We report the first experiments showing the reversibility of transient acoustic waves through high-order multiple scattering by means of an acoustic time-reversal mirror. A point source generates a pulse which scatters through 2000 steel rods immersed in water. The time-reversed waves are found to converge to their source and recover their original wave form, despite the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. Surprisingly, the observed resolution was one-sixth of the theoretical limit for the mirror's aperture.

Complex systems have the property that many competing behaviors are possible, and the system tends to alternate among them. In fact, the ability of a complex system to access many different states, combined with its sensitivity, offers great flexibility in manipulating the system's dynamics to select a desired behavior. By understanding dynamically how some of the complex features arise, we show that it is indeed possible to control a complex system's behavior. This is illustrated by using the noisy double rotor map as a paradigm.

We measure the local intensity and the intensity cross-correlation function of patterns emitted by a C${\mathrm{O}}_2$ laser with a Fresnel number around 60 and find evidence of weak turbulence. We show theoretically that the spatiotemporal dynamics of such a laser can be governed by two fields. One leads to a "turbulent" state principally through phase fluctuations, whereas the other yields a periodic modulation in space and time of the intensity. This can explain why the intensity of the laser is locally chaotic but the time-averaged intensity pattern retains the global symmetry of the system.

We study both the dynamics of dissolution of an equilibrium interface and phase separation in two-dimensional fluids using lattice Boltzmann simulations. Results for a liquid-gas system and a binary fluid are compared. For symmetric quenches in the liquid-gas system, single-phase domains grow like $t^{\alpha }$, where $\alpha =\frac{1}{2}$ for high viscosities (corresponding to early times), crossing over to $\alpha =\frac{2}{3}$ for low viscosities (later times). For a binary fluid the crossover is between $\alpha =\frac{1}{3}$ and $\alpha =\frac{2}{3}$. This behavior is compared to that for nonsymmetric quenches.

We address the striking coexistence of localized waves ("pulses") of different lengths, which was observed in recent experiments and full numerical simulations of binary-mixture convection. Using a set of extended Ginzburg-Landau equations, we show that this multiplicity finds a natural explanation in terms of the competition of two distinct, physical localization mechanisms; one arises from dispersion and the other from a concentration mode. This competition is absent in the standard Ginzburg-Landau equation. It may also be relevant in other waves coupled to a large-scale field.

We present a new approach to the measurement of coherence. By monitoring the quantum interference fluctuations in the population excited by a pair of time-delayed, randomly phased pulses, it is possible to extract information on internal dynamics, energy level splittings, and characteristic coherence decay times of the medium. As a proof of concept, we demonstrate the measurement of phase relaxation and doublet separation in atomic potassium. The principle of coherence observation by interference noise is very general, is shown to be robust and with inherent time resolution of a few optical cycles, and is proposed as an alternative to many interferometric applications.

A 3-cm-long curved Ge slab target was irradiated by 1.053 μm glass laser light with irradiance of 2.9 × ${10}^3$ W/${\mathrm{cm}}^2$. The 19.6 nm laser line due to the $3p\to 3s$, $J=0\mathrm{}\to 1$ transition in Ne-like Ge ions was found to be polarized parallel to the target surface, with a degree of polarization of -53%. This polarization is accounted for by the anisotropy in the radiation trapping of the resonance line in an expanding plasma, which has a velocity gradient in the direction normal to the target surface.

We show that drifting pulse solutions of a 1D complex Ginzburg-Landau equation can persist for positive growth rate $\varepsilon$ in a finite system. When $\varepsilon$ is increased, two different destabilization scenarios are observed. In sufficiently large systems, fluctuations grow out to form multiple pulses. In small systems, an increase in $\varepsilon$ eventually leads to a competition between fronts and pulses that results in a sharp transition to a state where the drifting pulse leaps forward in an incoherent fashion. Similar behavior is observed in a more realistic model.

We establish anomalous inertial range scaling of the structure functions for a model of homogeneous, isotropic advection of a passive scalar by a random velocity field. The velocity statistics is taken as Gaussian with decorrelation in time and velocity differences scaling as ${\left|\mathrm{x}\right|}^{\frac{\kappa }{2}}$ in space, with $0\le \kappa <2\mathrm{}$. The scalar is driven by a random forcing acting on spatial scale $L$. The structure functions for the scalar are well defined as the diffusivity is taken to zero and acquire anomalous scaling for large $L$. The anomalous exponent is calculated explicitly for the fourth structure function and for small $\kappa$.

Experiments on vertically oscillated granular layers in an evacuated container reveal a sequence of well-defined pattern bifurcations as the container acceleration is increased. Period doublings of the layer center of mass motion and a standing wave instability interact to produce hexagons and more complicated patterns composed of distinct spatial domains of different relative phase separated by kinks (phase discontinuities). A simple model displays quantitative agreement with the observed transition sequence.

We study quantum-mechanical tunneling between symmetry-related pairs of regular phase space regions that are separated by a chaotic layer. We consider the annular billiard, and use scattering theory to relate the splitting of quasidegenerate states quantized on the two regular regions to specific paths connecting them. The tunneling amplitudes involved are given a semiclassical interpretation by extending the billiard boundaries to complex space and generalizing specular reflection to complex rays. We give analytical expressions for the splittings, and show that the dominant contributions come from chaos-assisted paths that tunnel into and out of the chaotic layer.

An analytical method is developed describing the approach to a finite-time singularity associated with collapse of a narrow fluid layer in an unstable Hele-Shaw flow. Under the separation of time scales near a bifurcation point, a long-wavelength mode entrains higher-frequency modes, as described by a version of Hill's equation. In the slaved dynamics, the initial-value problem is solved explicitly, yielding the time and analytical structure of a singularity which is associated with the motion of zeros in the complex plane. This suggests a general mechanism of singularity formation in this system.

We introduce and study the first model of an experimentally realizable three-dimensional time-dependent nonturbulent fluid flow to display the phenomenon of global diffusion of passive-scalar particles at arbitrarily small values of the nonintegrable perturbation. This type of chaotic advection, termed resonance-induced diffusion, is generic for a large class of flows.

We calculate the Lyapunov exponents in a classical molecular dynamics framework. Yukawa and Slater-Kirkwood forces are considered in order to give an equation of state that resembles the nuclear and the atomic ${}^4$He equation of state, respectively, near the critical point for liquid-gas phase transition. The largest Lyapunov exponents $\lambda$ are always positive and can be very well fitted near the “critical temperature” with a functional form $\lambda \propto |{T-T}_c{|}^{-\omega }$, where the exponent $\omega =0.15\mathrm{}$ is independent of the system and mass number. At smaller temperatures we find that $\lambda \propto T^{0.4498}$, a universal behavior characteristic of an order to chaos transition.

An initial cloud of particles floating on the surface of a flowing fluid will often tend to a fractal pattern. If the wave-number spectrum of the pattern has an observable power law dependence $k^{-\rho }$, then the exponent $\rho$ is predicted to be $\rho {=D}_2-1\mathrm{}$, where $D_2$ is the correlation dimension of the fractal attractor. Numerical, experimental, and theoretical results are shown to support this prediction, but it is also found that, when the observable range in $k$ is limited, the predicted power law scaling can be obscured by fluctuations in the $k$ spectrum. The expected behavior can, however, be restored by use of averaging.

We present a new method for efficient, accurate calculations of transport properties of random media. It is based on the principle that the wave energy density should be uniform when averaged over length scales larger than the size of the scatterers. This scheme captures the effects of resonant scattering of the individual scatterer exactly, as well as the multiple scattering in a mean-field sense. It has been successfully applied to both “scalar” and “vector” classical wave calculations. Results for the energy transport velocity are in agreement with experiment. This approach is of general use and can be easily extended to treat different types of wave propagation in random media.

Atomic interferences are observed in the micromaser when the inversion of the atoms leaving the cavity is measured while the cavity frequency is scanned across the atomic resonance. The interferences are due to the nonadiabatic mixing of dressed states at the entrance and exit holes of the maser cavity. The interference structures are triangular shaped and approximately equidistant. They are associated with the dynamics of the atom-field interaction, show quantum jumps, and demonstrate bistability of the micromaser field.

We report the observation of the transition between positive and negative hexagons by using optical pattern formation in a sodium vapor cell with a feedback mirror. We show that because of a secondary instability of a developed hexagon against its spatial ultraharmonics a new pattern (ultrahexagon) appears. Pattern formation for an incident Gaussian light beam takes place in combination with switching effects that result in flowerlike structures. The data from analytical investigations and numerical simulations based on a microscopic theory are in good accordance with experiment.

The forces of capillary interaction between two thin cylindrical rods or two spherical particles placed at a liquid/air interface and immersed partially in the liquid are measured by a sensitive torsion microbalance. Cylindrical rods of radii 50, 165, and 365 μm are used. The spheres have radius 600 μm; they are modeling the lateral interaction between colloidal particles trapped in a thin liquid film. The measured forces range in magnitude between ${10}^{-4\mathrm{}}$ and ${10}^{-9\mathrm{}}\mathrm{N}$ depending on the particle size and the interparticle distance. The experimental data agree well with the theory of capillary immersion forces.

A Comment on the Letter by M. S. Kushwaha, et al., Phys. Rev. Lett. 71, 2022 (1993). The authors of the Letter offer a Reply.