Séminaires LPMMC 2013
Alain Gellé (Institut de Physique de Rennes)  Détails Fermer 
Étude ab initio des phonons cohérents dans le Bismuth le vendredi 20 décembre 2013 à 11:00 

Résumé : En physique des matériaux, les expériences de type pompessondes ont permis d’étudier des phénomènes ultrarapides comme les transitions de phases photoinduites ou la génération de phonons cohérents. Dans ce domaine, le bismuth apparaît comme un système modèle. En effet, dans ce matériau, une impulsion ultracourte excite de façon sélective un phonon en centre de zone de symétrie A1g, ce qui provoque de fortes variations de la réflectivité. Récemment, les expériences de diffraction X résolues en temps sont venues enrichir les mesures de réflectivité en permettant d’accéder directement aux déplacements atomiques liés au phonon. Cependant, ces résultats expérimentaux s’avèrent insuffisants si l’on souhaite comprendre en détail les mécanismes de génération des phonons cohérents.

Gregory Schehr (LPTMS)  Détails Fermer 
Maximal height of N nonintersecting Brownian excursions: from YangMills theory to interfaces in disordered media le vendredi 13 décembre 2013 à 11:00 

Résumé : Nonintersecting random walkers (or 'vicious walkers') have been studied in various physical situations, ranging from polymer physics to wetting and melting transitions and more recently in connection with random matrix theory or stochastic growth processes in the KardarParisiZhang (KPZ) universality class. In this talk, I will present a method based on path integrals associated to free Fermions models to study such statistical systems. I will use this method to calculate exactly the cumulative distribution function (CDF) of the maximal height of N nonintersecting Brownian excursions. I will show that this CDF is identical to the partition function of 2d Yang Mills (YM) theory on a sphere with the gauge group Sp(2N). I will show that, in the large N limit, the CDF exhibits a third order phase transition, akin to the DouglasKazakov transition found in 2d YM. I will also show that the critical behavior, close to the transition point, is described by the TracyWidom distribution for $eta = 1$, which describes the fluctuations of the largest eigenvalue of Random Matrices belonging to the Gaussian Orthogonal Ensemble. Liens :Gregory SchehrLPTMS 
Pavel Grigoriev (Landau Institute / Chercheur invite CPTG)  Détails Fermer 
Electronic transport as a tool to investigate the microscopic structure of a densitywave state le lundi 9 décembre 2013 à 14:00 

Résumé : The electronic transport is very sensitive to the microscopic structure of the densitywave state and may serve as a powerful tool for its investigation. In my talk I present several examples of how electronic transport reveals the microscopic structure of the DW state. In some cases the theory of these effects is evident; sometimes they are less trivial, being a longstanding puzzle. After a brief review I will talk about two recent examples of an unusual influence of a density wave on the electronic transport: (i) Spontaneous breaking of isotropy observed in the inplane conductivity of rareearth tritellurides, and (ii) the phase inversion of the Shubnikovde Haas oscillations after passing a transition to a densitywave state observed in organic metals. Liens :Pavel GrigorievLandau Institute / Chercheur invite CPTG 
Jean Christian Angles d'Auriac (Institut Neel)  Détails Fermer 
Locally selfsimilar phase diagram of a disordered Potts model on a hierarchical lattice le vendredi 6 décembre 2013 à 11:00 

Résumé : The Potts model with bond disorder is studied in the limit of a infinite number of states on a hierarchical lattice. This model is "academical" since neither the hierarchical lattice nor the infinite number of states are really physical. However the final result, ie the phase diagram of the model obtained by renormalization, is unusual and can justify this study. 
Cristina Bena (LPS Orsay)  Détails Fermer 
Majorana and Andreev bound states in topological wires in the proximity of superconductors le vendredi 29 novembre 2013 à 11:00 

Résumé : We study onedimensional topological SN and SNS long junctions obtained by placing a topological insulating nanowire in the proximity of either one or two SC finitesize leads. Using the Majorana Polarization order parameter we find that for a finitesize SN junction the ABS spectrum exhibits a zeroenergy extended state which carries a full Majorana fermion, while the ABS of long SNS junctions with a phase difference of pi transform into two zeroenergy states carrying two Majorana fermions with the same MP. We also study the effects of finite SC penetration depths in such junctions, as well as the effects of uniform phase gradients. Last but not least we analyze a more realistic model for the coupling between a superconducting substrate and a topological wire, the resulting proximity effect, and the role played by the Andreev bound states in the topological wire in such a setup. Liens :Cristina Bena et www.lps.upsud.fr 
Guillaume James (Laboratoire Jean Kuntzmann)  Détails Fermer 
Localized waves in granular crystals le vendredi 22 novembre 2013 à 11:00 

Résumé : Granular crystals consist of a collection of masses (typically steel beads) arranged on a regular lattice and interacting nonlinearly by contact. These systems display different types of nonlinear wave phenomena, such as the formation of localized waves (solitary waves or breathers) after an impact. The wave dynamics is strongly influenced by lattice properties (type of discrete elements, existence of confining potentials, precompression), which opens interesting possibilities to control stress waves. Granular crystals can be modeled by different types of lattice differential equations depending on their structural properties. In particular, onedimensional granular chains can lead to the FermiPastaUlam (FPU) model with Hertzian potential, mixed FPUKleinGordon lattices or the discrete pSchrödinger equation, a new asymptotic model obtained when confining potentials are present. We will illustrate the rich properties of localized waves in these models through numerical simulations and analytical results. 
Istvan Daruka (JKU  Linz)  Détails Fermer 
On the Growth and Phase Transitions of SelfAssembled Quantum Dots  Theoretical Studies le vendredi 15 novembre 2013 à 11:00 

Résumé : The talk discusses first some earlier analytic approaches on the growth and phase transitions of selfassembled quantum dots formed on semiconductor surfaces. Furthermore, current related research activity will be presented regarding a remarkable bulk morphological transition taking place in a layered prototype system. Our related Monte Carlo and CahnHilliard simulations reproduced all the experimentally observed morphological phases including the growth of pinholes, the formation of a percolation network and its breaking up into isolated quantumdots. The simulated dot sizes were in a quantitative agreement with the experimental values. The robustness of the results, i. e. the insensitivity to temperature and to the details of atomic exchange mechanisms and binding interaction types undoubtedly demonstrates the topological nature of this nanocapillarity forces driven transition. Liens :Istvan DarukaJKU  Linz 
Giuseppe Carleo (Laboratoire Charles Fabry)  Détails Fermer 
Universal superfluid transition and transport properties of twodimensional dirty bosons le vendredi 8 novembre 2013 à 11:00 

Résumé : The interplay of disorder and interactions has been at the center of intense theoretical and experimental activity the past decades. It sustains rich phase diagrams, relevant to a wide range of condensedmatter systems. Recent evelopments with ultracold atoms spark a renewed interest and open new challenging issues. For instance, in interacting systems on the continuum, the intervening phases are largely debated and still in a conjectural stage. In this talk I will discuss the phase diagram of disordered and interacting ultracold atoms in two dimensions, that we have obtained thanks to accurate, largescale Quantum Monte Carlo simulations.[I] I will show that the superfuid transition is strongly protected against disorder, up to the zerotemperature Boseglass transition. Most of its critical properties can be understood in terms of an universal BKT description with a simple scaling of the critical temperature versus the disorder strength. I will then address the strongly disordered regime at finite temperature where the possible existence of a (manybody) localized phase constitutes a challenging open question. Thanks to a taylored methodological improvement, we have gained direct access to the conducting properties. I will show that the finitetemperature insulating phase merging at large disorder strength is well described by a thermally activated behavior of the Arrhenius type. 
Nicolas Rougerie (LPMMC)  Détails Fermer 
BoseEinstein condensation of interacting particles and the quantum de Finetti theorem le vendredi 18 octobre 2013 à 11:00 

Résumé : The observation of BoseEinstein condensation in dilute atomic gases twenty years ago has given a new impetus to the theoretical study of large bosonic systems. Much of our current understanding of this physics is based on the meanfield approximation, which in this context roughly amounts to assuming that all particles behave independently of one another. That this approximation is a sensible one for a great variety of large bosonic systems is a remarkable fact, and I will argue that it can be seen as following from a very special structure property of the set of bosonic states, the quantum de Finetti theorem. I shall discuss the original theorem along with recent variants and applications to interacting bosonic systems. This is joint work with Mathieu Lewin and Phan Thành Nam. Liens :Nicolas RougerieLPMMC 
Jan von Delft (LMU)  Détails Fermer 
Microscopic Origin of the 0.7Anomaly in Quantum Point Contacts le jeudi 10 octobre 2013 à 11:00 

Résumé : Quantum point contacts, elementary building blocks of semiconductorbased quantum circuits, are narrow onedimensional constrictions usually patterned in a twodimensional electron system, e.g. by applying voltages to local gates. It is one of the paradigms of mesoscopic physics that the linear conductance of a point contact, when measured as function of its channel width, is quantized in units of GQ = 2e²/h. However, the conductance also exhibits an unexpected shoulder at ∼ 0.7 GQ, known as the "0.7anomaly", whose origin is still subject to debate. Proposed theoretical explanations have evoked spontaneous spin polarization, ferromagnetic spin coupling, the formation of a quasibound state leading to the Kondo effect, Wigner crystallisation and various treatments of inelastic scattering. However, explicit calculations that fully reproduce the various experimental observations in the regime of the 0.7anomaly, including the zerobias peak that typically accompanies it, are still lacking. Here we offer a detailed microscopic explanation for both the 0.7anomaly and the zerobias peak: their common origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest onedimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, magnetic spin susceptibility and inelastic scattering rate. We present theoretical calculations and experimental results that show good qualitative agreement for the dependence of the conductance on gate voltage, magnetic field, temperature, sourcedrain voltage (including the zerobias peak) and interaction strength. We also clarify how the lowenergy scale governing the 0.7anomaly depends on gate voltage and interactions. For low energies we predict and observe Fermiliquid behaviour similar to that known for the Kondo effect in quantum dots. At high energies, however, the similarities between 0.7anomaly and Kondo effect cease. Liens :Jan von DelftLMU 
Josef Kudrnovsky (Czech Science Academy)  Détails Fermer 
Theory of complex transport in magnetic metals and alloys le vendredi 4 octobre 2013 à 11:00 

Résumé : In this talk we will review a recent progress in the firstprinciples study of
transport properties of magnetic metals and alloys based on the KuboGreenwood
approach as formulated in the framework of the dirac version of the
tightbinding linear muffintin orbital method. A possible disorder in studied
systems is described by the coherent potential approximation.
Specifically, we will study: (i) The spindisorder resistivity (SDR) of
transition metal ferromagnets, rareearth metals, and Nibased Heusler alloys.
We identify the SDR at the Curie temperature with the residual resistivity of
the corresponding system evaluated in the framework of the disordered local
moment (DLM) model [1];
(ii) the anisotropic magnetoresistance (AMR) and the anomalous Hall effect (AHE)
of chosen Nibased transition metal alloys [2] as well as ordering Pdrich PdFe
alloys with complex lattice [3];
and the AHE in chosen halfmetallic Heusler alloys with native disorder [4].
Results of theoretical calculations will be compared with available experimental
data.

Pablo Arrighi (LIG)  Détails Fermer 
Discretetime, discretespace quantum theory le vendredi 27 septembre 2013 à 11:00 

Résumé : Physics usually formulates its laws in the language of Partial Differential Equations. But in order to solve these equations PDEs numerically, we usually have to discretize both time and space, thereby obtaining a discrete numerical model of the physical phenomenon that we are interested in. The broad category of computer models that are obtained in this way is called Cellular Automata. Thus, it could be said that we end up formulating physics phenomena in the language of computer programs. In this talk, we will see how much of that can be done with Quantum Physics. We will mention results about: the mathematical structure of discrete time discrete space models of quantum theory; the notion of simulation in those Quantum Cellular Automata; how to formulate some quantum physics phenomenon in the language of quantum computers programs. Liens :Pablo ArrighiLIG 
Claudio Attacallite (Institut Neel)  Détails Fermer 
Nonlinear optics from abinitio by means of the dynamical Berryphase le vendredi 20 septembre 2013 à 11:00 

Résumé : We present an abinitio realtime based computational approach to nonlinear
optical properties in Condensed Matter systems. The equation of motions, and in
particular the coupling of the electrons with the external electric field, are
derived from the Berry phase formulation of the dynamical polarization. The
zerofield Hamiltonian includes crystal local field effects, the renormalization
of the independent particle energy levels by correlation and excitonic effects
within the screened Hartree Fock selfenergy operator. The approach is
validated by calculating the secondharmonic generation of SiC and AlAs bulk
semiconductors: an excellent agreement is obtained with existing abinitio
calculations from response theory in frequency domain. We finally show
applications to the secondharmonic generation of CdTe, MoS2, hBN and the
thirdharmonic generation of Si.

D. W. Bukhvalov (KIAS)  Détails Fermer 
DFT modeling of the covalent functionalization of graphene: from ideal to realistic models le vendredi 6 septembre 2013 à 15:00 

Résumé : The production of multiple types of graphene, such as free standing, epitaxial graphene on silicon carbide and metals, graphene in solution, chemically grownand metals, graphene in solution, chemically grown graphenelike molecules, various graphene nanoribbons, and graphene oxide with different levels of reduction and various chemical composition, demonstrate the need for additional investigation beyond the basic principles of graphene functionalization for avoidance of occasionally contradictions between the predictions from firstprinciples simulations and experimental results. Herein, I report the current state of modeling of the different types of graphene using density functional theory (DFT) methods. The main focus is on the static (substrate, shape, curvature, strain and doping) and dynamic (starting point of functionalization, migration barriers and stability of configurations) aspects that provide a more correct and selective modeling of the chemisorption of various chemical species on the graphene scaffold. Based on the recent modeling of experimentally realized functionalization of different types of graphene we can conclude that the formation of uniform one or twosided functionalized graphene discussed in earlier studies is an exception to the typical scenarios of graphene chemistry. The presence of different substrates, defects and lattice distortions, such as ripples and strain, results in the formation of clusters or lines from the functional groups. Several configurations of the chemical species on the graphene substrate have been found to exist with ideal models but are only stable for graphene functionalized under special conditions. And finally employments of realistic models of graphenes for description of unexpected properties of graphene such as low dimensional ice formation or efficient catalysis of various reactions are also reported. Liens :D. W. BukhvalovKIAS 
Andrea di Ciolo (Georgetown University)  Détails Fermer 
Phase diagram of the frustrated spin1/2 XY and Heisenberg models on the honeycomb lattice le vendredi 6 septembre 2013 à 11:00 

Résumé : In this talk I present the phase diagram of the frustrated spin1/2 XY and
Heisenberg models on the honeycomb lattice, obtained by using accurate
correlated wave functions and Variational Monte Carlo simulations. Upon
increasing the frustration, these models show a very rich sequence of
spinordered phases and a spinliquid state is energetically favorable in a
small region of intermediate frustration.1 In my investigation, I consider an
unprecedented broad variety of spin (spiral) waves. These ordered phases are
represented by classically ordered states supplemented with a longrange Jastrow
factor, which includes relevant correlations and dramatically improves the
description provided by the purely classical solution of the models. The
construction of the spinliquid state is based on a spin decomposition in terms
of fermions, experiencing a Gutzwiller projection and longrange Jastrow
correlations. In comparison with the classical phase diagram, the quantum
fluctuations prolong the stability of the Néel antiferromagnet and favor a
stripe order for intermediate and quite strong frustration. The spiral waves are
ground state for strong frustration and the 120thorder becomes the
lowestenergy phase for very strong frustration. I also discuss connections with
experiments on magnetically frustrated systems.

Maurizia Palummo (Tor Vergata, Roma)  Détails Fermer 
Twodimensional nanosheets for optoelectronic applications le vendredi 12 juillet 2013 à 11:00 

Résumé : Twodimensional nanosheets for optoelectronic applications

Alexander Yaresko (MPI Stuttgart)  Détails Fermer 
Biquadratic exchange in Fe pnictides from band structure calculations le mardi 9 juillet 2013 à 11:00 

Résumé : Most undoped Fe arsenides, parent compounds for Fe based superconductors, undergo a transition into a collinear state with stripelike magnetic order in which antiferromagnetic (AFM) Fe chains are ferromagnetically ordered along the direction perpendicular to the chains. Two such collinear magnetic structures, characterized by ordering vectors (π,0) or (0,π), are connected by infinite number of noncollinear states with two AFM sublattices of second Fe neighbors rotated by an arbitrary angle with respect to each other. In the classical Heisenberg model all these states are degenerate. Band structure calculation show, however, that the degeneracy is lifted already at the mean field LSDA level and that in Fe arsenides (π,0) and (0,π) magnetic orders are separated by an energy barrier comparable to the energy difference between Neel and stripe AFM orders. The shape of the barrier can be reproduced by adding a biquadratic term to the Heisenberg model. We discuss the microscopic origin of the barrier and show that it is related to the peculiar band structure of Fe pnictides and nesting properties of their Fermi surfaces. The results for Fe arsenides are compared to BaMn_{2}As_{2} and hypothetical KFe_{2}Se_{2} for which we found that a noncollinear 90degree spin arrangement is more favorable than collinear ones. A doping dependence of the barrier is also discussed. 
Raphael Voituriez (LPTL)  Détails Fermer 
Firstpassage statistics and search strategies le vendredi 28 juin 2013 à 11:00 

Résumé : How long does it take a "searcher" to reach a "target" for the first time? This firstpassage time is a key quantity for evaluating the kinetics of various processes, and in particular chemical reactions involving "small" numbers of particles such as gene transcription, or at larger scales the time needed for animals to find food resources. I will present recent results which enable the evaluation of the distribution of firstpassage time for a wide range of random search processes evolving in a confined domain. This approach reveals a general dependence of the firstpassage time distribution on the geometry of the problem, which can become a key parameter that controls the kinetics of the search process. I will show how these results apply to transport in disordered and fractal media, and highlight their implications in transcription kinetics and other search processes at larger scales. Liens :Raphael VoituriezLPTL 
Martin Field (IBS)  Détails Fermer 
Hybrid Potential Simulation Methods for Studying Enzyme Catalysis le vendredi 21 juin 2013 à 11:00 

Résumé : An important goal of computational and theoretical biochemistry is helping elucidate how enzymes achieve their catalytic efficiency. The differing length and time scales of processes that contribute to catalysis, however, makes this a challenging task for molecular simulation techniques. An approach that has proved particularly powerful for the investigation of the chemical steps in enzymatic and other condensed phase reaction processes is the use of hybrid quantum chemical and molecular mechanical potentials. This talk will describe the types of hybrid potentials developed and implemented in the author's group and illustrate their use by a presentation of some recent applications to a variety of enzyme systems. Liens :Martin FieldIBS 
Ioannis Rousochatzakis (IFW)  Détails Fermer 
Frustrated magnetism and resonating valence bond physics in 2D kagomelike magnets with inequivalent loops le vendredi 14 juin 2013 à 11:00 

Résumé : Using a combination of exact diagonalization, analytical strongcoupling theories and resonating valence bond approaches we determine the phase diagram and the lowenergy physics of three kagomelike Heisenberg antiferromagnets with inequivalent resonance loops. At weak coupling the lattices become effectively bipartite, while at strong coupling heavily frustrated nets emerge. Competing tunneling amplitudes result in the intermediate coupling regime in shortranged spin correlations, the presence of a manyfold of lowlying singlets and the stabilization of valence bond crystal and spinnematic phases. Liens :IFW 
Allan MacDonald (University of Texas)  Détails Fermer 
Exciton Condensates are Super! le lundi 10 juin 2013 à 11:00 

Résumé : Electronic systems can have a type of order in which coherence is spontaneously established between two distinct groups of electrons. So far this (particlehole or exciton condensate) type of order has been found only in doublelayer twodimensional electron gas systems, and only in certain strong magnetic field limits. I will review some of the surprising superfluid transport effects that have already been observed in doublelayer exciton condensates, and speculate on others that may be observable in the future. 
Lorenzo Stella (EHU)  Détails Fermer 
Correlated ElectronIon Dynamics (CEID): An efficient method to model electronic (de)coherence from an atomistic point of view le vendredi 7 juin 2013 à 11:00 

Résumé : Quantum coherence between electronic and nuclear dynamics, as observed
experimentally in organic semiconductors, is the object of an intense
theoretical and computational effort. To simulate this kind of quantum coherent
dynamics, an efficient numerical scheme based on Correlated ElectronIon
Dynamics (CEID) has been recently devised [1]. In this talk, I describe a
further generalization of CEID [2] and its practical numerical implementation
[3]. To illustrate the capability of this extended CEID scheme, an atomistic
model of the electronic decoherence of a short conjugated oligomer is presented.
Finally, I discuss convergence and scaling properties of the extended CEID
scheme along with its applicability to larger systems, e.g., to investigate the
nonradiative relaxation of photoexcited conjugated polymers [4].
Liens :Lorenzo StellaEHU 
Saverio Moroni (SISSA)  Détails Fermer 
Normal fluid phases of He 3 in two dimensions le vendredi 31 mai 2013 à 11:00 

Résumé : I will present ongoing work, based on quantum Monte Carlo simulations, on the normal fluid phase of ³He in two dimensions, both for the strictly 2D case and for more realistic models of monolayers adsorbed on different substrates. We find close agreement with the experiment for both the static spin susceptibility and the dynamic structure factor. For weak enough alkali metal substrates, we predict a gasliquid phase transition not found for strictly 2D ³He. Liens :SISSA 
Sandro Sorella (SISSA)  Détails Fermer 
The new resonating valence bond method for abinitio electronic simulations le vendredi 24 mai 2013 à 11:00 

Résumé : The Resonating Valence Bond theory of the chemical bond was introduced soon after the discovery of quantum mechanics and has contributed to explain the role of electron correlation within a particularly simple and intuitive approach, where the chemical bond between two nearby atoms is described by one or more singlet electron pairs. We revisit the Pauling's resonating valence bond theory of the chemical bond within a new formulation, introduced by P.W. Anderson soon after the discovery of High Tc superconductivity. It is shown that this intuitive picture of electron correlation becomes now practical and efficient, and allows us to perform realistic simulations with correlated wavefunctions corresponding to several hundred atoms. Few examples will be given: i) in the Beryllium dimer we show the accuracy of the method for a particularly difficult case where single determinant approaches (DFT or HartreeFock) miserably fail, ii) recent finite temperature realistic simulations of liquid hydrogen and liquid water. Liens :Sandro SorellaSISSA 
Steve Simon (Oxford)  Détails Fermer 
Topological Matter and Why You Should Be Interested le vendredi 3 mai 2013 à 11:00 

Résumé : In two dimensional topological phases of matter, processes depend on gross topology rather than detailed geometry. Thinking in 2 et 1 dimensions, particle world lines can be interpreted as knots or links, and the amplitude for certain processes becomes a topological invariant of that link. While sounding rather exotic, we believe that such phases of matter not only exist, but have actually been observed in quantum Hall experiments, and could provide a uniquely practical route to building a quantum computer. Possibilities have also been proposed for creating similar physics in systems ranging from superfluid helium to strontium ruthenate to semiconductorsuperconductor junctions to quantum wires to spin systems to cold atoms. Liens :Steve SimonOxford 
Tobias Meng (University Basel)  Détails Fermer 
Spiral Luttinger liquids: helical nuclear spin order, Rashba nanowires, and their conductance le vendredi 26 avril 2013 à 11:00 

Résumé : When taking into account the nuclear spins, a quantum wire is a finite size Kondo lattice system. In such a system, the RKKY interaction can result in an ordered state at low temperatures, in which the Kondo lattice spins form one or several helices. These helices in turn induce a partial ordering of the electrons, and open up gaps in their spectrum. The helical order is a relatively stable phenomenon that persists even for multiple electronic subbands.
Liens :Tobias MengUniversity Basel 
Marco Schiro (Princeton)  Détails Fermer 
Quantum Many Body Physics with Strongly Interacting LightMatter Systems le lundi 8 avril 2013 à 15:30 

Résumé : In the physics of strongly correlated quantum systems the electromagnetic radiation has traditionally assumed the role of a spectroscopic probe and thus treated as a classical field. In recent years an increasing control over lightmatter interactions at the genuine quantum level has been achieved due to experimental developments in quantum optics and quantum electronics. This has brought forth a novel class of many body systems where elementary excitations are made by single quanta of light and matter. These hybrid setups are currently attracting a great experimental and theoretical interest, for the unique features they offer to explore quantum many body physics in novel far from equilibrium regimes. Motivated by the experimental effort, currently ongoing at Princeton, to realize these correlated systems of photons and atoms using superconducting circuits, in this talk I will discuss the physics of large arrays of microwave resonators coupled to superconducting qubits via the elementary Rabi nonlinearity. I will argue that the very nature of photon field and its interaction with matterlike excitations allows to stabilize finitedensity quantum phases of correlated photons out of the vacuum. I will discuss the properties of these phases and the quantum phase transition occurring between them and highlight the differences with the physics of interacting massive quantum particles. Liens :Marco SchiroPrinceton 
Hamza Jirari (Lyon)  Détails Fermer 
Quantum Brachistochrone le vendredi 5 avril 2013 à 11:00 

Résumé : We investigate the application of optimal control of a singlequbit coupled to an ohmic heat bath. For the weak bath coupling regime, we derive a BlochRedfield master equation describing the evolution of the qubit state parameterized by vectors in the Bloch sphere. By use of the optimal control methodology we determine the field that generates a single qubit rotation. We use the techniques of automatic differentiation to compute the gradient for the cost functional. We consider also the concept of Quantum Brachistochrone. Here the problem naturally arises of determining the minimal transition time between an initial state and a final state. The optimal control is of bangbang type and switches from the upper to the lower value of the control bounds. Liens : 
Wilhelm Zwerger (TU Munich)  Détails Fermer 
Scale Invariance in Atomic Physics: from Efimov states to Fermions at Unitarity le vendredi 22 mars 2013 à 11:00 

Résumé : The talk will provide an introduction to two examples in ultracold atom physics where scale invariance plays an important role: threebody Efimov states of bosonic atoms and Fermions at infinite scattering length. We discuss the issue of an apparently 'universal' threebody parameter in the Efimov context and both thermodynamics and transport properties of the unitary Fermi gas. Liens :Wilhelm ZwergerTU Munich 
Robert Whitney (LPMMC)  Détails Fermer 
Nanoscale Nonlinear Thermoelectricity  Cooling, Catastrophes and Carnot le vendredi 15 mars 2013 à 11:00 

Résumé : I start by summarizing thermoelectric effects, and how we might be able
to use them for refrigeration, perhaps to cool nanoscale systems to previously
unreachable temperatures (as low as a few mK). However quantum effects
cannot be ignored in such low temperature nanoscale systems. Thus, I develop a
quantum theory of thermoelectric effects, which is capable of dealing with the
highly nonlinear effects necessary for efficient refrigerators.
Liens :Robert WhitneyLPMMC 
Massimo Rontani (S3Modena)  Détails Fermer 
Visualizing electron correlation in nanoobjects using a scanning tunneling microscope: Molecules, quantum dots, carbon nanotubes le vendredi 8 mars 2013 à 11:00 

Résumé : Scanning tunnelling spectroscopy (STS) visualizes electron states in both
extended systems and nanoobjects, such as quantum dots, molecules, carbon
nanotubes. Whereas extended quantum states are insensitive to electron number
fluctuations, an energy gap opens each time a new electron is injected by the
STS tip into a nanoobject. This gap originates from the interaction of the next
incoming electron with the others already present in the system. Under this
Coulomb blockade condition, STS maps the wave function modulus of the electron
injected by the tip into the nanoobject. The obtained image is routinely
interpreted as the atomiclike or molecular orbital of the added electron, that
experiences the mean field of the other electrons already populating the system.
A fundamental question is whether features of the tunnelling map may appear due
to electronelectron correlation beyond mean field [1]. In this talk I will
demonstrate that the answer is positive, focusing on planar molecules with metal
centres [2], semiconductor quantum dots [3], quantum wires and carbon nanotubes [4].
Liens :Massimo RontaniS3Modena 
Ludger Santen (UniSaarland)  Détails Fermer 
Many Particle Models of Stochastic Transport le vendredi 1 mars 2013 à 11:00 

Résumé : Active transport is critical for cellular organization and function. Much of the intracellular longdistance transport is carried out by specialised proteins, socalled molecular motors. The molecular motors are connected to cargo like vesicles or cell organelles, and moving along the filaments of the cytoskeleton. Molecular motors of the kinesin and dynein family move in opposite direction along microtubulefilaments. Some of the cargo is even moved by kinesin and dynein motors, and frequently changes its directionality. In my talk I will discuss a few coorperative transport phenomena that are related to motor driven intracellular transport. In particular variants of stochastic many particle models of transport by molecular motors are discussed, which show a strong tendency to form macroscopic clusters on static lattices. Inspired by the fact that the microscopic tracks for molecular motors are dynamical, the influence of different types of lattice dynamics on stochastic bidirectional transport will be examined. Liens :Ludger SantenUniSaarland 
Stefano Giorgini (Trento University)  Détails Fermer 
Liquid and crystal phases of dipolar fermions in two dimensions le vendredi 22 février 2013 à 11:00 

Résumé : Quantum degenerate gases interacting with longrange dipolar forces have become a fascinating new research direction in the field of ultracold atoms. In the seminar I will briefly review some of the recent experimental and theoretical progresses on this topic and then I will focus on the properties of dipolar fermions in two spatial dimensions. I will report on results obtained using quantum Monte Carlo methods concerning the equation of state of the liquid and crystal phase at zero temperature which correspond, respectively, to the regime of low and high density. Results on the critical density of the liquid to solid quantum phase transition are presented and the possible existence of a stripe phase close to the freezing density is discussed. Preliminary results on a bilayer system with a dipolar impurity interacting with a system of dipolar fermions will also be discussed. 
Mario Barbatti (MPIMuelheim)  Détails Fermer 
Mechanistic aspects of ultrafast photoprocesses in bioorganic systems le vendredi 15 février 2013 à 11:00 

Résumé : Excitedstate dynamics of molecular systems plays a fundamental role in several fields, such as reactive scattering, hot chemical reactions, transport processes, photochemistry, and photophysics. In the last decade, computational simulations have become an important tool to unveil reaction mechanisms in these processes. In particular, semiclassical nonadiabatic dynamics simulations have revealed complex scenarios, where multiple reaction pathways are in constant competition among them and whose output is deeply dependent on details of a manifold of potentialenergy surfaces. In this talk, I will deliver an overview of recent achievements in this field, including a critical appraisal of the strengths and limitations of the available simulation methods. Special focus will be laid on the deactivation dynamics of UVexcited nucleobases, a phenomenon that may have played a central role for life evolution on Earth. Liens :Mario BarbattiMPIMuelheim 
JeanSébastien Caux (ITPAmsterdam)  Détails Fermer 
Dynamics in one dimension: from integrability to inelastic neutron scattering and beyond le vendredi 8 février 2013 à 11:00 

Résumé : Over the last few years, integrability has become a method of choice for the calculation of equilibrium dynamical correlation functions of systems such as spin chains and interacting atomic gases, its main strength being its ability to go beyond lowenergy effective theories. A brief review will be given of results on simple and more elaborate observables relevant to experiments such as inelastic neutron scattering, resonant inelastic xray scattering and their equivalents in cold atomic systems. Recent applications to outofequilibrium physics in cold atoms will also be discussed. Liens :JeanSébastien Caux 
Patrick Ilg (ETH Zurich)  Détails Fermer 
Structural features underlying the dynamics of supercooled dynamics le vendredi 1 février 2013 à 11:00 

Résumé : We apply instantaneous shear deformations to supercooled liquids to investigate the correlations between t he soft modes of the inherent structure belonging to the initial and final configurations and the isoconfi gurational DebyeWaller factors of the initial thermal configuration, as a function of the temperature and of the strain amplitude. The spatial distributions of nonaffine displacements (NAD) characterizing such response are correlated to the dynamical heterogeneities of the supercooled liquid, suggesting that partic les in regions of large NADs are likely to be more mobile than those belonging to small values of NADs. Moreover, our normal mode analysis shows that cooperative regions in NAD are strongly correlated to the lo w energy soft modes of the inherent structure of the supercooled liquid, responsible for the onset of plas ticity in the amorphous solid. In addition, we also observe a welldefined critical deformation amplitude, above which these correlations are lost. Liens :Patrick IlgETH Zurich 
Wolf von Klitzing (IESLFORTH)  Détails Fermer 
Breaking the flux limit: A novel atom laser using timedependent adiabatic potentials le vendredi 25 janvier 2013 à 11:00 

Résumé : Atom laser  coherent beams of matter originating from BoseEinstein condensates (BEC)  are one of the most prominent demonstrations of matter wave optics. They have been proposed as coherent atom sources for matterwave interferometry and direct atom lithography. Atom lasers are generated by coupling atoms from a trapped BEC into freespace using either a weak RF field [1] or a weak Bragg beam [2]. In this talk I will present a novel type of output coupling of an atomlaser from a BEC, which uses a strong RF field to create a timevarying adiabatic potential (TAP) in a magnetic IoffePritchared trap. In combination with gravity, the TAP opens a small hole is created in the very bottom of the trap from which the atombeam is allowed to escape. The TAP atom laser avoids the flux limits of the traditional laser based on weak coupling. This allowed us to demonstrate an increase in flux by more than one order of magnitude to 7 x 10^{7} atoms/s, whilst preserving some of the lowest divergences reported so far (6 mrad) [3]. The TAP also allowed us to generate thermal atom beams with record temperatures as low as 300 nK at a peakflux of up to 3 x 10^{8} atoms/s. In is talk will discuss the generation and limits of the TAP atom laser. References[1] I. Bloch, T.W. Hänsch, and T. Esslinger, Atom Laser With a CW Output a Coupler, Phys. Rev. Lett. 82 (15), 3008 (1999).[2] E.W. Hagley et al. A WellCollimated QuasiContinuous Atom Laser, Science 283 (5408), 17061709 (1999). [3] N.P. Robins et al. Achieving Peak Brightness in an Atom Laser, Phys. Rev. Lett. 96 140403 (2006) and J.E. Debs et al. Experimental comparison of Raman and rf outcouplers for highflux atom lasers, Phys. Rev. A 81 (2), 013618 (2010). Liens :Wolf von KlitzingIESLFORTH 
Manuel Donaire (LPMMC)  Détails Fermer 
The electromagnetic vacuum of random media le lundi 21 janvier 2013 à 11:00 

Résumé : In this talk I present an analytical approach to the study of several phenomena related to the vacuum field in dielectric random media. In the electric dipole approximation, exact expressions for the dipole emission rate and the van der Waals energy are derived as a function of the electrical susceptibility. Approximate expressions for the total vacuum energy are given, they all free of divergences. The role of local field factors is explained. The difference amongst the spectra of fluctuations which enter each of the aforementioned quantities is clarified. The results are compared with those obtained in the effective medium approximation. References[1]M. Donaire, Phys.Rev.A 83 022502 (2011).[2]M. Donaire, Phys. Rev. A 85 052518 (2012). [3]M. Donaire, Int. J. Mod. Phys. Conf. Ser. 14 291 (2012). Liens :Manuel DonaireLPMMC 
Elisabeth Agoritsas (Geneva University)  Détails Fermer 
Static fluctuations of a thick 1D interface in the 1 et 1 Directed Polymer formulation le vendredi 18 janvier 2013 à 11:00 

Résumé : The onedimensional KardarParisiZhang (KPZ) equation is at the crossroad
between a wide range of theoretical models and experimental systems such as
roughening phenomena and stochastic growth, the Burgers equation in
hydrodynamics or the 1 et 1 Directed Polymer, and the very definition and
implications of the KPZ universality class have been expanding since the 1980',
both in physicists and mathematicians communities.
Liens :Geneva University 
Ahsan Nazir (Imperial College)  Détails Fermer 
Title: Phononenhanced coherent scattering in a driven quantum dot le vendredi 11 janvier 2013 à 11:00 

Résumé : The recent experimental characterisation of excitonphonon interactions in a coherentlydriven semiconductor
quantum dot (QD) [1, 2], and their interpretation in terms of a twolevel system in contact with a bosonic
environment, have demonstrated that QDs offer a natural platform in which to explore dissipative dynamics in
the solidstate. In particular, the interplay between laserdriven coherent excitonic oscillations and incoherent
phononinduced processes leads to a rich dynamical behaviour, which can also have a profound effect on the dot
photon emission characteristics. In this talk, I shall explore the crucial role played by the solidstate environment
in determining the photon emission properties of a driven quantum dot [3]. In fact, I shall show that such
environmental interactions can lead to quantum dot emission characteristics that deviate fundamentally from
the wellestablished quantum optical behaviour of driven atoms. Specifically, for resonant driving, the coherently
emitted radiation field can actually increase with driving strength due to the quantum nature of the phonon bath.
This behaviour is in stark contrast to the conventional (quantum optical) expectation of a monotonically
decreasing fraction of coherent emission with stronger driving, and should be observable in experimentally
achievable regimes.
Liens :Ahsan NazirImperial College 
Arthur Goetschy (Physics department, Yale)  Détails Fermer 
Partial control of information and correlations in scattering media le lundi 7 janvier 2013 à 14:00 

Résumé : Random matrix theory as well as microscopic theory predict that the transmission eigenvalues of a disordered dielectric material have, in the diffusive regime, a bimodal distribution peaked around 0 and 1 that gives rise to the concept of closed and open eigenchannels. However, in a typical optical experiment where only a small fraction of the channels are excited or measured, the distribution of the transmission eigenvalues is not the bimodal but the MarchenkoPastur law. We propose an analytical theory that quantitatively describes the transition between these two distributions. In particular, we show that the reduction of the number of controlled input/output channels abruptly suppresses the open eigenchannels and then gradually yields to an effective loss of the correlations contained in the scattering matrix. This effect is illustrated with the study of the information capacity of a disordered waveguide. Finally, we show how the abrupt loss of the open eigenchannels can dramatically reduce the effect of coherent enhancement of absorption. 