Séminaires LPMMC 2008
MISSING (Département de physique théorique, Université de Genève) | Détails Fermer |
Electron-electron interaction effect in quantum point contacts le lundi 15 décembre 2008 à 11:00 |
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Résumé : The effect of electron-electron interactions on the transport properties of a quantum point contact (QPC) is considered. The basic point is that a QPC breaks translational invariance and therefore the electron-electron interaction process does not have to conserve momentum. This leads to a suppression of the current though the QPC. At low temperatures or bias voltages, the suppression is found perturbatively, but at higher temperatures or bias voltages perturbation theory breaks down. Therefore, we use a numerical self-consistent non-equilibrium Green's function approach to reach this regime for a simplified model. Liens :MISSINGDépartement de physique théorique, Université de Genève |
MISSING (Harish-Chandra Research Institute, Allahabad) | Détails Fermer |
Transport through superconducting junction of multiple one-dimensional quantum wires le jeudi 04 décembre 2008 à 11:00 |
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Résumé : We investigate transport properties of a superconducting junction of one-dimensional quantum wires. We include the effect of electron-electron interaction within the one-dimensional quantum wires using a weak interaction renormalization group (WIRG) procedure. Due to the proximity effect, transport across the junction occurs via direct tunneling as well as via crossed Andreev channel. We find that the fixed point structure of this system is far more rich than the fixed point structure of a normal metal-superconductor (NS) junction, where we only have two fixed points - the fully insulating fixed point or the Andreev fixed point. Even a two wire system with a superconducting junction ie a normal metal-superconductor-normal metal (NSN) structure, has non-trivial fixed points with intermediate transmissions and reflections. We demonstrate possible scenarios for production of pure spin current through such junctions and show that there are fixed points in the theory which correspond to the case of pure spin current. We have also shown that (a) effects due to inclusion of electron-electron interaction induced back-scattering in the wire, and (b) competition between the charge transport via the electron and hole channels across the junction give rise to a non-monotonic behavior of subgap conductance as a function of temperature. |
MISSING (Laboratoire MPQ, Université Paris Diderot) | Détails Fermer |
An interacting quantum impurity far from equilibrium le lundi 1er décembre 2008 à 11:00 |
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Résumé : "Quantum impurity models", describing a tiny structure coupled to macroscopic
ones, are having a second life. They are indeed well adapted to the description of situations where nanostructures are connected to macroscopic electronic reservoirs, a flourishing experimental field.
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MISSING (LPS Orsay) | Détails Fermer |
Atomes ultra-froids sur un réseau optique en échelle le lundi 24 novembre 2008 à 11:00 |
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Résumé : La réalisation expérimentale de réseaux optiques permet une étude approfondie de phases fortement corrélées dans les systèmes d'atomes froids. La dimensionnalité, l'intensité de l'effet tunnel comme celle des interactions sont autant de paramètres aisément contrôlables. De nombreux modèles théoriques, traditionnellement appliqués à la physique des solides, trouvent ici un nouveau champ d'application. Nous nous sommes particulièrement intéressés à l'étude d'un système d'atomes froids sur un réseau en forme d'échelle. Les échelles sont des systèmes quasi unidimensionnel ayant un diagramme de phase très riche, en fonction du champ magnétique et du remplissage. Je présenterai le calcul de plusieurs signatures expérimentales susceptibles d'être observées lors de mesures des corrélations de densité sur le nuage atomique, après relaxation du piège. Liens :LPS Orsay |
MISSING (University of Illinois) | Détails Fermer |
Mesoscopic 1x1 S matrices in the time domain le lundi 17 novembre 2008 à 14:00 |
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Résumé : Consider a disordered medium at finite temperature, and the incoherent reflection of an impulse in and out through a single channel. An ensemble of such reflections r(t) may be constructed using simple thermodynamic arguments. In the absence of inelastic interior processes, such a reflection would be interpreted as a coherent 1x1 S-matrix, S(t), with the usual properties of causality and unitarity. It is amusing to imagine identifying the ensemble of such S matrices that, when garbled by inelastic
scatterings, would lead to the stated ensemble of incoherent reflections
r(t). If this could be done, we would then have a procedure for constructing maximum entropy S matrices (and therefore Greens functions too) and determining the corresponding features of mesoscopic transport and diffusion in systems with arbitrary internal structure.
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MISSING | Détails Fermer |
Probing correlation effects in 1D systems via quantum Monte Carlo methods le lundi 20 octobre 2008 à 10:00 |
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Résumé : The interplay between strong correlation and quantum fluctuations makes one
dimensional (1D) systems very peculiar. Although a strict 1D system is an
ideal case, recently it has been possible to reach and probe 1D physics
both in semiconductor nanodevices and cold atom experiments. I will review
two paradigmatic cases in both fields, namely the localization in parallel
quantum wires[1], and the search for the FFLO state in very elongated traps
with spin imbalance[2]. I will show how a quantum Monte Carlo approach to
study these systems can predict accurately their phase diagram,
particularly when non-homogeneous effects due to the experimental
realization play an important role.
Liens : |
MISSING (LPTMS, Universite Paris Sud) | Détails Fermer |
Many-body physics with quantum gases: exotic superfluids and disordered condensates le lundi 13 octobre 2008 à 10:00 |
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Résumé : Recent experiments have shown that cold atoms can be used as toy models to
simulate complicated condensed matter systems, thus improving our understanding in terms of many-body theories. |
MISSING (LPMMC) | Détails Fermer |
Microscopics of disordered two-dimensional electron gas under high magnetic fields II le jeudi 02 octobre 2008 à 10:30 |
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MISSING (LPMMC) | Détails Fermer |
Microscopics of disordered two-dimensional electron gases under high magnetic fields le vendredi 26 septembre 2008 à 14:00 |
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MISSING (Institute for Theoretical Physics, Heidelberg) | Détails Fermer |
Nuclear Spin Dynamics: Implications of Microscopic Chaos le lundi 15 septembre 2008 à 11:00 |
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Résumé : Interaction between nuclear spins in solids forces each spin to perform a complicated dance in a time-dependent field created by neighboring spins. This leads to spin-spin relaxation observed by nuclear magnetic resonance (NMR). I briefly discuss the efforts to calculate NMR spin-spin relaxation from first principles, and then argue that the main obstacle to these efforts is the lack of the proper understanding of microscopic chaos. I proceed with presenting a theory, which invokes the notion of chaos and thereby predicts that NMR free induction decay and spin echoes observed in the same system
have identical exponential long-time behavior. This prediction was shown to be quantitatively correct by a very recent NMR experiment on hyperpolarized solid xenon. Such a lack of dependence of the long-time decay on the initial spin configuration reveals a new fundamental property of nuclear spin dynamics in solids. Namely, the quantum time evolution operator of a macroscopic system of nuclear spins 1/2 has isolated eigenmodes, which govern the long-time relaxation towards equilibrium. These eigenmodes decay on the ballistic microscopic timescale. Therefore, their existence cannot be predicted using the standard approximations of statistical physics. Such eigenmodes, however, are very reminiscent of Pollicott-Ruelle resonances in classical chaotic systems, and, in fact, were predicted on the basis of this analogy. Their discovery thus suggests that, even in the situations, when quasi-classical chaotic limit is not tenable, and the spacing between energy levels is not relevant to the observable properties, the notion of microscopic chaos can be defined for isolated many-body quantum systems at the level of Pollicott-Ruelle resonances.
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MISSING (LPMMC) | Détails Fermer |
Bose-Einstein condensate speckle in random potential le lundi 08 septembre 2008 à 14:00 |
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Résumé : Recent calculations [1] have shown that the image of a Bose-Einstein condensate (BEC) expanding diffusively in a random potential exhibits a complicated irregular pattern of particle density. This phenomenon is reminiscent to what we know as speckle in optics. We consider a BEC expanding in a quasi-1D waveguide and show that such speckle patterns show up long-range density correlations. These correlations are accessible experimentally and exhibit remarkable features such as a strong enhancement at long times. They can even take negative values for sufficiently distant points [2].
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MISSING | Détails Fermer |
Un modèle d'espace-temps discret le lundi 16 juin 2008 à 11:00 |
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Résumé : La physique dans son ensemble s'est construite sur une hypothèse de continuité de l'espace-temps dont les équations différentielles fondamentales sont l'expression. La tendance est aujourd'hui d'introduire un certain degré de discrétisation, comme dans la théorie des cordes, la théorie de la gravitation quantique à boucle etc. Mais ces tentatives sont basées sur la discrétisation d'un l'espace-temps supposé par ailleurs continu. Dans la présente approche l'univers physique est, dès l'origine, supposé être constitué d'éléments discrets si bien que tout doit être reconstruit, l'espace, le temps, les ondes, les particules etc. Dans ce séminaire on présentera un tel modèle d'univers discret et on discutera des hypothèses qui en forment la base. On évoquera, sans entrer dans les détails, les principales conséquences du modèle, reconstruction de l'espace-temps classique, justification de la mécanique quantique, explication du modèle standard, implications en cosmologie. Liens : |
MISSING (LPT Orsay) | Détails Fermer |
Percer à jour l'interaction forte - La chromodynamique quantique sur réseau le vendredi 13 juin 2008 à 11:00 |
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Résumé : Il y a 40 ans se déroulait une révolution scientifique: la fondation du modèle standard de la physique des particules. En son sein la très élégante chromodynamique quantique s'imposait comme la théorie des interactions fortes subnucléaires. Elle décrit le "confinement" des quarks et des gluons dans des états liés, neutres de "couleur", les "hadrons" (protons, neutrons, pions, etc.). A partir des quarks et des gluons elle promet, et souvent permet, de décrire de nombreuses expériences mettant en jeu ces "hadrons". La chromodynamique quantique sur réseau est la méthode de calcul la plus rigoureuse pour déduire les propriétés des hadrons de celles des quarks. Elle repose sur la discrétisation de l'espace-temps et est méthodologiquement proche de la physique statistique. Un petit échantillon de résultats sera montré. Cette discipline demandant de gros moyens de calcul, elle a donné lieu à des innovations dans le domaine des architectures d'ordinateurs. Liens : |
MISSING (Institut Néel) | Détails Fermer |
(titre non communiqué) le lundi 02 juin 2008 à 11:00 |
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Liens :MISSINGInstitut Néel |
MISSING (LPMMC) | Détails Fermer |
The 1D Bose-Hubbard model with a quasi-periodic potential le lundi 26 mai 2008 à 10:00 |
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Résumé : We determine the phase diagram and the momentum distribution for a one-dimensional Bose-Hubbard model in the presence of an incommensurate potential, by using a combined numerical (DMRG) and analytical (low-energy) analysis. The system displays a delocalized (superfluid) phase at small values of the height V2 of the second lattice and a localized (Bose glass-like) phase at larger strengths of the second lattice. We analyze the localization transition as a function of V2 beyond the known limits of free and hard-core bosons. We also analyze the emergence of a Bose-glass phase by looking at the evolution of the Mott-insulator lobes at varying V2. Finally, we characterize the superfluid phase by the momentum distribution. |
MISSING (LPTMC, UPMC, Jussieu, Paris) | Détails Fermer |
Metal-insulator transition in the two-dimensional fully polarized homogeneous electron gas from Hartree-Fock solutions le mardi 20 mai 2008 à 14:30 |
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Résumé : We determine the ground state of the two-dimensional, fully polarized electron gas within the Hartree-Fock approximation without imposing any particular symmetries on the solutions. At low electronic densities, the Wigner crystal solution is stable, but for higher densities (rs less than ~ 2.6) we obtain a ground state of different symmetry: the charge density forms a triangular lattice with about 11% more sites than electrons. We argue that this conducting state with broken translational symmetry remains the ground state of the high density region in the thermodynamic limit giving rise to a metal to insulator transition. |
MISSING (Department of Physics, University of L'Aquila, Italy) | Détails Fermer |
High pressure hydrogen: new predictions by coupled electron-ion Monte Carlo le lundi 19 mai 2008 à 11:00 |
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Résumé : The physical behaviour of hydrogen under megabar compression is still largely unknown because experimental difficulties prevent a systematic investigation of the phase diagram in the interesting range of pressures and temperatures. Ab-initio theoretical methods have been exploited to interpret the scattered experimental data and to make predictions. However the problem is particularly difficult because the energy scales of several different phenomena, such as electronic correlations, protonic quantum effects, finite size effects in the metallic phase and in the metal-insulator transition region, becomes comparable and need to be considered together in a non perturbative manner. Quantum Monte Carlo methods are unique in their ability to treat all those effects with high accuracy in simple systems. In recent years we have developed a new QMC method, the Coupled Electron-Ion Monte Carlo (CEIMC) [C. Pierleoni and D. M. Ceperley, Lecture Notes in Physics vol 741 pp 641-683 (2006), xxx.lanl.gov/abs/physics/0510254], which is particularly suitable to study hydrogen under high pressure.
Liens :MISSINGDepartment of Physics, University of L'Aquila, Italy |
MISSING (INRIA Grenoble) | Détails Fermer |
A computer modeling of the early immune response inside a lymph node le lundi 05 mai 2008 à 11:00 |
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Résumé : Lymphnodes play a key role in the development of an appropriate and efficient immune response. Once an pathological entity (Antigen,Ag) is detected inside the host organism it must be presented to specific lymphocytes to trigger, if it is the case, an immune response that gets rid of it. Ag-scanning must be highly efficient: at most in few hours it is necessary to find specific lymphocytes inside a repertoire that includes 106 different receptors. The way Nature organized itself to accomplish this goal represents a charming fine-tuned mechanism, based on a careful balance between diffusion properties, chemotaxis and receptor expression. And the lymphnode is the scene of such mechanisms. Liens :MISSINGINRIA Grenoble |
MISSING (LAPTH (Annecy)) | Détails Fermer |
Hubbard models in gauge theory and in condensed matter le lundi 28 avril 2008 à 11:00 |
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Résumé : We construct Hubbard-like models with general symmetries and super-symmetries; these models are integrable. Perturbative calculations in the large coupling regime show the connection with Heisenberg-like spin chains. The same regime is interesting in relation to a particular gauge theory in four dimensions. Integrability allows us to evaluate scattering properties and to work out energy eigenvalues. Methods of nonlinear integral equations are used to perform an accurate study of excitations of the antiferromagnetic vacuum. Liens :MISSINGLAPTH (Annecy) |
MISSING (ENS-Lyon) | Détails Fermer |
Ondes internes de gravité en océanographie physique le lundi 21 avril 2008 à 11:00 |
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Résumé : L'océan est un exemple important de fluides stratifiés, à l'intérieur desquels peuvent se propager des ondes internes de gravité. L'émission et la réflexion de ces ondes sont des mécanismes qu'il est important de comprendre pour expliquer la conversion de l'énergie des marées, ainsi que les processus de mélange observés entre les régions proches des fonds marins et à l'intérieur de l'océan. Ces ondes sont aussi très intéressantes pour le physicien car certaines de leurs propriétés défient notre intuition des phénomènes ondulatoires. Après avoir présenté les propriétés tout à fait inhabituelles de ces ondes ainsi que quelques résultats théoriques que nous avons obtenus, j'illustrerai leurs conséquences à l'aide de plusieurs expériences que nous avons réalisées au laboratoire de Physique de l'ENS Lyon ou à la plateforme Coriolis de Grenoble. |
MISSING (LPMMC) | Détails Fermer |
Casimir momentum le lundi 14 avril 2008 à 10:00 |
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Résumé : The effects of zero-point fluctuations such as the Van der Waals force or the Casimir force between a plate and a sphere, which is now accurately measured (with the best one is in the Néel lab!), are well known. Recent publications (Feigel, PRL92, 020404) claim that it is possible to transfert momentum from the quantum vacuum fluctuations, to matter and obtain mesurable velocity. We investigate this possibility via the coupling of an electric dipole and the free field in presence of applied crossed fields - to break the symmetries. A semi-classical approach has been done but we need a fully quantum mechanical description to confirm the latest and to validate a future experimental confirmation. |
MISSING (Institute of applied physics, Chisinau, Moldavie) | Détails Fermer |
Multi-quantum processes in quantum optics and condensed matter le mardi 08 avril 2008 à 11:00 |
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MISSING (LPMMC) | Détails Fermer |
Coulomb blockade for an oscillating tunnel junction le lundi 31 mars 2008 à 10:00 |
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Résumé : One of the key questions in the field of nano-mechanical systems is the effect of mechanical motion on the electric properties of the devices. In this presentation, We present the case of a tunnel junction formed between a fixed electrode and an oscillating one. Accumulation of the charge on the junction capacitor induces a force on the nano-mechanical oscillator. The junction is voltage-biased and connected in series with an impedance. We discuss how the picture of Coulomb blockade is modified by the presence of the oscillator. Quantum fluctuations of the mechanical oscillator modify the I-V characteristics particularly in the strong Coulomb blockade limit. We show that the oscillator can be taken into account by a simple modification of the effective impedance of the circuit. We discuss in some details the case of a single inductance and of a constant resistance. With little modifications the theory applies also to incoherent transport in Josephson junctions in the tunneling limit. |
MISSING (Institut Carnot, Université de Bourgogne) | Détails Fermer |
Control of atomic and molecular processes by designed external fields: From adiabatic to ultrashort pulse strategies le vendredi 07 mars 2008 à 11:00 |
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Résumé : I present tools and strategies for the control of atomic and molecular processes by external fields, such as laser and cavity fields, with parameters specifically designed to achieve particular goals. Applications to selective population transfer, tunneling effect, alignment of molecules, and quantum information processing are addressed. Liens :MISSINGInstitut Carnot, Université de Bourgogne |
MISSING (LPMMC) | Détails Fermer |
Quenching of moment of inertia in superfluid rotating fermions in traps le jeudi 06 mars 2008 à 10:00 |
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Résumé : In view of upcoming experimental determination of moment of inertia of rotating superfluid fermions at Innsbruck, I want to report on our work on this subject. We use a semiclassical approach to calculate currents and moment of inertia in such systems. A rich structure going from rigid rotation to ideal fluid flow is observed as a function of temperature and/or size of the system. The density profile of the superfluid component is determined. At low temperatures, due to superfluidity, the moment of inertia turns out to be quenched with respect to its rigid body value. |
MISSING (LPMMC) | Détails Fermer |
Properties of tunnel Josephson junctions with a ferromagnetic interlayer le lundi 25 février 2008 à 10:00 |
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Résumé : We investigate superconductor / insulator / ferromagnet / superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We formulate a quantitative model describing the oscillations of critical current as a function of thickness of the ferromagnetic layer and use this model to fit recent experimental data. We also calculate quantitatively the density of states (DOS) in this type of junctions and compare DOS oscillations with those of the critical current. |
MISSING (ENS-Lyon) | Détails Fermer |
Des bulles de laboratoire aux phénomènes volcaniques quasi-périodiques : que nous apprend l'acoustique dans des fluides non-newtoniens ? le lundi 11 février 2008 à 11:00 |
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Résumé : L'éclatement d'une bulle à la surface d'un fluide complexe peut produire des évènements violents, que ce soit la projection de purée ou de sauce lors d'expériences culinaires malheureuses, ou, à plus grande échelle et aux conséquences plus dramatiques, à la projection jusqu'à plusieurs centaines de mètres de fragments de lave, lors d'une éruption volcanique. Les signaux acoustiques émis par l'éclatement de la bulle en surface constituent un moyen non-intrusif (et, dans le cas des volcans, sécurisé) pouvant permettre d'accéder, en cas d'impossibilité de mesure directe, aux propriétés physiques du système. Mais quelles informations peut-on vraiment en tirer ? |
MISSING (Observatoire de Paris-Meudon) | Détails Fermer |
Superfluid neutron currents in the solid crust of a neutron star le vendredi 1er février 2008 à 11:00 |
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Résumé : In contrast with the core (which might consist of color superconducting quark matter) there is a consensus about the basic physics of the outer layers of a neutron star, which form a solid crust whose inner part is permeated by superfluid neutrons, whose local movement (describable in terms of nonrelativistic nuclear physics) relative to the metallic solid lattice can explain the rotation frequency glitches observed in pulsars (whose global description requires a general relativistic treatment). |
MISSING (LPMMC) | Détails Fermer |
Josephson effect in quantum gases le jeudi 31 janvier 2008 à 10:00 |
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Résumé : Starting from a quantum two-mode Bose-Hubbard Hamiltonian we determine the ground state properties, momentum distribution and dynamical evolution for a Bose Josephson junction realized by an ultracold Bose gas in a double-well trap. Varying the well asymmetry we identify Mott-like regions of parameters where number fluctuations are suppressed and the interference fringes in the momentum distribution are strongly reduced. We also show how Schroedinger cat states, realized from an initially phase coherent state by a sudden rise of the barrier among the two wells, will give rise to a destructive interference in the time-dependent momentum distribution. Liens :LPMMC |