CNRS Physics of Complex Systems

Amiens, France

CNRS Physics of Complex Systems

Amiens, France
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Barbero G.,Polytechnic University of Turin | Barbero G.,CNRS Physics of Complex Systems | Lelidis I.,National and Kapodistrian University of Athens
Journal of Physical Chemistry B | Year: 2011

The effect of the generation - recombination phenomenon on the electrical impedance of an electrolytic cell is investigated. We show that this phenomenon could be responsible for the appearance of a plateau in the real part of the impedance of the cell. The possibility to observe the plateau, arising from the generation - recombination phenomenon, is discussed in relation to the values of the association - dissociation coefficients. The analysis is done by assuming that the generation - recombination phenomenon can be described as a chemical reaction of first order, that the sample is in the shape of a slab, and that the electrodes of the cell are perfectly blocking. To simplify the analysis, the case where only one type of ions can move is considered. The extension of the results to the more general case, in which both types of ions are mobile, is also discussed. © 2011 American Chemical Society.

Antonopoulos C.,CNRS Physics of Complex Systems | Basios V.,CNRS Physics of Complex Systems | Bountis T.,University of Patras
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

We present results demonstrating the occurrence of changes in the collective dynamics of a Hamiltonian system which describes a confined microplasma characterized by long-range Coulomb interactions. In its lower energy regime, we first detect macroscopically the transition from a "crystallinelike" to a "liquidlike" behavior, which we call the "melting transition." We then proceed to study this transition using a microscopic chaos indicator called the smaller alignment index (SALI), which utilizes two deviation vectors in the tangent dynamics of the flow and is nearly constant for ordered (quasiperiodic) orbits, while it decays exponentially to zero for chaotic orbits as exp [- (λ1 - λ2) t], where λ1 > λ2 >0 are the two largest Lyapunov exponents. During the melting phase, SALI exhibits a peculiar stairlike decay to zero, reminiscent of "sticky" orbits of Hamiltonian systems near the boundaries of resonance islands. This alerts us to the importance of the Δλ= λ1 - λ2 variations in that regime and helps us identify the energy range over which "melting" occurs as a multistage diffusion process through weakly chaotic layers in the phase space of the microplasma. Additional evidence supporting further the above findings is given by examining the GALIk indices, which generalize SALI (= GALI2) to the case of k>2 deviation vectors and depend on the complete spectrum of Lyapunov exponents of the tangent flow about the reference orbit. © 2010 The American Physical Society.

Toledano P.,CNRS Physics of Complex Systems | Guennou M.,Luxembourg Institute of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

At variance with structural ferroic phase transitions which give rise to macroscopic tensors coupled to macroscopic fields, criteria defining antiferroelectric (AFE) phase transitions are still under discussion due to the absence of specific symmetry properties characterizing their existence. They are recognized by the proximity of a ferroelectric (FE) phase induced under applied electric field, with a double hysteresis loop relating the induced polarization to the electric field and a typical anomaly of the dielectric permittivity. Here, we show that there exist indeed symmetry criteria defining AFE transitions. They relate the local symmetry of the polar crystallographic sites emerging at an AFE phase transition with the macroscopic symmetry of the AFE phase. The dielectric properties of AFE transitions are deduced from a Landau theoretical model in which ferroelectric and ferrielectric phases are shown to stabilize as the result of specific symmetry-allowed couplings of the AFE order parameter with the field-induced polarization. © 2016 American Physical Society.

Zappone B.,CNR Institute for Chemical and Physical Processes | Meyer C.,CNRS Physics of Complex Systems | Bruno L.,University of Calabria | Lacaze E.,University Pierre and Marie Curie
Soft Matter | Year: 2012

We have created periodic lattices of close-packed non-toroidal focal conic domains (FCD) in smectic A liquid crystal films deposited on solid substrates that induce unidirectional planar anchoring, at the interface with air which induces homeotropic anchoring. Each cell of the quasi-hexagonal lattice contained a FCD with an eccentric ellipse aligned with the planar anchoring direction and tilted from the solid substrate towards the free interface. In contrast to common toroidal FCDs, the ellipse was truncated by both the substrate and the free interface, and its longest portions were virtual. Such geometry results from a complex balance between the distortion of the smectic layers, anchoring and surface tension achieved through multiple frustrations of the smectic layered structure and anchoring at the interfaces, and creation of additional defects at the boundaries between neighbouring FCDs in the lattice. © 2012 The Royal Society of Chemistry.

Allahverdyan A.E.,CNRS Physics of Complex Systems | Balian R.,CEA Saclay Nuclear Research Center | Nieuwenhuizen T.M.,New York University
Physics Reports | Year: 2013

The quantum measurement problem, to wit, understanding why a unique outcome is obtained in each individual experiment, is currently tackled by solving models. After an introduction we review the many dynamical models proposed over the years for elucidating quantum measurements. The approaches range from standard quantum theory, relying for instance on quantum statistical mechanics or on decoherence, to quantum-classical methods, to consistent histories and to modifications of the theory. Next, a flexible and rather realistic quantum model is introduced, describing the measurement of the z-component of a spin through interaction with a magnetic memory simulated by a Curie-Weiss magnet, including N ≫ 1 spins weakly coupled to a phonon bath. Initially prepared in a metastable paramagnetic state, it may transit to its up or down ferromagnetic state, triggered by its coupling with the tested spin, so that its magnetization acts as a pointer. A detailed solution of the dynamical equations is worked out, exhibiting several time scales. Conditions on the parameters of the model are found, which ensure that the process satisfies all the features of ideal measurements. Various imperfections of the measurement are discussed, as well as attempts of incompatible measurements. The first steps consist in the solution of the Hamiltonian dynamics for the spin-apparatus density matrix over(D, ̂) (t). Its off-diagonal blocks in a basis selected by the spin-pointer coupling, rapidly decay owing to the many degrees of freedom of the pointer. Recurrences are ruled out either by some randomness of that coupling, or by the interaction with the bath. On a longer time scale, the trend towards equilibrium of the magnet produces a final state over(D, ̂) (tf) that involves correlations between the system and the indications of the pointer, thus ensuring registration. Although over(D, ̂) (tf) has the form expected for ideal measurements, it only describes a large set of runs. Individual runs are approached by analyzing the final states associated with all possible subensembles of runs, within a specified version of the statistical interpretation. There the difficulty lies in a quantum ambiguity: There exist many incompatible decompositions of the density matrix over(D, ̂) (tf) into a sum of sub-matrices, so that one cannot infer from its sole determination the states that would describe small subsets of runs. This difficulty is overcome by dynamics due to suitable interactions within the apparatus, which produce a special combination of relaxation and decoherence associated with the broken invariance of the pointer. Any subset of runs thus reaches over a brief delay a stable state which satisfies the same hierarchic property as in classical probability theory; the reduction of the state for each individual run follows. Standard quantum statistical mechanics alone appears sufficient to explain the occurrence of a unique answer in each run and the emergence of classicality in a measurement process. Finally, pedagogical exercises are proposed and lessons for future works on models are suggested, while the statistical interpretation is promoted for teaching. © 2012 Elsevier B.V. All rights reserved.

Meyer C.,CNRS Physics of Complex Systems | Luckhurst G.R.,University of Southampton | Dozov I.,CNRS Physics of Complex Systems
Physical Review Letters | Year: 2013

We extend the twist-bend nematic (NTB) model to describe the electro-optics of this novel phase. We predict an electroclinic effect (ECE) subject to a dc electric field E applied perpendicular to the helix axis or wave vector q, with rotation of the NTB optic axis around E. This linear effect, with its flexoelectric origin, is a close analog to the electro-optic effects observed for chiral liquid crystals. However, in nematics composed of achiral molecules having a bent shape, it is the electro-optic signature of the NTB phase. We test our model experimentally in the low-temperature nematic phase of the odd liquid crystal dimer, CB7CB, with its molecules having, on average, a bent shape. The ECE measurements confirm the previously proposed twist-bend nematic structure of this phase, with its broken chiral symmetry, extremely short (<10 nm) doubly degenerate pitch and ultrafast, submicrosecond response times. © 2013 American Physical Society.

Meyer C.,CNRS Physics of Complex Systems | Luckhurst G.R.,University of Southampton | Dozov I.,CNRS Physics of Complex Systems
Journal of Materials Chemistry C | Year: 2015

We report precise birefringence measurements in the nematic, N, and in the twist-bend nematic, NTB, phases of the odd liquid crystal dimer CB7CB. The birefringence results obtained in large monodomains of the NTB phase strongly support its heliconical structure with doubly degenerate handedness and provide the temperature dependence of the conical tilt angle, . The birefringence measured in planar wall defects separating the monodomains with opposite sign of the chirality suggests a splay-bend structure of the nematic in this region, enabling a smooth transition between the adjacent right- and left-handed heliconical domains. This journal is © The Royal Society of Chemistry.

Meyer C.,CNRS Physics of Complex Systems
Liquid Crystals | Year: 2016

The recently discovered twist-bend nematic phase, NTB, has short-pitched heliconical structure with doubly degenerate handedness. In contrast to the classic nematic, in the NTB phase the director is spontaneously distorted, resulting in unusual elastic properties. The response of the NTB phase to external stimuli, like chiral doping or applied fields might provide further information about its structure and can find utilisation in practical applications. Here, the NTB behaviour is theoretically investigated under chiral doping and strong electric fields. We show that the chiral doping removes the NTB degeneration and modifies the conical tilt angle, leaving the pitch unchanged. Thus, the NTB helical twisting power is very high and strongly non-linear. Under electric field, we consider separately the ferroelectric, flexoelectric and dielectric couplings. We show that the experiments reported so far disagree with the ferroelectric behaviour, indicating that the NTB phase is not spontaneously polarised. On the contrary, the observed polar effects fit well with the flexoelectric coupling, confirming the degenerated heliconical structure of the phase. Under very strong fields, we predict a second-order twist-bend nematic – nematic phase transition due to the dielectric torque on the director. © 2016 Informa UK Limited, trading as Taylor & Francis Group

Basios V.,CNRS Physics of Complex Systems
European Physical Journal: Special Topics | Year: 2016

A novel case of probabilistic coupling for hybrid stochastic systems with chaotic components via Markovian switching is presented. We study its stability in the norm, in the sense of Lyapunov and present a quantitative scheme for detection of stochastic stability in the mean. In particular we examine the stability of chaotic dynamical systems in which a representative parameter undergoes a Markovian switching between two values corresponding to two qualitatively different attractors. To this end we employ, as case studies, the behaviour of two representative chaotic systems (the classic Rössler and the Thomas-Rössler models) under the influence of a probabilistic switch which modifies stochastically their parameters. A quantitative measure, based on a Lyapunov function, is proposed which detects regular or irregular motion and regimes of stability. In connection to biologically inspired models (Thomas-Rössler models), where strong fluctuations represent qualitative structural changes, we observe the appearance of stochastic resonance-like phenomena i.e. transitions that lead to orderly behavior when the noise increases. These are attributed to the nonlinear response of the system. © 2016, EDP Sciences and Springer.

Allahverdyan A.E.,CNRS Physics of Complex Systems | Allahverdyan A.E.,Yerevan Physics Institute | Wang Q.A.,CNRS Physics of Complex Systems
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

We study the minimal thermodynamically consistent model for an adaptive machine that transfers particles from a higher chemical potential reservoir to a lower one. This model describes essentials of the inhomogeneous catalysis. It is supposed to function with the maximal current under uncertain chemical potentials: if they change, the machine tunes its own structure fitting it to the maximal current under new conditions. This adaptation is possible under two limitations: (i) The degree of freedom that controls the machine's structure has to have a stored energy (described via a negative temperature). The origin of this result is traced back to the Le Chatelier principle. (ii) The machine has to malfunction at a constant environment due to structural fluctuations, whose relative magnitude is controlled solely by the stored energy. We argue that several features of the adaptive machine are similar to those of living organisms (energy storage, aging). © 2013 American Physical Society.

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