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.