# Séminaires de l’année 2008

 16 décembre à 11h Séminaire exceptionnel en l'honneur de l'éméritat de Jacques Treiner Philippe Nozières (ILL) Le problème à N corps : un défi permanent En un demi siècle le problème à N corps a connu des succès incontestables. Mais chaque avancée remet en question les concepts les mieux établis : la physique des corrélations fortes garde sa part de mystères. J'illustrerai cet état de fait sur trois exemples simples, (i) la transition métal-isolant de Mott, en particulier dans les semiconducteurs, (ii) l'interpolation de la supraconductivité entre couplage faible et couplage fort, (iii) la coexistence de deux ruptures de symétrie différentes (un supersolide, par exemple) : coopération ou conflit.
 12 décembre à 11h Séminaire du LPTMS Dmitrii Fil (Institute for Sinhle Crystals, Kharkov, Ukraine) Bose-Einstein condensation and superfluidity in a network of 1D wires: an application to the superfluidity of helium crystals We begin with a short review on recent experimental results on supersolidity of 4He crystals. Then we discus the mechanism of supersolids connected with superfluidify of vacancies in dislocation networks. We present simple microscopic explanation of superfluidity of screw dislocations. We propose a model of a decorated lattice that mimics a regular dislocation network with moving vacancies and compute the temperature of Bose-Einstein condensation (BEC) for the vacancies. We find that this temperature decreases under increase of the length of the segments of the network and the law of decrease depends significantly on the transparency of the vertexes of the network. We conclude that even at large densities of dislocations the BEC temperature is rather small. We argue that a quasi-superfluid state can emerge at temperatures much large than the BEC temperature. There is no genuine superfluidity in this state, but the time of decay of the flow is very large. The decay of superflow is connected with the vortex motion across the flow. On the example of a 2D network we show that such a motion is caused by an emergence of phase slipping centers in the segments and the decay time increases exponentially under decrease of the temperature.
 9 décembre à 11h Séminaire du LPTMS Yan Fyodorov (Scholl of Mathematical Sciences, University of Nottingham) Multifractality, freezing and extreme value statistics in Random Energy models with logarithmic correlations
 2 décembre à 11h Séminaire du LPTMS Paul L. Krapivsky (Department of Physics, Boston University) Zero-Temperature Dynamics of Ising Ferromagnets I will discuss Ising ferromagnets endowed with a zero-temperature non-conservative spin-flip dynamics. A ground state is surely reached only in one dimension, while more unexpected behaviors occur in higher dimensions. In the second part of the talk I will describe how interfaces evolve at zero-temperature. An interface separating different phases approaches (after re-scaling) to a deterministic limiting shape; in the simplest situations, e.g. starting from a quadrant, a finger, or a square of one phase in the sea of the other phase, I show how to compute these limiting shapes.
 26 novembre à 14h30 Séminaire du LPTMS Igor Melkhov (Université d'Innsbruck) Quantum optics with quantum gases
 25 novembre à 11h Séminaire du LPTMS Alfredo Ozorio de Almeida (CBPF, Rio de Janeiro) Double phase space scenario for quantum dissipation and decoherence Just as the unitary evolution of quantum states in Hilbert space corresponds to classical phase space motion, quantum operators may be portrayed semiclassically as evolving in double phase space. The restrictions on the classical doubled phase space Hamiltonian, that ensure unitarity, can then be relaxed to describe the evolution of the density operator for an open quantum system. Within the context of a Markovian environment, one obtains explicitly the extra "dissipative term" of the double Hamiltonian for the Linblad master equation in the Weyl representation. Semiclassical approximations then result from immediate generalisations of standard WKB (Van Vleck) theory, or, alternatively, from generalized wave packet evolution.
 18 novembre à 11h Séminaire du LPTMS Grisha Volovik (Lab. of Low Temperature, Helsinki University of Technology and Landau Institute) Principles of emergent physics with applications to relativistic quantum fields and gravity We discuss two sujets both inspired by the condensed matter experience. (1) The Fermi-point scenario of emergence of relativistic quantum fields and gravity. The main ingredients of Standard Models of particle physics (left-handed and right-handed fermions and gauge fields) naturally emerge in the quantum vacua, which contain the Fermi points -- topologically protected zeroes in the energy spectrum. Appearance of fermionic and bosonic quantum fields in the quantum vacua of this universality class is accompanied by emergence of general physical laws such as relativistic invariance: gauge invariance; gravity; relativistic spin; etc. Vacua of another universality class contain the Fermi surface and they are similar to metals. (2) The modification of the Einstein's theory of gravity. This sujet is based on the treatment of the quantum vacuum as an extended self-sustained system, which is similar to condensed matter, but is Lorentz-invariant. Thermodynamics of self-sustained vacua demonstrates that the vacuum energy appears in two forms: (i) the microscopic energy characterized by the Planck energy scale; (ii) and the considerably smaller macroscopic energy which contributes to the Einstein's equations as the cosmological constant. For vacua in full thermodynamic equilibrium, the macroscopic vacuum energy is automatically nullified without fine tuning. Dynamics of the self-sustained vacua demonstrates how, in a spatially flat Friedmann-Robertson-Walker universe, the effective cosmological constant'' relaxes from its natural Planck-scale value at early Planckian times to the natural value in the present Universe, which is by 120 orders of magnitude smaller.
 7 novembre à 11h Séminaire du LPTMS Maria Colomé-Tatché (LPTMS) Parametric excitation of a 1D gas in integrable and non-integrable cases We study the response of a highly excited 1D gas with point-like interactions to a periodic modulation of the coupling constant. We calculate the corresponding dynamic structure factors and show that they differ dramatically for integrable and non-integrable models. The non-integrable system is sensitive to excitations with a frequencies as low as the mean level spacing, which is exponentially small, whereas the threshold frequency in the integrable case is much larger and scales polynomially with the size of the system. This effect can be used as a probe of integrability for mesoscopic 1D systems and can be observed experimentally by measuring the heating rate of a parametrically excited gas.
 4 novembre à 11h Séminaire du LPTMS Damien Simon (LPS-ENS) Généalogies et arbres aléatoires dans des modèles d'évolution et de polymères dirigés
 28 octobre à 11h Séminaire du LPTMS Raphael Chétrite (ENS-Lyon) Des relations de fluctuations aux théorèmes de fluctuation-dissipation Pendant longtemps, la majorité des résultats que nous possédions en physique statistique hors d'équilibre étaient valables au voisinage de l'équilibre. C'est le cas par exemple du théorème de fluctuation-dissipation démontré sous sa forme moderne par Callen et Welton en 1951, des relations de réciprocité que Onsager démontra en 1930 et des relations de Green-Kubo datant de 1957. En 1993, Evans, Cohen et Morriss ont découvert lors d'une simulation numérique une symétrie dans la distribution de la création d'entropie d'une particule thermostatée soumis à une force extérieur. Cette découverte entraîna une grande quantité de travaux et de résultats : depuis les relation de Gallavotti et Cohen, de Jarzynski, de Crooks, jusqu'à l'application pour la manipulation de petites particules par les groupes de Peliti et Seifert entre autres. Chacune de ces relations concernant une quantité donnée : la contraction dans l'espace des phases, la production d'entropie, le travail reçue ou la chaleur reçue, qui souvent diffère d'un travail à l'autre. La disparités des formalismes et des quantités définies est la source des polémiques et conflits qui agitent régulièrement ce domaine. Lors de mon séminaire, mon premier but sera de montrer que tous ces résultats sont cohérents et peuvent être unifié dans le cadre des processus diffusifs et que la multiplicité des définitions possibles provient de la non unicité du renversement temporel d'un système. Je montrerais les liens entre les relations de fluctuations et les théorèmes de fluctuation-dissipation. Je discuterais aussi les applications à des modèles simples de turbulence ou de localisation d'Anderson.
 21 octobre à 11h Séminaire du LPTMS Deepak Dhar (Department of Theoretical Physics, Tata Institute, Mumbai, India) Exact entropy of dimer coverings for a class of lattices in three or more dimensions We construct a class of lattices in three and higher dimensions for which the number of dimer coverings can be determined exactly using elementary arguments. These lattices are a generalization of the two-dimensional kagome lattice, and the method also works for graphs without translational symmetry. The partition function for dimer coverings on these lattices can be determined also for a class of assignments of different activities to different edges.
 15 octobre à 14h30 Séminaire du LPTMS Mikhail Zvonarev (DPMC-MaNEP, Université de Genève) Spin dynamics in a one-dimensional Bose-Hubbard model We investigate the dynamics of transverse spin excitations in the slightly doped one-dimensional ferromagnetic Bose-Hubbard insulator. We combine effective field theory (Luttinger Liquid) approach with the exact (Bethe ansatz-like) resummation of formfactors. We calculate the threshold singularities of the spectral function and demonstrate its pronounced reconstruction with increasing doping. We explain why the observed unusual features cannot be seen in the fermionic models.
 14 octobre à 11h Séminaire du LPTMS Prof. G.M. Schütz (Institut für Festköperforschung Forschungszentrum, Jülich, Allemagne) Condensation in zero-range processes with generic perturbations The zero-range process is a stochastic interacting particle system that is known to exhibit a condensation transition. We present a detailed analysis of this transition in the presence of quenched disorder in the particle interactions. Using rigorous probabilistic arguments we show that disorder changes the critical exponent in the interaction strength below which a condensation transition may occur. The local critical densities may exhibit large fluctuations and their distribution shows an interesting crossover from exponential to algebraic behaviour.
 8 octobre à 14h30 Séminaire du LPTMS Karen Kheruntsyan (University of Queensland, Brisbane) Fermionic quantum atom optics Advances in the experimental control of ultracold quantum gases have now reached the stage where atomic correlations and quantum statistics can be accessed via the measurement of atomic shot-noise in absorption images and via direct position and time resolved atom detection [1-3]. Such measurements allow one to explore parallels with quantum optics, including applications similar to those that utilize the well-known source of entangled photon pairs -- the optical parametric down conversion. In this talk I will discuss the atom optics counterpart of down-conversion, which can be achieved through dissociation of a Bose-Einstein condensate of molecular dimers. In the case of fermionic atomic constituents, the molecular dissociation represents a new paradigm -- fermionic quantum optics. Here the analogies with the photonic down-conversion are not straightforward due to the different quantum statistics and the fundamentally different shot noise for fermions. I will present a theoretical model describing the quantum dynamics of dissociation and provide a qualitative account of recent atom correlation measurements at JILA [1]. In an idealized uniform case, the model gives rigorous upper bounds for fermionic pair correlation functions. We also analyse realistic non-uniform systems corresponding to dissociation of harmonically trapped molecular condensates. Our treatment addresses the role of the spatial inhomogeneity on the strength of atom-atom correlations in the short time limit. We obtain explicit analytic results for the density-density correlation functions in momentum space, and show that the correlation widths and the degree of relative number squeezing are determined merely by the shape of the source molecular condensate. Finally we study the dissociation of elongated molecular condensates and find evidence of directionality in Pauli blocking, which is the opposite effect to directionality due to bosonic stimulation in superradiance. [1] Greiner at al., "Probing pair-correlated fermionic atoms through correlations in atom shot noise", Phys. Rev. Lett. 94, 110401 (2005). [2] S. Fölling et al., "Spatial quantum noise interferometry in expanding ultracold atom clouds", Nature 434, 481-484 (2005). [3] M. Schellekens et al., "Hanbury Brown Twiss Effect for Ultracold Quantum Gases", Science 310, 648 (2005).
 17 septembre à 14h30 Séminaire du LPTMS Dmitry Kovrizhin (Oxford University) Multiparticle interference in electronic Mach-Zehner interferometers We present our theoretical investigation of the recently observed phenomena of the voltage dependent current oscillations of the Aharonov-Bohm interference in the electronic Mach-Zehnder interferometer constructed from the two chiral quantum Hall edge states by a group from Weizmann institute [1]. This unexpected behavior was believed to arise because of the interactions, but no clear explanation of this phenomenon was presented so far. We argue that the destructive interference of the paths with different number of particles in the interferometer arms is responsible for this behavior. Using perturbation theory in interactions and the exact solution of the problem we propose our theoretical explanation of this phenomenon. [1] I. Neder, M. Heiblum, Y. Levinson, D. Mahalu and V. Umansky, "Unexpected Behavior in a Two-Path Electron Interferometer", Phys. Rev. Lett. 96 016804 (2006)
 9 septembre à 11h Séminaire du LPTMS Lev Shchur (Landau Institute, Russia) Numerical estimation of the universal critical amplitude ratios in 2d Potts model Monte Carlo and series expansion data for the energy, specific heat, magnetization and susceptibility of the two-dimensional Potts model in the vicinity of the critical point are analysed. We estimate these amplitudes using the correction-to-scaling exponents predicted by conformal field theory. We also form effective ratios of the observables close to the critical point and analyze how they approach the universal critical-amplitude ratios. In particular, using the duality relation, we show analytically that for the Potts model with a number of states q<=4, the effective ratio of the energy critical amplitudes always approaches unity linearly with respect to the reduced temperature. This fact leads to the prediction of relations among the amplitudes of correction-toscaling terms of the specific heat in the low- and high-temperature phases. We present numerical and analytical support for the form of the first two correction-to-scaling terms. The role of logarithmic corrections for q=4 is discussed and an approach is proposed in order to account numerically for these corrections in the determination of critical amplitudes.
 16 juillet à 14h30 Séminaire du LPTMS Boris Altshuler (Columbia University and NEC Laboratories America) Optics of charge flows in gapless semiconductors : lensing, percolation and minimal conductivity
 9 juillet à 14h30 Séminaire du LPTMS Robert M. Ziff (University of Michigan) Measurements of harmonic measure for percolation hulls and diffusion-limited aggregation (DLA) An enhanced rare-event simulation technique to determine the harmonic measures of surfaces is introduced. The technique allows one for the first time to probe into the fjords of percolation hulls and see beyond the accessible boundary, which was previously studied theoretically by Duplantier and numerically by Meakin et al. We confirm Duplantier's prediction for the multifractal spectrum D(q) for the accessible (Grossman-Aharony) hull, including the prediction that the probability drops off with a power-law with exponent -24/25. For the complete hull, we find that that exponent is 1 over 200 orders of magnitude, and find the f(alpha) curve for the first time. New results for DLA highlight the importance of the accessible sites in this case also.
 30 juin à 11h Séminaire du LPTMS Pierre Meystre (University of Arizona) Cooperative light scattering in ultracold atomic samples
 17 juin à 11h Séminaire du LPTMS Alexios Polychronakos (Dep. of Physics, City College of New York) Noncommutative Wess-Zumino models and bosonization Noncommutative Wess-Zumino-Witten models arise as generalizations of the standard WZW models on noncommutative spaces or, interestingly, as exact bosonic descriptions of many-body fermion systems. A remarkable property of these models in 1+1 dimension is the existence of a "Seiberg-Witten"-type transformation that changes the noncommutativity parameter and yet leaves the action invariant. We extend this transformation to non-critical actions and demonstrate that it leads to standard bosonization in 1+1 dimensions and corresponding higher dimensional generalizations.
 10 juin à 11h Séminaire du LPTMS S.J. Rahi (Department of Physics, MIT) Nonmonotonic effects of parallel sidewalls on Casimir forces between cylinders We analyze the Casimir force between two parallel infinite metal cylinders with nearby metal plates using two methods. Surprisingly, the attractive force between cylinders depends nonmonotonically on the separation from the plate(s), and the cylinder-plate force depends nonmonotonically on the separation of the cylinders. These multibody phenomenona do not follow from simple two-body force descriptions. We can explain the nonmonotonicity with the screening (enhancement) of the interactions by the fluctuating charges (currents) on the two cylinders and their images on the nearby plate(s).
 4 juin à 14h30 Séminaire du LPTMS Leonid Levitov (MIT and KITP UCSB) Chiral electron dynamics in graphene nanostructures
 3 juin à 11h Séminaire du LPTMS Rodolfo Jalabert (IPCMS, Université de Strasbourg) Non-local interaction effects in the quantum conductance of one-dimensional wires We calculate the zero-temperature linear transport in one-dimensional systems using the embedding method, which is based on the relation between the system conductance and the persistent currents in large rings that contains the studied system. The persistent currents are calculated using the density matrix renormalization group algorithm that allows to obtain exact results which fully include interaction-induced electronic correlations.The crucial importance of the interactions is demonstrated by considering one-dimensional transport through an interacting region in series with a point-like one-body scatterer. When the conductance of the interacting region is perfect, independently of the interaction strength, a non-local interaction effect yields a total conductance of the composed system that depends on the interaction strength and is lower than the transmission of the one-body scatterer. This qualitative non-local effect allows to probe the dressing cloud of an interacting system by ideal noninteracting leads. The conductance correction increases with the strength of the interaction and the reflection of the one-body scatterer (attaining relative changes > 50%), and decreases with the distance between the interacting region and the one-body scatterer. Scaling laws are obtained and possible experimental realizations are suggested.
 27 mai à 11h Séminaire du LPTMS Alexei Tsvelik (Condensed Matter Theory Group, Brokhaven National Lab) Low-dimensional superconductivity-BEC; effects of disorder I discuss condensation of pairs in 1D and 2D models in the presence of chemical potential disorder. It is shown that in 2D the disorder reduces superconducting stiffness and hence reduces the Beresinskii-Kosterlitz-Thouless transition temperature. In 1D the disorder leads to localization of pairs.
 21 mai à 14h30 Séminaire du LPTMS Michael Hartung (Regensburg) Coherent backscattering of Bose-Einstein condensates in two-dimensional disorder potentials We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. We reproduce this result, obtained from the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic theory of weak localization in presence of a nonlinearity.
 20 mai à 11h Séminaire exceptionnel à l'occasion du 60ème anniversaire de Eugène Bogomolny Alain Connes (Collège de France et IHES) Thermodynamique des espaces noncommutatifs et fonction zêta Je décrirai comment la structure de l'hypothétique 'corps à un élément' conduit au système de thermodynamique quantique qui donne la réalisation spectrale des zéros de la fonction zêta ainsi que l'interprétation des formules explicites de Riemann-Weil comme une formule de traces.
 16 mai à 14h30 Séminaire du LPTMS Alex Kamenev (University of Minnesota) Dynamics of 1d quantum liquids The talk focuses on momentum and energy resolved correlation functions of 1d bosonic and fermionic liquids. The emphasis is on going beyond the quantum hydrodynamics (i.e. Luttinger liquid) approximation, which is restricted to a linearized dispersion relation. The non-linearity of the dispersion relation leads to, previously unacknowledged, power-law singularities in correlation functions with momentum-dependent exponents. I shall discuss possible manifestations of these singularities, such as e.g. excitation of dark solitons in 1d Bose condensates.
 14 mai à 14h30 Séminaire du LPTMS Dmitry V. Efremov (Institute for Theoretical Physics, Dresden, Germany) Non-analytic behavior of 2D Fermi-liquid and weakly ferromagnetic itinerant Fermi systems I will discuss recent works on the properties of 2D itinerant fermions near ferromagnetic quantum critical point. I will show that the free energy and the transverse and longitudinal static susceptibilities contain non-analyticities which destroy a continuous second-order transition. Depending on the parameters, the transition either becomes first-order, or occurs via an intermediate spiral phase.
 29 avril à 11h Séminaire du LPTMS Meera Paris (Princeton) Polarized atomic Fermi condensates Ultracold atomic fermions provide an exceptionally clean realization of the spin-imbalanced, two-component Fermi gas, since the interatomic interactions and atom spin are both independently tuneable. Indeed, there is hope that the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superfluid phase could be observed in such a system. I have investigated the finite temperature phase diagram of a polarized Fermi condensate and I find that phase separation dominates over the FFLO phase. However, I will show that the region of stability of the FFLO phase can be substantially enlarged when the Fermi gas is confined in a two-dimensional optical lattice.
 22 avril à 11h Séminaire du LPTMS Arnaud Ralko (Université Paul Sabatier, Toulouse) Spin liquids, exotic phases and hole doping in dimer systems Understanding electron pairing in high temperature superconductors is a major challenge in strongly correlated systems. In his milestone paper, Anderson proposed a simple connection between high temperature superconductors and Mott insulators. Electron pairs "hidden" in the strongly correlated insulating parent state as Valence Bond (VB) singlets lead, once fried to move at finite doping, to a superconducting behavior. A very good candidate of the insulating parent state is the resonating valence bond state (RVB), a state with only exponentially decaying correlations and no lattice symmetry breaking. A simple realization of RVB state has been proposed by Rokhsar and Kivelson (RK) in the framework of an effective quantum dimer model (QDM) with only local processes and orthogonal dimer coverings. In this talk, superfluidity, phase separation and flux quantization will be discussed in the case of the doped QDM on the square and the triangular lattices, showing a qualitative agreement with high-Tc superconductivity gauge theories. Since the nature of the parent insulating states is crucial for understanding the doped regime, the zero-doping phase diagrams will be first detailed. Especially, I will focus on the existence of a possible new second order phase transition scenario on the square lattice (generic in RK models) implying an anisotropic ordering, and on the RVB liquid crystallization via the condensation of ising-vortex-like topological excitations called visons.
 16 avril à 14h30 Séminaire du LPTMS Maxim Olshanii (University of Massachusetts, Boston, USA) Thermalization and its mechanism for generic isolated quantum systems Time dynamics of isolated many-body quantum systems has long been an elusive subject, perhaps most urgently needed in the foundations of quantum statistical mechanics. In generic systems, one expects the nonequilibrium dynamics to lead to thermalization: a relaxation to states where the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable through the time-tested recipe of statistical mechanics. The relaxation mechanism is not obvious, however; dynamical chaos cannot play the key role as it does in classical systems since quantum evolution is linear. Here we demonstrate [1], using the results of an ab initio numerical experiment with 5 hard-core bosons moving in a 5 × 5 lattice, that in quantum systems thermalization happens not in course of time evolution but instead at the level of individual eigenstates, as first proposed by Deutsch [2] and Srednicki [3]. [1] M. Rigol, V. Dunjko, and M. Olshanii, Nature, Apr 17 (2008) [2] J. M. Deutsch, Phys.Rev. A 43, 2046 (1991) [3] M. Srednicki, Phys. Rev. E 50, 888 (1994)
 15 avril à 11h Séminaire du LPTMS Anatole Kenfack (Max Planck Institute, Dresden) Quantum resonances, current reversals and chaos in ratchets systems The Ratchet effect, that is the possibility of directed transport without bias (net force) in systems with broken symmetries is discussed. I will first introduce the ratchet model, tracing it back to the famous gedanken experiment by Smulochowsky in 1912 to the its thermodynamic's validation by Feymann in 1963 - the epoch from which the ratchet effect started to really attract the scientific community. For illustration a variety of existing systems as well as few commonly studied ratchet models will be shown. In the first part of my talk, I will specifically present a recipes to conveniently measuring the current in a classically chaotic ratchet system. This is a contribution essentially clarifying a deeply debated misconception on the connection between current reversal and bifurcation[1-3]. In the second part of my talk, I will consider a model of a cold atom trapped in a periodic flashing on/off optical ratchet potential. This dissipationless asymmetry optical potential is currently experimentally achievable thanks to rapid recent advances in the design of optical ratchet potentials [4]. In this hamiltonian model, I will demonstrate that atoms can efficiently be transported. In particular by controlling the potential strength, high-order quantum resonances may naturally emerge and can induce not only current reversals, but also larger current than low-order resonances. Finally, I will also show that these intriguing quantum phenomena can only be observed when the classical counterpart of the system is fully chaotic [5]. [1] J. L. Mateos, Phys.Rev.Lett.84, 258 (2000). [2] M. Barbi and M. Salerno, Phys.Rev.E 62, 1988 (2000). [3] A. kenfack, S. M. Sweetnam and A. K. Pattanayak, Phys.Rev.E75, 056215 (2007) [4] G. Ritt, C. Geckeler, T. Salger, G. Cennini and M. Weitz, Phys. Rev.A74,063622 (2006). [5] A. Kenfack, J.Gong and A.K. Pattanayak, Phys.Rev.Lett.100, 044104 (2008).
 8 avril à 11h Séminaire du LPTMS Sébastien Burdin (MPI-PKS Dresden) Impuretés quantiques en Physique : des composants nanoélectroniques aux alliages d'Uranium L'interaction entre une impureté magnétique quantique et un bain électronique peut conduire à basse température à la formation de singulets Kondo. L'effet des corrélations entre impuretés sera décrit dans le contexte expérimental des alliages constitués d'actinides ou de lanthanides: température de cohérence, point critique quantique, comportement de non liquide de Fermi, élargissement de la surface de Fermi. Les spécificités d'un bain électronique mésoscopique, éventuellement chaotique, seront ensuite présentées, afin de décrire des systèmes tels que des nano-fils ou des impuretés magnétiques artificielles. Enfin, les effets de désordre et de frustration seront présentés, pouvant conduire à la formation d'un état de liquide de spin.
 1er avril à 11h Séminaire du LPTMS Fabio Toninelli (ENS Lyon) Disordered pinning/wetting models and Harris criterion Pinning/wetting models, with or without quenched disorder, are known to undergo a localization/delocalization transition. Let $\nu$ be the critical exponent which governs the vanishing of the free energy of the non-disordered model close to the critical point. The Harris criterion predicts that disorder is relevant if $\nu2$, while he case $\nu=2$ is controversial. These predictions can be rigorously proven for $\nu\neq 2$, and I will give a review of recent mathematical results. In the marginal case, $\nu=2$, I will show that disorder can be either marginally relevant or marginally irrelevant: this depends on finer details of the critical behavior of the non-disordered model (i.e., not just on $\nu$ but also on logarithmic corrections). A few challenging open problems remain (expecially for $\nu=2$)." This talk is based on work done in collaboration with B. Derrida, G. Giacomin and H. Lacoin.
 5 mars à 11h Séminaire du LPTMS Guillaume Roux (Aachen) Commensurability effects in 1D: examples from ladders and cold atoms with a quasi-periodic superlattice For particular fillings, interactions (an external potential) can lead to phases which break spontaneously (resp. explicitly) the translational symmetry. As a first example, the t-J model for two-leg ladders exhibits commensurate phases at fillings n=1/2 and 3/4. We show that the two phases have a different nature by means of density-matrix renormalization group calculations and describe the diamagnetic response of the system at low magnetic flux. A second example is a gas of cold atoms loaded in a 1D optical lattice to which an incommensurate lattice potential is superimposed. The competition between interactions and the quasi-periodic potential gives rise to a rich phase diagram. The physical properties of the system can be understood as intermediate between a true random potential and a standard commensurate one.
 19 mars à 14h30 Séminaire du LPTMS Piotr Deuar (LPTMS) Excitations in superfluid gases of fermionic dipoles The single-particle and collective excitations of a gas of fermionic dipoles have been determined for the case of a uniform, single-species, fully polarised gas below superfluid (BCS) critical temperature. Such Fermi gases are in many ways analogous to the never experimentally realised polar phase of Helium-3, and possibly some heavy-fermion superconductors in also having a vanishing energy gap on an entire line of the Fermi surface. Present-day advances in cooling heteronuclear gases make it appear likely that temperatures below Tc for such a dipole Fermi gas will be reached within a couple of years. The system exhibits rich behaviour in comparison with a standard two-species s-wave interacting BCS gas due to the long-range and anisotropic nature of the excitations. Notably only one species is needed for BCS pairing despite the fermionic nature, and collective modes remain damped even at T=0. The damping at T=0 and at T=Tc is strongly anisotropic, while the growth of corrections is polynomial with small T, which is much stronger than for the s-wave gas. Current response to external perturbations of the gap is also anisotropic and, interestingly, occurs at an angle to the applied probe.
 12 mars à 14h30 Séminaire exceptionnel du LPTMS Dmitri Khveshchenko (UNC Chapel Hill) Dirac fermions in a power-law correlated random vector potential We study localization properties of two-dimensional Dirac fermions subject to a power-law correlated random vector potential describing, e.g., the effect of "ripples" in graphene. By using a variety of techniques (low-order perturbation theory, self-consistent Born approximation, replicas, and supersymmetry) we compute the low-energy density of states and discuss a possible (pre)localization of the entire single particle spectrum.
 11 mars à 11h Séminaire du LPTMS Jean-Pierre Hansen (Department of Chemistry, University of Cambridge & LI2C, Université Pierre et Marie Curie) Polymères enn solution : des liquides de "blobs" ? Nous proposons une strategie systematique de reduction du nombre de degres de liberte de polymeres en interaction dans le regime des solutions concentrees (dites "semi-diluees"). La methode, basee sur la determination d'interactions effectives entre "blobs" contenant un grand nombre de monomeres, permet de simuler des systemes tres grands de polymeres tres longs, et de predire des comportements asymptotiques. La methode multi-echelles est appliquee a l'auto-assemblage de copolymeres en micro-phases micellaires.
 26 février à 11h Séminaire d'intérêt général du LPTMS Sébastien Balibar (LPS-ENS) Supersolidité et désordre Il est clairement paradoxal qu’un solide puisse être superfluide (on dit alors supersolide). Bien que différents théoriciens (Reatto, Chester, Andreev et Lifshitz, Leggett) aient discuté l’existence possible de supersolides dès 1969-70, ce n’est qu’en 2004 que Kim et Chan (Penn State University) ont découvert que l’hélium 4 solide présentait des propriétés de rotation non-classiques semblables à celles d’un superfluide. Ces expériences posent des questions fondamentales et ont déclenché une intense activité tant théorique qu’expérimentale sur la superfluidité éventuelle des solides quantiques ; je tenterai de la résumer rapidement malgré la difficulté de faire une synthèse de travaux souvent contradictoires entre eux. Il est apparu en 2006 que le désordre jouait un rôle essentiel dans les phénomènes observés. En particulier, Rittner and Reppy (Cornell) ont montré qu’un recuit des échantillons d’hélium faisait pratiquement disparaître les effets de rotation anormale. A l’ENS (Paris), nous avons montré que les joints entre grains des échantillons polycristallins permettaient un écoulement superfluide de matière à travers le solide[1]. Les propriétés de mouillage des joints de grains par le liquide sont des phénomènes importants en science des matériaux et nous avons entamé leur étude dans l’hélium 4 solide où ces joints constituent des systèmes quantiques bi-dimensionnels originaux[2]. Toutefois, nous avons aussi montré que, pour expliquer l’ensemble des phénomènes observés, il fallait considérer d’autres types de désordre (dislocations, régions vitreuses ou liquides éventuelles). Je tenterai donc de faire le point sur l’état actuel de la supersolidité, un phénomène dont on ne comprend toujours pas la nature, objet de controverses animées. [1] S. Sasaki, R. Ishiguro, F. Caupin, H.J. Maris, and S. Balibar, Science 313, 1098 (2006). [2] S.Sasaki, F. Caupin, and S. Balibar, Phys. Rev. Lett. 99, 205302 (2007)
 22 février à 14h30 Séminaire du LPTMS George I. Japaridze (Andronikashvili Institute of Physics, Tbilisi, Georgie) Metal-Insulator transitions in 1D half-filled electron system with next-nearest-neighbour hopping We study the quantum phase transition from an insulator to a metal in the ground state of the half-filled t-t' repulsive Hubbard model, Using the continuum-limit bosonization approach and density matrix renormalization group calculations. An effective low-energy Hamiltonian that describes the insulator-metal transition is derived. We find that the gross features of the phase diagram are well-described by the standard theory of commensurate-incommensurate transitions in a wide range of parameters. We also obtain an analytical expression for the insulator-metal transition line line t'_{c}(U,t). In the presence of a staggered ionic potential ? we find, that for particular values of the next-nearest-hopping amplitude t', with increasing on-site repulsion, at U_{c1} the model shows a transition from a band insulator into a metallic state and at larger U_{c2} there is another transition from a metal into a ferroelectric insulator.
 20 février à 11h30 Séminaire du LPTMS Iacopo Carusotto (Physics department, Trento University and INFM BEC Center) The non-equilibrium Bose-Einstein condensation phase transition in microcavity polariton systems Several recent experiments have provided evidence for the appearance of macroscopic coherence in polaritonic systems with a spontaneous symmetry breaking mechanism which is the non-equilibrium analog of the Bose-Einstein condensation phase transition. In this seminar we shall review our theoretical results on several aspects of this non-equilibrium phase transition, and we shall try to evidentiate analogies and differences from the usual BEC in equilibrium systems.
 19 février à 11h Séminaire du LPTMS John Clark (Washington University, St Louis et Université de Lisbonne) Statistical prediction of nuclear properties by support vector machine There exists a growing collection of excellent data on nuclear properties across an expanding chart of the nuclides. Radioactive ion-beam facilities are creating a wealth of nuclei far from stability, presenting new challenges to nuclear many-body theorists. In this climate it is of great interest to learn the extent to which the existing data, and only the data, determines the properties of yet undiscovered nuclides of given proton number $Z$ and neutron number $N$. For example, to what extent does the data, by itself, determine the physical mapping from $(Z,N)$ to the atomic mass $M$? Explorations of the predictive performance of global models of nuclear properties based on Support Vector Machines (SVMs) are providing a partial answer to such questions. SVMs have emerged as one the most powerful tools of statistical inference in problems of classification and function approximation. Results of their application to nuclear databases on atomic mass, beta-decay lifetimes, and ground-state spins and parities establish that "theory-thin" SVM models can provide a valuable complement to conventional "theory-thick" global macroscopic/microscopic parametrizations based on finite-range droplet models and density functional approaches. Use of SVMs to the model the discrepancies between the theory-thick predictions and experiment may yield insights into fundamental limitations on traditional global modeling. Recent results from state-of-the-art application of multilayer perceptrons to prediction of beta halflives will also be reported.
 14 février à 11h Séminaire du LPTMS Sandro Wimberger (Institute for Theoretical Physics, University of Heidelberg) Control of complex quantum dynamics with ultracold atom Modern atom-optics experiments allow one an unprecedented control of atomic degrees of freedom and, as a consequence, the clean realization of toy models used to explain transport phenomena in condensed matter physics. We present recent results on extended Bose-Hubbard systems. For reasonable lattice sizes, this model gives access to the full quantum spectrum, which allows us a complete characterization of "horizontal" (spatially) and "vertical" (energetic) quantum transport. Various dynamical regimes can be prepared, using atom-atom interactions, disorder, and external forces as control parameters. While for a disordered model we predict a crossover between quantum chaotic and localized dynamics, we show that the inter-band transport of atoms confined to a periodic lattice and subject to an additional tilting force is strongly dependent on the dynamics of the ground-state band. Analogies between Stark localization and Anderson localization are discussed, as well as predictions for experiments to observe signatures of complex dynamics with ultracold atoms.
 13 février à 14h30 Séminaire du LPTMS Xavier Barillier-Pertuisel (IPN Orsay) Boson-Fermion pairing on 1D optical lattices One of the most recent and exciting aspects in the field of cold atoms is the study of Bose-Fermi mixtures. Several boson-fermion mixtures have been realized and their properties have been theoretically studied using for instance Mean Field approximation or Quantum Monte Carlo (QMC) methods. The latter gives us exact results for one dimensional system. In our work we first considered BF pairing in a discrete environment of bosons and fully spin-polarized fermions. The system is modeled by a 1D Bose-Fermi Hubbard Hamiltonian with attractive BF interaction. One of the interests of such a system is to check the validity and limits of T-matrix approach, previously employed in the 3D case, by comparing with QMC results. In this talk, I will present Green's function formalism and the T-matrix approximation applied to a BF mixture for a discrete number of sites. I will thus show some results obtained for the ground state energy, the excitation energies and occupation numbers. I will also discuss the continuous case underlining the appearance of a stable weak coupling BF pairing mode. This Cooper-pair-like mode exists at any small value of the interaction due to the presence of a Fermi surface.
 12 février à 11h Séminaire du LPTMS Cristina Bena (SPhT, CEA Saclay) Challenges in low-dimensional systems I will review some of the important problems arising in the study of low-dimensional systems. In the first (and main) part of the talk I will focus on one-dimensional systems. In spite of intense theoretical and experimental efforts over the past years, many features of these systems have not yet been clarified. Their physics is dominated by interactions, which are expected to give rise to spectacular phenomena, such as charge fractionalization, fractional statistics, spin-charge separation, and non-Abelian statistics. I will discuss the non-equilibrium perturbative Keldysh formalism, that we use to study these phenomena, by computing and analyzing finite-frequency non-symmetrized shot noise. I will focus both on Fractional Quantum Hall Effect edge states, and on carbon nanotubes connected to metallic leads, and also compare our results to those obtained using integrability-based approaches. Finally, I will describe some upcoming measurements, which in conjunction with our results can give a definite confirmation of the existence of charge fractionalization in one dimensional systems. In the second part for the talk I will briefly discuss another low-dimensional system - graphene - and how its physics can help us understand the exotic physics of other two-dimensional systems such as high temperature superconductors.
 6 février à 14h30 Séminaire du LPTMS Raoul Santachiara (LPT ENS) Correlations of one-dimensional impenetrable anyons In this talk I will present some new results on 1D anyonic systems. The interest in these systems is generally motivated by the study of the edge transport in Fractional Quantum Hall liquids. The anyonic systems are characterized by the fact that the N-body wave function take a general phase under the permutation of two particles. The study of these systems offer then the possibility to study the correlation properties of indistinguishable particles in systems which interpolate between bosons and fermions. In particular I will discuss in some detail the properties of the one-particle reduced density matrix of a 1D gas of impenetrable anyons.
 23 janvier à 14h30 Séminaire du LPTMS Yuriy Bunkov (Institut Néel, Grenoble) Spin superfluidity and Bose-Einstein condensation of magnons Superfluid 3He can be considered a quantum vacuum carrying various types of quasiparticles and topological defects. The structure of this system shows many similarities to that of our Universe. It can act as a model system for the study of many types of general physics experiments, which are difficult or even impossible in Cosmology, Atomic or Nuclear physics. There is a complete analogy between the Bose-Einstein condensation of atomic gases and the Bose-Einstein condensation of magnons in superfluid 3He. Four different states of magnon condensation have been found; the homogeneously precessing domain (HPD) in 3He-B; the persistent signal, which is formed by a Q-ball in 3He-B at very low temperatures; coherent precession with fractional magnetization in 3He-B and coherent precession of magnetization in 3He-A and 3He-B in squeezed aerogel. All these cases are examples of the Bose-Einstein condensation of magnons with the interaction potential provided by specific spin-orbit coupling. The BEC phenomenon in the gas of magnons is readily accessible owing to the possibility of modifying the spin-orbit coupling. In some cases the BEC of magnons corresponds to almost 100% condensation. The BEC state of magnons is responsible for early observed spin superfluid phenomena; spin supercurrent which transports the magnetization on a macroscopic distance, spin current Josephson phenomena, phase-slip processes at the critical current and spin current vortex (a topological defect which is the analog of a quantized vortex in superfluids and of an Abrikosov vortex in superconductors), etc. 1. _Magnon condensation into a Q ball in 3He-B_ Yu.M. Bunkov and G.E. Volovik Phys. Rev. Lett. 98, 265302 (2007). 2. _Bose-Einstein condensation of magnons in superfluid 3He_ Yu.M. Bunkov and G.E. Volovik, Preprint in arXiv:0708.06631
 22 janvier à 11h Séminaire d'intérêt général du LPTMS Françoise Combes (Observatoire de Paris) La matière noire dans l'Univers On connaît aujourd'hui la composition de l'Univers à quelques pourcents près : pour les trois quarts de l'énergie noire, et le reste en matière. Pourtant la nature de l'énergie noire est un mystère, et même la nature de la matière reste un des défis les plus frustrants. La matière visible des galaxies ne représente qu'un pourcent de la matière totale. La majeure partie de la matière ordinaire (ou baryonique) ne rayonne pas, et il nous faut deviner quelle est sa forme, par des déductions indirectes, sur la formation et l'évolution des galaxies. Nous décrirons tous les indices qui nous sont apportés par les courbes de rotation des galaxies, par l'histoire de leur formation d'étoiles, par le fait que les galaxies évoluent d'un type morphologique à un autre sur une séquence de Hubble, qui représente les diverses étapes de la vie d'une galaxie. Les problèmes de formation des galaxies dans un univers en expansion ne peuvent être résolus que si l'on suppose l'existence de matière non-baryonique en quantité encore plus importante. L'existence de cette matière noire est confirmée par la cartographie de l'univers par les lentilles gravitationnelles, et l'observation des anisotropies du fond de rayonnement micro-onde. Nous décrirons les candidats possibles pour cette matière noire, avec leurs succès et leurs échecs. Enfin l'existence supposée de matière noire non-baryonique ne pourrait-elle pas plutôt être une conséquence de notre ignorance de la forme de la gravité à grande échelle ? Les perspectives passionnantes de cette hypothèse seront discutées.
 15 janvier à 11h Séminaire du LPTMS Yoshiyuki Kabashima (Tokyo Institute of Technology) Statistical mechanical approach to the linear vector channels in digital wireless communication The linear vector channel is one of the basic categories of the modern wireless communication. This includes Code Division Multiple Access (CDMA) scheme, which is employed in the third generation mobile phone systems and wireless LANs, and Multi-Input Multi-Output (MIMO) communication, which is an advanced scheme to enhance communication performance utilizing multiple antennas at both of the transmitter and receiver terminals. In the general scenario of the channel, multiple messages are transmitted to the receiver, being linearly transformed to multiple output signals by a random matrix and degraded by channel noise. This yields complicated dependence across message variables, which makes it nontrivial to infer the transmitted messages from the received output signals. In this talk, we show that the inference problem can be mapped to a virtual spin system governed by random interactions and, therefore, can be analyzed by techniques developed in statistical mechanics of disordered systems in the large system size limit. We also show that an integral formula of random matrix theory, which is sometimes referred to as the Itzykson-Zuber integral, plays an important roles in the analysis. K Takeda, S Uda and Y Kabashima, Europhys. Lett. 76, 1193-1199 (2006) K Takeda, A Hatabu and Y Kabashima, J. Phys. A: Math. Theor. 40, 14085-14098 (2007)
 14 janvier à 14h30 Séminaire exceptionnel du LPTMS Markus Mueller (Harvard University) Quantum electron glasses and electron assisted hopping Insulators close to the metal insulator transition exhibit interesting collective electronic phenomena which are still rather poorly understood. A prominent feature in such systems is the purely electronic nature of activated transport apparent in experiments, which is inconsistent with the theory of phonon-assisted hopping conduction and thus has remained an unexplained puzzle for decades. The situation became even less clear when recently theories of many body localization predicted a metal insulator transition at finite temperature for interacting electrons that are decoupled from phonons. In this talk I will address this problem for Anderson insulators with a single-particle localization length much larger than the mean distance between electrons. Under these circumstances Coulomb interactions drive the electrons into a strongly correlated quantum glass phase with non-trivial collective behavior. I will show that a typical metastable state exhibits a gapless spectrum of collective excitations which act as a bath with which individual electrons can exchange energy. In 2D systems, this results in a hopping transport mechanism with a nearly universal pre-exponential factor of order e^2/h. This is in good agreement with many recent experiments, and resolves the above mentioned discrepancies with other theories.
 8 janvier à 11h Séminaire du LPTMS Andrei A. Fedorenko (LPT-ENS) Elastic systems with correlated disorder We applied the functional renormalization group to elastic systems such as interfaces or lattices pinned by correlated quenched disorder considering two different types of correlations: columnar disorder and quenched defects correlated as \sim x^{-a} for large separation x. We computed the critical exponents and the response to a transverse field h to two-loop order. The correlated disorder violates the statistical tilt symmetry resulting in nonlinear response to a tilt. Elastic systems with columnar disorder exhibit a transverse Meissner effect: disorder generates the critical field h_c below which there is no response to a tilt, and above which the tilt angle behaves as \vartheta\sim(h-h_c)^{\phi} with a universal exponent \phi1. The obtained results is applied to the Kardar-Parisi-Zhang equation with temporally correlated noise. We also studied the long-distance properties of O(N) spin systems with long-range correlated random fields and random anisotropies. Below the lower critical dimension, there exist two different types of quasi-long-range-order with zero order-parameter but infinite correlation length.