Séminaires LPMMC 2012
Léonie Canet (LPMMC)  Détails Fermer 
Critical properties of a growing interface described by the KardarParisiZhang equation le vendredi 14 décembre 2012 à 11:00 

Résumé : The KardarParisiZhang equation is a stochastic nonequilibrium and nonlinear equation initially derived to describe the kinetic roughening of a growing interface. As it has turned out to map to many other important problems, such as directed polymers in random media or Burgers turbulence, it has emerged as a fundamental model to investigate nonequilibrium scaling phenomena and phase transitions. The theoretical understanding of the interface properties in the rough phase, corresponding to a strongcoupling regime, requires nonperturbative methods. I will show, from a detailed analysis of the KPZ symmetries, how to construct an effective action to study the KPZ growth, and derive its NonPerturbative Renormalization Group flow. I will then present results for critical properties in the stationary regime: critical exponents and correlation and response functions. I will show that these functions exhibit generic scaling, determine the associated universal scaling functions and universal amplitude ratios. Liens :Léonie CanetLPMMC 
Pavel Grigoriev (Landau Institute)  Détails Fermer 
Interlayer magnetoresistance in strongly anisotropic quasi2D compounds le vendredi 7 décembre 2012 à 11:00 

Résumé : The angular and field dependence of magnetoresistance (MR) is a very powerful tool to determine the details of electronic spectrum and of Fermisurface geometry of various compounds, which is crucial for understanding their electronic properties. Recently, the magnetic quantum oscillations (MQO) and angular magnetoresistance oscillations were applied to extract the Fermisurface geometry and the electron dispersion in cuprate and pnictide hightemperature superconductors. Both these effects are traditionally used to study the electron dispersion in layered organic metals and most other metallic compounds. As the anisotropy of the compounds increases, the standard 3D theory of magnetoresistance becomes inapplicable, and a new theoretical description is required.

Adriano Amaricci (SISSA)  Détails Fermer 
Nonequilibrium stationary states in the driven Hubbard model le lundi 3 décembre 2012 à 14:00 

Résumé : I shall present a recent work concerning the nonequilibrium dynamics of a strongly correlated electrons systems in a static electric field, with the aim of identifying the conditions to reach a nonequilibrium stationary state (NSS). I show that, for a generic electric field, the convergence to a stationary state requires the coupling to a thermostating bath, absorbing the work done by the external force. By following the realtime dynamics of the system, I also show that coupling to bath provides an essentially sufficient condition, i.e. NSS can be reached for almost any value of dissipation. I characterize the properties of the NSS in terms of some physical observables, pointing out the existence of an analogue of the Pomeranchuk effect. Finally, I map out a phase diagram of the system and I identify a dissipation regime for which steady current is largest for a given field. Liens :Adriano AmaricciSISSA 
Sourav Pal (National Chemical Laboratory, India)  Détails Fermer 
Shape resonance in electron atom and electronmolecule molecule collision: Complex absorbing potential based coupledcluster theory le mercredi 28 novembre 2012 à 11:00 

Résumé : Electronatom and electronmolecule shape resonance leads to metastable resonant states, which become unnormalizable. Thus, bound state quantum chemistry techniques can not be applied. However, using complex absorbing potential and complex scaling, resonant states can be normalized. Using bound state coupledcluster methods to the complex Hamiltonian, resonance energies can be obtained as real part of complex electron affinity and resonance widths can be obtained as imaginary part. Test results to prototype systems are presented. 
Marcelo Rozenberg (LPS Orsay)  Détails Fermer 
Resistive switching in oxides le vendredi 23 novembre 2012 à 11:00 

Résumé : Resistive random access memory (RRAM) composed of a transtition metal oxide dielectric in a capacitor like structure is a candidate technology for next generation nonvolatile memory devices [1]. We introduce a model that accounts for the bipolar resistive switching phenomenom observed in many perovskite transition metal oxides [2]. The numerical study of the model predicts that strong electric fields develop in the highly resistive dielectricelectrode interfaces, leading to a spatially inhomogeneous distribution of oxygen vacancies and a concomitant nonvolatile resistance memory effect. The theoretical results of the model are validated by successful comparison with nontrivial resistance hysteresis loops measured in cuprate YBCO and manganite PCLMO samples. Insights from the model simulations are used to propose a novel multilevel and nonvolatile memory cell. We shall present results for an implementation of a 6bit multileve memory cell device [3]. Liens :Marcelo RozenbergLPS Orsay 
Nicolas Wschebor (Universidad de la Republica, Montevideo)  Détails Fermer 
Going beyond perturbation theory in reactiondiffusion problems le vendredi 16 novembre 2012 à 11:00 

Résumé : The simplest reactiondiffusion problems are considered. They consist on particles of a single species A that can diffuse with diffusion constant D and that can perform annihilation (2A>nothing) or branching (A>2A) or, alternatively, A>3A. It is shown that mean field methods and perturbative expansions around it completely miss some simple properties as the structure of the phase diagram as found by Monte Carlo simulations. It is shown that methods that go beyond perturbation theory allows to quantitatively reproduce Monte Carlo results in a very simple way. Liens :Nicolas WscheborUniversidad de la Republica, Montevideo 
Mathieu Lewin (Université de CergyPontoise)  Détails Fermer 
How much energy does it cost to make a hole in an ideal Fermi gas? le vendredi 9 novembre 2012 à 11:00 

Résumé : The change in energy of an ideal Fermi gas when a local onebody potential is inserted into the system, or when the density is changed locally, are important quantities. In this talk I will explain that the wellknown semiclassical approximation provides a lower bound to the correct quantum mechanical energy of the perturbed Fermi sea, up to a universal constant. This work generalizes a famous estimate of Lieb and Thirring, in the vacuum.
It is a collaboration with R. Frank, E.H. Lieb (Princeton, USA) and R. Seiringer (McGill, Montréal, Canada). 
JeanFrançois Joanny (Institut Curie)  Détails Fermer 
Mechanics and instabilities of healthy and cancerous tissues le vendredi 26 octobre 2012 à 11:00 

Résumé : In this talk, we present some theoretical and experimental results on the growth and mechanical properties of healthy and cancerous tissues. 
Ng Hui Khoon (University of Singapore)  Détails Fermer 
Transport in 1D revisited: A simple, exact solution for phase disorder le vendredi 19 octobre 2012 à 11:00 

Résumé : I will revisit the baby problem of singlechannel scattering in a onedimensional chain of scatterers, with uniform phase disorder. I will present a simple, exact analytical solution that applies even in the strong scattering limit. This permits the easy computation of all moments of the conductance and resistance, and I will show how one obtains answers wellknown from weakscattering limit from our exact solution. I will also discuss features in the probability distribution for the conductance missing from the usual weak scattering treatment. This is work done together with Berge Englert from the Centre for Quantum Technologies, Singapore. 
Pablo Cornaglia (Bariloche Atomic Center, Argentina)  Détails Fermer 
Tunable spin and charge Seebeck effects in magnetic molecular junctions le vendredi 12 octobre 2012 à 11:00 

Résumé : The increasing interest in the thermoelectric properties of materials and the quest for high Seebeck coefficients is motivated by the promise of more efficient solid state refrigerators and the conversion of waste heat into electricity. The Seebeck effect refers to the generation of a charge current (or a voltage drop) by a temperature gradient applied across a metal, and the spinSeebeck effect, concerns the thermal generation of pure spin currents. The recent experimental observation of the Seebeck effect in different nanostructures, in particular in molecular junctions and quantum dots, opened new routes to study these phenomena. On the one hand, the high sensitivity of these systems to external fields, their scalability and tunability make them potential candidates for a variety of technological applications. On the other hand, thermoelectric and thermomagnetic effects provide a unique probe of electron correlation effects and are a useful tool to gain further insight on fundamental problems like the Kondo regime where the energy transfer is dominated by spin fluctuations.
I'll present results for the charge and spin Seebeck effects of a spin1 molecular junction as a function of temperature (T), applied magnetic field (H), and molecular magnetic anisotropy (D) obtained using Wilson's numerical renormalization group [1]. A hardaxis magnetic anisotropy produces a large enhancement of the charge Seebeck coefficient Sc (~ k_{B}/e) whose value only depends on the residual interaction between quasiparticles in the low temperature Fermiliquid regime. In the underscreened spin1 Kondo regime, the high sensitivity of the system to magnetic fields makes it possible to obtain a sizable value for the spin Seebeck coefficient even for magnetic fields much smaller than the Kondo temperature. Similar effects can be obtained in C60 junctions where the control parameter is the gap between a singlet and a triplet molecular state. I'll also discuss briefly the thermoelectric properties of an SU(4) Kondo resonance, that describes the low temperature transport through clean C nanotubes [2].
Liens :Pablo CornagliaBariloche Atomic Center, Argentina 
Giancarlo Strinati (University of Camerino)  Détails Fermer 
Density and Spin Response of a Fermi Gas in the Attractive and QuasiRepulsive Regime le vendredi 5 octobre 2012 à 11:00 

Résumé : Recent experimental advances in ultracold Fermi gases allow for exploring response functions under different dynamical conditions. In particular, the issue of obtaining a quasirepulsive regime starting from a Fermi gas with an attractive interparticle interaction while avoiding the formation of the twobody bound state is currently debated. Here, we provide a calculation of the density and spin response for a wide range of temperature and coupling both in the attractive and quasirepulsive regime, whereby the system is assumed to evolve nonadiabatically toward the upper branch of the Fermi gas. A comparison is made with the available experimental data for these two quantities. 
Denis Basko (LPMMC, Grenoble)  Détails Fermer 
Chaos and transport in disordered classical nonlinear chains le vendredi 28 septembre 2012 à 11:00 

Résumé : Chaotic behavior is the reason for destruction of Anderson localization by a weak nonlinearity in a classical system. A disordered onedimensional chain of coupled weakly anharmonic classical oscillators is one of the simplest models where this effect can be studied. I will analyze the probability for appearance of chaos in such chains, and then I will discuss the consequences of chaos for transport of conserved quantities along the chain. Liens :Denis BaskoLPMMC, Grenoble 
Joakim Bergli (University of Oslo)  Détails Fermer 
Relaxation processes in Coulomb glasses le vendredi 21 septembre 2012 à 14:00 

Résumé : Coulomb glasses are materials with electron states localized by the disorder under conditions of longrange interactions between their particles. One realization of a Coulomb glass is a doped semiconductor at low temperatures. Another example is granular metals. Coulomb glasses show complex dynamics typical for other complex systems: sluggish, nonexponential, relaxation of the conductance as well as aging and memory effects similar to those observed in structural glasses. We report dynamical Monte Carlo simulations of relaxation processes in a Coulomb glass. Both the relaxation to equilibrium following an initial temperature quench and during and after a driving by a strong current is studied. We see that out of equilibrium there is an effective electron temperature established on a short timescale, and this relaxes slowly to the bath temperature. We also study the response of the system to an external perturbation and observe how it relaxes after such a perturbation. Both from a random state and after a perturbation from equilibrium we find that the effective temperature relaxes logarithmically. 
Abhiram Soori (Indian Institute of Science, Bangalore, India)  Détails Fermer 
Conductance in systems of TomonagaLuttinger Liquid systems with resistances le jeudi 20 septembre 2012 à 14:00 

Résumé : Conductance of quantum wires connected to Fermi liquid leads has been extensively studied in literature. In such 1dimensional systems, effects of interactions are taken into account by the technique of Bosonization. We have developed a phenomenological formalism that allows us to study the effect of resistances in quantum wires[1]. Rayleigh dissipation function is combined with the technique of Bosonization to model the power dissipation. We use this formalism to study the conductance and the power dissipated in systems of a quantum wire and junction of three quantum wires connected to Fermi liquid leads. Further, we combine two Yjunctions to study the conductance of a parallel combination of resistances[2]. We find that the effective resistance of a parallel combination is quite different from that of its classical counterpart. Liens :Abhiram SooriIndian Institute of Science, Bangalore, India 
Vincenzo Savona (École polytechnique fédérale de Lausanne)  Détails Fermer 
The superfluidinsulator phase diagram of 1D weakly interacting bosons in a disorder potential le vendredi 14 septembre 2012 à 11:00 

Résumé : A onedimensional system of noninteracting particles in presence of disorder is always in an insulating state at zero temperature. Interactions can induce a quantum phase transition to a superfluid state, characterized by quasi longrange order. This Boseglass to superfluid transition has been the subject of intense theoretical studies and of several recent experiments carried out on ultracold atomic clouds. I will present a theoretical study of the weakly interacting Bose gas at zero temperature in a disorder potential with finite spatial correlation length. To leading order in the interaction strength, the boundary between the superfluid and insulating phases can be determined from a symmetrybreaking approach: the extended Bogolyubov model. This approach accounts correctly for diverging low energy phase fluctuations  that occur in a lowdimensional system and are ultimately responsible for the suppression of superfluidity. In this context, the phase diagram on the interactiondisorder plane (U,D) can be characterized by inspecting the longrange behaviour of the onebody density matrix as well as the drop in superfluid fraction. It turns out in particular that the phase boundary between the two phases follows two different power laws on the phase diagram in the whitenoise and ThomasFermi limits respectively. This feature is peculiar of a spatially correlated disorder and is expected to govern the behaviour of disordered ultracold atomic clouds in current experiments. The insitu density profile is perhaps the feature of an ultracold atomic cloud that can be most easily measured in an experiment. I will show that a direct link exists between the fragmentation of the density profile and the occurrence of the phase transition. This link is given by the probability distribution of the gas density. In particular, the appearance of a superfluid fraction coincides with a vanishing probability distribution in the limit of zero density, namely with the disappearance of fragmentation. This analysis sets the intuitive relation between fragmentation and insulating behaviour into a rigorous framework, and opens the way to the experimental detection of the phase transition. Liens :Vincenzo SavonaÉcole polytechnique fédérale de Lausanne 
Charles Mathy (Department of physics, Harvard)  Détails Fermer 
Quantum flutter of supersonic particles in onedimensional quantum liquids le jeudi 31 mai 2012 à 11:00 

Résumé : I will present exact results on the dynamics of an impurity injected at a supersonic velocity into a onedimensional quantum liquid [1]. Two main surprising results are revealed in the analysis. Firstly, the momentum of the impurity does not decay to zero, instead it forms a strongly correlated state that propagates at a reduced velocity. Secondly, the system undergoes coherent oscillations in which the impurity vibrates with respect to its correlation hole. These oscillations, which we call quantum flutter, have their origin in a longlived coherent quantum superposition of two families of quasiequilibrium states in the system. These results provide the first example of new physics due to supersonic propagation in a strongly coupled nonrelativistic manybody system. Furthermore, the main features are robust to changes in parameters of the system, suggesting that they may be a generic feature of onedimensional quantum systems. 
Giovanni Lombardi (Département de physique, Université de Padova, Italie)  Détails Fermer 
Itinerant magnetism in 2D ultracold Fermi atoms with SpinOrbit coupling: variational approach le jeudi 10 mai 2012 à 11:00 

Résumé : The exploration of Rashba spinorbit (SO) interaction is stimulated by its increasing interest for spintronic applications. The first experimental evidences of SO interactions have been found in semiconductor devices. We investigate the behaviour of a 2D ultracold Fermi gas in presence of a Rashbalike spinorbit coupling. At ultralow temperatures the fermions interact via contact (repulsive) interactions. We determine the ground state of the system perturbatively up to second order terms in the interaction strength. First we employ the functional integral method that enables us to highlight clearly the similarities with the well known (3D) Stoner model. Then, to overcome some difficulties related to the dimensionality, we analyse the behaviour of the 2D system with a variational approach, both without and with the SO coupling. We focus in particular on the aspects related to the magnetization, and as main result we show how the presence of the Rashba coupling frustrates the emergence of a magnetized phase. Liens : 
Charles Poli (Instituto de Física, Universidad Autónoma de Puebla)  Détails Fermer 
Chaos Ondulatoire en milieux ouverts : une généralisation de la distribution de Wigner le mardi 24 avril 2012 à 11:00 

Résumé : Initialement introduite en 1951 par E. Wigner pour décrire le spectre d’énergie de noyaux lourds, la distribution des écarts entre plus proches niveaux a par la suite été intensivement utilisée pour décrire les propriétés statistiques de systèmes ondulatoires chaotiques. Cependant, la distribution de Wigner, basée sur l’hypothèse forte d’un système fermé, ne s’applique plus pour les systèmes ouverts, c’estàdire des systèmes pour lesquels le couplage avec l’extérieur ne peut pas être négligé. En reprenant l’approximation du modèle à 2 niveaux utilisé par Wigner et en prenant explicitement en compte le couplage avec l’extérieur, je présenterais une expression analytique exacte de la distribution des écarts dans le cas particulier d’un canal de perte. Puis, je montrerais qu’en considérant la force de couplage comme un paramètre libre cette distribution est aussi une excellente approximation dans le cas d’un nombre quelconque de canaux de pertes. Ainsi, l’ensemble de ces résultats, appuyé par des simulations numériques et des données expérimentales, peut être vu comme une généralisation de la distribution de Wigner aux systèmes ouverts. 
Marie Piraud (Institut d'optique, Orsay)  Détails Fermer 
Matter wave transport and Anderson localization in anisotropic 3D disorder le mardi 10 avril 2012 à 11:00 

Liens :Institut d'optique, Orsay 
Martina Flöser (Institut Néel)  Détails Fermer 
Transport and Classical Percolation in the High Temperature Regime of the Quantum Hall Effect le jeudi 5 avril 2012 à 11:00 

Résumé : The quantized Hall resistance plateaus observed at high magnetic fields in twodimensional electron gases are known to vanish rapidly with increasing temperature. We wish to show that dissipative transport remains however nontrivial at high temperatures, as it is dominated by classical percolation physics. For this purpose, we use a diagrammatic formalism based on a local conductivity approach to investigate the interplay of inelastic phonon scattering with the random drifting of electronic trajectories [1,2]. This provides a microscopic derivation of the universal transport critical exponent in the hightemperature regime of quantum Hall transitions, that was up to now only conjectured from qualitative geometrical arguments.
We also derive microscopic expressions for the dependences in temperature and magnetic field of the longitudinal conductance. This allows us to test our predictions with recent experiments and to extract the transport critical exponent from experimental data [3].
Liens :Institut Néel 
Gregory Berkolaiko (Maths Department, Texas A & M University)  Détails Fermer 
Universality in chaotic quantum transport: the concordance between random matrix and semiclassical theories le mardi 6 mars 2012 à 11:00 

Résumé : Electronic transport through chaotic quantum dots exhibits universal, system independent properties which are consistent with random matrix theory. The observable quantities can be expressed, via the semiclassical approximation, as sums over the classical scattering trajectories. Correlations between such trajectories are organized diagrammatically and have been shown to yield universal answers for some observables. We develop a general combinatorial treatment of the semiclassical diagrams by relating them to factorizations of permutations. Taking previously calculated answers (Heusler et al, 2006) for the contribution of a given diagram, we prove agreement to all orders between the semiclassical and random matrix approaches for all moments (linear or nonlinear) of the transmission amplitudes for systems with and without time reversal symmetry. This explains the mathematics behind the applicability of random matrix theory to chaotic quantum transport. The streamlined calculation could also pave the way for inclusion of nonuniversal effects. Liens :Gregory BerkolaikoMaths Department, Texas A & M University 
Zoran Ristivojevic (LPT, ENS Paris)  Détails Fermer 
Interacting electrons in one dimension beyond the Luttinger liquid paradigm: relaxation rates and transport le mardi 24 janvier 2012 à 11:00 

Résumé : In contrast to higher dimensional systems where pair collisions provide finite relaxation rate and lifetime, the situation in one dimension is peculiar. A onedimensional electron gas requires threeparticle collisions for finite relaxation due to constraints imposed by the conservation laws. At zero temperature the fastest relaxation is provided by the interbranch processes which enable energy exchange between counterpropagating particles. At sufficiently high temperatures the leading mechanism is due to the intrabranch scattering of comoving electrons. We derive the corresponding relaxation rates that strongly depend whether one considers screened or unscreneed Coulomb interaction. The abovementioned relaxation processes are responsible for interactioninduced modifications of electrical and thermal conductance in quantum wires. Our approach is based on the Boltzmann equation that is beyond the Luttingerliquid theory 
Vitaly Golovach (LPMMC)  Détails Fermer 
Spinselective tunneling in quantum dots confining holes le mardi 17 janvier 2012 à 11:00 

Résumé : Holes confined in SiGe nanocrystals have recently been studied in
dc transport experiments [1]. Motivated by the experimental work,
I will consider the tunneling of a charge carrier from a metallic
lead into the valence band of the semiconductor. In a magnetic
field, the spinorbital splitting of the valence band gives rise
to spinselective tunneling. I will give a working explanation of
the phenomenon based on an expansion of the Luttinger Hamiltonian
around the 2D limit. I will formulate a simple model for the tunnel
contact (the Zmodel), in which the effect of the spin selectivity
is maximally strong. Our results demonstrate that spinselective
tunneling in semiconductor nanostructures can be achieved without
the use of ferromagnetic contacts. This effect can be used to
induce spin currents and measure Rabi oscillations in singledot
devices with strong spinorbit interaction. Liens :Vitaly GolovachLPMMC 