Spiral waves, a type of ‘‘reentrant excitation,’’ are believed to be associated with the most dangerous cardiac arrhythmias, including ventricular tachycardia and fibrillation. Recent experimental findings have implicated defective regions as a means of trapping spirals which would otherwise drift and (eventually) disappear. Here, we model the myocardium as a simple excitable medium and study via simulation the interaction between a drifting spiral and one or more such defects. We interpret our results in terms of a criterion for the transition between trapped and untrapped drifting spirals.

The influence of parametric noise on a class of a growing-rate-type model is studied. The formal time-dependent solution is found analytically for any linearly coupled white noise. The Mellin-Barnes technique is used in order to extend this solution on a whole parameter space in the case of Gaussian white noise and white shot noise. The asymptotic behavior in different regions of parameter space is examined. A digital averaging is done which supports the analytical results. It is found that, in contrast to the deterministic case, the relaxation in the presence of the noise needs not be purely monotonic.

Quantum coherence and its destruction (dephasing) by coupling to a dissipative environment plays an important role in time-resolved nonlinear optical response as well as nonadiabatic transitions and tunneling processes in condensed phases. Generating functions of density-matrix elements and multitime coordinate and momentum correlation functions related to these phenomena are calculated using a pathintegral approach by performing functional integration. The dissipative environment is assumed to be an ensemble of harmonic oscillators and is taken into account by using Feynman-Vernon influence functional. Closed-form expressions for generating functions in terms of the bath spectral density are derived. The present theory generalizes earlier calculations of these quantities to arbitrary temperatures, any dependence of the transition coupling on coordinates (non-Condon effects), and arbitrary order in the interstate coupling. Conditions for factorizing the Liouville-space generating functions that allow a reduced description based on the classical Langevin equation are established. Possible applications to four-wave-mixing spectroscopy and nonadiabatic rate processes are discussed.

We propose a dynamic model for granular flows. Starting from the equation of motion for a grain together with the equation of continuity, we derive a Langevin equation describing the time evolution of the granular system. We show how the model displays scale invariance and reduces to several existing models in appropriate limits. In particular, we also point out the possibility of different behaviors in granular systems according to their sizes.

We obtain the conditions that ensure the stability of spatially and temporally periodic orbits of coupled-map lattices. The stability matrices can be put in a circulant and block circulant form. This allows us to reduce the problem to smaller matrices corresponding to the building blocks of spatial periodicity. We find that additional conditions are imposed as we expand the size of the lattice. For the traveling-wave solution the analysis is considerably simplified. We have analyzed both the one-dimensional and higher-dimensional lattices.

The time evolution of the mean deviation of initially close trajectories in a stochastic dynamical system is investigated. It is shown both for additive and linearly coupled multiplicative noise that the mean deviation loses its dependence on initial conditions for long times. For shorter times a power law is found for certain types of additive noise processes, in sharp contrast to the exponential separation of initially nearby trajectories in deterministic chaotic systems. Exponential time evolution is obtained for linearly coupled multiplicative noise after an initial transient during which more complex regimes, including a superexponential stage, can take place.

Characteristic recurrence phenomena that are strongly dependent on initial conditions but do not depend much on the numbers of degrees of freedom are observed in the study of a system consisting of identical harmonic oscillators coupled with the Nosé-Hoover thermostat. The time evolution of the energy of each oscillator in this system at large Q (Q is a parameter controlling the speed of the response of the thermostat) is composed of a secular periodic motion with frequency of order 1/Q and fast fluctuations around it with frequency of order 1. The latter is the natural frequency of oscillators. The secular part depends very sensitively on initial conditions. The dynamical behaviors of the system are analyzed by the perturbation treatment with respect to 1/Q, and a Hamiltonian H’={${\mathit{tsum}}_{\mathit{i}}$${\mathit{tsum}}_{\mathit{j}}$ ${\mathrm{p}}_{\mathit{i}}$${\mathit{p}}_{\mathit{j}}$cos[2(${\mathit{q}}_{\mathit{i}}$-${\mathit{q}}_{\mathit{j}}$)]}/16, which describes the behavior of the secular part in the original system, is derived of first order in 1/Q. The coordinate ${\mathit{q}}_{\mathit{i}}$ and the conjugated momentum ${\mathit{p}}_{\mathit{i}}$ in the Hamiltonian H’ are the slowly changing part of the phase ${\mathrm{\phi }}_{\mathit{i}}$ of the oscillator and the energy ${\mathit{H}}_{\mathit{i}}$. I found that this Hamiltonian H’ is completely integrable and can be solved analytically. The solutions thus obtained describe very well the dynamical behaviors in the original system.

A restricted variational formulation for extended irreversible thermodynamics is proposed. Then, it is applied to a heat- and electricity-conducting viscous fluid, and the resulting equations are reduced to order two to be compared with those well known of kinetic theory. A better understanding of the phenomenological coefficients than in previous reported works is obtained.

We explore the rich behavior of vibrated powders, focusing on the relaxation of transients between vibrational steady states, on the diffusion of independent particles, and on ‘‘global’’ and ‘‘local’’ angular distributions of contacting particles. When examined as a function of vibrational intensity, these are shown to be explicable in terms of a model of independent-particle and collective excitations. Our data provide a wealth of information about topics as diverse as consolidation under packing, the roughness of granular piles, and the local structure of bridges and arches in shaken packings.

We consider mean first-passage times (MFPT’s) for systems driven by non-Markov gamma and McFadden dichotomous noises. A simplified derivation is given of the underlying integral equations and the theory for ordinary renewal processes is extended to modified and equilibrium renewal processes. The exact results are compared with the MFPT for Markov dichotomous noise and with the results of Monte Carlo simulations.

We study the probability distribution of a passive scalar undergoing turbulent mixing in the presence of a mean scalar gradient. Kerstein’s model, which describes the turbulent mixing process as a collection of instantaneous local rearrangements of the scalar, is argued to be a plausible approximation when the size of the system is much larger than the velocity correlation length, and after the description has been coarse grained over a correlation volume. In the physical range of parameters, we find numerically that the fluctuations of the scalar θ are close to exponentially distributed when a linear mean scalar gradient is imposed. The phenomenological mean-field-like theory of Pumir, Shraiman, and Siggia [Phys. Rev. Lett. 66, 2984 (1991)] is derived heuristically beginning with the Kerstein model. This theory predicts strictly exponential tails for the probability distribution of the scalar fluctuations P(θ), i.e., P(θ)∼exp(‖θ‖). We also consider a simplified version of the Kerstein model which is analytically fully tractable and gives qualitatively similar results numerically. Under conditions of spatial homogeneity and imposed linear mean scalar gradient, this simplified model and the full Kerstein model are described by the same mean-field theory. However, for the simplified model, the large-‖θ‖ asymptotic behavior of P(θ) is Poisson-like, i.e., P(θ)∼exp[-‖θ‖ln‖θ/(const)‖].

In recent numerical experiments on series arrays of overdamped Josephson junctions, Nichols and Wiesenfeld [Phys. Rev. A 45, 8430 (1992)] discovered that the periodic states known as splay states are neutrally stable in all but four directions in phase space. We present a theory that accounts for this enormous degree of neutral stability. The theory also predicts the four non-neutral Floquet multipliers to within 0.1% of their numerically computed values. The analytical approach used here may be appli- cable to other globally coupled systems of oscillators, such as multimode lasers, electronic oscillator circuits, and solid-state laser arrays.

The correlation functions ${\mathit{C}}_{\mathrm{\phi }}$ and ${\mathit{C}}_{\mathrm{\phi }}^2$, associated with the order-parameter field φ→(r,t) and its square, respectively, are discussed using heuristic arguments and an approximate analytical approach. Topological defects (walls, strings, monopoles) in the field, seeded by a quench from the high- to the low-temperature phase, lead to singular short-distance behavior in the scaling functions, and power-law tails in the corresponding structure factors. For superfluid helium, the structure factor ${\mathit{S}}_{\mathrm{\phi }}^2$(k,t) is measurable in principle using small-angle scattering (whereas ${\mathit{S}}_{\mathrm{\phi }}$ is inaccessible). It is predicted to exhibit a power-law tail, ∼[${\mathit{a}}^4$/L(t${)}^2$](lnka${)}^2$/k, where L(t) is the characteristic scale at time t after the quench and a is the core size of a vortex line. Correlation functions for the defect density are also discussed.

The motion of a proton in a double-well potential created by the potentials of two heavy ions in a molecular chain is considered. The topology of the potential changes depending on the distance between the molecules. Both forms, double well and single well, are possible. Individual chaotic proton motion is triggered by the oscillations of the lattice. Depending on the system parameters, both the one-well and the cross-well attractors can be either periodic or chaotic. This has some interesting consequences for the interpretation and understanding of propagation of ionic defects in hydrogen-bonded chains in the presence of external oscillating fields. A frequency-locked propagating kink is found.

Dynamic light scattering is applied to the glass phase of nonaqueous suspensions of sterically stabilized colloidal spheres. The short-ranged steric repulsion ensures that the particle interactions are close to hard sphere. This is supported by the observation that the equilibrium phase behavior of these suspensions agrees with that predicted for the hard-sphere atomic system. We verify a model for a nonergodic medium, which assumes that the particles are localized during an experiment and which allows the intermediate scattering function to be calculated from a single measurement of the time-averaged intensity autocorrelation function. Intermediate scattering functions are obtained for several concentrations over a range of wave vectors around the main diffraction peak. The measured nonergodicity parameters are in good agreement with the predictions of mode-coupling theory for the hard-sphere glass. The comparison involves no adjustable parameters. At long times the intermediate scattering functions can be scaled to a single curve for over 2.5 decades in time. This, combined with the results that the nonergodicity parameters and critical amplitudes required for the scaling are in quantitative agreement with mode-coupling theory, provides a convincing verification of the predicted factorization property of the β process in the glass phase.

The self-avoiding random walk on lattices with quenched random site energies is studied using exact enumeration in d=2 and 3. For each configuration we compute the size R and energy E of the minimum-energy self-avoiding walk (SAW). Configuration averages yield the exponents ν and χ, defined by ${\mathit{R}}^2$¯∼${\mathit{N}}^{2\nu }$ and δ${\mathit{E}}^2$¯∼${\mathit{N}}^{2\mathrm{\chi }}$. These calculations indicate that ν is significantly larger than its value in the pure system. Finite-temperature studies support the notion that the system is controlled by a zero-temperature fixed point. Consequently, exponents obtained from minimum-energy SAW’s characterize the properties of finite temperature SAW’s on disordered lattices.

The unstable periodic orbits of a chaotic NMR laser have been identified and classified according to their stability properties. The dynamical control method by Ott, Grebogi, and Yorke (OGY) [Phys. Rev. Lett. 64, 1196 (1990)] has been successfully applied in a six-dimensional embedding space, as required for a proper reconstruction of the attractors. An improved method has then been implemented in order to circumvent the occurrence of several, possibly complex, unstable eigenvalues in the standard OGY procedure and has shown superior performance. Finally, an extension to several control stations per Poincaré time is discussed.

The Mori-Lee formalism for solving the Liouville (or Heisenberg) equation of motion for Hermitian systems demonstrates that the Laplace-transformed dynamical correlations in canonical ensembles can be written as (in)finite continued fractions. We show that a model-independent direct summation method allows accurate numerical evaluation of all known classes of these (in)finite continued fractions that arise in dynamics problems and thus provides a powerful technique to study the dynamics of many-body and few-body systems. Some studies on dynamical correlations in s=1/2 quantum spin chains are cited as applications of the method presented.

In a recent Letter [Phys. Rev. Lett. 67, 664 (1991)] we found semiclassical propagation to be remarkably accurate in the chaotic stadium billiard long after classical fine structure had developed on a scale much smaller than ħ. We give a complete account of that work and derive an approximate time scale for the validity of the semiclassical approximation as a function of ħ.

We extend the master-equation treatment of dynamical evolution of a system-plus-reservoir configuration including the propagation of initial correlations as a noise source. Specializing into the quantum harmonic oscillator coupled to a fermionic heat bath, we develop a model for the diffusion matrix in the space of diagonal density operators. It can be shown that mean values of observables undergo Langevin-like motion and, in particular, that the mean value and dispersion of the oscillator quanta approach the canonical equilibrium values. A final interpretation of the characteristics and role of the noise source is given.