Séminaires LPMMC 2017
Etienne Jussiau (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 21 décembre 2017 à 11h00 

Liens :[LPMMC] 
Cécile Répellin (MIT)  Détails Fermer 
Stability of the spin1/2 kagome ground state with breathing anisotropy le lundi 18 décembre 2017 à 11h00 

Résumé : Quantum spin liquids (QSLs) are strongly correlated phases which cannot be characterized by a spontaneous symmetry breaking at zero temperature. Their exotic features (such as fractionalized excitations and topological properties) and the prospect of realizing them in frustrated magnets have aroused a lot of interest. The kagome lattice antiferromagnet is one the main model candidates which may realize a QSL. It poses a challenge to theorists and experimentalists alike: materials such as herbertsmithite can be approximated by this model but additional terms and disorder may change the nature of the ground state entirely. On the theoretical front, the ground state of the ideal model is generally admitted to be a QSL whose precise nature remains one of the most debated questions of the field, the (gapped) topological Z2 spin liquid and (gapless) dirac spin liquid being two of the strongest candidates. We study the spin1/2 breathing (or trimerized) kagome lattice. In this variation of the kagome Heisenberg antiferromagnet (which appears in a recently synthesized vanadium compound), the spins belonging to upward and downward facing triangles have different coupling strengths. Beyond the experimental motivation, connecting the ideal kagome ground state to its fully trimerized counterpart may bring important insight into the nature of the kagome ground state, as strong coupling approaches have suggested the importance of the trimerized model as an effective model capturing most low energy degrees of freedom. Using DMRG and exact diagonalizations, we show the large stability of the kagome ground state upon introducing the breathing anisotropy. Exploration of the entanglement properties of the ground state confirm this picture, and reveal the persistence of signatures of Dirac excitations even for relatively large breathing anisotropy. Finally, we closely examine the limit of strong breathing anisotropy and find indications of a transition to a nematic phase. 
Nicolas Victorin (LPMMC)  Détails Fermer 
Bosonic double lattice ring under a gauge field le jeudi 14 décembre 2017 à 10h30 

Liens :[LPMMC] 
Leonardo Mazza (ENS)  Détails Fermer 
Majorana fermions in particleconserving settings le vendredi 8 décembre 2017 à 11h00 

Résumé : The paradigmatic condensedmatter models where zeroenergy localized Majorana fermions have been studied so far have the distinguishing feature of not conserving the number of fermions. Moreover, the accepted definition of Majorana fermion naturally belongs to this scenario. Is it possible to discuss Majorana fermions in canonical particleconserving settings? In this seminar I will present several exact and numerical results on Majorana fermions in particleconserving scenarios. I will start from the discussion of a model for bosons and fermions where, in a proper limit, the physics of the celebrated Kitaev chain appears. I will continue by presenting exact results on Majorana fermions in ladder models where the two legs of the system can only exchange pair of particles. Finally, I will comment on the possibility of making experiments with Majorana fermions in particleconserving settings. References: Iemini, LM, Rossini, Fazio and Diehl, PRL 115, 156402 (2015) Iemini, LM, Fallani, Zoller, Fazio, Dalmonte, PRL 118, 200404 (2017) 
Piero Naldesi (LPMMC)  Détails Fermer 
Soliton interferometry in a ring le jeudi 7 décembre 2017 à 11h00 

Liens :[LPMMC] 
Michele Fillipone  Détails Fermer 
The Interacting Mesoscopic Capacitor Out of Equilibrium le vendredi 1er décembre 2017 à 11h00 

Résumé : The mesoscopic capacitor has revealed an efficient quantum device to achieve the triggered emission of single coherent electrons in solid state systems. The role of electronelectron interactions in these systems strongly driven outofequilibrium still requires clarification. We consider the full nonequilibrium response of a mesoscopic capacitor in the large transparency limit, exactly solving a model with electronelectron interactions appropriate for a cavity. For a cavity coupled to the electron reservoir via an ideal point contact, we show that the response to any timedependent gate voltage V_{g}(t) is strictly linear in V_{g}. We analyze the charge and current response to a sudden gate voltage shift, and find that this response is not captured by a simple circuit analogy. In particular, in the limit of strong interactions a sudden change in the gate voltage leads to the emission of a sequence of multiple charge pulses, the width and separation of which are controlled by the chargerelaxation time τ_{c}=h C_{g}/e^{2} and the time of flight τ_{f}. Our results are compared with recent noise measurements in HongOuMandel experiments (Freulon et al. Nat. Comm. 6, 6854 (2015)). Our approach justifies the presence of an unexplained dip in the noise as a function of the time delay of activation of the two sources and highlights the unexpected importance of interaction effects in the dynamics of quantum cavities in these experiments. We also consider the effect of a finite reflection amplitude in the point contact, which leads to nonlinearingatevoltage corrections to the charge and current response. Reference 
Aleksandr Svetogorov (LPMMC)  Détails Fermer 
Coherent quantum phaseslips in onedimensional inhomogeneous superconductors le jeudi 30 novembre 2017 à 11h00 

Liens :[LPMMC] 
Artur Slobodeniuk  Détails Fermer 
Fine structure of multilayer TMDC: when more is different le jeudi 23 novembre 2017 à 11h00 


Jiri Minar (University of Nottingham, School of Physics and Astronomy)  Détails Fermer 
Localization phenomena and topological properties of atomic lattice gases with longrange interactions le vendredi 17 novembre 2017 à 11h00 

Résumé : Recent experimental progress in cold atomic gases has allowed for creation of arbitrary lattice geometries at unit filling [1,2] which opens exciting ways for studies of the dynamics of quantum spin Hamiltonians [3]. In this talk I will discuss two examples of dynamics in spin systems with longrange interactions. Firstly, I will discuss the dynamics of Rydberg excitations in an optical tweezer array under the socalled facilitation condition [4]. Here, the presence of positional atomic disorder results in a correlated disorder in the interatomic interaction strengths and drastically affects the facilitation dynamics. To shed light on the role of disorder in a manybody setting we show that here the dynamics is governed by an AndersonFock model, i.e., an Anderson model formulated on a lattice with sites corresponding to manybody Fock states. We first consider a onedimensional atom chain in a limit that is described by a onedimensional AndersonFock model with disorder on every other site, featuring both localized and delocalized states. Next, we consider a situation in which the system maps on a twodimensional AndersonFock model and observe a clear suppression of excitation propagation, which we ascribe to the localization of the manybody wave functions in Hilbert space. With the help of the developed framework we then study the excitation dynamics in the ladder geometry. Secondly, I will describe the topological properties of a twodimensional atomic lattice gas, where the coupling of the atoms to the radiation field gives rise to dissipation and longrange interactions beyond a simple power law [5]. This has for a consequence energy spectra with onesided divergences in the Brillouin zone or possible breaking of the standard bulkboundary relation in topological insulators. We show that under certain conditions, the topological properties, such as the transport of an excitation along the edge of the lattice, remain robust with respect to the presence of lattice defects and dissipation. [1] D. Barredo, S. de Léséleuc et al., Science 354, 1021 (2016) [2] M. Endres, H. Bernien et al., Science aah3752 (2016) [3] H. Labuhn, D. Barredo et al., Nature 534, 667 (2016) [4] M. Marcuzzi, J. Minář et al., Phys. Rev. Lett. 118, 063606 (2017) [5] R. Bettles, J. Minář et al., Phys. Rev. A 96, 041603(R) (2017) 
Pierre Nataf (ETH Zürich)  Détails Fermer 
Numerical methods to investigate Heisenberg SU(N) lattice models. le jeudi 16 novembre 2017 à 11h00 

Résumé : Systems of multicolor fermions have recently raised considerable interest due to the pos
sibility to experimentally study those systems on optical lattices with ultracold atoms [1].
To describe the Mott insulating phase of Ncolors fermions, one can start with the SU(N)
Heisenberg Hamiltonian. In the case of one particule per site, the SU(N) Heisenberg Hamiltonian takes the form of a Quantum permutation Hamiltonian H = J∑_{*lt;i,j>}P_{ij} , where the
transposition operator P_{ij} exchanges two colors on neighboring sites.
We have developped a method[2] to implement the SU(N) symmetry in an Exact Diagonalization algorithm. In particular, the method enables one to diagonalize the Hamiltonian
directly in the irreducibe representations of SU(N), thanks to the use of standard Young
tableaux[3], which are shown to form a very convenient basis to diagonalize the problem. It
allowed us to prove that the ground state of the Heisenberg SU(5) model on the square lat
tice is long range color ordered [2] and it provided evidence that the phase of the Heisenberg
SU(6) model on the Honeycomb lattice is a plaquette phase [4]. Finally, SU(N) chiral phases
on the triangular lattice with artificial gauge fields are also investigated and characterized
through ED[5]. Liens :[Pierre Nataf][ETH Zürich] 
Itai Arad (Physics Department, Technicon)  Détails Fermer 
Efficient representation of manybody ground states le vendredi 10 novembre 2017 à 11h00 

Résumé : Quantum Hamiltonian Complexity is a branch of quantum information which looks at quantum manybody Hamiltonians, the backbone of condensedmatter physics, through the lenses of quantum information and computational complexity. In this talk I will demonstrate this unique perspective by asking to what extent ground states of quantum spin systems are quantum or classical. I will study this question by defining "classical states" as those which can be wellapproximated using only a polynomial number of degrees of freedom, despite the fact that they live in an exponentially large Hilbert space. I will explain why this problem was essentially solved for the case of one dimensional spin chains, and why it is still wide open for systems in higher dimensions. I will also explain how it is related to the existence of area laws and tensornetworks description. Finally, I will present some recent progress on this question for a class of frustrated 2D systems. 
Massimo Boninsegni (University of Alberta, Edmonton)  Détails Fermer 
The fate of microemulsions in twodimensional systems: yes, sometimes numbers matter le lundi 6 novembre 2017 à 14h00 

Résumé : Twodimensional systems of particles interacting via a purely repulsive potential, decaying at large distances as the cubic inverse power of the distance, have been the subject of much theoretical and experimental investigation. On general grounds, an ordinary fluid to crystal phase transition should take place at zero temperature, on increasing the density. However, an elegant argument was provided a decade ago [1] to the effect that no conventional first order phase transition could occur, as ordinary coexistence of fluid and crystal phases separated by a microscopic interface would be energetically unstable at low temperature. For, the system could lower its energy by forming a socalled microemulsion, namely a novel phase of matter featuring large solid clusters ("bubbles") floating in the fluid. This intriguing prediction could in principle be observed experimentally in assemblies of cold dipolar atoms, but also in excitonic systems in semiconductor quantum wells. Failure to observe this scenario experimentally or in numerical simulation prompted a reexamination of the original argument; as it turns out, the characteristic size of the bubbles in the micro motion crucially depends on specific features of the phase transition, such as melting and freezing densities, as well as on the energy per unit length of a macroscopic interface separating the two phases. No reliable estimate was available of any of the three quantities until recently, when the first first principle calculation was completed. I shall describe it in this talk, and argue that based on its results the micro emulsion scenario is of "academic" interest only, due to the astronomically large length scales involved. [1] B. Spivak and S. Kivelson, Phys. Rev. B 70, 155114 (2004) 
Matthieu Vanicat (Department of Theoretical Physics, Ljubljana)  Détails Fermer 
Matrix ansatz in integrable nonequilibrium models le jeudi 2 novembre 2017 à 11h00 

Résumé : I will present new examples of exactly solvable exclusion processes. They are models of particles in interaction on a one dimensional lattice with L sites. The particles are evolving randomly on the lattice following simple stochastic rules. The lattice is connected at its extremities to particle reservoirs with different densities which drive the system outofequilibrium. I will explain how to compute exactly the stationary distribution (which does not obey a Boltzmann statistics) in a matrix product form. This will allow us to compute analytically physical quantities such as particle current and correlation functions. We will also be able to make connection with an hydrodynamic description: the Macroscopic Fluctuation Theory. 
Jordi Boronat (Departament de Fisica, Universitat Politecnica de Catalunya, Barcelona, Spain)  Détails Fermer 
Ultradilute drops of bosons le vendredi 20 octobre 2017 à 11h00 

Résumé : Strongly interacting systems of dipolar bosons in three dimensions confined by harmonic traps are analyzed using the exact path integral groundstate Monte Carlo method. By adding a repulsive twobody potential, we find a narrow window of interaction parameters leading to stable groundstate configurations of droplets in a crystalline arrangement. We find that this effect is entirely due to the interaction present in the Hamiltonian without resorting to additional stabilizing mechanisms or specific threebody forces. In a different context, we will show preliminary results on the formation of dilute liquid drops in Bose mixtures with interspecies attraction. 
Jose Maria Escalante Fernandez (LPMMC)  Détails Fermer 
Anderson localization in classical waves : the role played by its vector character le jeudi 19 octobre 2017 à 11h00 

Liens :[LPMMC] 
Maxim Olshanyi (University of Massachusetts Boston, Boston, USA)  Détails Fermer 
Scattering of a GrossPitaevskii breather off a barrier: the Inverse Scattering Transform made tangible le vendredi 13 octobre 2017 à 11h00 

Résumé : A key observable signature of integrabilityof the existence of infinitely many "higher" conservation lawsin a system supporting solitons is the fact that a collision between solitons does not change their shape or size. But then, if solitons meet on top of a strong integrabilitybreaking barrier, one would expect the solitons to undergo some process consistent with energy conservation but not with higher conservation laws, such as the larger soliton cannibalizing the smaller one. However, here we show that when a stronglycoupled "breather" of the integrable nonlinear Schrodinger equation is scattered off a strong barrier, the solitons constituting the breather separate but survive the collision: as we launch a breather with a fixed impact speed at barriers of lower and lower height, at first all constituent solitons are fully reflected, then, at a critical barrier height, the smallest soliton gets to be fully transmitted, while the other ones are still fully reflected. This persists as the barrier is lowered some more until, at another critical height, the second smallest soliton begins to be fully transmitted as well, etc., resulting in a staircaselike transmission plot, with _quantized_ plateaus. We show how this effect makes tangible the _inverse scattering transform_: the powerful, but otherwise physically opaque mathematical formalism for solving completely integrable partial differential equations. Supported by the NSF, ONR, and USIsrael BSF. In collaboration with V. Dunjko. 
Sathishkumar Rangaswamy Kuppuswamy, Achille Mauri  Détails Fermer 
Satish : Modelling of Exciton Binding Energy in Semiconductor Nanocrystals + Achille: Dielectric Properties of Luttinger Semimetals le jeudi 12 octobre 2017 à 13h30 


Dario Ballarini  Détails Fermer 
BKT phase transition in 2D polariton condensates le jeudi 12 octobre 2017 à 11h00 

Résumé : Polariton condensation has been observed in many different systems, ranging from standard inorganic 2D microcavities and 1D wires to 0D confined systems. The coherence build up process behind the condensate formation has been widely studied but the presence of an exciton reservoir and the unavoidable effects of the finite size of the excitation area are detrimental and in some cases have been heavily underestimated. Thanks to a sample with very long polariton lifetime and with a marked spatial homogeneity it is possible to generate an extended condensed state outside of the laser spot. Here we show the fascinating exhibition of an equilibrium BerezinskiiKosterlitzThouless (BKT) phase, despite the intrinsic dissipation character of polariton quasiparticles, characterised by a powerlaw coherence decay both in time and space domain. Such a combined observation opens the doors to the study of the excitations spectrum of drivendissipative condensates with high energy resolution. 
Thierry Jolicoeur (LPTMS, Orsay)  Détails Fermer 
Quantum coherence in bilayer graphene structures le vendredi 6 octobre 2017 à 11h00 

Résumé : Macroscopic quantum coherence is known to happen in liquid Helium, in superconductors and also in systems with more delicate entities like polaritons and excitons. The exciton condensation in semiconductor devices is known to happen in the quantum Hall regime. It happens in the realm of "quantum Hall ferromagnetism" where degeneracies due to spin and valley degrees of freedom play a major role. Monolayer and bilayer graphene in the quantum Hall regime have such degeneracies and may display numerous phases with various types of condensation. After givng an overview of these ideas I will describe the status of experimental/theoretical understanding of bilayer graphene at various integer filling factors in the quantum Hall regime. 
Andrew Mitchell (University College Dublin)  Détails Fermer 
Correlated quantum transport through dots and molecules le vendredi 29 septembre 2017 à 11h00 

Résumé : When nanoscale components are incorporated into external circuits, electronic transport can exhibit striking quantum phenomena with no classical analogue  such as entanglement, quantum interference, and fractionalization. In this seminar I discuss recent theoretical progress in understanding two related classes of nanoelectronic device: semiconductor quantum dots and singlemolecule junctions. The new chargeKondo quantum dot design paradigm allows unprecedented opportunities to engineer exotic quantum critical states, with beautiful agreement between experiment and theory. On the other hand, the fundamental physics of molecular junctions is often obfuscated by orbital complexity. Here I present a theoretical framework to understand such devices, and novel predictions for a new Kondo Blockade effect. [1] Mitchell, Pedersen, Hedegaard, Paaske, Nature Communications, 8, 15210 (2017) [2] Mitchell, Landau, Fritz, Sela, Phys. Rev. Lett. 116, 157202 (2016) [3] Iftikhar, Anthore, Mitchell, et al, arXiv:1708.02542 
Davide Squizzato (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 28 septembre 2017 à 11h00 

Liens :[LPMMC] 
Florian Eich (Max Planck Institute for the Structure and Dynamics of Matter, Hamburg)  Détails Fermer 
Charge and Energy Transport at the Nanoscale: A DensityFunctional Theory Perspective le vendredi 22 septembre 2017 à 11h00 

Résumé : The theoretical description of charge and energy flow at atomic length and time scales has received renewed interest due to advances in experimental techniques. For example, recent experiments demonstrate the ability to measure temperatures at the nano scale. This raises a host of fundamental question such as: Can we define a local temperature at atomic length scales? When do quantum mechanical effects, such as interference, become important? How can we use the notion of temperature, which is welldefined in the context of statistical physics, to describe nonequilibrium phenomena? In my talk I will try to address these questions and provide an overview over a novel nonequilibrium densityfunctional approach, dubbed timedependent thermal densityfunctional theory, which aims at an efficient description of charge and energy transport phenomena at the nanoscale. 
Biagio Lucini (Swansea University)  Détails Fermer 
Density of states and numerical simulations: from Statistical Mechanics to Gauge Theories le vendredi 15 septembre 2017 à 11h00 

Résumé : Determining the density of states through a Monte Carlo method allow us to rephrase numerical simulations into a framework that circumvents importance sampling and with it some of the drawbacks of importance sampling methods. In this work, we shall present a recently proposed algorithm based on a nonMarkovian process which has been conceived to determine efficiently continuous density of states. Using as prototype the Potts model and the U(1) Lattice Gauge Theory system, we shall show that the method is very effective at avoiding ergodicity problems related to strong metastabilities at first order phase transition points. We then present more preliminary results on the direct computation of partition functions, on the avoidance of topological trapping in systems where topological objects play a crucial role in the dynamics and on the “solution” of the sign problem. 
Gianluca Rastelli  Détails Fermer 
Dissipative phase transition with quantum frustration le vendredi 8 septembre 2017 à 11h00 

Résumé : We study a quantum dissipative rotor model in which each local phase difference and each local momentum are uniformly coupled to two different baths. Such systems can represent e.g. a chain of resistively shunted Josephson junctions and capacitively coupled to a diffusive metal. The first dissipative coupling quenches the quantum phase fluctuations favoring the longrange phase order (i.e. superconducting ground state) whereas the second one quenches momentum fluctuations destroying phase coherence (insulating ground state). Using the selfconsistent harmonic approximation, we calculate the zero temperature phase diagram as determined by the two dissipative coupling constants and the bare zero point fluctuations. As an effect of the quantum frustration for the two canonical conjugate observables, we obtain an rich phase diagram with a nonmonotonic behavior: for instance, the ground state can change from superconducting to insulating and back to the superconducting phase by increasing the dissipation. 
Julien Varignon (CNRSThalès)  Détails Fermer 
Firstprinciples study of strongly correlated oxide perovskites le mardi 5 septembre 2017 à 11h00 

Résumé : Transition metal oxides with a ABO3 perovskite structure have attracted widespread interest over the last decades, both from an academic and industrial point of view. This is ascribed to their wide range of functionalities going from superconductivity, ferroelectricity, magnetism, orbitalorderings or thermoelectricity for instance. This diversity in their physical behaviour comes from the interplay between lattice, charge, orbital and magnetic degrees of freedom allowed by the transition metal [1]. Among all perovskites, a special emphasis has been dedicated to systems with a 3d transition metal element on the B site. Indeed, partly filled d shells allow for strong electronic correlations and/or ionic (lighter elements) or covalent (heavier elements) effects that in turn can have a dramatic influence on the properties of the materials. In the context of understanding the microscopic mechanism underlying these phenomenon, as well as engineering novel properties and functionalities with perovskites, Density Functional Theory (DFT) has already demonstrated its efficiency and appears nowadays as an essential tool in solid state physics. In this seminar, I will present two different studies involving correlated and ionic or covalent systems and based on firstprinciples calculations. In the first part, on the basis of universal symmetry arguments, I will show how to couple lattice mode distortions through the recent “hybrid improper ferroelectricity” mechanism [2,3,4] and enable an electric field control of orbitalorderings and related electronic properties. This concept will be illustrated in rare earth titanates or rareearth vanadates based superlattices [5,6] and in highly strained bulk phases of popular perovskites such as BiFeO3 or SrTiO3 [7], where orbital orders can be produced irrespective of electronic degeneracies. In the second part, I will address the problem of rareearth nickelates and I will evidence that these systems sit at the border line of ionic and covalent characters [8]. Then, I will highlight that covalence can be a powerful lever to control and engineer electronic and magnetic phases both in bulk and at oxide interfaces [9]. [1] Zubko et al, Annu. Rev. Condens. Matter Phys. 2, 41 (2011). [2] Varignon et al, C.R. Physique 16, 153 (2015). [3] Bousquet et al, Nature 452, 732 (2008). [4] Rondinelli et al, Adv. Mater. 24, 1928 (2012). [5] Bristowe, JV et al, Nat. Commun. 6, 6677 (2015). [6] Varignon et al, Sci. Rep. 5, 15364 (2015). [7] Varignon et al, Phys. Rev. Lett. 116, 057602 (2016). [8] Varignon et al, npj Quantum Materials 2, 21 (2017). [9] Grisolia, JV et al, Nat. Phys. 12, 484 (2016). 
Zhengqiao Li (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 13 juillet 2017 à 13h30 

Liens :[LPMMC] 
John Helm (University of Otago)  Détails Fermer 
Spinorbit coupled interferometry with ring–trapped Bose–Einstein condensates le lundi 10 juillet 2017 à 13h30 

Résumé : We propose a method of atominterferometry using a spinor Bose–Einstein (BEC) and the wellestablished
experimental technique of timevarying magnetic fields as a coherent beamsplitter. Our protocol creates longlived
superpositional counterflow states, which are of fundamental interest and can be made sensitive to both the
Sagnac effect and magnetic fields on the sub microGauss scale. We split a ringtrapped condensate, initially in the
mf = 0 hyperfine sublevel, into superpositions of both internal spin state and condensate superflow [1], which are
spinorbit coupled. After interrogation a relative phase accumulation can be inferred from a population transfer to
the mf = 1 states [2]. We present numerical and analytical treatments of our system [3].
Liens :[University of Otago] 
Josh Myers (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 6 juillet 2017 à 13h30 

Liens :[LPMMC] 
Andrew Jordan  Détails Fermer 
The arrow of time for continuous quantum measurements le vendredi 30 juin 2017 à 11h00 

Résumé : The question of the time reversibility of quantum mechanics with measurements is one that has been debated for some time. In this talk, I will present new work exploring our ability to distinguish the forward from the timereverse measurement records of continuous quantum measurements. The question involves both the conditions for the timereversibility of the quantum trajectory equations of motion, as well as statistical distinguishability of the arrow of time. For a continuous qubit measurement example, we demonstrate that timereversed evolution is physically possible, provided that the measurement record is also negated. Despite this restoration of dynamical reversibility, a statistical arrow of time emerges, and may be quantified by the loglikelihood difference between forward and backward propagation hypotheses. We then show that such reversibility is a universal feature of nonprojective measurements, with forward or backward Janus measurement sequences that are timereversed inverses of each other. J. Dressel, A. Chantasri, A. N. Jordan, A. N. Korotkov, arXiv:1610.03818 
Luis MoralesMolina  Détails Fermer 
Maximally entangled states, pairsuperfluidity and MORE in a manybody interacting system le jeudi 29 juin 2017 à 13h30 

Résumé : In this talk I will present interesting results about the study of quantum correlations
between two species of ultracold bosons living on a ring lattice. In the first part, I
going to show that the presence of synthetic magnetic fields can lead to the formation
of entangled states between pair of qudits (high dimensional qubits). Notably,
maximally entangled eigenstates are possible to find for welldefined values of the
AharonovBohm phase of the synthetic magnetic field, which are zeroenergy
eigenstates of both the kinetic and interacting parts of the BoseHubbard Hamiltonian
[1]. This latter property makes them exeptional and robust for applications. In the
second part, I will focus on the eigenstates of the lowestenergy band in the regime
of large interaction where a pairsuperfluid phase naturally emerge for the ground
state. In this scenario, the analysis of the interference pattern in the momentum
distribution indicates a strong connection between entanglement and the pair
superfluid phase. This is further highlighted by the fact that for maximally entangled
eigenstates any single order tunneling process is naturally suppressed [2]. Thus the
observation of features of a pairsuperfluid behavior can be used as a signature of
the presence of entanglement. This might be an important tool for the
characterization of the entanglement in the ground state. Finally, I will discuss the
perspective of using this setting with two type of particles as a benchmark to
investigate the connection between phase coherence and entanglement in many
body quantum systems.
References

Nicola Lo Gullo ((thermodynamique))  Détails Fermer 
Groundstate and asymptotic dynamical properties of 1D ultracold gases in the presence of a mobility edge le lundi 26 juin 2017 à 13h30 

Résumé : In the first part of the talk we explore the groundstate properties of cold atomic gases focusing on the cases of noninteracting fermions and hardcore (TonksGirardeau) bosons, trapped by the combination of two potentials (bichromatic lattice) with incommensurate periods. In the tightbinding limit, the singleparticle states in the lowest occupied band show a localization transition, as the strength of the second potential is increased above a certain threshold. In the continuum limit, when the tightbinding approximation does not hold, a mobility edge is found, instead, whose position in energy depends upon the strength of the second potential. Here, we study how the crossover from the discrete to the continuum behavior occurs, and prove that signatures of the localization transition and mobility edge clearly appear in the generic manybody properties of the systems. Specifically, we evaluate the momentum distribution, which is a routinely measured quantity in experiments with cold atoms, and demonstrate that, even in the presence of strong bosonboson interactions (infinite in the TonksGirardeau limit), the singleparticle mobility edge can be observed in the groundstate properties. In the second part we study the dynamical manybody response of for a onedimensional fermionic gas in a mono and bichromatic optical potential following the sudden switchingon of a deltalike barrier at some at the center of the system. Specifically we look at the Loschmidt echo as a figure of merit to characterize the response of the system and its long time behavior. In order to evaluate the echo we employ two complementary approaches: (1) functional determinants (Levitov) which gives the exact numerical solution for time and therefore frequencyresolved responses and (2) a perturbative approach (Linked Cluster Expansion) which provides an accurate evaluation of the contribution of different physical processes involved in the dynamics. Again we focus on the two limits of tightbinding and continuum showing that the phenomenon of the orthogonality catastrophe can be observed in such systems which, unlike their condensed matter counterpart, are nowadays created and controlled with a very high accuracy. 
Karyn Le Hur (CPHT Ecole Polytechnique, Palaiseau,)  Détails Fermer 
ManyBody Quantum Physics with Photons le vendredi 23 juin 2017 à 11h00 

Résumé : We review recent developments in the context of manybody quantum physics with microwave photons in superconducting quantum electrodynamics networks and Josephson junction arrays. First, we show how the JaynesCummings lattice model yields an analogy with the BoseHubbard model and can allow to engineer a Mottsuperfluid transition of photons. We discuss the challenges to achieve such a transition, requiring the coupling to AC perturbations and the necessity to include dissipation effects. We also discuss progress in methods and probes. Then, we discuss realizations of topological phases and robust photonics by analogy to progress in quantum materials and ultracold atoms, and address disorder and interaction effects. We also show the simulation of novel topological chain devices with superconducting and Josephson circuits. Experimental progress and realizations are discussed. Such systems also offer novel platforms to address and probe dissipative and driven quantum impurity physics, such as the Kondo effect. 
Benoît Vermersch (LPMMC)  Détails Fermer 
Quantum optics with manybody systems of atoms and photons: From quantum networks to entanglement measurement le jeudi 22 juin 2017 à 13h30 

Résumé : The physics of lightmatter interactions plays a fundamental role for two important research areas in quantum technology: quantum information and quantum simulation. On the one hand, quantum optics theory allows to design robust protocols for the processing of quantum information in quantum networks. On the other hand, in the context of quantum simulation, lightassisted interactions between atoms provide the toolbox to prepare and probe manybody phases of complex Hamiltonians, which are related to longstanding problems in condensed matter (e.g. quantum magnetism or fractional quantum Hall states). In this talk, I will discuss these two topics. I will present our recent results on quantum information processing in quantum networks, and on the engineering of new tools for quantum simulation. At the technical level, I will show how we combine atomic physics, quantum optical techniques and numerical methods borrowed from condensed matter physics (such as MatrixProductState (MPS) techniques) to study these types of complex open manybody systems. In the first part I will discuss some of our works related to photonic quantum networks. After a general introduction, I will present a theoretical description of these systems based on the formalism of waveguide QED. This will then allow me to present recent results on the development of robust Quantum State Transfer protocols [1,2,3], and to introduce our MPS techniques for the description of the dynamics of quantum networks beyond the standard treatment of quantum optics. The second part of the seminar will be devoted to quantum simulators. I will first explain the challenge of measuring entanglement, which is essential to characterize various phases in condensed matter physics (such as Fractional Quantum Hall effect or Haldane phase). I will then show how to measure the entanglement spectrum of ground states of generic Hamiltonians based on direct engineering of the entanglement Hamiltonian [4]. Our method, based on the BisognanoWichmann theorem [5], allows one to measure entanglement spectra via standard spectroscopy and can be implemented in all quantum simulation platforms. I will provide numerical examples to support this result and give examples of AMO implementations of entanglement Hamiltonians. If time allows, I will present a complementary method based on Random Matrix Theory, which would allow to measure the entanglement growth in a manybody localised (MBL) system [6].
Liens :[LPMMC] 
Vincent Rossetto (LPMMC)  Détails Fermer 
Differential imaging in heterogeneous media (2) le jeudi 15 juin 2017 à 13h30 

Liens :[LPMMC] 
Harold Baranger (Duke University)  Détails Fermer 
Nonlinear IV Curve at a Quantum Critical Point and Quantum Noise le vendredi 9 juin 2017 à 11h00 

Résumé : Manybody systems that are either driven far from equilibrium or simply subjected to quantum noise exhibit complex interplay between the manybody correlations and the external variables, and so are attracting increasing attention. I shall discuss a system that is particularly advantageous for studying these effects: it exhibits impurity quantum criticality, it is amenable to detailed experimental study (and initial experiments have been done), and it is simple enough theoretically that analytical results can be obtained. (i) First, I briefly survey the experimental system and initial results. The system consists of a spinpolarized carbon nanotube quantum dot connected to resistive leads via tunable tunnel barriers. A quantum critical point (QCP) occurs when a level in the dot is resonant with the leads and the dot is symmetrically coupled to them. (ii) Second, I present our calculation of the nonlinear IV curve at the QCP and show remarkable agreement with the experiment. This result has a simple interpretation as an environmental blockade, albeit one involving a strange barrier between two chiral fermion modes and a strange environment that involves a nonlinear combination of the original electrons and environment. (iii) Third, turning to a more complicated structure, I discuss the case of two dots in the Kondo regime connected to leads in series. In this system, we find that the (equilibrium) quantum noise from the resistive leads stabilizes a nonFermi liquid QCP. While it is natural to suppose that quantum noise will suppress manybody correlations, this is a striking counterexample in which the noise "rescues" the quantum phase transition. 
Katarina Rojan (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 8 juin 2017 à 13h00 

Liens :[LPMMC] 
Gertrud Zwicknagl (Institut fuer Mathematische Physik, TU Braunschweig)  Détails Fermer 
The utility of band theory in strongly correlated electron systems le vendredi 2 juin 2017 à 11h00 

Résumé : Band structure calculations are an important tool in modern material science. Theory and simulation have been shown to provide useful guidelines for materials discovery, design, and optimization. Understanding the collective electronic properties of emergent materials with strong correlations, however, remains a great challenge to condensedmatter theory. Important examples are transition metal oxides, metals containing lanthanide or actinide atoms, and organic conductors. At low temperatures, these materials exhibit novel phenomena like metaltoinsulator transitions, heavy fermions, unconventional superconductivity and unusual magnetism which may eventually provide new functionalities. The complex behavior and the high sensitivity with respect to external fields result from the fact that the quantum mechanical (ground) states are determined by subtle quantum correlations not captured by standard methods of electronic structure calculations. I will review how the band approach can be modified to incorporate the typical manybody effects. Of particular interest is the question when and why standard band theory based on Density Functional Theory may predict the correct Fermi surfaces in many heavy fermion compounds and what we can learn from this agreement. I will present recent results on the evolution with magnetic field of the Fermi surface in heavy fermion systems and magneticfieldinduced Lifshitz transitions. 
Eiji Kawasaki (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 1er juin 2017 à 13h30 

Liens :[LPMMC] 
Jordan Hervy (LPMMC)  Détails Fermer 
Microtubule decoration by tau proteins le jeudi 18 mai 2017 à 13h30 

Liens :[LPMMC] 
Achim Rosch (University of Cologne)  Détails Fermer 
Pumping spinchains le jeudi 18 mai 2017 à 11h00 

Résumé : Weak perturbations can drive an interacting manyparticle system far from its initial equilibrium state if one is able to pump into degrees of freedom approximately protected by conservation laws. This concept has for example been used to realize BoseEinstein condensates of photons, magnons, and excitons. Integrable quantum system like the onedimensional Heisenberg model are characterized by an infinite set of conservation laws. Here we develop a theory of weakly driven integrable systems and show that pumping can induce huge spin or heat currents even in the presence of integrability breaking perturbations, since it activates local and quasilocal approximate conserved quantities. We suggest to realize novel heat or spin pumps using spinchain materials driven by THz radiation. 
Christophe Mora (Laboratoire Pierre Aigrain, Ecole Normale Supérieure)  Détails Fermer 
Photons and electrons in quantum circuits le vendredi 12 mai 2017 à 11h00 

Résumé : Current progresses in implementing quantum electronic devices open exciting perspectives for investigating unexplored regimes of quantum optics with microwave light. Playing with linear or nonlinear, dissipative or dissipationless elements, quantum circuits involve the transport of electrons but can also be engineered to manipulate the state of the surrounding electromagnetic field. The electronphoton crosstalk is potentially enhanced by two means, either by significantly increasing the effective fine structure constant characterizing matterlight interaction, or by building superconducting highfinesse resonators in which photons remain coherently trapped for very long times. Equipped with these tools and taking advantage of the offered strong nonlinearities in quantum circuits, many experiments have designed protocols to create and probe nonclassical states of microwave photons, such as Fock states, squeezed states or even cat states. Producing these typical nonclassical states is known to be a key step towards quantum communication with scalable solidstate devices. After a general introduction to the field of quantum circuits, we will discuss the fact that dissipation due to electron transport is not necessarily detrimental to the realization of coherent nonclassical states such as squeezed vacuum. A tunnel junction will be shown to be able to generate a squeezed steady state in a microwave cavity when excited parametrically by a classical AC voltage source. Photonassisted tunneling of electrons is accompanied by the emission of pairs of photons in the cavity, thereby engineering a driven squeezed state. The mechanism leading to squeezing differs from parametric amplifiers as it is steered by dissipation in the spirit of the reservoir engineering techniques used in quantum optics. We will finally mention ways to improve significantly the squeezing properties of radiation. References [1] U. C. Mendes and C. Mora, Cavity squeezing by a quantum conductor, New J. Phys. 17, 113014 (2015) [2] U. C. Mendes and C. Mora, Electronphoton interaction in a quantum point contact coupled to a microwave resonator, Phys. Rev. B 93, 235450 (2016) [3] C. Mora, C. Altimiras, P. Joyez, F. Portier, Quantum Properties of the radiation emitted by a conductor in the Coulomb Blockade Regime, Phys. Rev. B 95, 125311 (2017) 
Vincent Rossetto (LPMMC)  Détails Fermer 
Differential imaging in heterogeneous media le jeudi 11 mai 2017 à 13h30 

Liens :[LPMMC] 
Duncan O'Dell (McMaster University)  Détails Fermer 
Quantum catastrophes le vendredi 5 mai 2017 à 11h00 

Résumé : Catastrophe theory provides a unified description of a broad range of singularities and defects in fields. A key idea is that of scale: at large scales the singularity appears truly singular but at smaller scales it is smoothed, e.g. by wave interference. In 2004 Michael Berry and Mark Dennis suggested that waves might themselves display singularities which are only smoothed by the fundamental discreteness of quantum field excitations (e.g. photons). In this talk I will give examples of such “quantum catastrophes” appearing in the dynamics of cold atom systems following a quench. Quantum catastrophes resemble classical wave catastrophes at large quantum numbers, but the quantization of excitations leads to an intrinsic granularity. This alters the morphology of the classic catastrophes, particularly the network of dislocations that underlie them. I will emphasize that, owing to the structural stability of catastrophes and their scaling properties, quantum catastrophes represent a universal aspect of dynamics in quantum fields. 
Malo Tarpin (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 4 mai 2017 à 13h30 

Liens :[LPMMC] 
Camille Aron (LPTENS)  Détails Fermer 
Stronglycorrelated electrons driven out of equilibrium by a voltage bias: resistive switchings le vendredi 14 avril 2017 à 11h00 

Résumé : A variety of correlated oxides experience a sudden change of resistivity by several orders of magnitude when subject to a strong voltage bias. This nonequilibrium phase transition, referred as resistive switching (RS), shows hysteretic IV characteristics essential for new electronic memory/switching devices. Before addressing this poorly understood complex phenomenon, I will start with the dissipative dynamics of a simple Hubbard model driven by a constant electric field. In this context, I will introduce new theoretical tools needed address nonequilibrium steady states of stronglyinteracting systems, bypassing the transient dynamics. I will detail the fate of Mott physics in the nonlinear regime: dimensional crossover and dielectric breakdown. Afterwards, I will propose a minimal microscopic model to describe and reproduce most of the RS phenomenology in ordered correlated insulators. 
Piero Naldesi (Université de Trento)  Détails Fermer 
Detecting a manybody mobility edge with quantum quenches le jeudi 13 avril 2017 à 13h30 

Résumé : The manybody localization (MBL) transition is a quantum phase transition involving highly excited eigenstates of a disordered quantum manybody Hamiltonian, which evolve from "ergodic" to "localized". The MBL transition can be driven by the strength of disorder in a given spectral range, or by the energy density at fixed disorder when the system possesses a manybody mobility edge. A possible method to study the latter mechanism is via quantum quenches of variable width which prepare the state of the system in a superposition of eigenstates of the Hamiltonian within a controllable spectral region. Studying numerically a chain of interacting spinless fermions in a quasiperiodic potential, we argue that this system has a manybody mobility edge; and we show that its existence translates into a clear dynamical transition in the time evolution immediately following a quench in the strength of the quasiperiodic potential, as well as a transition in the scaling properties of the quasistationary state at long times. 
Hugo Flayac (EPFL Lausanne)  Détails Fermer 
(titre non communiqué) le mardi 11 avril 2017 à 15h00 


José Lebreuilly (INOBEC Center, Universite de Trento)  Détails Fermer 
Stabilizing incompressible quantum fluids in photonic devices via a nonMarkovian reservoir le lundi 10 avril 2017 à 13h30 

Résumé : Over the last decade, a growing community has started to investigate the possibility of stabilizing strongly correlated
photon fluids in various platforms such as cavity QED and superconducting quantum circuits. A particular emphasis
has been placed on the stabilization of incompressible quantum phases such as the celebrated Mott Insulator state.
This phase has been predicted and observed in isolated systems and appears at integer densities and low temperatures,
but still represents conceptually and experimentally an important challenge in optical devices, where the particle
number is not conserved and heating effects cannot be neglected.
In order to tackle the intrinsic nonequilibrium nature of photonic systems, we investigate the effect of a frequency
dependent, ie., nonMarkovian incoherent pump in order to compensate particle losses and refill selectively the photonic
manybody states, and propose to implement this scheme via a reservoir of populationinverted twolevel emitters
with a broad distribution of transition frequencies [1, 2]. In the simplest case of a Lorentzian emission spectrum [1],
this pump allows for the selective generation of photonic Fock states with a welldefined particle number. For the
novel case of a squareshape spectrum [2], this scheme is predicted to stabilize a nonequilibrium steady state sharing
important features with a zerotemperature equilibrium state with a tunable chemical potential. We demonstrate
numerically for finite sytem sizes the existence of an incompressible MottInsulator state of arbitrary integer density,
which is robust against tunneling and losses, and exhibits a crossover towards a coherent state reminescent of the
superfluid phase.
Liens :[José Lebreuilly][INOBEC Center], INOBEC Center, Universite de Trento 
Gergely Zarand (Budapest University of Technology and Economics)  Détails Fermer 
Designed Quantum Criticality le vendredi 7 avril 2017 à 11h00 

Résumé : Recent years witnessed an astonishing increase of control over quantum systems. Today it is possible to design and construct quantum systems with unprecedented control, realize novel quantum states, and study their dynamical and out of equilibrium behavior using nanotechnological and optical tools. In my talk, I will demonstrate the power of this approach on a few specific examples of artificially designed quantumcritical systems, mostly from the nanophysics perspective. I will review how one can gain insight to certain paradigmatic quantum phase transitions by combining artificial atoms with resistive elements, carbon nanotubes, or topological edge states, and how a combination of detailed field theoretical modeling and accurate experiments may lead to a complete characterization. I will also discuss future perspectives such as, e.g., interfacing nanotechnological circuits with cold atomic or cavity QED elements. References:

Charles Downing (IPCMS Strasbourg)  Détails Fermer 
Energy transport and topological aspects of collective plasmons in chains of metallic nanoparticles le jeudi 6 avril 2017 à 13h30 

Résumé : One of the primary goals of plasmonics is to confine light at subwavelength scales. This aim is motivated by the desire to both transport and manipulate light over macroscopic distances. While metallic nanostructures have been proposed and widely studied to achieve such "plasmonic circuits", both radiative and nonradiative losses inherent to metals are rather significant. Hence, the possible applications for energy and information transport at the nanoscale are seemingly limited. Understanding the different damping mechanisms in radiativelycoupled metallic nanostructures is thus of paramount importance to the field of plasmonics, from both a fundamental point of view and in order to increase the efficiency of signal transmission.
We investigate the collective plasmonic modes in chains of spherical metallic nanoparticles that are coupled by nearfield interactions. These dipolar interactions between the nanoparticles gives rise to collective plasmons, which are extended over the whole plasmonic lattice. We study both a simple chain composed of regularlyspaced nanoparticles, which displays phenomena fundamental to all onedimensional nanoparticle arrays, and a bipartite chain, which exhibits nontrivial topological features.
We obtain the size and momentumdependent nonradiative Landau damping and radiative decay rates, which determine the excitation propagation along the regular chain [1]. We find that the behavior of the radiative decay rate as a function of the plasmon wavelength leads to a transition from an exponential decay of the collective excitation for short distances to an algebraic decay for large distances. Importantly, we show that the exponential decay is of a purely nonradiative origin. These findings constitutes an important step in the quest for the optimal conditions for plasmonic propagation in nanoparticle chains.
We also study a bipartite chain constituted by metallic nanoparticle dimers [2]. We find an effective Dirac Hamiltonian describing the collective plasmons, and show that the corresponding spinor eigenstates represent Diraclike massive bosonic excitations. We show that the system is governed by a topologically nontrivial Zak phase, which predicts the manifestation of edge states in the chain. When two bipartite chains with different topological phases are connected, we find the appearance of a bosonic version of a JackiwRebbi midgap state. We investigate losses of the collective plasmonic excitations in the bipartite chain, and comment on the challenges for experimental realization of the topological effects found theoretically.

Kris van Houcke (LPS, ENS, Paris)  Détails Fermer 
Solving fermionic manybody problems by summing Feynman diagrams le vendredi 31 mars 2017 à 11h00 

Résumé : It is commonly believed that in quantum Monte Carlo (QMC) approaches to fermionic manybody problems, the infamous sign problem generically implies prohibitively large computational times in the thermodynamic limit. I will point out that for convergent (or subject to resummation) Feynman diagrammatic series evaluated with the Monte Carlo algorithm of [Rossi, arXiv:1612.05184], the computational time increases only polynomially with the inverse error on thermodynamiclimit quantities. I will discuss the computational complexity problem for different QMC approaches: conventional techniques (auxiliaryfield, pathintegral and diffusion QMC) and diagrammatic Monte Carlo approaches. I will also report on recent progress of Diagrammatic Monte Carlo simulation of the resonant Fermi gas and the homogeneous electron gas. 
Alexandre Svetogorov (LPMMC)  Détails Fermer 
(titre non communiqué) le jeudi 30 mars 2017 à 13h30 

Liens :[LPMMC] 
Régis Mélin (NEEL)  Détails Fermer 
Simple FloquetWannierStarkAndreev viewpoint for multiterminal Josephson junctions le vendredi 24 mars 2017 à 11h00 

Résumé : Three superconductors contacted within a narrow region form a threeterminal Josephson junction, controlled by two independent voltages, and by two independent phase differences. Coherent DC multipair currents can flow at resonance, for commensurate voltage bias values [1,2]. The amplitude of those currents depends on the value of a welldefined static phase mode. After introducing the nonlocal quartets, I will present the results from the recent Grenoble experiment in Lefloch group [3], as well as the more recent ones from the Weizmann group [4]. Those experiments provide evidence for an anomaly in the voltage dependence of the differential resistance, compatible with the quartets. In addition, the noise crosscorrelations [5] data of the Weizmann group [4] are compatible with LandauZenerStueckelberg transitions inducing random change in the direction of the quartet flow, and thus large and positive current crosscorrelations. In the second part of the talk, a simple physical picture of the steady state will be developed [6], using Floquet theory. The later will be introduced on the example of a driven qubit, starting from the rotating wave approximation, and going beyond with Floquet theory. The equilibrium Andreev bound states (for V=0) evolve into nonequilibrium FloquetWannierStarkAndreev (FWSAndreev) ladders of resonances (for nonzero V). Those resonances acquire a finite width due to multiple Andreev reflection processes. The effect of an extrinsic linewidth broadening on the quantum dot will also be considered, and introduced through a Dynes phenomenological parameter. The dcquartet current manifests a crossover between the extrinsic relaxation dominated regime at low voltage to an intrinsic relaxation due to MAR processes at higher voltage. Three important lowenergy scales will be identified, and a perspective is to relate those lowenergy scales to the crosscorrelation experiment of the Weizmann group [4]. Finally, future directions of research will be mentioned. [1] A. Freyn, B. Douçot, D. Feinberg and R. Mélin, Phys. Rev. Lett. 106, 257005 (2011) [2] R. Mélin, D. Feinberg and B. Douçot, Eur. Phys. J. B 89:67 (2016) [3] A.H. Pfeffer, J.E. Duvauchelle, H. Courtois, R. Mélin, D. Feinberg and F. Lefloch, Phys. Rev. B 90, 075401 (2014) [4] Y. Cohen, Y. Ronen, J.H. Kang, M. Heiblum, D. Feeinberg, R. Mélin and H. Shtrikman, arXiv:1606:08441 [5] R. Mélin, M. Sotto, D. Feinberg, J.G. Caputo and B. Douçot, Phys. Rev. B 93, 115436 (2016) [6] R. Mélin, J.G. Caputo, K. Yang and B. Douçot, Phys. Rev. B 95, 085415 (2017) 
Manfred Sigrist (ETH Zurich, Suisse)  Détails Fermer 
Unconventional Superconductivity  A matter of Symmetry and Topology le vendredi 17 mars 2017 à 11h00 

Résumé : Unconventional superconducting phases incorporate most intriguing features through the symmetry and topological properties of their order parameters, as already several decades ago has been found in the superfluid He3. Among the known unconventional superconductors only few are considered as good candidates to realize topological phases. The most prominent cases are the socalled chiral superconductors, such as Sr2RuO4 most likely with chiral pwave and SrPtAs possibly with chiral dwave pairing. Cooper pairs form here with finite angular momentum. We will discuss the basic phenomenology of the two systems and give an overview of the status of experiments attempting to probe their topological properties. Finally other cases of topological superconductivity will be briefly discussed. ATTENTION : LIEU INHABITUEL 
Bernard Bernu (LPTMC, UPMC, Jussieu, Paris) Annulé  Détails Fermer 
Specific heat and magnetic susceptibility of spin systems le vendredi 10 mars 2017 à 11h00 

Annulé 
Pavel Grigoriev (Landau Institute for Theoretical Physics, Moscow)  Détails Fermer 
Slow quantum oscillations without finegrained Fermi surface reconstruction in cuprate superconductors le vendredi 17 février 2017 à 11h00 

Résumé : The Fourier transform of the observed magnetic quantum oscillations (MQO) in YBaCuO hightemperature superconductors has a prominent lowfrequency peak with two smaller neighbouring peaks. The separation and even the position of these three peaks is almost independent of doping. This pattern has been explained previously by rather special, exquisitely detailed, Fermisurface reconstruction. We propose that these MQO have a different origin, and their frequencies are related to the bilayer and interbilayer electron hopping rather than directly to the areas of tiny Fermisurface pockets. Such socalled "slow oscillations" explain more naturally many features of the observed oscillations and allow us to estimate the interlayer transfer integrals and inplane Fermi momentum. 
Jamir Marino  Détails Fermer 
Driven Markovian quantum criticality le jeudi 16 février 2017 à 13h30 

Résumé : I will discuss the realisation of a drivendissipative analogue of quantum criticality, arising from the onset of a diffusion Markovian noise in a onedimensional driven open Bose gas. Salient features of the novel fixed point are the persistence of both nonequilibrium conditions as well as quantum coherence close to criticality. This provides a sharply distinct situation from more generic driven systems where both effective thermalisation as well as asymptotic decoherence ensue, paralleling classical dynamical criticality. Time permitting, I will also outline a diagrammatic comparison between the characteristic instances of classical and quantum dynamical field theories, employed to study critical phenomena out of equilibrium. 
Lev Ioffe  Détails Fermer 
Nonergodicity in many body systems: consequences for the Josephson junction chain le vendredi 10 février 2017 à 11h00 

Résumé : I argue that the chaotic behavior does not always imply ergodicity at realistic time scales for many classical and quantum systems. In particular, at very high disorder a generic closed quantum systems becomes completely localized that is highly nonergodic. I argue that this (manybody) localization is preempted by a wide regime of nonergodic behavior that displays a number of unusual properties. A good system to study these effects is onedimensional Josephson junction array in a somewhat unusual regime. I review the physics of these arrays and give the arguments for the existence of the novel phase appearing at relatively high temperatures. I will argue that these phases are robust with respect to the presence of the ubiquitous random charges and thus allow experimental observation. I will sketch the analytical theory of the nonergodic phase using Random Graph models. 
Guillaume Lang (LPMMC)  Détails Fermer 
Correlations in lowdimensional quantum gases le jeudi 9 février 2017 à 13h30 

Liens :[LPMMC] 
Roberto Raimondi (Università Roma Tre, Italy)  Détails Fermer 
Charge and spin in a twodimensional electron gas: always an exciting encounter le vendredi 3 février 2017 à 11h00 

Résumé : The spin Hall effect, first predicted in 1971 by Dyakonov and Perel, is the generation of a spin current in response to an applied electric field. The spin galvanic effect arises from the coupling between charge current and spin polarization. Both effects, which arise as a consequence of spinorbit coupling, are now at the forefront of spintronics research, which aims to develop new device functionalities based on spincharge conversion mechanisms. In this talk I will give an overview of the results obtained over the last few years in the theory of the spincharge coupling effects in a twodimensional electron gas. In particular, I will show that the formulation of the Rashba spinorbit coupling as a SU(2) gauge field provides an elegant description of the spin Hall and spin galvanic effects. I will also consider the effect of spinorbit coupling from impurities and the specific interplay with the Rashba spinorbit coupling. A mention of the role of spinorbit coupling due to phonon scattering will also be made. 
Juan Polo (LPMMC)  Détails Fermer 
Tunneling dynamics of ultracold atoms le jeudi 2 février 2017 à 13h30 

Résumé : In this talk I will present some of the projects in which I have been involved during my PhD at the Autonomous University of Barcelona. In particular, we studied tunnelingrelated phenomena in ultracold atom systems by means of analytical approaches, numerical simulations and semianalytical models. The aim of these works has been to contribute to fields such as Atomtronics and Quantum Technologies with applications including, for instance, a proposal to build a solitonbased matterwave interferometer or protocols to load and transport ultracold atoms with high efficiency and robustness in concentric ring potentials via spatial adiabatic passage processes. In addition, we also explore more fundamental issues like the determination of the boundaries in two component BoseEinstein condensates, the generation of complex tunnelings for ultracold atoms carrying orbital angular momentum trapped in sidedcoupled cylindrically symmetric potentials and the creation of single atom edgelike states in ribbons. Liens :[LPMMC] 
Fabien Bruneval (Service de Recherches de Métallurgie Physique, CEA Saclay, France)  Détails Fermer 
Electronic excitations in molecules with manybody perturbation theory le vendredi 27 janvier 2017 à 11h00 

Résumé : The description of excited states is most easily understood in terms of Green's functions. The working approximations to obtain the Green's function have historically been developed targeting to condensed matter systems. For instance, the GW approximation [1] to the electron selfenergy has been shown to yield accurate crystal band structures [2] and the BetheSalpeter equation is known to describe very well the excitons in solids [3]. However, until recently, little was known about the performance of manybody perturbation theory for atoms, molecules, and clusters. Our inhouse code named MOLGW [4] addresses the efficient and accurate calculations of electronic excitations for finite systems. This code, based on standard quantum chemistry Gaussian basis sets, is conceptually simple, since it does not require any other convergence parameter besides the initial choice of the basis set. The code works efficiently in parallel and is opensource: it can be freely downloaded on the web [5]. With this unique tool, we have demonstrated the concavity error of the GW approximation [6] and we have explored the accuracy of the quasiparticle energy calculations within the GW approximation for organic molecules as compared to photoemission spectroscopy or to high level quantum chemistry references [7,8]. We have also measured the quality of the optical excitations obtained from the BetheSalpeter equation [9]. Recently, we have implemented selfenergies that go beyond the standard GW approximation, the socalled “vertex corrections”. [1] L. Hedin, Phys. Rev. 139, A796 (1965). [2] M.S. Hybertsen and S.G. Louie, Phys. Rev. B 34, 5390 (1986). [3] G. Onida, L. Reining, and A. Rubio, Rev. Mod. Phys. 74, 601 (2002). [4] F. Bruneval, T. Rangel, S.M. Hamed, M. Shao, C. Yang, and J.B. Neaton, Computer Phys. Comm. http://dx.doi.org/10.1016/j.cpc.2016.06.019 (2016). [5] http://www.molgw.org [6] F. Bruneval, J. Chem. Phys. 136, 194107 (2012). [7] F. Bruneval and M.A.L. Marques, J. Chem. Theory Comput. 9, 324 (2013). [8] T. Rangel, S.M. Hamed, F. Bruneval, and J.B. Neaton, J. Chem. Theory Comput. 12, 2834 (2016). [9] F. Bruneval, S.M. Hamed, and J.B. Neaton, J. Chem. Phys. 142, 244101 (2015). Liens :[Fabien Bruneval] 
Michael Pasek (Laboratoire KasslerBrossel)  Détails Fermer 
Anderson localization of cold atoms in optical disordered potentials le jeudi 26 janvier 2017 à 13h30 

Résumé : Three recent experiments have claimed the observation of Anderson localization of cold atoms exposed to
3D optical disordered potentials. However, the estimated mobility edge, namely the critical value of energy
separating the localized and ergodic phase, is observed to be significantly larger than the current best theoretical
and numerical predictions. I will try to shed some light on this matter, in particular regarding the effect on the
mobility edge of the local probability distribution and longrange spatial correlations of the disordered potential.
I will finally discuss some recent (and unpublished) experimental results on the measurement of spectral functions
of cold atoms in disordered potentials by the Atom Optics group at Laboratoire Charles Fabry.

Neil Drummond (Department of Physics, Lancaster University)  Détails Fermer 
HighPressure Phase Diagram of Solid Molecular Hydrogen le vendredi 13 janvier 2017 à 11h00 

Résumé : Establishing the phase diagram of hydrogen is a major challenge for theoretical and experimental physics. We have used the highly accurate diffusion quantum Monte Carlo method to calculate staticlattice energies for solid hydrogen at pressures up to 400 GPa, to which we have added anharmonic vibrational energies calculated within density functional theory (DFT). We have focused on the observed highpressure phases II, III and IV, which we have modelled using structures found in DFT searches. We find good agreement with experiment for the stabilisation of phase IV. The calculated pressure for the transition between phases II and III is larger than found in experiment, and we suggest possible reasons for this. The isotope dependence of the IIIII transition is wellreproduced. Our calculations show that the metallic structure that is strongly favoured in DFT at high pressures is not energetically competitive, resolving an outstanding disagreement between theory and experiment. 
Hridis Pal (Laboratoire de Physique des Solide, Orsay)  Détails Fermer 
Do quantum oscillations always arise from the Fermi surface? le vendredi 6 janvier 2017 à 11h00 

Résumé : Quantum oscillations are conventionally understood to arise from the Fermi level; hence, they are considered to be a proof of the existence of an underlying Fermi surface. This fact forms the basis for experiments measuring these oscillations to study metallic systems and map the Fermi surface. In this talk, I will show that this conventional understanding is not always true: in certain situations quantum oscillations can also arise from inside the Fermi sea. The necessary condition and possible scenarios for such unusual behavior will be pointed out. These unconventional oscillations are not described by the standard LifshitzKosevich theory valid for metals. Their temperature dependence is drastically different from that in metals. Additionally, oscillations in thermodynamic quantities (de Haasvan Alphen effect) and transport quantities (Shubnikov deHaas effect) are found to behave differently, in contrast to that in metals. Such new insights open the door to the possibility of using quantum oscillations to study features in systems traditionally thought to be outside the scope of this techniqueI will point out some realistic examples where such unconventional oscillations could show up. 