• Séminaire LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : In this talk, I will give an overview of our works on multiple scattering of interacting bosons in random media. Starting first with the non-interacting case, the effects of coherent backscattering and weak localization are introduced as the main interference corrections to diffusive transport in weak disorder. Then, I will extend the underlying theoretical concepts (ladder and crossed diagrams) to the case of many interacting particles (beyond the mean field approximation), and derive nonlinear transport equations which finally characterize the effect of interactions on diffusive transport and coherent backscattering. Furthermore, I will show that, by combining many-particle transport theory with quantum optical methods, our theory is also able to describe multiple scattering of intense laser light by dilute clouds of cold atoms.

Liens :Thomas WellensPhysikalisches Institut, Freiburg

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : As recently discovered [1], Anderson localization (AL) in a bulk disordered system triggers the emergence of a coherent forward scattering (CFS) peak in momentum space, which twins the well-known coherent backscattering (CBS) peak observed in weak localization experiments. Going beyond the perturbative regime, we address here the long-time dynamics of the CFS peak in a 1D and 2D random systems [2]. Focusing on the 2D case, we show that CFS generally arises due the confinement of the wave in a finite region of space, and explain under which conditions it can be seen as a genuine signature of AL. In the localization regime, our numerical results show that the dynamics of the CFS peak is governed by the level repulsion between localized states, with a time scale related to the Heisenberg time. This is in perfect agreement with recent findings based on the nonlinear sigma model. In the stationary regime, the width of the CFS peak in momentum space is inversely proportional to the localization length, reflecting the exponential decay of the eigenfunctions in real space, while its height is exactly twice the background, reflecting the Poisson statistical properties of the eigenfunctions.

Références

1. PRL 109 190601 (2012)
2. PRA 90, 043605 (2014), arXiv:1410.0774

Liens :Christian MiniaturaCenter for quantum technologies, Singapour

• Soutenance de thèse
• Personne à contacter :
• Maison des magistères

Liens :LPMMC

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : Répétion soutenance de thèse

Liens :LPMMC

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : In this talk I will discuss the dynamics of interacting fermionic systems in the mean-field regime. Compared to the bosonic case, fermionic mean-field scaling is naturally coupled with a semiclassical scaling, making the analysis more involved. From a physical point of view, as the number of particles grows one expects the quantum evolution of the system to be effectively described by Hartree-Fock theory. The next degree of approximation is provided by a classical effective dynamics, corresponding to the Vlasov equation. I will consider initial data which are close to quasi-free states, both at zero and at positive temperature, with an appropriate semiclassical structure. Under some regularity assumptions on the interaction potential I will show that the time evolution of such initial data stays close to a quasi-free state, with reduced one-particle density matrix given by the solution of the time-dependent Hartree-Fock equation. The result holds for all (semiclassical) times, and gives effective bounds on the rate of convergence towards the Hartree-Fock dynamics as the number of particles goes to infinity.

Liens :Marcello Porta

• Séminaire LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : We study energy and particle transport for one-dimensional strongly interacting bosons through a single channel connecting two atomic reservoirs. We show the emergence of particle- and energy-current separation, leading to the violation of the Wiedemann-Franz law. As a consequence, we predict different time scales for the equilibration of temperature and particle imbalances between the reservoirs. Going beyond the linear spectrum approximation, we show the emergence of thermoelectric effects, which could be controlled by either tuning interactions or the temperature. Our results describe in a unified picture fermions in condensed matter devices and bosons in ultracold atom setups. We conclude discussing the effects of a controllable disorder.

Liens :Freie Universität Berlin

• Colloque CPTGA
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : Kondo effect (KE) is omnipresent and universal phenomenon arising in situations where a nanoobject with internal degrees of freedom interacts with a Fermi sea. Starting with brief history of evolution of KE from metals doped with magnetic impurities to complex quantum dots, we concentrate on its realizations in ultra-cold Fermi gases in optical lattices. These systems give unique possibility of achieving overscreening non-Fermi liquid regime for KE, which is highly elusive in other situations where the multichannel KE is the source of Kondo screening.

Liens :Konstantin Kikoin

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Liens :LPMMC

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : In this talk I will present the phase diagram of the strongly-interacting Bose-Hubbard model defined on a two-leg ladder geometry in the presence of a homogeneous flux. Our work was motivated by a recent experiment [1], which studied the same system, in the complementary regime of weak interactions. Based on extensive density matrix renormalization group simulations and a bosonization analysis, we have fully explored the parameter space spanned by filling, inter-leg tunneling, and flux. As a main result, we demonstrate the existence of gapless and gapped Meissner and vortex phases, with the gapped states emerging in Mott-insulating regimes. We calculate experimentally accessible observables such as chiral currents and vortex patterns and study their dependence on model parameters.

[1] Atala et al., Nature Phys. 10, 588 (2014)

Liens :Marie PiraudTheoretical Nanophysics, Munich

• Soutenance de thèse
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : L’effet Hall quantique, qui apparaît dans les gaz d’électrons bidimensionnels soumis à un champ magnétique perpendiculaire et à basses températures, a été un sujet de recherche intense pendant les derniers trente ans, en particulier, à cause des manifestations spectaculaires de la mécanique quantique dans les propriétés de transport à l’échelle macroscopique. Dans cette thèse, on étend l’horizon de la recherche au niveau théorique sur ce sujet, en considérant les effets du couplage spin-orbite et l’interaction électron-électron de façon analytique dans ce régime. Dans la première partie de ce manuscrit, on considère l’effet simultané du couplage spin-orbite de type Rashba et l’interaction Zeeman dans le régime de l’effet Hall quantique entier. Pour cela, on étend un formalisme de fonctions de Green basé sur des états de vortex cohérents avec l’objectif d’inclure le couplage entre les degrés de liberté orbitaux et de spin dans les états de dérive électroniques. Puis, comme première application, on montre comment obtenir analytiquement, nonperturbativement et de manière contrôlée des fonctionnelles quantiques (spectre et densité d’états locale) pour des potentiels électrostatiques arbitraires et localement plats. Les fonctionnelles sont ensuite analysées dans différents régimes de températures et comparées aux données expérimentales obtenues à partir des sondes de spectroscopie locales. Comme seconde mise en pratique du formalisme, on étudie en profondeur les propriétés de transport de charge et de spin dans un régime hydrodynamique d’équilibre local (ou quasi-équilibre) et dérive des expressions analytiques qui incorporent les caractères non-relativiste et relativiste des gaz d’électrons avec couplage spin-orbite de type Rashba. Dans la deuxième partie de la thèse, on s’occupe du problème de traiter analytiquement les fortes interactions électron-électron dans le régime de l’effet Hall quantique fractionnaire. À cette fin, on étudie un problème à deux corps généralisé avec du désordre et des corrélations électroniques, en utilisant une nouvelle représentation d’états de vortex cohérents. Des corrélations à longue portée entre les particules sont incor- porées de manière topologique à travers la présence d’une métrique non-Euclidienne. Subséquemment, on montre que ces états de vortex forment bien une base d’un espace de Hilbert élargi, puis on dérive l’équation du mouvement pour la fonction de Green. Enfin, on vérifie la consistance de notre théorie pour tout niveau de Landau de paire et on discute la nécessité d’aller au-delà de la limite semiclassique (à champ magnétique infinie) pour obtenir des gaps dans chaque niveau de énergie.

Liens :Daniel Hernangomez-PerezLPMMC

• Séminaire LPMMC
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : The recent impressive experimental progress in the field of quantum simulation paved the way to address the out-of-equilibrium dynamics of many-body systems with an unprecedented degree of control and tunability. Particularly suitable candidates in this respect are cold atoms in optical lattices and arrays of coupled QED cavities. I will provide an excerpt of my recent theoretical results on the non-equilibrium quantum physics and quantum transport, in both closed and dissipative systems. I will put emphasis on the short- and the long-time dynamics after a sudden quench, emphasizing how strong local correlations between quantum objects are able to non-perturbatively drive the system's behavior. Moreover, I will highlight how the presence of dissipation, naturally driving the system under non-equilibrium conditions, may drastically change the scenario.

Liens :Davide RossiniÉcole normale supérieure de Pise

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In this talk I will present an overview of two exciting, and rather different, recent applications of entanglement in the study of quantum many-body systems, inspired by the formalism of "tensor networks" and their potential use in quantum computing. I will show that entanglement provides a way to understand the topologically-ordered phases of matter with exotic anyon excitations, such as those arising in the context of the fractional quantum Hall effect. Here entanglement acts as a link between conformal field theory, solvable models, and the "matrix-product state" formalism that enables efficient computational studies of these systems. In the second example, I will illustrate the role of entanglement in understanding the properties of "many-body localized" phases that have attracted some attention recently as a novel class of ergodicity-breaking phases that fail to thermalize. It will be shown how entanglement helps us understand the emergent local integrals of motion which characterize the many-body localized phases and strongly constrain their non-equilibrium dynamics, perhaps leading to possible applications as platforms for "quantum memories".

Liens :Zlatko PapicPerimeter Institute

• Séminaire théorie
• Personne à contacter :
• Salle Mott, D420, Institut Neel, 3eme etage, Bat D

Résumé : The electronic transport trough a tunnel junction (normal, magnetic or superconducting) is affected by its electromagnetic environment. The hallmark of such an interaction is the Dynamical Coulomb Blockade (DCB), a quantum phenomenon associated with the activation of a Coulomb gap at finite voltage due to inelastic tunneling processes assisted by the environmental degrees of freedom [1]. On the other hand, the light emitted into the environment is a direct witness of the electronic transport, offering an alternative way to access it [2]. In the first part of the talk, I will study the magnetization dynamics in ferromagnet|insulator|ferromagnet and ferromagnet|insulator|normal metal ultra-small tunnel junctions, and the associated voltage drop in the presence of an electromagnetic environment assisting the tunneling processes. We find that voltages comparable to the driving frequency ω can be reached even for small magnetization precession cone angles θ, in stark contrast to the case where the environment is absent [3,4]. We stress some possible applications of such an effect, such as the detection of local magnetization precessions in textured ferromagnets. In the second part, we investigate the properties of the light emitted by a voltage-biased Josephson junction coupled to a specific electromagnetic environment [2,5,6]. We show that depending on the particular implementations [2,6], the resulting photons emitted by the junction can show pronounced non-classical behavior that is quantified by the second-order photonic correlation function g²(τ). We calculate explicitly the g²(τ) function in different limits, and point out that our theoretical results are in very good agreement with the recent experimental work in Ref. [6]. Finally, we discuss some further theoretical and experimental extension of our work such as the production and detection of photonic squeezed states and photonic entanglement.

[1] G. L. Ingold and Y. Nazarov, Single charge tunneling (Plenum, Berlin Heidelberg, 1992).
[2] M. Hofheinz, F. Portier et al, Phys. Rev. Lett, 106, 217005 (2011).
[3] T. Moriyama, R. Cao, et al, Phys. Rev. Lett., 100, 067602 (2008).
[4] Mircea Trif and Pascal Simon, arXiv:1405.5744 (to appear in PRB).
[5] Mircea Trif and Pascal Simon, (unpublished).
[6] O. Parlavecchio, F. Portier et al, (to be published).

Liens :Mircea TrifLPS Orsay

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : répétition soutenance de thèse

Liens :LPMMC

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : We evaluate Renyi entropy flows from generic quantum heat engines (QHE) to a weakly-coupled probe environment kept in thermal equilibrium. We show the flows are determined by two quantities: heat flow and fictitious dissipation that manifest the quantum coherence in the engine. This pertains also the common Shanon entropy flow. The results appeal for revision of the concept of entropy flows in quantum non-equilibrium thermodynamics.

Liens :Yuli Nazarov

• Séminaire théorie
• Personne à contacter :
• Amphitéâtre, maison des Magistères

Résumé : The last few years have seen a growing experimental and theoretical activity aiming at extending the simulation potential of cold atoms to transport properties. In this presentation, I will show how one can benefit from the high tunability of cold atom setups to engineer optimal transport properties. In this framework, I will discuss the proposal and experimental realization of the investigation of thermoelectric effects with ultracold atoms, and the possibility to use well designed transport properties to cool Fermi gases deep into the degeneracy regime.

Liens :Charles Grenier

• Soutenance HDR
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Liens :Denis BaskoLPMMC

• Séminaire théorie
• Personne à contacter :
• Salle Mott, D420, Institut Néel, 3e étage, bât. D

Résumé : In the presence of even weak amounts of disorder, low dimensional topological band insulators turn into topological Anderson insulators (tAI). Translational invariance being absent, the tAI must be described by concepts different from the clean limit band structures classification schemes. In this talk we argue that much of the universal physics of the tAI is contained in the system size dependent flow of two parameters, the first of which is an average transport coefficient (at any finite size, the tAI is a conductor), and the second the mean value of a now statistically distributed topological index. These two parameters exhibit flow similar to that of the Pruisken-Khmelnitskii flow diagram of the quantum Hall insulator. Specifically, the flow describes describes quantum criticality at topological phase transitions, the approach towards an insulating configuration away from criticality, and, along with it, the emergenece of a self averaging integer index. For some symmetry classes, that flow can be established in closed analytic form. However, we argue that the overall picture is of more general validity and provides a unified framework to describe both the bulk and the surface physics of the topological Anderson insulator.

Liens :Alex Altland

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Liens :LPMMC

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In the first part of the talk, we study the competition between magnetic and spin-liquid phases in the Hubbard model on the anisotropic triangular lattice, which is described by two hopping parameters in different spatial directions, and is relevant for layered organic charge-transfer salts and for the inorganic compounds Cs2CuBr4 and Cs2CuCl4.[1,2] By using variational wave functions which include both Jastrow and backflow terms, we provide solid evidence that two spin-liquid phases are stabilized in the strongly correlated regime, while states with spiral magnetic order and a non trivial pitch vector are found close to the isotropic point. Two different kinds of collinear orders are found in a wide region of the phase diagram close, respectively, to the limits of square lattice and decoupled one-dimensional chains. We also introduce another family of organic charge-transfer salts where a fully anisotropic triangular-lattice description produces importantly different results, including a significant lowering of the critical U of the spin-liquid phase.[3]

In the second part of the talk, we consider the extended Hubbard model on the isotropic triangular lattice as a function of filling and interaction strength. The complex interplay of kinetic frustration and strong interactions leads to exotic phases where charge order, antiferromagnetic order, and metallic conductivity can coexist. Our variational Monte Carlo simulations show that three kinds of ordered metallic states are stable as a function of nearest-neighbor interaction and filling. In one of the phases, the coexistence of conductivity and order is explained by a separation into two functional classes of particles: part of them contributes to the stable order, while the other part forms a partially filled band on the remaining substructure.[4]

[1] Tocchio, Feldner, Becca, Valenti, Gros, PRB 87, 035143 (2013)
[2] Tocchio, Gros, Valenti, Becca, PRB 89, 235107 (2014)
[3] Jacko, Tocchio, Jeschke, Valenti, PRB 88, 155139 (2013)
[4] Tocchio, Gros, Zhang, Eggert, arXiv:1402.3160

Liens :

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Liens :

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Annulé
L'orateur est malade.

Liens :Peter PuschnigUni Graz

• Séminaire LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : In general, Quantum Monte Carlo methods suffer from a sign problem when simulating fermionic systems. This causes the efficiency of a simulation to decrease exponentially with the number of particles and inverse temperature. To circumvent this issue, a nodal constraint is often implemented, restricting the Monte-Carlo procedure from sampling paths that cause the many-body density matrix to change sign. Unfortunately, this high-dimensional nodal surface is not a priori known unless the system is exactly solvable, resulting in uncontrolled errors. We will discuss two possible routes to extend the applicability of finite-temperature path integral Monte Carlo. First we extend the regime where signful simulations are possible through a novel permutation sampling scheme. Afterwards, we discuss a method to variationally improve the nodal surface by minimizing a free energy during simulation. Applications of these methods will include both free and interacting electron gases, concluding with discussion concerning extension to inhomogeneous systems.

Liens :Lausanne

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : The non-interacting topological insulators (TIs) have attracted great interest in the last decade. While this class of materials is today well-understood, the effect of electron-eletron interactions in such systems remains in general elusive. In this talk, I will address and discuss two different aspects of interactions in 2D topological band structures: (i) in some cases, strongly interacting TI models can be used to describe the exotic magnetic properties of certain transition metal oxides. (ii) in other cases, strong interactions can drive a TI into spin-liquid phases.

Liens :Stephan RachelTU Dresden

• Séminaire interne LPMMC
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Liens :LPMMC

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, maison des Magistères

Résumé : Feshbach resonances - namely resonances between an unbound two-body state (atomic state) and a bound (molecular) state - are a unique tool to tune the interaction properties of ultracold atoms. In this talk, I will show that the coherent coupling of the atomic and molecular state, can lead to a novel insulating phase - the Feshbach insulator - for bosons in an optical lattice close to a narrow Feshbach resonance. This new state of quantum matter appears around the resonance, preventing the system from collapsing when the effective atomic scattering length becomes negative. Surprisingly enough, the transition from condensate to Feshbach insulator has a characteristic first-order nature, due to the simultaneous loss of coherence in the atomic and molecular components. Our realistic study shows that these features appear clearly in the ground-state phase diagram of e.g. rubidium 87 around the 414 G resonance, and they are therefore directly amenable to experimental observation.

Liens :Laurent de Forges de ParnyENS Lyon

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Quasicrystals can be modelled with a collection of polygons (tiles) that cover the whole plane, so that each pattern (a sub-collection of tiles) appears up to translation with a positive frequency, but the tiling is not periodic. The frequency of presence of each pattern determines the spectrum of the system, and this is the subject of the gap labelling theory [1].

Using a microwave realization of a tight-binding Penrose-tiled quasicrystal [2], we measured the gap labelling and the spatial energy distribution of each eigenstate [3]. The energy-scaling behaviour of the hopping terms in this particular system, allowed us to identify the main patterns that determine the first hierarchical structure of the spectrum . Our energy-scaling analysis enabled us not only a straightforward interpretation of the gap labelling but also a full understanding of the wavefunction behaviour observed in each band.

The next goal will be the realization of a light Penrose-tiled quasicrystal for ultracold atoms, the underlying idea being to have access to samples with a lager number of tiles and thus to have the opportunity to observe the hierarchical effect of larger and larger patterns in the gap labelling. We have shown that this may be possible by mapping the gap labelling on a Brillouin zone (BZ) labelling and measure the areas of the extended BZs or the Bragg peak intensity distribution via different time-of-flight techniques [4].

[1] J. Bellissard in "From Number Theory to Physics", 538 (Springer, 1993).
[2] M. Bellec, et al., Phys. Rev. Lett. 110, 033902 (2013).
[3] P. Vignolo et al. , in preparation.
[4] J.M. Gambaudo, P. Vignolo, New J. Phys. 16, 043013 (2014).

Liens :Patrizia VignoloInstitut Non Lineaire de Nice

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In this talk I will show some applications of the Time Dependent Density Functional Theory and Many Body Perturbation Theory (GW/BSE) on molecular systems of technological and biological interest (natural dyes, retinal chromophore, Peridinin carotenoid), focusing on the effects of intrinsic structural fluctuations and environment (solvent, protein) on their optical properties. I will illustrate how these effects dramatically affect the absorption spectra of these systems, and how they can be reasonably taken into account in TDDFT and MBPT framework via multi-scale techniques or hybrid quantum-classical approximations using quantum Monte Carlo(QMC) method for the structural properties and the Bethe Salpeter equation to simulate the excitations of these molecules in their "natural” environment.

Liens :Daniele VarsanoLa Sapienza - Roma

• Séminaire théorie
• Personne à contacter :
• Salle de lecture 2, Maison des Magistères

Résumé : Dans les années 40, C.R. Rao a réalisé que la matrice d'information de Fisher, connue en statistique paramétrique, se transformait comme une métrique riemannienne. Cette réalisation a permis l'application de la géométrie riemannienne en statistique paramétrique, et peut être considérée comme le point de départ de la géométrie de l'information. L'exposé présente une nouvelle approche de la classification bayésienne des populations gaussiennes univariées, qui s'appuie sur la géométrie riemannienne de Rao. Il est basé sur un article récemment publié, par S. Said, L. Bombrun et Y. Berthoumieu [1]. Dans une première partie, la définition de la métrique riemannienne de Rao est rappelée. Un résultat classique montre que le modèle gaussien univarié, muni de cette métrique, s'apparente au demi-plan de Poincaré, (espace hyperbolique de dimension 2). On introduit ensuite une nouvelle classe de distributions de probabilité sur le demi-plan de Poincaré, qu'on appelle a prioris riemanniens, et on montre comment les paramètres de ces distributions peuvent être estimés. La partie finale de l'exposé présente en détail l'application des deux premières parties au problème de la classification des populations gaussiennes univariées. Dans la limite où les populations en question sont grandes, l'approximation de Laplace permet de se ramener au problème d'estimation de paramètres, présenté dans la deuxième partie de l'exposé.

[1] Salem Said, Lionel Bombrun and Yannick Berthoumieu, New Riemannian Priors on the Univariate Normal Model, Entropy 16, 4015-4031 (2014)

Liens :Groupe Signal et Image, IMS Bordeaux

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : A first-principles implementation of the Kadanoff-Baym Equation (KBE) for extended systems is presented. The KBE describes both the carriers dynamics and the polarization dynamics by means of the out-of-equilibrium Green’s function [1]. When the polarizarion is considered, the inclusion of the static self-energy and the changes in the time-dependent Hartree potentials give, in the linear regime limit, the time dependent extension of the well-known Bethe-Salpeter equation [2]. These effects also ensures a correct coupling between the electronic system and the laser pulse, that is between the field intensity and the number of electrons injected in the conduction band. The terms describing the dynamical correlation instead, also known as scattering terms, are the key players in the relaxation process. In our approach they are included within the out of equilibium extensions of the GW, for the electron-electron interaction, and FAN, for the electron-phonon interaction self-energy [3]. In particular we describe how equilibrium is restored in bulk Silicon, when carriers are injected in the conduction band by an ultra-short laser pulse. The excited electrons and holes relax towards two Fermi distributions, within about one hundred femtoseconds. While the two Fermi distributions are created the energy gained by the electronic system is dissipated to the lattice. The whole process is completed on the pico-seconds time-scale. The correct balance between the electronic and the phononic scattering processes is obtained thanks to a double grid sampling of the Brillouin zone. The results of the simulations are also compared with recent pump-probe measurments with femtosecond laser pulses [4–7]

[1] H. W. J. Haug, and A.-P. Jauho, Quantum kinetics in transport and optics of semiconductors, Springer series in solid-state science, 123, Springer-Verlag Berlin Heidelberg 1997, 2nd edition(2008)
[2] C. Attaccalite, M. Gruning, and A. Marini, Phys. Rev. B 84, 245110 (2011)
[3] A. Marini, J. Phys.: Conf. Ser. 427, 012003 (2013)
[4] D. Sangalli, and A. Marini, Ultra–fast degenerate L→L scattering versus inter–valley L→X scattering in bulk Silicon: a parameters free approach, manuscript in preparation (2014)
[5] D. Sangalli, and A. Marini, Ultra-fast carriers dynamics in semi-conductors: a first-principles many-body approach, manuscript in preparation (2014)
[6] A. Marini, C. Hogan, M.Gruning, and D. Varsano, Comp. Phys. Comm. 180, 1392 (2009)
[7] T. Ichibayashi, S. Tanaka, J. Kanasaki, and K. Tanimura, Phys. Rev. B 84, 235210 (2011)

Liens :Davide SangalliPolitecnico Milan

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Matter occurs in various phases with different properties. Usually these phases are characterized in terms of symmetry breaking. A major discovery in the 1980s was the quantum Hall effect which forms a new kind of “topological” order. This order represents exotic phases with unusual properties and cannot be understood in terms of symmetry breaking. Since then, a growing number of instances of topological phases has accumulated, and important applications – not least topological quantum computers – have been proposed, but a characterization and classification of these new phenomena has been slow to emerge. In parallel, DMRG has arrived as a powerful numerical method with ex- tensions to two dimensional systems and time-dependent phenomena. I will show how to use DMRG to develop new frameworks that help to understand topologically ordered systems. For example, it is now possible to extract characterizing properties of the anyonic excitations di- rectly from the ground state of fractional quantum Hall systems. This approach further makes contact with “measurable” quantities (Hall viscosity) and field theories (central charge at critical points). Other remarkable examples are symmetry protected topological phases in one-dimensional systems for which DMRG provides a complete characterization.

Liens :Frank PollmannMPI Dresden

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : The new challenges posed by the need of finding strong rare-earth free magnets demand methods that can predict magnetization and magnetocrystalline anisotropy energy (MAE). In this talk, I will review the general status of electronic structure approaches to this problem. I will then argue that correlated electron effects, which are normally underestimated in band structure calculations, play a crucial role in the development of the orbitalcomponent of the magnetic moments. Because magnetic anisotropy arises from this orbital component, the ability to include correlation effects has profound consequences on our predictive power of the MAE of strong magnets. I will show that incorporating the local effects of electronic correlations with dynamical mean-field theory provides reliable estimates of the orbital moment, the mass enhancement and the MAE of YCo5.

Liens :Jian-Xin ZhuLos Alamos National Lab

• Colloque CPTGA
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Mesons are bound states of a quark and an antiquark. The four neutral mesons B_d, B_s, D, and K have the unique property that they mix with their antiparticles. This means that a meson-antimeson system forms a quantum-mechanical two-state system, exhibiting the familiar oscillations between the two states. These neutral meson sytems are gold mines for the study of the CP symmetry, which links the interactions of particles to those of antiparticles. Since 1964 it is known that CP is violated and today CP asymmetries are experimentally studied with high precision. These investigations might reveal new laws of nature well ahead of collider experiments which aim at the production of new particles in high-energy collisions.

Liens :Ulrich NiersteKIT

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Nous étudions la susceptibilité orbitale de systèmes 2D multibandes avec une paire de points de Dirac et interpolant entre le nid d'abeille (graphène) et le réseau dé (aussi appelé T3). Bien qu'ayant le même spectre d'énergie en champ magnétique nul, ces systèmes présentent des différences spectaculaires dans leur susceptibilité magnétique orbital, allant du dia- au para-magnétisme au niveau des points de Dirac (i.e. à demi-remplissage ou dopage nul). Nous montrons que ce comportement étonnant est lié à une phase de Berry topologique variant continûment de π (graphène) à 0 (dé). Ce dernier aspect contraint fortement les effets interbandes, entrainant une dépendance inhabituelle de la réponse magnétique dans toute la bande (i.e. à dopage fini).

A. Raoux, M. Morigi, J.N. Fuchs, F. Piéchon, G. Montambaux, "From Dia- to Paramagnetic Orbital Susceptibility of Massless Fermions", Phys. Rev. Lett. 112, 026402 (2014)

Liens :Jean-Noel Fuchs

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : The development of solar cells is driven by the need for clean and sustainable energy. Organic and dye sensitized cells (DSC) are considered as promising alternatives for traditional single crystal silicon cells, particularly for large area, low cost applications. However, the efficiency of such cells is still far from the theoretical limit.
First-principles quantum mechanical simulations may be used for computer-aided design of new materials, material combinations, and nano-structures for more efficient organic and dye-sensitized cells. To this end, it is important to obtain an accurate description of the electronic structure, including the fundamental gaps and energy level alignment at interfaces. This requires a treatment beyond ground-state density functional theory (DFT). Within the framework of many-body perturbation theory (MBPT), these properties may be calculated using the GW approximation, where G is the one-particle Green's function and W is the dynamically screened Coulomb potential.
In this talk I will provide an introduction to GW methods and demonstrate their applications to the components of organic and dye-sensitized solar cells: TiO2 clusters [1], organic semiconductors [2,3], dyes [4,5], and dye-sensitized TiO2 clusters [6,7].

References:
[1] N. Marom, M. Kim, and J. R. Chelikowsky, Phys. Rev. Lett. 108, 106801 (2012)
[2] T. Körzdörfer and N. Marom, PRB 86, 041110(R) (2012)
[3] N. Marom, F. Caruso, X. Ren, O. Hofmann, T. Körzdörfer, J. R. Chelikowsky, A. Rubio, M. Scheffler, and P. Rinke, PRB 86 245127 (2012)
[4] N. Marom, X. Ren, J. E. Moussa, J. R. Chelikowsky, and L. Kronik, Phys. Rev. B 84, 195143 (2011)
[5] E. Salomon, P. Amsalem, N. Marom, M. Vondracek, L. Kronik, N. Koch, and T. Angot, Phys. Rev. B 87 075407 (2013)
[6] N. Marom, J. E. Moussa, X. Ren, A. Tkatchenko, and J. R. Chelikowsky, Phys. Rev. B 84, 245115 (2011)
[7] N. Marom, T. Körzdörfer, X. Ren, A. Tkatchenko, and J.R. Chelikowsky, to be published

Liens :Noa MaromTulane University

• Colloque CPTGA
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Ferroelectricity in ABO3 perovskites and related compounds has been a topic of intensive research for more than 60 years. Recently, the coupling of the ferroelectric mode with other structural distortions has attracted an increasing interest since it offers promising and still widely unexplored possibilities to couple ferroelectricity with other functional properties and even to produce unusual phenomena. In this context, the trilinear coupling between ferroelectric and oxygen rotational modes in naturally-occuring and artificial layered perovskites emerged as a practical way to produce unusual dielectric properties or to achieve enhanced magneto-electric coupling. Focusing on titanate and vanadate systems, we will discuss here how even more exciting new phenomena can appear when additional orbital and charge orders enter into play!

Liens :Philippe GhosezUniversite de Liege

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In most systems that exhibit order at low temperatures, the order occurs in the elementary degrees of freedom such as spin or charge. Prominent examples are magnetic or superconducting states of matter. In contrast, emergent order describes the phenomenon where composite objects exhibit longer range correlations. Such emergent order has been suspected to occur in a range of correlated materials. One specific example are spin systems with competing interactions, where long-range discrete order in the relative orientation of spins may occur. Interestingly, this order parameter may induce other phase transitions as is the case for the nematic transition in the iron pnictides. In this talk, we introduce and discuss a system with emergent Z6 symmetry, a two-dimensional frustrated Heisenberg antiferromagnet on the windmill lattice consisting of interpenetrating honeycomb and triangular lattices [1,2]. The multiple spin stiffnesses can be captured in terms of a four-dimensional metric tensor, and the renormalization group flow of the stiffnesses is described by the Ricci flow of the metric tensor. The key result is a decoupling of an emergent collective degree of freedom given by the relative phase of spins on different sublattices. In particular, our results reveal a sequence of two Berezinskii-Kosterlitz-Thouless phase transitions that bracket a critical phase.

[1] P. P. Orth, P. Chandra, P. Coleman, J. Schmalian, Phys. Rev. Lett. 109, 237205 (2012). [2] P. P. Orth, P. Chandra, P. Coleman, J. Schmalian, Phys. Rev. B 89, 094417 (2014).

Liens :Peter OrthKIT

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Upon application of an external tuning parameter, a magnetic state can be driven to a normal metal state at zero temperature. This phenomenon is known as quantum criticality and leads to fascinating responses in thermodynamics and transport of the compound. In the standard picture, a single quantum critical point occurs at zero temperature, which results in a nontrivial critical behaviour in its vicinity. Here we show that in two dimensions the scenario is considerably more complex due to the enormous amount of quantum fluctuations. Instead of the single point separating the antiferromagnet from the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterised by a superposition of d-wave superconductivity and a quadrupole-density wave, id est a state in which an electron quadrupole density spatially oscillates with a period drastically different from the one of the original spin-density wave. At moderate temperatures both orders co-exist at short distances but thermal fluctuations destroy the long-range order. Below a critical temperature the fluctuations are less essential and superconductivity becomes stable. This new phenomenon may shed some light on the origin of the mysterious pseudogap state and of the high-temperature transition into the superconducting state in cuprates. Our results demonstrate that quantum phase transitions between antiferromagnets and normal metals in layered materials may be the proper playground for search of new high temperature superconductors.

Liens :Catherine PépinIPhT

• Colloque CPTGA
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Quantum annealing - a finite temperature version of the quantum adiabatic algorithm - combines the classical technology of slow thermal cooling with quantum mechanical tunneling, to try bring a physical system towards its ground state. The Canadian company D-Wave systems has recently built and sold programmable devices that are designed to use this effect to find solutions to hard optimization problems. I will present results of experiments designed to shed light on crucial questions about these controversial devices: are these devices quantum or classical? Are they faster than classical devices? What is their potential?

Liens :Matthias TroyerETHZ

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Helical edge states are the electronic conducting states emerging at the boundaries of a 2D topological insulator, which are currently under the spotlight due to their peculiar properties. These channels are characterized by a tight connection between the electron group velocity and the spin orientation, and exhibit a perfect transmission, as recently observed in CdTe/HgTe and InAs/GaSb quantum wells. When a constriction is realized in the 2D quantum well, electron tunneling between helical edge states occurs via two types of channels allowed by time-reversal symmetry, namely spin-preserving (p) and spin-flipping (f) tunneling processes. Determining and controlling the effects of these two channels is crucial to the application of helical edge states in spintronics. We show that, despite the Hamiltonian terms describing these two processes do not commute, the scattering matrix entries of the related 4-terminal setup always factorize into products of p-terms and f-terms contributions. Such factorization provides an operative way to determine the transmission coefficient Tp and Tf related to each of the two processes, via transconductance measurements. Furthermore, these transmission coefficients are also found to be controlled independently by a suitable combination of two side gate voltages applied across the junction. This result holds for an arbitrary profile of the tunneling amplitudes, including disorder in the tunnel region, enabling us to discuss the effect of the finite length of the tunnel junction, and the space modulation of both magnitude and phase of the tunneling amplitudes.

Reference:
P. Sternativo and F. Dolcini, Phys. Rev. B 89, 035415 (2014)

Liens :Fabrizio DolciniNQS - Politecnico di Torino

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : We investigate the dynamics of a critical XXZ spin-1/2 chain prepared in an inhomogeneous initial state with different magnetizations on the left and right halves. We simulate the real-time evolution using the time-evolving block decimation (TEBD) method. We follow the front propagation by measuring the magnetization and entanglement entropy profiles, and we focus on the situation where the initial state is not fully polarized. At long times, as in the free fermion case (T. Antal et al. 1999), a large central region develops where correlations become time-independent and translation invariant. The shape and speed of the fronts is studied numerically and we evaluate the stationary current as a function of initial magnetic field and as a function of the anisotropy \Delta. We compare the results with the conductance of a Tomonaga-Luttinger liquid, and with the exact free-fermion solution at \Delta=0. We also investigate the two-point correlations in the stationary region and find a good agreement with the "twisted" form obtained by J. Lancaster and A. Mitra (2010) using bosonization. Some deviations are nevertheless observed for strong currents.

Liens :Gregoire MisguichIPhT

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In this talk I will focus on inducing topological state from regular, or irregular scattering in (i) p-wave superconducting and (ii) proximity coupled Rashba wires. In particular, I will discuss how, while being detrimental to the topological order in p-wave wires, the disorder can {it induce} topological state in Rashba wires contrary to common expectations. The total phase space area of the topological state is conserved for long disordered wires, and can even be increased in an appropriately engineered superlattice potential. I will also discuss how, in finite wires, the Majorana state oscillates as a function of chemical potential and magnetic field. I will conclude with a discussion of recent experiments, which was conducted on dirty Rashba wires, in the light of these new results.

Liens :Inanc AdagideliSabanci University

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : We present an accurate method to determine ground-state properties of strongly-correlated electrons decribed by lattice-model Hamiltonians. In lattice density-functional theory (LDFT) the basic variable is the one-particle density matrix $gamma$. From the HK theorem, the ground state Energy $E_{gs}[gamma_{gs}] = min_{gamma} E[gamma]$ is obtained by minimizing the energy over all the representable $gamma$. The energy functional can be divided into two contributions: the kinetic-energy functional, which linear dependence on $gamma$ is axactly known, and the correlation-energy functional $W[gamma]$, which approximation constitutes the actual challenge. Within the framework of LDFT, we develope a numerical approach to $W[gamma]$, which involves the exact diagonalisation of an effective many-body Hamiltonian of a cluster surrounded by an effective field. This effective Hamiltonian depends on the density matrix $gamma$. In this talk we discuss the formulation of the method and its application to the Hubbard and single-impurity Anderson models in one and two dimensions. The accuracy of the method is deponstrated by comparison with the Bethe-Ansatz solution (1D), density-matrix renormalization group calculations (1D), and quantum Monte Carlo simulations (2D).

Liens :Matthieu Saubanere

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : We study [1,2] the tunneling density of states (TDOS) of a disordered electronic system with Coulomb interaction on both the metallic and insulating sides of an Anderson-localization metal-insulator transition (MIT). We discuss how the zero-bias anomaly on the metal side transforms into the Coulomb-gap suppression of the TDOS in the localized phase of the MIT. For tunneling into the insulating phase, the average TDOS shows a critical behavior at high energies, with a crossover to a soft Coulomb gap Δ at low energies. We demonstrate that the single-particle excitations experience a localization transition (which belongs to the noninteracting universality class) at an energy E=±E_c. The mobility edge E_c scales with the distance μ_c−μ from the interacting critical point according to E_c∝(μ_c-μ)^{nu z}, where nu and z are the interacting localization-length and the dynamical critical exponents. Our theoretical expectations and the "phase diagram" of the Anderson MIT in the presence of Coulomb interaction are in an overall agreement with recent experimental results [3] for the average TDOS in a device with a tunable doping level across the MIT.

We further explore mesoscopic fluctuations and correlations of the local density of states (LDOS) near the MIT. It is shown that the LDOS multifractality survives in the presence of Coulomb interaction. Specifically, the LDOS shows strong fluctuations and long-range correlations which reflect the multifractality associated with interacting and noninteracting fixed points as well the localization of low-energy excitations. By using the onlinear sigma-model approach, we calculate the spectrum of multifractal exponents of a Coulomb-interacting system without time-reversal and spin symmetries in 2 et ϵ spatial dimensions and show that it differs from that in the absence of interaction. The multifractal character of fluctuations and correlations of the LDOS can be studied experimentally by scanning tunneling microscopy of two-dimensional and three-dimensional disordered structures. Our results are in an overall agreement with the experimental data of Ref. [4]. In addition to MIT and transitions between different phases of topological insulators, we envision a possibility to extend our analysis also to superconductor-insulator transitions.

[1] I.S. Burmistrov, I.V. Gornyi, and A.D. Mirlin, Phys. Rev. Lett. 111, 066601 (2013).
[2] I.S. Burmistrov, I.V. Gornyi, and A.D. Mirlin, Phys. Rev. B 89, 035430 (2014).
[3] A. Mottaghizadeh, Q. Yu, P. L. Lang, A. Zimmers, and H. Aubin, Phys. Rev. Lett. 112, 066803 (2014).
[4] A. Richardella, P. Roushan, S. Mack, B. Zhou, D.A. Huse, D.D. Awshalom, and A. Yazdani, Science 327, 665 (2010).

Liens :Igor GornyiKIT

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : The quantum coherence effects between electrons in nanodevices lead to a rich variety of phenomena in quantum transport. One of these is the violation of Kirchhoff's laws in the quantum RC-circuit. In this system, a metallic lead exchanges electrons coherently with a quantum dot driven dynamically by a top metallic gate. In the non interacting case, the charge relaxation resistance of the system differs from the usual dc-transport resistance given by the Landauer formula. The charge relaxation resistance is universally fixed to h/(2e2) for a single mode conductor, regardless of the transmission of the mode. I will show that the Fermi liquid behavior of these systems at low energy explains this universality even in the presence of strong interactions in the dot. Moreover, I will discuss the emergence of a giant dissipation regime associated to the breaking of the Kondo singlet for Zeeman energies of the order of the Kondo temperature. I will provide a comprehensive analytical description of the peak of the charge relaxation resistance associated to this giant dissipation and demonstrate its persistence out of the Kondo regime.

Liens :FU-Berlin

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In one and two spatial dimensions there is a logical possibility for identical quantum particles different from bosons and fermions, obeying intermediate or fractional (anyon) statistics. I will consider applications of recent bounds for the energy of a many-particle wave function in terms of its density, so called Lieb-Thirring inequalities, to models of anyons in two dimensions, as well as to models in one dimension of Lieb-Liniger and Calogero-Sutherland type. These bounds follow from a local form of the exclusion principle valid for such generalized exchange statistics modeled by interactions. This is joint work with Jan Philip Solovej.

Liens :Douglas LundholmKTH

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : A phenomenological analysis of resistance and diamagnetic response in the LaAlO3/SrTiO3 or LaTiO3/SrTiO3 (LXO/STO) heterostructures suggests the occurrence of a low-dimensional (e.g., filamentary or fractal) structure of the superconducting cluster with small long-distance connectivity embedded in the two-dimensional system [1,2]. To explain the systematic occurrence of such mesoscopically disordered regions, we model the electron gas at the interface of oxide heterostructures considering a twodimensional electron gas in the presence of a sizable Rashba spin-orbit coupling (RSOC). Under simple general assumptions, we show that an electronic phase separation occurs for realistic values of the RSOC and of the band parameters [3,4]. This could provide an intrinsic mechanism for the recently observed inhomogenous phases at the LAO/STO or LTO/STO interfaces and opens the way to new interpretations of the quantum critical behaviour of LTO/STO [5]. We investigate the effects of temperature and magnetic field on the charge instability finding a novel type of quantum critical point related to the vanishing of the critical temperature of the electronic phase separation [4].

[1] D. Bucheli, S. Caprara, C. Castellani, and M. Grilli, New Journal of Physics 15, 023014 (2013).
[2] S. Caprara, J. Biscaras, N. Bergeal, D. Bucheli, S. Hurand, C. Feuillet-Palma, A. Rastogi, R. C. Budhani, J. Lesueur, and M. Grilli, Phys. Rev. B 88, 020504(R) (2013).
[3] S. Caprara, F. Peronaci, and M. Grilli, Physical Review Letters 109, 196401 (2012)
[4] D. Bucheli, M. Grilli, F. Peronaci, G. Seibold, and S. Caprara, arXiv:1307.5427.
[5] J. Biscaras, N. Bergeal, S. Hurand, C. Feuillet-Palma, A.Rastogi, R. C. Budhani, M. Grilli, S.Caprara, J. Lesueur, arXiv:1209.6464, and Nature Materials 12, 542 (2013)

Liens :Marco GrilliSapienza University

• Séminaire théorie
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : Over the last few years, there has been an upsurge of interest in materials in which exotic states may emerge as the result of relativistic spin-orbit interactions. We will discuss insulating iridium oxides from this perspective. We show that the strong spin-orbit coupling, through the entanglement of spin and orbital spaces, leads to a variety of interesting Hamiltonians ranging from the Heisenberg model to the Kitaev or quantum compass models, for different lattice geometries [1]. Based on these effective Hamiltonians, we present a comprehensive theoretical study [3,4] of the rich phase behavior and dynamics observed in layered iridium oxides such as tetragonal Sr2IrO4 and Sr3Ir2O7 and hexagonal A2IrO3 (A=Na, Li). We suggest that the hexagonal iridates might be close to the Kitaev spin-liquid state. We also discuss the layered tetragonal vanadate Sr2VO4 and argue that magnetically-hidden octupolar order, driven by spin-orbit coupling, is realized in this compound [2].

[1] G.Jackeli and G.Khaliullin, Phys.Rev.Lett. 102, 017205 (2009).
[2] G.Jackeli and G.Khaliullin, Phys.Rev.Lett. 103, 067205 (2009).
[3] J.Chaloupka, G.Jackeli, and G.Khaliullin, Phys.Rev.Lett. 105,
027204 (2010).
[4] J.Chaloupka, G.Jackeli, and G.Khaliullin, Phys.Rev.Lett. 110, 097204 (2013).

Liens :George JackeliMax Planck Institute - Stuttgart

• Colloque CPTGA
• Personne à contacter :
• Amphithéâtre, Maison des Magistères

Résumé : In this talk I will focus on passive and active motion in blood suspension and other suspensions. I will first discuss how to model blood flow from microscopic considerations (i.e. by taking blood element explicitly into account, e.g. red blood cells). I will show some dynamics of individual red blood cells, and then present collective dynamics and some intringuing rheological behaviors in microcirculation. I will then discuss some issues related to the so-called ameboid swimming. Microorganisms, such as bacteria, algae, or spermatozoa, are able to propel themselves forward thanks to flagella or cilia activity. By contrast, other organisms employ pronounced changes of the membrane shape to achieve propulsion, a prototypical example being the Eutreptiella gymnastica. Cells of the immune system as well as dictyostelium amoebas, traditionally believed to crawl on a substratum, can also swim in a similar way. A model will be presented and it will be shown that fast propulsion can be achieved with adequate shape adaptations. The autopropulsion distance over one cycle is a universal linear function of a simple geometrical dimensionless quantity A/V^(2/3) (V and A are the cell volume and its membrane area). This study captures the peculiar motion of Eutreptiella gymnastica with simple force distribution.

Liens :Chaouqi MisbahLIPHY