Autour de la formule de Landauer-Büttiker
le vendredi 14 décembre 2007 à 11h00
Séminaire théorie
Personne à contacter :
Lieu : Maison des Magistères
Résumé : Le formalisme de Landauer-Büttiker est largement utilisé en nanophysique. Il permet, en supposant négligeable toute interaction entre porteurs de charge, de réduire les problèmes de transport dans les systèmes électroniques au calcul de la matrice de collision d'un système à un électron. Je montrerai que ce formalisme est un cas particulier d'une approche générale, proposée par Ruelle, de la construction d'états stationnaires hors-équilibre par la théorie des collisions. Ces résultats sont le fruit d'une collaboration avec W. Aschbacher (TU-Munich), V. Jaksic (McGill) et Y. Pautrat (Orsay).
Dark solitons in three dimensional dipolar Bose-Einstein condensates
le lundi 10 décembre 2007 à 11h00
Séminaire LPMMC
Personne à contacter :
Lieu : Maison des Magistères, salle MAG1
Résumé : We study the dynamical stability of dark solitons in dipolar
Bose-Einstein condensates. In the absence of non-locality due to the
dipolar interaction, stationary dark solitons (nodal planes) are unstable
against transversal excitations (snake instability) in 2D and 3D. On the
contrary, due to its non local character, the dipolar interaction allows
for stable 3D stationary dark solitons. We discuss in detail the
conditions to achieve this stability, which demand the use of an
additional optical lattice.
Generalities on disordered system in one dimension: Anderson localization versus Bose glass
le lundi 3 décembre 2007 à 10h00
Séminaire interne LPMMC
Personne à contacter :
Lieu : Maison des Magistères, salle MAG 1
Résumé : The problem of disorder in one dimension is an extremely challenging one. For free electrons it's well known that disorder can give rise to very strong effects such as Anderson localization. When interactions are included the question is largely open and even more challenging in the presence of quantum fluctuations associated to reduced dimensionality. We give a brief review of tools to the analysis of disorder in the framework of the Luttinger liquid models treated by means of bosonization and renormalization group approach.
Résumé : La question de contrôlabilité d'un système quantique consiste à montrer
l'existence d'un hamiltonien optimal permettant de transférer le système d'un
état initial fixé vers un état final désiré durant un intervalle de temps
donné. Dans un premier temps, nous introduirons la théorie du contrôle optimal
qui nous permet de donner une formulation mathématique rigoureuse de ce
problème. Cette formulation consiste à minimiser ou maximiser une fonctionnelle
de coût traduisant l'objectif physique à atteindre en tenant compte de
l'équation d'évolution du système quantique interprétée comme une contrainte
dynamique. Nous donnerons ensuite une description de l'approche numérique dite
indirecte adoptée pour optimiser la fonctionnelle de coût. Comme application,
nous considérons un système quantique à N-niveaux d'énergies à base de
jonctions Josephson. Plus précisement, nous nous intéressons à un circuit
supraconducteur impliquant un SQUID dc polarisé en courant. La dynamique de
phase d'un tel circuit est équivalente au mouvement d'une particule quantique
soumise à un potentiel local anharmonique. Dans ce cas, le problème de contrôle
optimal est de déterminer le profil temporel d'un champ radio-fréquence
permettant de conduire la particule d'un état initial fixé vers un état final
désiré. Nous présenterons les résultats numériques dans les cas du transfert de
la particule de l'état fontamental vers un état excité arbitrairement choisi.
Nous présenterons aussi une analyse de la sensibilité de la solution optimale
par rapport aux fluctuations des paramètres internes de l'hamiltonien décrivant
la particule quantique. Une éventuelle implémentation expérimentale sera
discutée.
Strong electronic correlations far from thermal equilibrium
le vendredi 12 octobre 2007 à 11h00
Séminaire théorie
Personne à contacter :
Lieu : Maison des Magistères
Résumé : Confined nanostructures, such as semiconductor and nanotube quantum dots, have become
an important tool for investigating fundamental aspects of correlated electron systems. The
exquisite control of microscopic parameters in such devices have opened new possibilities
not accessible in bulk systems. Perhaps the most exciting of these is the study of strong
electronic correlations far from thermal equilibrium, whether under steady-state conditions
or subject to real-time dynamics. The description of strong electronic correlations far from
thermal equilibrium is one of the outstanding open questions in the field. Many of the
theoretical approaches that have proven so successful in equilibrium are simply inadequate
once the system is driven out of thermal equilibrium. Basic concepts, such as that of the
renormalization group, are still lacking. In this talk I will review some of our recent
advancements toward filling in this gap. In particular, the development of a numerical
renormalization-group approach to track the real-time dynamics of quantum impurity
systems such as small quantum dots. Applications of the approach to quantum dissipation
and to the decoherence of an impurity spin due to its coupling to the environment will be
discussed.
String theory, extra dimensions and hidden symmetries
le vendredi 5 octobre 2007 à 11h00
Séminaire théorie
Personne à contacter :
Lieu : Maison des Magistères, amphi Mag
Résumé : String theory aims at describe nature at the smallest length scales by replacing the concept of point-like elementary particles by extending strings. Originally developed as a model of strong interactions it has become a leading candidate for a fundamental unified theory of particle physics and quantum gravity. In this talk I will review history, problems and progress of string theory. In particular, I will address recent developments on the holographic correspondence that provide new insight into the nature of quantum field theories.
Résumé : Cet exposé portera sur une classe de modèles sur réseau pour lesquels tous les états à une particule sont localisés par un effet de frustration géométrique (cages d'Aharonov-Bohm). En présence d'interactions, on peut s'attendre à des phénomènes surprenants, comme l'existence d'états liés délocalisés pour une paire de particules (y compris dans le cas d'une interaction répulsive). Je montrerai que le comportement à densité finie de tels systèmes dépend crucialement de l'existence d'une symétrie locale résiduelle, compatible avec les interactions. Ces idées seront illustrées en particulier sur l'exemple des réseaux de jonctions Josephson, qui doivent ainsi permettre de stabiliser des condensats exotiques (avec une charge élémentaire 4e), de simuler des théories de jauge sur réseau, et peut-être de réaliser des mémoires quantiques protégées contre la décohérence.
Introduction au groupe de renormalisation non-perturbatif
le jeudi 21 juin 2007 à 10h00
Séminaire Méso
Personne à contacter :
Lieu : Maison des Magistères, salle Mag 1
Résumé : Je proposerai une brève introduction aux techniques du groupe de renormalisation non-perturbatif ou fonctionnel (développées principalement par C. Wetterich et T. Morris). Ces méthodes ont été utilisées dans divers contextes, de la physique des hautes énergies à la physique statistique et la physique de la matière condensée, où elles ont permis d'obtenir des résultats importants car elles permettent d'explorer des régimes 'non-perturbatifs', en général inaccessibles par les développements perturbatifs usuels. Je dériverai l'équation de base de ce formalisme et illustrerai, sur l'exemple simple du modèle d'Ising, comment l'exploiter et les résultats qu'elle permet d'obtenir.
Influence of electro-mechanical effects on resonant electron tunneling through small carbon nano-peapods
le jeudi 14 juin 2007 à 10h00
Séminaire Méso
Personne à contacter :
Lieu : Maison des Magistères, amphi Mag
Résumé : Short single-wall carbon nanotubes (CNTs) can behave at small temperatures as quantum dot and the presence of fullerene molecules inside such a nanotubes can deeply change their transport properties.
In this work, we discuss the influence of fullerene molecules trapped inside a single-wall CNT on resonant electron transport at low temperatures, that is at temperatures much smaller than the mean level spacing in the CNT. The strong hybridization strength between the electronic states on the NT ant the unoccupied molecular orbitals of the fullerenes makes possible the hopping of delocalized electrons from the CNT to the local fullerene states. Particularly, we suggest that the strong coupling between the electronic states on the CNT shell and the localized states on the fullerenes affects the resonant tunneling transport of electrons through the metallic peapod system in a manner that is experimentally observable. We propose that resonant electron transport due to polaronic effects provides a method for studying the fullerene nanmechanical dynamics through the temperature dependence of the resonant conductance peaks. The comparison with some experimental results will be shown.
Résumé : Present day technology has reached the level when the study of the quantum mechanical behavior of individual objects becomes feasible. These include detection and mesaurement of single electron spin, superconducting and charge qubits, and even tiny ("nano") mechanical systems. While the former systems do not have classical analogs, the latter does. This opens an interesting possibility to study the interface between the classical and quantum behaviors. In this talk I will describe (1) the state of the art experimental results of the nanomechanical systems, (2) theoretical proposals to achieve their ground-state cooling, (3) issues of quantum measurements, i.e. how the measurements affects systems, and (4) some applications.
Detecting the fluctuations of a nanomechanical resonator
le jeudi 3 mai 2007 à 10h00
Séminaire Méso
Personne à contacter :
Lieu : Maison des Magistères, amphi Mag
Résumé : In this talk I will review my work on the theory of the detection of the fluctuations of a nanomechanical oscillator. The last years have seen the shrinking of mechanical components to micrometer size and below. Such small devices are of interest for very sensitive force detection as well as for fundamental physics. One active area of investigation is the cooling of a nanomechanical resonator close to its ground state. An interesting question in this context is how to measure the temperature of the oscillator, without disturbing it too much. I will present a model system, an oscillator coupled to a tunnel junction, and show how the noise in the tunnel junction can act as a meter for the oscillator occupation number.
Résumé : I consider fluctuation corrections to transport characteristics of a quantum dot (or metallic grain) in the limit of large conductance, taking into account the attractive interaction in the Cooper channel inside the dot, when it is coupled to normal leads by means of tunnel contacts. Under this condition the dot undergoes a phase transition to a superconducting state at some critical temperature Tc. Then at temperatures just above Tc the conductance, shot noise and generally higher order cumulants of the transferred charge (electron full counting statistics) develop singular corrections due to fluctuations of the order parameter. In particular, the fluctuation correction to the current noise appeared to be more singular and pronounced, than the correction to the differential conductance.
The effect of the noise enhancement is of rather general nature and should hold for the diffusive wires and films as well. The possible experimental verification of this phenomenon, using the bridge geometry, when a short diffusive wire is connecting two superconducting films, placed at temperature slightly above Tc, is discussed.
Résumé : I will discuss the dynamics of a system in which a nanomechanical resonator is coupled to a superconducting single-electron transistor (SSET). Such systems have been realized experimentally by Keith Scwab and co-workers (Cornell). When the SSET is biased in the vicinity of a transport resonance, the dynamics of the resonator is very sensitive to which side of the resonance is chosen as the operating point of the SSET. When the SSET is biased on one side of the resonance, the SSET acts on the resonance like a thermal bath whose effective temperature is surprisingly low. In contrast, on the opposite side of the resonance the SSET can drive the resonator into a variety of different oscillatory states. In this case useful analogies can be made with quatum optical systems such as the micromaser.
Résumé : We consider low-temperature behavior of weakly interacting electrons in disordered conductors in the regime when all single-particle eigenstates are localized by the quenched disorder. The key result is that in the absence of coupling of the electrons to any external bath dc electrical conductivity exactly vanishes as long as the temperature T does not exceed some finite value Tc. At the same time, it can be also proven, that at high enough T the conductivity is finite. These two statements imply that the system undergoes a finite temperature metal-insulator transition, which can be viewed as Anderson-like localization of many-body wave functions in the Fock space. As a result, in the insulating phase electron-electron interaction alone is unable to cause the relaxation and establish equilibrium.
Electron Dynamics on the interface dielectric/organic (theoretical aspects)
le mercredi 7 février 2007 à 10h00
Séminaire Méso
Personne à contacter :
Lieu : Maison des Magistères, amphi Mag
Résumé : Recent experiments have demonstrated that the performances of organic
FETs strongly depend on the dielectric properties of the gate material [1].
In particular, it has been shown that the temperature dependence of the
mobility evolves from metallic-like to insulating-like upon increasing the
dielectric constant of the gate insulator.
In this talk I will present a microscopic model that consistently explains the
experimental observations in terms of the formation of polarons, due to
the interaction of the charge carriers with the phonons at the interface with
the gate material.
The theory shows that increasing the dielectric polarizability of the gate
material results in a crossover from the weak to the strong polaronic coupling
regime, where the carriers become self-trapped on individual molecular units,
giving rise to a thermally activated mobility.
The microscopic parameters extracted from the mobility data are in good
agreement with the known parameters of the devices, and indicate that the
polaronic carriers are located in the uppermost molecular layers of the
crystal, close to the interface.
[1] "Tunable Frohlich Polarons in Organic Single-Crystal Transistors'
Hulea, Fratini, Xie, Mulder, Iossad, G.R. , Ciuchi, and Morpurgo,
Nature Materials 2006
Laboratoire de Physique et Modélisation des Milieux Condensés
