, Castelldefels (Barcelona), Spain)


High for Low: High Energy ideas and phenomena in quantum simulation.


After reviewing idea and motivations for quantum simulation, I will introduce the simulation of synthetic (background) gauge fields with ultracold atoms as paradigmatic example. Such quantum simulators allow to study quantum Hall effects, as well as the simulation of relativistic physics and other topological systems. Of all the possible strategies developed in order to engineer synthetic gauge fields, I will detail the one based on "synthetic dimensions" that we introduced in analogy to spacetime compactifications, and that has been recently realized in two different experiments. In the last part of the talk, I will briefly comment about other branches of my program, namely, how it is possible to simulate (certain) lattice gauge theories, i.e. models in which the synthetic gauge field becomes dynamical and quantum, and artificial gravity backgrounds. 


 Jeudi 10 mars 2016, 13h30, Salle 4.13



(LMPT, Université Francois Rabelais, Tours)


Equations de Painlevé et blocs conformes.


L'équation de Painlevé VI décrit les déformations isomonodromiques de systèmes Fuchsiens de rang 2 à 4 singularités régulières sur la sphère de Riemann. J'expliquerai comment construire explicitement sa solution générale ainsi que la solution du problème de Riemann-Hilbert isomonodromique associé en utilisant les blocs conformes de l'algèbre de Virasoro.


Jeudi 11 février 2016, 13h30, Salle 4.13



(Institute for Theoretical Physics, Goethe-Universitaet Frankfurt)


Effects of doping and pressure in copper based kagome materials: spin liquid and strongly correlated Dirac metal.


Herbertsmithite ZnCu3(OH)6C12 is one of the best candidates for a quantum spin liquid; its Cu(2+) ions form a kagome lattice. With half-filled Cu dx2-y2 bands, it is a Mott insulator. We have determined the parameters of the underlying Heisenberg Hamiltonian using density functional theory (DFT) methods. We propose to dope herbertsmithite with 1/3 extra electron per Cu by substituting Ga for Zn. At the 4/3 filling level, the kagome Hamiltonian exhibits topologically protected Dirac points. Ga-herbertsmithite would be a rare example of a material with strongly correlated Dirac electrons at symmetry-protected locations in the Brillouin zone. Such a metal with strongly correlated Dirac point electrons would have rather unique magnetic and transport properties. We investigate the stability of herbertsmithite variants by calculating formation energies. We predict that doping to the 2/3 filling level, into the flat band of the kagome lattice band structure, would yield a ferromagnetic material that displays a quantum anomalous Hall effect. An interesting polymorph of herbertsmithite is kapellasite which orders magnetically and has a ferromagnetic nearest neighbour exchange coupling. We discuss the positioning of kapellasite ZnCu3(OH)6C12, haydeeite MgCu3(OH)6C12 and hypothetical CdCu3(OH)6C12 in the phase diagram of the J1-J2-J3 Heisenberg model. We find kapellasite and haydeeite to be near the boundaries between magnetically ordered and disordered phases, and we predict that moderate pressures could bring the materials into the paramagnetic regime. 


 Mardi 9 février 2016, 11h15, Salle 4.13



(Fachgruppe Physik , Bergische Universität Wuppertal, RFA)


Correlation functions of integrable quantum spin chains.


Integrable systems like the spin-1/2 Heisenberg chain provide the rare opportunity of calculating exact results like energies and thermodynamics for finite size and in the thermodynamic limit. We know many integrable systems for many decades, but actual calculations of physical properties have only rather recently been done if at all. A notorious problem is posed by the computation of correlation functions. For the infinite volume system, even static correlators present a major challenge. Currently, we know how to calculate short range correlators for the spin-1/2 Heisenberg chain at arbitrary temperature and magnetic field, where the first results for zero temperature and zero magnetic field have been obtained about 20 years ago. A central result of Jimbo, Miwa, Smirnov and others is the factorization of general static correlators into sums over products of nearest-neighbour correlators similar to the Wick theorem for ideal quantum systems however with much more complicated structure factors. There are applications in the field of non-equilibrium systems such as interaction quenches. I will talk about applications of the computational methods for the study of the stationary state approached through the equilibration process. A major subject of my talk will be the generalization of the study of correlation functions in direction of higher spin-S chains with su(2) symmetry and into the direction of higher rank symmetry like su(3).


 Jeudi 28 janvier 2016, 14h00, Salle 4.13



(LPS Université Paris Sud)


An incompressible state of a two dimensional electron gas.


Two-dimensional electrons in a magnetic field can form new states of matter characterized by topological properties and strong electronic correlations as displayed in the integer and fractional quantum Hall states. In these states, the electron liquid displays several spectacular characteristics, which manifest themselves in transport experiments with the quantization of the Hall resistance and a vanishing longitudinal conductivity or in thermodynamic equilibrium when the electron fluid becomes incompressible. Several experiments have reported that dissipationless transport can be achieved even at weak, non-quantizing magnetic fields when the electrons absorb photons at specific energies related to their cyclotron frequency. Here we perform compressibility measurements on electrons on liquid helium demonstrating the formation of an incompressible electronic state under these resonant excitation conditions. This new state provides a striking example of irradiation-induced self-organization in a quantum system.

Jeudi 21 janvier 2016, 14h00, Salle 4.13


Stefano NEGRO

(LPT ENS Paris)


The fermionic basis in sin(h)-Gordon model.


Recently a powerful approach to the computation of one-point functions in the quantum XXZ spin 1/2 chain has been proposed by Boos, Jimbo, Miwa and Smirnov; this framework relies on the existence of a particular basis in the state space of the theory: the fermionic basis. I will present the construction of these fermions for the scaling limit of inhomogeneous XXZ spin 1/2 chain, the sine-Gordon model, and for its twin, the sinh-Gordon model. If time allows it, I will briefly present a possible interpretation of the fermionic basis and one-point functions in terms of the action of a modified version of the sinh-Gordon model.



 Calendrier 2015


 Jeudi 10 décembre 2015, 14h00, Salle 4.13



(Institut de Mathématiques de Bourgogne, Dijon)


Spin chains with generic boundaries, separation of variables and scalar products.


In this talk I'll consider the XXZ spin chain with the most general boundary terms. It will be shown that the separation of variables method permits to construct the complete set of eigenstates and to describe the spectrum in terms of the solutions of the inhomogeneous version of the Baxter T-Q equation. In the constrained case this equation reduces to the usual homogeneous T-Q equation. Finally I’ll discuss the ways to compute the correlation functions, overlaps and the form factors for the models solvable by the separation of variables technique using a simpler example of the anti-periodic spin chains.


 Jeudi 26 novembre 2015, 14h00, Salle 4.13



(I.T.P. Universität Innsbruck, Autriche)


Entanglement Spectroscopy of Quantum Matter.


The entanglement spectrum, i.e. the logarithm of the eigenvalues of reduced density matrices of quantum many body wave functions, has been the focus of a rapidly expanding research endeavor recently. Initially introduced by Li & Haldane in the context of the fractional quantum Hall effect, its usefulness has been shown to extend to many more fields, such as topological insulators, fractional Chern insulators, spin liquids, continuous symmetry breaking states, etc. > After a general introduction to the field we review some of our own contributions to the field, in particular the perturbative structure of the entanglement spectrum in gapped phases, the entanglement spectrum across the Mott-insulator transition in the Bose-Hubbard model, and the relation of the entanglement spectrum of (1+1) dimensional quantum critical systems to the operator content of their underlying CFT.


 Jeudi 19 novembre 2015, 14h00, Salle 4.13



(CMPMS Department, Brookhaven National Laboratory, Upton, NY)


Kondo chain model: the transport and everything else.


We study a model of Kondo chain with anisotropic exchange interaction in the regime where the RKKY exchange dominates over the Kondo screening. Depending on the anisotropy the model has two phases one of which has broken helicity symmetry. The influence of disorder on low energy excitations is dramatically different in different phases. The phase with broken helicity has a long localization length.


Jeudi 5 novembre 2015, 14h00, Salle 4.13



(LPTM, Cergy)


Quantized conductance with bosonic atoms.


We analyze theoretically the quantization of conductance occurring with cold atoms trapped in two reservoirs connected by a constriction within which an attractive gate potential is added. We show that the Bose statistics strongly enhances the effect compared to fermions. We focus on temperatures slightly above the condensation threshold in the reservoirs, which contain a thermal gas. The Bose statistics leads to conductance plateaux whose large values depend on the thermodynamics of the gas in the reservoirs. We highlight the key role of weak repulsive interactions between the bosons in preventing them from collapsing into the constriction. We also point out the differences in the thermoelectric effects occurring with bosons and fermions in the quantized conductance regime.


 Jeudi 15 octobre 2015, 14h00, Salle 4.13


Antoine TILLOY

(LPT, ENS Paris)


Quantum jumps from continuous quantum trajectories.


When a quantum system is subjected to a continuous measurement, its evolution becomes stochastic and in a proper limit, it can be described by a continuous equation with Gaussian noise. On the other hand, it is known since Bohr that a quantum system subjected to successive von Neumann measurements undergoes rare quantum jumps. The objective of this talk is to show how this simple jumpy behavior can be obtained as a limit of the finer continuous picture. Starting from repeated interaction schemes, my first objective will be to introduce smoothly the formalism of quantum trajectories to explain what a continuous measurement even means in a quantum context. Then I will show, numerically, heuristically and perhaps even analytically that when the measurement rate increases, the evolution of a continuously monitored quantum system becomes "jumpy". I will show how the jump rates can be computed from the system parameters in the general case and I will finally demonstrate on an example that the continuous picture is much finer than what the naive quantum jump limit would suggest even in the infinitely strong measurement limit.


 Jeudi 8 octobre 2015, 14h00, Salle 4.13



(Brown University Physics Department, USA)


Duality and construction of higher spin gravity.


An overview of Higher Spin Gravity will be given with emphasis on its Duality with critical Large N Quantum Field Theories. The basis of this Duality will be explained through a first quantized spinning particle representation.


Jeudi 24septembre 2015, 14h00, Salle 4.13


Salvatore MANMANA

(Institut für Theoretische Physik Universität Göttingen, Allemagne)


Mott Quantum Criticality in the Anisotropic 2D Hubbard Model.


We present evidence for Mott quantum criticality in an anisotropic two-dimensional system of coupled Hubbard chains at half filling. In this scenario emerging from variational cluster approximation and cluster dynamical field theory, the interchain hopping acts as control parameter driving the second-order critical endpoint of the metal-insulator transition down to zero. Below the volume of hole and electron Fermi pockets of a compensated metal vanishes continuously at the Mott transition. Above this value, the volume reduction of the pockets is cut by a first-order transition. We discuss the relevance of our findings to a putative quantum critical point in layered organic conductors whose location remains elusive so far.


Jeudi 17 septembre 2015, 14h00, Salle 4.13


Herbert FRIED

(Brown University, Providence RI, USA)


A sequential description of the birth and death of a universe.


Beginning with a new QED/QFT-based model of Dark Energy, an extension to encompass Inflation provides an immediate candidate for Dark Matter. A few moments' further thought suggests a simple model for the Why and the How of the Big Bang, and the subsequent Birth of a New, and Death of the Old Universe.


Lundi 29 juin 2015, 14h00, Salle 4.13



(GPS/IJL, Université de Lorraine)


Quantum quenches and the arctic circle.


We analyze a one-dimensional system of particles evolving from a domain-wall initial state. This 'quantum quench' problem is well-known in real-time; here we analyze in imaginary time. The main interest of this model is that it exhibits the 'arctic-circle phenomenon' originally discovered in dimer models by [Propp, Jokusch, Shor], namely a spatial phase separation between a critically fluctuating region and a frozen region. Large-scale correlations inside the critical region are expressed in terms of correlators in a (euclidean) massless Dirac theory. It is observed that this theory is inhomogenous: the metric is position-dependent, so it is in fact a Dirac theory in curved two-dimensional space. The technique used to solve the toy-model can be extended to deal with the transfer matrices of other models: dimers on the honeycomb lattice, on the square lattice, and the six-vertex model at the free fermion point ($\Delta=0$). In all cases, explicit expressions are given for the long-range correlations in the critical region, and for the underlying Dirac action.


Jeudi 18 juin 2015, 11h00, Salle 4.13



(UFSCAR CCET , Sao Carlos, Brésil)


Modified algebraic Bethe Ansatz for the XXX Heisenberg chain on the segment.


The exact solution of the isotropic spin$-\frac{1}{2}$ Heisenberg chain on the segment is revisited. By means of the modified algebraic Bethe ansatz approach, the Bethe vector and its dual are constructed in different settings, corresponding to distinct forms of similar-transformed reflection matrices.


Jeudi 4 juin 2015, 14h45, Salle 4.13


Sahbi EL HOG

(LPTM Cergy - Séminaire Docotrant)


Blume-Emery-Griffiths Model In Thin Films of Stacked Triangular Lattices.


We study in this paper the Blume-Emery-Griffiths model in a thin film of stacked triangular lattices. The model is described by three parameters: bilinear exchange interaction between spins J, quadratic exchange interaction K and single-ion anisotropy D. The spin Si at the lattice site i takes three values (−1, 0, +1). This model can describe the mixing phase of He-3 (Si = +1, −1) and He-4 (Si = 0) at low temperatures. Using Monte Carlo simulations, we show that there exists a critical value of D below (above) which the transition is of second-(first-)order. In general, the temperature dependence of the concentrations of He-3 is different from layer by layer. At a finite temperature in the superfluid phase, the film surface shows a deficit of He-3 with respect to interior layers. However, effects of surface interaction parameters can reverse this situation. Effects of the film thickness on physical properties will be also shown as functions of temperature.


Jeudi 4 juin 2015, 14h00, Salle 4.13



(LPTM Cergy - Séminaire Docotrant)


"Phase diagram" of a mean field game.


Mean field games were introduced by J-M.Lasry and P-L. Lions in the mathematical community, and independently by M. Huang and co-workers in the engineering community, to deal with optimization problems when the number of agents becomes very large. In this article we study in detail a particular example called the 'seminar problem' introduced by O.Guéant, J-M Lasry, and P-L. Lions in 2010. This model contains the main ingredients of any mean field game but has the particular feature that all agent are coupled only through a simple random event (the seminar starting time) that they all contribute to form. In the mean field limit, this event becomes deterministic and its value can be fixed through a self consistent procedure. This allows for a rather thorough understanding of the solutions of the problem, through both exact results and a detailed analysis of various limiting regimes. For a sensible class of initial configurations, distinct behaviors can be associated to different domains in the parameter space . For this reason, the 'seminar problem' appears to be an interesting toy model on which both intuition and technical approaches can be tested as a preliminary study toward more complex mean field game models. 


Lundi 1er juin 2015, 14h00, Salle 4.13


Michio JIMBO

(Rikkyo University, Ikebukuro, Tokyo, Japon)


Toroidal gl(1) Algebra and Bethe Ansatz.


Since the seminal work of Bazhanov, Lukyanov and Zamolodchikov, diagonalization of integrals of motion has been an important issue in conformal field theory. Feigin,Kojima,Shiraishi and Watanabe introduced a commutative family of operators for the q-deformed Virasoro algebra. It turns out that their first Hamiltonian (and most likely all others) is derived from the transfer matrix acting on Fock representations of the toroidal gl(1) algebra. Making use of the shuffle algebra realization, we show that its spectrum is described by a Bethe-type equation. This is a joint work with B.Feigin, T.Miwa and E.Mukhin. 


 Jeudi 28 mai 2015, 14h00, Salle 4.13



(LMPT, Université de Tours)


Formulation q-hypergéométrique pour la classe de systèmes intégrables associées à l'algèbre q-Onsager.


Par analogie avec l'exemple élémentaire de l'oscillateur harmonique, des modèles intégrables de la classe Calogero-Sutherland-Moser, ou encore la chaîne de Toda ouverte pour lesquels une formulation (q-)hypergéométrique des fonctions propres (en termes des polynômes d'Hermite à une variable, Macdonald-Koornwinder multivariables ou cas limites type (q-)Whittaker ou Hall-Littlewood) est connue, je montrerai que la famille bispectrale des polynômes multivariables orthogonaux de Gasper-Rahman (généralisant ceux d'Askey-Wilson) offrent une base q-hypergéométrique permettant de construire les fonctions propres du Hamiltonien de la classe des modèles intégrables générés par l'algèbre q-Onsager (ex: Ising, chiral Potts superintegrable, XY, chaîne XXZ ouverte avec bords génériques,...). Dans une première partie, la structure des polynômes de Gasper-Rahman et leurs propriétés (orthogonalité, bispectralité) seront rappelées. Dans une seconde partie, la construction systématique de modules de dimension infinie et finie de l'algèbre q-Onsager en termes de ces polynômes sera décrite. Dans une troisième partie, l'action de la sous-algèbre Abélienne (génératrice des quantitiés conservées) dans la base polynomiale sera considérée. En particulier, les conditions d'existence de sous-espaces invariants seront identifiées. Les conséquences et perspectives pour les modèles intégrables précités seront brièvement décrites. Travail en collaboration avec X. Martin (LMPT Tours).


 Jeudi 16 avril 2015, 14h00, Salle 4.13


Alexander C. TIEGEL

(Institute for Theoretical Physics, University of Göttingen, Germany)


Spectral functions of one-dimensional quantum magnets.


We present numerical results for experimentally relevant spectral functions of one-dimensional strongly correlated quantum systems at both zero and finite temperature. As an example, we study the the electron spin resonance (ESR) intensity of spin-1/2 XXZ Heisenberg chains with Dzyaloshinskii-Moriya interactions in magnetic fields. The spectral functions are computed directly in the frequency domain via a Chebyshev expansion of the Green's function in a density-matrix renormalization group (DMRG) framework using matrix product states (MPS). At finite temperature, the method is based on a purification of the density operator by exploiting a Liouville space formulation of the dynamics. Our results are discussed in the context of experimentally observed bound states and their field-theoretical descriptions.


Jeudi 22 janvier 2015, 14h00, Salle 4.13


Françoise CORNU

(LPTHE Orsay)


First passage fluctuation relations ruled by cycles affinities.


For a non-equilibrium stationary state described by stochastic thermodynamics it is well-known that the entropy production rate can be expressed in terms of the affinity associated with every transition bond in the graph representation of the master equation. We exhibit the invariance of cycle affinities in finite state Markov processes under various natural probabilistic constructions : for instance under conditioning and under a new combinatorial construction that we call ``drag and drop''. We also show that cycle affinities have a natural probabilistic meaning related to first passage fluctuation relations. Indeed, for semi-Markovian processes whose corresponding graph is made of a single cycle, we establish that the cycle current obeys a fluctuation relation for first passage times at integer winding numbers, which is dual to the fluctuation relation for the cycle current at fixed time : contrarily to seminal fluctuation relations about the probability for measuring a cumulative current during a given time, the latter fluctuation relations deal with the probability for the time needed for one cycle to be performed in one sense or in the opposite one.


Mardi 20 janvier 2015, 14h00, Salle 4.13


Stanislao GUALDI

(Ecole Centrale Paris - Laboratoire MAS)


Tipping points and monetary policy in a stylized macroeconomic agent-based model.


Traditional approaches in economics rely on the assumption that economic agents are identical, non-interacting and rational. Within this framework, economic instabilities would require large exogenous shocks, when in fact small local shocks can trigger large systemic effects when heterogeneities and interactions are taken into account. The need to include these effects motivate the development of agent-based models (ABMs), which are extremely versatile and allow to take into account more realistic behavioural rules. In this talk we introduce a simple ABM, explore the possible types of phenomena that it can reproduce and propose a methodology that characterizes a model through its “phase diagram”. We then generalize the model with the aim of investigating the role and efficacy of the monetary policy of a ‘Central Bank’. We show that the existence of different equilibrium states of the economy can cause the monetary policy itself to trigger instabilities and be counter-productive.


Jeudi 15 janvier 2015, 14h00, Salle 4.13



(IPhT, CEA Saclay)


Distances dans les cartes planaires aléatoires et intégrabilité discrète.


Les propriétés métriques des cartes (graphes plongés dans des surfaces) aléatoires ont été beaucoup étudiées ces dernières années. Dans cet exposé, je présenterai une approche combinatoire à ces questions, exploitant des bijections entre les cartes et certains arbres étiquetés. Grâce à un phénomène inattendu d'«intégrabilité discrète», il est possible de compter exactement les cartes ayant deux ou trois points marqués à distances prescrites, et plus encore. Je parlerai ensuite des applications probabilistes à l'étude de la carte brownienne (obtenue comme limite d'échelle des cartes planaires aléatoires) et des cartes planaires uniformes infinies (obtenues comme limites locales). Si le temps le permet, j'expliquerai également l'origine combinatoire de l'intégrabilité discrète, liée au développement en fraction continue de la «résolvante» du modèle à une matrice. L'exposé repose sur des travaux en communs avec E. Guitter et P. Di Francesco.


Jeudi 8 janvier 2015, 14h00, Salle 4.13



(LPTHE Jussieu)


Markov stochastic processes with multiplicative white noise:

formalism and applications to the dynamics of a magnetic moment.


I will discuss several subtle aspects of Markov Langevin equations with multiplicative white noise, putting special emphasis on the celebrated Landau-Lifshitz-Gilbert-Brown equation for the dynamics of a magnetic moment. I will derive equilibrium and out of equilibrium fluctuation theorems from the generating functional formalism. The numerical integration of these equations also needs special care. I will present an algorithm that deals with all subtleties in a rather simple way and I present results for a simple physical situation that justifies its correctness.




 Calendrier 2014


 Vendredi 12 décembre 2014, 13h30, Salle 4.13



(Condensed Matter Theory Group, Brookhaven National Laboratoty, Upton, NY, USA)


Topological Kondo effect.


 Jeudi 27 novembre 2014, 14h00, Salle 4.13



(INO–CNR BEC Center and Dipartimento di Fisica, Universita di Trento, Italy)


Transport of Bose Gases Through Constrictions: Cooling by Heating and Fast Thermalisation.


The transport of an ultracold gas through a constriction connecting two reservoirs presents pluridisciplinary issues related to mesoscopic physics, disorder, and superfluidity. This geom- etry holds promises for the realization of a superleak for superfluid gases. First, assuming that a superleak is available, I shall describe a novel adiabatic cooling mechanism [1] which is related to the fountain effect of liquid helium. Second, I shall analyze theoretically the transport of a uniform weakly–interacting Bose gas through a constriction [2]. The transport of the superfluid part is assumed to be hydrodynamic, and the ballistic transport of the nor- mal part is described using the Landauer–Bu ̈ttiker formalism. I shall show that Helmholtz (plasma) oscillations can be observed at finite temperatures below Tc. Furthermore, because of its strong compressibility, the Bose gas is characterized by a fast thermalisation compared to the damping time for plasma oscillations, accompanied by a fast transfer of the normal component. I shall also outline the possible realization of a superleak through the inclusion of a disordered potential.


Jeudi 6 novembre 2014, 14h00, Salle 4.13



(DGM, INSA Rouen)


Nonlinear waves in networks: model reduction for the sine-Gordon equation.


The propagation of localized waves in nonlinear networks is an ubiquitous problem. Examples are fluxon motion in arrays of Josephson junctions, pulse propagation in the circulatory system.. Modeling such problems is difficult and it is helpful to simplify the equation and the geometry. We will illustrate these issues with the analysis of the propagation of sine-Gordon waves through Y junctions.


Lundi 22 septembre 2014, 11h30, Salle 4.13



(DPMC, Université de Genève)


Quantum magnets as quantum simulators.


The ability to control the properties of magnetic insulators by magnetic fields large enough to fully polarize the system has opened a host of possibilities. In addition to the intrinsic interest of such questions for magnetic systems, is has been shown that such systems could be efficiently used as quantum simulators to emulate problems pertaining to itinerant fermionic or bosonic systems. The magnetic field can then be viewed as similar to a gate voltage controlling the number of ``particles'' allowing an unprecedented level of control. In parallel with the experimental developments, progress on the theoretical front both on the numerical and the analytical side, have allowed a remarkable level of accuracy in obtaining the physical properties and in particular the correlation functions of these systems. A comparison between theoretical predictions without adjustable parameters or fudging with results from NMR, Neutrons or other probes such as ESR is thus now possible. This has allowed for example to test \emph{quantitatively} the physics of Tomonaga-Luttinger liquids and also to tackle the effects of the interactions between spinons by comparing the physics of weak rung ladders with the one of strong rung ones. I will review the recent results obtained in this domain with the different experimental compounds and will discuss the open questions and challenges. This concerns in particular the issues of finite temperatures, higher dimensional systems and effects of disorder.


Mardi 24 juin 2014, 14h00, Salle 4.13


Anastasia DOIKOU

(Heriot-Watt University, Edinburgh, Scotland)


Algebraic formulation of classical & quantum integrable defects.


A systematic approach to classical integrable defects is proposed, based on an underlying Poisson algebraic structure. Local integrals of motions are constructed as well as the time components of the corresponding Lax pairs for the sine-Gordon model. Continuity conditions imposed upon the time components of the Lax pair to all orders give rise to sewing conditions, which turn out to be compatible with the hierarchy of charges in involution. At the quantum level, using the Bethe ansatz methodology, we extract the transmission matrices for the XXZ model for two distinct types of defects. These describe the interaction between the particle-like excitations displayed by the model and the spin impurity. In the attractive regime of the XXZ model, we also derive the breather's transmission amplitude. 


Lundi 19 mai 2014, 14h00, Salle 4.13


Benjamin DOYON

(King's College, London)


Entropie d'intrication dans les systèmes étendus.


Je donnerai une revue des résultats principaux sur le sujet de l'entropie d'intrication dans les systèmes étendus en une dimension d'espace. En particulier, je discuterai de ceux que j'ai obtenus avec mes collaborateurs J. Cardy et surtout O. A. Castro Alvaredo: dans les théories des champs quantiques en une dimension, et dans les situations générales avec brisure de symétrie continue. J'expliquerai le concept de "branch-point twist field" que nous avons introduit en théorie des champs et dans les chaînes de spin pour calculer l'entropie d'intrication. Si le temps le permet, j'expliquerai nos résultats récents qui généralisent toutes les formules d'intrication critique connues jusqu'ici aux théories conformes non-unitaires.


Mardi 15 avril 2014, 14h00, Salle 4.13


Herbert M. FRIED

(Brown University Physics Department)


Inflation as the Precursor of Dark Energy.


Beginning with a new, QED-based Model of Dark Energy, it is possible to encompass Inflation within the same Model if one makes the unusual assumption that, for every electrically- charged lepton and quark pair fluctuating in the Quantum Vacuum, there is a massive, electrically-charged, fermionic tachyon and anti-tachyon pair fluctuating in the QV. (This assumption, while perhaps distasteful, cannot be negated by any experimental test.) Then one finds immediate agreement between cosmological requirements of times and energy densities at the beginning and end of Inflation. And one also obtains an automatic explanation of Dark Matter, since a charged, fermionic, tachyon of very high energy is a perfect candidate for a dark-matter particle. Finally, considerations of what can happen upon the very rare event of a very high energy T-Tbar annihilation, provide a possible understanding of the Why and the How of the Big Bang, leading to the Birth of a New Universe, and to the probable Death of the Old Universe.


Mardi 15 avril 2014, 14h00, Salle 4.13



(LIPN, Paris XIII Villetaneuse)


Combinatorics and geometry for random tensor models.


Random tensor models, seen as field theoretical models, represent a natural generalization of the celebrated 2-dimensional matrix models. These matrix models are known to be connected to 2-dimensional quantum gravity, and one of the main results of their study is that their perturbative series can be reorganized in powers of 1/N (N being the matrix size). The leading order in this expansion is given by planar graphs (which are dual to triangulations of the 2-dimensional sphere S^2).


Jeudi 10 avril 2014, 14h00, Salle 4.13



(ITP Universität Göttingen, RFA)


Dynamique des chaînes quantiques à température finie.


Nous discutons les propriétés dynamiques des chaînes de spins quantiques à température finie. Du point de vue des expériences, p. ex. les analyses de diffusion inélastique des neutrons des systèmes quantiques de basse dimensionnalité nécessitent une interprétation de l'évolution thermique des intensités de la diffusion. Du point de vue de la théorie, on sait peu sur la dépendance en température de la dispersion d’une particule dans de tels systèmes et encore moins concernant la dynamique de $N$ particules à température finie. Le calcul de la dynamique à température finie reste toujours un défi. Nous faisons un résumé des méthodes numériques modernes, y compris la diagonalisation exacte, le développement de moment, l’évolution en temps réel dans le contexte du « density-matrix renormalization group », et notamment des récents progrès sur la réalisation de ces calculs directement dans l'espace des fréquences en utilisant une formulation en espace Liouville de la dynamique à température finie. Comme une première application, nous discutons le cas de chaînes de spins quantiques. Nous présentons ici des résultats concernant la fusion de l'état fondamental et les formes des lignes thermales. En second lieu, nous discutons l’exemple d'une échelle de spin très frustrée. Dans ce cas - et également dans une certaine mesure dans les chaînes de spin simples – des caractéristiques spectrales des excitations persistent à température étonnamment élevée et même infini.


Mardi 8 avril 2014, 14h00, Salle 4.13



(ICCUB, Universidad Barcelona, Espagne)


Markov chains for tensor network states.


Markov chains for probability distributions related to matrix product states and 1-dimensional quantum Hamiltonians are introduced. With appropriate 'inverse temperature' schedules, these chains can be combined into a random approximation scheme for ground states of such Hamiltonians. Numerical experiments suggest that a linear, i.e. fast, schedule is possible in non-trivial cases. A natural extension of these chains to 2-dimensional quantum Hamiltonians is next presented and tested. This extension is stable by construction and the obtained results compare well with euclidean evolution. The proposed Markov chains are inherently sign problem free (even for fermionic degrees of freedom), and the random approximation scheme can be tailored to escape local minima.


Vendredi 4 avril 2014, 14h00, Salle 4.13


Patrick NAVEZ

(Fakultãt fũr Physik, Universitãt Duisburg Essen, RFA)


Large coordination number expansion for quantum lattice systems.


Quantum lattice systems are encountered in many field of physics: solid states, ultracold gas and photonic band gap materials. We establish a set of hierarchy equations describing the time evolution of the N-points spatial correlation reduced density matrix for such systems and apply it specifically to Bose and Fermi Hubbard gases or spin Heisenberg magnets. This set of equations is solved through a 1/Z expansion where Z is the coordination number i.e. number of interaction of a site with its nearest neighbors [1,2]. For a large class of quantum systems, we show how the generic leading order equation for the one-site reduced density matrix allows to derive linearized equation of motion for quasi-particle excitation operators whose solutions reproduce correctly the spectra found in the literature. In the next order in 1/Z, using the variable separation technique for the two-sites reduced correlated density matrix, we find that these excitations can be virtually produced in pairs in order to generate quantum fluctuations. We illustrate the powerfulness of these general concepts for several cases such as virtual particle-hole pairs in the Hubbard models that lower the ground state energy in the Mott phase or two virtual magnons of opposite spins in Heisenberg models that tend to reduce antiferromagnetism [3].


Jeudi 3 avril 2014, 14h00, Salle 4.13


Christophe OGUEY

(LPTM, Université de Cergy-Pontoise)


Topological moves in foams and liquid crystalline mesophases.


Topological movements are inherent to structured fluids when the structure undergoes large deformations, when it flows. These movements are dissipative and responsible for the special, non-Newtonian, rheology of complex fluids. In the first part, we'll study the time evolution of T1 flips in soap froths. Occasionally, when bubbles switch neighbours, the non-equilibrium configuration relaxes on several times scales. The process involves mechanical flow and surfactant concentration variations, coupled by the Marangoni effect. In the second part, we'll explore the types of elementary topological transformations that a 3-arm mikto copolymer mesophase can undergo. These star copolymers have three mutually immiscible branches. They self-assemble in a rich variety of ways which can be described as 3-coloured space partitions.


Jeudi 20 mars 2014, 14h00, Salle 4.13





Dislocation Dynamics in Helium Crystals.


The microscopic mechanisms responsible for mechanical dissipation peaks in metals are controversial. We studied the shear modulus and dissipation of 4He crystals because their extreme purity simplifies the interpretation of their mechanical response. We found that dislocation motion in solid helium causes very unusual behavior, including an 80% decrease in the shear modulus with no stress threshold or yield point. This behavior occurs because the only impurities in 4He crystals are 3He atoms, and they are mobile even at zero temperature because they move by quantum tunneling. Because our dilution refrigerator has windows, we were able to make video recordings of the growth of each crystal and determine its orientation, allowing us to study the anisotropy of its shear modulus. By measuring the shear modulus and dissipation of our crystals as a function of temperature, drive frequency, drive amplitude and 3He impurity concentration, we showed that there are distinct regimes in which the dislocations interact with 3He impurities or thermal phonons in different ways. 


 Jeudi 20 mars 2014, 11h15, Salle 4.13



(ICFO – The Institute of Photonic Sciences, Castelldefels (Ba), Espagne)


Physics of the 1D long range Ising model in a transverse field.


Long range interacting systems can show different behaviour from their short range versions. Recent experiments with trapped ions have started to investigate them Among them the simplest is the long range Ising model in a transverse field in 1D. The ground state and low energy excitations will be discussed with special focus on the complexity of the ground state wave function in terms of entanglement and more traditional spin correlation functions [1]. We will also illustrate that, out of equilibrium, long range interacting systems could display violations of causality when the long range interactions decay sufficiently slow with the distance [2]. These predictions have been confirmed by recent experiments with trapped ions [3,4].


Lundi 17 mars 2014, 14h00, Salle 4.13


 Nicolas PAVLOFF

(LPTMS, Orsay)


Trous noirs soniques et rayonnement de Hawking dans les condensats de Bose-Einstein.


L'analogue d'un trou noir peut être réalisé par l'écoulement d'un liquide dans un tuyau: si le flot est super-sonique dans une région de l'espace, une onde sonore émise dans cette région ne pourra plus remonter le courant. On parle de "trou muet". En 1981 B. Unruh a suggéré que les trous muets doivent permettre d'observer le rayonnement des trous noir qui est un effet quantique prévu par S. Hawking dans les années 70. Cette idée a récemment connu un regain d'intérêt dans le domaine de la condensation de Bose-Einstein. Une première raison en est la précision du contrôle expérimental qu'on peut obtenir sur ces systèmes (je présenterai des expériences en cours). Il y a également une motivation théorique que je discuterai en détail: il a été montré récemment que l'étude des corrélations de densité permet d'identifier le rayonnement de Hawking. Je présenterai certains des analogues de trous noirs gravitationnels (vagues en eau peu profonde, ondes lumineuses dans un milieu non-linéaire), puis je proposerai des configurations réalistes permettant d'observer le rayonnement de Hawking dans des condensats de Bose-Einstein atomiques et polaritoniques.


Vendredi 14 mars 2014, 15h00, Salle 4.13





Equation of state for ultracold fermions.


The field of ultracold fermions has been rapidly growing in the last ten years. In these systems, atomic fermionic isotopes can be in two internal states ("spin one half fermions"), like electrons in solids. The physics of ultracold fermions is particularly rich, since one can experimentally tune the strength of the interaction, or "polarize" the system. After a general introduction, including the presentation of the BEC-BCS crossover, I will present my work on the calculation of the equation of state in different situations. These calculations will be compared to accurate measurements on ultracold Li6 gases performed at ENS and MIT. 


 Mardi 11 mars 2014, 14h00, Salle 4.13


Christophe TEXIER

(LPTMS, Orsay)


Wigner time­ delay distribution in chaotic cavities and freezing transition.


The Wigner time delay is a useful concept capturing temporal aspects of a scattering process. It is also of great interest because it provides a measure of the density of states in the scattering region. In particular it was shown to be a central concept for describing charging effects in mesoscopic conductors (mesoscopic capacitance, charge relaxation resistance, etc). We analyse the statistical properties of the Wigner time delay in chaotic cavities within a random matrix theory approach. Using the joint distribution for proper time­ delays (eigenvalues of the Wigner­Smith time­delay matrix) derived by Brouwer, Frahm & Beenakker [Phys. Rev. Lett. 78, 4737 (1997)], we obtain, in the limit of large number of conducting channels, the large deviation function for the distribution of the Wigner time­delay (the sum of proper times) by a Coulomb gas method. The distribution is shown to present a rich structure. In particular, we show that the existence of a power law tail originates from narrow resonance contributions, related to a (second order) freezing transition in the Coulomb gas.



Jeudi 6 mars 2014, 11h00, Salle 4.13



(LPTMS, Orsay)


Effet Kondo dans les boites quantiques: fluctuations mésoscopiques et effets de taille finie.


L'effet Kondo décrit les conséquences du couplage entre le degré de liberté de spin d'une impureté et les électrons de conduction d'un métal ou d'un semi-conducteur. Il y a eu un regain d'intérêt pour ce problème classique de la physique du solide en relation avec les boites quantiques, et plus généralement dans le contexte de la physique mésoscopique. Une raison pour cela est que les boites quantiques peuvent être fabriquées de manière à jouer le rôle d'atomes artificiels, et en particulier d'impuretés magnétiques très versatiles dont les paramètres peuvent être choisis et modifiés à volonté. En plus de cet aspect, l'effet Kondo dans ce contexte pose des questions nouvelles associée aux effets de taille finie, soit à l'échelle de l'énergie de Thouless (fluctuations mésoscopiques), soit à l'échelle de l'espacement moyen entre niveaux d'énergie. Dans cet exposé, après une brève revue du rôle de l'effet Kondo dans la physique des boites quantiques, j'aborderais plus en détails les questions associées au confinement du gaz d'électron.



 Jeudi 20 février 2014, 14h00, Salle 4.13



(LMPT, Université de Tours)


Progrès récents sur l'effet tunnel résonnant.


Après avoir présenté le contexte, les problématiques et les enjeux de l'étude de l'effet tunnel en présence de résonances classiques, je présenterai quelques idées-clef, introduites ces dernières années, qui ont permis d'obtenir des résultats suffisamment significatifs pour faire progresser notablement notre compréhension de l'effet tunnel dans les systèmes complexes.


 Jeudi 13 février 2014, 14h00, Salle 4.13


 Jean-Philippe KOWNACKI

(LPTM, Université de Cergy-Pontoise)


Transitions de phase dans les membranes polymérisées fantômes et renormalisation non perturbative.


Les surfaces aléatoires sont des objets physiques bidimensionnels dont la géométrie fluctue. Parmi ceux-ci, les membranes polymérisées, dont l'intérêt a été ravivé récemment pour leur lien avec le graphène, ont une structure interne fixe dûe aux forces entre les molécules qui les composent. L'élasticité qui en résulte donne lieu à l'existence d'une phase haute température froissée et d'une phase basse température plate. Les membranes sont dites fantômes si l'encombrement stérique n'est pas pris en compte. Dans cet exposé, on passera d'abord rapidement en revue les principaux résultats obtenus ces dernières décennies concernant la phase plate et la transition de froissement des membranes polymérisées fantômes. Puis, on présentera plus en détail l'approche récente de ces problèmes par le groupe de renormalisation non pertubative. On développera en particulier des résultats très récents qui suggèrent fortement que la transition de froissement est du premier ordre.


 Jeudi 23 janvier 2014, 14h00, Salle 4.13



(IMAG Grenoble, France)


Entanglement between Demand and Supply in Markets with Bandwagon Goods.


Whenever customers' choices (e.g. to buy or not a given good) depend on others choices (cases coined 'positive externalities' or 'bandwagon effect' in the economic literature), the demand may be multiply valued: for a same posted price, there is either a small number of buyers, or a large one -- in which case one says that the customers coordinate. This leads to a dilemma for the seller: should he sell at a high price, targeting a small number of buyers, or at low price targeting a large number of buyers? In this paper we show that the interaction between demand and supply is even more complex than expected, leading to what we call the curse of coordination: the pricing strategy for the seller which aimed at maximizing his profit corresponds to posting a price which, not only assumes that the customers will coordinate, but also lies very near the critical price value at which such high demand no more exists. This is obtained by the detailed mathematical analysis of a particular model formally related to the Random Field Ising Model and to a model introduced in social sciences by T C Schelling in the 70's.


 Jeudi 16 janvier 2014, 14h00, Salle 4.13



(LPTMS, Université Paris Sud Orsay, France)


Top Eigenvalue of a Random Matrix: Large Deviations.


The statistical properties of the largest eigenvalue of a random matrix are of interest in diverse fields such as in the stability of large ecosystems, in disordered systems, in statistical data analysis and even in string theory. In this talk I'll discuss some recent developments in the theory of extremely rare fluctuations (large deviations) of the largest eigenvalue using a Coulomb gas method. Such rare fluctuations have also been measured in recent experiments in coupled laser systems. I'll also discuss recent applications of this Coulomb gas method in three different problems: entanglement in a bipartite system, conductance fluctuation through a mesoscopic cavity and the vicious random walkers problem.


 Vendredi 10 janvier 2014, 14h00, Salle 5.54


 Jean-Marie STEPHAN

(Dept. of Physics, University of Virginia, Charlottesville VA, USA)


Entropy and Mutual information in low-dimensional classical and quantum critical systems.


In studies of new and exotic phases of quantum matter, the Renyi entanglement entropy has established itself as an important resource. For example it is universal at one-dimensional quantum critical points: the leading term can be used to extract the central charge $c$ of the underlying conformal field theory, and thus identify the universality class. In this talk I will show how an analogous quantity defined for classical systems, the Renyi Mutual Information (RMI), can be used to access universality classes in 2d. In particular for a rectangle cut into two rectangles, the shape dependence of the RMI can be computed exactly and is proportional to $c$. This makes it possible to extract $c$ from (transfer-matrix) Monte Carlo simulations. I will also discuss how this Mutual information is related to the entanglement entropy of certain resonating valence bond states in 2d, as well as other basis-dependent entropies in 1d quantum systems.


 Jeudi 9 janvier 2014, 14h00, Salle 4.13



(James Franck Institute, University of Chicago, USA)


Universal thermodynamics of the superfluid to Mott insulator transition.


The superfluid--Mott-insulator transition of a Bose gas in an optical lattice, when it occurs at constant density, belongs to the universality class of the quantum XY model. We discuss the thermodynamics of the two-dimensional quantum O(N) model for $N\geq 2$ in the vicinity of its zero-temperature quantum critical point, and in particular the universal scaling function ${\cal F}_N$ which determines the pressure $P(T)$. We show that the large-$N$ approach is unable to predict the (non-monotonuous) shape of ${\cal F}_N$ for $N\lesssim 10$, but ${\cal F}_N$ can be computed from a non-pertubative renormalization- group approach. Finally, we discuss the spectral function of the amplitude mode (equivalent to a Higgs mode in presence of a gauge field) close to the quantum critical point and how the well defined mode at small N disappears as N increase.