Séminaires de l’année 2006

19 décembre à 11h LPTMS-Bât.100-ORSAY
Julia Meyer (Ohio State University)
Electron-electron interaction effects in quasi-one-dimensional quantum wires
Due to their quasi-one-dimensional structure, the conductance of quantum wires is expected to be quantized in units of the conductance quantum G0=2e2/h. However, a number of recent experiments report deviations from perfect conductance quantization, such as the so-called 0.7-anomaly observed below the first plateau. These experiments have stimulated much theoretical interest in the physics of one-dimensional conductors not captured by the Luttinger-liquid theory. The talk concentrates on the transition from a one-dimensional to a quasi-one-dimensional state. In the absence of interactions between electrons, this corresponds to filling up the second subband of transverse quantization. On the other hand, strongly interacting one-dimensional electrons form a Wigner crystal, and the transition corresponds to it splitting into two chains (zigzag crystal). The evolution of the system and the electronic excitation modes in the vicinity of the transition are studied as the interaction strength changes [1]. Furthermore, the spin properties in the Wigner crystal regime are addressed. While the spin properties of the one-dimensional crystal are relatively simple, we find several interesting spin structures in the zigzag crystal [2]. In particular, the spontaneous spin polarization of the ground state, which was proposed as a possible origin of the conductance anomalies, is realized in a certain range of electron densities. [1] J.S. Meyer, K.A. Matveev, and A.I. Larkin, preprint cond-mat/0612101. [2] A.D. Klironomos, J.S. Meyer, and K.A. Matveev, Europhys. Lett. 74, 679 (2006).
12 décembre à 11h LPTMS-Bât.100-ORSAY
Philippe Lecheminant (Cergy)
Atomes froids fermioniques de spin à une dimension : une approche de basse énergie.
La possibilité de synthétiser des gaz de fermions piégés dans des réseaux optiques représente un développement important pour la physique de la matière condensée avec notamment la perspective d'étudier des phases quantiques exotiques prévues dans le contexte des systèmes d' électrons fortement corrélés. Une question importante est l'effet des degrés de liberté internes supplémentaires, naturellement obtenus en combinant le spin nucléaire I et le spin électronique S en un état hyperfin de moment angulaire total F1/2, sur des systèmes fermioniques. Dans ce séminaire, nous discuterons du diagramme de phase à une dimension des systèmes de fermions froids de spin hyperfin général F1/2 en utilisant une approche de théorie des champs conformes et des calculs numériques (Monte Carlo quantique et DMRG) pour le cas F=3/2. En particulier, nous verrons que la physique obtenue est très différente du cas F=1/2 avec en particulier la possibilité de formation d'une phase superfluide de type moléculaire comme par exemple une phase quartet, état lié de paires de Cooper, pour le cas F=3/2.
5 décembre à 11h LPTMS-Bât.100-ORSAY
Emmanuel Trizac (LPTMS)
Painleve classification, counter-ion condensation and some physical properties of bio-polymers
Onsager-Manning condensation is a manifestation of the long range character of the Coulombic potential in 2D geometry. This phenomenon is the cornerstone of our understanding of bio-polymers. Beyond a charge density threshold, counterions "condensate" onto the charged polymer. With salt (added electrolyte), which is the experimentally relevant situation,the phenomenon is more complex and has received little attention. We will show that mathematical advances from the 1980s allow to obtain analytical results in a large range of salt content. We will in particular discuss the implications of this work for the Coulombic elasticity of bio-polymers through their persistence length.
29 novembre à 14h30 LPTMS-Bât.100-ORSAY
Zoran Hadzibabic (LKB, ENS, Paris)
Bose gas in Flatland
Physics of a Bose gas in 2D is quite different from the usual 3D situation. In a homogeneous 2D fluid of identical bosons long-range order is always destroyed by long wavelength thermal fluctuations, but the system can nevertheless become superfluid at a finite critical temperature. This phase transition does not involve any symmetry breaking and in the Berezinskii-Kosterlitz-Thouless (BKT) paradigm it is explained in terms of binding and unbinding of pairs of vortices with opposite circulations. Above the critical temperature, proliferation of unbound vortices is expected. In a trapped atomic gas, the harmonic confinement can make the superfluid phase more robust, and the nature of the low temperature state is a topic of some theoretical debate. Using optical lattice potentials we can create two parallel, independent 2D atomic clouds with similar temperatures and chemical potentials. When the clouds are suddenly released from the trapping potential and allowed to freely expand, they overlap and interfere. This realizes a matter wave heterodyning experiment which gives direct access to several features of the phase distributions in the two planes. Long wavelength phase fluctuations create a smooth and random variation of the interference fringes and free vortices appear as sharp dislocations in the interference pattern. Temperature study of these effects supports the BKT picture of the development of quasi-long-range coherence in these systems. However, there are still many open questions related to the superfluidity of the sample and the effects of the trapping geometry.
28 novembre à 11h LPTMS-Bât.100-ORSAY
Michael Kearney (University of Surrey)
Scaling in compact cluster growth models and related queueing systems
Consideration is given to analysing the scaling behaviour of various compact cluster growth models whose underlying structure relates to the enumeration of partition polygons, stack polygons and staircase polygons. In particular, using generating function techniques the asymptotics of the cluster size distribution is established in each case. Through simple mappings it is shown how the results provide new insights into several well known queueing systems as regards the integrated waiting time incurred during a busy period. Physical examples of interest include traffic jams and Abelian sandpile (avalanche) models in the compact directed percolation universality class.
21 novembre à 11h LPTMS-Bât.100-ORSAY
Boris Narozhny (ICTP Trieste)
Interaction effects in low-dimensional electron systems.
Recent advances in semiconductor and nano-technology allow manufacturing of a wide variety of novel low dimensional and hybrid systems. These systems can be made as small as few nanometres or comprise several layers of two-dimensional electron gases. These systems are widely used to study thermodynamic and transport properties of electrons in low dimensions with the focus on the interplay between disorder and electron-electron interaction, as well as the mesoscopic effects. In this talk I will describe theoretical ideas aimed at constructing a theory, which would take into account the effects of electron-electron interaction and disorder on equal footing and would be applicable to most experimental systems currently available. The point is that long distance physics is independent of the details of impurity potentials. Thus such theory should be able to capture the basic physics of disordered systems, while at the same time could be adapted to particular geometries of systems of interest. Focusing on few examples, I will describe theoretical methods used to construct such a theory and in particular show how seemingly different approaches can be used in a complementary fashion in order to understand complex phenomena beyond applicability limits of well-established theories.
15 novembre à 14h30 LPTMS-Bât.100-Orsay
Giuliano Orso (LPTMS)
1D attractive Fermi gases with unequal spin populations
We investigate two-component attractive Fermi gases with density imbalance in trapped quasi-1D configurations. The ground state properties are determined within local density approximation, starting from the exact Bethe-ansatz equations for the homogeneous case. We predict that the atoms are distributed according to a two-shell structure with a partially polarized phase in the center of the trap and either a fully paired or a fully polarized phase in the wings. The size of the cloud as well as the critical polarization needed to suppress the fully paired shell, are calculated as a function of the coupling strength. We compare our results with the 3D case, where experimental data are already available.
30 octobre à 14h30 LPTMS-Bât.100-ORSAY
Avinash Khare (Institute of Physics, Bhubaneswar, India)
Linear Superposition for Nonlinear Equations and New Identities for Jacobi Elliptic Functions
In this talk I shall show that a kind of linear superposition works for many nonlinear equations in case their periodic solutions are in terms of Jacobi elliptic functions. This happens because Jacobi elliptic functions satisfy novel identities, many of which were not known before. The structure and the derivation of these identities will be briefly discussed. I will also demonstrate that exact solutions of several discrete nonlinear equations (including discrete Nonlinear Schrodinger equation) can be obtained by using these identities.
24 octobre à 11h LPTMS-Bât.100-ORSAY
Vincent Pasquier (SPhT, Saclay)
Une déformation des nombres de Razumov-Stroganov
Après une introduction aux conjectures de Razumov et Stroganov, je présenterai une approche du problème utilisant les représentations polynomiales du groupe des tresses et de leur extension affine. Je montrerai que ces polynomes sont des déformations de fonctions d'ondes de l'effet Hall quantique et qu'ils sont caractérisés par des conditions d'annulations. Je conclurai par de nouvelles conjectures qui permettront peut-être de relier ce sujet à d'autres domaines comme les invariants de Gromov-Witten, la conjecture n!....
10 octobre à 10h30 SEMINAIRE EXCEPTIONNEL DU LPTMS EN L'HONNEUR DE HUBERT KRIVINE
Jean-Pierre Kahane (Departement de Mathematiques, Orsay)
Retrospective sur Fourier et les questions qui s'y rattachent
Fourier est l'un des noms propres les plus fréquents dans la littérature scientifique. La rétrospective montrera qu'il a été longtemps mésestimé en France. Actuellement il n'en est plus ainsi,et on s'interrogera pourquoi. Sa vision de la science nous est plus proche, et les voies qu'il a ouvertes s'avèrent fructueuses, qu'il s'agisse de notions générales (le flux en physique, l'intégrale définie en mathématiques), de l'équation de la chaleur, ou des séries de Fourier.
3 octobre à 11h LPTMS-Bât.100-Orsay
Xavier Waintal (SPEC Saclay)
Fusion quantique d'un cristal
Les électrons dans les métaux (trois dimensions) ou dans les heterostructures semi-conductrices (deux dimensions) sont habituellement à l'état liquide (liquide de Fermi). A basse densité cependant, on observe une transition vers une phase cristaline (cristal de Wigner). Cette transition a longtemps été considérée comme une simple transition du premier ordre. A l'aide d'une fonction d'onde variationnelle et de calculs de Monte-Carlo quantique, nous avons montré que la physique autour de la transition est en fait beaucoup plus riche: comprendre la fusion du cristal (espace réel), c'est aussi comprendre la fusion (dans l'espace réciproque) de la surface de Fermi; le point de transition étant en quelque sorte le point "d'incertitude maximum". Le système se trouve ainsi dans un état hybride, ni cristal ni liquide, que nous avons chercher à caractériser.
7 juillet à 11h LPTMS-Bât.100-ORSAY
Ivan Deutsch (University of New Mexico)
Coherence control of ultra-cold atoms
17 mai à 14h30 LPTMS-Bât.100-ORSAY
Hans-Peter Buchler (University of Innsbruck)
Atomic quantum simulator for lattice gauge theories and ring exchange models
We present the design of a ring exchange interaction in cold atomic gases subjected to an optical lattice using well understood tools for manipulating and controlling such gases. The strength of this interaction can be tuned independently and describes the correlated hopping of two bosons. We discuss a setup where this coupling term may allows for the realization and observation of exotic quantum phases, including a deconfined insulator described by the Coulomb phase of a three-dimensional U(1) lattice gauge theory.
2 mai à 11h LPTMS-Bât.100-ORSAY
Jon Keating (University of Bristol)
A Different Orthogonal/Symplectic-to-Unitary transition in RMT
I will discuss spectral statistics of random orthogonal and symplectic matrices as a function of position in the spectrum. I will focus in particular on how these behave when one is far from spectral symmetry points. And I will describe some applications in number theory.
11 avril à 11h LPTMS-Bât.100-ORSAY
Cristina Toninelli (LPTMS)
Rigorous results for relaxation times of finite dimensional models with glassy dynamics
We present a new analytical technique which allows to study the long-time behavior for some finite dimensional models with glassy dynamics. We apply this tool to derive new and rigorous results for the dynamics of some kinetically constrained models which are relevant for liquid/glass and jamming transitions. In particular, we derive the size and density dependence of relaxation times, the asymptotic form of correlation and persistence functions and explain how one can establish a firm connection between time-scales and dynamical heterogeneties for cooperative models of fragile liquids.
5 avril à 14h30 LPTMS-Bât.100-ORSAY
Temo Vekua (LPT Strasbourg et Tbilissi Georgie)
Quantum properties of zigzag chains in magnetic field
We will review recent theoretical (analytical as well as numerical) results (magnetic phase diagram) for a specific one-dimensional frustrated quantum magnet, the spin-1/2 chain with frustrating antiferromagnetic next nearest neighbor exchange J2 ('J1-J2' chain). This model can be alternatively described as two separate antiferromagnetic chains which are coupled by a zig-zag like ferromagnetic exchange. The system is in the class of geometrically frustrated low dimensional quantum spin systems which are currently under intensive investigations both by theoretical as well as by experimental groups (for both ferromagnetic as well as antiferromagnetic $J_1$). From the theoretical point of view models of this type are highly challenging because of the interplay of several competing interactions which drastically complicate the analysis and often give rise to unconventional phases. Due to the reduced dimensionality quantum fluctuations are enhanced and standard perturbative expansions (e.g. the spin wave expansion) fail. As a result many of the interesting problems in the regime of strongly correlated electrons are dominated by frustration and their solution requires truly nonperturbative approaches. The talk will mainly be on effective field theoretical methods based on non linear sigma model approach and two distinct and in a sense complementary bosonization procedures, (i) the modern form of the Luther-Peschel bosonization formalism and (ii) the mapping to dilute gas of hard core bosons.
4 avril à 14h30 LPTMS-Bât.100-ORSAY
Sergey Matveenko (Institut Landau, LPTMS)
Instanton theory of subgap interchain tunneling in quasi 1D conductors
We present our (with S. Brazovskii) theory of internal coherent tunneling in the pseudogap region where the applied voltage is below the free electron gap. We address quasi 1D systems where the gap is originated by spontaneous lattice distortions (Peierls effect) like in CDWs or in polyacetylene, as well as generically gapful systems like conjugated polymers, semiconducting nanotubes and quantum wires of semiconductors. Their common property is a deep selftrapping of electrons and their pairs into solitons, polarons, bipolarons. The instanton approach allows to calculate the interchain tunneling current both in single electron (polarons) and bi-electron (bipolarons, solitons pairs) channels.
29 mars à 14h30 LPTMS-Bât.100-ORSAY
Peter Schlagheck (Université de Regensburg)
Resonance-and chaos-assisted tunneling
We present a quantitative semiclassical theory of dynamical tunneling in quantum systems with mixed regular-chaotic classical phase space structure. The theory applies to the tunneling-induced level splitting between near-degenerate states that are localized on a pair of symmetry-related regular islands in phase space, and is based on the presence of a prominent nonlinear resonance which induces the coupling of the regular states to the chaotic domain. Good agreement between this semiclassical approach and the exact quantum tunneling rates is obtained in various one-dimensional systems that are subject to periodic driving. We discuss the extension of our approach to open systems and show that the decay rates of nondispersive wave packets in driven hydrogen can thereby be reproduced.
21 mars à 11h LPTMS-Bât.100-ORSAY
Shmuel Fishman (Technion)
Quantum Chaos, Resonances and Atom Optics: From Experiments to Number Theory
In the field of Quantum Chaos, the quantum dynamics of systems that are chaotic in the classical limit is explored. The Kicked Rotor is the standard model for the exploration of such systems with time dependent Hamiltonians. It was first realized for laser cooled atoms by Mark Raizen and coworkers at Austin, Texas. Since then various aspects of Quantum Chaos were studied for such systems. Resonances are among the most typical manifestations of quantum mechanical behavior and are very sensitive to values of parameters (for example in standard atomic spectroscopy). In 1999 resonances, that are stable with respect to variation of parameters, were discovered experimentally by the Oxford group for the dynamics of laser cooled Cesium atoms when driven in the direction of gravity. A theoretical explanation of this surprising observation, based on a novel pseudo-classical limit, will be presented. The relation between quantum and classical resonances will be discussed. Theoretical predictions that were verified experimentally by several groups will be presented. In particular it is found that the experimental results depend on the number theoretical properties of the values of parameters. The theory makes use of invariance properties of the system, that are similar to the ones of solids, of scaling theory as well as of the theory of dynamical systems.
14 mars à 11h LPTMS-Bât.100-ORSAY
Paul Zinn-Justin (LPTMS)
Combinatorial properties of integrable stochastic processes
We consider a class of quantum integrable models which can be reformulated as (one-dimensional) Markov processes. The simplest example is the XXZ spin chain at anistropy parameter Delta=-1/2, which is equivalent to a Markov process on configurations of arches (pairings of points on a line). A number of conjectures concerning the equilibrum state have been formulated in recent work; an overview will be given of the current status on these conjectures and the methods to prove some (if not all) of them. The connection to some areas of mathematics (combinatorics, algebraic geometry) will be sketched.
7 mars à 11h LPTMS-Bât.100-ORSAY
Markus Buttiker (University of Geneva)
Two-particle phase of shot noise correlations
The quantization of charge and quantum diffraction leads to current fluctuations known as shot noise. Of particular interest are correlations of currents at different contacts of a conductor. Such correlations are a signature of two particle processes. We discuss a two-particle Aharonov-Bohm effect under conditions when there is no single particle Aharonov-Bohm effect. Observation of the two particle Aharonov-Bohm effect permits a violation of a Bell inequality. We investigate two-particle phases in correlations of dynamically excited electron-hole pairs. Excitation of electron-pairs with out-of-phase voltages applied to two contacts permits to trace out the two particle-phase in a four-terminal conductor. We give the statistical distribution of the two-particle phase for a chaotic cavity.
28 février à 11h LPTMS-Bât.100-ORSAY
Didina Serban (SPhT, CEA Saclay)
Integrability in the planar N=4 gauge theory and the Hubbard model
It was established recently that a certain integrable long-range spin chain describes the dilatation operator of N=4 gauge theory in the su(2) sector to at least three-loop order, while exhibiting BMN scaling to all orders in perturbation theory. I shall explain how this spin chain can be obtained as an approximation to an integrable short-ranged model of correlated fermions, namely the Hubbard model.
23 février à 10h LPTMS-Bât.100-ORSAY
Anna Posazhennikova (Karlsruhe)
Conductance of a spin-1 quantum dot: two stage Kondo effect
In the talk we discuss the low-temperature transport of a spin-1 quantum dot (QD), which is governed by a Kondo physics. The presence of a large spin of the dot leads to the multi-channel Kondo effect (in our case 2 channels). First we discuss the conductance of such a dot, when one channel is decoupled. Interestingly, this situation is then described in terms of underscreened Kondo effect and results in a singular behaviour of conductance. In case of two channel, complex interference effects arise and we discuss them for the case of elastic scattering and show how conductance gets suppressed at low temperatures. For the spin we use Schwinger boson representation and make the calculations within the large-N approximation.
22 février à 14h30 LPTMS-Bât.100-ORSAY
Sergio Gaudio (Los Alamos)
Consequences of induced interactions in two component atomic Fermi gases to s-wave resonances : Is a triplet superfluid possible ?
In this talk, I will demonstrate that starting with a two component Fermi gas in the normal phase and in the quantum degenerate regime, where direct p-wave pairing is suppressed, a tripled interaction may still arise due solely to quantum fluctuations, close to the unitarity regime. I will then answer the question whether this interaction is strong enough to drive the system into a triplet superfluid transition within temperatures obtainable in cold atom traps.
14 février à 11h LPTMS-Bât.100-ORSAY
Frédéric Hospital et Olivier Martin (LPTMS)
Recombinaison et statistique multilocus en génétique
En génétique, les croisements sont utilisés pour établir des cartes génétiques, pour faire de l'inference d'association et pour détecter des QTL. On fera un panorama de ces questions qui tournent toutes autour de la recombinaison. Puis on se focalisera sur le problème de fréquences multilocus pour différents types de croisement et les méthodes algorithmiques que nous avons développé.
8 février à 15h LPTMS-Bât.100-ORSAY
Dimitry Gangardt (LPTMS)
Long range correlations in Calogero Sutherland Model
In this talk I present recent progress in understanding correlation properties of exactly solvable Calogero Sutherland model of 1D particle interacting with inverse square potential. In particular, the long-range diagonal and off-diagonal correlations can coexist in some range of interactions and thus make this model partcularly interesting for studies of the super-solid phase.
31 janvier à 11h LPTMS-Bât.100-ORSAY
Léonie Canet (Physics Department, Manchester)
Systèmes hors de l'équilibre et groupe de renormalisation non perturbatif
L'étude des phénomènes critiques dans les systèmes loin de l'équilibre thermique --- où les relations de bilan détaillé sont violées --- a montré que ceux-ci s'avèrent beaucoup plus riches qu'à l'équilibre. Néanmoins, leur compréhension en reste beaucoup plus vague, principalement parce que les outils qui ont mené à la classification complète des comportements critiques à l'équilibre (invariance conforme, groupe de renormalisation (RG)) ne s'appliquent plus ou donnent des résultats limités. Nous allons nous intéresser à, d'une part, des processus de réaction-diffusion et d'autre part, des phénomènes de croissance, que nous explorerons par une approche non perturbative (le NPRG). Nous allons montrer qu'en effet, les aspects non perturbatifs s'avèrent jouer un rôle crucial dans ces systèmes. Par exemple, cette approche va nous permettre de caractériser la transition de la classe d'universalité dite PC (parite conservée) jusqu'alors inaccessible en théorie de perturbation. Nous allons montrer que de plus, même pour la classe d'universalité la plus répandue dans les processus de réaction-diffusion (celle de la percolation dirigée), l'existence même d'une transition ne peut être déterminée que non perturbativement. Enfin, l'étude de l'équation de Kardar-Parisi-Zhang par le NPRG va nous permettre de mettre en évidence des effets non triviaux, comme l'existence de non-analycites, qui conditionnent fortement ses propriétés critiques.
24 janvier à 11h LPTMS-Bât.100-ORSAY
Lev Ioffe (Rutgers University)
Quantum error correction by hardware in Josephson junction networks.
I argue that any realization of macroscopic quantum computer requires that individual logical qubits are realized as quantum systems with degenerate ground states which are protected from the effects of the environment. This protection can be achieved by a highly redundant system, similarly to a classical error correction in the noisy environment in which one checks for errors comparing values of different bits at regular intervals and correct the erroneous ones. Alternatively, the protection can be achieved by the proper choice of the Hamiltonian that automatically projects the quantum state of a large many body system into the error-free subspace. Because the quantum errors can happen both in phase and in amplitude the quantum error correction of the first type requires a huge overhead that makes it rather unrealistic for the existing physical implementations of logical qubits. In contrast, the Hamiltonian approach is not too difficult to realize in a broad class of models. The real challenge is to find the feasible implementation of these models. I will show that the novel class of Josephson array with non-trivial symmetry or topology satisfies these requirements. For these "topologically protected" arrays the effect of noise is exponentially small in array size.
17 janvier à 11h LPTMS-Bât.100-ORSAY
Vladimir Akulin (Laboratoire Aimé Cotton, Orsay)
La description de l'intrication quantique par des polynômes d'opérateurs nilpotents
We propose a general method for introducing extensive characteristics of quantum entanglement. The method relies on polynomials of nilpotent raising operators, that create entangled states acting on a reference vacuum state. By introducing the notion of tanglemeter (the logarithm of the state vector represented in a special canonical form and expressed via polynomials of nilpotent variables), we show how this description provides a simple criterion for entanglement as well as a universal method for constructing the invariants characterizing entanglement. We compare the existing measures and classes of entanglement with those emerging from our approach. We derive the equation of motion for the tanglemeter and, in representative examples of up to four-qubit systems, show how the known classes appear in a natural way within our framework. We extend our approach to qutrits and higher-dimensional systems, and make contact with the recently introduced idea of generalized entanglement. Ref: A. D. Mandilara, V. M. Akulin, A. V. Smilga, L. Viola, Description of Quantum Entanglement with Nilpotent Polynomials, quant-ph/0508234 31 Aug 2005
10 janvier à 11h LPTMS-Bât.100-ORSAY
Thierry Mora (LPTMS)
Amas de solutions dans le problème de satisfiabilité
Ces dernières années, la méthode de la cavité, qui présente une alternative à la méthode des répliques, s'est avérée particulièrement fructueuse dans l'étude de problèmes de satisfaction de contraintes, tels que le problème de satisfiabilité ou le problème du coloriage de graphes aléatoires. Les progrès qu'elle a permis sont tant sur le plan analytique qu'algorithmique. La propriété d'agglomération en amas, qui prévoit une phase "vitreuse" où l'espace des solutions se sépare en un grand nombre d'amas éloignés les uns des autres, est l'une des hypothèses les plus cruciales qui sous-tendent cette théorie. Je tenterai de montrer comment, à partir d'arguments probabilistes élémentaires, cette propriété peut être prouvée rigoureusement dans le cas du problème K-SAT, avec K ? 8.
10 janvier à 11h LPTMS-Bât.100-ORSAY
Gunnar Möller (LPTMS)
Formation de Paires de Fermions Composites dans les Systèmes à Effet Hall Quantique Bicouches à Facteur de Remplissage nu_T = 1
On présentera une étude des systèmes à Effet Hall Quantique bicouches à proximité du facteur de remplissage total nu = 1/2 + 1/2, qui avait pour but de comprendre la transition de phase de l'état électronique compressible réalisé aux grandes séparations des couches d, vers un état incompressible au dessous d'une valeur critique d_c. Des fonctions d'onde d'essai décrivant ce système sont proposés qui s'appuyant sur l'image de deux fluides interpénétrants de particules composites: fermions composites (FC) et bosons composites (BC). On montre qu'en tenant compte de la formation de paires du type BCS des FC, on obtient un accord exceptionnel avec les solutions obtenues par calculs numériques de diagonalisations exactes pour des petites systèmes, mettant en évidence l'existence des paires de FC dans le fluide quantique examiné.
6 janvier à 11h LPTMS-Bât.100-ORSAY
Harold Baranger (LPTMS, Duke University)
Interference and Interactions in Electronic Nanostructures
The interplay between quantum mechanical interference and electron-electron interactions is one of the great themes of condensed matter physics. Recently, this topic has arisen in the context of nanostructures - quantum dots, metallic nanoparticles, carbon nanotubes, single conducting molecules, etc. I will introduce the general issue of interference and interaction in nanostructures, and then illustrate by choosing two examples from recent work in our theoretical nanophysics group at Duke. (1) Coupling between the spatial interference and spin direction in ferromagnetic nanoparticles: spin-orbit coupling is treated using a random matrix theory model to give excellent agreement with the experimental results of Gueron, et al. (2) Conduction through single organometallic molecules: a DFT plus Green function approach to transport through single molecules shows that a single metal atom can have a large impact on the conductance of a large organic molecule.