The dynamic behavior of spiral-shaped excitation patterns in the Belousov-Zhabotinskii reaction was investigated under the influence of externally applied direct current. Spiral centers drift towards the anode. The chirality of the spirals determines the direction of an additional perpendicular drift. A deformation of the Archimedian spiral shape was observed. Both effects were studied quantitatively and were reproduced in simulations with a simple reaction-diffusion model.

The Ott-Grebogi-Yorke control method is analyzed in the case that the attractor is reconstructed from a time series using time delay coordinates. It turns out that the control formula of Ott, Grebogi, and Yorke should be modified in order to apply to experimental systems if time delay coordinates are used. We reveal that the experimental surface of section map depends not only on the actual parameter but also on the preceding one. In order to meet this dependence two modifications are introduced which lead to a better performance of the control. To compare their control abilities they are applied to simulations of a Duffing oscillator.

We show that far-infrared radiation can be generated in the depletion field near semiconductor surfaces via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This mechanism is conceptually different from those previously proposed and accounts for many recent experimental observations.

Nearly-zero-field magnetoconductance measurements have been used to deduce the conduction-band spin splitting of GaAs. The dominant spin-scattering mechanism is the randomization of spin precession due to elastic scattering. By independently controlling electron density and mobility, it is observed that the crystal-field-induced spin splitting is the cause of the spin-orbit scattering. This technique is used to infer the band-structure splitting parameter, ${\mathit{a}}_{42}$=26.1±0.9 eV A${\mathrm{̊}}^3$.

We report the observation of an induced absorption on the high-energy side of spectral hole burning in band-to-band transitions in GaAs. This effect appears in the early stage of photoexcitation by an intense femtosecond pump pulse. This feature can be explained by edge singularities in the presence of a ‘‘nonequilibrium Fermi sea.’’

The dielectric permittivity of ${\mathrm{Fe}}_3$${\mathrm{O}}_4$/NiO superlattices reveals frequency-dependent features, not found in the constituent materials, which we identify as due to interfacial (Maxwell-Wagner) electrical polarization. Large modulations of the displacement field within 34 Å of an interface have thus been observed. We find further evidence that the low-frequency dielectric constant is half the value and the interfacial relaxation time is ∼10 times larger in a 34 Å/34 Å superlattice than expected suggesting that the short superlattice wavelength is less than the length scale required to define ‘‘bulk’’ polarization and conductivity.

We have observed large specific-heat anomalies around 18 mK in two single-crystal samples of ${\mathrm{UPt}}_3$ of different origin. These indicate additional forms of order which still take place in the electronic system of the heavy-fermion compound at these low temperatures. Since magnetic-susceptibility measurements showed that the samples remain superconducting, the new ordering coexists with superconductivity.

We calculate exactly the one-particle spectral densities and Hall conductivity for the two-dimensional Hubbard model in the large-U limit, for two different finite clusters. The Fermi surface obtained from the one-particle spectral functions is consistent with Luttinger’s theorem, but the Hall conductivity changes from electronlike for a dilute system to holelike for a nearly half-filled system, indicating the deconfinement of charge and spin excitations in a two-dimensional system.

We show that in the unconventional pairing state of a superconductor, i.e., the state with nodes in its excitation spectrum, the dynamical structure factor S(q,ω) has anomalous resonance peaks in q space. These resonance peaks are due to quasiparticle scattering between different nodes on the Fermi surface. The symmetry of the order parameter and the Fermi-surface geometry uniquely determine the resonance pattern. Observation of such peaks by neutron scattering will unambiguously determine the symmetry of the superconducting pairing state. Quantitative calculations are carried out in two dimensions to demonstrate this unusual effect. Applications to high-${\mathit{T}}_{\mathit{c}}$ oxide superconductors are discussed.

State-selected molecular beams of ${\mathrm{ND}}_3$ in antisymmetric inversion states (‖JKM〉‖inversion〉=‖111〉‖-〉 or ‖222〉‖-〉+‖333〉‖-〉) are scattered on a graphite (0001) surface. A preponderance of symmetric inversion states, ‖+〉, is observed in the directly scattered molecules. The observed effect is an interference between the ‘‘left,’’ C-${\mathrm{ND}}_3$, and ‘‘right,’’ C-${\mathrm{D}}_3$N, inversion configurations of a single molecule on a surface potential. This molecular self-diffraction on a surface is related to the orientational asymmetry in the molecule-surface interaction.

We have searched for the reaction ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\tau }}^{+}$${\mathrm{\tau }}^{\mathrm{-}}$ff¯, where f is either an electron, muon, or charged pion, at √s =29 GeV using the Mark II detector at the SLAC PEP storage ring. One candidate event is found while 2.3 events are expected from known processes. We would expect to see 11 events if the cross section for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\tau }}^{+}$${\mathrm{\tau }}^{\mathrm{-}}$ff¯ at √s =29 GeV were enhanced by the factor of 4.7 which the ALEPH Collaboration reports for √s =91 GeV. We also look for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ff¯ and ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$ff¯, and for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\tau }}^{+}$${\mathrm{\tau }}^{\mathrm{-}}$γ, using a similar analysis procedure, and see the number of events predicted by the standard model.

The light-cone nucleon momentum distributions obtained from nonrelativistic spectral functions or given by nuclear-binding/pion models are often used to analyze high-${\mathit{Q}}^2$ quasielastic and deep-inelastic (e,e’) reactions. We demonstrate that in such models the presence of non-nucleonic components causes the scattering from forward and backward moving target protons to be significantly different. Other models do not have this property. The sensitivity of current (e,e’p) and (p,pp) color transparency experiments is sufficient to observe these differences.

Using resonant multiphoton laser excitation in combination with temporarily applied electric fields we have excited high-lying planetary states in strontium. Thereby, the angular momenta were high enough to prevent penetration of the ${\mathrm{Sr}}^{++}$ core, as well as the mutual penetration of the two excited wave functions. Analysis of the spectra (quantum defects and oscillator strength modulations) clearly revealed the dipole structure of such a ‘‘planetary atom,’’ where the outer electron is bound to either a permanent or an induced dipole formed by the inner electron and the ionic core.

The order ${\mathrm{\alpha }}^4$ ln${\mathrm{\alpha }}^{\mathrm{-}1}$ ${\mathit{f}}_{\mathrm{RYD}}$ corrections to the n=1 and n=2 energy levels of positronium are calculated using the conventional Bethe-Salpeter formalism. The Barbieri-Remiddi potential provides a suitable zeroth-order kernel about which perturbation theory is developed. A simple algorithmic method which amounts to an expansion of the interaction kernel in powers of the momentum divided by the mass is shown to suffice to extract the ln${\mathrm{\alpha }}^{\mathrm{-}1}$ terms from most of the interaction terms.

We present a new scheme, the ion-ripple laser, for generating tunable coherent radiation ranging from microwaves to the ultraviolet. A relativistic electron beam obliquely propagating through an ion ripple excites electromagnetic radiation which is coupled to a negative-energy electrostatic wave via backward Raman scattering. The radiation peaks at a resonance frequency of ω∼2${\gamma }^2$${\mathrm{\kappa }}_{\mathit{i}\mathit{r}}$c cosθ. We derive the fluid theory dispersion relation for wave coupling and estimate the nonlinear saturation level. A 1 2/2 D PIC simulation code is used to verify the theory and scaling laws.

The dynamics of transverse particle oscillations in the Fermilab Tevatron, artificially made nonlinear, is studied by observing beam profiles over periods up to an hour. A ‘‘diffusive’’ model with amplitude-dependent diffusion coefficient gives a quantitatively accurate description of beam evolution. The evolution is influenced strongly by nonlinear resonance.

It it pointed out that the continuing advocacy of electrostatic double layers as particle accelerators in the aurora and other space and astrophysical plasmas is fundamentally unsound. It is suggested furthermore that there is little reason to invoke static or quasistatic electric fields as the cause of auroral electron acceleration. Stochastic acceleration by electrostatic wave turbulence appears to present a natural explanation for this and for electron acceleration in other space anfd astrophysical plasmas.

The electron-energy-distribution function (EEDF) in low-pressure argon and helium rf discharges has been measured during the discharge transition from the low-voltage to the high-voltage mode using a recently developed high-resolution Langmuir probe technique. When the transition occurs, the EEDF becomes Maxwellian because of increased electron-electron collisions arising from a sharp rise in electron density and a steep fall in electron temperature caused by γ-electron injection.

The incident-ion energy distribution in a collisionless radio-frequency (rf) sheath is derived analytically for a general electric-field configuration in the high-frequency regime. The analysis is based on a two-time-scale asymptotic expansion of the ion equation of motion, where the ratio of the ion transit frequency ${\mathrm{\omega }}_{\mathrm{tr}}$ to the rf frequency ω is assumed to be small. It is shown that the ‘‘ponderomotive force’’ due to the rf modulation of the electric field exerts a retarding effect on the ion motion, counteracting the dc-bias field. The results obtained here are applicable to rf-discharge-based process tools used in microelectronics fabrication, where the conditions of low collisionality and high rf frequency are usually satisfied.

The simultaneous observations of a beat-wave-excited plasma wave and of energetic electrons are reported. A plasma wave is excited when the beat-wave frequency of a laser with two wavelengths 10.6 and 9.57 μm equals the plasma frequency. The Stokes sideband measurement gives a wave amplitude δn/${\mathit{n}}_0$ of ∼5%. Plasma electrons with the energies of more than 10 MeV are observed at the resonant density.