# Séminaires de l’année 2009

 16 décembre à 14h30 Séminaire du LPTMS Pavel Grigoriev (Institut Landau) Properties of superconductivity on the density wave background. We investigate the properties and the microscopic structure of superconductivity (SC), coexisting and sharing the common conducting band with density wave (DW). Such coexistence may take place when the nesting of the Fermi surface (FS) is not perfect. Then in the DW state some quasiparticle states remain on the Fermi level and lead to the Cooper instability. The soliton band in the DW state also leads to the Copper instability. The dispersion of the quasiparticles on the Fermi level in the DW state strongly differs from that without DW if the nesting is almost perfect, and so do the properties of SC on the DW background. The upper critical field Hc2 in such a SC state increases as the system approaches the critical pressure, where the ungapped quasiparticle states and superconductivity just appear. This upper critical field may considerably exceed the usual Hc2 value without DW. The SDW background strongly suppresses the singlet SC pairing, while it does not affect so much the triplet SC transition temperature. The results obtained may explain some experimental observations in layered organic metals (TMTSF)2PF6 and a-(BEDT-TTF)2KHg(SCN)4, where SC appears in the DW states under pressure and shows many unusual properties. Publications: [1]. L.P. Gor'kov, P.D. Grigoriev, " Nature of superconducting state in the new phase in (TMTSF)2PF6 under pressure.", Phys. Rev. B 75, 020507 (2007). [2]. P.D. Grigoriev, “Properties of superconductivity on the density wave background with small ungapped Fermi surface pockets”, Phys. Rev. B 77, 224508 (2008). [3]. P.D.Grigoriev, “Superconductivity on the density-wave background with soliton-wallstructure”, PhysicaB 404, 513–516 (2009).
 15 décembre à 11h Séminaire du LPTMS Mohammed Rajabpour (Université de Turin) Ashkin-Teller model on the iso-radial graphs Ashkin-Teller model as a spin system with a rich phase diagram is interesting as a lab for investigating many different ideas in two dimensional statistical mechanics such as renormalization group, conformal field theory and Schramm Loewner evolution. In this talk we will define AT on the iso-radial graphs and we will discuss the different features of the critical line of the model on the different lattices. We will also stress on the mathematical importance of iso-radial graphs and the concept of the universality.
 8 décembre à 11h Séminaire du LPTMS Valeri Shikin (Institute for Solis State Physics, Chernogolovka) Delta-percolation and Quantum Hall Effect We define a particular type of percolation phenomena which deserves the name “delta-percolation” for its peak-like appearance. One common systems demonstrating the delta-percolation behavior is the 2D electron (hole) system in the high magnetic field. We discuss behavior of extended states, whose appearance within the “sea” of electronic orbits localized by the random potential is the necessary condition for appearance of the integer Quantum Hall Effect (QHE). We consider observable consequences of the suggested interpretation of properties of extended states in the theory of integer QHE.
 3 décembre à 11h Séminaire "Systèmes complexes et Physique biologique" du LPTMS Jure Dobnikar (Department of Chemistry, University of Cambridge and Jozef Stefan Institute, Ljubljana, Slovenia) Field-induced self-assembly of suspended colloidal membranes We report experiments that probe self-assembly of micrometer-size colloids into one-particle-thick, robust, and self-healing membranes. In a magic-angle precessing magnetic field, superparamagnetic spheres experience isotropic pair attraction similar to the van der Waals force between atoms. But the many-body polarization interactions among them steer an ordered aggregation pathway consisting of growth of short chains, cross-linking and network formation, network coarsening, and consolidation of membrane patches. We theoretically explain membrane stability, their elastic and self-healing properties and the observed aggregation pathway. This generic scenario can be induced in any particles of large enough susceptibility.
 1er décembre à 11h Séminaire du LPTMS Dmitry Kovrizhin (Rudolf Peirels Centre for Theoretical Physics , Oxford University) Phase transitions and nonequilibrium dynamics of quantum Hall states from cold atoms to semiconductor nanostructures. Novel methods and applications. I will talk about recently developed methods to study phase transitions in quantum Hall systems and dynamics of IQHE edge states on examples of Bose-Einstein condensates under extreme rotation and electronic interferometers far from equilibrium. I will give an overview of application of the Density Matrix Renormalization group (DMRG) to systems of fast rotating bosons and present results of the calculations for various geometries. Then I will discuss recent experiments on interferometry and spectroscopy of IQHE edge states in semiconductor nanostructures and show how these systems can be studied using a new analytical method based on bosonization. I will present details of the calculations on an example of exactly solved model for electronic Mach-Zehnder interferometers. [1] D. L. Kovrizhin and J. T. Chalker, Phys. Rev. B 80, 161306(R) (2009) [2] D. L. Kovrizhin, arXiv:0909.3693 [3] S. I. Matveenko, D. Kovrizhin, S. Ouvry, G. V. Shlyapnikov, arXiv:0908.2172
 17 novembre à 11h Séminaire du LPTMS Pierpaolo Vivo (ICTP, Trieste) Phase transitions in the quantum conductance problem Linear statistics on ensembles of random matrices occur frequently in many applications. We present a general method to compute probability distributions of linear statistics for large matrix size N. This is applied to the calculation of conductance and shot noise for ballistic scattering in chaotic cavities, in the limit of large number of open channels. The method is based on a mapping to a Coulomb gas problem in Laplace space, displaying phase transitions as the Laplace parameter is varied. As a consequence, the sought distribution displays a central Gaussian region flanked on both sides by non-Gaussian tails, and weak non-analytical points at the junction of the two regimes.
 3 novembre à 11h Séminaire du LPTMS Mikhail Zvonarev (LPTMS) Spectral properties of one-dimensional quantum liquids I will report on a theoretical progress in understanding the spectral properties of one-dimensional quantum liquids. While for small energy and momentum transferred the spectral properties are explained (with notably exceptions) by the Luttinger model, their description at arbitrary energy/momentum remained an open problem until very recently. The situation changed in the last three years, and at present there exist several theories explaining the spectral properties of 1D systems away from the Luttinger Liquid regime. I will discuss these theories and open questions.
 27 octobre à 11h Séminaire du LPTMS Martin Lenz (Institut Curie, Université Paris 6) Polymerization and mechanochemical action of dynamin Living organisms are fundamentally open systems, which continuously exchange matter with their surroundings. Dynamin is used by cells to split membrane compartments apart during such exchanges. Recruitment and assembly Dynamin polymerizes around membrane nanotubes into long (~10 µm) helical polymers. Membrane elasticity imposes a critical radius for dynamin polymerization, which could trigger its recruitment in vivo. Surprising facts about the nucleation and growth of the helix yield information about dynamin assembly and interactions with the membrane. Mechanochemical action and severing Using the energy from GTP hydrolysis, dynamin polymers twist, constrict and pull on the membrane, and thus eventually sever it. Combining experiments and a generalized hydrodynamics description, we show that the dynamin helix changes conformation as one rigid body, suggesting that mechanical stresses induce its breakage. We predict its dynamics and account for seemingly contradictory experimental results. Our formalism is a powerful tool for further investigations about dynamin's membrane severing action.
 13 octobre à 11h Séminaire du LPTMS Michel Ferrero (École Polytechnique) Nodal/antinodal dichotomy in cuprate superconductors: a valence-bond dynamical mean-field approach The selective destruction of coherent quasiparticle excitations in the "antinodal" region of momentum-space is a striking property of the normal state of underdoped cuprate superconductors, and a key challenge for theory. In this talk, I will review a recent attempt to understand this phenomenon from a minimal cluster extension of the dynamical mean-field theory, based on a valence bond self-consistently embedded in a effective medium. I will show that the destruction of quasiparticles at the antinodes, the opening of a pseudogap, and the formation of Fermi arcs, can be viewed as an orbital-selective Mott transition in momentum-space. Comparison to tunneling and angular-resolved photoemission experimental data will be discussed.
 6 octobre à 11h Séminaire du LPTMS Evgeny Burovskiy (LPTMS) Multi-particle composites in density-imbalanced quantum fluids We consider two-component one-dimensional quantum gases with density imbalance. While generically such fluids are two-component Luttinger liquids, we show that if the ratio of the densities is a rational number, p/q, and mass asymmetry between components is sufficiently strong, one of the two eigenmodes acquires a gap. The gapped phase corresponds to (algebraic) ordering of (p+q)-particle composites. In particular, for attractive mixtures, this implies that the superconducting correlations are destroyed. We illustrate our predictions by numerical simulations of the fermionic Hubbard model with hopping asymmetry. ( Mostly based on: EB, Guiliano Orso, and Thierry Jolicoeur, arXiv:0904.0569 and arXiv:0907.1533 )
 29 septembre à 11h Séminaire du LPTMS Lev Shchur (Landau Institute for THeoretical Physics, Chernogolovka, Russia) Phase diagram for diffusion limited aggregation growth in two dimensions We present results of the intensive simulations and careful analysis of the diffusion-limited aggregation (DLA) model in two dimensions. We use previously developed the off-lattice killing-free algorithm for the cluster generation and the method of variable size probing particles for the cluster analysis. We generate clusters with the $m$-fold symmetry of sticking directions and with different values of the noise reduction parameter $n$. We propose the phase diagram in the plane $(m,n)$ with the two regimes of the DLA cluster growth. Clusters in the first regime are fractal crystals with the well defined $m$-fold symmetry and with the value of the asymptotic fractal dimension $D=3/2$. Clusters in the second regime are the random fractals with $D=1.710\ldots$. The global direction of the cluster axes changes by the angle $180/m$ at the critical line.
 22 septembre à 11h Séminaire du LPTMS Guilhem Semerjian (ENS, Paris) Le modèle de Bose-Hubbard sur le réseau de Bethe Les modèles de physique statistique définis sur les réseaux de Bethe admettent habituellement une solution exacte, qui s'obtient à partir de la résolution d'une équation de récursion. Dans le cas de modèles de spins classiques (Ising ou Potts par exemple) cette équation est d'une nature relativement simple, et cette approche peut être étendu, sous le nom de méthode de la cavité, pour traiter des modèles avec du désordre gelé, voire de la frustration et des propriétés vitreuses. Dans ce séminaire je discuterai une autre direction plus récente de généralisation de la méthode, pour des modèles quantiques, i.e. spins 1/2 en champ transverse et modèles de bosons sur réseau. Dans ce cas on peut aussi obtenir une équation de récursion dont la solution décrit les propriétés thermodynamiques du modèle ; elle est toutefois un peu plus compliquée que dans le cas classique, mais peut être résolue numériquement avec une précision arbitraire. Le réseau de Bethe étant une implémentation de l'approximation de Bethe pour les modèles de dimension finie, on comparera aussi, sur l'exemple du modèle de Bose-Hubbard, les prédictions ainsi obtenues avec d'autres traitements de ce modèle en dimension finie.
 15 septembre à 11h Séminaire du LPTMS Evgeny Kozik (University of Massachussetts) Diagrammatic Monte Carlo for correlated fermions We show that Monte Carlo sampling of the Feynman diagrammatic series (DiagMC) can be used for tackling hard fermionic quantum many-body problems in the thermodynamic limit by presenting accurate results for the repulsive Hubbard model in the correlated Fermi liquid regime. Using perturbative Feynman diagrams (rather than skeleton, or "bold-line", graphs) for the single-particle self-energy we can study moderate values of the onsite repulsion (U/t ~ 4) and temperatures down to T/t = 1/40. We compare our results with high temperature series expansion and with single-site and cluster dynamical mean-field theory. We find that going from single site to large clusters is essential for obtaining the same level of accuracy as provided by DiagMC.
 30 juin à 11h Séminaire du LPTMS Boris Altshuler (Columbia University) Insulating State of Disordered Superconducting Films and Josephson Arrays I will remain main concepts of the Superconductor-Insulator transition in disordered films and discuss basic properties of the insulating state of these systems. Recent experiments on charge transport in highly resistive films of superconducting materials revealed quite unexpected behavior of these systems: Ahrrenius law in temperature dependence of conductivity accompanied with low temperature anomalies. More recently bistabilities that leads to huge jumps in the resistivity were observed in nonlinear transport. We will see that an assumption that electronic system is overheated with respect to phonons provides natural quantitative description of these singularities. Overheating of the electrons in an insulator is remarkable because it is not compatible with the conventional models on phonon-assisted transport. It looks like most of existing experimental results can be understood at least qualitatively on the basis of the many-body localization concept. I will discuss these theoretical predictions, its relevance to experiments and open problems in this field.
 26 juin à 11h30 Séminaire du LPTMS : Physique Biologique Roberto Mulet (Physics Faculty, University of Havana) Estimating the size of the solution space of metabolic networks Cellular metabolism is one of the most investigated system of biological interactions. While the topological nature of individual réactions and pathways in the network is quite well understood there is still a lack of comprehension regarding the global functional behavior of the system. In the last few years flux-balance analysis (FBA) has been the most successful and widely used technique for studying metabolism at system level. This method strongly relies on the hypothesis that the organism maximizes an objective function. However only under very specific biological conditions the cell seems to obey such optimization law. A more refined analysis not assuming extremization remains an elusive task for large metabolic systems due to algorithmic limitations. In this work we propose a novel algorithmic strategy that provides an efficient characterization of the whole set of stable fluxes compatible with the metabolic constraints. The algorithm, based on the well-known Bethe approximation, can be used to approximately compute the volume of a non full-dimensional convex polytope in high dimensions. We first compare the accuracy of the predictions with an exact algorithm on small random metabolic networks. We also verify that the prédictions of the algorithm match closely those of Monte Carlo based methods in the case of the Red Blood Cell metabolic network. Then we test the effect of gene knock-outs on the size of the solution space in the case of {\it E-Coli} central metabolism. Finally we analyze the statistical properties of the average fluxes of the reactions in the {\it E-Coli} metabolic network.
 23 juin à 11h Séminaire du LPTMS Cyril Petitjean (University Regensburg) A semiclassical approach to the ac-conductance of quantum chaotic cavities Due to progress in the control and manipulation of mesoscopic structures driven by high frequency periodic voltages, the ac regime has recently been experimentally investigated [1] and consequently theoretical interest in it has been renewed. We consider a quantum chaotic cavity that is coupled via tunnel barriers and gates to a macroscopic circuit which contains ac-sources. For the transparent barrier, our semiclassical techniques permit us to include the Ehrenfest time in the weak-localization correction to the screened conductance, previously obtain by the random matrix theory [2]. Then by extending the recent semiclassical theory in presence of tunnel barriers [3] to the ac-transport, we investigate the effect of dephasing on the relaxation resistance of a chaotic capacitor in the linear low frequency regime. This last investigation is in principle rrelevant to the recent measurements of the admittance at zero magnetic flux of a mesoscopic capacitor [1,4]. Works in collaboration with D.Waltner, J.Kuipers,I.Adagideli and K.Richter See eprint : arXiv:0906.1791v1. [1] J. Gabelli et al., Science {\bf 313}, 499 (2006). [2] P.W.~Brouwer and M.~B\"uttiker, Europhys. Lett. {\bf 37}, 441 (1997). [3] R.S.~Whitney, Phys. Rev. {\bf B, 75}, 235404 (2007). [4] S.~Nigg and M.~B\"uttiker, Phys. Rev. {\bf B 77}, 085312 (2008).
 17 juin à 11h Séminaire du LPTMS Ennio Arimondo (Université de Pise) BEC in temporally modulated optical lattices
 16 juin à 11h Séminaire du LPTMS Tommaso Roscilde (ENS Lyon) Dynamical emergence of supersolidity in asymmetric boson mixtures in an optical lattice Supersolidity - the simultaneous appearance of Bose condensation and crystalline order in a degenerate system of bosons - is a long-sought and elusive phenomenon in condensed matter. Cold-atom realizations of a supersolid can be envisioned in optical lattices, in which spontaneous crystallization of atoms occurs in presence of finite-range or long-range interactions, or in the case of interactions mediated by a secondary atomic species. In this talk we will theoretically discuss how (quasi-)condensation an emerge dynamically in a mixture of two hardcore boson species with mass and number imbalance in a one-dimensional optical lattice. Starting from a "molecular crystal" of trimers (made of two heavy and one light particle) and suddenly changing the Hamiltonian of the system, we observe that (quasi-)condensation appears in both atomic species without disrupting the crystalline order imprinted in the system. An extensive study of the ground state phase diagram of this model shows that this supersolid state has no equilibrium counterpart. This suggests the intriguing possibility of engineering novel many-body states via a controlled collective quantum evolution in cold-atom systems.
 9 juin à 11h Séminaire du LPTMS Dragi Karevski (Université de Nancy) Quantum Non-Equilibrium Steady States Induced by Repeated Interactions We study the steady state properties of a finite XX chain coupled at its boundaries to quantum reservoirs. The reservoirs are made of free spins that interact one after the other with the boundary sites of the chain. We show that the dynamical equations governing the time evolution of the two-point correlation matrix are of Lindblad type in the continuous interaction limit. Under XY coupling with the reservoir spins, the steady state correlations are calculated exactly. It is shown that the relevant physical quantities characterizing completely the steady state are the magnetization profile and the associated current. Except at the boundary sites, the magnetization is equal to the average of the reservoirs magnetizations. The steady state current, proportional to the difference in the reservoirs magnetizations, shows a non-monotonous behavior with respect to the system-reservoir coupling strength, with an optimal current state for a finite value of the coupling. Finally, the steady state density matrix is shown to be of generalized Gibbs form. At small current linear irreversible thermodynamics predictions are recovered.
 2 juin à 11h Séminaire du LPTMS Dimitri Gangardt (University of Birmingham) Mobile impurities in 1D quantum liquids: Bloch oscillations and dissipation Dynamical properties of impurity particles immersed in a 1D quantum liquid are strongly modified by the interactions. The collective nature of the excitations manifests itself in periodic dispersion typical for lattices and may lead to Bloch oscillations. Experiments with ultracold atoms are currently underway to observe this phenomenon. At the same time the quantum and thermal fluctuations always present in quantum liquids provide a mechanism for dissipation and lead to viscous friction force acting on the moving impurity. A naïve estimate shows that typically the dissipation is rather large and may prevent observation of Bloch oscillations. We evaluate viscosity in terms of the quantum fluid parameters and show how the dissipation is drastically suppressed for nearly integrable systems due to quantum interference.
 20 Mai à 15h Séminaire du LPTMS, Physique biologique Jorge V. José (Physics Dep., Physiology and Biophysics, University of Buffalo) Non-equilibrium biophysical model of self-organized spindle formation In eukaryotic cells, separation of duplicated chromosomes is done via the mitotic spindle. The mitotic spindle is a regular structure within a cell consisting of oriented microtubule fibers. Forming a spindle pattern is a major structural step towards cell division or mitosis. We have developed biophysical non-equilibrium thermodynamic models to describe in vitro chromosome driven spindle formation experiments in Xenopus extracts. Our first 2D model calculations [1] successfully described the order of events seen in some of the Xenopus extracts experiments, where the chromosomes are replaced by chromatin-covered micrometer magnetic beads. I will describe more realistic 3D improvements in our modeling analysis, which include microtubule contact forces and excluded volume [2, 3]. There are, however, a number of challenges that must be addressed for spindle modeling to continue to be a useful tool for understanding this fundamental biological process. In this talk I will describe some important problems needing better biological date and hypothesis, as well as using random graph analysis to better characterize spindle geometric patterns. [1] S. C. Schaffner and J. V. José, PNAS, 103, 11166 (2006), [2] ibid in “Methods in Cell Biology” (Elsevier-Academic Press) (2008) and [3] ibid (to be published).
 18 Mai à 14h30 Séminaire du LPTMS Alexei Tsvelik (Brookhaven National Laboratory) Visual demonstration of the spinon confinement in quasi-1D spin ladder: CaCu2O3 CaCu2O3 is a quasi-1D antiferromagnetic insulator. Its magnetic subsystem can be well described as formed by weakly coupled pairs of spin-1/2 chains (spin ladders). The peculiar fact about CaCu2O3 is that the exchange interaction on rangs of the ladders is much smaller than the interaction along the chains. This allows to observe a crossover from the regime of independent chains dominated by spin-1/2 excitations (spinons) to the strong coupling low energy regime regime with S=0 and S=1 excitations. I present the experimental data and discuss their relation to the theory.
 13 Mai à 14h30 Séminaire du LPTMS Alessio Recati (University of Muenchen and Trento INFM BEC Center) Unitary polarized Fermi gas under adiabatic rotation We discuss the effect of an adiabatic rotation on the phase separation between the superfluid and normal component of a trapped polarized Fermi gas at unitarity and zero temperature, under the assumption that quantized vortices are not formed. We show that the Chandrasekhar-Clogston limit n_down/n_up characterizing the local polarization in the normal phase at the interface is enhanced by the rotation as a consequence of the centrifugal effect. The density profiles (local and column integral) of the two spin species are calculated as a function of the angular velocity for different values of the polarization. The critical value of the angular velocity at which the superfluid exhibits a spontaneous quadrupole deformation is also calculated for the unpolarized case.
 12 Mai à 11h Séminaire du LPTMS Luigi Cantini (LPTENS) Magic in the ground state of the dense O(1) loop model The interplay between statistical mechanics and combinatorics has always been of great interest both for physicists and for mathematician. The observations of Razumov and Stroganov, that the properly normalized components of the ground state of the dense O(1) model enumerate classes of alternating sign matrices, is an instance of such an interplay. The boundary conditions on the O(1) model are re?ected on the symmetries of the FPL. After giving a general overview on the problem and its ramifications, we present some recent progress in the analytical construction of the ground state of the O(1) model on a strip, with the most general boundary conditions.
 6 Mai à 16h Séminaire du LPTMS, Physique biologique William Bialek (Princeton University) Physics problems in early embryonic development One of the most beautiful phenomena in nature is the emergence of a fully formed, highly structured organism from a single, undifferentiated cell, the fertilized egg. Over the past decades, biologists have shown that in many cases the « blueprint » for the body is laid out with surprising speed and is readable as variations in the concentration of particular molecules (the expression levels of particular genes). In this lecture I’ll give a brief review of this biological background, and then show how, as we try to make quantitative sense out of this qualitative picture, we encounter a number of interesting physics problems: How can spatial patterns in the concentration of these molecules scale with the size of the egg. What insures that the spatial patterns are reproducible from one embryo to the next? Since the concentrations of all the relevant molecules are small, does the random behavior of individual molecules set a limit to the precision with which patterns can be constructed? Has nature found strategies to minimize the impact of this noise and maximize the flow of information through the system? I will try to give not just a formulation of these problems, but also report on recent progress toward solutions, which has involved considerable exchange between theory and experiment.
 5 Mai à 11h Séminaire du LPTMS Christophe Texier (LPTMS) Diffusion classique unidimensionnelle dans un champ de force aléatoire en présence de sites absorbants. One-dimensional model of classical diffusion in a random force field with a weak concentration $\rho$ of absorbers is studied. The force field is taken as a Gaussian white noise with $\mean{\phi(x)}=0$ and $\mean{\phi(x)\phi(x')}=g\,\delta(x-x')$. Our analysis relies on the relation between the Fokker-Planck operator and a quantum Hamiltonian in which absorption leads to breaking of supersymmetry. Using a Lifshits argument, it is shown that the average return probability is a power law $\smean{P(x,t|x,0)}\sim{}t^{-\sqrt{2\rho/g}}$ (to be compared with the usual Lifshits exponential decay $\exp{-(\rho^2t)^{1/3}}$ in the absence of the random force field). The localisation properties of the underlying quantum Hamiltonian are discussed as well.
 28 Avril à 11h Séminaire du LPTMS Julien Vidal (LPTMC) Anyons, fermions, et ordre topologique dans un système de spins 1/2: le modèle de Kitaev En 1977, Leinaas et Myrheim [1] suggéraient l'existence de statistiques quantiques différentes de celles de Bose-Einstein et de Fermi-Dirac. Quelques décennies plus tard, les particules obéissant à ces statistiques, les anyons, n'ont toujours pas été observées expérimentalement. Le modèle de Kitaev [2] qui décrit des spin 1/2 en interaction sur réseau hexagonal est sans aucun doute un des meilleurs candidats à la detection de tels objets. En effet, le spectre de ce système contient des excitations anyoniques, abéliennes et non-abéliennes, intimement liées à l'ordre topologique sous-jacent. Ces anyons sont localisés dans l'espace ce qui permet d'envisager simplement les manipulations indispensables à la mise en évidence expérimentale de leur statistiques [3]. Cependant, le spectre contient également des excitations fermioniques susceptibles de polluer les processus de détection. Je présenterai une analyse perturbative de ce modèle autour de la limite de dimères isolés permettant de comprendre les problèmes liées à la coexistence de ces deux types de particules, anyons et fermions, au sein d'un même spectre [4]. [1] J. M. Leinaas et J. Myrheim, Nuovo Cimento Soc. Ital. Fis. B 37, 1 (1977). [2] A. Kitaev, Ann. Phys. 321, 2 (2006). [3] L. Jiang et al., Nature Physics 4, 482, (2008). [4] J. Vidal, K. P. Schmidt et S. Dusuel, Phys. Rev. Lett. 100, 177204 (2008).
 15 Avril à 14h30 Séminaire du LPTMS Kris Van Houcke (Ghent University) Diagrammatic Monte Carlo: towards an exact solution for strongly correlated fermions Diagrammatic Monte-Carlo (DiagMC) is an exact technique that allows one to simulate quantities specified in terms of diagrammatic expansions, the latter being a standard tool of many-body quantum statistics. The sign problem, that is typically fatal to Monte-Carlo approaches, appears to be manageable with DiagMC. We introduce a general DiagMC scheme for strongly interacting fermions. As an illustrative example, we discuss the application of DiagMC to the Fermi-Hubbard model, and benchmark the technique against state-of-the-art numerical tools for strongly correlated fermions. In addition, we discuss the thermodynamic properties of a Fermi gas at unitarity, obtained through DiagMC simulation.
 14 Avril à 11h Séminaire du LPTMS Christian Hagendorf (LPTENS) The winding of loop-erased random walks The geometry of random planar objects such as boundaries of spin clusters or random walks can often be studied in terms of conformal mappings taking them into some reference shape. The growth of these objects then leads to a random evolution of the mapping, which is often easier to analyse than the originial process. One of the simplest growth processes is given by Schramm-Loewner evolutions (SLE), which provide a description of random curves and interfaces in statistical mechanics models. This talk will focus on the SLE description of planar loop erased random walks (LERWs). It will outline how to study its winding and passage properties via SLE computations, and contrast them to the case of simple random walks. In particular, it will be explained how LERWs on doubly connected domains can surprisingly be related to a solvable diffusion-advection problem for the Burgers equation in (1+1)-dimensions.
 07 Avril à 14h Séminaire du LPTMS David Feder (LPTMS et Université de Calgary) Can ground states of low-dimensional spin systems be useful for quantum computation? In measurement-based quantum computation (MBQC), quantum algorithms are performed solely by making sequential measurements of qubits in a suitably entangled state. The main advantage of this approach is that it is not necessary to perform any entangling operations, which are often difficult to implement in experiments. Ideally, this entangled 'universal resource' state would be the (strongly correlated) non-degenerate ground state of some simple spin Hamiltonian on a lattice, preferably in low dimensions. I will describe a set of simple criteria that can be used to determine if a given state is indeed a universal resource for MBQC, and suggest possible candidate Hamiltonians.
 07 Avril à 11h Séminaire du LPTMS Benoit Estienne (LPTHE, Paris) Parafermionic conformal field theories Conformal field theory is a very powerful tool to describe critical statistical systems in two dimensions. Many lattice models, such as the Ising model, the 3-states Potts model or the Ashkin-Teller model, enjoy a discrete ZN symmetry. Such systems are described by parafermionic conformal field theories, and the additional symmetries are encoded in a set of chiral fields with fractional spins called parafermions. After reviewing the general properties of this class of CTFs and the corresponding lattice models, I will present some recent results about the second parafermionic field theory.
 03 Avril à 11h30 Séminaire du LPTMS, Physique Biologique Christophe Sola (IGEPE,Institut de Génétique et microbiologie, Orsay) Transforming raw genetic data into knowledge. Infectious diseases transmission dynamics : the tuberculosis paradigm The study of infectious diseases history can be reconstructed by genetic means, whereas the surveillance, control and prevention of outbreak extensions could also be predicted using adequate data and models. We will concentrate on a disease as old as human kind : tuberculosis, as a model of a global infectious disease. We will review the state of the art knowledge on the history of this disease and will present some recent data that suggest that studies combining new formal and new experimental approaches could allow to boost our knowledge on human-bacilli interactions and their consequences on population structures and history.
 01 Avril à 14h30 Séminaire du LPTMS Mauro Antezza (LKB, ENS) Atom-Light Elementary Excitations in a Periodic Ensemble of Ultracold Atoms We derive the excitation spectrum for light coupled to atoms in an optical lattice, close to a 0 -> 1 atomic transition, for the branches that are mainly of atomic nature. We explicitly include the quantum atomic motion, which leads to remarkable features such as a finite lifetime of the elementary excitations and their energy dependence with the lattice depth, that will show up in a real experiment as we discuss. It also makes the theory naturally divergence-free and resolves a controversy for the occurrence of a spectral gap.
 31 Mars à 11h Séminaire du LPTMS Philippe Lecheminant (Université de Cergy-Pontoise) Dualité et classification des phases quantiques à une dimension Un des problèmes théoriques majeurs de la matière condensée est la classification des phases de la matière que l'on peut stabiliser à température nulle. Récemment, la classification des phases de fermions sans interaction en toute dimension a été proposée par Kitaev. A une dimension spatiale, on peut utiliser des méthodes analytiques puissantes comme la théorie des champs conformes et l'intégrabilité qui permettent de tenir en compte de manière non-perturbative les effets des interactions entre électrons. En particulier, il est possible d'effectuer une classification des phases critiques grâce à la symétrie conforme. Cependant, la situation est beaucoup moins claire pour les phases des unidimensionnels quantiques avec un gap spectral. Dans ce séminaire, je discuterai d'une possible approche de ce problème qui repose sur l'existence de symétries de dualités cachées non-perturbatives dans la théorie des champs effective de basse énergie des unidimensionnels quantiques. Des illustrations de la méthode aux atomes froids fermioniques à une dimension et aux échelles électroniques seront discutées.
 27 Mars à 11h30 Journal Club du LPTMS Lucas Kroc (Cornell University, Ithaca USA) Probabilistic Inference Methods for Solving Combinatorial Problems Combinatorial problems are at the core of many tasks with direct practical relevance, such as hardware and software verification, planning, and scheduling to name a few. The two main solution paradigms for solving such problems are based on backtrack-style search and local search. However, recently, a new powerful technique, called survey propagation (SP), was introduced. SP can solve certain classes of problem instances with millions of variables and constraints. SP incrementally constructs solutions using guidance from approximate statistical information derived from the underlying combinatorial constraints. This talk will discuss a new derivation of SP based on purely combinatorial insights. Our method also enables us to derive SP-style procedures for a diverse range of constraint satisfaction problems, and provides insights into clusters of solutions. In the second part of this talk, we will discuss several other combinatorial methods based on combining statistical inference, systematic search, and local search. These methods are the most scalable algorithms for hard to solve classes of Boolean satisfiability (SAT), model counting (#SAT), and maximum satisfiability (MaxSAT) problems.
 25 Mars à 14h30 Séminaire du LPTMS Victor Gurarie (University of Colorado) Many particle generalization of the Landau-Zener problem An old theorem in quantum mechanics states that if the parameters of a system are changed sufficiently slowly, the system remains in its ground state. However, if the system is large, a natural question arises, how slow is slow enough. In this talk, I will introduce an interesting many particle system which does not want to stay in its ground state unless its parameters change at an exceedingly slow rate. The system, a version of a time-dependent Dicke model, arises in a variety of contexts in cold atoms, quantum optics, and condensed matter physics.
 24 Mars à 11h Séminaire du LPTMS Nigel Cooper (University of Cambridge) Correlated Phases of Atomic Bose Gases on a Rotating Lattice While dilute atomic Bose gases are typically in a weakly interacting regime, they can be driven into regimes of strong correlations by various means. A very interesting possible way to do this is by the use of a rotating optical lattice. I shall discuss the nature of the groundstate of such systems, as described by the Bose-Hubbard model in a uniform effective magnetic field. The relevant physics involves the interplay between the fractional quantum Hall effect for bosons and the "Hofstadter butterfly" spectrum. I will explain how that this interplay can lead strongly correlated phases that have no counterpart in the continuum, and present numerical evidence for the existence of these phases.
 20 Mars à 11h30 Séminaire du LPTMS, Physique Biologique Christophe Deroulers, IMNC, Orsay Modéliser la croissance de tumeurs du cerveau : vers la prise en compte des interactions entre cellules En l'état actuel des techniques médicales, les cancers du cerveau sont très souvent incurables. Si tous ne croissent pas aussi vite (leur développement peut s'étaler sur des périodes de quelques semaines à plusieurs décennies), tous semblent évoluer inéluctablement vers des formes de plus en plus malignes, jusqu'à entraîner le décès du patient. La malignité ne se manifeste pas, comme c'est souvent le cas pour d'autres types de cancers, par l'essaimage de métastases qui, transportées par les différents systèmes circulatoires, envahissent progressivement tout l'organisme. En revanche, les cellules cancéreuses ont tendance à migrer à travers les tissus sains qui entourent la tumeur sur des distances relativement importantes, ce qui empêche une ablation chirurgicale exhaustive et provoque des rechutes quasiment systématiques. La migration cellulaire est donc un phénomène clef dans les cancers du cerveau, encore très mal connu, et qui a des conséquences à l'échelle de l'organe entier alors qu'il a lieu à l'échelle des cellules. Dans ce contexte, il peut être utile d'utiliser une approche de modélisation, d'abord dans des cas simples, puis si possible sur des tumeurs entières en situation réaliste, avec le double but de mieux comprendre le phénomène complexe qu'est la croissance d'un cancer du cerveau et à plus long terme d'améliorer les traitements ou d'en proposer de nouveaux.Après une introduction au phénomène médical et biologique qui va nous occuper, nous rappellerons quels types de données expérimentales sont à disposition et nous citerons quelques-uns des travaux de modélisation entrepris ces dernières années par différentes équipes. Ensuite, nous nous intéresserons plus précisément à l'influence des interactions de contact entre certaines cellules qui interviennent dans ces phénomènes sur la migration des cellules cancéreuses. Grâce à un modèle simpliste sur réseau discret, et sur la base de quelques expériences in vitro en attendant des données in vivo adéquates, nous montrerons que ces interactions de contact influencent la migration. Grâce à un calcul (approché) de limite hydrodynamique, nous argumenterons que cela peut avoir des conséquences à l'échelle macroscopique. En particulier, ce travail mène à critiquer que l'hypothèse très répandue d'une diffusion de type brownien des cellules cancéreuses. Remarque amusante, ce modèle constitue en fait une famille de "modèles à contraintes cinétiques" bien connus des spécialistes de la physique des phénomènes vitreux. Référence bibliographique : arxiv:0812.4708 (sous presse dans Physical Review E)
 17 Mars à 11h Séminaire du LPTMS Vadim Cheianov, University of Lancaster Ordered states of dilute adatom systems in graphene We investigate the electron-mediated interaction between atoms adsorbed on the surface of monolayer graphene. The interaction is shown to favour ordered states, whose order parameter depends on the subgroup of the lattice symmetry group preserved by an individual impurity. The corresponding order-disorder phase transitions are studied numerically and the corresponding critical temperatures are calculated. The signatures of the phase transitions in transport and optical properties of the system are discussed.
 13 Mars à 11h30 Séminaire exceptionnel du LPTMS, Systèmes complexes Zdzislaw Burda, Department of theory of complex systems, Pologne Localization of maximal entropy random walk We define a new class of random walk processes which maximize entropy. This maximal entropy random walk is equivalent to generic random walk if it takes place on a regular lattice, but it is not if the underlying lattice is irregular. In particular, we consider a lattice with weak dilution. We show that the stationary probability of finding a particle performing maximal entropy random walk localizes in the largest nearly spherical region of the lattice which is free of defects. This localization phenomenon, which is purely classical in nature, is explained in terms of the Lifshitz states of a certain random operator.
 11 Mars à 14h30 Séminaire du LPTMS Long Dang, University of Alberta, Canada Disorder induced Superfluidity We use quantum Monte Carlo simulations to study the phase diagram of hard-core bosons with short-ranged attractive interactions, in the presence of uniform diagonal disorder. It is shown that moderate disorder stabilizes a glassy superfluid phase in a range of values of the attractive interaction for which the system is a Mott insulator, in the absence of disorder. A transition to an insulating Bose glass phase occurs as the strength of the disorder increases.
 10 Mars à 11h Séminaire du LPTMS Israel Klich, University of Virginia The Entanglement Entropy of Fermions Entanglement entropy has been of interest lately as a measure of quantum many body correlations between the parts of a many body system. In this talk I will discuss the problem of entanglement entropy of fermions. I will explain it's relation to Widom's conjecture and to the area law. I will also describe a universal relation between entanglement entropy and statistics of current flowing through a quantum point contact which provides a way to experimentally measure entanglement entropy of fermions. Finally, I will discuss the role of super-selection rules on the accessible ("useful") part of the entropy.
 6 Mars à 11h30 Journal Club du LPTMS Fabio Caccioli, SISSA , Trieste Non-equilibrium mean-field theories on scale-free networks Many non-equilibrium processes on scale-free networks present anomalous critical behavior that is not explained by standard mean-field theories. We propose a systematic method to derive stochastic equations for mean-field order parameters that implicitly account for the degree heterogeneity. The method can be used to correctly predict the dynamical critical behavior of some binary spin models and reaction-diffusion processes. The validity of this approach is supported by showing its relation with the generalized Landau theory of equilibrium critical phenomena on networks. Florent Krzakala & Lenka Zdeborova, Laboratoire PCT, ESPCI ; LANL, Los Alamos The Structure of Gibbs States in Glassy Mean Fields Problem We introduce a formalism which allows to follow Gibbs states when an external parameter, such as the temperature or the magnetic fields, is changed in glassy mean field systems. We discuss how this can be used to obtain non trivial and hiterto unreachable informations on the static and dynamic behavior of mean field glass formers and random constraint satisfaction models. In particular, we explicitly demonstrate the presence of temperature chaos, level crossings and of the Kovacs effect; we show how to bound the residual energy after a very slow annealing, and explicit the connection between success of local algorithms and the presence of frozen variables. Finally we also show how to create instances of the problem together with an equilibrated configuration, a technique which have many applications in monte-carlo simulations.
 3 Mars à 11h Séminaire du LPTMS David Fichera and Piotr Deuar (LPTMS) Brief presentations of LPTMS research activities
 20 Février à 11h30 Séminaire du LPTMS, Physique Biologique Johannes Berg, Institut de Physique, Freiburg Statistical physics in quantitative biology: The regulation of gene expression Fluctuations in biological systems span a wide range; from the evolutionary time-scales of population dynamics, to stochastic effects in biochemical processes. All these time-scales enter the problem of when and where a gene is read out to produce RNA molecules and proteins. I will discuss several paradigmic cases, from the generation of transcription factor binding sites in regulatory DNA to the out-of-equilibrium dynamics of gene expression levels.
 17 Février à 11h Séminaire du LPTMS Pasquale Calabrese, Dipartimento di Fisica Enrico Fermi, Université de Pise Quantum quenches in extended systems Understanding the non-equilibrium dynamics of extended quantum systems after the trigger of a sudden, global perturbation (quench) represents a daunting challenge, especially in the presence of interactions. The main difficulties stem from both the vanishing time scale of the quench event, which can thus create arbitrarily high energy modes, and its non-local nature, which curtails the utility of local excitation bases. We show that the time-dependence of correlation functions in an extended quantum system in d dimensions following a quantum quench, may be extracted using methods of boundary critical phenomena in d+1 dimensions. For d=1 particularly powerful results are available using conformal field theory. They may be explained in terms of a picture, valid more generally, whereby quasiparticles, entangled over regions of the order of the correlation length in the initial state, then propagate classically through the system.
 11 Février à 14h30 Séminaire du LPTMS Kurt Gibble, PennState University Physics with atomic clocks : beyond what time is it? Atomic clocks realize the most accurate measurements of any kind and are extremely sensitive to incredibly small perturbations. The current generation of atomic clocks uses laser-cooling and, after circumventing some new problems, these will realize 100 fold improvements in clock accuracies. I will describe the basic physics of clocks and several of the new problems. These include frequency shifts due to collisions of the cold atoms, the size of the recoil of an atom when it absorbs a photon, and juggling many atoms in fountains. Looking forward, the next generation of atomic clocks will utilize optical frequency transitions and an elegant technology to count the optical frequency (1015 Hz) ticks of the clock.
 10 Février à 11h Séminaire du LPTMS Gregory Schehr, LPT, Orsay From vicious walkers to random matrix theory : a path integral approach Vicious, i.e. non-intersecting, random walkers have been studied in various physical situations, ranging from wetting and melting to stochastic growth processes. In this talk, I will present a method based on path integrals associated to free Fermions models to study such statistical models. In particular I will show how this allows for a physical derivation of the connection between certain constrained vicious walkers and random matrix theory. Besides I will use this method to calculate exactly the distribution of the maximal height of p vicious walkers with and without a wall.
 06 Février à 11h30 Séminaire du LPTMS, Physique Biologique Chris Wiggins, Department of Applied Physics and Applied Mathematics, Columbia University Learning Networks from Biology, Learning Biology from Networks Both the 'reverse engineering' of biological networks (for example, by integrating sequence data and expression data) and the analysis of their underlying design (by revealing the evolutionary mechanisms responsible for the resulting topologies) can be re-cast as problems in machine learning: learning an accurate prediction function from high-dimensional data. In the case of inferring biological networks, predicting up- or down- regulation of genes allows us to learn ab intio the transcription factor binding sites (or motifs') and to generate a predictive model of transcriptional regulation. In the case of inferring evolutionary designs, quantitative, unambiguous model validation can be performed, clarifying which of several possible theoretical models of how biological networks evolve might best (or worst) describe real-world networks. In either case, by taking a machine learning approach, we statistically validate the models both on held-out data and via randomizations of the original dataset to assess statistical significance. By allowing the data to reveal which features are the most important (based on predictive power rather than overabundance relative to an assumed null model) we learn models which are both statically validated and biologically interpretable.
 03 Février à 11h Séminaire du LPTMS Pietro Faccioli, Dipartimento di Fisica and I.N.F.N., Universite degli studi di Trento Dominant Reaction Pathways in Thermally-Actived transitions of Bio-Molecules The standard technique to investigate the out-of-equilibrium dynamics of biomolecules is represented by molecular dynamics (MD) simulations. Unfortunately, such a method becomes very inefficient, when it is applied to characterize rare, thermally-activated transitions, such as the protein folding reaction. The reason is that most of the computational time is invested in simulating the thermal oscillations of the system, in the (meta-) stable states. On the other hand, one is mostly interested in the reaction pathways, i.e. in the trajectories which connect the different (meta-) stable states. In this talk, we present a recently developed theoretical and computational framework - denominated Dominant Reaction Pathways (DRP) - to rigorously identify the most statistically significant transition pathways, in rare bio-molecular conformational reactions. The method is completely general and is based on the analogy between the out-of-equilibrium stochastic dynamics and the quantum evolution in imaginary time. In such an analogy, the most probable thermally-activated transitions correspond to the instantons of quantum mechanics. The idea is therefore to apply the instanton theory to the path integral representation of the conditional Fokker-Planck probability. This way one derives an equation for the transition pathways which can be very efficiently solved on the existing computers. The main advantage is that one avoids investing computational time in simulating the local thermal motion in the meta-stable configurations. We shall present the formalism of the DRP technique, and its first applications from simple systems to real proteins, made of several hundreds atoms.
 30 Janvier à 11h Séminaire du LPTMS Tung-Lam Dao, CPHT, Ecole Polytechnique Polarized superfluidity in the attractive Hubbard model with population imbalance We study a two-component Fermi system with attractive interactions and different populations of the two species in a cubic lattice. For an intermediate coupling, we find a uniformly polarized superfluid which is stable down to very low temperatures. The momentum distribution of this phase closely resembles that of the Sarma phase, characterized by two Fermi surfaces. This phase is shown to be stabilized by a potential energy gain, as in a BCS superfluid, in contrast with the unpolarized Bose-Einstein condensate which is stabilized by kinetic energy. We present general arguments suggesting that preformed pairs in the unpolarized superfluid favor the stabilization of a polarized superfluid phase.
 27 Janvier à 11h Séminaire du LPTMS Olivier Giraud, LPT, Toulouse Quantum simulation of multifractal quantum systems It has been realized over the past twenty years that the principles of quantum mechanics allow to realize new computational devices which can be more efficient than their classical counterparts. It has been shown that quantum algorithms can be devised which are asymptotically faster than classical algorithms. However, despite years of effort in the field, not so many efficient quantum algorithms have been found, and it is still unclear which problems can be treated faster on a quantum computer and how efficiently. Feynman’s idea of simulating complex physical systems on a quantum computer has proved fruitful in that respect. Quantum chaos in particular has been an inspiring source for construction of efficient quantum algorithms simulating chaotic systems. It turns out that systems which are intermediate between chaotic and integrable systems possess features interesting both from a theoretical and a quantum-computational point of view. Here I will discuss multifractal properties of intermediate quantum maps. I will show how entanglement of localized or fractal random states can be related to localization properties or multifractal exponents of such states. Finally I will discuss the construction of efficient quantum algorithms based on the quantum wavelet transform.
 21 Janvier à 15h Séminaire du LPTMS, Physique Biologique Ginestra Bianconi, ICTP, Trieste Flux distribution of metabolic networks Bioinformatics methods, and in particular flux-balance-analysis give good predictions of the metabolic fluxes in bacteria. This framework assume steady state of the reactions and otimization of the biomass. In this talk we focus on the analytical derivation of the flux distribution of metabolic networks assuming i) only the steady state of the reactions and ii) both the steady state of the reactions and the optimization of the biomass. The problem of predicting the flux distribution can be cast in an Hamiltonian problem of soft variables.Different statistical mechanics techniques have been devised for solving the two problems: we make use of the Gaussian replica symmetric ansatz in one case and we make an expansion close to the critical point for predicting the critical exponents in the other case. The conclusions is that the flux distribution becomes power-law close to optimal biomass production with exponents independent on the degree distribution of the network. The predicted power-law exponent is in agreement with in silico simulations of flux-balance-analysis but differs form the one find in experiments.
 20 Janvier à 11h Séminaire du LPTMS Luca dell'Anna, SISSA, Trieste Interaction effects o transport in disordered d-wave superconductors : a study of several universality classes We study the localization properties of disordered d-wave superconductors by means of fermionic replica trick method, deriving the effective non-linear sigma-model for the spin diffusive modes. According to the presence of certain symmetries and the range of imputity potential, we provide a detailed classification for the behavior of some physical quantities, like the density of states, the spin and the quasiparticle charge conductivities. Following the Finkel'stein approach, we finally extend the effective functional method to include residual quasiparticle interactions, at all orders in the scattering amplitudes, obtaining the complete RG equations for the full set of charges of the field theory.
 14 Janvier à 14h30 Séminaire du LPTMS Barbara Capogrosso-Sansone (Harvard University) Sign-Alternating Interaction Mediated by Strongly-Correlated Lattice Bosons We reveal a generic mechanism of generating sign-alternating inter-site interactions mediated by strongly correlated lattice bosons. The ground state phase diagram of the two-component hard-core Bose-Hubbard model on a square lattice at half-integer filling factor for each component, obtained by worm algorithm Monte Carlo simulations, features solid+superfluid phase for strong anisotropy between the hopping amplitudes. The new phase is a direct consequence of effective interactions between heavy'' atoms mediated by the `light'' superfluid component. Due to their sign-alternating character, mediated interactions lead to a rich variety of yet to be discovered quantum phases
 13 Janvier 2009 Séminaire du LPTMS Saburo Higuchi (LPTMS - Ryukoku University) Gauillaume Roux (LPTMS) Raoul Santachiara (LPTMS) Short presentation of research activity We give a brief description of our actual research activity
 6 Janvier à 11h Séminaire du LPTMS Maribel Garcia de Soria (LPTMS) Fluctuations of global quantities in dissipative systems In the last years, there has been a great interest in the study of the properties of granular media. It has been seen that the presence of the inelasticity, which is the main feature of grain interactions, gives rise to a very interesting phenomenology. More recently, interest has increased about systems in which not only the energy but also the density and momentum is not conserved due to the annihilation of the colliding particles. We have studied the correlations and its influence in the fluctuations of the total energy for a granular gas in two different situations: evolving freely and heated by a stochastic thermostat. We have also studied the fluctuations of the total number of particles, momentum and energy in the ballistic annihilation model.