Density of states and numerical simulations: from Statistical Mechanics to Gauge Theories
le vendredi 15 septembre 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Lecture 2, maison des Magistères

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

Dissipative phase transition with quantum frustration
le vendredi 8 septembre 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : We study a quantum dissipative rotor model in which each local phase difference and each local momentum are uniformly coupled to two different baths.
Such systems can represent e.g. a chain of resistively shunted Josephson junctions and capacitively coupled to a diffusive metal.
The first dissipative coupling quenches the quantum phase fluctuations favoring the long-range phase order (i.e. superconducting ground state)
whereas the second one quenches momentum fluctuations destroying phase coherence (insulating ground state).
Using the self-consistent harmonic approximation, we calculate the zero temperature phase diagram as determined by the two dissipative coupling
constants and the bare zero point fluctuations.
As an effect of the quantum frustration for the two canonical conjugate observables, we obtain an rich phase diagram with a non-monotonic behavior:
for instance, the ground state can change from superconducting to insulating and back to the superconducting phase by increasing the dissipation.

First-principles study of strongly correlated oxide perovskites
le mardi 5 septembre 2017 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Salle Weil E424, Institut Néel, Bâtiment E, 3ème étage

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

Spin-orbit coupled interferometry with ring–trapped Bose–Einstein condensates
le lundi 10 juillet 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We propose a method of atom-interferometry using a spinor Bose–Einstein (BEC) and the well-established
experimental technique of time-varying magnetic fields as a coherent beam-splitter. Our protocol creates longlived
superpositional counterflow states, which are of fundamental interest and can be made sensitive to both the
Sagnac effect and magnetic fields on the sub micro-Gauss scale. We split a ring-trapped condensate, initially in the
mf = 0 hyperfine sub-level, into superpositions of both internal spin state and condensate superflow [1], which are
spin-orbit coupled. After interrogation a relative phase accumulation can be inferred from a population transfer to
the mf = 1 states [2]. We present numerical and analytical treatments of our system [3].

References
[1] T. Isoshima, M. Nakahara, T. Ohmi, and K. Machida, Phys. Rev. A 61, 063610 (2000).
[2] P. L. Halkyard, M. P. A. Jones, and S. A. Gardiner, Phys. Rev. A 81, 061602 (2010).
[3] J. L. Helm, T. P. Billam, A. Rakonjac, S. L. Cornish, S. A. Gardiner, arXiv:1701.02154.

The arrow of time for continuous quantum measurements
le vendredi 30 juin 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The question of the time reversibility of quantum mechanics with
measurements is one that has been debated for some time. In this
talk, I will present new work exploring our ability to distinguish the
forward from the time-reverse measurement records of continuous
quantum measurements. The question involves both the conditions for
the time-reversibility of the quantum trajectory equations of motion,
as well as statistical distinguishability of the arrow of time. For a
continuous qubit measurement example, we demonstrate that
time-reversed evolution is physically possible, provided that the
measurement record is also negated. Despite this restoration of
dynamical reversibility, a statistical arrow of time emerges, and may
be quantified by the log-likelihood difference between forward and
backward propagation hypotheses. We then show that such reversibility
is a universal feature of non-projective measurements, with forward or
backward Janus measurement sequences that are time-reversed inverses
of each other.
J. Dressel, A. Chantasri, A. N. Jordan, A. N. Korotkov, arXiv:1610.03818

Maximally entangled states, pair-superfluidity and MORE in a many-body interacting system
le jeudi 29 juin 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Dominique Spehner ()

Lieu : Salle de lecture 2, maison des Magistères

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

References

[1]
S. A. Reyes, L. Morales-Molina, M. Orszag, and D. Spehner, EPL 108, 20010
(2014).

[2] L. Morales-Molina, S. A. Reyes and E. Arevalo, EPL 115, 36004 (2016).

Ground-state and asymptotic dynamical properties of 1D ultracold gases in the presence of a mobility edge
le lundi 26 juin 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : In the first part of the talk we explore the ground-state properties of cold atomic gases focusing on the cases of
noninteracting fermions and hard-core (Tonks-Girardeau) bosons, trapped by the combination of two potentials
(bichromatic lattice) with incommensurate periods. In the tight-binding limit, the single-particle states in the
lowest occupied band show a localization transition, as the strength of the second potential is increased above a
certain threshold. In the continuum limit, when the tight-binding approximation does not hold, a mobility edge is
found, instead, whose position in energy depends upon the strength of the second potential. Here, we study how
the crossover from the discrete to the continuum behavior occurs, and prove that signatures of the localization
transition and mobility edge clearly appear in the generic many-body properties of the systems. Specifically, we
evaluate the momentum distribution, which is a routinely measured quantity in experiments with cold atoms,
and demonstrate that, even in the presence of strong boson-boson interactions (infinite in the Tonks-Girardeau
limit), the single-particle mobility edge can be observed in the ground-state properties. In the second part we
study the dynamical many-body response of for a one-dimensional fermionic gas in a mono- and bi-chromatic
optical potential following the sudden switching-on of a delta-like barrier at some at the center of the system.
Specifically we look at the Loschmidt echo as a figure of merit to characterize the response of the system and
its long time behavior. In order to evaluate the echo we employ two complementary approaches: (1) functional
determinants (Levitov) which gives the exact numerical solution for time- and therefore frequency-resolved
responses and (2) a perturbative approach (Linked Cluster Expansion) which provides an accurate evaluation
of the contribution of different physical processes involved in the dynamics. Again we focus on the two limits
of tight-binding and continuum showing that the phenomenon of the orthogonality catastrophe can be observed
in such systems which, unlike their condensed matter counterpart, are nowadays created and controlled with a
very high accuracy.

Karyn Le Hur (CPHT Ecole Polytechnique, Palaiseau,)

Many-Body Quantum Physics with Photons
le vendredi 23 juin 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : We review recent developments in the context of many-body quantum physics with microwave photons in superconducting quantum electrodynamics networks and Josephson junction arrays. First, we show how the Jaynes-Cummings lattice model yields an analogy with the Bose-Hubbard model and can allow to engineer a Mott-superfluid transition of photons. We discuss the challenges to achieve such a transition, requiring the coupling to AC perturbations and the necessity to include dissipation effects. We also discuss progress in methods and probes. Then, we discuss realizations of topological phases and robust photonics by analogy to progress in quantum materials and ultra-cold atoms, and address disorder and interaction effects. We also show the simulation of novel topological chain devices with superconducting and Josephson circuits. Experimental progress and realizations are discussed. Such systems also offer novel platforms to address and probe dissipative and driven quantum impurity physics, such as the Kondo effect.

Quantum optics with many-body systems of atoms and photons: From quantum networks to entanglement measurement
le jeudi 22 juin 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé :

The physics of light-matter interactions plays a fundamental role for two important research areas in quantum technology: quantum information and quantum simulation.
On the one hand, quantum optics theory allows to design robust protocols for the processing of quantum information in quantum networks.
On the other hand, in the context of quantum simulation, light-assisted interactions between atoms provide the toolbox to prepare and probe many-body phases of complex Hamiltonians, which are related to long-standing problems in condensed matter (e.g. quantum magnetism or fractional quantum Hall states).

In this talk, I will discuss these two topics. I will present our recent results on quantum information processing in quantum networks, and on the engineering of new tools for quantum simulation.
At the technical level, I will show how we combine atomic physics, quantum optical techniques and numerical methods borrowed from condensed matter physics (such as Matrix-Product-State (MPS) techniques) to study these types of complex open many-body systems.

In the first part I will discuss some of our works related to photonic quantum networks.
After a general introduction, I will present a theoretical description of these systems based on the formalism of waveguide QED.
This will then allow me to present recent results on the development of robust Quantum State Transfer protocols [1,2,3], and to introduce our MPS techniques for the description of the dynamics of quantum networks beyond the standard treatment of quantum optics.

The second part of the seminar will be devoted to quantum simulators. I will first explain the challenge of measuring entanglement, which is essential to characterize various phases in condensed matter physics (such as Fractional Quantum Hall effect or Haldane phase).
I will then show how to measure the entanglement spectrum of ground states of generic Hamiltonians based on direct engineering of the entanglement Hamiltonian [4].
Our method, based on the Bisognano-Wichmann theorem [5], allows one to measure entanglement spectra via standard spectroscopy and can be implemented in all quantum simulation platforms.
I will provide numerical examples to support this result and give examples of AMO implementations of entanglement Hamiltonians.
If time allows, I will present a complementary method based on Random Matrix Theory, which would allow to measure the entanglement growth in a many-body localised (MBL) system [6].

[1] C Dlaska, BV and P. Zoller et al Quantum Sci. Technol. 2 015001 (2017).

[2] BV, PO Guimond, H. Pichler and P. Zoller et al Phys. Rev. Lett. 118, 133601 (2017).

[3] Berit Vogell, BV, T. Northup, B. Lanyon and C. Muschik arxiv:1704.06233.

[4] M. Dalmonte, BV and P. Zoller, in preparation.

[5] Bisognano and Wichmann, J. Math. Phys. 17, 303 (1976).

Nonlinear I-V Curve at a Quantum Critical Point and Quantum Noise
le vendredi 9 juin 2017 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

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

The utility of band theory in strongly correlated electron systems
le vendredi 2 juin 2017 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

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

Résumé : Weak perturbations can drive an interacting many-particle system far from its initial equilibrium state if one is able to pump into degrees of freedom approximately protected by conservation laws. This concept has for example been used to realize Bose-Einstein condensates of photons, magnons, and excitons. Integrable quantum system like the one-dimensional Heisenberg model are characterized by an infinite set of conservation laws. Here we develop a theory of weakly driven integrable systems and show that pumping can induce huge spin or heat currents even in the presence of integrability breaking perturbations, since it activates local and quasi-local approximate conserved quantities. We suggest to realize novel heat or spin pumps using spin-chain materials driven by THz radiation.

Christophe Mora (Laboratoire Pierre Aigrain, Ecole Normale Supérieure)

Photons and electrons in quantum circuits
le vendredi 12 mai 2017 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

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

Quantum catastrophes
le vendredi 5 mai 2017 à 11h00

Séminaire théorie

Personne à contacter : Robert Whitney ()

Lieu : Amphithéâtre, maison des Magistères

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

Strongly-correlated electrons driven out of equilibrium by a voltage bias: resistive switchings
le vendredi 14 avril 2017 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : A variety of correlated oxides experience a sudden change of
resistivity by several orders of magnitude when subject to a strong
voltage bias. This nonequilibrium phase transition, referred as
resistive switching (RS), shows hysteretic I-V characteristics
essential for new electronic memory/switching devices.
Before addressing this poorly understood complex phenomenon, I will
start with the dissipative dynamics of a simple Hubbard model driven
by a constant electric field. In this context, I will introduce new
theoretical tools needed address non-equilibrium steady states of
strongly-interacting systems, bypassing the transient dynamics. I will
detail the fate of Mott physics in the non-linear regime: dimensional
crossover and dielectric breakdown. Afterwards, I will propose a
minimal microscopic model to describe and reproduce most of the RS
phenomenology in ordered correlated insulators.

Detecting a many-body mobility edge with quantum quenches
le jeudi 13 avril 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : The many-body localization (MBL) transition is a quantum phase transition involving highly excited eigenstates of a disordered quantum many-body Hamiltonian, which evolve from "ergodic" to "localized". The MBL transition can be driven by the strength of disorder in a given spectral range, or by the energy density at fixed disorder when the system possesses a many-body mobility edge. A possible method to study the latter mechanism is via quantum quenches of variable width which prepare the state of the system in a superposition of eigenstates of the Hamiltonian within a controllable spectral region. Studying numerically a chain of interacting spinless fermions in a quasi-periodic potential, we argue that this system has a many-body mobility edge; and we show that its existence translates into a clear dynamical transition in the time evolution immediately following a quench in the strength of the quasi-periodic potential, as well as a transition in the scaling properties of the quasi-stationary state at long times.

Stabilizing incompressible quantum fluids in photonic devices via a non-Markovian reservoir
le lundi 10 avril 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Over the last decade, a growing community has started to investigate the possibility of stabilizing strongly correlated
photon fluids in various platforms such as cavity QED and superconducting quantum circuits. A particular emphasis
has been placed on the stabilization of incompressible quantum phases such as the celebrated Mott Insulator state.
This phase has been predicted and observed in isolated systems and appears at integer densities and low temperatures,
but still represents conceptually and experimentally an important challenge in optical devices, where the particle
number is not conserved and heating effects cannot be neglected.
In order to tackle the intrinsic non-equilibrium nature of photonic systems, we investigate the effect of a frequency-
dependent, ie., non-Markovian incoherent pump in order to compensate particle losses and refill selectively the photonic
many-body states, and propose to implement this scheme via a reservoir of population-inverted two-level emitters
with a broad distribution of transition frequencies [1, 2]. In the simplest case of a Lorentzian emission spectrum [1],
this pump allows for the selective generation of photonic Fock states with a well-defined particle number. For the
novel case of a square-shape spectrum [2], this scheme is predicted to stabilize a non-equilibrium steady state sharing
important features with a zero-temperature equilibrium state with a tunable chemical potential. We demonstrate
numerically for finite sytem sizes the existence of an incompressible Mott-Insulator state of arbitrary integer density,
which is robust against tunneling and losses, and exhibits a crossover towards a coherent state reminescent of the
superfluid phase.

[1] J. Lebreuilly, I. Carusotto, and M. Wouters, C. R. Phys. 17, 836 (2016), (preprint on arxiv arXiv:1502.04016 ).
[2] J. Lebreuilly, A. Biella, F. Storme, D. Rossini, R. Fazio, C. Ciuti, I. Carusotto, ’Stabilizing strongly correlated photon fluids
with a non-Markovian reservoir’: soon on arxiv.

Designed Quantum Criticality
le vendredi 7 avril 2017 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

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

R.M. Potok, I.G. Rau, H. Shtrikman, Y. Oreg, David Goldhaber-Gordon, Nature 446, 167-171 (2007)

G. Zaránd, C.H. Chung, P. Simon, M. Volta, Physical Review Letters 97, 166802 (2006)

H.T. Mebrahtu, I.V. Borzenets, H. Zheng, Y.V. Bomze, A.I. Smirnov, S. Florens, H.U. Baranger, and G. Finkelstein, Nature Physics 9, 732 (2013)

J. Simon, W.S. Bakr, R. Ma, M.E. Tai, P.M. Preiss, and M. Greiner, Nature 472, 307–312 (2011)

B. Dóra, F. Pollmann, J. Fortágh, G. Zaránd, Physical Review Letters 111, 046402 (2013)

Energy transport and topological aspects of collective plasmons in chains of metallic nanoparticles
le jeudi 6 avril 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

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

[1] A. Brandstetter-Kunc, G. Weick, C. A. Downing, D. Weinmann, R. A. Jalabert, Nonradiative limitations to plasmon propagation in chains of metallic nanoparticles, Phys. Rev. B94, 205432 (2016)

[2] C. A. Downing and G. Weick, Topological collective plasmons in bipartite chains of metallic nanoparticles, Phys. Rev. B95, 125426 (2017)

Solving fermionic many-body problems by summing Feynman diagrams
le vendredi 31 mars 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : It is commonly believed that in quantum Monte Carlo (QMC) approaches to fermionic many-body problems, the infamous sign problem generically implies prohibitively large computational times in the thermodynamic limit. I will point out that for convergent (or subject to resummation)
Feynman diagrammatic series evaluated with the Monte Carlo algorithm of [Rossi, arXiv:1612.05184], the computational time increases only polynomially with the inverse error on thermodynamic-limit quantities. I will discuss the computational complexity problem for different QMC approaches: conventional techniques (auxiliary-field, path-integral and diffusion QMC) and diagrammatic Monte Carlo approaches. I will also report on recent progress of Diagrammatic Monte Carlo simulation of the resonant Fermi gas and the homogeneous electron gas.

Simple Floquet-Wannier-Stark-Andreev viewpoint for multiterminal Josephson junctions
le vendredi 24 mars 2017 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

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

Résumé : Unconventional superconducting phases incorporate most intriguing features through the symmetry and topological properties of their order parameters, as already several decades ago has been found in the superfluid He-3. Among the known unconventional superconductors only few are considered as good candidates to realize topological phases. The most prominent cases are the so-called chiral superconductors, such as Sr2RuO4 most likely with chiral p-wave and SrPtAs possibly with chiral d-wave pairing. Cooper pairs form here with finite angular momentum. We will discuss the basic phenomenology of the two systems and give an overview of the status of experiments attempting to probe their topological properties. Finally other cases of topological superconductivity will be briefly discussed.
ATTENTION : LIEU INHABITUEL

Bernard Bernu (LPTMC, UPMC, Jussieu, Paris)
Annulé

Slow quantum oscillations without fine-grained Fermi surface reconstruction in cuprate superconductors
le vendredi 17 février 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The Fourier transform of the observed magnetic quantum oscillations (MQO) in YBaCuO high-temperature superconductors has a prominent low-frequency peak with two smaller neighbouring peaks. The separation and even the position of these three peaks is almost independent of doping. This pattern has been explained previously by rather special, exquisitely detailed, Fermi-surface reconstruction. We propose that these MQO have a different origin, and their frequencies are related to the bilayer and inter-bilayer electron hopping rather than directly to the areas of tiny Fermi-surface pockets. Such so-called "slow oscillations" explain more naturally many features of the observed oscillations and allow us to estimate the inter-layer transfer integrals and in-plane Fermi momentum.

Driven Markovian quantum criticality
le jeudi 16 février 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : I will discuss the realisation of a driven-dissipative analogue of quantum criticality, arising from the onset of a diffusion Markovian noise in a one-dimensional driven open Bose gas.
Salient features of the novel fixed point are the persistence of both non-equilibrium conditions as well as quantum coherence close to criticality. This provides a sharply distinct situation from more generic driven systems where both effective thermalisation as well as asymptotic decoherence ensue, paralleling classical dynamical criticality. Time permitting, I will also outline a diagrammatic comparison between the characteristic instances of classical and quantum dynamical field theories, employed to study critical phenomena out of equilibrium.

Non-ergodicity in many body systems: consequences for the Josephson junction chain
le vendredi 10 février 2017 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I argue that the chaotic behavior does not always imply ergodicity at realistic time scales for many classical and quantum systems. In particular, at very high disorder a generic closed quantum systems becomes completely localized that is highly non-ergodic. I argue that this (many-body) localization is preempted by a wide regime of non-ergodic behavior that displays a number of unusual properties.
A good system to study these effects is one-dimensional Josephson junction array in a somewhat unusual regime. I review the physics of these arrays and give the arguments for the existence of the novel phase appearing at relatively high temperatures. I will argue that these phases are robust with respect to the presence of the ubiquitous random charges and thus allow experimental observation.
I will sketch the analytical theory of the non-ergodic phase using Random Graph models.

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

Tunneling dynamics of ultracold atoms
le jeudi 2 février 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : In this talk I will present some of the projects in which I have been involved during my PhD at the Autonomous University of Barcelona. In particular, we studied tunneling-related phenomena in ultracold atom systems by means of analytical approaches, numerical simulations and semi-analytical models. The aim of these works has been to contribute to fields such as Atomtronics and Quantum Technologies with applications including, for instance, a proposal to build a soliton-based matter-wave interferometer or protocols to load and transport ultracold atoms with high efficiency and robustness in concentric ring potentials via spatial adiabatic passage processes. In addition, we also explore more fundamental issues like the determination of the boundaries in two component Bose-Einstein condensates, the generation of complex tunnelings for ultracold atoms carrying orbital angular momentum trapped in sided-coupled cylindrically symmetric potentials and the creation of single atom edge-like states in ribbons.

Electronic excitations in molecules with many-body perturbation theory
le vendredi 27 janvier 2017 à 11h00

Séminaire théorie

Personne à contacter : Valerio Olevano ()

Lieu : Attention : lieu inhabituel : CNRS, Institut Néel, salle Remy Lemaire K223

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

Anderson localization of cold atoms in optical disordered potentials
le jeudi 26 janvier 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

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

References:
M. P., Z. Zhao, D. Delande, and G. Orso, Phys. Rev. A 92, 053618 (2015)
M. P., G. Orso, and D. Delande, arXiv:1609.01065 (2016)

High-Pressure Phase Diagram of Solid Molecular Hydrogen
le vendredi 13 janvier 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Establishing the phase diagram of hydrogen is a major challenge for theoretical and experimental physics. We have used the highly accurate diffusion quantum Monte Carlo method to calculate static-lattice energies for solid hydrogen at pressures up to 400 GPa, to which we have added anharmonic vibrational energies calculated within density functional theory (DFT). We have focused on the observed high-pressure phases II, III and IV, which we have modelled using structures found in DFT searches. We find good agreement with experiment for the stabilisation of phase IV. The calculated pressure for the transition between phases II and III is larger than found in experiment, and we suggest possible reasons for this. The isotope dependence of the II-III transition is well-reproduced. Our calculations show that the metallic structure that is strongly favoured in DFT at high pressures is not energetically competitive, resolving an outstanding disagreement between theory and experiment.

Hridis Pal (Laboratoire de Physique des Solide, Orsay)

Do quantum oscillations always arise from the Fermi surface?
le vendredi 6 janvier 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

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