From KPZ equation with correlated noise to Burgers turbulence
le jeudi 15 décembre 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : KPZ equation traditionally describes the dynamics of a driven interface. It can however be recast as Burgers' equation. Then it describes the hydrodynamics of a fully compressible fluid.
In this talk, I will introduce both equations and show how they are related. Then I will discuss the phenomenology of Burgers turbulence. I will present preliminary results that I obtained with the Non Perturbative Functional Renormalization Group applied to KPZ equation with spatially correlated noise. I will discuss in particular two types of forcing correlation: Exponentially decreasing correlations with a large correlation length and power-law correlation with a positive exponent. Finally, I will discuss the effect of Galilei symmetry and its possible spontaneous breaking.

Quantum-Hall-effect physics with synthetic ladders
le lundi 12 décembre 2016 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Synthetic ladders pierced by a magnetic field realized with one-dimensional alkaline-earth(-like) fermionic gases represent a promising environment for the investigation of quantum-Hall physics with ultracold atoms. We unveil the existence of a hierarchy of fractional insulating and conducting states by means of both analytical and numerical methods based on the density-matrix renormalization group algorithm. We show that such states can be exploited for constructing a topological Thouless pump where the charge transported after one cycle is quantized to fractional values and demonstrate this behavior with a full many-body time-dependent calculation. We conclude with some results on Laughlin-like states in both fermionic and bosonic systems.

1 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, Nat. Commun. 6, 8134 (2015)
2 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, New J. Phys. 18, 035010 (2016)
3 - L. Taddia, E. Cornfeld, D. Rossini, L. Mazza, E. Sela, R. Fazio, arXiv:1607.07842 (2016)

Ground-state cooling a mechanical oscillator by spin-dependent transport and Andreev reflection
le vendredi 9 décembre 2016 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : We study the ground-state cooling of a mechanical oscillator coupled to the charge or the spin of a quantum dot inserted between spin-polarised
or a normal metal and a superconducting contact. Such a system can be realized e.g. by a suspended carbon nanotube quantum dot with a suitable
coupling between a vibrational mode and the charge or spin. We show that ground-state cooling of the mechanical oscillator can be achieved for
many of the oscillator's modes simultaneously as well as selectively for single modes. We discuss different modes of operation which also include
single mode cooling by resonance, which is tunable by a magnetic field. We finally discuss how the oscillator’s state can be detected in the
current-voltage characteristic.
[1] P. Stadler, W. Belzig and G. Rastelli, Phys. Rev. Lett. 113, 047201 (2014).
[2] P. Stadler, W. Belzig and G. Rastelli, Phys. Rev. B 91, 085432 (2015).
[3] P. Stadler, W. Belzig and G. Rastelli, Phys. Rev. Lett. 117, 197202 (2016).

Momentum space topology, anomalous quantum Hall effect, and the absence of equilibrium static chiral magnetic effect
le jeudi 8 décembre 2016 à 13h30

Séminaire LPMMC

Personne à contacter : Thierry Champel ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Using derivative expansion applied to the Wigner transform of the two -
point Green function we analyze the anomalous quantum Hall effect (AQHE),
and the chiral magnetic effect (CME). The corresponding currents are
proportional to the momentum space topological invariants. We reproduce
the conventional expression for the Hall conductivity in the ideal tight
binding models of (2+1) D topological insulators. At the same time
using this method we prove, that in the equilibrium (3+1) D theory the
static CME is absent in a certain class of solids, as well as in the
properly regularized relativistic quantum field theory.

Artificial gauge fields in ultracold gases and photonics
le mardi 6 décembre 2016 à 13h30

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : I discuss recent developments in the study of artificial gauge fields in synthetic quantum systems such as ultracold gases and photonics, with an emphasis on the works I have done in the field. There have been a lot of activities in using synthetic quantum systems, such as ultracold atomic gases and photonics, to realize and study various models. One important and nontrivial ingredient in studying phenomena originally known in solid-state electron systems using synthetic quantum systems is the effect of gauge fields such as magnetic fields and spin-orbit couplings. Since ultracold gases and photons are neutral, one needs to artificially engineer the effect of gauge fields so that the neutral particles behave as if they are charged particles in the presence of gauge fields.
In this talk, I first discuss the artificial spin-orbit coupling in ultracold Bose gases in a continuous space. In particular I discuss the type of spin-orbit coupling called the Rashba spin-orbit coupling. In the presence of the Rashba spin-orbit coupling, the single-particle ground states become infinitely degenerate, which gives rise to various non-trivial many-body effects.
I then discuss artificial magnetic fields in ultracold gases in the presence of lattice potentials. In tight-binding models, the effect of gauge fields is represented as complex hopping phases. I discuss recent developments of the idea of the synthetic dimension, which is to use an internal degree of freedom of an atom as an extra synthetic dimension to simulate tight-binding models with hopping phases. I discuss our proposal on how to realize four-dimensional quantum Hall effect in ultracold gases using synthetic dimensions.
Finally, I discuss how artificial magnetic fields can be obtained also in an array of photonic cavities. I present my recent proposal on how to realize synthetic dimensions also in photonics. I discuss how both the two-dimensional and four-dimensional quantum Hall effects can be observed in photonics.

Boris Svistunov (Department of Physics, University of Massachusetts, Amherst)

Trapping Centers at the Superfluid—Mott-insulator Criticality: Transition between Charge-quantized States
le vendredi 2 décembre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Under the conditions of superfluid—Mott-insulator criticality in two dimensions, the trapping centers—i.e., local potential wells and bumps—are
generically characterized by an integer charge corresponding to the number of trapped particles (if positive) or holes (if negative). Varying the strength of the center leads to a transition between two competing ground states with charges differing by $\pm1$. The hallmark of the transition scenario is a splitting of the number density distortion into a half-integer core and a large halo carrying the complementary charge $\pm 1/2$. The sign of the halo changes across the transition. The radius of the halo diverges on the approach to the critical strength of the center.

Drude weight fluctuations in many-body localized systems
le jeudi 1er décembre 2016 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Many-body localized systems are supposed to be perfectly insulating for temperatures below a critical temperature Tc. In this talk, we will discuss some of the transport properties of systems showing a transition to a many-body localized phase. In particular, we numerically investigate the distribution of Drude weights D of many-body states in disordered one-dimensional interacting electron systems across the transition. Drude weights are proportional to the spectral curvatures induced by magnetic fluxes in mesoscopic rings. They offer a method to relate the transition to the many-body localized phase to transport properties. In the delocalized regime, we find that the Drude weight distribution at a fixed disorder configuration agrees well with the random-matrix-theory prediction P(D)∝ (γ^2+D^2)^(-3/2), although the distribution width γ strongly fluctuates between disorder realizations. A crossover is observed towards a distribution with different large-D asymptotics deep in the many-body localized phase, which remarkably is not reproduced by the expected Cauchy distribution. We show that the average distribution width <γ>, rescaled by LΔ, Δ being the average level spacing in the middle of the spectrum and L the systems size, is an efficient probe of the many-body localization transition, as it increases/vanishes exponentially in the delocalized/localized phase.

Critical behavior of open quantum systems
le lundi 28 novembre 2016 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : I will discuss intriguing features of dissipative phase transitions in open
quantum systems. In particular, I will present recent results [1] about the
critical properties of two-dimensional lattices of spins interacting via an
anisotropic Heisenberg Hamiltonian and subject to incoherent spin flips. Us-
ing the recently developed corner-space renormalization method [2], I will
show the finite-size scaling and critical exponent of the magnetic linear sus-
ceptibility. I will also present results for the Von Neumann entropy and the
quantum Fisher information across the transition, showing that a dissipative
phase transition can share properties of both thermal and quantum phase
transitions.

Anderson localization on a Bethe lattice: new phases and new phase transitions
le vendredi 25 novembre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Motivated by the problem many-body localization and the hierarchical structure of connections in the Hilbert space of many-body systems, we revisit the problem of sigle-particle localization on a Bethe lattice. We give both analytical and numerical evidence of existence of the new phase of non-ergodic extended (NEE) states with multifractal statistics of wave function coefficients in addition to the usual ergodic extended (EE) phase
and the localized phase. We discuss the phase diagram on the Bethe lattice and the phase transitions between the three phases.

Quantum Galilean Cannon as a Schrodinger Cat
le jeudi 10 novembre 2016 à 13h30

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A quantum Galilean cannon is a 1D sequence of $N$ hard-core particles with special mass ratios, and a hard wall; conservation laws due to the reflection group $A_N$ prevent both classical stochastization and quantum diffraction. It is realizable through specie-alternating mutually repulsive bosonic soliton trains. We show that an initial disentangled state can evolve into one where the heavy and light particles are entangled, and propose a sensor, containing $N_a$ atoms, with a $\sqrt{N_a}$ times higher sensitivity than in a one-atom sensor with $N_a$ repetitions.
Joint work with Thibault Scoquart, Vanja Dunjko, and Steven Glenn Jackson.
Supported by NSF, ONR, and IFRAF

Experiments and theory of the quasi-periodic Bose-glass
le vendredi 4 novembre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : We present results on the comparison between experiments and theory on the bichromatic lattice for cold atoms gases. This setup allows us to study the physics of disordered bosons where superfluid, Mott and glass phases compete. We first review the model and its phase diagram obtained from numerical calculations. The localization mechanism, which is different from the one for uncorrelated disordered, is introduced and its possible experimental signatures are proposed. Then, experimental results covering the full range of the disorder vs interaction diagram are confronted to numerical results. The effect of the trapping potential and of temperature are analyzed to explain the observed momentum distribution. We conclude on ongoing experiments using a similar setup.

A microscopic theory for quantum dynamics and propagation of monopoles in classical spin ice materials
le vendredi 28 octobre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The spin ice pyrochlores, Dy2Ti2O7 (DTO) and Ho2Ti2O7 (HTO), are frustrated magnets where the excitations above the ice-like degenerate ground state emerge as effective magnetic monopoles. The diffusion of such monopoles proceeds via flipping of large electronic spins whose local Ising anisotropy, ultimately, originates from the coupling of the RE-ions to their crystalline environment. Despite the success of the classical model, experimental evidence suggests that, at temperatures relevant for spin ice physics, quantum dynamics regulate the spin-flip mechanism otherwise suppressed by the strength of crystal-field (CF) barrier.
This seminar will present a study, for both DTO and HTO, of the microscopic mechanisms which contribute to the tunnelling of an individual spin (with total angular momentum $ J > 1/2 $). Starting from the quantum mechanical interaction of a spin ice RE-ion with its surrounding CF environment, it will be emphasised the nature of the single-ion quantum states selected by the CF symmetry. Then, by means of perturbation theories, it will be illustrated how the realisation of spontaneous spin-dynamics depends on the coupling between such symmetries and the effective magnetic fields resulting in the presence of monopoles in the system. Monopoles in facts, in contrast to their mutual long ranged interactions, manifest with a very localised impact only on the spins immediately adjacent to them. In loose words, they act stimulating quantum fluctuations on the neighbouring RE-ions, and allowing their quantum spin-tunnelling otherwise inhibited in the frozen spin ice ground state.
This study is part of a broader project on competition and balance between quantum and classical mechanisms in condensed matter systems. In the context of spin ice materials, it represents the first attempt in providing a quantitative description for the quantum-tunnelling of a RE-spin, and it unlocks a theory of monopole hopping with timescales which do not depend on external tuneable parameters but only on structural properties of each material.

Artficial antiferromagnets, THz radiation and room temperature quantum interferences
le vendredi 21 octobre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The oscillatory interlayer (RKKY) interaction between two magnetic layers separated by a metallic spacer is one of the few coherent quantum phenomena that persist at room temperature. In this talk, I will introduce the concept of dynamical control of interference pattern with ultra fast voltage pulses. I will show that the RKKY interaction can be controlled dynamically by illuminating the sample (e.g. a spin valve) with radiation in the 10-100Thz range. This new sort of (dynamical) spin torque can be strong enough to induce magnetic reversal, possibly with very little dissipating power.

Quantum correlations unveiled
le vendredi 14 octobre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Correlations and coherence are distinctive features of quantum many-body states. When considering quantum ground states, they are essentially synonymous -- quantum correlations manifest the existence of coherent quantum fluctuations in the system, and are the necessary condition for the existence of entanglement between any two subsystems. But, when moving to mixed states, the situation becomes blurred, as correlations acquire also a classical, incoherent nature, and entanglement is no longer unambiguously quantified.
In this seminar I will introduce a general separation scheme between quantum/coherent correlations and thermal/incoherence correlations for mixed states, which is deeply motivated by the equilibrium statistical mechanics of quantum many-body systems. Quantum correlations are associated with the violation of a classical fluctuation-response identity, invoking the full structure of the imaginary-time propagator, and lending themselves to a direct experimental measure. The existence of quantum correlations among two subsystems A and B negates a physically motivated form of separability (namely of absence of entanglement), in which correlations are generated by a "hidden" classical source. Most strikingly, two-point quantum correlations in extended quantum systems are numerically found to decay exponentially at any finite temperature, exhibiting a novel quantum coherence length which is completely independent of the ordinary correlation length. This new length is found to be a very sensitive probe of the quantum critical "fan" of systems exhibiting a quantum phase transition.

Chaos, Metastability and ergodicity in low dimensional superfluid circuits
le jeudi 13 octobre 2016 à 13h30

Séminaire LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We show that the standard Landau and Bogoliubov superfluidity criteria fail in low-dimensional Bose-Hubbard Circuits. Proper determination of the superfluidity regime-diagram must account for the crucial role of chaos, an ingredient missing from the conventional stability analysis. We clarify the role of "chaos" for the metastability criteria of flow states [1], and for the possibility to
witness Rabi oscillations in a SQUID like setup [2].
[1] G. Arwas, A. Vardi, D. Cohen, Scientific Reports 5, 13433 (2015).
[2] G. Arwas, D. Cohen, New Journal of Physics 18, 015007 (2016).

Landau-Zener-Stueckelberg physics in a quantum SINIS turnstile
le vendredi 7 octobre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I will discuss the dynamical quantum problem of a driven discrete energy level coupled to a semi-infinite continuum whose density of states has a square-root-type singularity, such as states of a free
particle in one dimension or quasiparticle states in a BCS superconductor.
The system dynamics is strongly affected by the quantum-mechanical
repulsion between the discrete level and the singularity, which gives
rise to a bound state, suppresses the decay into the continuum, and can produce Stueckelberg oscillations.
This quantum coherence effect may limit the performance of mesoscopic
superconducting devices, such as quantum electron turnstile.

Signatures of the 3D Anderson localization of elasticc waves
le mercredi 5 octobre 2016 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Note : the multiple announcements that you have received yesterday were caused by a reconfiguration of the e-mail servers of the polygone. The organisers of the seminars are not to be blamed for them.

Lev B. Ioffe (LPTHE, Universite Pierre et Marie Curie, Paris, France)
Annulé

Theory of the liquid-liquid phase transition in high pressure hydrogen
le vendredi 23 septembre 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : abstract:
The phase diagram of high pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications[1]. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted[2,3,4]. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition[5,6,7]. Theoretical results based on density functional theory are also very scattered[7]. We report highly accurate coupled electron-ion Monte Carlo calculations of this transition finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25-30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity and loss of electron localization[8].
We discuss the difference observed with respect to the predictions of a different Quantum Monte Carlo method [9,10].
references:
[1] J.M. McMahon, M.A. Morales, C. Pierleoni and D.M. Ceperley, “The properties of hydrogen and helium under extreme conditions”, Review of Modern Physics 84, 1607 (2012).
[2] M.A. Morales, C. Pierleoni, E. Schwegler and D.M. Ceperley “Evidence for a first-order liquid-liquid transition in high-pressure hydrogen from ab initio simulations”, PNAS 107, 12799 (2010).
[3] M.A. Morales, J.M. McMahon, C. Pierleoni, D.M. Ceperley, “Nuclear quantum effects and nonlocal exchange-correlation functionals applied to liquid hydrogen at high pressure”, Phys. Rev. Lett. 110, 065702 (2013).
[4] W. Lorenzen, B. Holst, R. Redmer, “First-order liquid-liquid phase transition in dense hydrogen” Phys. Rev. B 82, 195107 (2010).
[5] Zaghoo M, Salamat A, Silvera IF, “A first-order phase transition to metallic hydrogen”, (2015) arXiv:1504.00259
[6] Ohta K et al. ”Phase boundary of hot dense fluid hydrogen” Scientific Reports 5:16560 (2015).
[7] Knudson MD et al. ”Direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium” Science 348, 1455 (2015).
[8] C. Pierleoni, M.A. Morales, G. Rillo, M. Holzmann and D.M. Ceperley, “Liquid-liquid phase transition in hydrogen by Coupled Electron-Ion Monte Carlo Simulations”, PNAS 113, 4953–4957(2016).
[9] G. Mazzola, Yunoki S, Sorella S “Unexpectedly high pressure for molecular dissociation in liquid hydrogen by electronic simulation”, Nature Communications 5, 3487 (2014).
[10] Mazzola G, Sorella S, “Distinct metallization and atomization transitions in dense liquid hydrogen”, Phys Rev Lett 114, 105701 (2015).

Electronic friction in metals and insulators: quantum molecular dynamics beyond the Born-Oppenheimer approximation
le vendredi 9 septembre 2016 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A 14 MeV fusion neutron in a tokamak may penetrate far into the reactor wall before colliding with a nucleus. From then on, a cascade of inter-atomic collisions shares the neutron's kinetic energy between millions of atoms, and the creation of electronic excitations produces a kind of electronic friction that slows atoms gradually. Radiation damage cascades have been well explored at a classical level, but much less is known about the quantum mechanical energy transfer from moving atoms to electrons [1].
We have used large-scale time-dependent tight-binding simulations and state-of-the-art time-dependent density functional simulations within the Ehrenfest approximation to investigate electronic friction in real solids. In metals, it is often assumed that electronic friction can be ignored when the atomic velocities are low. We show that this is not the case. In insulators, the presence of an energy gap suggests that electronic excitation may be impossible below a threshold atomic speed. Experimental data on the threshold effect is confused, with some groups seeing it and others not. By simulating a single projectile atom moving through an otherwise perfect crystal, we identify a new excitation mechanism, the "electron elevator", that produces electronic friction below the band-gap threshold [2]. At low projectile speeds, the elevator provides the dominant contribution to electronic stopping.
[1] C.P. Race, D.R. Mason, M.W. Finnis, W.M.C. Foulkes, A.P. Horsfield, and A.P. Sutton, Rep. Prog. Phys. 73, 116501 (2010).
[2] A. Lim, W.M.C. Foulkes, A.P. Horsfield, D.R. Mason, A. Schleife, E.W. Draeger, and A.A. Correa, Phys. Rev. Lett. 116, 043201 (2016).

Topological superconducting phases in chains of magnetic adatoms
le vendredi 1er juillet 2016 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In a recent experiment, Nadj-Perge et al. [Science 346, 602 (2014)] provide possible evidence for Majorana bound states in chains of magnetic adatoms on a conventional superconductors. The formation of topological superconductivity in this system relies on ferromagnetic order of the magnetic moments and spin-orbit coupling of the substrate superconductor.
In this talk, I will begin with a general introduction to the physics of Majorana bound states. The second part of the talk will discuss the physical picture underlying this and related experiments, including a possible explanation of the unexpectedly strong localization of the observed end states, and give a critical discussion of the evidence for Majorana bound states in this system.

Résumé : Goldstone-mode-preserving, non-perturbative expansion scheme for
multi-orbital band ferromagnets and other aspects of quantum magnetism in
correlated electron systems will be discussed, including spin-charge and
spin-orbital coupling effects in ferromagnetic manganites, onset and
melting of local orbital order, hole dynamics in cuprate antiferromagnets,
multi-orbital quantum antiferromagnetism in iron pnictdes, and magnetic
excitations in strongly spin-orbit coupled iridates.

Aleksandr Svetogorov (Moscow Institute for Physics and Technology)

Non-adiabatic geometric phases and dephasing in an open quantum system
le jeudi 23 juin 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Denis Basko ()

Lieu : Salle de lecture 2

Résumé : We analyze the influence of a dissipative environment on geometric phases in a quantum system subject to non-adiabatic evolution. We find dissipative contributions to the acquired phase and modification of dephasing, considering the cases of weak short-correlated noise as well as of slow quasi-stationary noise. Motivated by recent experiments, we find the leading non-adiabatic corrections to the results, known for the adiabatic limit.

Ultracold Fermi gases in periodic and in disordered potentials
le vendredi 17 juin 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The experiments performed with ultracold atomic gases trapped in optical lattices or in optical speckle patterns provide us with a new versatile and tunable experimental setup to investigate the combined effect due to interactions and external periodic or random potentials in many-fermion systems. I will present some recent results on the Stoner instability, i.e., the ferromagnetic transition induced by strong repulsive interactions, obtained via continuous-space quantum Monte Carlo (QMC) simulations.
First, I will compare QMC predictions for clean Fermi gases with very recent experimental results obtained at LENS. Then, I will discuss the effect of shallow optical lattices, highlighting the difference with respect to the predictions based on the single-band approximation. Finally, I will discuss disordered Fermi gases, analyzing the interplay between the Stoner instability and the Anderson localization induced by disorder.

Superfluid flow past an obstacle
le mercredi 15 juin 2016 à 11h00

Séminaire LANEF

Personne à contacter :

Lieu : Salle du bâtiment accueil, CEA - 17 rue des Martyrs

Résumé : I will consider a turbulent superfluid flow past an obstacle within the Gross-Pitaevskii equation model. Certain basic turbulence properties like the condensate destruction and healing, four- and three-wave interactions, vortex generation and annihilation and reconnections, vortex bundles, Kelvin waves and sound radiation will be discussed using this example. In the end I will discuss an idea of using Gross-Pitaevskii equation model for modeling a finite-temperature turbulent superfluid flow past a hot object with a view to model the hotwire probes.

Photophysics in a gem stone: A route to excitation spectra and spin physics from first principles theory
le vendredi 10 juin 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The beauty of a gem stone lets forget that its color is often lend by the quantum mechanics of impurities. In fact, color centers in such and related solids show a rich photo physics. Possessing a total electron spin they may be utilized as light sources, for sensing, and to store quantum information and thus pave way for the development of solid state quantum computing. Optical spin manipulation is enabled by radiative and non-radiative processes. Unravelling the physics of the coupled electron spin dynamics is still a challenge for both experiment and theory.
Although the photo physics of a vast variety of systems, including defects, was successfully addressed by many body perturbation theory (GW and BSE) and time dependent density functional theory, direct access to the important excited multiplet states is not provided. Towards the spin physics of defects we developed an ab initio configuration-interaction approach based on hybrid density functional theory and an effective screened coulomb interaction within the constrained random phase approximation (CRPA). The talk outlines our theoretical approach within DFT, many-body theory, and CI-CRPA towards the quantitative description of defects in semiconductors and insulators, such as color centers. The principles of their photo physics will be discusses for exemplary defects in diamond and silicon carbide.

Evolutionary Games of Condensates
le mardi 31 mai 2016 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Condensation phenomena occur in many systems, both in classical and quantum mechanical contexts. Typically, the entities that constitute a system collectively concentrate in one or multiple states during condensation. For example, particular strategies are selected in zero-sum games, which are generalizations of the children’s game Rock-Paper- Scissors. These winning strategies can be identified with condensates.
In our work, we apply the theory of evolutionary zero-sum games to explain condensation in bosonic systems when quantum coherence is negligible. Only recently has it been shown that a driven-dissipative gas of bosons may condense not only into a single, but also into multiple non-degenerate states. This phenomenon may occur when a system of non-interacting bosons is weakly coupled to a reservoir and is driven by an external time-periodic force (Floquet system). On a mathematical level, this condensation is described by the same coupled birth-death processes that govern the dynamics of evolutionary zero-sum games. We illuminate the physical principles underlying the condensation and find that the vanishing of relative entropy production determines the condensates. Condensation proceeds exponentially fast, but the system of condensates never comes to rest: The occupation numbers of condensates oscillate, which we demonstrate for a Rock-Paper-Scissors game of condensates.

The making and breaking of non-abelian anyons in electronic systems
le vendredi 27 mai 2016 à 11h00

Colloque CPTGA

Personne à contacter : Manuel Houzet ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Under certain conditions quantum electronic systems form non-abelian states of matter. In these states an interchange of two fundamental excitations ("non-abelian anyons") does not merely multiply the wave function by a sign, but rather transforms the system from one quantum state to another, where the transformation is determined by topological considerations. If realized, such states may form the basis to "Topological Quantum Computation", with a very high degree of immunity to de-coherence. In my talk I will describe what non-abelian states of matter are, how electrons may be engineered to form such states, what are the observable consequences of non-abelian anyons, and how non-abelian anyons may be fractionalized. While I will mention Majorana fermions as the simplest example, I will focus on the physics of more complicated non-abelian anyons, and on what is needed for their realization.
The talk will assume no prior knowledge of this scary-looking concepts.

Topological matter
le vendredi 20 mai 2016 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : One of the main focus points of condensed matter research in the past decade has been on the topological classification of media. This is a rather new viewpoint for the characterization of the electronic properties of matter, and its successes include the prediction and observation of new classes of elements beyond the usual metals and insulators, for example topological insulators, nodal line semimetals and most recently Weyl semimetals. This approach also unites different branches of physics, as it gives a unified view on the emergent low-energy properties of matter. In my talk, I will show how the Berry phase analysis of the spectrum of a lattice periodic Hamiltonian leads to such a classification of elements. I will also demonstrate how the topological robustness shows up in the formation of protected (invariant to local deformations) surface and edge states in topological media and how the topological nature of the bulk dispersion shows up in terms of bulk anomalies. The surface states have intriguing properties, and especially in nodal line semimetals the interactions may lead to ordered states - such as superconductivity - with a very high critical temperature.

Atomic physics meets nanophotonics: creating complex quantum states of matter and light
le vendredi 13 mai 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé :
Significant efforts have been made to interface cold atoms with micro- and nano-photonic systems in recent years. Originally, it was envisioned that the migration to these systems from free-space atomic ensemble or macroscopic cavity QED experiments could dramatically improve figures of merit and facilitate scalability for applications such as quantum information processing. However, a more interesting scenario would be if nanophotonic systems could yield new paradigms for controlling quantum light-matter interactions, which have no obvious counterpart in macroscopic settings.
Here, we describe one paradigm for novel physics, based upon the coupling of atoms to photonic crystal structures. In particular, we show that atoms can become dressed by localized photonic "clouds" of tunable size. This cloud behaves much like an external cavity, but which is attached to the position of the atom. This dynamically induced cavity can then mediate long-range spin interactions or forces between atoms, yielding an exotic quantum material where spins, phonons, and photons are strongly coupled.

Many-body physics with quantum gases in disorder
le vendredi 29 avril 2016 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Salle K223 Remy Lemaire, Institut Néel

Résumé : I will first give a brief overview of the studies of ultracold quantum gases in disorder and then turn to one-dimensional interacting disordered bosons at finite temperatures. It will be shown that they may undergo an interaction-induced non-conventional insulator-fluid transition, and I will present the phase diagram. The emphasis will be put on the recent work for strongly interacting one-dimensional disordered bosons, where I will demonstrate the presence of two insulator-fluid transitions. The next issue will be interacting bosons in the quasiperiodic potential (superposition of two incommensurate one-dimensional lattices). I will demonstrate the presence of finite temperature many-body localization-delocalization transition induced by the interaction between the bosons. It will be shown that in a wide range of parameters an increase in temperature favors the insulator state, so that in this sense one has an anomalous “freezing with heating” phenomenon.
ATTENTION : LIEU INHABITUEL !

Journée des jeunes physiciens
le jeudi 14 avril 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Steven, and have been selected and will each give a 14 minutes presentation at the "Rencontres des jeunes physiciens", a one day event for young scientist only, to be hold on the 15th of April. The titles of their presentations are

Variational Density-Matrix Monte Carlo calculations for fermions
le jeudi 7 avril 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : We develop the new quantum Monte Carlo method which allows one to calculate ab initio the spectral functions from imaginary time correlations. The method is based on the imaginary time - finite temperature analogy. This formalism describes the correlations between different imaginary times introducing the thermal density matrix.
As an example we consider the electron gas in two dimensions and calculate the density fluctuation function in the imaginary time. The Fourier transformation of this function allows one to obtain dynamic local field factor, which is important characteristic of the electron gas.
The extension of the zero temperature RPA form of the Jastrow function to finite temperature is used in order to describe the configurations at different imaginary times. We present the QMC results at different interaction strengths which demonstrate the applicability of the method.

Transport in multi-terminal systems consisting of topological and superconducting elements
le vendredi 25 mars 2016 à 11h00

Séminaire théorie

Personne à contacter : Mireille Lavagna ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : This talk will focus on speaker's work on (i) junctions of interacting quantum wires and (ii) junction of topological insulator and superconductor. (i) It is well known that the conductance matrix of a junction of three interacting quantum wires. We have studied this problem using the current splitting matrix (M-matrix) to describe the junction in the bosonised language. An important result of our analysis is that conductance matrix depends on the interaction parameter only when time reversal symmetry is broken at the junction [1]. Further, the effective resistance of a parallel combination of two resistances (which consists of two Y-junctions) deviates from that in a classical circuit (which is obtained using Kirchoff's - current and voltage laws). The expression for effective resistance can be highly non-trivial depending on the choice of the M-matrices of the two Y-junctions [2]. (ii) A junction of topological insulators with a p-wave superconductor is interesting since such a set-up can drive a non-zero spin current into the superconductor. Results on zero-bias peak, and novel satellite peak in such a set-up will be discussed [3].
[1] A. Soori and D. Sen, EPL 93, 57007 (2010).
[2] A. Soori and D. Sen, Phys. Rev. B 84, 035422 (2011).
[3] A. Soori, O. Deb, K. Sengupta and D. Sen, Phys. Rev. B 87, 245435 (2013).

Superfluid and quantum features in the hydrodynamic flow of a fluid of light
le jeudi 24 mars 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : In the presence of a significant Kerr optical nonlinearity, a many-photon light beam can behave as a quantum fluid of interacting bosons.
This has opened the way to active experimental and theoretical investigations of hydrodynamic and many-body quantum features in
photon-based systems, the research field of the so-called quantum fluids of light [1]. A promising platform to study photon-fluid physics
consists in the paraxial propagation of a quasimonochromatic light wave through a nonabsorbing cavityless nonlinear optical medium of
Kerr type. In contrast to semiconductor-planar-microcavity architectures where driving and dissipation play a major role in the evolution
of the fluid of light, the photon field in a cavityless, propagating, geometry obeys a fully conservative quantum dynamics. The statistical
properties of the beam of light entering the dielectric fix the initial conditions of the problem and the ones of the light exiting the medium
determine the final state.
The first part of my talk will be dedicated to a review of a very general quantum theory of light propagation in such a configuration
[2]. As a first application of the formalism, we will see in a second part that the occurrence of a frictionless flow of superfluid light
may be revealed from the dramatic suppression of the optomechanical deformation of a solid dielectric immersed into a nonlinear
optical liquid, demonstrating that, in the optical case also, superfluidity is associated with a drop in the force exerted by the fluid on an
obstacle stymying its flow [3]. In a third part, I will show that the paraxial-propagation geometry constitutes a very simple platform to
investigate quantum-quench physics in closed systems of many interacting bosons [2], including, e.g., the dynamical Casimir effect, the
light-cone-like spreading of the two-body correlations following a quantum quench [2], or the emergence of prethermalization features
in one-dimensional configurations [4]. Before concluding, I will present ongoing experiments aiming at measuring the optical analog
of the Bogoliubov dispersion law in a one-dimensional nonlinear optical waveguide [5] and at detecting superfluid features in the flow
of a photon fluid past a localized optical defect [6]. In conclusion, I will briefly sketch an in-progress study of the thermalization of a
quantum fluid of light towards the Bose–Einstein statistics [7] and perspectives about the investigation of the strong-interaction regime in
cavityless geometries.

[1] I. Carusotto and C. Ciuti, Quantum fluids of light, Rev. Mod. Phys. 85, 299 (2013).
[2] P.-É. Larré and I. Carusotto, Propagation of a quantum fluid of light in a cavityless nonlinear optical medium: General theory and response to
quantum quenches, Phys. Rev. A 92, 043802 (2015).
[3] P.-É. Larré and I. Carusotto, Optomechanical signature of a frictionless flow of superfluid light, Phys. Rev. A 91, 053809 (2015).
[4] P.-É. Larré and I. Carusotto, Prethermalization in a quenched one-dimensional quantum fluid of light: Intrinsic limits to the coherent propagation of
a light beam in a nonlinear optical fiber, arXiv:1510.05558 (to appear in Eur. Phys. J. D).
[5] S. Biasi, M. Borghi, M. Mancinelli, S. Manna, A. Trenti, F. Turri, P.-É. Larré, I. Carusotto, L. Pavesi, and F. R. Manzano (in preparation).
[6] M. Bellec, C. Michel, M. Albert, and P.-É. Larré (in preparation).
[7] A. Chiocchetta, I. Carusotto, and P.-É. Larré, Thermalization of quantum light in bulk nonlinear media (to appear on arXiv.org).

What can we do when Boltzmann-Gibbs entropy and statistical mechanics fail?
le mercredi 9 mars 2016 à 14h30

Colloque CPTGA

Personne à contacter :

Lieu : LEGI, 1209-1211 rue de la piscine, Domaine Universitaire Amphi K (1er étage)

Résumé : The celebrated Boltzmann-Gibbs additive entropy and its associated statistical mechanics, together with Maxwell theory for electromagnetism and classical, quantum and relativistic mechanics, constitute one of the pillars of contemporary physics. Nevertheless, and in spite of its tremendous power, this theory fails when the system does not satisfy some basic hypothesis such as ergodicity. This is the case of a plethora of natural, artificial and social complex systems. What can we then do? The introduction of nonadditive entropies and the generalization of the BG theory to which they lead can be shown to be a satisfactory solution for a wide range of such anomalous systems. After some brief introduction to the main concepts, we will illustrate its applications with recent analytical, experimental, observational and numerical results.

Tracer particles in a Laughlin liquid as emergent anyons
le jeudi 3 mars 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Anyons are by definition particles with quantum statistics different from those of bosons and fermions. They can occur only in low dimensions, 2D being the most interesting case. They have hitherto remained hypothetical, but there is good theoretical evidence that certain quasi-particles occuring in quantum Hall physics should behave as anyons.
In this talk I shall consider the case of tracer particles immersed in a Laughlin liquid. I will argue that, under certain circumstances, these become anyons. This has been guessed a long time ago by Arovas-Schrieffer-Wilczek, based on a Berry phase calculation. I shall review this argument, along with the reservations I have about it. I will then discuss a new argument, which seems more convincing to me.
Part of the motivation for giving this informal talk is to check if other people might understand the historical argument better than I do, or share my reservations about it, and/or the impression that they are completely lifted by the new derivation.

Bad Metal Behavior and Mott Quantum Criticality
le vendredi 19 février 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : According to early ideas of Mott and Anderson, the interaction-driven metal-insulator transition – the Mott transition – remains a sharp T=0 phase transition even in absence of any spin or charge ordering. Should this phase transition be regarded as a quantum critical point? To address this question, here we examine the phase diagram and transport properties of the maximally frustrated half-filled Hubbard model, in the framework of dynamical mean-field theory (DMFT). We identify a “quantum Widom line” (QWL) which defines the center of the corresponding quantum critical region associated with Mott metal-insulator transition in this model. The evolution of resistivity with temperature is then evaluated along trajectories following (parallel to) the QWL, displaying remarkable scaling behavior characteristic of quantum criticality. Precisely this kind of behavior was found in very recent experiments on organic Mott systems [1,2]. In the case of the doping-driven Mott transition, we show that the mysterious “Bad Metal” behavior (T-linear resistivity around the Mott-Ioffe-Regel limit) coincides with the Quantum Critical region of the Mott transition.
[1] Quantum criticality of Mott transition in organic materials, Tetsuya Furukawa, Kazuya Miyagawa, Hiromi Taniguchi, Reizo Kato & Kazushi Kanoda, Nature Physics, 9 Feb. 2015; doi:10.1038/nphys3235.
[2] See also: http://condensedconcepts.blogspot.com/2015/03/quantum-criticality-near-mott.html

Laurent de Forges de Parny (Laboratoire de Physique, ENS Lyon)

Recent advances in ultracold atomic gases: bosonic mixtures and quantum magnetism
le jeudi 18 février 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : This pedagogic seminar focuses on ultracold atoms in optical lattices.
The strong interactions between particles give rise to surprising behaviors, unequalled in condensed matter physics. In particular, recent progress in the field have allowed the study of bosonic systems with multiple components, such as atomic and molecular mixtures and bosons with spin degree of freedom. The richness of these systems comes from the competition between different terms of the Hamiltonian, leading to multi-component Bose-Einstein condensates, multiple transitions and quantum magnetism.
Two systems, involving conversions between the different species of particles trapped, particularly drew my attention: an atomic and molecular model and a spin-1 bosonic model. I will show that these systems exhibit intriguing phases (e.g. Nematic and ferromagnetic superfluids, Feshbach insulator), as well as many phase transitions (e.g. quantum first order, singlet-to-nematic, Ising transition). Some of my numerical results discussed in this seminar, obtained by using the exact quantum Monte Carlo approach, have just been experimentally observed.

Modelling diffusion in complex systems of biological and soft matter
le vendredi 12 février 2016 à 11h00

Séminaire théorie

Personne à contacter : Markus Holzmann ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Diffusion is an essential process in biological and soft matter, representing nutrient and reactant transport as well as biological function of macromolecules. Multiple experimental techniques access diffusion processes at different length and time scales, and numerous theoretical approaches to diffusion provide well-established predictions for model systems such as suspensions of hard colloids. Systems of biological and soft matter, however, often show more complex structural features, which challenge the transfer of theoretical results for simple model systems.
In the talk, two examples will be presented in an integrated discussion of experimental results and theoretical modelling.
First, protein diffusion in concentrated solutions will be shown to be reasonably well described by colloid theory. Second, diffusion of water in the vicinity of membranes will be shown to be anomalous and anisotropic, presenting challenging problems for analytical modelling, as e.g. desired for the implementation of fit functions for experimental and computational results.

Synthetic gauge field and synthetic dimension in interacting ultracold Fermi gases
le lundi 8 février 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2

Résumé : The recently introduced concept of synthetic gauge field in synthetic dimension opens the possibility of studying the properties of (quasi) (d+1)-dimensional systems in a classical gauge field by means of d-dimensional multi-species lattice gases, both theoretically [1] and experimentally [2]. I will show how the presence of atomic interactions affects the physics of the system, leading to properties resembling fractional quantum Hall effect [3,4].

[1] A. Celi, P. Massignan, J. Ruseckas, N. Goldman, I. B. Spielman, G. Juzeliūnas, and M. Lewenstein, Phys. Rev. Lett.112, 043001 (2014).
[2] M. Mancini, G. Pagano, G. Cappellini, L. Livi, M. Rider, J. Catani, C. Sias, P. Zoller, M. Inguscio, M. Dalmonte, and L. Fallani, Science349, 1510 (2015).
[3] S. Barbarino, L. Taddia, D. Rossini, L. Mazza, and R. Fazio, Nat. Comm.6, 8134 (2015).
[4] S. Barbarino, L. Taddia, D. Rossini, L. Mazza, and R. Fazio, arXiv:1510.05603.

Mathematical and theoretical physics approaches to the KPZ equation

Proving conformal invariance in critical scalar theories in any dimension
le vendredi 29 janvier 2016 à 11h00

Colloque CPTGA

Personne à contacter : Léonie Canet ()

Lieu : Lecture 2, maison des Magistères

Résumé : Recent progress in the study of three-dimensional Ising model has been made by the use of the "conformal bootstrap [1] that assumes the existence of conformal symmetry in critical phenomena. In this way the best estimates of critical exponents in the three-dimensional Ising model have been obtained. Due to these results we study a necessary condition for the existence of conformal invariance in scale invariant systems. By using the Lebowitz inequalities [2], we prove that this necessary condition is fulfilled in all dimensions for the Ising universality class [3]. This shows, in particular, that scale invariance implies conformal invariance for the three-dimensional Ising model.

Quantum superpositions of causal relations: |A causes B> + |B causes A>
le jeudi 28 janvier 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : The notion of causality is deeply rooted in our understanding of the classical world: we typically understand the relationship between events in terms of causal relations, where some events are causes for other events, their effects.
Now, we know that the quantum world is full of weird phenomena. What happens to causal relations? Do they look the same as in the classical world, or could their be new types of causal relations? E.g., in the same way that quantum objects can be in a superposition of two incompatible states (allowing a cat to be both dead and alive at the same time!), could we have superpositions of causal orders? A situation of the kind |A causes B > + |B causes A > ? In this talk I will present a new formalism that allows us to tackle these questions, and discuss some possible applications in quantum information science.

Antiferro quadrupolar and Ising-nematic orders of a frustrated bilinear-biquadratic Heisenberg model and implications for the magnetism of FeSe
le vendredi 22 janvier 2016 à 11h00

Colloque CPTGA

Personne à contacter : Tommaso Roscilde ()

Lieu : Lecture 2, maison des Magistères

Résumé : Motivated by the magnetic properties of the iron chalcogenides, we study the phase diagram of a generalized Heisenberg model with frustrated bilinear-biquadratic interactions on a square lattice. We identify zero-temperature phases with antiferroquadrupolar and Ising-nematic orders. The effects of quantum fluctuations and interlayer couplings are analyzed. We propose the Ising-nematic order as underlying the structural phase transition observed in the normal state of FeSe, and discuss the role of the Goldstone modes of the antiferroquadrupolar order for the dipolar magnetic fluctuations in this system. Our results provide a considerably broadened perspective on the overall magnetic phase diagram of the iron chalcogenides and pnictides, and are amenable to tests by new experiments.

[1] R. Yu and Q. Si, Phys. Rev. Lett. 115, 116401 (2015)

Rong Yu obtained his Ph.D. degree from University of Southern California in 2007. He was a postdoctoral research associate at University of Tennessee, Knoxville (2007–2009) and at Rice University (2009-2013). Since 2013 he has been an associate professor at Department of Physics, Remin University of China. He has been working on theory of correlated electronic systems. Current main areas of his research includes phase transitions in heavy fermion systems, frustration and disorder effects in quantum magnets, superconductivity and correlation effects in iron-based superconductors.

Quantum spins as quantum simulators
le jeudi 21 janvier 2016 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Salle Rémy Lemaire K223, Bât. K, Institut Néel

Résumé : Quantum magnets in insulating structures have proven to be remarkable systems
when subjected to a strong magnetic field.
In addition to their own intrinsic interest they can be mapped on model systems
of itinerant quantum particles. This has allowed to use them as quantum simulators for
studying the properties of interacting hard core bosons. In particular I will focus
on two recent ladder compounds for which a combination of numerical studies and
analytical ones has allowed to obtain fully the dynamical correlation functions.
I will review the recent results in that respect, in particular some of the
experiments and the corresponding theories for phenomena such as Bose-Einstein
condensation and Tomonaga-Luttinger liquids as observed by neutrons, NMR and
also ESR.
I will discuss the recent successes in this domain as well as several of the
open problems and perspectives offered by such compounds such as the possibility
to study dimensional crossover, disorder effects etc.

A thermomagnetic mechanism for self-cooling cables
le vendredi 15 janvier 2016 à 11h00

Séminaire théorie

Personne à contacter : Markus Holzmann ()

Lieu : Lecture 2, maison des Magistères

Résumé : A solid state mechanism for cooling high-current cables is proposed based on the Ettingshausen effect, i.e. the transverse thermoelectric cooling generated in magnetic fields [1]. The intense current running in the cable generates a strong magnetic field around it, that can be exploited by a small current running in a coating layer made out of a strong ”thermomagnetic” material to induce a temperature difference between the cable core and the environment. Both analytical calculations and realistic numerical simulations for Bismuth coatings in typical magnetic fields are presented. The latter yield temperature drops ≃60K and >100K for a single- and double-layer coating respectively. These encouraging results should stimulate the search for better thermomagnetic materials, in view of applications such as self-cooled superconducting cables working at room temperature.
[1] Luca de Medici, arXiv:1506.01674 (2015).

Germanium as a optical gain material
le jeudi 14 janvier 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : In the talk I will speak about
the potential of Ge as an optical
gain material and one of the most
interesting approaches to achieve
the emission of germanium.

Periodic striped ground states in Ising models with competing interactions
le vendredi 8 janvier 2016 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Lecture 2, maison des Magistères

Résumé : We consider Ising models in two and three dimensions, with short range
ferromagnetic and long range, power-law decaying, antiferromagnetic
interactions. We let J be the ratio between the strength of the
ferromagnetic to antiferromagnetic interactions. The competition between
these two kinds of interactions induces the system to form domains of
minus spins in a background of plus spins, or vice versa. If the decay
exponent p of the long range interaction is larger than d+1, with d the
space dimension, this happens for all values of J smaller than a critical
value J_c(p), beyond which the ground state is homogeneous. In this talk,
we give a characterization of the infinite volume ground states of the
system, for p>2d and J in a left neighborhood of J_c(p). In particular,
we report a proof that the quasi-one-dimensional states consisting of
infinite stripes (d=2) or slabs (d=3), all of the same optimal width and
orientation, and alternating magnetization, are infinite volume ground
states. We shall explain the key aspects of the proof, which is based on
localization bounds combined with reflection positivity. Joint work with
Robert Seiringer.

Modeling the Tau protein - Microtubule interaction
le jeudi 7 janvier 2016 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : Tauopathies such as Alzheimer’s disease or Parkinson syndromes are examples of disorders associated with a dysfunction of tau proteins to properly regulate the network of microtubules (MTs) in axons. Understanding how and by which means Tau proteins affect the microtubule dynamics represent therefore a major challenge in understanding Tauopathies. In this work, we were interested in the dynamic interaction between a Tau protein and a non-dynamic microtubule. Based on the experimental observations, we constructed a model describing the interaction Tau - MT as a dynamical process "on - off" in which the free Tau ("off") may be confined ("on") on the MT which it moves along a one-dimensional diffusion process and then followed by either an attachment on the MT or a detachment in the "off" state. This model is characterized by 5 parameters that can be extracted from experiments : k_{on} the confinement rate on the MT for a free Tau, D the diffusion coefficient of the one-dimensional diffusion for a Tau confined on the MT, k_{b} the attachment rate for a Tau confined along the MT, k_{c} the detachment rate of Tau bound on the MT, and k_{f} the escape rate of Tau protein. We have derived the analytical expressions of the main characteristics of the Tau - MT interaction.