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Comment développer l’intelligence collective grâce à l’information sociale

Une équipe interdisciplinaire de chercheurs impliquant un physicien et un doctorant du LPT vient de publier dans les Proceedings of the National Academy of Sciences USA les résultats d’expériences et de modélisation quantifiant comment les gens prennent en compte l’opinion des autres pour corriger leur réponse initiale dans des tâches d’estimation. Communiqué de presse nationale du CNRS

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Topological chiral spin liquid groundstate of a frustrated antiferromagnetic Heisenberg model

A simple spin-12 frustrated antiferromagnetic Heisenberg model (AFHM) on the square lattice—including chiral plaquette cyclic terms—was argued to host a bosonic Kalmeyer-Laughlin (KL) fractional quantum Hall ground state. Here, we construct generic families of chiral projected entangled pair states (chiral PEPS) with low bond dimension which, upon optimization, provide better variational energies than the KL Ansatz. The optimal PEPS exhibits chiral edge modes described by the (...)

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How disorder can turn into long-range order at high magnetic field ?

Building on NMR experiments at high magnetic field, physicists have demonstrated that impurity-induced disorder in a Mott insulator can lead to the revival of long-range order.

Phys. Rev. Lett. 118, 067203 (2017)

Phys. Rev. Lett. 118, 067204 (2017)

See also CNRS news INP website

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Local injection of pure spin current generates electric current vortices

In a paper, published by Applied Physics Letters, Yaroslaw Bazaliy of the University of South Carolina and Revaz Ramazashvili of the LPT have studied local injection of pure spin current into an electrically disconnected ferromagnetic-normal sandwich. Curiously enough, it turns out that local spin injection inevitably induces electric current vortices, that are centered at the normal-ferromagnetic interface. This finding invites us to revisit various aspects of spin injection in spintronic devices.

Appl. Phys. Lett. 110, 092405 (2017)

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Temperature dependence of the NMR relaxation rate 1/T1 for quantum spin chains

Maxime Dupont, PhD student in LPT, together with Sylvain Capponi and Nicolas Laflorencie, studied dynamical correlations at finite temperature in quantum spin chains using matrix product state techniques, thus allowing to extract the NMR relaxation rate. This work addresses the delicate question of the low-temperature dimensional crossover in realistic materials, such as coupled S=1 chains material DTN.

Phys. Rev. B 94, 144409 (2016) (Editors’ Suggestion)

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Dynamics of a Mobile Impurity in a One-Dimensional Bose Liquid

We developed a microscopic theory of a quantum impurity propagating in a one-dimensional Bose liquid. As a result of scattering off thermally excited quasiparticles, the impurity experiences the friction. We found that, at low temperatures, the resulting force scales either as the fourth or the eighth power of temperature, depending on the system parameters. For temperatures higher than the chemical potential of the Bose liquid, the friction force is a linear function of temperature.

Aleksandra Petković and Zoran Ristivojevic Phys. Rev. Lett. 117, 105301 (2016)

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Physique théorique : particules sur la grille

En physique des solides, le comportement des systèmes dits fortement corrélés est un sujet d’intérêt à la fois d’un point de vue théorique et expérimental. Ces matériaux ont, en effet, des propriétés électroniques particulières : les électrons situés sur la couche périphérique sont si fortement liés à leur atome que, pour les déplacer, il faut une énergie non négligeable alors que, dans les matériaux courants, ils peuvent se déplacer presque librement. A basse température, ces (...)

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Dependence of DNA Persistence Length on Ionic Strength of Solutions with Monovalent and Divalent Salts

A Joint Theory-Experiment Study : collaboration between the LPT and the IPBS and the LMGM. Using high-throughput tethered particle motion (TPM) single-molecule experiments, the double-stranded DNA persistence length, Lp, is measured in solutions with Na+ and Mg2+ ions of various ionic strengths. The data are compared to available theoretical models. No decisive theory is found which fits all the Lp values for the two ion valencies. Using high-throughput tethered particle motion (...)

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Développements non perturbatifs à bases de graphes

Les techniques de perturbations basées sur un développement en graphes sont très communes en physique. Elles se heurtent à un problème générique du fait des symétries réduites des graphes : par exemple, l’absence de symétrie par translation, alors qu’elle est présente pour un système infini homogène. Une collaboration entre Sylvain Capponi et le groupe de Kai Schmidt à Dortmund a permis de dépasser cette difficulté.

Reference : K. Coester et al. 2015 EPL 110 20006 (sélectionné comme Editors’ Choice)

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Many-body localization edge in the random-field Heisenberg chain

Using a massively parallel exact diagonalization technique, David Luitz, Nicolas Laflorencie and Fabien Alet have shown the existence of an extensive many-body localization edge for the random field Heisenberg chain, separating a thermal (ergodic) phase from a many-body localized (non-ergodic) regime.

Reference : Phys. Rev. B 91, 081103(R) (2015) (Editors’ Suggestion)

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Real space imaging of spin polarons

Thanks to state-of-the-art NMR experiments performed in Japan on Zn doped frustrated spin compound, SrCu2(BO3)2, theorists from the LPT in collaboration with Pr. Mila in Lausanne have been able to interpret the real space profile as a magnetic polaron.

Reference : Phys. Rev. Lett. 114, 056402 (2015)

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Interactions entre cultures et les personnalités les plus influentes sur Wikipédia en 24 langues - Interactions of cultures and top people of Wikipedia in 24 language editions

Deux physiciens du LPT et leurs collaborateurs ont identifié les 100 personnalités les plus influentes dans les versions en 24 langues de Wikipédia, en utilisant des outils avancés d’analyse des réseaux complexes, apportant ainsi un éclairage nouveau sur l’interaction entre cultures (English version).

Référence : PLoS ONE 10(3) : e0114825 (2015) ; Communiqué de presse CNRS

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Multifractality of quantum wave functions facing perturbation

26 juin 2014

Physicists from LPT together with colleagues in Orsay, Belgium and Argentina have shown that the destruction of quantum multifractality by a perturbation can follow two different scenarios.

Reference : Phys. Rev. Lett. 112, 234101 (2014) ; Communiqué CNRS

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Étude des phases prédites par un modèle de bancs de poissons déduit d’expériences - Swarming, schooling, milling : phase diagram of a data-driven fish school model

Un modèle de dynamique de bancs de poissons déduit d’expériences réalisées par le CRCA Toulouse reproduit les différentes phases usuellement observées (comme les « moulins » de la photo), et une phase rare extrêmement allongée (English version).

Référence : New J. Phys. 16, 015026 (2014) (incluant un résumé vidéo)

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Spin-Wave Localization for 2D Bosons in a Random Potential

Left : Diagram Disorder - Energy of the Spin-Wave spectrum of 2D hard-core bosons in a random potential. A mobility edge separates extended from localized modes.

In Phys. Rev. Lett. 111, 160403 (2013), J.P Alvarez Zuniga and N. Laflorencie from LPT, have studied the 2D Superfluid to Bose-Glass transition using a spin-wave approach. Upon increasing the disorder strength, an insulating but gapless state emerges : the Bose-glass phase (a state absent in mean-field). Furthermore, a mobility edge in the spin-wave excitation spectrum is found at a finite frequency Ωc, decreasing with disorder, and presumably vanishing in the Bose-glass.

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Gapless spin liquid in the frustrated square lattice Heisenberg quantum antiferromagnet

Left : Singlet bond weight distribution in the Liang-Doucot-Anderson picture with a linear color scale for a 8 X 8 torus.

Magnetic frustration in quantum magnets may be responsible for exotic spin liquid ground state. Using novel Tensor Network techniques we provide strong evidence that frustrating antiferromagnetic interaction stabilizes a gapless spin liquid in the Heisenberg quantum antiferromagnet on the square lattice. Such a state can be viewed as a Resonating Valence Bond (RVB) state with a distribution of singlet bonds at all distances. Our framework also enables to investigate the topological properties of this novel spin liquid.

PRL 111, 037202 (2013)

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Symmetry-protected topological phases of cold atoms in one dimension

Cartoon of two possible exotic phases for two-component alkaline-earth cold atoms with nuclear spin I=1/2 and SU(4) symmetry. These Haldane phases for nuclear spin and orbital degrees of freedom present a dilute hidder order that can be seen here as an alternation of sites with total (pseudo)spin component ±1, diluted with an arbitrary number of sites with (pseudo)spin component 0.

EPL 102, 37008 (2013),

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Fractionalization of Itinerant Anyons in One-Dimensional Chains

Left : Anyonic fusion trees for electrons, Ising σ (Majorana) anyons, and Fibonacci anyons.

The theoretical prediction of (non-abelian) Majorana particles in topological insulators and closely related systems by Kane and Mele has recently boosted the quest for the discovery of emergent non-abelian particles beyond the realm of the quantum Hall effect. Didier Poilblanc (LPT) and collaborators (Microsoft, ETH-Zurich) have constructed models of interacting itinerant non-Abelian anyons moving along one-dimensional chains.

PRB 87, 085106 (2013) PRL 108, 207201 (2012)

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Coulomb meets Néel

An example of a quantum wave-function, displaying at the same time antiferromagnetic and Coulomb-like dipolar behaviors, was recently evidenced.

Phys. Rev. Lett. 109, 147204 (2012),

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Quantum Data Fitting

Phys. Rev. Lett. 109, 050505 (2012) ; accompanying synopsis in "Physics" by the editors of PRL

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Ultimate quantum limits to sensitivity of nano-oscillator based "artificial noses"

In an article to appear in Europhysics Letters, a researcher from the LPT has derived the ultimate bounds of sensitivity of molecular mass measurements with nano-mechanical oscillators.

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Google matrix of the world trade network

L.Ermann and D.L.Shepelyansky

Recent results obtained at LPT for new way of ranking of international world trade commerce (see here) have been highlighted at launched by Antoine Sire (see here) .

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Heisenberg-limited sensitivity with decoherence-enhanced measurements

Quantum-enhanced measurements use quantum mechanical effects to enhance the sensitivity of measurements. However, the required quantum states are generally highly entangled, difficult to produce, and very prone to decoherence. In work that has now appeared in Nature Communications, Daniel Braun of the LPT and John Martin (University de Liege) show how Heisenberg-limited measurements can be achieved without the use of entangled states by coupling the quantum resources to a common (...)

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Quantum oscillations in antiferromagnetic conductors

Discovered in 1930, magnetic quantum oscillations were the second (after superconductivity) experimentally found macroscopic manifestation of the quantum nature of the electron : in a magnetic field, the electron energy levels in a metal become discrete, which leads to oscillations of various quantities with varying the magnetic field. For over fifty years now, magnetic quantum oscillations have been a direct and precise probe of the electron physics in metals. The scope of the (...)

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Drag forces in fluctuating fields

Inclusions, or defects, moving at constant velocity through free classical fields have been shown to be subject to a drag force which depends on the field dynamics and the coupling of the inclusion to the field. The results can be used to predict the drag exerted on inclusions, such as proteins, in lipid membranes due to their interaction with height and composition fluctuations. The force, measured in Monte Carlo simulations, on a point like magnetic field moving through an Ising (...)

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Ionic Capillary Evaporation in Weakly Charged Nanopores

Three members of the Statistical Physics group of the LPT have proposed in an article appearing in Physical Review Letters that a novel ionic “liquid-vapor” phase transition occurring for electrolytes in weakly charged nanopores could explain the time-dependent electrical conductivity fluctuations observed across certain biological and artificial ion channels.

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Un algorithme quantique pour l’échantillonnage Monte Carlo exact

Trois chercheurs du LPT de Toulouse et du LPTMS d’Orsay ont récemment publié dans Physical Review Letters (de juin 2010) un travail où ils proposent une version quantique de l’algorithme dit de « couplage depuis le passé », proposé il y a une quinzaine d’années par James Propp et David Wilson.

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Two-dimensional ranking of Wikipedia articles

A.O.Zhirov, O.V.Zhirov and D.L.Shepelyansky

Recent results obtained at LPT on two-dimensional ranking of Wikipedia articles are highlighted at Atelier.FR (see here), More information about results can be find at arxiv1006.4270 and webpage. The results of the research are submitted to Eur. Phys. J. B.

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Quantifying Quantumness and the Quest for Queens of Quantum

What are the "most quantum" states that one can possibly produce and what are their properties ? Experimental progress, which has led to the coherent manipulation of a small number of quantum systems, such as trapped ions, or superconducting qubits, has rendered these questions highly relevant not only for the foundations of physics, but also for practical implementation of quantum information processing. In an article that has now appeared in the New Journal of Physics, we make the (...)

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Computing fidelity at magnetic quantum phase transitions

Quantum phase transitions (QPT) play an important role in condensed matter systems. Traditional investigations of those QPT usually need the definition of an order parameter, which itself already requires a deeper understanding of the physical system. When a system undergoes a quantum phase transition, the ground-state wave-function shows a change of nature, which can be monitored using the fidelity concept.

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Why DNA chains are "kinked" when observed on surfaces

DNA elastic properties on surfaces differ from ones in solution.

Atomic Force Microscopy (AFM) is widely used to observe double-stranded DNA adsorbed on surfaces. In recent experiments by Wiggins et al., « anomalies » have been detected in the distribution of bending angles along DNA (which measures its flexibility) : an over-abundance of large angles were found which are not predicted by the traditional statistical model of DNA chains, the Worm-Like Chain model (see figure). N. Destainville, M. Manghi and J. Palmeri of the statistical group of the (...)

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Effective Theory of Magnetization Plateaux in the Shastry-Sutherland Lattice

The Shastry-Sutherland lattice and its realization in the material SrCu2(BO3)2 have been attracting a lot of attention due to its fascinating behavior in a magnetic field, namely that magnetization plateaux have experimentally been observed for values of 1/8, 1/4 and 1/3 of the saturation value. Because of the lack of powerful numerical techniques or unbiased analytical tools to tackle 2D frustrated systems, a promising approach consists in deriving an effective hardcore bosonic model. (...)

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A gauge theory picture of an exotic transition in a dimer model

We study a phase transition in a 3D lattice gauge theory, a coarse-grained version of a classical dimer model. The dimer model on a cubic lattice, first studied by F. Alet and collaborators, displays a continuous transition between an ordered columnar phase at low temperature and a disordered phase at high temperature where dimer-dimer correlations show an algebraic decay. This is rather unusual as the standard Ginzburg-Landau theory of phase transitions generally predicts an exponential (...)

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The Pairing Glue in High-Temperature Superconductors

Didier Poilblanc

The question of whether one should speak of a “pairing glue” in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an “instantaneous glue,” this (...)

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What is the true statistics of holons ?

Didier Poilblanc

The exact nature of holons, the charged spinless excitations of the (doped) celebrated RVB state, has been debated over the last two decades. In particular, their statistics (do they behave as fermions or bosons ?) was still controversial. A doped two-dimensional quantum dimer model describing a doped Mott insulator and retaining the original Fermi statistics of the electrons is defined and analyzed by numerical studies on small clusters. This model shows a rich phase diagram including a d-wave hole-pair unconventional superconductor at small enough doping and a bosonic superfluid at large doping. The hole kinetic energy is shown to favor binding of topological defects (similar to vortices or "visons") to the bare fermionic holons turning them into bosons, in agreement with arguments based on RVB wave-functions. Results may have interesting connections with the physics of cuprates superconductors

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10th Anniversary Highlight Article in New Journal of Physics

D. Braun

Decoherence is the main concern in experimental realizations of quantum computers. In order to control the quantum computer, a certain amount of interaction with its environment is needed that destroys in general the phase coherence of the computer very quickly. An article in NJP, which was now selected as one of the 10th Anniversary Highlights articles by the editors of the journal, explains how this dilemma can in principle be avoided.

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Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models

Arnaud Ralko, Didier Poilblanc and Roderich Moessner

Two members of the strongly correlated system group (A. Ralko and D. Poilblanc), in collaboration with R. Moessner from MPI-PKS (Dresden), revisit the phase diagram of Rokhsar-Kivelson models, which are used in fields such as superconductivity, frustrated magnetism, cold bosons, and the physics of Josephson junction arrays. From an extended height effective theory, we show that one of two simple generic phase diagrams contains a mixed phase that interpolates continuously between columnar and plaquette states.

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Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder

Low-dimensional quantum magnets are fascinating objects from both experimental and theoretical points of view. This work deals with the case of weakly ferromagnetically coupled inequivalent chains and reveals the emergence of a new energy scale that is interpreted in terms of a Suhl-Nakamura interaction.

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Valence Bond Entanglement Entropy

The notion of quantum entanglement in condensed matter systems provides a new route to study and understand the effect of strong correlations and the nature of exotic phases at low temperature. Following these lines, members of the lab have recently shown that the entanglement present in some quantum magnets can be studied in the framework of Resonating Valence Bond physics, known to play a crucial role for instance in the physics of spin liquids or high Tc superconductivity.

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Phase separation and flux quantization in the doped quantum dimer model on the square and triangular lattices

Arnaud Ralko, Frédéric Mila and Didier Poilblanc

Two members of the strongly correlated system group (A. Ralko and D. Poilblanc), in collaboration with F. Mila from EPF Lausanne, have studied the doped quantum dimer model, a simplified model that is expected to capture the essence of hole motion in an antiferromagnet. The physics of hole motion in an antiferromagnet is of broad interest and of very high relevance for illuminating high-Tc superconductivity and quantum spin liquids in general. In this work, superfluidity, phase separation and flux quantization are discussed, giving new insights on the physics of doped Mott insulators.

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DNA bubbles and bending : how conformational fluctuations modify its thermal denaturation

Three members of the statistical physics group have studied theoretically thermal DNA denaturation by illuminating the role of statistical non-uniform bending inside a DNA chain.

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Entanglement-screening by nonlinear resonances

A member of the Laboratory, in collaboration with researchers from Australia, United States and Germany, has recently shown that nonlinear resonances in a classically mixed phase space allow to define generic, strongly entangled multi-partite quantum states. The robustness of their multipartite entanglement was shown to increase with the particle number, i.e. in the semiclassical limit, for those classes of diffusive noise which assist the quantum-classical transition.

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Predicting the maximum or the minimum of a random signal

What is the probability that a stock prize does not fall below a certain level ? What is the probability that the total magnetization of a spin system does not change sign ? Predicting the maximum or the minimum of a random temporal signal is a challenging problem recently addressed by a member of the laboratory, which commonly appears in physics, biology, finance, and image or data processing.

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Exceeding the Pauli limit

Superconducting doped spin ladders under a magnetic field parallel to their plane reveal interesting novel phases and phenomena, as recently shown by members of the Laboratory, including irrational magnetization plateaus and exceeding of the Pauli limit.

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An unconventional phase transition

Phase transitions occupy a central role in physics, due both to their experimental ubiquity and their fundamental conceptual importance. The explanation of universality at phase transitions was the great success of the theory formulated by Ginzburg and Landau, and extended through the renormalization group by Wilson. However, recent theoretical suggestions have challenged this point of view.

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Measuring the size of Schrödinger’s cat

Imagine you are supposed to keep the memory in your computer with its millions of bytes in a quantum superposition of two very different contents, such as the memory full of zeros and the memory full of ones. For how long you can you do this in principle, if at all ?

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How to reproduce bacterial propulsion in a biomimetic way ?

By coupling the elastic properties of a rotative semi-flexible nanorod and hydrodynamic interactions, which are the source of propulsion in Stokes flows, a member of the Laboratory, in collaboration with German researchers, has shown theoretically that bacterial propulsion can be reproduced in a biomimetic way. Moreover, this work sheds light on the major role played by elasticity in flagellar motion of bacteria as E. coli.

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An argentinian-french collaboration : a novel mechanism for fractional magnetization plateaus !

A novel (quite general) mechanism for the appearance of magnetization plateaus in quasi-one-dimensional quantum spin systems is proposed where the coupling to the underlying lattice plays an essential role.

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