Critical properties of a growing interface described by the Kardar-Parisi-Zhang equation
le vendredi 14 décembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The Kardar-Parisi-Zhang equation is a stochastic non-equilibrium and non-linear equation initially derived to describe the kinetic roughening of a growing interface. As it has turned out to map to many other important problems, such as directed polymers in random media or Burgers turbulence, it has emerged as a fundamental model to investigate non-equilibrium scaling phenomena and phase transitions.
The theoretical understanding of the interface properties in the rough phase, corresponding to a strong-coupling regime, requires non-perturbative methods.
I will show, from a detailed analysis of the KPZ symmetries, how to construct an effective action to study the KPZ growth, and derive its Non-Perturbative Renormalization Group flow. I will then present results for critical properties in the stationary regime: critical exponents and correlation and response functions. I will show that these functions exhibit generic scaling, determine the associated universal scaling functions and universal amplitude ratios.

Interlayer magnetoresistance in strongly anisotropic quasi-2D compounds
le vendredi 7 décembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The angular and field dependence of magnetoresistance (MR) is a very powerful tool to determine the details of electronic spectrum and of Fermi-surface geometry of various compounds, which is crucial for understanding their electronic properties. Recently, the magnetic quantum oscillations (MQO) and angular magnetoresistance oscillations were applied to extract the Fermi-surface geometry and the electron dispersion in cuprate and pnictide high-temperature superconductors. Both these effects are traditionally used to study the electron dispersion in layered organic metals and most other metallic compounds. As the anisotropy of the compounds increases, the standard 3D theory of magnetoresistance becomes inapplicable, and a new theoretical description is required.

The theory of magnetoresistance is developed in the "weakly incoherent" limit, when the electron interlayer tunneling conserves the in-plane momentum, but the time of this tunneling is much longer than mean scattering time by impurities and/or cyclotron period [1-3]. This theory predicts several new qualitative features. First, the nonoscillating part of interlayer magnetoresistance increases as a square root of interlayer component of magnetic field. This contradicts the standard theory of magnetoresistance, which states that only a perpendicular-to-current component of magnetic field affects this current. The magnetic quantum oscillations are stronger damped in this limit and the angular dependence of magnetoresistance changes compared to the predictions of the standard theory. These new features in magnetoresistance appear because the effect of impurity scattering is enhanced by low-dimensionality, naturally appearing in layered conductors. The experiments on magnetoresistance in layered organic metal have been performed to check the predictions of the new theory. The agreement turned out to be very nice, especially for clean samples.[4] In strongly anisotropic compounds with low electron concentration a magnetic field may lead to even steeper field-dependence of interlayer magnetoresistance because of the Coulomb anomaly [5,6].

Non-equilibrium stationary states in the driven Hubbard model
le lundi 3 décembre 2012 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Rémy Lemaire (salle K223), bât. K 1er étage - Institut Neel

Résumé : I shall present a recent work concerning the non-equilibrium dynamics of a strongly correlated electrons systems in a static electric field, with the aim of identifying the conditions to reach a non-equilibrium stationary state (NSS).
I show that, for a generic electric field, the convergence to a stationary state requires the coupling to a thermostating bath, absorbing the work done by the external force. By following the real-time dynamics of the system, I also show that coupling to bath provides an essentially sufficient condition, i.e. NSS can be reached for almost any value of dissipation. I characterize the properties of the NSS in terms of some physical observables, pointing out the existence of an analogue of the Pomeranchuk effect. Finally, I map out a phase diagram of the system and I identify a dissipation regime for which steady current is largest for a given field.

Shape resonance in electron- atom and electron-molecule molecule collision: Complex absorbing potential based coupled-cluster theory
le mercredi 28 novembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Weil E424 3etage - Institut Neel

Résumé : Electron-atom and electron-molecule shape resonance leads to metastable
resonant states, which become unnormalizable. Thus, bound state quantum
chemistry techniques can not be applied. However, using complex
absorbing potential and complex scaling, resonant states can be
normalized. Using bound state coupled-cluster methods to the complex
Hamiltonian, resonance energies can be obtained as real part of complex
electron affinity and resonance widths can be obtained as imaginary
part. Test results to prototype systems are presented.

Resistive switching in oxides
le vendredi 23 novembre 2012 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Resistive random access memory (RRAM) composed of a transtition metal oxide dielectric in a capacitor- like structure is a candidate technology for next generation non-volatile memory devices [1]. We introduce a model that accounts for the bipolar resistive switching phenomenom observed in many perovskite transition metal oxides [2]. The numerical study of the model predicts that strong electric fields develop in the highly resistive dielectric-electrode interfaces, leading to a spatially inhomogeneous distribution of oxygen vacancies and a concomitant nonvolatile resistance memory effect. The theoretical results of the model are validated by successful comparison with non-trivial resistance hysteresis loops measured in cuprate YBCO and manganite PCLMO samples. Insights from the model simulations are used to propose a novel multi-level and non-volatile memory cell. We shall present results for an implementation of a 6-bit multi-leve memory cell device [3].<br/><br/>
[1] M. Rozenberg, <a href="http://dx.doi.org/10.4249/scholarpedia.11414">Scholarpedia 6(4), 11414 (2011)</a><br/>
[2] M. Rozenberg, M. J. Sanchez, R. Weht, C. Acha, F. G. Marlasca, and P. Levy, <a href="http://dx.doi.org/10.1103/PhysRevB.81.115101">Phys. Rev. B 81, 115101 (2010)</a>.<br/>
[3] P. Stoliar et al. (submitted).

Going beyond perturbation theory in reaction-diffusion problems
le vendredi 16 novembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The simplest reaction-diffusion problems are considered. They consist on particles of a single species A that can diffuse with diffusion constant D and that can perform annihilation (2A->nothing) or branching (A->2A) or, alternatively, A->3A. It is shown that mean field methods and perturbative expansions around it completely miss some simple properties as the structure of the phase diagram as found by Monte Carlo simulations. It is shown that methods that go beyond perturbation theory allows to quantitatively reproduce Monte Carlo results in a very simple way.

How much energy does it cost to make a hole in an ideal Fermi gas?
le vendredi 9 novembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The change in energy of an ideal Fermi gas when a local one-body potential is inserted into the system, or when the density is changed locally, are important quantities. In this talk I will explain that the well-known semiclassical approximation provides a lower bound to the correct quantum mechanical energy of the perturbed Fermi sea, up to a universal constant. This work generalizes a famous estimate of Lieb and Thirring, in the vacuum.
It is a collaboration with R. Frank, E.H. Lieb (Princeton, USA) and R. Seiringer (McGill, Montréal, Canada).

References:
[1] Energy Cost to Make a Hole in the Fermi Sea, Phys. Rev. Lett. 106 (2011), 150402
[2] A positive density analogue of the Lieb-Thirring inequality, Duke Math. Journal, (2012), in press

Mechanics and instabilities of healthy and cancerous tissues
le vendredi 26 octobre 2012 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : In this talk, we present some theoretical and experimental results on the growth and mechanical properties of healthy and cancerous tissues.
We first show that because of the coupling between cell division and the local stress, a tissue can be considered as a visco-elastic liquid with a relaxation time smaller than the cell division time.
We then show recent experiments on spheroid cell aggregates on the effect of mechanical stress on tissue growth.
Finally, we discuss situations where a tissue in unstable.
For a thick epithelial tissue, we propose a hydrodynamic-like instability of the basement membrane of the tissue driven by cell division.
We also discuss the steady state structure of villis which are the protrusions of the surface of the intestine or the colon. We describe the formation of villis as a buckling instability of a polar cell monolayer. The polarity of the layer does not seem to play a role in the intestine where the villis are arranged in a square array but it is important in the colon where they are organized in a hexagonal array. Similar instabilities occur as well for tube-like cellular structures.

Transport in 1D revisited: A simple, exact solution for phase disorder
le vendredi 19 octobre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : I will revisit the baby problem of single-channel scattering in a one-dimensional chain of scatterers, with uniform phase disorder. I will present a simple, exact analytical solution that applies even in the strong scattering limit. This permits the easy computation of all moments of the conductance and resistance, and I will show how one obtains answers well-known from weak-scattering limit from our exact solution. I will also discuss features in the probability distribution for the conductance missing from the usual weak scattering treatment. This is work done together with Berge Englert from the Centre for Quantum Technologies, Singapore.

Tunable spin and charge Seebeck effects in magnetic molecular junctions
le vendredi 12 octobre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The increasing interest in the thermoelectric properties of materials and the quest for high Seebeck coefficients is motivated by the promise of more efficient solid state refrigerators and the conversion of waste heat into electricity. The Seebeck effect refers to the generation of a charge current (or a voltage drop) by a temperature gradient applied across a metal, and the spin-Seebeck effect, concerns the thermal generation of pure spin currents. The recent experimental observation of the Seebeck effect in different nano-structures, in particular in molecular junctions and quantum dots, opened new routes to study these phenomena. On the one hand, the high sensitivity of these systems to external fields, their scalability and tunability make them potential candidates for a variety of technological applications. On the other hand, thermoelectric and thermomagnetic effects provide a unique probe of electron correlation effects and are a useful tool to gain further insight on fundamental problems like the Kondo regime where the energy transfer is dominated by spin fluctuations.
I'll present results for the charge and spin Seebeck effects of a spin-1 molecular junction as a function of temperature (T), applied magnetic field (H), and molecular magnetic anisotropy (D) obtained using Wilson's numerical renormalization group [1]. A hard-axis magnetic anisotropy produces a large enhancement of the charge Seebeck coefficient Sc (~ k_{B}/|e|) whose value only depends on the residual interaction between quasiparticles in the low temperature Fermi-liquid regime. In the underscreened spin-1 Kondo regime, the high sensitivity of the system to magnetic fields makes it possible to obtain a sizable value for the spin Seebeck coefficient even for magnetic fields much smaller than the Kondo temperature. Similar effects can be obtained in C60 junctions where the control parameter is the gap between a singlet and a triplet molecular state. I'll also discuss briefly the thermoelectric properties of an SU(4) Kondo resonance, that describes the low temperature transport through clean C nanotubes [2].

Density and Spin Response of a Fermi Gas in the Attractive and Quasi-Repulsive Regime
le vendredi 5 octobre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Recent experimental advances in ultracold Fermi gases allow for exploring response functions under different dynamical conditions. In particular, the issue of obtaining a quasirepulsive regime starting from a Fermi gas with an attractive interparticle interaction while avoiding the formation of the two-body bound state is currently debated. Here, we provide a calculation of the density and spin response for a wide range of temperature and coupling both in the attractive and quasirepulsive regime, whereby the system is assumed to evolve nonadiabatically toward the upper branch of the Fermi gas. A comparison is made with the available experimental data for these two quantities.

Chaos and transport in disordered classical nonlinear chains
le vendredi 28 septembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Chaotic behavior is the reason for destruction of Anderson localization by a weak nonlinearity in a classical system. A disordered one-dimensional chain of coupled weakly anharmonic classical oscillators is one of the simplest models where this effect can be studied. I will analyze the probability for appearance of chaos in such chains, and then I will discuss the consequences of chaos for transport of conserved quantities along the chain.

Relaxation processes in Coulomb glasses
le vendredi 21 septembre 2012 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Coulomb glasses are materials with electron states localized by the disorder under conditions of long-range interactions between their particles. One realization of a Coulomb glass is a doped semiconductor at low temperatures. Another example is granular metals. Coulomb glasses show complex dynamics typical for other complex systems: sluggish, non-exponential, relaxation of the conductance as well as aging and memory effects similar to those observed in structural glasses. We report dynamical Monte Carlo simulations of relaxation processes in a Coulomb glass. Both the relaxation to equilibrium following an initial temperature quench and during and after a driving by a strong current is studied. We see that out of equilibrium there is an effective electron temperature established on a short timescale, and this relaxes slowly to the bath temperature. We also study the response of the system to an external perturbation and observe how it relaxes after such a perturbation. Both from a random state and after a perturbation from equilibrium we find that the effective temperature relaxes logarithmically.

Conductance in systems of Tomonaga-Luttinger Liquid systems with resistances
le jeudi 20 septembre 2012 à 14h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères, salle MAG-1

Résumé : Conductance of quantum wires connected to Fermi liquid leads has been extensively studied in literature. In such 1-dimensional systems, effects of interactions are taken into account by the technique of Bosonization. We have developed a phenomenological formalism that allows us to study the effect of resistances in quantum wires[1]. Rayleigh dissipation function is combined with the technique of Bosonization to model the power dissipation. We use this formalism to study the conductance and the power dissipated in systems of a quantum wire and junction of three quantum wires connected to Fermi liquid leads. Further, we combine two Y-junctions to study the conductance of a parallel combination of resistances[2]. We find that the effective resistance of a parallel combination is quite different from that of its classical counterpart.

The superfluid-insulator phase diagram of 1-D weakly interacting bosons in a disorder potential
le vendredi 14 septembre 2012 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Weil (Institut Néel) bâtiment E, 3e étage

Résumé : A one-dimensional system of non-interacting particles in presence of disorder is always in an insulating state at zero temperature. Interactions can induce a quantum phase transition to a superfluid state, characterized by quasi long-range order. This Bose-glass to superfluid transition has been the subject of intense theoretical studies and of several recent experiments carried out on ultracold atomic clouds.
I will present a theoretical study of the weakly interacting Bose gas at zero temperature in a disorder potential with finite spatial correlation length. To leading order in the interaction strength, the boundary between the superfluid and insulating phases can be determined from a symmetry-breaking approach: the extended Bogolyubov model. This approach accounts correctly for diverging low energy phase fluctuations - that occur in a low-dimensional system and are ultimately responsible for the suppression of superfluidity. In this context, the phase diagram on the interaction-disorder plane (U,D) can be characterized by inspecting the long-range behaviour of the one-body density matrix as well as the drop in superfluid fraction. It turns out in particular that the phase boundary between the two phases follows two different power laws on the phase diagram in the white-noise and Thomas-Fermi limits respectively. This feature is peculiar of a spatially correlated disorder and is expected to govern the behaviour of disordered ultracold atomic clouds in current experiments.
The in-situ density profile is perhaps the feature of an ultracold atomic cloud that can be most easily measured in an experiment. I will show that a direct link exists between the fragmentation of the density profile and the occurrence of the phase transition. This link is given by the probability distribution of the gas density. In particular, the appearance of a superfluid fraction coincides with a vanishing probability distribution in the limit of zero density, namely with the disappearance of fragmentation. This analysis sets the intuitive relation between fragmentation and insulating behaviour into a rigorous framework, and opens the way to the experimental detection of the phase transition.

Quantum flutter of supersonic particles in one-dimensional quantum liquids
le jeudi 31 mai 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères

Résumé : I will present exact results on the dynamics of an impurity injected at a supersonic velocity into a one-dimensional quantum liquid [1]. Two main surprising results are revealed in the analysis. Firstly, the momentum of the impurity does not decay to zero, instead it forms a strongly correlated state that propagates at a reduced velocity. Secondly, the system undergoes coherent oscillations in which the impurity vibrates with respect to its correlation hole. These oscillations, which we call quantum flutter, have their origin in a long-lived coherent quantum superposition of two families of quasi-equilibrium states in the system. These results provide the first example of new physics due to supersonic propagation in a strongly coupled non-relativistic many-body system. Furthermore, the main features are robust to changes in parameters of the system, suggesting that they may be a generic feature of one-dimensional quantum systems.

[1] C.J.M. Mathy, M.B. Zvonarev and E. Demler. Quantum flutter of supersonic particles in one-dimensional quantum liquids. arXiv:1203.4819.

Itinerant magnetism in 2D ultracold Fermi atoms with Spin-Orbit coupling: variational approach
le jeudi 10 mai 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères

Résumé : The exploration of Rashba spin-orbit (SO) interaction is stimulated by its increasing interest for spintronic applications. The first experimental evidences of SO interactions have been found in semiconductor devices. We investigate the behaviour of a 2D ultracold Fermi gas in presence of a Rashba-like spin-orbit coupling. At ultralow temperatures the fermions interact via contact (repulsive) interactions. We determine the ground state of the system perturbatively up to second order terms in the interaction strength. First we employ the functional integral method that enables us to highlight clearly the similarities with the well known (3D) Stoner model. Then, to overcome some difficulties related to the dimensionality, we analyse the behaviour of the 2D system with a variational approach, both without and with the SO coupling. We focus in particular on the aspects related to the magnetization, and as main result we show how the presence of the Rashba coupling frustrates the emergence of a magnetized phase.

Charles Poli (Instituto de Física, Universidad Autónoma de Puebla)

Chaos Ondulatoire en milieux ouverts : une généralisation de la distribution de Wigner
le mardi 24 avril 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères

Résumé : Initialement introduite en 1951 par E. Wigner pour décrire le spectre d’énergie de noyaux lourds, la distribution des écarts entre plus proches niveaux a par la suite été intensivement utilisée pour décrire les propriétés statistiques de systèmes ondulatoires chaotiques. Cependant, la distribution de Wigner, basée sur l’hypothèse forte d’un système fermé, ne s’applique plus pour les systèmes ouverts, c’est-à-dire des systèmes pour lesquels le couplage avec l’extérieur ne peut pas être négligé.
En reprenant l’approximation du modèle à 2 niveaux utilisé par Wigner et en prenant explicitement en compte le couplage avec l’extérieur, je présenterais une expression analytique exacte de la distribution des écarts dans le cas particulier d’un canal de perte. Puis, je montrerais qu’en considérant la force de couplage comme un paramètre libre cette distribution est aussi une excellente approximation dans le cas d’un nombre quelconque de canaux de pertes. Ainsi, l’ensemble de ces résultats, appuyé par des simulations numériques et des données expérimentales, peut être vu comme une généralisation de la distribution de Wigner aux systèmes ouverts.

Transport and Classical Percolation in the High Temperature Regime of the Quantum Hall Effect
le jeudi 5 avril 2012 à 11h00

Séminaire interne LPMMC

Personne à contacter :

Lieu : Maison des Magistères, salle de lecture 2

Résumé : The quantized Hall resistance plateaus observed at high magnetic fields in two-dimensional electron gases are known to vanish rapidly with increasing temperature. We wish to show that dissipative transport remains however non-trivial at high temperatures, as it is dominated by classical percolation physics. For this purpose, we use a diagrammatic formalism based on a local conductivity approach to investigate the interplay of inelastic phonon scattering with the random drifting of electronic trajectories [1,2]. This provides a microscopic derivation of the universal transport critical exponent in the high-temperature regime of quantum Hall transitions, that was up to now only conjectured from qualitative geometrical arguments.
We also derive microscopic expressions for the dependences in temperature and magnetic field of the longitudinal conductance. This allows us to test our predictions with recent experiments and to extract the transport critical exponent from experimental data [3].

Universality in chaotic quantum transport: the concordance between random matrix and semiclassical theories
le mardi 6 mars 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères, salle commune LPMMC

Résumé : Electronic transport through chaotic quantum dots exhibits
universal, system independent properties which are consistent with
random matrix theory. The observable quantities can be expressed, via
the semiclassical approximation, as sums over the classical scattering
trajectories. Correlations between such trajectories are organized
diagrammatically and have been shown to yield universal answers for
some observables.
We develop a general combinatorial treatment of the semiclassical
diagrams by relating them to factorizations of permutations. Taking
previously calculated answers (Heusler et al, 2006) for the
contribution of a given diagram, we prove agreement to all orders
between the semiclassical and random matrix approaches for all moments
(linear or nonlinear) of the transmission amplitudes for systems with
and without time reversal symmetry. This explains the mathematics
behind the applicability of random matrix theory to chaotic quantum
transport. The streamlined calculation could also pave the way for
inclusion of non-universal effects.

Interacting electrons in one dimension beyond the Luttinger liquid paradigm: relaxation rates and transport
le mardi 24 janvier 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères

Résumé : In contrast to higher dimensional systems where pair collisions provide finite relaxation rate and lifetime, the situation in one dimension is peculiar. A one-dimensional electron gas requires three-particle collisions for finite relaxation due to constraints imposed by the conservation laws. At zero temperature the fastest relaxation is provided by the interbranch processes which enable energy exchange between counterpropagating particles. At sufficiently high temperatures the leading mechanism is due to the intrabranch scattering of comoving electrons. We derive the corresponding relaxation rates that strongly depend whether one considers screened or unscreneed Coulomb interaction. The abovementioned relaxation processes are responsible for interaction-induced modifications of electrical and thermal conductance in quantum wires. Our approach is based on the Boltzmann equation that is beyond the Luttinger-liquid theory

Spin-selective tunneling in quantum dots confining holes
le mardi 17 janvier 2012 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Maison des Magistères

Résumé : Holes confined in SiGe nanocrystals have recently been studied in
dc transport experiments [1]. Motivated by the experimental work,
I will consider the tunneling of a charge carrier from a metallic
lead into the valence band of the semiconductor. In a magnetic
field, the spin-orbital splitting of the valence band gives rise
to spin-selective tunneling. I will give a working explanation of
the phenomenon based on an expansion of the Luttinger Hamiltonian
around the 2D limit. I will formulate a simple model for the tunnel
contact (the Z-model), in which the effect of the spin selectivity
is maximally strong. Our results demonstrate that spin-selective
tunneling in semiconductor nanostructures can be achieved without
the use of ferromagnetic contacts. This effect can be used to
induce spin currents and measure Rabi oscillations in single-dot
devices with strong spin-orbit interaction.

[1] G. Katsaros et al., Nature Nanotech. 5, 458 (2010).