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

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. Source

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

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. Source

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

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. Source

Herbert C.,French Atomic Energy Commission | Herbert C.,Institute Pierre Simon Laplace | Dubrulle B.,French Atomic Energy Commission | Chavanis P.H.,CNRS Physics of Complex Systems | Paillard D.,Institute Pierre Simon Laplace
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

The large-scale circulation of planetary atmospheres such as that of the Earth is traditionally thought of in a dynamical framework. Here we apply the statistical mechanics theory of turbulent flows to a simplified model of the global atmosphere, the quasigeostrophic model, leading to nontrivial equilibria. Depending on a few global parameters, the structure of the flow may be either a solid-body rotation (zonal flow) or a dipole. A second-order phase transition occurs between these two phases, with associated spontaneous symmetry breaking in the dipole phase. This model allows us to go beyond the general theory of marginal ensemble equivalence through the notion of Goldstone modes. © 2012 American Physical Society. Source

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

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. Source

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