We study the consequences of interface roughness on folded acoustic phonon Raman spectra in GaAs/AlAs quantum wells and superlattices. Layer thickness fluctuations cause a partial breakdown of crystal momemtnum conservation along and perpendicular to the growth direction. Folded phonons from the whole minizone contribute to a continuous Raman background with intensity anomalies at zone edge and internal dispersion gaps. The observations are explained by invoking the dependence of the Raman efficiency on crystal momentum. Typical length scales of interface roughness are determined.

We propose a new analytical technique, cycle length analysis (CLA), which quantifies all intermittent periodic modes of a data set. CLA is especially well suited to the treatment of extended measurements of complex physical, chemical, and biological systems when subjected to variations in underlying dynamical parameters which cannot be controlled precisely or monitored accurately. We demonstrate the application of CLA using (1) a coupled oscillator model under the conditions of continuously varying control parameters and (2) clinical cardiac rhythm data.

The focusing nonlinear Schrödinger equation is numerically integrated over moderate to long time intervals. In certain parameter regimes small errors on the order of roundoff grow rapidly and saturate at values comparable to the main wave. Although the constants of motion are nearly preserved, a serious phase instability (chaos) develops in the numerical solutions. The instability is found to be associated with homoclinic structures and the underlying mechanisms apply equally well to many Hamiltonian wave systems.

The short-time behavior of an unstable particle is examined in a realistic field theoretical model. The onset of linear decreasing of the nondecay probability is shown to be extremely rapid, so rapid as to exclude any relevance of regimes quadratic in time (‘‘Zeno’’ quantum paradox). The result applies to super-renormalizable, renormalizable, and nonrenormalizable cases, particularly to the proton decay problem. We discuss also the deviations from the exponential decay law in small-Q-value decays.

It is shown that an interacting theory, defined on a regular lattice, must have a vectorlike spectrum if the following conditions are satisfied: (a) locality, (b) relativistic continuum limit without massless bosons, and (c) pole-free effective vertex functions for conserved currents. The proof exploits the zero-frequency inverse retarded propagator of an appropriate set of interpolating fields as an effective quadratic Hamiltonian, to which the Nielsen-Ninomiya theorem is applied.

We analyze the possibility of universality violation in diagonal leptonic decays of the Z boson, in the context of interfamily ‘‘seesaw’’ models. In a minimal extension of the standard model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the current experiments at the CERN ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ collider LEP.

We demonstrate that expected efficiencies and purities for b tagging at the detectors at the Superconducting Super Collider and the CERN Large Hadron Collider may allow detection of the standard model Higgs boson in tt¯H production, with H→bb¯ decay, for 80≲${\mathit{m}}_{\mathit{H}}$≲130 GeV, provided ${\mathit{m}}_{\mathit{t}}$≳130 GeV.

High resolution measurements of the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(γ,n) at ${\mathit{E}}_{\gamma }$∼58 MeV are presented. The distribution of strength to the resolved bound final states in ${}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ is compared with that of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ obtained in previous analogous (γ,p) measurements and the implications for the theoretical description of (γ,N) reactions are discussed. These new results confirm the importance of two-nucleon effects in intermediate energy photon absorption and highlight inadequacies in state-of-the-art microscopic calculations of (γ,N) reactions.

We report absolute cross sections for K- and L-shell ionization of silver and gold targets, respectively, by lepton (electron, positron) impact in the threshold region. The experiments were performed at the slow positron source TEPOS facility at the linac of the Strahlenzentrum in Giessen. A different behavior of positron and electron impact near threshold was observed. In comparison with theoretical calculations, our results suggest that the differences between electrons and positron impact are mainly caused by the acceleration or deceleration of the incident projectile in the target nuclear field.

A continuous time dynamic model of a d-dimensional lattice of coupled localized m-component chaotic elements is solved exactly in the limit m→∞. A self-consistent nonlinear partial differential equation for the correlations in space and time is derived. Near the onset of spatiotemporal disorder there are solutions that exhibit a novel space-time symmetry: the corresponding correlations are invariant to rotations in the d+1 space-time variables. For d<3 the correlations decay exponentially at large distances or long times. For d≥3 the correlations exhibit a power law decay as the inverse of the distance or time.

We derive here an expression for the length spectrum, Σδ(l-${\mathit{l}}_{\mathit{j}}$) of periodic orbits in chaotic billiards. Our analysis yields the average exponential proliferation of lengths superimposed by fluctuations which arise as a sum over contributions from ‘‘quantal energies.’’ We further show that these fluctuations are non-Poisson. Long-orbits, however, mimic the Poisson result for the spectral rigidity, Δ(L).

The balance of one-point and two-point statistical characterics of vorticity, is considered on the basis of the Navier-Stokes equations. It is shown that within the inertial range of scales (L ${\mathrm{Re}}^{\mathrm{-}3/4}$≪r≪L, L external scale, Re Reynolds number) there is a physically distinguished scale ${\mathit{l}}_{\mathit{s}}$∼L ${\mathrm{Re}}^{\mathrm{-}3/10}$. The balance of vortical correlations with scales r≥${\mathit{l}}_{\mathit{s}}$ is directly affected by the large-scale motion. ${\mathit{l}}_{\mathit{s}}$ is a natural length scale for the ‘‘vortex strings,’’ observed experimentally and numerically in three-dimensional turbulent flows. The twist of vortex lines in the internal structure of vortex strings is also briefly discussed.

Several recent experiments have produced an ordered chain of ions which are confined along a one-dimensional axis. Here we examine the rate of irreversible energy transfer between degrees of freedom describing motions transverse and parallel to this axis. Because of the strong transverse confinement, the transverse motions are much higher frequency than the parallel motion, and so a many-particle adiabatic invariant exists which greatly reduces the rate of thermal equilibration.

Laser pulses with a power of ${10}^{12}$ W and a duration of ${10}^{\mathrm{-}13}$ s were focused onto both gas and solid targets. Strong emission of pulsed radiation at terahertz frequencies was observed from the resulting plasmas. The most intense radiation was detected from solid density targets and was correlated with the emission of MeV x rays and electrons. Results indicate that radiative processes in such plasmas are driven by ponderomotively induced space charge fields in excess of ${10}^8$ V/cm. This work constitutes the first direct observation of a laser-induced wake field.

The phase diagram of dipolar hard spheres has been determined by Monte Carlo simulation for reduced densities ${\mathrm{\rho }}^{\mathrm{*}}$ ranging from 0.02–0.3 and reduced temperatures ${\mathit{T}}^{\mathrm{*}}$ from 0.08–0.25. For ${\mathit{T}}^{\mathrm{*}}$≲0.15 the particles are found to associate to form chains which at the highest density are in a polarized ferroelectric state.

Light-induced defect creation is demonstrated in hydrogenated polycrystalline silicon (poly-Si:H). The newly created defects are metastable as in hydrogenated amorphous silicon (a-Si:H). However, unlike a-Si:H the magnitude of the light-induced degradation decreases with repeated illumination and anneal cycles and is restored upon reexposure to monatomic hydrogen. This unique response arises from the inherent structural inhomogeneity of the polycrystalline material and establishes that hydrogen directly contributes to the metastability in poly-Si:H.

We argue that the critical forest-fire model (FFM) introduced by Drossel and Schwabl [Phys. Rev. Lett. 69, 1629 (1992)] is a critical branching process in the mean-field approximation where the number s of trees burned in a forest fire is power-law distributed with exponent ${\mathrm{\tau }}_{\mathit{s}}$=5/2. The mean-field model of the FFM in finite dimension is the percolation model and, as in percolation, the upper critical dimension is 6. Simulations show that the FFM becomes increasingly percolationlike with increasing dimension d and is, within error bars, fully consistent with the percolation results when d≥3.

A stochastic ‘‘forest-fire’’ model is considered. Sites are filled individually at a constant mean rate; also, ‘‘sparks’’ are dropped at a small rate k, and instantaneously burn up the entire cluster they hit. I find nontrivial critical exponents in the self-organized critical limit k→0, contrary to earlier results of Drossel and Schwabl. Spatial correlation functions and a site occupancy correlation exponent are measured for the first time. Scaling relations, derived by analogy to uncorrelated percolation, are used extensively as numerical checks. Hyperscaling is violated in this system.

The localization of a directed polymer onto an extended defect (such as a line or a plane) in the presence of competing bulk disorder is examined. Based on scaling ideas and exact analysis on a hierarchical lattice, we develop a new renormalization scheme to study the directed polymer localization problem. We establish absence of delocalization transition for attractive columnar defect in the marginal dimension ${\mathit{d}}_{\mathit{c}}$=2, and for attractive planar defect in d=3. For columnar defect in three dimensions, our simulations yield a localization length exponent ${\nu }_{\mathrm{\perp }}$=1.8±0.6.

We analyze the statistics of water droplet avalanches in a continuously driven system. Distributions are obtained for avalanche size, lifetime, and time between successive avalanches, along with power spectra and return maps. For low flow rates and different wave viscosities, we observe a power-law scaling in the size and lifetime distributions of water droplet avalanches, indicating that a state with no characteristic time and length scales was reached. Higher flow rates resulted in an exponential behavior with characteristic scales.