18 dÃ©cembre Ã  11h LPTMS-BÃ¢t.100-ORSAY KallÃ© KytÃ¶lÃ¤ (LPTMS) LERW as an example of near critical SLEs Schramm-Loewner evolutions (SLE) are random, conformally invariant curves that provide a mathematical description of continuum limits of interfaces in 2-d statistical physics at criticality. After a brief introduction to SLEs, I will discuss the question of extending this method to near critical models, in which one lacks conformal invariance because of finite correlation length. As a concrete model I will use perturbed loop-erased random walks (LERW). LERWs correspond to a c=-2 conformal field theory of symplectic fermions and a field theory interpretation will be given to the boundary operators and the perturbation.
 14 dÃ©cembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Scott Hoffmann (University of Queensland, Brisbane, Australia) A Hybrid Phase-Space Method for Interacting Bose Fields We introduce a new phase-space method that treats some modes of a multimode system with the Wigner representation and the remainder with the positive-P representation. We tested the method on a two-mode system with applications to pulses interacting in an optical fibre and a quantum nondemolition measurement scheme. Future work will apply the method to the problem of quantum Brownian motion of an impurity in a Bose-Einstein condensate.
 12 dÃ©cembre Ã  14h LPTMS-BÃ¢t.100-ORSAY Alexei Tsvelik (Brookhaven National Laboratory) Friedel oscillations in the local density of state of Mott Insulators.
 11 dÃ©cembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Robert Whitney (Institut Laue Langevin) The rivalry between interference and tunneling in quantum transport through chaotic dots We consider interference effects (weak-localization, shot-noise, etc) in quantum transport through large chaotic quantum dots (billiards). We look at various situations with tunnel barriers on the leads, or in the chaotic dot itself. We show there is a remarkable interplay between the two best known consequences of quantum mechanics; interference and tunnelling. Using the recently developed trajectory-based theory of quantum chaos (beyond the diagonal approximation). We show that the weak-localization correction to the conductance is strongly suppressed by tunnel barriers, while shot-noise is usually enhanced. We then discuss a newly observed massive enhancement of conductance in mirror-symmetric billiards, showing that it is due to a "co-operation" between interference and tunnelling.
 5 dÃ©cembre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Giuliano Orso (LPTMS) BCS-BEC crossover in a random external potential We present recent results [1] on the ground state properties of a disordered superfluid Fermi gas across the BCS-BEC (Bose Einstein condensate) crossover. We show that, for weak disorder, both the depletion of the condensate fraction of pairs and the normal fluid density exhibit a non-monotonic behavior as a function of the interaction parameter $1/k_Fa$, reaching their minimum value near unitarity. In the BCS limit the condensate fraction is strongly suppressed by disorder, despite the fact that the superfluid order parameter is unaffected by impurities, in agreement with Anderson's theorem. [1] G. Orso, arXiv:0709.2621, Phys. Rev. Lett. in press.
 27 novembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Brice Chung - Maria ColomÃ©-TatchÃ© - Julien Randon-Furling - David Papoular - JÃ©rÃ´me Dubail - Paolo Giglioli - Alvaro Rojo - Jasper Franke - Maria-Isabel Garcia de Soria - Giuliano Orso - Revaz Ramazashvili. PrÃ©sentation des doctorants, prÃ©-doctorant et post-doctorants
 23 novembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Andrei Derevianko (Physics Department, University of Nevada, Reno, USA) Bose-Einstein condensates of polar molecules ; anisotropic interactions = anisotropic mass So far the theory of Bose-Einstein condensates (BEC) of polar molecules was based on an ad hoc generalization of equations for spherical atoms. Here I adopt a rigorous pseudopotential approach to low-energy dipolar interactions and derive a non-linear mean-field SchrÃ¶dinger equation for a harmonically-trapped condensate. I show that, effectively, the dipolar interactions alter molecular mass. The resulting effective mass is anisotropic: to the leading order the mass is altered only for the motion along the polarizing field. For a typical BEC of spin-polarized magnetically-interacting alkali-metal atoms the effective atomic mass is reduced by 10% from its bare value. For a BEC of polar molecules the massmay be reduced by a factor of a 1000.
 21 novembre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Revaz Ramazashvili (LPTMS) Zeeman electric-dipole resonance in antiferromagnetic conductors Essential dependence of the electron g-factor on the quasiparticle momentum is a fundamental and, so far, largely overlooked property of antiferromagnetic conductors. It leads to a number of remarkable phenomena, such as excitation of spin flip transitions by AC *electric* field. Absorption intensity of these transitions exceeds that of the electron spin resonance (ESR) by some four orders of magnitude. I develop a theory of this phenomenon in a weakly doped antiferromagnetic insulator. The predictions may be relevant for a number of antiferromagnetic conductors, ranging from chromium to electron-and hole-doped cuprates, to organic conductors with spin density wave, and to heavy fermion antiferromagnetic metals.
 20 novembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Alexei S. Ioselevich (Landau Institute, Moscow) Statistical models of porous materials: topological phase transitions and nouniversal critical behavior We introduce and study several kinetic models for the porous material fabrication process. These models involve consecutive removing of randomly chosen nodes (or links) of the initially solid network. After each step of this process the system is locally adjusted to secure the connectivity: clearly, in a real physical 3d system disconnected clusters can not be mechanically stable without support from the "mainland". Depending on details of the adjustment procedure different scenarios are possible. In the simplest models the system experiences a second order topological phase transition to the "tree-like phase" with the increase of porosity, which has dramatic physical consequences. In the tree-like phase the network does not have {\it a backbone}, and, therefore, it should be non-conducting and extremely fragile. This phase transition is nonuniversal: corresponding critical indices depend on details of the model. In more realistic models for the adjustment procedure the phase transition is smoothed and the high-porosity state pertains finite (though, extremely poor) conductivity and elasticity. The reason for this smoothing is "backbone reinforcement effect": the number of links, restored in each adjustment act, dramatically increases at low backbone density. Some principal results (e.g., the existence and characteristics of the phase transition in the simplest model) were obtained analytically, but the main instrument of our study for more sophisticated models was numerical simulations. Our results may give an insight for the problems, which technologists encounter, when dealing with highly porous materials.
 7 novembre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Mikhail Zvonarev (UniversitÃ© de GenÃ¨ve) Spin dynamics in a one-dimensional ferromagnetic Bose gas We investigate the propagation of spin excitations in a one-dimensional (1D) ferromagnetic Bose gas. While the spectrum of longitudinal spin waves in this system is sound-like, the dispersion of transverse spin excitations is quadratic making a direct application of the Luttinger Liquid (LL) theory impossible. By using a combinationof different analytic methods we derive the large time asymptotic behavior of the spin-spin dynamical correlation function for strong interparticle repulsion. The result has an unusual structure associated with a crossover from the regime of trapped spin wave to an open regime and does not have analogues in known low-energy universality classes of quantum 1D systems.
 6 novembre Ã  11h LPTMS-BÃ¢t.100-ORSAY Mathias Albert - Pierre Carmier - RÃ©my Dubertrand - Lenka Zdeborova PrÃ©sentation des doctorants
 24 octobre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Teimuraz Vekua (LPTMS) Critical properties of attractive two-component fermions at the onset of imbalance We study low energy properties of the one dimensional mixture of two component attractive Fermi gas as an imbalance sets in. Using the weak coupling bosonization approach, including the curvature effects, supplemented by non-perturbative analysis we work outhow the presence of a gapless density fluctuations influences the nature of the phase transition, in which another (spin) mode is involved directly. Our results differ qualitatively from the ones obtained by assuming spin- charge separation.
 23 octobre Ã  11h LPTMS-BÃ¢t.100-ORSAY Giulio Casati ((Center for Nonlinear and Complex Systems, Universita_dell_Insubria, Como, Italy) Classical and quantum chaos, and understanding and control of heat Flow The understanding of the underlying dynamical mechanisms which determines the macroscopic laws of heat conduction is a long standing task of non-equilibrium statistical mechanics. A better understanding of such mechanism may also lead to potentially interesting applications based on the possibility to control the heat flow. Indeed, a model of thermal rectifier has been recently proposed in which heat can flow preferentially in one direction. Although this model is far away from a prototype realization, it is based on a mechanism of very general nature and, as such, is suitable of improvement and may eventually lead to real applications. More recently, a different thermal diode model has been proposed in which, even though the underlying physical mechanism is similar to the previous model, there is a new crucial element which allows to improve the efficiency by more than two orders of magnitude. Finally we briefly discuss the possibility to build a thermal transistor. Of particular interest is the problem, almost completely unexplored, of the derivation of Fourier law from quantum dynamics. To this end we discuss heat transport in a model of a quantum interacting spin chain and we provide clear numerical evidence that Fourier law sets in above the transition to quantum chaos.
 17 octobre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Michael Urban (IPN, Orsay) Temperature effects in collective modes of superfluid trapped Fermi gases At finite temperature, the hydrodynamic motion of the superfluid component of a trapped Fermi gas is coupled to to the motion of the normal component, which in the BCS limit behaves like a collisionless normal Fermi gas. The coupling between the superfluid and the normal component can be treated in the framework of a semiclassical transport equation for the quasiparticle distribution function, combined with a hydrodynamic equation for the superfluid component. A numerical method for solving these equations is presented, in which the quasiparticle distribution function in phase space is approximated by a finite number of test particles, which have the properties of "classical" Bogoliubov quasiparticles. As applications of the semiclassical theory, the case of a slowly rotating trap and several collective modes of a trapped gas are discussed
 10 octobre Ã  15h30 LPTMS-BÃ¢t.100-ORSAY Evgeny Burovskiy (Institut fÃ¼r Physik, UniversitÃ¤t Greifswald) Fermi gas in the unitarity limit We discuss the physics of a a fermi gas with the spin degrees of freedom. It is well known theoretically and experimentally that there is a smooth corssover between the BCS regime and the BEC regime between the two sides of a feshbach resonance. Right at resonance, the s-wave scattering length disappera from the problem and we are left with a problem of fundamental physics where all relevant quantities are essentialy pure numbers. One issue is the computation of the critical temperature at this point. I discuss the use of modern algorithms applied to the lattice Hubbard model with attractive interaction in the dilute limit and also to a continuum model. I give estimate of he critical temperature and compare with alternative estimates.
 10 octobre Ã  14h30 LPTMS-BÃ¢t.100-ORSAY Victor Fleurov (Tel Aviv University) Temporal dynamics of tunneling Hydrodynamic approach
 24 juillet Ã  11h LPTMS-BÃ¢t.100-ORSAY Sushanta Dattagupta (Indian Institute of Science Education & Research, Kolkata) Dissipative diamagnetism -- Gibbs and Einstein approaches A quantum system, which is otherwise coherent, starts behaving like a classical system when it is put in contact with a large environment. This quantal to classical crossover is an interesting topical issue in many areas, including condensed matter physics. In the latter subject, an old problem is that of Landau Diamagnetism (LD), exhibited by a statistical collection of electrons moving in a magnetic field. LD is a purely quantum phenomenon. What happens when the electrons additionally interact with a dissipative bath? We will examine this question within both Einstein and Gibbs approaches to statistical physics. The result is that LD goes over to Bohr-Van Leeuwen (BV)like expression when the environmental coupling is strong. The role of the confining boundary within which the electrons move will be examined, interalia, in this LD to BV transition.
 3 juillet Ã  11h LPTMS-BÃ¢t.100-ORSAY Mehran Kadar (MIT) The elusiveness of polymer knots Knots are a topological property of closed curves, and while not rigorously defined, are also expected to occur in sufficiently long open polymers. We argue, however, that energetic and entropic factors favor tight knots, with the entanglements localized on relatively small segments. Knots are also statistically rare in swollen (coil) polymers, but abundant in compact (globular) configurations. Yet, our investigation of the Protein Data Bank reveals very few knotted structures in globular proteins. We shall discuss some particularly intriguing examples of this set, including the most complicated protein knot appearing in Human ubiquitin hydrolase.
 19 juin Ã  11h LPTMS-BÃ¢t.100-ORSAY Tannie Liverpool (DÃ©partement de Maths, Leeds) Soft Active Matter : from polymer physics to the cell cytoskeleton We study the dynamics of active solution of self-propelled polar colloidal particles. We derive generic phenomenological hydrodynamic equations for coarse-grained order parameters from a variety of microscopic mechanisms. Using these equations we investigate the phase behaviour and mechanical properties in different homogeneous states as a function of non-equilibrium driving and filament concentration. Finally we consider the response of the active filament suspension to applied shear deformation in the isotropic phase, non-equilibrium polarized and nematic liquid crystalline states.
 12 juin Ã  15h30 LPTMS-BÃ¢t.100-ORSAY Lev Ioffe (Rutgers) Toward experimental realization of protected q-bits The main challenge of quantum computation is how to protect the macroscopical object implementing the quantum bit from the decoherence. An attractive possibility is to use the topological protection that implies that information is stored non-locally so that matrix elements of all physical operators vanish. I discuss simplest mathematical models providing the topologically protected quantum bits that are characterized by exponentially weak coupling to the environment. I show that these models can be realized as a low energy sector of some special Josephson junction arrays. I discuss physical effects and additional degrees of freedom that are present in the physical arrays and derive the conditions that should be met for these arrays to reproduce the mathematical models with the topological protection. I will show and discuss recent experimental data obtained on these structures by Rutgers experimental group that are in agreement with theoretical expectations and the ones that are not.
 5 juin Ã  11h LPTMS-BÃ¢t.100-ORSAY Malte Henkel (UniversitÃ© de Nancy) Extensions supersymetriques de l'invariance schroedingerienne Nous considerons les symetries dynamique de l'equation de Schroedinger libre qui, en considerant aussi les masses comme des nouvelles variables, s'etend vers une invariance conforme en d+2 dimensions. Une construction analogue est possible pour les equations Levy/Leblond, qui decrivent une particule non relativiste de spin 1/2. Ensuite, l'extension supersymetrique N=2 de ces equations sera presentee qui meme, respectivement, aux modeles super schroedingeriens' et (3|2)-supersymetrique'. Leur supersymetries dynamiques sont donnes par les superalgebres de Lie osp(2|2)*_s sh(2|2) et osp(2|4). L'algebre de Schroedinger et ses analogues supersymetriques sont les sous-algebres maximales de dimension finie d'une famille des superalgebres de Lie de dimension infinie qui se construisent systematiquement dans le cadre des superalgebres de Poisson. Les algebres Schroedinger-Neveu-Schwarz (sns(N)) avec N supercharges en sont des cas speciaux. En vue des applicaitons, des fonctions a deux point covariants sous des sous-algebres differents sont calculees. Si on a aux moins N=2 supercharges et invariance sous des inversions d'espace-temps, les valeurs des dimensions d'echelle sont fixees.
 29 mai Ã  11h LPTMS-BÃ¢t.100-ORSAY Vladas Sidoravicius (IMPA, Rio de Janeiro) Columnar effects and pinning for a few oriented polymer models We present new results on the effect of a "columnar defect" (i.e. of a perturbation of the environment along a line) on various models related to directed polymers driven by a random two-dimensional Poisson process. Related to these are the so-called "slow-bond problem" for the asymmetric exclusion process, and a new process defined in terms of interacting and annihilating random walks. The main tool used in the study of these problems, even though the specific details differ in each case, is that of "influence percolation", i.e. a way to relate these problems to one of one-dimensional, dependent, long-range percolation.
 22 mai Ã  11h LPTMS-BÃ¢t.100-ORSAY Hugh Montgomery (University of Michigan) Primes in short intervals, zeta zeros, and random matrix theory By using the Hardy-Littlewood prime k-tuple conjecture as a guide, one can develop a conjecture concerning the asymptotics of the higher moments of the number of primes in a short interval (x, x+h] where h is a fractional power of x. The can in turn be interpreted in terms of sums over zeros of the zeta function, where the analogous behavior for sums over eigenvalues of random unitary matrices has been determined by Rains.
 7 mai Ã  11h LPTMS-BÃ¢t.100-ORSAY SÃ©bastien Burdin (Max-Planck-Institut PKS, Dresden, Germany) Universality and energy scale(s) of Kondo systems The general concepts underpinning the single impurity Kondo effect will be introduced, focusing on the universality of the low- temperature Fermi-liquid Kondo regime. This can be contrasted with the behavior of a dense Kondo lattice, for which the Kondo and Fermi liquid temperatures may be different. New results concerning random Kondo alloys will then be presented. These results reveal a crossover between the diluted and dense Kondo regimes, occurring when the magnetic impurity concentration is equal to the density of conducting electrons. The presentation will close with a discussion of other experimental realizations of the Kondo effect, in mesoscopic systems.
 30 avril Ã  11h LPTMS-BÃ¢t.100-ORSAY Raoul Santachiara (LPT ENS Paris) Quantum correlations in 1D critical systems : from random critical spin chains to 1D anyonic fluids I will consider universal quantities in 1D critical quantum systems. In the first part of the talk, I will briefly show the behavior of entanglement entropy for random spin chains. Some recent speculations on possible extensions of the c-theorem to the systems with strong randomness will be discussed. In the second part, I will focus on the recent results on the one-particle correlation function of 1D interacting anyonic systems. The interest in these systems is motivated by the study of the edge excitations in quantum Hall fluids.