Demystifying the holographic mystique
le vendredi 18 décembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Despite great efforts, the progress towards a systematic study and classification of various 'strange' metallic states of matter has been rather slow. It's been argued, however, that the recent proliferation of the ideas of holographic correspondence originating from string theory might offer a possible way out of the stalemate. This discussion aims at ascertaining the true (as opposed to the desired) status of the applications of holography to condensed matter systems and elucidating the conditions under which it might indeed work.

Thermal transport in the disordered electron liquid
le vendredi 11 décembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In this talk, I will present a theoretical study of thermal transport in the disordered two-dimensional electron liquid. At temperatures smaller than the impurity scattering rate, in the diffusive regime, thermal conductivity acquires non-analytic quantum corrections. Our approach to this problem is based on an analysis of the heat density-heat density correlation function. In a two-stage procedure, a renormalization group calculation is supplemented with a perturbative study of scattering processes induced by the Coulomb interaction in the sub-temperature energy range. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. As an application, I intend to discuss thermal transport on the metallic side of the metal-insulator transition in Si MOSFETs.
References: G. Schwiete and A.M. Finkel’stein, PRB 90, 060201 (2014), PRB 90, 155441 (2014), arXiv:1509.02519, arXiv:1510.06529

Jörg Schmalian ( Institute for Theoretical Condensed Matter Physics, Karlsruhe Institute of Technology, Karlsruhe, G)

Nematic order in iron superconductors - who is in the driver's seat?
le vendredi 4 décembre 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Understanding the low-energy excitations of the normal state of the iron based materials is fundamental to unveil the pairing mechanism responsible for its superconducting state. Experiments have revealed that below the tetragonal-to-orthorhombic transition line, which closely tracks the magnetic transition line, the system displays a strongly anisotropic behavior that cannot be attributed only to the small lattice distortion. As a result, it has been suggested that electronic degrees of freedom, dubbed nematic, drive the structural transition. In this talk, I will present a theoretical model for the low-energy fluctuations associated with this nematic degrees of freedom, studying its impact on different properties of the iron pnictides. I will generalize the concept of vestigial order to other correlated systems. Finally, we will discuss the role of nematic fluctuations to the pairing interaction of the iron pnictides.

Thermodynamic of trajectories for quantum open systems. From full-counting statistics and dynamical phase transition tThermodynamic of trajectories for quantum open systems. From full-counting statist
le jeudi 3 décembre 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : The description of the dynamics resulting from the interaction of a quantum system with its environment is one of the key goals of modern quantum physics. The formal description of the evolution of an open system, especially in a quantum context, is typically tackled through master equation approach. Recently, a promising approach came to light, combining the quantum master equation and large-deviation theory. Unlike others, this approach applies to any dissipative quantum systems, paving the way to a standard description of dynamic of open quantum system in terms of thermodynamics of trajectories.
From two different systems, I will explore the possibility given by this approach. Starting with a small interacting spin ring, we will see how thermodynamic of trajectories predict bistable dynamical behaviour. Next I will consider a paradigmatic system in quantum mechanics, a quantum harmonic oscillators connected to various baths. I will present how our approach, based on quantum optics methods yields an analytical expression for the large- deviation function encoding the full-counting statistics of exchange between the system and the environment. Furthermore, the same approach, generalised to any network of harmonic oscillator undergoing linear dynamics allows us to, efficiently derive numerically the behaviour of energy-exchange processes between the system in a steady state and the environment. From it we can access to possible fluctuation theorem, a key thermodynamic quantities for a large variety of open systems.

Realization of strongly interacting topological phases on lattices
le jeudi 26 novembre 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : While fractional quantum Hall effect (FQHE) was realized experimentally thirty years ago in semiconductor heterostructures, strongly interacting chiral topological phases are still at the center of an important research effort, both as they serve as building blocks of more exotic phases such as fractional topological insulators and as a realization outside of semi-conductor physics is still missing. In this talk, I will describe realizations of these phases in cold atoms gases and in frustrated spins systems. I will first introduce optical flux lattices, which are continuous models that exhibit topological flat bands with a tunable Chern number and host fractional states beyond the FQHE. Then, I will focus on chiral spin liquids whose emergence on the kagomé lattice using local Hamiltonians has been shown very recently. Unlike itinerant particle systems where FQHE can be understood as a consequence of interactions in a partially filled topological band, I will show that such a picture does not hold for this chiral spin liquid. however, it can be described by model states obtained using a parton construction.

First principles calculations in magnetic and disordered compounds: recovery of finite lifetime effects in a Green function approximation framework
le vendredi 20 novembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The electronic structure and low-lying excitations' spectrum of a material can represent an essential tool to understand experiments. Relying on the foundations of density functional theory, a variety of techniques are available for the study of these dispersion relationships with good degree of predictivity across a variety of systems. I will concentrate on a Green function formalism, in its suitability to deploy approximation schemes that can also capture finite lifetime effects. Discussion will focus in particular on the examples of electronic states in disordered solids, such as alloys or doped compounds, and collective spin excitations undergoing specific damping processes.
Basic features of the methods adopted for such studies will be outlined, and illustrated through some real materials' case studies and simplified models prototypes.

New decoupling scheme of the (quantal) BBGKY hirarchy of density matrices at the 3-body level with applications to exactly solvable models for a test
le jeudi 19 novembre 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : The equations of motion couple 1-body densities to 2-body ones. 2-body
densities are coupled to 3-body ones, etc. We show that the 3-body
correlators can approximately but very naturally be expressed as a
quadratic form of the 2-body correlators closing thus the hirarchy on the
2-body level. Neglecting the 2-body correlations leads to
Time Dependent Hartree-Fock. Its small amplitude limit gives RPA (with
excahnge). The small amplitude limit of the coupled equations for 1-body
and 2-body densities leads to equations where the RPA is augmented to
contain ground state correlations self-consistently (SCRPA) which in turn
is coupled to a higher RPA of the 2-body sector. The scheme is conserving,
fullfills f-sum rule, and Goldstone theorem.
Applications to exactly sovable models like 1D Hubbard show very promising
results.

Quantum phase transition and transport in a Kondo Chain
le vendredi 13 novembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I will discuss the low energy physics of a Kondo chain
where electrons from a 1d band interact with magnetic
moments via an anisotropic exchange interaction.
I will show that the anisotropy gives rise to two
different phases which are separated by a quantum
phase transition. In the phase with easy plane
anisotropy, Z_2 symmetry between sectors with
different helicity of the electrons is broken.
As a result, localization effects are suppressed
and the dc transport acquires (partial) symmetry
protection. This effect is similar to the protection
of the edge transport in time-reversal invariant
topological insulators. The phase with easy axis
anisotropy corresponds to the Tomonaga-Luttinger
liquid with a pronounced spin-charge separation.
The slow charge density waves have no protection
against localizatioin.

Dynamics of ultracold atoms in coupled ring potentials
le jeudi 12 novembre 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : lecture 2

Résumé : In this talk we will discuss spatial adiabatic passage processes for ultracold atoms in tunnel coupled cylindrically symmetric potentials. Specifically, we investigate the matter-wave analogue of the Rapid Adiabatic Passage and of the Stimulated Ramann Adiabatic Passage techniques for the loading of a single atom and its transport between different potentials. The dynamics is described by means of a two- and a three-state models, obtaining good agreement with the corresponding numerical simulations of the two-dimensional Schrödinger equation.
In addition, we will present the dynamics of angular momentum states for a single ultracold atom trapped in two dimensional systems of sided coupled ring potentials. We will show that the tunneling amplitudes between different ring states with variation of winding number are complex. In particular, we demonstrate that in a triangular ring configuration, spatial dark states can be geometrically engineered to manipulate the transport of angular momentum via quantum interference.

Gianluca Stefanucci (Dipartimento di Fisica Università di Roma )

NEGF approach to time-resoved quantum transport
le vendredi 6 novembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In the first part of the talk I give an introduction to the fundamentals of the Nonequilibrium Green's Function (NEGF) formalism, highlighting important aspects of the theory like how to incoroporate initial correlations, how to generate conserving approximations, and how to make the scheme practical through the solution of the Kadanoff-Baym equations. In the second part of the talk the NEGF formalism is applied to nanoscale junctions to derive a generalization of the Meir-Wingreen formula that includes initial correlations and memory effects. I present numerical results through correlated model systems and discuss the performance of different conserving approximations in the description of level broadenings, level renormalization, and bistability. If time permits, I will also discuss a recent development to drastically reduce the computational cost of the simulations.
Gianluca Stefanucci is the author of a book on Non-Equilibrium Green Functions (NEGF) theory (http://ebooks.cambridge.org/ebook.jsf?bid=CBO9781139023979) and a contributed author to the book "Molecular and Nano Electronics: Analysis, Design and Simulation" (http://www.sciencedirect.com/science/bookseries/13807323/17), with interests on applications of NEGF to both steady-state and transient response Quantum Transport.

Strongly correlated one-dimensional quantum gases: pinning Mott transition and variational description of dynamical properties
le lundi 2 novembre 2015 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Strong correlations naturally emerge in interacting one-dimensional quantum systems, with remarkable consequences on both their static and dynamical properties.
In this talk I will first focus on the consequences on static properties, and present our recent study of the superfluid-insulator transition of one-dimensional interacting bosons in both deep and shallow periodic potentials [1].
Our theoretical analysis is based on quantum Monte-Carlo simulations in continuous space and on the Luttinger liquid approach. We provide the first quantitative determination of the phase diagram at arbitrary lattice heights, finding the regimes of validity of widely used approximate models, namely the Bose-Hubbard and sine-Gordon models.
Experimental results have also been obtained at the LENS laboratory in Florence, and are in excellent agreement with the theoretical predictions.
In the remainder of my talk I will introduced a novel variational approach to accurately describe the dynamical properties of many-body systems in continuous space: the continuous-space time-dependent Variational Monte Carlo [2].
We obtain variational results for both statics and dynamics of interacting 1D quantum gases. The approach we introduce is a few orders of magnitude more accurate than existing state-of-the-art variational approaches, which are the continuous-space counterpart of DMRG. I will also discuss how it can be applied to regimes and dimensionalities traditionally unaccessible to accurate many-body methods.
[1] G. Boéris, L. Gori, M. D. Hoogerland, A. Kumar, E. Lucioni, L. Tanzi, M. Inguscio, T. Giamarchi, C. D’Errico, G. Carleo, G. Modugno, and L. Sanchez-Palencia, arXiv:1509.04742 (2015)
[2] G. Carleo, L. Cevolani, L. Sanchez-Palencia, and M. Holzmann, in preparation (2015)

Classification of topological quantum matter with symmetries
le vendredi 23 octobre 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Topological materials have in recent years become a subject of intense research due to fundamental considerations as well as potential use for technical applications in device fabrication and quantum information. One of the hallmarks of topological materials is the existence of protected exotic zero-energy surface states, which arise as a consequence of a nontrivial topology of the bulk wave functions. In this talk, starting from the ten-fold classification of topological insulators and superconductors, I will survey recent developments, with a particular emphasis on the topological classifications of fully gapped and gapless materials in terms of crystal reflection symmetries. As concrete examples, I discuss the Dirac materials Ca3PbO and Sr3PbO and show that these antiperovskites are reflection-symmetry-protected topological insulators. The Dirac surface states of these materials are protected by a non-zero mirror Chern number, which can take on only even values. As an example of a reflection-symmetry-protected topological semimetal, I will present results about the recently discovered compound Ca3P2 and demonstrate that this system exhibits surface states which are almost dispersionless. If time permits, I will also discuss some results about the topological properties of noncentrosymmetric superconductors and their Majorana flat-band states.

Geometric and algorithmic methods of quantum optimal control
le jeudi 22 octobre 2015 à 13h30

Séminaire LPMMC

Personne à contacter :

Lieu : lecture 2

Résumé : Quantum optimal control is a growing multidisciplinary field of research that aims at developing mathematical and numerical tools to control quantum systems more efficiently. This talk will first give an overview of the field and its applications. Then we will focus on two examples of methods: the geometric approach with applications to NMR and IMR, and the numerical approach named Gradient Optimization of Analytic conTrols (GOAT), applied to Josephson junctions.

Charge, heat and spin transport in superconducting hybrid structures
le lundi 19 octobre 2015 à 14h00

Soutenance HDR

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The thermoelectric transport in superconducting hybrid structures is studied. In these systems the superconductors are placed in electric contact with other materials, for example normal metals, ferromagnets, etc. The proximity and Josephson effects in superconductor/ ferromagnet hybrid structures were studied in diffusive limit. Some novel effects were predicted. On the basis of this research the thermal transport in normal metal/ superconductor micro-refrigerators was studied. There was proposed the model of a micro-refrigerator, which may overcome the existing limitations of the cooling process. Finally, novel project on the electric transport in superconductor/ topological insulator hybrid structures was proposed.
The buffet will be served at 17:00.

Higgs-Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering
le vendredi 9 octobre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A theory for Raman scattering on 2D quantum antiferromagnets is presented which is based on
an effective $O(3)$ - model. Well within the N\'eel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs
contribution, which are calculated diagramatically. The full Raman spectrum is determined completely by the antiferromagnetic exchange coupling $J$ and a dimensionless Higgs mass. Due to the momentum dependence of the Raman vertex,
the contribution from the Higgs mode shows up as separate peak in the spectrum only for intermediate values of the Higgs mass.
The dominant two-magnon excitations give rise to a broad, asymmetric peak at $\omega\simeq 2.44\, J$,
which is a result of magnon-magnon interactions mediated by the Higgs mode.
Experimental Raman spectra of undoped cuprates and Iridates turn out to be in very good agreement with the theory.
They provide a clear signature of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.

From the quantum quenches to the microwaverefrigerator
le vendredi 2 octobre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The study of the many-body quantum eigenstates has recently revealed
a novel dynamical transition, dubbed many-body localization, intimately related to the mechanism of quantum thermalization, and resulting from the competition between disorder and interaction. When the disorder becomes sufficiently strong, ergodicity is not ensured and standard thermodynamics does not apply. Here, we show manifestations of the weak/strong disorder regime in an open and driven quantum system. We consider a minimal model for Dynamic Nuclear Polarization (DNP), the most effective technique to increase nuclear polarization by doping a compound with unpaired electrons. These electron spins are subject to random fields and dipolar couplings, and are driven out-of-equilibrium by microwave irradiation. We show the emergence of two distinct dynamical phases: for strong interactions the irradiated electron spins behave as a thermal bath, cooling down the nuclear spins to their extremely low effective temperature. For weak interaction, hyperpolarization loses efficiency and the description in terms of an effective temperature breaks down.

QUANTUM MAGNETS and the SPIN BATH
le jeudi 17 septembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Lecture 2, maison des Magistères

Résumé : In the ‘Quantum Ising” model a set of coupled spins can quantum fluctuate between “up” and “down” states. Of particular interest is the case where the interspin couplings are dipolar – this describes the most widely-used type of quantum computer, a huge variety of rare earth and transition metal-based spin systems, and has also been used to describe some superconducting networks. It is also of more general interest in quantum field theory.
I begin by discussing the general problem of decoherence and quantum relaxation for a magnetic system coupled to a ‘spin bath’ environment. These environments are the main problem preventing us from making useable quantum computers. Particular attention is paid to these phenomena for Quantum Ising systems. Experimental and theoretical work on both quantum relaxation, decoherence, and quantum phase transitions is summarized for the LiHo system, and for several molecular magnetic quantum Ising systems - the implications for adiabatic quantum computers are also discussed. Finally, I discuss a new technique involving strong high-frequency applied AC fields which will allow quantum control of the effective Hamiltonian of these systems, in principle allowing the complete suppression of the coupling to the spin bath environment.

The boundary conformal field theories of the 2D Ising critical points
le lundi 14 septembre 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : We present a numerical method to identify the Boundary Conformal Field Theories (BCFT) describing the critical behavior of the 2D-Ising Model on the Strip. This method is based on the measuring of the low-lying excitation energies spectra of the Ising Quantum Chain for different Boundary conditions in order to compare them with those of the possible Boundary Conformal Field Theories of the (A2;A3) minimal model.

Efimov Physics: from three- to N-particle universality
le vendredi 11 septembre 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Lecture 2, maison des Magistères

Résumé : Efimov physics refers to the universal behaviour of three-body particles in systems where the interaction is tuned, by nature or by scientists, close to the verge of binding a two-body subsystem. The most striking aspect of this physics is the Efimov effect, that is the appearance of an infinite number of three-body bound states that accumulate at the zero-energy threshold in the limit where the two-body-scattering length diverges (unitary limit). Moreover, the ratio between two consecutive bound-state energies tends to a constant that, for identical bosons, is universal. The limit is exact for all of the three body states in the case of zero-range potentials, an ideal and pathological limit where the infinite tower of three-body bound states is unbounded from below (Thomas collapse). For real potential, the range of the force is finite, the system has a well-defined three-body-ground state, and the limit receives non-universal corrections - finite-range corrections.
After an introduction to Efimov physics, I will show how finite-range corrections can be taken into account in the theory, allowing to map both real-potential calculations and experimental results onto the universal zero-range theory.
In addition, I'll show that the same finite-range analysis allows to analyse the N-body spectrum and to map it onto the same universal prediction of the zero-range three-body theory. In a sense still to be clarified, I'll show evidences that the three-, four-, five-, and six-body shallow systems belong to the same class of universality.

Quantum spin chains and Bethe Ansatz
le vendredi 10 juillet 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Quantum integrable systems have a long history. Originally, solving such models was done through the Coordinate Bethe Ansatz, while the underlying mathematical structure was not manifest. In the eighties, R-matrices, solutions of the celebrated Yang-Baxter equation, has become a cornerstone of the resolution of such systems. R-matrices contain the Hamiltonian of the system and constitute the basic ingredient of the Algebraic Bethe Ansatz that allows for solving the model. After presenting and comparing the two ansatz, we review some of the strategies that can be implemented to infer an R-matrix from the knowledge of its Hamiltonian, and apply this framework to the case of 19-vertex models in the context of spin chains.

Higgs modes in condensed matter physics
le vendredi 3 juillet 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : We present solid evidence for the existence of an amplitude (Higgs) mode in two- dimensional relativistic field theories based on analytically continued results from quantum Monte Carlo simulations of the Bose-Hubbard model in the vicinity of the superfluid-Mott insulator quantum critical point, featuring emergent particle-hole symmetry and Lorentz invariance. The Higgs boson, seen as a well-defined low-frequency resonance in the spectral density, is quickly pushed to high energies in the superfluid phase and disappears by merging with the broad secondary peak at the characteristic interaction scale. Simulations of a trapped system of ultra-cold Rb-87 atoms demonstrate that the low-frequency resonance feature is lost for typical experimental parameters, while the characteristic frequency for the onset of strong response is preserved. We compute the universal scaling function and comment on the agreements and disagreements with three dimensions and observations of Higgs particles in more traditional solid state experiments.

Meron-antimeron Crystals with Spin Scalar Chiral Stripes
le jeudi 2 juillet 2015 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : ILL: Salle de reunion 401, 4eme etage, Science Building (SB, ILL)

Résumé : Skyrmion, a topologically-nontrivial magnetic structure with a
noncoplanar spin configuration, has been extensively studied not only
for the intriguing transport phenomena but also potential applications
to electronic devices. The spin-orbit coupling plays an important role
in stabilizing the noncoplanar spin textures and also in determining the
size of skyrmions through the competition between the
Dzyaloshinskii-Moriya interaction and the exchange interaction. As the
spin-orbit coupling is an intrinsic parameter in each material, it is
rather difficult to control the stability of skyrmions and to manipulate
their sizes. For more variety and flexibility, it is desired to explore
another class of nontrivial spin textures arising from different origins.
In this study, we propose a new mechanism for stabilizing the
topologically-nontrivial magnetic order, in which the size of
noncoplanar spin texture is flexibly varied. Instead of the spin-orbit
coupling, we utilize the instability originating in the Fermi surface
structure, which exists ubiquitously in itinerant electron systems. In
this mechanism, the ordering vector is determined by the size and shape
of the Fermi surface.We demonstrate this insimple spin-charge coupled
systems, a family of square-lattice Kondo lattice models. Performing an
efficient numerical simulation based on the Langevin dynamics, we find
that a meron-antimeron crystal with spin scalar chiral stripes can have
lower energy than the helical order that is inferred from the
Ruderman-Kittel-Kasuya-Yosida interaction. We also investigate the
stabilizing mechanism by the variational calculation, the truncation of
the spin scattering processes, and the perturbation theory in terms of
the spin-charge coupling and the degree of noncoplanarity. Our results
show that the formation of meron-antimeron crystals is a dominant
instability in the spin-charge coupled systems in a wide range of
electron filling fractions.

Anisotropic expansion and collective modes of trapped Fermi gases
le vendredi 26 juin 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The expansion of a unitary Fermi gas after its release from an anisotropic trap and the damping of collective modes have been used to determine experimentally the viscosity of this system. However, the viscous hydrodynamic approach is only valid if the mean free path is much smaller than the system size. Actually, experimental results show that the trapped gases are often closer to the ballistic than to the hydrodynamic regime. In this seminar, I will present a detailed analysis of the expansion and the collective modes of a normal-fluid Fermi gas in the framework of the Boltzmann equation with in-medium effects (mean field, cross section). After discussing approximate solutions of the Boltzmann equation based on phase-space moments, I will describe the fully numerical solution with the test-particle method, which agrees very well with experimental results. Some results for shock-wave formation will be shown, too.

Topological pumping in non-equilibrium periodically driven systems
le vendredi 12 juin 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Periodically driven quantum systems, such as semiconductors subject to light and cold atoms in optical lattices, provide a novel and versatile platform for realizing topological phenomena. Among these are analogs of topological insulators and superconductors, attainable in static systems. However, some of these phenomena are unique to the periodically driven case. I will describe how the interplay between periodic driving, disorder, and interactions gives rise to new steady states exhibiting robust topological phenomena, with no analogues in static systems. Specifically, I will show that disordered two dimensional driven systems admit an “anomalous" phase with chiral edge states that coexist with a fully localized bulk. This phase serves as a basis for a new topologically protected non-equilibrium transport phenomenon: quantized non-adiabatic charge pumping. I will make a comparison to interacting one dimensional driven systems, and show that despite the fact that they cannot support such a phenomenon, they do harbor current carrying states with excessively long life times.

Anomalous scaling at non-thermal fixed points of Gross-Pitaevskii and KPZ turbulence
le vendredi 29 mai 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : The concept of Non-thermal Fixed Point has been used to describe far-from-equilibrium quasi-stationary ultra-cold Bose gases. In particular in the strong-wave turbulence regime scaling of energy and particle spectra was analysed successfully.
Classical hydrodynamic turbulence is related to super-fluid turbulence by means of the density and phase decomposition of the Bose gas wave function. Phase dynamics is then described by the stochastic Kardar-Parisi-Zhang (KPZ) equation. This relation in between ultra-cold Bose gases and KPZ dynamics is explored here for d=1, 2 and 3 and is shown to have remarkable consequences. In particular, it enables the use of calculations from the KPZ literature to read off the anomalous scaling exponent of the ultra-cold Bose gas spectra at a Non-Thermal Fixed Point.

Many-body physics of bosons in (quasi)disorder
le vendredi 29 mai 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Anderson's localization was introduced more than fifty years ago in the context of single-particle physics: the eigenstates of the Shrödinger equation with a random potential may be localized in space leading to the absence of particle transport. It was realized quite recently that the localization idea is actually much more general and applies to a variety of situations. This program was put forward to solve the problem of electron lifetime in a quantum dot and lead to the discovery of the many-body localization physics (Anderson localization in the many-body Fock space). In the solid state context this approach has proven to be very useful to tackle the long-standing problem of the transport of interacting localized single-particle states in the absence of phonons. This gave rise to the demonstration of the energy threshold between the insulating and metallic regimes [1]. The physics of interacting bosons [2] is also very interesting particularly in connection with ongoing experiments on cold atomic gases. In this seminar I would like to show to you very recent results on the finite-temperature fluid-insulator transition of bosons in one dimension (1D). On the one hand I will present the case of the quasiperiodic potential (superposition of two incommensurate periodic potentials) and give predictions regarding the transport phase diagram including the unexpected freezing with heating behaviour [3]. On the other hand I will talk about the transport of strongly- interacting 1D bosons in the random potential and I will show the reentrance of the insulating state at strong interaction, hence completing the transport phase diagram of interacting disordered bosons at finite temperature [4].
References:
[1] D.M. Basko, I.L. Aleiner, and B.L. Altshuler, Metal–insulator transition in a weakly interacting many-electron system with localized single-particle states, Annals of Physics 321, 1126 (2006). See references therein.
[2] I. L. Aleiner, B. L. Altshuler, and G. V. Shlyapnikov, A finite-temperature phase transition for disordered weakly interacting bosons in one dimension, Nature Physics 6, 900 (2010).
[3] V.P. Michal, B.L. Altshuler and G.V. Shlyapnikov, Delocalization of Weakly Interacting Bosons in a 1D Quasiperiodic Potential, Phys. Rev. Lett. 113, 045304 (2014).
[4] V.P. Michal, I.L. Aleiner, B.L. Altshuler, G.V. Shlyapnikov, Finite-Temperature Fluid-Insulator Transition of Strongly Interacting 1D Disordered Bosons, arXiv:1502.00282.

Universal features of quantum dynamics: quantum catastrophes
le vendredi 22 mai 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : Rainbows, bright lines on the bottom of swimming pools, rogue waves at sea, gravitational lensing: these are all examples of natural focusing. In optics they are known as caustics and are the places where geometric ray theory predicts infinite intensity forcing us to use wave theory get sensible answers. Since the 1970s it has been realized that there is an underlying order to these structures given by catastrophe theory [R. Thom (1975), V.I. Arnol’d (1975)]. There are only seven distinct catastrophes that can occur in 3+1 dimensions. They are generic (need no special symmetry) and they are insensitive to perturbations ("structurally stable") which is what makes them universal. In this talk I will argue that catastrophe theory should be extended to include quantum catastrophes, i.e. places where wave theory fails and we are forced to second-quantize to remove a singularity (in the case of optics this forces us to introduce the photon). I will give examples from the dynamics of Bose-Einstein condensates in lattices following a quench to illustrate the basic idea.

Nonequilibrium dynamics of fermions: from resonant Xray scattering in solids to ultracold atoms
le vendredi 22 mai 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Many new experimental techniques in condensed matter physics go beyond the paradigm of linear response measurements. I will use example of resonant Xray scattering in high Tc cuprates to demonstrate how new insights into experimental results can be gained by considering their nonequilibrium aspects. I will also discuss on-going experiments with ultracold atoms that can help address open problems of quantum dynamics of many-body fermionic systems.

Ingénierie quantique des matériaux et dispositifs à forte corrélations électroniques
le lundi 4 mai 2015 à 14h00

Cours

Personne à contacter :

Lieu : Amphithéâtre du CNRS

Résumé : Le cours de cette année présente une introduction à la physique des systèmes quantiques à fortes corrélations dans une perspective large, ainsi qu'un point de vue sur de nouvelles thématiques émergentes dans ce domaine. Plusieurs types de systèmes physiques seront abordés : oxydes de métaux de transition principalement, mais également nanostructures de type points quantiques ou atomes froids dans les réseaux optiques. Un premier cours mettra en lumière les phénomènes et concepts communs à ces systèmes : blocage de Coulomb, transition de Mott, etc., et introduira aussi quelques aspects généraux de la structure électronique des oxydes. Dans un second cours, on abordera des développements récents cherchant à contrôler de manière sélective par des impulsions Tera-Hertz les propriétés structurales et électroniques de matériaux aux propriétés remarquables (manganites, cuprates,...). Un dernier cours décrira le regain d’intérêt actuel pour les effets thermoélectriques dans le domaine des conducteurs mésoscopiques et des gaz atomiques ultra-froids.

The complex life at the border between competing quantum condensates
le vendredi 24 avril 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Much of the complexity of Nature results from the combined action of three concepts: symmetry breaking, topological stability, and emergence. Symmetry breaking and emergence of new symmetries are counteracting principles, the result of which is complexity, ultimately a precondition for the existence of life. Two major fields within condensed-matter physics that reflect these principles are magnetism and superconductivity. Both phenomena are essentially many-body quantum effects, the study of which is one of the grand challenges of modern science. The combination of superconducting quantum coherence with spin-ordered or spin-correlated states is one of the promising new developments in condensed matter physics, giving, for example, rise the the emerging field of "Superconducting Spintronics". I will discuss examples of physical phenomena emerging from the competition between and coexistence of superconductivity and magnetism.

Strong correlation effects in topological phase transition: Emergence of a thermodynamic character.
le jeudi 16 avril 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Mott, D420, Institut Neel, 3eme etage, Bat D

Résumé : Topological quantum phase transitions are characterised by changes in global
topological invariants beyond the paradigm of spontaneous symmetry breaking.
For non-interacting electrons, such transitions are continuous and always
accompanied by a gap-closing in the energy spectrum.
The inclusion of electronic interaction may however lead to the emergence of a
new scenario.
After having briefly discussed the topological transition in a paradigmatic
model for a 2D topological insulator, I will show that strong electronic
interactions can superimpose to the topological transition a conventional
(i.e. thermodynamic) critical behavior, even though the transition still does
not correspond to any long-range order in the ordinary thermodynamic sense.
Contrary to conventional expectations, such a topological quantum phase
transition occurs without closure of the energy gap. Indeed, this theoretical
study reveals the existence of a quantum critical endpoint associated with an
orbital instability on the transition line between a 2D topological insulator
and a trivial band insulator. Remarkably, this phenomenon entails unambiguous
signatures associated to the orbital occupations that can be detected
experimentally.

Spreading of quantum correlations in long-range interacting systems
le vendredi 10 avril 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The study of the dynamics of correlated quantum systems is attracting considerable attention sparked by
the emergence of new quantum systems that combine long coherence times, slow dynamics, and precise control
of parameters. They include ultracold atoms, artificial ion crystals, electronic circuits, spin chains in organic conductors,
and quantum photonic systems. In ultracold-atom systems for instance, major assets are the possibility to engineer
out-of-equilibrium initial states and to dynamically change some microscopic parameter(s) of the system, hence realizing a
so-called quench. The latter constitutes a unique tool to study the out-of-equilibrium physics of correlated quantum systems.
One of the most fundamental feature of the dynamics of quantum systems is the existence of so-called Lieb-Robinson bounds
to the propagation of correlations. While universal bounds have been confirmed in short-range interacting systems, the effect
of long-range interactions remains a very debated subject and contrasting results have been reported. Here, we study the out-of-equilibrium dynamics of two long-range quantum models, describing lattice bosons and spins. For sufficiently fast decaying long-ranged potentials, we find that the quantum limit set by the long-range Lieb-Robinson bounds is never attained and a purely ballistic behavior is found. For slowly decaying potentials, a radically different scenario is observed in the two models. In the bosonic case, a remarkable local spreading of correlations is still observed, despite the existence of infinitely fast traveling excitations in the system. This is in marked contrast with the spin case, where locality is broken. We provide a microscopic justification of the different regimes observed and of the origin of the protected locality in bosonic models.

Open Quantum Walks: Microscopic Derivations and Applications
le vendredi 10 avril 2015 à 09h00

Cours

Personne à contacter :

Lieu : Salle de lecture 2, Maison des Magistères

Résumé : Over the last few years dynamical properties and limit distributions of Open Quantum Walks (OQWs), quantum walks driven by dissipation, have been intensely studied [S. Attal et. al. J. Stat. Phys. 147, Issue 4, 832 (2012)]. For some particular cases of OQWs central limit theorems have been proven [S. Attal, N. Guillotin, C. Sabot, Ann. Henri Poincaré 16 (2015), no. 1, 15–43]. However, only recently the connection between the rich dynamical behavior of OQWs and the corresponding microscopic system-environment models has been established. The microscopic derivation of an OQW as a reduced system dynamics on a 2-nodes graph [I. Sinayskiy, F. Petruccione, Open Syst. Inf. Dyn. 20, 1340007 (2013)] and its generalization to arbitrary graphs allow to explain the dependance of the dynamical behavior of the OQW on the temperature and coupling to the environment. Also, Open Quantum Walks show potential for applications in Quantum Biology and Dissipative Quantum Computing.

An ensemble theory of ideal quantum measurement processes
le jeudi 9 avril 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A minimalist theory of ideal quantum measurements is presented. The tested system S and the apparatus A are treated as a compound, isolated system, and the process is identified with the establishment of a generalised thermodynamic equilibrium. The results can be found on the level of thermodynamics, with a qualitative account of the relaxation mechanisms, but they can also be derived through detailed dynamic calculations based on standard quantum statistical mechanics. A quantum formalism without interpretation is used, where density operators encode knowledge about properties of a statistical ensemble, and also of its subensembles. The analysis of the measurement involves three steps. The first one deals with the dynamics of the density matrix of S+A associated with a large set of runs; it involves both the disappearance of the off-diagonal blocks (by decoherence or dephasing) and the establishment of correlations between S and the pointer of A in the diagonal blocks. The desired form for this density matrix at the end of the process is thus obtained, under some specified conditions to be fulfilled by the Hamiltonian. However, due to a quantum ambiguity, this is not sufficient to account for the occurrence of a well defined outcome for each
individual run of the ensemble. Therefore, in a second step, a stronger result is established, concerning all possible subensembles of runs. Their associated density operators are shown to relax towards the required structure owing to a specific mechanism that acts near the end of the process. In the third step, the equations thus formally obtained are interpreted by means of postulates which relate macrophysics to microphysics and pertain more to A than to S. The properties currently attributed to ideal measurements are thereby recovered most economically, and the status of Born’s rule is re-evaluated.

Open Quantum Walks: Microscopic Derivations and Applications
le jeudi 9 avril 2015 à 10h30

Cours

Personne à contacter :

Lieu : Salle de lecture 2, Maison des Magistères

Résumé : Over the last few years dynamical properties and limit distributions of Open Quantum Walks (OQWs), quantum walks driven by dissipation, have been intensely studied [S. Attal et. al. J. Stat. Phys. 147, Issue 4, 832 (2012)]. For some particular cases of OQWs central limit theorems have been proven [S. Attal, N. Guillotin, C. Sabot, Ann. Henri Poincaré 16 (2015), no. 1, 15–43]. However, only recently the connection between the rich dynamical behavior of OQWs and the corresponding microscopic system-environment models has been established. The microscopic derivation of an OQW as a reduced system dynamics on a 2-nodes graph [I. Sinayskiy, F. Petruccione, Open Syst. Inf. Dyn. 20, 1340007 (2013)] and its generalization to arbitrary graphs allow to explain the dependance of the dynamical behavior of the OQW on the temperature and coupling to the environment. Also, Open Quantum Walks show potential for applications in Quantum Biology and Dissipative Quantum Computing.

The string theory – condensed matter flirtation: an eyewitness account
le vendredi 3 avril 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A quake is rumbling through the core of physics: the empiricisms of condensed matter physics and the mathematics of string theory appear to have some deep relations. For the initiated this has an unusually strong allure, but since this cocktail involves some of the most impenetrable areas of physics it is not easy to communicate the excitement to the community at large. I will attempt to get some of it across by telling the story from the perspective of a condensed matter theorist who learned string theory only quite recently. How string theory evolved from a reductionist’s enterprise into some modern incarnation of statistical physics, equipped with general relativity turbo’s and quantum information boosters in the form of the “AdS/CFT” holographic duality. How the universality of general relativity turned into a classification method for phases of matter, including new forms of “quantum” matter characterized by dense quantum entanglements on the macroscopic scale. How the latter reveal highly unusual traits having eerie resemblances with the mysterious experimental observations, with as highlight the famous linear resistivity measured in the strange metal phase of the high Tc supercondcutors.

Anderson localization of matter-waves in tunable diagonal and off-diagonal disorder with an optical lattice
le vendredi 27 mars 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Salle de lecture 2, Maison des Magistères

Résumé : We consider disorder formed by immobile hard-core bosons in one dimensional optical lattice. We show how fast periodic modulations of interspecies interactions between immobile and mobile particles allow us to produce an effective model with diagonal and off-diagonal disorder. In this seminar we point out that periodic modulation of interaction allow disorder to work as a tunable band-pass filter for momenta.

Theory of quartet condensation (alpha-particles, bi-excitons, ...) with applications to nuclear systems and life on earth.
le vendredi 27 mars 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Bound states of four fermions with the possibility of their condensation in a many body system are not so frequent in physics. There exist speculations since long that bi-excitons rather than excitons may condense in semiconductors but no experimental verification has been found so far. In cold atom physics there may exist the future possibility to trap four different fermions what would allow to study the many body physics of quartets. Quite a few theoretical works, predicting a superfluid phase of quartets, exist on the marked considering exact solutions of 1D four flavor Hubbard models. In nuclear physics exists the strongly bound quartet in form of the alpha particle. There may be the possibility that in compact star physics a transient phase of superfluid alpha particles exists. In finite nuclei several states can be explained as being formed by a losely interacting gas of alpha particles. This also bears the connection to 'life on earth'. I will present general theory with applications to nuclear systems.

Maximally entangled states for bosonic atoms on a ring lattice
le vendredi 20 mars 2015 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : We study the generation of entanglement between two species of neutral cold atoms living on an optical ring lattice. Synthetic magnetic fields are exploited to create an entangled state between the two species. Maximally entangled eigenstates are found for well defined values of the Aharonov-Bohm phase, which are zero energy eigenstates of both the kinetic and interacting parts of the Bose-Hubbard Hamiltonian, making them quite exceptional and robust against certain non-perturbative fluctuations of the Hamiltonian. We propose a protocol to reach the maximally entangled state starting from an initially prepared ground state. An indirect method to detect this state by measuring the current of particles is proposed.

Artificial spin-ice and vertex models
le vendredi 20 mars 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Classical frustrated magnets are classical and quantum systems in which
the interactions in combination with the lattice structure impede the
spins to order in an optimal configuration at zero temperature. This
occurs in natural spin-ice samples in which the spin interactions mimic
the frustration of proton positions in water ice. The theoretical interest
in these systems has been boosted by the artificial manufacture of
materials that share these properties and are of flexible design.
I will present a simple modelling of two dimensional spin-ice materials
based on non-integrable extensions of the celebrated vertex models of
statistical mechanics. I will describe (1) their static properties,
obtained with an extension of the Bethe-Peierls or cavity method; (2)
their stochastic dynamics, that display metastability, coarsening of
stripes in the ferromagnetic phases, diffusion of topological defects, and
other interesting features. The comparison to experiments will also be
developed in the talk.
My work on this subject has been done in collaboration with L. Foini, G.
Gonnella,
D. Levis, A. Pelizzola and M. Tarzia.

Superconductivity in doped or pressurized insulators : cuprates and organics
le vendredi 13 mars 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The insulating state that arises from electron-electron repulsion is called a Mott insulator. Pressure or doping can cause a transition from an insulating state to a metal, the Mott transition. The doping-induced transition in two dimensions is peculiar and particularly relevant for high-temperature superconductors (cuprates). In this talk I show, for the one-band Hubbard model, that the Mott metal-insulator transition obtained by doping goes through an intermediate phase, the pseudogap phase. The transition between the pseudogap phase and the metal is first order, and the associated Widom line, discovered by Sordi in Grenoble, explains many of the crossovers observed in cuprates. One also finds a superconducting phase. The topology of the resulting phase diagram has many similarities with experiment. The importance of retardation for superconductivity, even in the strong correlation case, is discussed. When applied to the appropriate one-band Hubbard model for layered BEDT organics, the cellular dynamical mean-field methods used above for cuprates also yield good agreement with experiment in organics.

Quantum Mechanics in a Glass of Water
le vendredi 6 mars 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A new energy decomposition analysis for periodic systems based on absolutely localized molecular orbitals is presented [1,2]. In combination with the recently developed second generation Car-Parrinello molecular dynamics [3,4], this not only allows for ab-initio molecular dynamics simulations on previously inaccessible time and length scales, but also provide unprecedented insights into the nature of hydrogen bonding between water molecules. The effectiveness of this new combined approach is demonstrated on liquid water, ice and the water/air interface [5-7]. Our simulations reveal that although a water molecule forms, on average, two strong donor and two strong acceptor bonds, there is a significant asymmetry in the energy of these contacts. We demonstrate that this asymmetry is a result of small instantaneous distortions of hydrogen bonds and show that the distinct features of vibrational and X-ray absorption spectra originate from molecules with high instantaneous asymmetry [1,2,7,8].

References

T. D. Kühne & R. Z. Khaliullin, Nature Comm. 4, 1450 (2013).

R. Z. Khaliullin & T. D. Kühne, Phys. Chem. Chem. Phys. 15, 15746 (2013).

T.D. Kühne, M. Krack, F. Mohamed & M. Parrinello, Phys. Rev. Lett. 98, 066401 (2007).

T. D. Kühne, WIREs Comput. Mol. Sci. 4, 391 (2014).

T. D. Kühne, M. Krack & M. Parrinello, J. Chem. Theory Comput. 5, 235 (2009).

T. D. Kühne, T. A. Pascal, E. Kaxiras & Y. Jung, J. Phys. Chem. Lett. 2, 105 (2011).

R. Z. Khaliullin & T. D. Kühne, J. Am. Chem. Soc. 136, 3395 (2014).

C. Zhang, R. Z. Khaliullin, D. Bovi, L. Guidoni & T. D. Kühne, J. Chem. Phys. Lett. 4, 3245 (2013).

Résumé : The basic idea that parity measurements could be used to create entanglement is theoretically established by more than 10 years. Moreover, parity measurements can be realized in different solid state implementations, e.g. for superconducting qubits in resonant superconducting cavities and for quantum dots coupled to an electronic Mach-Zehnder interferometer or to a quantum point contact. A major problem in all these implementations is that the unavoidable back-action of the measurement hinder the realization of certain maximally entangled states. Importantly, such a decoherence has been recently reported in experiments with superconducting qubits in the groups of Dicarlo in Delft and Siddiqi in Berkeley.

This talk will put forward a scheme to overcome this dangerous decoherence effect: in fact we propose to use two different quantum feedback protocols to stabilize the steady state of the system to any maximally entangled
states defined a priori. I will present a microscopic derivation of the detector outcome and its back-action,
which allows to show that a direct feedback based on parity measurement allows to
overcome the measurement-induced dephasing within each parity subspace, leading to a larger and longer-lasting entanglement. While this scheme is inefficient for external noise sources (e.g. gate voltage fluctuations), we show that a more elaborate scheme requiring two simultaneous measurements and a single feedback channel leads to a
steady state entanglement, whose amount depends on the feedback efficiency. By combining these continuous feedback schemes with parity measurements, this work provides a further step towards the key goal of entanglement generation and qubits stabilization for quantum information processing.

Phénomènes thermo-électriques dans des nanostructures supraconductrices
le lundi 2 mars 2015 à 14h00

Soutenance de thèse

Personne à contacter :

Lieu : Salle Mott, D420, Institut Néel, 3e étage, bât. D

Résumé : This Ph.D. thesis deals with the theoretical study of the electric and thermal phenomena which occur at low temperatures in nano-structured devices comprising a tunnel junction between normal and/or superconducting metals. When a thin insulating layer separates the two metallic electrodes of a junction, both electric and heat currents flowing through this system are governed by the so-called quantum tunneling effect. Under the proper conditions, the electrons can overcome the insulating barrier carrying electric charge as well as energy (heat). Typically, these tunneling processes are affected by the exchange of energy with the high-temperature external circuit in which the nano-devices are embedded. In my thesis, I analyze the effect of such a thermal environment on the charge transport in three different nano-structures based on superconducting tunnel junctions and I study the heat tunneling in a superconducting cascade electronic nano-refrigerator.

Anyonics: Designing exotic circuitry with non-Abelian anyons
le vendredi 27 février 2015 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Non-Abelian anyons are widely sought for the exotic fundamental physics they harbour as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states, s-wave superconductors and insulators with strong spin-orbit coupling. Similarly to a Majorana wire, the ends of our “quantum wire” would bind "parafermions", exotic non-Abelian anyons which can be viewed as fractionalized Majorana zero modes.

I will briefly discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors".

Non equilibrium dynamics and entanglement in the transverse field Ising chain
le vendredi 20 février 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I will discuss the long time dynamics of some relevant observables in the quantum spin Ising chain that evolves unitarily from an initial out of equilibrium configuration. The aim is that of understanding the behaviour of out of equilibrium many body quantum systems, observed in recent ultra-cold atom experiments, in terms of few laws coming from quantum field theory or quantum mechanics, in a similar way as classical thermodynamics is based on statistical mechanics and ensemble formulation.

(titre non communiqué)
le vendredi 13 février 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : "Using the formal analogy between the statistics of work in
non-equilibrium statistical mechanics, large deviation principle and the
phenomenon of multifractality of random eigenfunctions in the field of
Anderson localization we generalize the Jarzynski equality by specifying
the low-temperature behavior of the work generating function.
We checked the new relations experimentally by measuring the dissipated
work in a driven single electron box and found a remarkable
correspondence. The results represent an important universal feature of
the work statistics in systems out of equilibrium and help to understand
the nature of the symmetry of multifractal exponents in the theory of
Anderson localization.

The preprint of this work can be found on arxiv.org/abs/1411.1852.

Light-cone effect and supersonic correlations in one- and two-dimensional bosonic superfluids
le vendredi 30 janvier 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In this talk I will present some recent results on the out-of-equilibrium dynamics of interacting
lattice bosons [1]. In particular, we study how (and how fast) correlations can spread in
a quantum system abruptely driven out of equilibrium by a quantum quench. This protocol can be
experimentally realized with ultra-cold atoms, which allow to address fundamental questions concerning
the quasi-locality principle in isolated quantum systems [2, 3]. We focus on the spreading of
density-density correlations in Bose-Hubbard models after a quench of the interaction strength, using
time-dependent variational Monte Carlo simulations [4]. This method gives access to unprecedented
long propagation times and to dimensions higher than one. In both one and two dimensions, we
demonstrate the existence of a "light-cone", characterized by the ballistic spreading of correlations
with a ﬁnite propagation time. We extract accurate values of the correlation-cone velocity in
the superﬂuid regime and show that the spreading of correlations is generally supersonic. Further, we
show that in two dimensions the correlation spreading is highly anisotropic and presents nontrivial
interference effects.
[1] G. Carleo, F. Becca, L. Sanchez-Palencia, S. Sorella, and M. Fabrizio, Phys. Rev. A 89, 031602(R) (2014).
[2] M. Cheneau et al., Nature 481, 484 (2012).
[3] T. Langen et al., Nat. Phys. 9, 640 (2013).
[4] G. Carleo, F. Becca, M. Schiro, and M. Fabrizio, (Nature) Sci. Rep. 2, 243 (2012).

Henry Glyde (Department of Physics and Astronomy, University of Delaware, Theory Group, Institut Laue Langevin)

Superfluidity, Bose-Einstein Condensation and Phonon-Roton modes of Liquid 4He confined in Nanopores
le vendredi 23 janvier 2015 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Liquid 4He confined in small pore media has been extensively investigated in the past ten years [1]. It represents a model of a dense Bose liquid at nanoscales, possibly in reduced dimension and in disorder. Superfluidity is substantially modified by this confinement while Bose-Einstein condensation (BEC) and the characteristic phonon-roton modes are less modified. The onset temperature for superflow, TC, is suppressed to very low temperatures while that for BEC and well-defined modes is little changed. There is a temperature range where there is BEC but no superflow. The low TC has been interpreted as a signal that the apparent superfluidity is a frequency dependent response characteristic of a 1D liquid showing Luttinger Liquid (LL) properties. We [2] present Path Integral Monte Carlo calculations of the superfluid fraction and the One Body Density matrix (OBDM). The aim is to see how well the observed superfluid fraction and BEC can be reproduced by a standard, zero frequency picture. It is to determine the effective dimensionality of the fluid from the scaling of the superfluid fraction and OBDM. At the pore radii typically used in experiments (e.g. 28 pore diameter, liquid radius 9 Å), we find scaling characteristic of a 2D liquid with a cross over to 3D scaling for somewhat larger pores. Scaling characteristic of a 1D, LL liquid is obtained only for very small pores (liquid radii of atomic dimensions (3 Å)). TBEC for BEC lies above TC as observed. The impact of disorder will be discussed.

[1] e.g. Evidence for a common physical origin of the Landau and BEC theories
of superfluidity, S. O. Diallo et al., Phys. Rev. Lett. 113, 205302 (2014).
[2] L. Vranjes Markic and H. R. Glyde, (to be published)

Detecting the inelastic scattering of photons propagating in a Josephson junction chain coupled to a non-linear element
le vendredi 16 janvier 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : We study the inelastic scattering of photons that propagate through a Josephson junction (JJ) chain coupled with a superconducting circuit that forms a non-linear element. Our motivation is predicting how the inelastic processes can be detected in the current-voltage characteristics(CVC) measured in a probing JJ in series with the chain, with the help of so called environmental P(E) theory. In this work, the non-linear superconducting circuit is modeled as an oscillator with a weak anharmonicity. Our results predict a peak around eV=ℏ ω'_{s}, corresponding to oscillator's modified resonant frequency, in the CVC of the probing JJ that is characteristic of the photon-photon interaction in the system.

Quench dynamics of a strongly correlated Bose gas in a split trap
le vendredi 9 janvier 2015 à 14h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Salle de lecture 2

Résumé : We study the center of mass dynamics of an interacting one-dimensional Bose gas confined in an harmonic potential with a localized barrier at the center, after a small quench of the center of the trap. We show that the frequency of the periodic dipole mode that is established strongly depends on the interaction strength between the particles, in contrast to what happens for a purely harmonic confinement, because of the interplay of effects due to the barrier, the interaction and quantum fluctuations, that even give rise to a parity effect in the strongly correlated regime.