Reversible transitions between synchronization states of the cardiorespiratory system

Phase synchronization between cardiac and respiratory oscillations is investigated during anesthesia in rats. Synchrograms and time evolution of synchronization indices are used to show that the system passes reversibly through a sequence of different phase-synchronized states as the anesthesia level changes, indicating that it can undergo phase transitionlike phenomena. It appears that the synchronization state may be used to characterize the depth of anesthesia.     

Noise-induced phase synchronization and synchronization transitions in chaotic oscillators

Whether common noise can induce complete synchronization in chaotic systems has been a topic of great relevance and long-standing controversy. We first clarify the mechanism of this phenomenon and show that the existence of a significant contraction region, where nearby trajectories converge, plays a decisive role. Second, we demonstrate that, more generally, common noise can induce phase synchronization in nonidentical chaotic systems. Such a noise-induced synchronization and synchronization transitions are of special significance for understanding neuron encoding in neurobiology.     

Experimental evidence of anomalous phase synchronization in two diffusively coupled Chua oscillators

We study the transition to phase synchronization in two diffusively coupled, nonidentical Chua oscillators. In the experiments, depending on the used parameterization, we observe several distinct routes to phase synchronization, including states of either in-phase, out-of-phase, or antiphase synchronization, which may be intersected by an intermediate desynchronization regime with large fluctuations of the frequency difference. Furthermore, we report the first experimental evidence of an anomalous transition to phase synchronization, which is characterized by an initial enlargement of the natural frequency difference with coupling strength. This results in a maximal frequency disorder at intermediate coupling levels, whereas usual phase synchronization via monotonic decrease in frequency difference sets in only for larger coupling values. All experimental results are supported by numerical simulations of two coupled Chua models.     

Experimental evidence for the complete saturation phase in the argon neutral system

An experimental study of the population densities of excited states confirms the existence of the complete excitation saturation phase in the argon neutral system. Collisional radiative coefficients r⁽¹⁾_m are independent of n_e and decrease with decreasing ionization energy. At higher n_e-values the levels are observed to come consecutively into Saha equilibrium (PLTE).     

Synchronization in the human cardiorespiratory system

We investigate synchronization between cardiovascular and respiratory systems in healthy humans under free-running conditions. For this aim we analyze nonstationary irregular bivariate data, namely, electrocardiograms and measurements of respiratory flow. We briefly discuss a statistical approach to synchronization in noisy and chaotic systems and illustrate it with numerical examples; effects of phase and frequency locking are considered. Next, we present and discuss methods suitable for the detection of hidden synchronous epochs from such data. The analysis of the experimental records reveals synchronous regimes of different orders n:m and transitions between them; the physiological significance of this finding is discussed.     

Three types of transitions to phase synchronization in coupled chaotic oscillators

We study the effect of noncoherence on the onset of phase synchronization of two coupled chaotic oscillators. Depending on the coherence properties of oscillations characterized by the phase diffusion, three types of transitions to phase synchronization are found. For phase-coherent attractors this transition occurs shortly after one of the zero Lyapunov exponents becomes negative. At rather strong phase diffusion, phase locking manifests a strong degree of generalized synchronization, and occurs only after one positive Lyapunov exponent becomes negative. For intermediate phase diffusion, phase synchronization sets in via an interior crises of the hyperchaotic set.     

Topology and phase transitions: paradigmatic evidence

We report upon the numerical computation of the Euler characteristic chi (a topologic invariant) of the equipotential hypersurfaces Sigma(v) of the configuration space of the two-dimensional lattice varphi(4) model. The pattern chi(Sigma(v)) versus v (potential energy) reveals that a major topology change in the family Sigma(v)(vinR) is at the origin of the phase transition in the model considered. The direct evidence given here-of the relevance of topology for phase transitions-is obtained through a general method that can be applied to any other model.     

Paramagnetic resonance evidence for phase transitions in bilayers of pure spin-labeled lipids

The paramagnetic resonance lineshapes for lipid bilayers composed of neat spin-labeled lipids have been studied as a function of temperature and structure of the lipid spin label. Evidence for phase transitions in the bilayers of two paramagnetic lipids is presented. Very stable hysteresis in the phase transition on heating and cooling of one of the lipids is observed and the effect of additives on the hysteresis is investigated. A plot of chemical potential versus temperature for lipid phases of different geometry is compatible with the observed hysteresis.     

Photoinduced phase transitions in a Peierls system

A theory is constructed that explains photoinduced phase transitions in a Peierls system being irradiated by light with a finite width of the optical spectrum and a central frequency close to the upper van Hove singularity of the first kind in the combined density of electron states. The electron spectrum and the matrix elements of the dipole-moment operator are calculated by Bogolyubov’s method of canonical transformations. The interaction with the light is described by the Liouville equation for the density matrix of the electron subsystem in the dipole approximation. The light field is considered a quasimonochromatic time-independent random process with a Lorentzian spectrum. The derived equations are analyzed for two limits: (1) when the width of the optical spectrum tends to zero (a monochromatic light field), and (2) when the width of the optical spectrum is close to the upper limit (a bifurcation point) at which a photoinduced phase transition can still be observed. An existence criterion for such a transition is obtained, and the main parameters of the transition (the critical points and the size of the hysteresis loop) are calculated. The broadening of the optical spectrum of the incident light is shown to narrow the range of values of the central frequency of the light field and to reduce the size of the hysteresis loop. Finally, near the phase transition point, cavityless optical bistability sets in in the system, with light absorption increasing in the process. Zh. éksp. Teor. Fiz. 114, 1407–1420 (October 1998)     

Experimental evidence of synchronization of time-varying dynamical network

We investigate synchronization of time varying networks and stability conditions. We derive interesting relations between the critical coupling constants for synchronization and switching times for time-varying and time average networks. The relations are based on the additive property of Lyapunov exponents and are verified experimentally in electronic circuit.     

Experimental observation of direct current voltage-induced phase synchronization

The dynamics of two uncoupled distinct Chua circuits driven by a common direct current voltage is explored experimentally. It was found that, with increasing current intensity, the dominant frequencies of these two Chua circuits will first vary at different speeds, approach an identical value for a certain current intensity and then separate. Techniques such as synchronization index and phase difference distribution were employed to analyze the phase coherence between these two Chua circuits.     

Stochastic phase synchronization in the crayfish mechanoreceptor/photoreceptor system

The two light-sensitive neurons in the crayfish's abdominal sixth ganglion ("caudal photoreceptors," or CPRs), are both primary light sensors and secondary neurons in a mechanosensory pathway. Pei et al. (1996) demonstrated that light enhances the transduction of weak, periodic hydrodynamic stimuli (measured as an increase in the signal-to-noise ratio at the stimulus frequency in the power spectrum of the recorded neural spikes). This has been interpreted as a stochastic resonance effect, in which added light increases the noise intensity of the input to the photoreceptor (possibly through fluctuations in membrane potential), leading to an enhancement of the signal-to-noise ratio (SNR). Here, we discuss the recent demonstration (Bahar et al., 2002) of the correlation between a stochastic-resonance-like effect and an increase in stochastic phase synchronization between the neural response and a periodic mechanical stimulus. We also discuss a novel effect (Bahar et al., 2002) in which light increases the SNR of the second higher harmonic of a periodic input signal, effectively rectifying the input signal. This "second harmonic effect" can also be interpreted in terms of stochastic phase synchronization (Bahar et al., 2002). We review other recent results on the role of stochastic phase synchronization in mediating sensory responses in the crayfish nervous system.     

Experimental studies of phase transitions in isolated metal clusters

The reactions of isolated, neutral transition metal clusters with small molecules are used to probe cluster structure and to identify changes in structure with cluster size. Examples are presented of reactivity, adsorbate uptake, and product composition studies. The general conclusion is that transition metal clusters seem to have structure (are “solid”) under typical experimental conditions, and that their structure, i.e., the way the atoms pack, can change many times in the growth sequence from small clusters to bulk metal. These phase changes are often accompanied by dramatic changes in both chemical and physical properties. Evidence is presented for the existence of isomers of certain cluster sizes for some metals. In a few cases, the chemical evidence can be used to propose possible cluster structure; this is illustrated for iron and nickel clusters.     

Experimental evidence for a high-pressure isostructural phase transition in osmium

We have measured the isothermal equation of state (EOS) of osmium to 75 GPa under hydrostatic conditions at room temperature using angle-dispersive x-ray diffraction. A least-squares fit of this data using a third-order Birch-Murnaghan EOS yields an isothermal bulk modulus K0=411+/-6 GPa, showing osmium is more compressible than diamond. Most importantly, we have documented an anomaly in the compressibility around 25 GPa associated with a discontinuity in the first pressure derivative of the c/a ratio. This discontinuity plausibly arises from the collapse of the small hole-ellipsoid in the Fermi surface near the L point.     

Experimental evidence of the phase reversal in diffraction patterns

In a diffracted wavefield occurs in patterns phase variations. The most familiar example is the Fraunhofer diffraction pattern through a single slit: the theory predicts that the side bands of the diffraction pattern alternate in phase. This paper deals with phase reversal phenomenon in diffraction patterns. Two interesting systems are recognized: the diffraction through a half plane and the diffraction through a single slit.     

Explosive synchronization transitions in scale-free networks

Explosive collective phenomena have attracted much attention since the discovery of an explosive percolation transition. In this Letter, we demonstrate how an explosive transition shows up in the synchronization of scale-free networks by incorporating a microscopic correlation between the structural and the dynamical properties of the system. The characteristics of the explosive transition are analytically studied in a star graph reproducing the results obtained in synthetic networks. Our findings represent the first abrupt synchronization transition in complex networks and provide a deeper understanding of the microscopic roots of explosive critical phenomena.     

脉冲驱动离散混沌系统同步的实验与理论研究

设计了一个具有混沌和超混沌特性的二维离散混沌系统，对其动力学特征进行了分析，从电 路实验上实现了脉冲驱动离散混沌系统的同步, 用数学方法进行分析, 从理论上给出系统实 现脉冲同步的条件, 实验结果与理论分析和数值计算结果一致-另外，还考虑到弱噪声影响 ，实验结果表明：脉冲同步方法仍能保持电路系统的混沌同步，说明系统具有一定的鲁棒性 - 关键词： 脉冲同步 离散系统 电路实验     

Study on phase synchronization of stochastic chaotic system

This paper detects and characterizes the diverse roles played by bounded noise in chaotic phase synchronization (CPS) of weakly coupled nonlinear stochastic systems. Analysis of a paradigmatic model of two bidirectional coupled three-level food chains is carried out by various statistical measures such as Shannon entropy and mutual information. The results indicate that inside the synchronous regime, CPS is considerably reduced under the influence of bounded noise; near the onset of phase synchronization, temporal phase locking is diversely changed with the increase of noise, i.e., either weak or strong noise also degrades the degree of CPS, while intermediate noise enhances CPS remarkably, and an optimal noise intensity is detected that maximizes the enhancement.     

First-order superradiant phase transitions in a multiqubit cavity system

We predict the existence of novel first-order phase transitions in a general class of multiqubit cavity systems. Apart from atomic systems, the associated superradiant phase transition should be observable in a variety of solid-state experimental systems, including the technologically important case of interacting quantum dots coupled to an optical cavity mode.     

Electronically driven phase transitions in a quasi-one-dimensional adsorbate system

A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function χ(q, 0) at the connecting vector q is expected. With 1/2G_x < q < 2/3G_x one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for Θ_Br = 0.5 ML. Cooling of a defect-free c(2×2)-Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2×1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of ~200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2k_F interaction in the substrate.