# Séminaires de l’année 2010

 15 décembre à 14h30 Séminaire du LPTMS Francesco Intravaia (Los Alamos National Laboratory) Looking inside the Casimir Effect The fluctuations of the coupled system electromagnetic field + matter lead to an interaction between two bodies that is generally called Casimir effect or van der Waals force, depending on the distance between the objects. By definition, this fluctuation-induced force is strongly connected with concepts of condensed matter physics like plasmon-polaritons and diffusive currents. The knowledge of the interplay between (quantum) thermodynamics and these phenomena is crucial to understand the the strength and the sign of the Casimir force. We investigate thermodynamical aspects of the contributions of surface plasmons and Foucault currents to the Casimir effect putting in evidence some unusual behaviors. We also show how these phenomena could be used in order to taylor the strength and the sign of the force.
 14 décembre à 11h Séminaire du LPTMS Dmitry Kovrizhin (Max-Planck Institute for the Physics of Complex Systems, Dresden) Equilibration of integer quantum Hall edge states (work with J.T. Chalker, Theoretical Physics, Oxford University) I will present results of our recent work [3] on equilibration of quantum Hall edge states at integer filling factors which was motivated by experiments involving point contacts at finite bias [1,2]. Idealizing the experimental situation and extending the notion of a quantum quench, I will discuss the time evolution of a non-equilibrium state in a translationally invariant system. It will be shown that electron interactions bring the system into a steady state at long times, which is, strikingly, not a thermal one. For filling factor \nu=1 I will consider relaxation arising from finite-range and Coulomb interactions between electrons in the same channel, and for filling factor \nu=2 due to contact interactions between electrons in different channels. Comparison of the results with experiments [1,2] will be presented.[1] "Energy Relaxation in the Integer Quantum Hall Regime" H. le Sueur, C. Altimiras, U. Gennser, A. Cavanna, D. Mailly, F. Pierre, Phys. Rev. Lett. 105, 056803 (2010). [2] "Non-equilibrium edge-channel spectroscopy in the integer quantum Hall regime", C. Altimiras, H. le Sueur, U. Gennser, A. Cavanna, D. Mailly and F. Pierre, Nature Physics 6, 34 (2009). [3] "Equilibration of integer quantum Hall edge states", D.L. Kovrizhin, J.T. Chalker, arXiv:1009.4555
 10 décembre à 11h30 Séminaire "Fluides quantiques" Mark Raizen (Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin) Towards Atomic Fock States In this talk I will report on our progress towards the creation of isolated atomic Fock states "on demand" with ultra-cold Fermionic atoms. I show that realization of this step is an enabling resource for the study of scalable quantum entanglement and quantum computing. This system will also enable a "bottom-up" approach to quantum simulations of interacting fermions.
 8 décembre à 10h30 Séminaire du LPTMS Jurgen Jost (Max Planck Institut, Leipzig) Quantitative and qualitative properties of networks I shall introduce methods from information geometry and spectral graph theory to analyze the qualitative and quantitative properties of individual networks and of network ensembles.
 7 décembre à 11h Séminaire du LPTMS Alexei Kolezhuk (National Academy of Sciences, Institute of Magnetism, Ukraine) Effective theories of multicomponent cold atom systems with enhanced symmetry I discuss the phase diagrams and effective field-theoretical description for spin-1 bosons and spin-3/2 fermions in low-dimensional optical lattices at unit filling in the Mott regime. The focus is on the physics around points/lines exhibiting enhanced SU(N) symmetry, which can be perturbed by the varying scattering length ratio as well as by the quadratic Zeeman field. For spin-1 bosons, the phase diagram around the ferromagnetic SU(3) point is presented. For spin-3/2 fermions in the vicinity of the staggered antiferromagnetic SU(4) point it is shown that within 1-loop RG the Sp(4) perturbation favors the ordered phase.
 3 décembre à 14h Séminaire "Systèmes complexes" Imre Kondor (Eotvos University, Budapest) Long range random correlations in individual spin glass samples Various combinations of spin-spin correlations averaged over the randomness in spin glasses have long been known to be long-ranged, that is falling off like a power of the distance. This implies that correlations must be long ranged in some sense already before averaging. Since long range correlations seem to be a common feature of several complex systems, and in some of the real life complex systems not only the averaged behaviour, but also the individual samples are important, we decided to look into the the distribution of correlations in the paradigmatic case of spin glasses. Numerical simulations in the mean field model and in low dimensional Euclidean lattices show that the distribution of the simplest spin-spin correlations is very broad at low temperatures in all cases. The dependence of the correlations on distance in Euclidean lattices is random, with anomalously large absolute values showing up with finite probability even for large distances. The correlations strongly depend on the realization of the random couplings and on the boundary conditions, but any averaging (over the randomness, or over some spherical regions around a reference site) washes away all this structure. The talk will conclude with some speculations concerning the possible relevance of these findings to real life complex systems, such as traffic, electric power, or financial networks.
 2 décembre à 15h30 Séminaire "Fluides quantiques" M. Yu Kagan (Kapitza Institute, Moscou) Nanoscale phase separation in magnetic oxydes Different mechanisms of small-scale electron phase separation in CMR materials are analyzed using a simple ferromagnetic Kondo lattice model with Coulomb repulsion or orbital t-J model. We predict the instability of magnetic (charge or orbital) homogeneous ordering in these models with respect to the formation of droplet structure (magnetic or orbital polarons of nanoscale dimension) for much of the phase diagram of these magnetic oxide materials. Various types of magnetic polarons of Mott-Nagaev or de Gennes type are studied in 3D and layered materials. The transport characteristics and 1/f noise spectrum of phase separated materials are discussed and beautiful analogies with mesoscopic physics are revealed. Experimental confirmations of the proposed picture are presented.
 1er décembre à 14h30 Séminaire du LPTMS Yacine Ikhlef (Université de Genève) An intégrable model for localisation The Chalker-Coddington (CC, 1988) model is a simple 2D disordered lattice model for the Integer Quantum Hall Effect (IQHE), and its exact solution is still an open problem. Using the supersymmetric path-integral formulation of the CC model, we propose a truncation procedure, leading to a loop model with loop fugacity n=0. In the same loop model with general n, we identify an integrable manifold by a mapping to the dilute Birman-Wenzl-Murakami (BWM) algebra. Moreover, a discrete parafermion exists exactly on this manifold, as it is the case for several other loop models. The correspondence with BWM allows us to conjecture a conformal field theory description for specific values of n, not including n=0. We discuss numerical results at n=0, suggesting that it does not belong to the IQHE universality class. However, it constitutes an integrable case for the first step of another truncation procedure of the CC model, studied by Marston, Kondev and Tsai. Joint work with John Cardy (Oxford) and Paul Fendley (Virginia)
 30 novembre à 11h Séminaire du LPTMS Alain Barrat (CPT Marseille) Réseaux sociaux réels, réseaux sociaux en ligne Dans ce séminaire, je présenterai quelques évolutions récentes de l'étude des réseaux sociaux. De nouveaux outils technologiques permettent en effet d'obtenir des données d'une grande richesse et d'affronter des thématiques jusqu'ici difficiles d'accès par manque de données empiriques. D'une part, la collecte de données sur des réseaux sociaux en-ligne permet leur étude détaillée, et en fait une sorte de "laboratoire", permettant par exemple de tester des théories sociologiques. D'autre part, les nouvelles technologies permettent de collecter des informations sur la mobilité des individus et leurs interactions. Ces technologies permettent ainsi d'obtenir une description dynamique des interactions sociales avec une grande résolution temporelle. Le croisement de ces deux types de technologies permet de plus la comparaison de comportements en ligne et dans la vie réelle. Dans ce cadre, je présenterai tout d'abord une étude empirique d'un réseau social en ligne, puis des résultats obtenus lors de déploiements de l'infrastructure développée par le projet SocioPatterns (sociopatterns.org), qui mesure la proximité face-à-face d'individus dans des contextes tels que des conférences scientifiques. Je montrerai enfin les résultats de la comparaison de réseaux en ligne avec les interactions observées entre participants à une conférence: les participants rencontrent plus souvent et plus longuement d'autres participants avec qui ils ont une relation en-ligne, ou qui sont des co-auteurs; il devient alors possible d'utiliser les observations de rencontres dans une conférence pour prédire l'existence ou l'absence de relations en ligne.
 26 novembre à 16h Séminaire des post-docs du LPTMS Maxim Efremov, François Sausset, Alexej Weber
 24 novembre à 14h30 Séminaire des thésards du LPTMS Aurélien Decelle Community detection in graph Detecting community structure from network topology is a well known problem with many possible applications. A large number of studies was conducted over the last decade, but a principal approach that would for instance output that a random graph does not have any community structure is still missing. Based on a random graph model for a community structure I will first show the existence of a phase transition between possible and impossible community inference. This phase transition is related to some known results from statistical physics of spin glasses, for optimal inference the partition function of a corresponding spin glass model needs to be computed. Then I will turn to real-world networks and inspired by the theoretical results I introduce a new message passing algorithm which is able to learn parameters of the community structure (number of communities, ...), and to infer the most likely community assignment. As an application I will present some results on real-world networks. Simon Moulieras Superfluid Motion of Light Superfluidity, the ability of a fluid to move without dissipation, is one of the most spectacular manifestations of the quantum nature of matter. It is at the basis of many actual and potential applications, with limitations coming mainly from the very low temperatures required for its manifestation. Here we analyze a different system, the room-temperature propagation of light in a nonlinear medium. We show that there are regimes where the motion is superfluid, e.g. the scattering from defects is suppressed. We explicitly describe an experimental realization of this new phenomenon based on the transverse motion of light through a photonic lattice.
 23 novembre à 14h30 Séminaire Fluides Quantiques Gedminas Juzeliunas (Université de Vilnius) Light-induced gauge potentials for cold atoms In the initial part of the talk we shall review general methods enabling to produce an artificial magnetic field (an artificial Lorentz force) acting on electrically neutral atoms. Subsequently we shall consider specific schemes for creating the gauge potentials for ultracold atoms using light fields. In particular we discuss recent studies of the non-Abelian light-induced gauge potentials and their manifestations such as the quasi-relativistic behaviour and negative reflection of the atoms.
 23 novembre à 11h Séminaire du LPTMS Vera Spillner (Institut für Philosophie (Bonn) and Institut für Theoretische Physik(Heidelberg)) Does Nobody Understand Quantum Mechanics? Richard Feynman once stated that "if you think you understand quantum theory, you don't understand it." Is the theory really impossible to understand? In my talk, I will propose a new definition for what 'scientific understanding' can mean. Furthermore I will take a look at three famous interpretations of quantum mechanics: The Kopenhagen Interpretation, An American Philosopher's (Tim Maudlin's) View and the Theory of David Bohm. While they all refer to the same mathematical formalism and are therefore a matter of taste for physicists, one of them will turn out to produce more 'understanding' by my definition. Despite being a philosophical result, this could be relevant if physics still aims at increasing humans' understanding of the world around us.
 19 novembre à 11h Séminaire du LPTMS Michele Castellana (LPTMS) The Renormalization Group for Disordered Systems Both for spin glasses and structural glasses, the mean field theory of disordered systems provides a suggestive picture of laboratory glassy phenomena as the reflection of an ideal thermodynamic phase transition. Notwithstanding this, the implementation of a renormalization group approach for such a disordered systems describing the glassy phase beyond mean-field is still missing. In this seminar, we will present our work on a particular class of non-mean field disordered models called hierarchical models, for which a renormalization group transformation can be implemented explicitly. Our results will be presented with a view to understanding the physics of the spin-glass phase in finite dimension, and to implementing a real space renormalization group approach for realistic non-mean field models, as opposed to the ordinary field-theoretical approaches based on the replica method.
 18 novembre à 14h Séminaire Physique Biologique et Systèmes Complexes Olga Valba, Sergei Nechaev (LPTMS) Sequence matching algorithms and pairing of noncoding RNAs A new statistical approach to alignment of two heteropolymers which can form hierarchical cloverleaf-like secondary structures is proposed. This offers a new constructive algorithm for quantitative determination of binding free energy of two noncoding RNAs with arbitrary primary sequences. The alignment of ncRNAs differs from the complete alignment of two RNA sequences: in ncRNA case we align only the sequences of nucleotides which constitute pairs between two different RNAs, while the secondary structure of each RNA comes into play only by the combinatorial factors affecting the entropc contribution of each molecule to the total scoring function. We have demonstrated how our algorithm enables to solve the structure recovery'' problem. Namely, we can predict in zero-temperature limit the cloverleaf-like (i.e. secondary) structure of interacting ncRNAs by knowing only their primary sequences.
 17 novembre à 14h Séminaire du LPTMS Maxim Yu Kagan (Kapitza Institute, Moscou) BCS-BEC crossover and chiral anomaly in p-wave superfluids with a symmetry of the A1 phase We solve the Leggett equations for the BCS-BEC crossover in a three dimensional resonance p-wave superfluid with the symmetry of the A1 phase. We calculate the sound velocity, the normal density, and the specific heat for the BCS domain (? > 0), for the BEC domain (? < 0), and close to the important point ? = 0 in the 100% polarized case. We find the indications of a quantum phase transition close to the point ?(T = 0) = 0. Deep in the BCS and BEC domains, the crossover ideas of Leggett, Nozieres, and Schmitt-Rink work quite well. We discuss the spectrum of orbital waves, the paradox of intrinsic angular momentum and the complicated problem of chiral anomaly in the BCS A1 phase at T = 0. We present two different approaches to the chiral anomaly, based on supersymmetric hydrodynamics and on the formal analogy with the Dirac equation in quantum electrodynamics.
 16 novembre à 14h30 Séminaire du LPTMS Gernot Akemann (Brunel, London) Non-Hermitian Random Matrices: New Results and Applications The applications of Random Matrix Theory are not restricted to Hermitian Hamiltonians, describing the spectral properties of its real eigenvalues it in some limit. There exist a number of examples where complex eigenvalues naturally occur, like for time lagged covariance matrices or in quantum field theories with non-zero chemical potential. I will present some new results on non-Hermitian generalisations of so-called Wishart ensembles, in particular with real matrix elements. On the mathematical side their solution can be written in terms of Laguerre polynomials orthogonal in the full complex plane with some non-standard weight function. I will present the recent solutions of all three classical non-Hermitian ensembles and comment on generalisations. Comparisons to spectra from numerical simulations in Lattice Gauge Theory will illustrate these findings.
 15 novembre à 11h Séminaire Physique Biologique et Systèmes complexes Rosalind Allen (University of Edinburgh) Modelling microbial evolution with migration Almost all living organisms can move from place to place. It is an important question in evolutionary biology how this migration affects the evolution of populations. We have used a statistical physics approach to study how migration affects the evolution of microbial populations. This is important because the evolution of microbes can lead to new diseases as well as resistance to existing treatments (such as antibiotics). We find that different evolutionary mechanisms (mutations versus horizontal gene transfer) are affected in different ways by migration. We also introduce a quasispecies evolution model which shows a dynamical phase transition at a critical value of the migration rate.
 3 novembre à 14h30 Séminaire du LPTMS PierPaolo Vivo (ICTP) How many eigenvalues of a Gaussian matrix are positive? The index of a random matrix (i.e. the number of positive or negative eigenvalues)is a random variable providing information about the stability of stationary points in high-dimensional potential landscapes. For a Gaussian matrix model of large size N, typically half of the eigenvalues are positive and half negative (Wigner's semicircle law), however atypical fluctuations away from the semicircle are quite interesting for a number of reasons. Using a Coulomb gas technique and functional methods,we compute analytically the full large deviation function of the index and find that it has a quadratic form modulated by a logarithmic singularity around the 'peak'. The distribution of the index has thus a Gaussian form near the peak, but its variance increases logarithmically with the matrix size. This finding is compared with an exact finite N result based on the Andrejeff integration formula and a recent result by Prellberg.
 29 octobre à 14h30 Séminaire du LPTMS Khandker Muttalib (University of Florida) In search of a solvable "Levy-like" random matrix ensemble We introduce a family of rotationally invariant unitary random matrix ensembles characterized by a parameter lambda. While the Gaussian unitary ensembles are described by the well-known Hermite orthogonal polynomials, the lambda ensembles are constructed such that lambda = 1 will correspond to critical ensembles described by the so-called q - Hermite' polynomials. For lambda not equal to 1, we describe the ensembles by novel generalizations of the q - orthogonal polynomials. In particular, lambda <1 corresponds to distributions with tails falling off faster than 1/x. However, they turn out to be not quite "Levy-like".
 27 octobre à 14h30 Séminaire du LPTMS Pascal Naidon (Japan Science & Technology, The University of Tokyo) The Efimov effect in lithium 6 Forty years ago, Vitaly Efimov predicted the existence of a universal 3-body attraction for any 3-particle quantum system interacting via short-range interactions, whenever these interactions are resonant, i.e. the corresponding scattering lengths become very large. This force can bind the particles and form trimers with remarkable properties, with no classical counterparts. This curious effect was demonstrated in ultracold gases, in particular in the case of lithium 6, where an Efimov trimer was directly observed for the first time. I will present our latest experimental results and their theoretical analysis.
 26 octobre à 11h Séminaire du LPTMS Sergei Nechaev (LPTMS) On shock's statistics in "Tetris" game We consider a (1 + 1) dimensional ballistic deposition process with next-nearest neighbor interaction, which belongs to the KPZ universality class, and introduce for this discrete model a variational formulation similar to that for the randomly forced continuous Burgers equation. This allows to identify the characteristic structures in the bulk of a growing aggregate (clusters'' and crevices'') with minimizers and shocks in the Burgers turbulence, and to introduce a new kind of equipped Airy process for ballistic growth. We dub it the hairy Airy process' and investigate its statistics numerically. We also identify scaling laws that characterize the ballistic deposition patterns in the bulk: the law of `thinning'' of the forest of clusters with increasing height, the law of transversal fluctuations of cluster boundaries, and the size distribution of clusters. The corresponding critical exponents are determined exactly based on the analogy with the Burgers turbulence and simple scaling considerations.
 8 octobre à 11h Séminaire du LPTMS David Campbell (Université de Boston) Filtering Coherent Atomic Beams: The Peierls-Nabarro Energy Landscape of the Nonlinear Trimer Atomic Bose-Einstein condensates (BECs) trapped in optical lattices (OLs) have been the subject of great recent experimental and theoretical interest, both in their own right and as analog models of certain solid state systems. Recent studies of the leakage of a BEC trapped in an OL have shown that localized nonlinear excitations known as “Discrete Breathers” (DBs) can prevent atoms from reaching the leaking boundaries, thereby slowing the decay of the condensate. To understand the mechanism by which these DBs enhance the trapping, we study the case of atom transport—“tunneling”—through a DB on a nonlinear trimer. We show that this transport is related to the destabilization and subsequent motion of DB and that there exists a threshold in the total energy on the trimer that controls this destabilization. We find that this threshold and the resultant tunneling can be described analytically by defining a two-dimensional “Peierls-Nabarro” energy landscape which restricts the dynamics of the trimer to a limited region of phase space. We further establish that the value of the threshold is related to the Peierls-Nabarro barrier of a single DB. Our results suggest a possible means for controlling the transmission of coherent atomic beams in interferometry and other processes. This work has been carried out in collaboration with Holger Hennig and Jerome Dorignac.
 7 octobre à 14h Séminaire Physique Biologique et Systèmes complexes Hervé Rouault (LPS, ENS) Cell fate determination depends in part on the establishment of specific transcriptional programs of gene expression. These programs result from the interpretation of the genomic cis-regulatory information by sequence-specific factors. Decoding this information in sequenced genomes is an important issue. We developed statistical analysis tools to computationally identify the cis-regulatory elements (CRMs) that control gene expression in a set of co-regulated genes. Starting with a small number of functional sequences, we computationally predicted transcription factor binding sites (TFBSs) and genomic CRMs underlying co-regulation. This method was applied to the gene expression program active in Drosophila melanogaster sensory organ precursor cells (SOPs), a specific type of neural progenitor cells. The statistics and phylogeny-based tools described here can be more generally applied to identify the cis-regulatory elements of specific gene regulatory networks in any family of related species with sequenced genomes.
 5 octobre à 11h Séminaire du LPTMS Jan von Delft (LMU) The Kondo exciton: a quantum quench towards strong spin-reservoir correlations We study a quantum quench for a semiconductor quantum dot coupled to a Fermionic reservoir, induced by the sudden creation of an exciton via optical absorption. The subsequent emergence of correlations between spin degrees of freedom of dot and reservoir, culminating in the Kondo effect, can be read o from the absorption - lineshape and understood in terms of the three xed points of the single-impurity Anderson model. At low temperatures the lineshape is dominated by a power-law singularity, with an exponent that depends on gate voltage and, in a universal, - asymmetric fashion, on magnetic field, indicative of a tunable Anderson orthogonality catastrophe. We will also report on recent experimental progress towards observing this physics.
 10 septembre à 11h Séminaire du LPTMS Pavel Lushnikov (University of New Mexico, Albuquerque) Collapse and stable self-trapping for Bose-Einstein condensates with 1/r^b type attractive interatomic interaction potential We consider dynamics of Bose-Einstein condensates with long-range attractive interaction proportional to 1/r^b and arbitrary angular dependence. It is shown exactly that collapse of Bose-Einstein condensate without contact interactions is possible only for b greater or equal to 2. Case b=2 is critical and requires number of particles to exceed critical value to allow collapse. Case b>2 is supercritical with expected weak collapse which traps rapidly decreasing number of particles during approach to collapse. For b<2 singularity at r=0 is not strong enough to allow collapse but attractive 1/r^b interaction admits stable self-trapping even in absence of external trapping potential.
 6 septembre à 11h Séminaire du LPTMS Oleg Yevtushenko (Ludwig-Maximilians-Universität München) Critical Scaling at the Anderson Localization Transition in the Strong Multifractality Regime We study dynamical scaling (DS) in disordered systems at (or close to) the point of the Anderson localization transition. Wave functions of such systems are fractal. DS is connected to strong spatial correlations of the wave functions. These correlations are particularly nontrivial in the strong fractality regime where fractals are very sparse. It has been conjectured [1] that there exists an exact relation between the exponent of DS and the 2nd fractal dimension. To the best of our knowledge, neither existence of DS nor the relation between the exponents were checked analytically. We study DS and the critical exponents in the strong fractality regime using the model of almost diagonal random matrices with fractal eigenstates [2] by analyzing asymptotic behavior of the return probability in the long time limit. Since the nonlinear \sigma-model cannot be solved in the strong fractality regime we use an alternative field theoretical method: the SuSy virial expansion in a number of interacting energy levels [3]. We have proven the DS to hold true up to the leading terms of 2nd loop of RG. We discuss necessary conditions for the exact relation between the critical exponents. [1] J.T. Chalker and G.J. Daniell, Phys. Rev. Lett. 61, 593 (1988); J.T. Chalker, Physica A 167, 253 (1990). [2] A.D. Mirlin, Y.V. Fyodorov, F.M. Dittes, J. Quezada, T.H. Seligman, Phys. Rev. E 54, 3221 (1996). [3] O. Yevtushenko, A. Ossipov, J. Phys. A: Math. Theor. 40 (2007) 4691; S. Kronmueller, O.M. Yevtushenko, E. Cuevas, J. Phys. A: Math. Theor. 43 (2010), 075001.
 13 juillet à 11h Séminaire du LPTMS Todor Mishonov (University of Sofia) Superconductivity of overdoped cuprates : the modern face of the ancestral two-electron exchange The single-site two-electron exchange amplitude J_sd between the Cu 4s and Cu 3d_{x^2-y^2} states is found to be the pairing mechanism of high-T_c overdoped cuprates. The noninteracting part of the Hamiltonian spans the copper Cu 4s, Cu 3d_{x^2-y^2} and oxygen O 2p_x and O 2p_y states. Within the standard BCS treatment an explicit expression for the momentum dependence of the gap Delta_p is derived and shown to fit the angle-resolved photoemission spectroscopy (ARPES) data. The basic thermodynamic and electrodynamic properties of the model (specific heat C(T), London penetration depth lambda(T)) are analytically derived. These are directly applicable to cuprates without complicating structural accessories (chains, double CuO_2 planes, etc.). We advocate that the pairing mechanism of overdoped and underdoped cuprates is the same, as T_c displays smooth doping dependence. Thus, a long-standing puzzle in physics is possibly solved.
 8 juillet à 11h Séminaire du LPTMS Olga Goulko (DAMTP, Cambridge University, UK) The imbalanced Fermi gas at Unitarity Lattice field theory is a useful tool for studying strongly interacting theories in condensed matter physics. A prominent example is the unitary Fermi gas: a two-component system of fermions interacting with divergent scattering length. With Monte Carlo methods this system can be studied from first principles. In presence of an imbalance (unequal number of particles in the two components) a sign problem arises, which makes conventional algorithms inapplicable. We will show how to apply reweighting techniques to generalise the recently developed worm algorithm to the imbalance case, and present results for thermodynamic observables, in particular the critical temperature, for equal, as well as unequal number of fermions in the two spin components.
 6 juillet à 11h Séminaire du LPTMS Fabian Heidrich-Meisner Out-of-equilibrium dynamics of interacting fermions in one dimension In this talk, I will discuss the application of the time-dependent density matrix renormalization group (TD-DMRG) technique to the problem of calculating steady-state currents in interacting nano-structures and more generally, the non-equilibrium dynamics of interacting fermions in one dimension. A paradigmatic model that captures essential features of quantum dots with spin and charge fluctuations is the single-impurity Anderson model. Although experiments on semi-conductor structures that are believed to be well described by the SIAM have been available for a long time, no full solution for the current-voltage characteristics is available yet. Here, I present TD-DMRG results for this problem, providing accurate data in the zero-temperature limit [1]. Besides steady-state currents, we are further interested in properties of the steady state such as spin and charge fluctuations. Moreover, particles flowing through a nanostructure give rise to a rapid increase of entanglement between the leads, which I shall also discuss. Similar results will be presented for the case of non-equilibrium transport through an extended interaction region, namely a Mott insulator [2]. Here, I will use TD-DMRG to study the dielectric breakdown of this state. Finally, I shall discuss a simple, yet still experimentally example, namely the expansion of interacting fermions, released from a confining potential, into a one-dimensional lattice [3-5]. This set-up is currently being used in experiments with cold atoms to study ballistic and diffusive dynamics far from equilibrium. Here, I will illustrate several intriguing and at times counterintuitive phenomena that one can encounter, such as metastable states [4]. [1] Heidrich-Meisner, Feiguin, Dagotto, Phys. Rev B 79, 235336 (2009) [2] Heidrich-Meisner, Gonzalez, Al-Hassanieh, Feiguin, Rozenberg, Dagotto, in preparation. [3] Heidrich-Meisner, Rigol, Muramatsu, Feiguin, Dagotto: Phys. Rev. A 78, 013620 (2008) [4] Heidrich-Meisner, Manmana, Rigol, Muramatsu, Feiguin, Dagotto, Phys. Rev. A 80, 041603(R) (2009) [5] Langer, Heidrich-Meisner, Gemmer, McCulloch, Schollwöck, Phys. Rev. B 79, 214409 (2009)
 29 juin à 11h Séminaire du LPTMS Céline Nadal (LPTMS) Phase transitions in the distribution of bipartite entanglement of a random pure state Entanglement is a crucial resource in quantum information and computation. To exploit non-classical correlations to the maximum advantage, it is desirable to create states with large entanglement. Bipartite random pure states are good candidates as they are known to have almost maximal average entanglement entropy. Here we compute analytically the distribution of Renyi and Von Neuman entanglement entropies for such a random pure state. Mapping this quantum problem to a random matrix problem, we use a Coulomb gas method, a well-known method in random matrix theory. We show that there are two critical points at which the entropy probability changes shape. These changes are a direct consequence of two phase transitions in the associated Coulomb gas problem. Alvaro Rojo (LPTMS) Reconstruction par l'effet de champ de Dislocations d'une Jonction comportant une Onde de Densité de Charge Incommensurable Pouvoir générer des particules élémentaires à partir des champs continus est un paradigme en physique. Les solitons (particules élémentaires dans les processus transitoires) déterminent les propriétés électroniques s’ils sont énergétiquement favorables par rapport aux électrons. Leur existence a déjà été identifiée expérimentalement dans différents systèmes, particulièrement dans des cristaux électroniques, comme les ondes de densité de charge (ODC). Dans ce cas, la période de l’onde de densité de charge n’est pas fixée et peut être modifié en absorbant des électrons ajoutés ou retirés de l’état fondamental. Cette physique donne l’accès à la reconstruction de l’état fondamental dans le dispositif nanométrique par l’intermédiaire d’un réseau de vortex ODC. L’analyse des spectres de basse énergie donne en outre la preuve dont le courant tunnel procède par des solitons de phase doublement chargés. Dans ce travail, nous réalisons une simulation numérique de l'état fondamental de l'Onde de Densité de Charge Incommensurable consécutive à l'application d'une différence de potentiel entre des chaînes conductrices. Nous confirmons la reconstruction de la jonction en rangées des dislocations, dont le nombre augmente par pas discrets avec la différence de potentiel. Le cœur des dislocations concentre la majorité de la diminution de potentiel à travers la jonction ce qui donne lieu à un effet tunnel effectif. Les résultats présentés ici confirment, illustrent et donnent des détails sur les concepts développés lors des expériences d'effet tunnel sur NbSe3 et TaS3 (Yu. Latyshev et al, 2006).
 25 juin à 15h Séminaire exceptionnel du LPTMS Paolo Solinas (Aalto University, Espoo, Finland) Decoherence of adiabatically steered quantum system In the seminar I will present our results about the effect of Markovian environmental noise on the dynamics of a two-level quantum system which is steered adiabatically by an external driving field. I will discuss the approximations to arrive to a master equation and, in particular, the secular approximation and its consequences on the predicted dynamics. The master equation will be used to describe the superconducting Cooper pair pumping in presence of environmental noise. I will show how, in the adiabatic limit, the ground state Cooper pair pumping is robust against zero-temperature environment. I will also discuss the possibility to engineer the environmental noise in the above physical set-up and future experiments.
 24 juin à 16h Séminaire du LPTMS Gunnar Boldhaus (Bioinformatics, University of Leipzig, Germany) Knockouts, Robustness and Cell Cycles The response to a knockout of a node is a characteristic feature of a networked dynamical system. Knockout resilience in the dynamics of the remaining nodes is a sign of robustness. Here we study the effect of knockouts for binary state sequences and their implementations in terms of Boolean threshold networks. Beside random sequences with biologically plausible constraints, we analyze the cell cycle sequence of the species Saccharomyces cerevisiae and the Boolean networks implementing it. Comparing with an appropriate null model we do not find evidence that the yeast wildtype network is optimized for High knockout resilience. Our notion of knockout resilience weakly correlates with the size of the basin of attraction, which has also been considered a measure of robustness.
 22 juin à 11h Séminaire du LPTMS Martin Lenz (James Franck Institute, Chicago) Cross-linking actin into function Actin filaments are paramount players in the shaping and dynamics of eukaryotic cells, and can form a variety of structures as they are regulated by a host of associated proteins. Here we focus on some spectacular examples of such constructs: stereocilia and stress fibers, which are bundles formed under the influence of actin cross-linking proteins. The relatively large scale of these structures implies that their overall organization crucially involves the collective behavior of actin and its cross-linkers, into which a theoretical approach yields some interesting insights. We first propose that the cross-linker espin may regulate the shape of stereocilia, which are protrusions of cells of the inner ear crucial for hearing. Very good agreement with a range of experimental results is obtained by fitting only one parameter: the detachment rate of espin. Our model displays a transition from finite-length structures to unbounded growth, some universal features of which could help classify cellular growth processes. We next focus on biomimetic bundles containing the cross-linking motor myosin. We show that the widely accepted filament sliding model is not sufficient to predict whether these bundles tend to extend or contract, whereas the latter is always observed experimentally. We explore the hypothesis that this symmetry could be broken through the nonlinear elastic behavior of actin filaments.
 18 juin à 14h Séminaire du LPTMS : Physique biologique et systèmes complexes Guillaume Romet-Lemonne (LEBS, Gif) Actin dynamics and mechanics : from single filaments to networks Actin proteins assemble to form filaments, which can elongate and disassemble from both ends, with different reaction rates. Each actin monomer binds an ATP, which is hydrolyzed into ADP after incorporation of the monomer into the filament. This results in a complex system, where ATP hydrolysis is coupled to the dynamics of assembly/disassembly, and modulates the interaction of the filament with regulatory proteins. I will present experimental data and theoretical analysis on the dynamics of individual filaments in vitro,and show that we can determine the ATP/ADP profile in the filament,and get information on this complex mechanism. In cells, actin filaments are organized in a variety of networks, as they interact with regulatory proteins. Branched networks for example, can generate a force as they grow against a surface, such as the leading edge of a migrating cell, or the rear of an endosome or a bacterium within the cytoplasm. By reconstituting this machinery in vitro, to propel artificial vesicles, we get valuable insight in the underlying mechanism. I will show how the diffusion of proteins bound to a the lipid membrane plays an important role in regulating the growing network and the resulting movement, and how this provides a better understanding of protein-protein interactions in this system.
 15 juin à 11h Séminaire du LPTMS Rhoda Hawkins ( LPTMC, Jussieu) Mécanisme physiques de motilité et de polarisation cellulaire Nous présentons deux mécanismes physiques permettant de modéliser deux fonctions cellulaires essentielles: la motilité; et la polarisation cellulaire. Dans un premier temps, Nous proposons un nouveau mécanisme de la motilité; cellulaire, qui repose sur le couplage de la polymérisation de l'actine avec le confinement géométrique. Nous montrons que le confinement joue un rôle crucial et peut donner lieu à des vitesses de déplacement des cellules potentiellement plus grandes que la vitesse de polymérisation, et ce même en absence d'adhésion spécifique. Notre modèle correspond qualitativement à des expériences récentes qui montrent que des cellules sans protéines d'adhésion sont mobiles dans des environnements confinés (comme les canaux micro fluidique) alors qu'elles sont incapables de se déplacer sur un substrat plan. Il pourrait nous aider à comprendre les mécanismes de migration cellulaire dans des géométries plus complexes tels que les tissus vivants où les effets de confinements sont omniprésents. Dans un deuxième temps, nous présentons un mécanisme général de polarisation cellulaire - c'est à dire conduisant à l'émergence d'une distribution inhomogène des marqueurs de polarité - reposant sur un couplage des marqueurs moléculaires avec le cytosquelette. Notre modèle montre que la géométrie des filaments du cytosquelette, qui peuvent être nucléés soit sur la membrane (e.g. actine) soit au centre de la cellule (e.g. microtubules) détermine si le système est capable de se polariser spontanément ou seulement grâce à des signaux asymétriques extérieurs. Ce modèle est en accord avec des expériences récentes de polarisation cellulaire pour deux systèmes biologiques très différents: la levure et le neurone.
 8 juin à 11h Séminaire du LPTMS Luigi Cantini (LPT ENS) Proof of the Razumov-Stroganov conjecture. In 2001 Razumov and Stroganov conjectured that the (properly normalized) components of the ground state of the dense O(1) loop model on a semi-infinite cylinder enumerate fully-packed loop (FPL) configurations on the square, with alternating boundary conditions, refined according to the link pattern for the boundary points. This conjecture has arisen a lot of interest both in the physics and in the mathematics community. In this talk, after reviewing the main background, I will present a proof of this conjecture. The main idea is to recognize the fundamental role of ?gyration?, an operation that can be performed on FPL, which was already the key in Wieland?s proof of the rotational symmetry of the FPL enumerations.
 4 juin à 11h Séminaire du LPTMS : Physique biologique et systèmes complexes Yoshiyuki Kabashima (Tokyo Institute of Technology, Japan) A statistical mechanical approach to compressed sensing We consider the problem of reconstructing an N-dimensional continuous vector x from P constraints which are generated from its linear transformation under the assumption that the number of non-zero elements of x is typically limited to \rho N (0 \le \rho \le 1). Problems of this kind are very relevant for various modern sensing (sampling, imaging, etc.) techniques such as radar measurement, computed tomography and so on. In efforts toward designing efficient reconstruction schemes exploiting the prior knowledge of sparsity, minimization of a cost function with respect to the so-called Lp-norm of x under the P constraints has been actively studied these days [1].In this talk, we introduce our recent attempt [2], which is based on the replica method of statistical mechanics, for evaluating a critical relation between \alpha = P/N and \rho for successfully reconstructing the original vector x by the minimization.
 1 juin à 16h Séminaire du LPTMS Uzy Smilansky (Weizmann Institute, Israel) Quantum Chaos and Combinatorics on discrete graphs I shall discuss the spectrum and eigenfunctions of the discrete Laplacian on d-regular graphs. After presenting some trace formulae, I shall show the intimate connection between spectral statistics and the statistics of the number of cycles on the graphs. Finally I shall discuss nodal domains and percolation of level-sets of the eigenvectors. Using a "random waves conjecture" I will compute the nodal distribution and the critical level as a function of the degree and the eigenvalue.
 18 mai à 11h Séminaire du LPTMS Henk van Beijeren (Utrecht) Exact solutions of TASEPs describing two-way traffic at several speeds As shown by Karimipour[1] the Matrix Product Ansatz method for solving the totally asymmetric exclusion process can be generalized to a system of several species of particles. For this to be possible the following conditions must hold: 1) Define the free velocity vi of a particle of species i as plus or minus the exchange rate with an empty site to its right respectively to its left depending on the sign of vi . Then the exchange rate of a particle of type i with a particle of type j to its right must be vi ? vj if vi ? vj > 0 and zero otherwise. 2) Define i and i as the injection respectively extraction rate for species i, with injection on the left and extraction on the right for species of positive velocity and the other way around for species of negative sign. Then the extraction rates have to satisfy the conditions vi ? vj = i ? j if vi and vj have the same sign (in contrast to what is stated in [1] no conditions have to be imposed on the injection rates). In a grand ensemble formalism one easily postulates a general form for the partition function with a few parameters that can be identified easily. From this one immediately obtains all the currents and densities as functions of the exchange, injection and extraction rates. Like in the 1-species case, depending on these rates one will usually have at least three different phases, one determined by the injection and extraction rates on the left (the low density phase), one determined by the corresponding rates on the right (the high density phase) and one or more that depend only on ratio’s between injection rates (the maximum current phase in the one-species system). V. Karimipour, Phys. Rev. E 59 (1999) 205; M. Khorrami and V. Karimipour, J. Stat. Phys. 100 (2000) 999
 12 mai à 14h30 Séminaire du LPTMS Alessio Recati (CNR-INO, BEC Center, University of Trento, Italie) Bogoliubov theory of a dumb-hole in Bose-Einstein condensates and density correlation as a probe of analog "Hawking radiation" Sonic analogous of black holes have been proposed in the 80's by B. Unruh. In these set-ups, the flow of a fluid becomes super-sonic in some region of space, and no sound issued from the super-sonic region can reach the sub-sonic one. One speaks of a dumb hole. Dumb holes have been considered as potential tools to get hints in black hole radiation which is a quantum effect predicted by S. Hawking in the 70's. Most of the analysis rely on a formal analogy between hydrodynamics in presence of a sonic horizon and quantum fields in curved space-times. We apply the microscopic Bogoliubov theory to analyze the fluctuations of the flow of a dilute atomic condensate in the presence of a sonic horizon. We show that the density-density correlation has a peculiar long-distance behavior which represents a new and better way to detect what is called analog Hawking radiation. The analytical results are in good quantitative agreement with first principle numerical calculations.
 11 mai à 11h Séminaire du LPTMS Olga Dimitrova (LPTMS) Infrared catastrophe in two-quasiparticle collision integral Relaxation of a non-equilibrium state in a disordered metal with a spin-dependent electron energy distribution is considered. The collision integral due to the electron-electron interaction is computed within the approximation of a two-quasiparticle scattering. We show that the spin-flip scattering processes with a small energy transfer may lead to the divergence of the collision integral for a quasi one-dimensional wire. This divergence is present only for a spin-dependent electron energy distribution which corresponds to the total electron spin magnetization M = 0 and only for non-zero interaction in the triplet channel. We found the infrared cut-off to be proportional to the inverse dephasing time by taking into account the corrections to the collision integral from the many-particle processes.
 4 mai à 11h Séminaire du LPTMS Alexander Ossipov (The University of Nottingham) Multifractal eigenfunctions in random matrix ensembles Random matrix models is an efficient tool to study the Anderson metal-insulator transition. In this talk we focus on two such models: the power-law banded random matrices and the ultrametric random matrices. Using the weak disorder virial expansion we show how the multifractal critical exponents can be calculated analytically. Relations between different critical exponents, universality of the results as well as some open problems are discussed.
 28 avril à 15h Séminaire du LPTMS Yoseph Imry (Weizmann Institute of Science) Slow relaxation and aging (mainly) in electron glasses, a universal random distance matrix description The phenomena of slow relaxation and “aging” in glasses, with emphasis on electronic ones, is briefly reviewed. It is shown that a spectrum of relaxation rates, ?, behaving approximately as 1/ ? (for small ? ) can explain these effects, while producing a universal description in terms of a simple function. We find that this description holds also for other glasses for different physical properties. We obtained this spectrum before, based on approximations for the hopping model. Here we consider the related random “distance” matrices where the matrix elements depend exponentially on the distance between uniformly and randomly distributed points. This model arises naturally in various further physical contexts, such as the diffusion of particles, and scalar phonon localization. Using a combination of a renormalization group procedure and a direct moment calculation, we find the exact eigenvalue distribution, for low densities and the localization properties of the eigenmodes, for arbitrary dimension. The results agree perfectly with numerics. Finally, we discuss the physical implications of the results vis a vis the universal relaxation mentioned above.
 27 avril à 11h Séminaire du LPTMS Daniel Cabra (La Plata) Quantum disordered phase on the frustrated honeycomb lattice We study the phase diagram of the Heisenberg model on the honeycomb lattice with antiferromagnetic interactions up to third neighbors along the line $J_2=J_3$ that includes the point $J_2=J_3=J_1/2$, corresponding to the highly frustrated point where the classical ground state has macroscopic degeneracy. Using the Linear Spin-Wave, Schwinger boson technique followed by a mean field decoupling and exact diagonalization for small systems we find an intermediate phase with a spin gap and short range N\'eel correlations in the strong quantum limit (S=1/2). All techniques provide consistent results which allow us to predict the existence of a quantum disordered phase, which may have been observed in recent high-field ESR measurements in manganites.
 20 avril à 11h Séminaire du LPTMS Emmanuel Schenck (Ipht, Saclay) Décroissance de l'énergie des ondes amorties Nous considérons un cas modèle d'équation d'ondes dissipative dans une variété où la dynamique de rayons (ou flot géodésique) présente des propriétés chaotiques. L’énergie des ondes décroit alors vers zéro lorsque le temps va à l’infini : le problème de la stabilisation consiste a déterminer la vitesse de cette décroissance en fonction de la géométrie du problème. Sous une hypothèse dynamique reliée au flot géodésique, nous obtenons une décroissance exponentielle de l’énergie pour toutes données initiales suffisamment régulières. La particularité de ce résultat est sa validité dans certains cas où la condition de contrôle géométrique n’est pas satisfaite, c'est à dire lorsqu'il existe des trajectoires classiques non-amorties.
 14 avril à 14h30 Sémianire du LPTMS Alfredo Ozorio de Almeida (Rio de Janeiro) Blind spots between quantum states and the Loschmidt echo The overlap between a quantum state and its image under tiny translations oscilates swiftly. Complete orthogonality occurs generically at isolated points: quantum blind spots. The cases of superpositions of coherent states and of Bohr-quantized states will be analyzed. Decoherence in the Markovian approximation lifts the overlap minima from zero. Generalizations of blind spots for continuous two-parameter groups of unitary transformations reveal novel aspects of the quantum Loschmidt echo.
 13 avril à 11h Séminaire du LPTMS David Papoular (LPTMS) Microwave-Induced Feshbach Resonances Controlling the strength of the interatomic interactions is an important ingredient in many experiments involving ultracold atomic gases. This is usually done using Feshbach resonances induced by static magnetic fields. We propose an alternative to these static-field resonances, where the coupling is achieved using a resonant microwave magnetic field. Our results yield optimistic prospects for experiments with Sodium, Potassium, Rubidium, and Cesium. Elia Zarinelli (LPTMS) Surface tension in Kac glass models
 9 avril à 14h Séminaire du LPTMS : Physique biologique et systèmes complexes Jean Lehmann (Institut de Génétique et Microbiologie, Université Paris-Sud 11) Polymérisation des acides aminés et phénomène de codage Lors de ce séminaire, le phénomène de polymérisation des acides aminés sera discuté de façon générale. En se basant sur un modèle élémentaire de polymérisation dans lequel l’ARN joue le rôle de catalyseur, nous verrons que les propriétés de ces deux types de molécules peut en principe conduire cette polymérisation vers un régime de décodage. Dans ce type de régime, les unités d’interaction le long d’un brin d’ARN (les codons) catalysent l’ajout spécifique d’acides aminés selon leurs propriétés, ceci même si les médiateurs de la polymérisation (les tRNA) ne sont pas chargés spécifiquement avec ces acides aminés.
 30 mars à 11h Séminaire du LPTMS Cécile Monthus (IPhT Saclay) Anderson loalization and Many-body localization: statistics of renormalized hoppings For Anderson localization one-particle models, there exists an exact real-space renormalization procedure (Aoki 1980) that can be used numerically to obtain the statistical properties of the renormalized hopping between two sites separated by a distance L. We will describe the results for the Anderson tight-binding model in dimensions d=2 and d=3. In the localized phase, we obtain the same universality class as the strong disorder phase of the directed polymer in a random medium. At criticality in d=3, the statistics of renormalized hoppings becomes multifractal, in direct correspondence with the multifractality of individual eigenstates. Many-body localization problems can be studied similarly via an exact renormalization procedure in configuration space. For a one-dimensional lattice model of interacting fermions with disorder, we have studied numerically the statistical properties of the renormalized hopping between two configurations separated by a distance L in configuration space (distance being defined as the minimal number of elementary moves to go from one configuration to the other). We find a many-body localization transition at a finite disorder strength, with a localization length diverging as a power law, and with an essential singularity in the delocalized phase.
 23 mars à 16h Séminaire du LPTMS Markus Mueller (ICTP TRieste) Criticality, avalanches and Barkhausen noise in spin glasses The low temperature phase of spin glasses exhibits algebraically decaying spin-spin correlations at all T demonstrate in a direct measurement. The Barkhausen noise in magnetic hysteresis might provide a new diagnostic tool for this criticality. Indeed, at T=0, when the magnetization is traced as a function of external field H, field-induced spin flips can trigger an avalanche of further spin flips. Here, criticality is non-trivially encoded in the power law distribution of the size of such events. I will present an analytical theory for this phenomenon in the case of the SK spin glass, and discuss analogies with other glassy systems.
 23 mars à 11h Séminaire du LPTMS Vladimir Kravtsov (ICTP Trieste) Horizon in Random Matrix Theory, Hawking radiation and flow of cold atoms It is shown that level correlations in random matrix ensembels with log-square confinement are equivalent to (equal-time) density correlations in a Luttinger liquid in 1+1 curved space-time with the event horizon. Both of them show an anomalous cross-correlation peak which has also been predicted in the sonic analogy of a black hole in a one-dimensional flow of cold atoms. We discuss the origin of this phenomenon and trace it back to the exponential unfolding in the random matrix ensemble and the exponential red-shift in the black hole physics.
 19 mars à 11h Séminaire du LPTMS Anatoly Kamchatnov (Institute of Spectroscopy, Russian Academy of Science) Dispersive shock waves in Bose-Einstein condensate In typical situations, if dissipative and dispersive effects are neglected, propagation of large amplitude waves in nonlinear fluids leads to formation of singularities in distributions of density and/or flow velocity. In classical shock wave theory the dissipation effects regularize this singularity by means of formation of (viscous) shock – sharp jump of parameters of the wave. However, there are situations where dissipation effects are negligibly small compared with dispersion ones, and in these cases an oscillatory structure – dispersive shock wave - is formed instead of a usual shock. In particular, such dispersive shocks have been observed in experiments on nonlinear dynamics of Bose-Einstein condensates. In this talk short introduction is given to the theory of dispersive shocks based on the Whitham approach to nonlinear waves modulations. The developed method is applied to several typical problems related with generation of nonlinear waves by the flow of Bose-Einstein condensate past an obstacle.
 16 mars à 11h Séminaire du LPTMS Marcello Civelli ( ILL, Grenoble) Evolution de la fonction de pairing à travers le diagramme de phase des supraconducteurs à haute température Selon la plupart des expériences et théories sur les supraconducteurs haute-Tc basés sur le cuivre, les propriétés physiques de ces matériaux sont assez anomales (non BCS) à bas dopage et plus standard (BCS) à haut dopage. Notre étude montre qu’en ce qui concerne la formation des paires de Cooper (qui est le phénomène à la base de l’état supraconducteur) cette règle n’est pas respectée. Nous montrons de fait que la fonction de pairing est très anomale à haut dopage alors qu’elle a une forme plus standard à bas dopage. Ce changement de comportement en fonction du dopage correspond au dopage optimal (pour lequel la température critique est maximale) et est relié à l’apparition d’un « pseudogap » dans la densité spectrale des électrons. Nous montrons aussi que ces propriétés de la fonction de pairing pourraient être mesurées dans des expériences de spectroscopie à effet tunnel « scanning tunnelling spectroscopy » (STM).
 12 mars à 16h Séminaire du LPTMS Boris Altshuler (Columbia University) Anderson Localization: looking forward Localization of the eigenfunctions of quantum particles in a random potential was discovered by P.W. Anderson more than 50 years ago. In spite of its respectable age and rather intensive theoretical and experimental studies this field is by far not exhausted. Anderson localization was originally discovered and studied in connection with spin relaxation and charge transport in disordered conductors. Later this phenomenon was observed for light, microwaves, sound, and more recently for cold atoms. Moreover, it became clear that the domain of applicability of the concept of localization is much broader. For example, it provides an adequate framework for discussing the transition between integrable and chaotic behavior in quantum systems. This talk is an introduction into the current understanding of the Anderson localization and its manifestation in different systems. In particular, we will see that the ideas developed for understanding quantum mechanics of a single particle can be extended to attack many-body problems in the presence of disorder.
 9 mars à 11h Séminaire du LPTMS Christian Hagendorf (University of Virginia) The Gaussian free field and SLE(4) on doubly connected domains In this talk I consider the Gaussian free field whose level lines are known to be related to SLE(4). It is shown how this relation allows to define chordal SLE(4) processes on doubly connected domains, describing traces that are anchored on one of the two boundary components. The precise nature of the processes depends on the conformally invariant boundary conditions imposed on the second boundary component. Starting from the free field theory I will show how to generalise Schramm's formula for left-/right-passage with respect to a point to doubly connected domains for different boundary conditions, and point out a relation to first-exit problems for Brownian bridges. For the free field compactified at the self-dual radius, the extended su(2)_1 symmetry leads to a class of conformally invariant boundary conditions parametrised by elements of SU(2). I will explain how to extend SLE(4) to this setting. This allows for a derivation of new passage probabilities a la Schramm that interpolate continuously from Dirichlet to Neumann conditions.
 02 mars 11h Séminaire du LPTMS Henrik Johannesson (Université de Goteborg) Quantum Spin Hall Physics: Interactions at the Edge I will give an elementary introduction to the physics of a quantum spin Hall (QSH) system, and then report on recent work on interaction effects at its edge. Using bosonization methods I will show that a Rashba spin-orbit interaction may open a scattering channel that causes localization of the edge modes for a weakly screened electron-electron interaction. The localization transition is found to belong to the universality class of the quantum sine-Gordon model, with the dynamically generated mass gap scaling with an interaction-dependent exponent. As another example of interaction effects I will also briefly discuss the tunneling between two edges of a QSH device. The low-bias tunneling current and the differential conductance will be shown to exhibit scaling with voltage and temperature that again depend on an interaction-dependent exponent. Implications for edge state transport in HgTe quantum wells will be discussed.
 17 février 14h30 Séminaire du LPTMS Victor Fleurov (Tel Aviv University, Israel) Dynamics of fluctuations in an optical Laval nozzle Using the analogy between the description of coherent light propagation in a medium with Kerr nonlinearity by means of nonlinear Schrodinger equation and that of a dissipationless liquid, we propose an optical analogue of the Laval nozzle. The optical Laval nozzle will allow one to form a transonic flow in which one can observe and study very unusual dynamics of classical and quantum fluctuations, including an analogue of the Hawking radiation of real black holes. Theoretical analysis of this dynamics is supported by numerical calculations, and estimates for a possible experimental realization are presented.
 16 février 11h Séminaire du LPTMS Sylvain Prolhac (Technische Universität München) Fluctuations du courant dans le modèle d'exclusion asymétrique The asymmetric simple exclusion process (ASEP) features lattice hard-core particles driven in a preferred direction by an external field. This model has an out of equilibrium stationary state, with a macroscopic current flowing through the system. In the one-dimensional case, an exact study of the current fluctuations is possible using Bethe Ansatz. In particular, all the cumulants of the current have been calculated for the periodic model, which allows to understand precisely the crossover between the non-driven model, which belongs to the universality class of the Edwards-Wilkinson equation, and the driven model, which belongs to the universality class of the Kardar-Parisi-Zhang equation.
 11 février 14h Séminaire du LPTMS : Physique Biologique et Systèmes Complexes Vittoria Colizza (ISI Turin) Mobility networks and global epidemic spread A key issue dominating the recent discussions on containment plans against a possible pandemic influenza is the effectiveness of restriction measures on the international air travel. Recent findings based on detailed data-driven numerical simulations pointed out the limited benefit provided by feasible travel flows reductions in slowing down the global spread of an infectious disease not contained at the source. I will introduce a theoretical framework in terms of reaction-diffusion processes for the study of the propagation of epidemics in spatially structured systems and show how the complex structure of realistic mobility patterns impacts the invasion dynamics. The presented results provide a general framework for the understanding of the effect of travel restrictions in epidemic containment.
 9 février 11h Séminaire du LPTMS François Ghoulmié (Australian National University) Limit order book market complexity: statistical physics approaches In this talk, I explain the logics and structures of my research project and specify my discoveries during research works in the investment and academic industries. Part of my research consists in building solid theoretical market models that are able to explain the origins of observed statistical regularities in terms of economic behavior of market participants and their interactions: this is a challenging aggregation problem where I explain the behavior of macro- variables such as prices and trading volumes, starting from micro-variables, i.e. individual agents behavior. In the context of limit order book markets that characterizes the modern electronic functioning of financial markets, I show how in a mean field Ising style market model with novel adaptive modeling of agents behavior, the interaction between heterogeneous trading strategies and market impact lead to the major empirical price properties such as excessive fluctuations in assets returns and to characteristics specific to these markets regarding the profile of the book. By using a Boltzmann-Gibbs rule for the dynamic asset allocation in the model, I reconcile seemingly contradictory notions in finance, the informational efficiency characterized by the absence of autocorrelation in assets returns and the long memory in market orders buy and sell flows. I finally discuss future directions on these topics.
 2 février 11h Séminaire du LPTMS Andréa Sportiello (Université de Milan) Potts model, O(n) non-linear sigma-models and spanning forests In modern language, Kirchhoff Matrix-Tree Theorem (of 1847) puts in relation the partition function for spanning trees on a graph, to the one of a (scalar, complex, massless) fermionic free field. A trivial extension is to the 'massive' perturbation of the fermionic field, leading to 'rooted' spanning forests. We generalize these facts in many respects. We show that a fermionic theory with a 4-fermion interaction gives the generating function for 'unrooted' spanning forests, corresponding to the limit of the Potts Model for q -> 0. Remarkably, this theory coincides with the perturbative theory originated from a non-linear sigma-model with OSP(1|2) symmetry, which, in Parisi-Sourlas correspondence, is expected to coincide with the O(n) non-linear sigma-model, in analytic continuation to n -> -1. The relation between spanning forests and the fermionic theory can be proven directly with combinatorial methods. However, the underlying OSP(1|2) symmetry leads to the definition of a subalgebra of Grassmann Algebra: the scalars under global rotations. This leads to a set of combinatorial corollary properties that simplify and enlighten all the proofs.
 27 janvier 14h Séminaire du LPTMS Sébastien Dusuel ( Lycée Saint Louis, Paris) Quasi-particules anyoniques et transition de phase topologique: le code torique en champ magnétique Je commencerai par décrire qualitativement les propriétés élémentaires des systèmes quantiques bidimensionnels possédant des excitations anyoniques (particules de statistique fractionnaire,n'étant ni des bosons ni des fermions), en relation avec les domaines récents des qubits topologiquement protégés et du calcul topologique quantique. Ensuite, je donnerai une introduction pédagogique du modèle de spins 1/2 le plus simple qui présente de tels excitations exotiques et de l'ordre topologique, à savoir le code torique de Kitaev avec anyons émergents Z_2 [1]. Finalement, la robustesse de la phase topologique du code torique à la perturbation locale la plus simple (un champ magnétique) sera estimée, en donnant une image physique en termes de quasi-particules anyoniques [2,3]. En fonction de la direction du champ magnétique : - l'ordre topologique est détruit par une transition de phase quantique du premier ou du second ordre, d'où un diagramme de phase riche - le système peut posséder une pléthore d'états liés [1] Kitaev, Ann. Phys 303, 2 (2003) [2] Vidal, Dusuel & Schmidt, Phys. Rev. B 79, 033109 (2009) [3] Vidal, Thomale, Schmidt & Dusuel, Phys. Rev. B 80, 081104(R) (2009)
 25 janvier 14h30 Séminaire du LPTMS Yusuke Nishida (MIT) Universal few-body and many-body physics in mixed dimensions We propose to study a two-species Fermi gas (e.g. 40K-6Li mixture) in which one species is confined in 2D planes or 1D lines and interacts with the other species in the 3D space by a tunable short-range interaction. We show that such a Fermi gas in "mixed dimensions" has rich physics both in few-body and many-body physics. In few-body physics, the confinement acting only on one species can induce a new type of resonances both in 2-body and 3-body scatterings. In particular, the 3-body resonances are due to the emergence of the Efimov effect that does not exist in the 40K-6Li mixture in a free space. In many-body physics, the system has a very rich phase diagram in the plane of the effective scattering length and the layer separation. Resulting phases include an interlayer s-wave pairing, an intralayer p-wave pairing, a dimer Bose-Einstein condensation, and a Fermi gas of Efimov-like trimers. reference: [1] "Universal Fermi gases in mixed dimensions" Phys. Rev. Lett. 101, 170401 (2008) [http://arxiv.org/abs/0806.2668] [2] "Confinement-induced Efimov resonances in Fermi-Fermi mixtures" Phys. Rev. A 79, 060701(R) (2009) [http://arxiv.org/abs/0903.3633] [3] "Induced p-wave superfluidity in two dimensions" Annals Phys. 324, 897 (2009) [http://arxiv.org/abs/0810.1321] [4] "Phases of a bilayer Fermi gas" [http://arxiv.org/abs/0906.4584]
 20 janvier 14h Séminaire du LPTMS Thomas S. Jackson (Yale) Fractal Geometry of Minimal Spanning Trees The minimal-cost spanning tree (MST) problem is one of the oldest combinatorial optimization problems in computer science. Given an edge-weighted graph, it asks for a subset of edges containing no cycles (a tree) which leaves all vertices connect to each other while minimizing the total weight of the edges on the tree. We study scaling properties of average-case solutions to this problem when the edge weights are quenched random variables, using a correspondence between bond percolation and Kruskal's greedy algorithm which constructs the MST. We construct a mean field theory by studying the MST problem on the Bethe lattice with appropriate boundary conditions and find that its upper critical dimension is 6, contrary to previous claims. Below this dimension we are able to construct a diagrammatic expansion for corrections to mean-field theory and show that this expansion may be renormalized. We obtain an epsilon expansion for the fractal dimension of paths on (subsets of) the MST.
 19 janvier 11h Séminaire du LPTMS N.S. Jamal Rahi (MIT) The scattering theory approach to electrodynamic Casimir interactions We have derived methods for calculating the Casimir force to arbitrary accuracy, for any number of objects, arbitrary shapes, susceptibility functions, and separations. The technique is applicable to objects immersed in media other than vacuum, nonzero temperatures, and spatial arrangements in which one object is enclosed in another. The method combines each object's classical electromagnetic scattering amplitude with universal translation matrices, which convert between the bases used to calculate scattering for each object, but are otherwise independent of the details of the individual objects. We have explored various novel geometries using these techniques, including objects interacting with cavities and objects with sharp edges. The method is further used to examine whether fluctuation-induced forces can lead to stable levitation. Neglecting permeabilities, we find that any equilibrium position of items subject to such forces is also unstable if the permittivities of all objects are higher or lower than that of the enveloping medium; the former being the generic case for ordinary materials in vacuum. Thus, the Casimir force cannot producestable equilibria in vacuum for ordinary materials.
 12 janvier 11h Séminaire du LPTMS Alexandre Lefevre (Spth, Saclay) When does aging stop? The existence of a rough energy landscape is a key ingredient in the dramatic slowing down undergone by glass forming materials at low temperature (or density). The formal setting associated to this picture corresponds to some mean field models of spin glasses, which analytical resolution in the 90's made it possible to define unambiguously important concepts such as effective temperatures. However, the theory is not complete, and discrepancies remain with experimental measurements. The following questions remain to be answered to. Is the relaxation of one time quantities (energy, enthalpy, etc) really featureless ? Why are dynamical laws predicted by theory almost never observed ? How is aging interrupted in the vicinity of the glass transition ? Is there a age-temperature superposition principle ? On which length scales is the dynamics correlated in old samples ? In this talk, I will propose an answer to these questions, reconciling theory and experiments and showing the central role played by the relaxation of the energy. On the way, new and possibly analytical surprising results about dynamicas within mean field models will be shown.
 5 janvier à 11h Séminaire du LPTMS Eugène Sukhorukov (Université de Genève) Decay of a metastable state activated by non-Gaussian noise: A critical review of the generalized Kramers problem I will review the problem of the noise activated escape from a metastable state in the presence of non-Gaussian noise, and present connections between various theoretical approaches. I will also respond to criticism of our work by Tomas Novotny concerning the weak damping limit. Finally, I will discuss the application of the Kramers problem to mesoscopic threshold detectors of non-Gaussian noise, and will review recent experimental achievements