Electrolyte based on fluorinated cyclic quaternary ammonium ionic liquids

Ionic liquids, ILs, based on fluorinated pyrrolidinium and piperidinium ammonium cations and imide anion were prepared and characterized. The physicochemical and electrochemical properties of these ILs including melting point, glass transition and degradation temperatures, viscosity, ionic conductivity, and electrochemical stability were determined and compared to alkyl pyrrolidinium and piperidinium ILs. The incorporation of a CF₃ group instead of a CH₃ induces an increase of the IL viscosity, thus a conductivity decrease. However, good ionic conductivity is obtained with fluorinated pyrrolidinium IL. Cyclic amine ILs with propyl alkyl chain or fluorinated ammonium exhibit very high electrochemical stability toward oxidation. The effect of the addition of LiTFSI on the IL properties was studied with the same methodology.     

Retracting fronts induce spatiotemporal intermittency

The intermittent route to spatiotemporal complexity is analyzed in simple models which display a subcritical bifurcation without hysteresis. A new type of spatiotemporal complex behavior is found, induced by fronts which "clean" the perturbations around an unstable state. The mechanism which generates these "retracting fronts" through nonlinear dispersion is analyzed in the frame of the complex Ginzburg-Landau equation. For sufficiently strong nonlinear dispersion the effects also occur for a supercritical bifurcation.     

Onset of itinerant ferromagnetism associated with semiconductor-metal transition in Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Nb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>CoSn half Heusler solid solution compounds

In this paper, the magnetic and transport properties of the Ti _x Nb_{1 − x} CoSn solid solution compounds with half Heusler cubic MgAgAs-type structure have been studied. This work shows the onset of ferromagnetism associated with a semiconductor to metal transition. The transition occurs directly from ferromagnetic metal to semiconducting state as it is the case in the TiCo _x Ni_{1 − x} Sn series studied previously. A weak quantity of Ti in NbCoSn is suffcient to allow the appearance of ferromagnetic order and metallic state. The variations of the Curie temperature as a function of saturation and effective paramagnetic moments are related to the itinerant ferromagnetism model. A comparison is made with the TiCoSn _x Sb_{1 − x} series (also studied previously), where the transition from TiCoSn ferromagnetic metal to non-magnetic semiconductor TiCoSb occurs through an intermediate metallic Pauli-like state.      

On the existence of thermodynamics for the random energy model

Derrida's random energy model is considered. Almost sure andL ^P convergence of the free energy at any inverse temperature are proven. Rigorous upper and lower bounds to the finite size corrections to the free energy are given.     

Etude d'impulsions lumineuses breves par interferometrie a deux ondes

We have studied the evolution of the light transmitted through matter in which an optical breakdown is taking place. With a two-waves interferometer made a birefringent plate between polarizers we get informations about the time correlation function of the fields. With an interferometer made of two identical harmonic generating plates separated by a dispersion medium, we learn about the correlation function of the square of the fields. Both systems act as spectral filters with a notch cosine-shaped centered at the incident frequency. We have also observed a transient angular spreading of the beam not previously emphasized which brings a further complication in the picture.     

Kondo lattice and heavy fermions in Heusler phases: CeInAg2−xCux

Magnetic properties, electrical resistivity, specific heat and magnetic excitations have been investigated in Heusler phases CeInAg_2–xCu_x. The hybridization continuously increases from CeInAg₂ (antiferromagnetic Kondo lattice) to CeInCu₂ (heavy fermion compound). The specific heat coefficient for this last compound is found to reach 1.2 J/mole. K² at 1.4 K, the Kondo temperature is 6 K and the Wilson ratio is close to 2.     

Interface motion in a two-dimensional Ising model with a field

We determine by Monte Carlo simulations the width of an interface between the stable phase and the metastable phase in a two-dimensional Ising model with a magnetic field, in the case of nonconversed order parameter (Glauber dynamics). At zero temperature, the width increases ast with–1/3, as predicted by earlier theories. As temperature increases, the value of the effective exponent that we measure decreases toward the value 1/4, which is the value in the absence of magnetic field.     

Statistics of transfer matrices for disordered quantum thin metallic slabs

In the quantum transport problem of a tight-binding Anderson model, the statistics of eigenvalues for the transfer matrices of thin disordered slabs is studied. Numerical simulations indicate that the probability distribution of nearest neighbor eigenvalue spacing and the ₃ statistics have already become close to that of the Gaussian orthogonal ensemble for sample lengths of the order of the mean free path, provided that transverse localization effects are not important. An intuitive argument is given why this should occur independently of the size of the matrix. Therefore, good mixing of the channels is not essential for obtaining Gaussian orthogonal ensemble type statistics and universal conductance fluctuations.     

Convergence rates for steady-state derivative estimators

We describe various derivative estimators for the case of steady-state performance measures and obtain the order of their convergence rates. These estimatorsdo not use explicitly the regenerative structure of the system. Estimators based on infinitesimal perturbation analysis, likelihood ratios, and different kinds of finite-differences are examined. The theoretical results are illustrated via numerical examples.     

On the spectral function of the Poisson–Voronoi cells

Denote by (t)=∑_n1e^−λ_nt, t>0, the spectral function related to the Dirichlet Laplacian for the typical cell of a standard Poisson–Voronoi tessellation in . We show that the expectation E(t), t>0, is a functional of the convex hull of a standard d-dimensional Brownian bridge. This enables us to study the asymptotic behaviour of E(t), when t→0⁺,+∞. In particular, we prove that the law of the first eigenvalue λ₁ of satisfies the asymptotic relation lnP{λ₁t}−2^dω_dj_(d−2)/2^d·t^−d/2 when t→0⁺, where ω_d and j_(d−2)/2 are respectively the Lebesgue measure of the unit ball in and the first zero of the Bessel function J_(d−2)/2.