Depolarized hyper Rayleigh scattering of para-nitroaniline (${\mathit{C}}_{2\mathit{v}}$ symmetry) and nitrocalix[4]arene (${\mathit{C}}_{4\mathit{v}}$ symmetry) in solution has been measured. Using linearly and circularly polarized fundamental radiation information about the ratios between the several hyperpolarizability tensor components, including their sign, was obtained. Results are consistent with the theory developed for both symmetry groups. Comparison between experimental depolarization ratios and ratios obtained from ab initio calculated hyperpolarizability tensor components shows good agreement.

A model of a two-dimensional autonomous system subject to external noise is investigated. Without noise the system has a stable limit cycle in a certain region of control parameter. Various noise-induced effects have been found numerically, such as a noise-induced frequency shift in the presence of the deterministic limit cycle, and noise-induced coherent oscillations in the absence of the deterministic limit cycle. An interesting result is that the stochastic resonance phenomenon appears in a system without an external signal and when the asymptotic state of the deterministic system is stationary.

We analyze the ensemble of surfaces surrounding critical clusters at T=${\mathit{T}}_{\mathit{c}}$ in the 3D Ising model. We find that ${\mathit{N}}_{\mathit{g}}$(A), the number of surfaces of genus g and area A, behaves as ${\mathit{A}}^{\mathit{x}\left(\mathit{g}\right)}$${\mathit{e}}^{\mathrm{-}\mathrm{\mu }\mathit{A}}$. We show that μ is constant and x(g) is approximately linear; the sum ${\mathit{tsum}}_{\mathit{g}}$ ${\mathrm{N}}_{\mathit{g}}$(A) scales as a power of A. The cluster volume is proportional to its surface area. We discuss similar reuslts for the ordinary spin clusters of the 3D Ising model and for 3D bond percolation.

We study dilatonic domain walls specific to superstring theory. Along with the matter fields and metric the dilaton also changes its value in the wall background. We found supersymmetric (extreme) solutions which in general interpolate between isolated superstring vacua with nonequal value of the matter potential; they correspond to the static, planar domain walls with flat metric in the string (sigma model) frame. We point out similarities between the global space-time of dilatonic walls and that of charged dilatonic black holes. We also comment on nonextreme solutions corresponding to expanding bubbles.

Fractional superstrings in the tensor-product formulation experience ‘‘internal projections’’ which reduce their effective central charges. Simple expressions for the characters of the resulting effective world sheet theory are found. All states in the effective theory can be consistently assigned definite spacetime statistics. The projection to the effective theory is shown to be described by the action of a dimension-three current in the original tensor-product theory.

Using big bang nucleosynthesis limits on the number of light neutrinos we derive a cosmological lower bound of ≊2 TeV on the scale of extended technicolor interactions (or any other new interactions) for the third family in models where heavy gauge bosons couple to both left- and right-handed neutrinos.

A precise measurement of the ${\mathit{D}}_{\mathit{s}}^{\pm }$ meson lifetime is reported. The data were accumulated by the high energy photoproduction experiment E687 at Fermilab in the 1990–1991 fixed target run. The measurement has been done using 900 fully reconstructed ${\mathit{D}}_{\mathit{s}}^{\pm }$→φ${\mathrm{\pi }}^{\pm }$ decays. The lifetime of the ${\mathit{D}}_{\mathit{s}}^{\pm }$ meson is measured to be 0.475±0.020±0.007 ps.

No evidence for the neutrinoless double-β decay of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ has been found in a search using a segmented Si(Li) detector with source foils enriched to 97% ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$. After 0.2664 mole years of exposure, we report a 1σ lower limit on the half-life for the ${\mathit{J}}^{\mathit{P}}$=$0^{+}$→$0^{+}$ transition of 0.44×${10}^{23}$ years.

The azimuthal angle distributions of neutral pions at midrapidity from Au+Au reactions at 1 GeV/nucleon incident energy have been measured. An enhanced emission of ${\mathrm{\pi }}^0$’s perpendicular to the reaction plane is observed. The azimuthal asymmetry is dependent on the ${\mathrm{\pi }}^0$ momentum: the ${\mathrm{\pi }}^0$ spectrum perpendicular to the reaction plane is harder than in the reaction plane. The strength of the observed asymmetry appears to be more pronounced for ${\mathrm{\pi }}^0$ than for charged particles and neutrons.

We reexamine the multiple-time-scale method as applied to ergodic adiabatic Hamiltonian systems. Solving for the evolution of the phase space density to first order in the slowness parameter, we find a term previously overlooked. The inclusion of this term resolves a standing discrepancy between the multiple-time-scale approach to this problem, and an approach using a Fokker-Planck equation. We apply our solution to the dynamics of a ‘‘slow’’ system coupled to an ensemble of ‘‘fast’’ systems following chaotic trajectories.

In this Letter, we show that coherent structures are related to folds of horseshoes which are present in chaotic systems. We develop techniques that allow us to construct coherent structures by manipulating folds in three prototypical problems: a 1D chaotic map, a 2D chaotic map, and a chaotically advected fluid. The ability to construct such structures is of practical importance for the control of chaotic or turbulent extended systems such as fluids, plasmas, and coupled oscillator arrays.

We present a model for a Saffman-Taylor finger advancing into a viscoelastic medium, along with solutions for the dynamics of the top of the finger coupled to a ‘‘fractured zone.’’ The fractured zone is defined as a region of the medium which the finger can penetrate, and ahead of which stress build up, eventually leading to fracture of the medium. The model predicts a self-sustained oscillation in the tip velocity arising via a Hopf bifurcation.

The large electric fields associated with sources of space charge have led to numerous proposals for possible collective (space-charge-based) accelerators. Traditionally, these schemes involve either magnetic fields or background ions (a plasma) to contain the space charge. This Letter proposes a new technique for particle acceleration that directly uses the large space charge fields of freely moving electron beams. Such free electron accelerators promise eventual accelerating gradients in excess of 10 GV/m.

Energetic ions can drive Alfvén gap modes unstable, causing large losses of fast ions. Toroidicity-induced Alfvén eigenmodes (TAE) were expected to disappear into the shear Alfvén continuum and become stable as the plasma beta increased. Although TAE modes may disappear, another dangerous instability with similar properties but approximately half the TAE frequency appears in a spectral gap that is created by finite beta effects. The measured frequency of the new mode agrees with the theoretical frequency of beta-induced Alfvén eigenmodes.

Statistical analysis of a large set of current profile data obtained for sawtoothing discharges in the DIII-D tokamak indicates that the safety factor (inverse rotational transform) in the plasma center, ${\mathit{q}}_0$, is close to 1.0. The increase of ${\mathit{q}}_0$ associated with the sawtooth crash is found to be of the order of 0.05. The results of time resolved measurements of the current profile and the soft x-ray emissions support a model of the sawtooth instability based on total reconnection of the m=1 magnetic island.

Period doubling and concomitant subharmonic generation have been reported in rf-excited plasmas. We show that this phenomenon is caused by the nonlinear interaction of the plasma-sheath capacitance with the external rf power circuits. As a result, changes in the external circuits can be used to control period doubling. Sheath capacitance can be derived from electrical measurements of period doubling.

The temperature dependent thicknesses of ultrathin polystyrene films under vacuum on Si (111) substrates were investigated via x-ray reflectivity in situ. The contraction of ultrathin polymer films was directly observed for the first time to the author’s knowledge. The degree of contraction depends on the initial thickness of the ultrathin polystyrene film, with the magnitude of contraction increasing with decreasing initial film thickness. This contraction ranged from 0–17 % and occurred at temperatures well below the reported bulk polystyrene glass transition temperature.

Information on the temperature dependence of the atomic correlation functions in quasicrystals has been obtained from Mössbauer and high resolution neutron spectroscopies. A pronounced nonlinear decrease is observed at high temperature for the purely elastic contribution. This indicates localized vibrational states within a length scale of about 10 Å.

Detailed infrared studies have been carried out on proton- and deuteron-implanted Si. A dominant trigonal defect involving a pair of inequivalent hydrogen atoms has been identified, with local modes at 2061.5, 1838.3, 1599.1, and 817.2 ${\mathrm{cm}}^{\mathrm{-}1}$. The structure, the local modes, and the isotopic shifts of the ${\mathrm{H}}_2^{\mathrm{*}}$ defect have been calculated using ab initio pseudopotential cluster theory. The structure is consistent with channeling and uniaxial stress experiments. The calculated frequencies and isotopic shifts are in close agreement with those observed.

We have studied NMR spin echoes in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ liquid in a magnetic field ${\mathit{B}}_0$=8 T for temperatures T≥4.5 mK. The data are analyzed in terms of the Leggett-Rice effect to obtain the transverse spin-diffusion coefficient ${\mathit{D}}_{\mathrm{\perp }}$(T). For T<20 mK we find that ${\mathit{D}}_{\mathrm{\perp }}$ is less than measured in earlier experiments at lower ${\mathit{B}}_0$. This phenomenon has been predicted for degenerate Fermi systems, due to the phase space created for quasiparticle scattering by spin polarization. The effect we have measured is more pronounced than suggested by scaling a dilute-gas theory result by a Fermi-liquid factor.