Collective oscillations in disordered neural networks

We investigate the onset of collective oscillations in a network of pulse-coupled leaky-integrate-and-fire neurons in the presence of quenched and annealed disorder. We find that the disorder induces a weak form of chaos that is analogous to that arising in the Kuramoto model for a finite number N of oscillators [O.V. Popovych at al., Phys. Rev. E 71} 065201(R) (2005)]. In fact, the maximum Lyapunov exponent turns out to scale to zero for N going to infinite, with an exponent that is different for the two types of disorder. In the thermodynamic limit, the random-network dynamics reduces to that of a fully homogenous system with a suitably scaled coupling strength. Moreover, we show that the Lyapunov spectrum of the periodically collective state scales to zero as 1/N^2, analogously to the scaling found for the `splay state'.Comment: 8.5 Pages, 12 figures, submitted to Physical Review

Epidemic Threshold in Continuous-Time Evolving Networks

Current understanding of the critical outbreak condition on temporal networks relies on approximations (time scale separation, discretization) that may bias the results. We propose a theoretical framework to compute the epidemic threshold in continuous time through the infection propagator approach. We introduce the {\em weak commutation} condition allowing the interpretation of annealed networks, activity-driven networks, and time scale separation into one formalism. Our work provides a coherent connection between discrete and continuous time representations applicable to realistic scenarios.Comment: 13 pages, 2 figure

Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice

We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe$_2$. The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents.Comment: 5 pages, 4 figure

Superconductivity and magnetism in RbxFe2-ySe2: Impact of thermal treatment on mesoscopic phase separation

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed RbxFe2-ySe2 single crystals is presented. Magnetization experiments evidence that annealing of RbxFe2-ySe2 at 413 K, well below the onset of phase separation Tp=489 K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above Tp, suppresses the superconducting transition temperature Tc and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at T=488K=Tp increases Tc up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below Tp: a magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens just at Tp does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at Tp, leading to an improvement of the superconducting properties of RbxFe2-ySe2 and an enhancement of Tc.Comment: 13 pages, 12 figure

Transitions/relaxations in polyester adhesive/PET system

The correlations between the transitions and the dielectric relaxation processes of the oriented poly(ethylene terephthalate) (PET) pre-impregnated of the polyester thermoplastic adhesive have been investigated by differential scanning calorimetry (DSC) and dynamic dielectric spectroscopy (DDS). The thermoplastic polyester adhesive and the oriented PET films have been studied as reference samples. This study evidences that the adhesive chain segments is responsible for the physical structure evolution in the PET-oriented film. The transitions and dielectric relaxation modes’ evolutions in the glass transition region appear characteristic of the interphase between adhesive and PET film, which is discussed in terms of molecular mobility. The storage at room temperature of the adhesive tape involves the heterogeneity of the physical structure, characterized by glass transition dissociation. Thus, the correlation between the transitions and the dielectric relaxation processes evidences a segregation of the amorphous phases. Therefore, the physical structure and the properties of the material have been linked to the chemical characteristics

Influence of secondary phases during annealing on re-crystallization of CuInSe2 electrodeposited films

Electrodeposited CuInSe2 thin films are of potential importance, as light absorber material, in the next generation of photovoltaic cells as long as we can optimize their annealing process to obtain dense and highly crystalline films. The intent of this study was to gain a basic understanding of the key experimental parameters governing the structural–textural-composition evolution of thin films as function of the annealing temperature via X-ray diffraction, scanning/transmission electron microscopy and thermal analysis measurements. The crystallization of the electrodeposited CuInSe2 films, with the presence of Se and orthorhombic Cu2−xSe (o-Cu2−xSe) phases, occurs over two distinct temperature ranges, between 220 °C and 250 °C and beyond 520 °C. Such domains of temperature are consistent with the melting of elemental Se and the binary CuSe phase, respectively. The CuSe phase forming during annealing results from the reaction between the two secondary species o-Cu2−xSe and Se (o-Cu2−xSe+Se→2 CuSe) but can be decomposed into the cubic β-Cu2−xSe phase by slowing down the heating rate. Formation of liquid CuSe beyond 520°C seems to govern both the grain size of the films and the porosity of the substrate–CuInSe2 film interface. A simple model explaining the competitive interplay between the film crystallinity and the interface porosity is proposed, aiming at an improved protocol based on temperature range, which will enable to enhance the film crystalline nature while limiting the interface porosity

Random Hamiltonian in thermal equilibrium

A framework for the investigation of disordered quantum systems in thermal equilibrium is proposed. The approach is based on a dynamical model--which consists of a combination of a double-bracket gradient flow and a uniform Brownian fluctuation--that `equilibrates' the Hamiltonian into a canonical distribution. The resulting equilibrium state is used to calculate quenched and annealed averages of quantum observables.Comment: 8 pages, 4 figures. To appear in DICE 2008 conference proceeding

Study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) is a semi--crystalline polymer that can be crystallized to different degrees heating from the amorphous state. Even when primary crystallization has been completed, secondary crystallization can take place with further annealing and modify the characteristics of the amorphous interlamellar phase. In this work we study the glass transition of highly crystallized PET and in which way it is modified by secondary crystallization. Amorphous PET samples were annealed for 4 hours at temperatures between 140C and 180C. The secondary crystallization process was monitored by differential scanning calorimetry and the glass transition of the remaining interllamelar amorphous phase was studied by Thermally Stimulated Depolarization Currents measurements. Non--isothermal window polarization is employed to resolve the relaxation in modes with a well--defined relaxation time that are subsequently adjusted to several standard models. Analysis of experimental results, show that cooperativity is reduced to a great extend in the interlamellar amorphous regions. The evolution of the modes on crystallization temperature reveals that large scale movements are progressively replaced by more localized ones, with higher frequency, as crystallization takes place at higher temperatures. As a consequence, the glass transition temperature of the amorphous interlamellar phase tends to lower values for higher annealing temperatures. Evolution of calorimetric scans of the glass transition are simulated from the obtained results and show the same behaviour. The interpretation of these results in terms of current views about secondary crystallization is discussed.Comment: 30 pages, 5 tables, 12 figures; figure 5 modifie