Tapping spin glasses and ferromagnets on random graphs.

We consider a tapping dynamics, analogous to that in experiments on granular media, on spin glasses and ferromagnets on random thin graphs. Between taps, zero temperature single spin flip dynamics takes the system to a metastable state. Tapping corresponds to flipping simultaneously any spin with probability p. This dynamics leads to a stationary regime with a steady state energy E(p). We analytically solve this dynamics for the one-dimensional ferromagnet and +/-J spin glass. Numerical simulations for spin glasses and ferromagnets of higher connectivity are carried out; in particular, we find a novel first order transition for the ferromagnetic systems.     

Complex dynamics in the Oregonator model with linear delayed feedback

The Belousov-Zhabotinsky (BZ) reaction can display a rich dynamics when a delayed feedback is applied. We used the Oregonator model of the oscillating BZ reaction to explore the dynamics brought about by a linear delayed feedback. The time-delayed feedback can generate a succession of complex dynamics: period-doubling bifurcation route to chaos; amplitude death; fat, wrinkled, fractal, and broken tori; and mixed-mode oscillations. We observed that this dynamics arises due to a delay-driven transition, or toggling of the system between large and small amplitude oscillations, through a canard bifurcation. We used a combination of numerical bifurcation continuation techniques and other numerical methods to explore the dynamics in the strength of feedback-delay space. We observed that the period-doubling and quasiperiodic route to chaos span a low-dimensional subspace, perhaps due to the trapping of the trajectories in the small amplitude regime near the canard; and the trapped chaotic trajectories get ejected from the small amplitude regime due to a crowding effect to generate chaotic-excitable spikes. We also qualitatively explained the observed dynamics by projecting a three-dimensional phase portrait of the delayed dynamics on the two-dimensional nullclines. This is the first instance in which it is shown that the interaction of delay and canard can bring about complex dynamics.     

A simple electromagnetic method of cyclic loss measurement for superconducting wires in a combined alternating transverse magnetic field and transport current

We measured cyclic losses in a superconducting wire, carrying alternating transport current, simultaneously exposed to an alternating transverse magnetic field. Samples of Bi-2223 Ag-sheathed tapes have configuration of a double-layer non-inductive coil, which itself is a pickup coil to measure the AC losses. Potential taps were attached to both terminals of the sample coil. The external field was applied along the axis of the sample coil. In this procedure, we can estimate an averaged Poynting's vector on a cylindrical surface between the two layers by means of signals from a pair of the potential taps and from pickup coils for the external magnetic field and the transport current. We can also measure a magnetization and an extended transport-current components of AC losses in addition to a total cyclic loss for a combined alternating external field and transport current. Obtained results are compared with numerical predictions of the critical state model taking into account the magnetic field dependence of critical current density.     

Complex dynamics of droplet traffic in a bifurcating microfluidic channel: periodicity, multistability, and selection rules

The binary path selection of droplets reaching a T junction is regulated by time-delayed feedback and nonlinear couplings. Such mechanisms result in complex dynamics of droplet partitioning: numerous discrete bifurcations between periodic regimes are observed. We introduce a model based on an approximation that makes this problem tractable. This allows us to derive analytical formulae that predict the occurrence of the bifurcations between consecutive regimes, establish selection rules for the period of a regime, and describe the evolutions of the period and complexity of droplet pattern in a cycle with the key parameters of the system. We discuss the validity and limitations of our model which describes semiquantitatively both numerical simulations and microfluidic experiments.     

Droplet actuation induced by coalescence: Experimental evidences and phenomenological modeling

This paper considers the interaction between two droplets placed on a substrate in immediate vicinity. We show here that when the two droplets are of different fluids and especially when one of the droplet is highly volatile, a wealth of fascinating phenomena can be observed. In particular, the interaction may result in the actuation of the droplet system, i.e. its displacement over a finite length. In order to control this displacement, we consider droplets confined on a hydrophilic stripe created by plasma-treating a PDMS substrate. This controlled actuation opens up unexplored opportunities in the field of microfluidics. In order to explain the observed actuation phenomenon, we propose a simple phenomenological model based on Newton’s second law and a simple balance between the driving force arising from surface energy gradients and the viscous resistive force. This simple model is able to reproduce qualitatively and quantitatively the observed droplet dynamics.     

Chaotic capture of vortices by a moving body. II. Bound pair model

Previously, we have presented a simple model for the interaction of a fluid vortex structure with a moving bluff body, and demonstrated the existence of a trapping mechanism related to chaotic scattering. This single point vortex model required explicit perturbation to generate chaos and the subsequent complex dynamics. Here, we present a model which attempts to introduce internal degrees-of-freedom in the vortex structure in the simplest manner, by replacing the single vortex with a like-signed pair. We show that this model exhibits chaotic trapping without the need of explicit perturbation, however, the region of parameter space for which trapping occurs is exceedingly small due to the spatially dependent form of the perturbation. We claim that this result explains some the behavior observed in Navier-Stokes simulations of the same vortex-body system, where we find close correspondence between the dynamics of an extended vorticity distribution and the single vortex model. Finally, we generalize the model to unequal strength vortex pairs, and find more complex behavior which includes "partial" capture of the weaker vortex by the body. (c) 1994 American Institute of Physics.     

Giant electrical fluctuations in metallic disordered packings

We have experimentally studied granular arches through electrical measurements. The packing is composed of 2d metallic pentagons and is submitted to small taps. Large electrical fluctuations are observed and they are distributed along power laws. This indicates the presence of long-time memory effects even the packing density remains constant around a value ρ = 0.72±0.02. Large electrical fluctuations should be associated with the breaking/creation of granular arches. Received 3 October 2000     

Opinion dynamics as a movement in a bistable potential

In this paper, we investigate the Sznajd model of opinion dynamics with anticonformity on a complete graph. We show that below some threshold value of anticonformal behavior spontaneous reorientations occur between two stable states. Dealing with a complete graph allows us also for an analytical treatment. We provide analytical calculations both for the infinite and finite systems. We show that opinion dynamics can be understood as a movement of a public opinion in a symmetric bistable effective potential. We focus also on the spontaneous transitions between stable states in the case of the finite system and show that a typical waiting time can be observed.     

The role of time scale separation in a nonequilibrium roughening transition

In this work we analyze the role of time scale separation between the external driving and the avalanche relaxation dynamics in a one-dimensional model of propagation of innovations among economic agents. When the time scales are separated the model presents a nonequilibrium roughening transition. We show that when avalanche overlapping is permitted, only a rough phase is observed.     

Risk bubbles and market instability

We discuss a simple model of correlated assets capturing the feedback effects induced by portfolio investment in the covariance dynamics. This model predicts an instability when the volume of investment exceeds a critical value. Close to the critical point the model exhibits dynamical correlations very similar to those observed in real markets. Maximum likelihood estimates of the model's parameter for empirical data indeed confirms this conclusion. We show that this picture is confirmed by the empirical analysis for different choices of the time horizon.     

Dynamic interacting bubble simulation (DIBS): an agent-based bubble model for reacting fluidized beds

In this paper we explore the global dynamics of an agent-type model for bubbles in gas-fluidized beds and demonstrate that these features are consistent with experimentally observed behavior. The model accounts for the simultaneous interactions of thousands of individual bubbles and includes mass-transfer and first-order reactions between the gas and solids so that the impact of the dynamics is reflected in reactant conversion. We start with model parameters that have been demonstrated to produce time average behavior consistent with experimental reactor measurements. By observing the temporal variations of spatially averaged bubble properties, we are able to clearly distinguish the onset of global low-dimensional features that appear to be consistent with previous observations. The most prominent of these features is a large-scale oscillation that exhibits intermittency with power-law scaling in the vicinity of a critical gas flow. We show that the oscillation occurs as the result of a globally synchronized horizontal movement of the bubbles toward the center of the reactor. The oscillation appears to be consistent with the occurrence of the so-called "slugging" phenomenon, which is known to have large effects on fluidized bed reactor performance. Although this model can clearly be further improved, its success in replicating several of the key features of slugging indicates that this approach can serve as a useful tool for understanding and possibly controlling fluidized bed dynamics. We also conjecture that this model may be useful for more generally understanding the occurrence of global features in high-dimensional, multi-agent systems.     

Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.     

A study of contact interface and wear diagnosis for hand taps using ultrasonic method

This work was to study the contact interface between a set of used hand taps and another new one based on the regional scanning of ultrasound. The contact image was a novel disclosure for hand taps contact. The objective of this work was to provide a wear diagnosis by making a comparison of contact area between the used and the new hand taps. The 2D maps showed an apparent change not only in area sizes but also in contact shapes between the used and the new hand taps. The 3D contact images also provided useful information to show the degree of contact.The contact area between the tap and the workpiece was calculated using an image analysis software package. The range of contact areas varied from 2.49 mm² to 35.31 mm² for the used hand taps and from 1.19 mm² to 28.55 mm² for the new taps, depending on the definition of the contrast ratio. The result provided another scientific data for users to decide a correct timing for the tool replacement. In addition, maps of reflection coefficient and pressure contour distribution were presented. The range of contact pressure varied from 2.5 Mpa to 4.2 Mpa.     

一个人类行为动力学模型及其精确解

对于Baraba′si提出的基于人类行为的排队模型,比较了Baraba′si的解与Va′zquez的解,指出这两个解存在差异.提出一个服务台具有服务时间的人类行为动力学模型.通过严格的解析,计算出事件的等待时间分布,得到事件在系统中的逗留时间分布(这个指标在Baraba′si模型中没有被获得).结果表明,按照优先权选择协议,这个模型的等待时间分布是指数γ=2的幂律分布,这与钱学森通信回复时间分布的指数2.1非常接近.     

Mechanical properties of thin confined polymer films close to the glass transition in the linear regime of deformation: Theory and simulations

Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature T(g) can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk T(g) and T(g) + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to T(g) + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.     

Number theoretic example of scale-free topology inducing self-organized criticality

In this Letter we present a general mechanism by which simple dynamics running on networks become self-organized critical for scale-free topologies. We illustrate this mechanism with a simple arithmetic model of division between integers, the division model. This is the simplest self-organized critical model advanced so far, and in this sense it may help to elucidate the mechanism of self-organization to criticality. Its simplicity allows analytical tractability, characterizing several scaling relations. Furthermore, its mathematical nature brings about interesting connections between statistical physics and number theoretical concepts. We show how this model can be understood as a self-organized stochastic process embedded on a network, where the onset of criticality is induced by the topology.     

Bubble dynamics relaxation in aqueous foam probed by multispeckle diffusing-wave spectroscopy

We study the bubble rearrangement dynamics in aqueous foam during the passage from liquidlike to solidlike behavior which follows a transient shear deformation that perturbs the bubble packing. The local dynamics is probed using multispeckle diffusing-wave spectroscopy. We show that following the perturbation the average time between rearrangements relaxes exponentially, with time elapsed since the end of the perturbation. The observed scaling of the characteristic relaxation time with perturbation amplitude and foam age is explained by a schematic coarse-grained model based on the scaling state hypothesis.     

Asymptotic chaos

We provide in table I a list of normal forms of ordinary differential equations describing the dynamics of physical systems in conditions near to the simultaneous onset of up three instabilities. The first (quadratic) terms in the Taylor series for the nonlinear terms in these amplitude equations (as they are called in fluid dynamics) are given in each case. We focus on a particular example involving three instabilities and derive an asymptotic version of the corresponding normal form as a limit of small dissipation is approached. The numerical investigation of this asymptotic normal form strongly suggests that chaotic behavior occurs as close as one wants to the onset of the triple instability. This chaotic behavior is also exhibited by a return map constructed by direct numerical experiments on the amplitude equation. We also derive by analytic methods a model return map that qualitatively reproduces much of the dynamics observed numerically in the solutions of the asymptotic normal form in nearly homoclinic conditions. In the limit of strong contraction, this model map of the plane reduces to a unidimensional map that is valuable in understanding the dynamics of the original system.     

Entanglement routers using macroscopic singlets

We propose a mechanism where high entanglement between very distant boundary spins is generated by suddenly connecting two long Kondo spin chains. We show that this procedure provides an efficient way to route entanglement between multiple distant sites. We observe that the key features of the entanglement dynamics of the composite spin chain are well described by a simple model of two singlets, each formed by two spins. The proposed routing mechanism is a footprint of the emergence of a Kondo cloud in a Kondo system and can be engineered and observed in varied physical settings.