Quantitative predictions for the doubly differential detachment collisions, 0.5-MeV ${\mathrm{H}}^{-}+\mathrm{He}\to \mathrm{H}\mathrm{}(n=2\mathrm{})+{\mathrm{He}}^{*}+e^{-}$, are provided by use of hyperspherical wave functions for the H-$e^{-}$ system. Individual H-$e^{-}$ hyperspherical channels are identified as producing distinct "cusp," "shape resonance," and/or "shoulder" features in the laboratory-frame cross sections. Moreover, the first quantitative predictions of the expected Gailitis-Damburg dipole-field-induced oscillations in the cross sections above the $\mathrm{H}\mathrm{}(n=2\mathrm{})\mathrm{}$ threshold are presented and distinguished from ${}_{}{}^1{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}$ shape resonance effects.

A new type of electron-photon coincidence experiment is presented which utilizes stepwise electron and laser excitation of the target atoms. The technique has been applied to the measurement of atomic-collision parameters for $e^{-}-\mathrm{Hg}\left(6\mathrm{}{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}\right)$ collisions at an incident electron energy of 16.0 eV.

A transient, time-delayed, four-wave mixing experiment is performed in strontium vapor under stochastic excitation conditions. The first two broad-bandwidth laser pulses are correlated and their delay is adjustable. Thus they combine into a non-Markovian excitation process with adjustable memory time. One of the correlated pulses is allowed to be strong. In contrast with previous investigations, it is shown both theoretically and experimentally that under strong-field conditions the atoms are not able to keep the excitation memory over time intervals longer than the inverse laser bandwidth.

It is shown that the degree of polarization of a beam in electron storage rings can be limited by nonlinear effects. The strength of a nonlinear effect depends on the amplitude of the particles. Particles performing synchrotron and betatron oscillations with high amplitudes can contribute overproportionally to depolarization. As a result, the emittance of a beam is not allowed to exceed certain boundaries, otherwise the beam will become more and more depolarized. This limit is called nonlinear spin acceptance.

A well-defined order-disorder transition occurs in the capillary waves on a fluid layer driven by vertical oscillation. The transition is characterized by a sharp decline in both the translational correlation length and the long-range orientational order of the pattern, and an onset in the characteristic frequency $f^{*}$ of chaotic fluctuations varying approximately as ${(A-A_d)}^{\frac{1}{2}}$, for driving amplitudes above a threshold $A_d$. The transition is geometry dependent even when the container size is 50-100 times the wavelength.

A new relativistic, bounce-averaged Fokker-Planck-quasilinear code shows that for experimentally relevant rf power $\frac{p_{\mathrm{rf}}(\mathrm{W}/{\mathrm{cm}}^3)}{{\left[n_e\right({10}^{13} {\mathrm{cm}}^{-3\mathrm{}}\left)\right]}^2}\gtrsim 0.5$, electron-cyclotron current-drive efficiency (and absorptivity) is power dependent; at given plasma radius the current-drive efficiency increases above linear values for absorption of the rf on the low-field side of the cyclotron resonance layer, but decreases and even passes through zero for absorption on the high-field side. Thus, electron-cyclotron current drive by high-field launch will generally not be suitable.

A precursor to the $L$-to-$H$ transition is identified in magnetic fluctuations of the ASDEX $H$-mode discharges which are initiated without a sawtooth oscillation. This precursor is the $m=4\mathrm{}$, $n=1\mathrm{}$ mode which is uncoupled with the $m=1\mathrm{}$, $n=1\mathrm{}$ internal mode. This precursor appears also near the edge localized mode and $H$-to-$L$ transition. The behavior in time of the amplitude suggests that these transitions are governed not by the edge electron temperature, but by the edge current density.

We report on the first measurements of the tokamak-safety-factor profile by means of Alfvén waves. The waves were launched by use of the TCA tokamak Alfvén-wave heating antennae. The associated density oscillation profiles provided the Alfvén-wave resonance layer positions which depend on the local value of the safety factor, $q$. Our results show a time-averaged $q$ profile with a flat central region with $q$ close to unity.

We have used an ultrasonic technique to measure the superfluid density and the dissipation in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on porous Vycor glass. The superfluid transition is quite sharp at the lowest frequency (5 MHz) but broadens and shifts to higher temperatures as the frequency is increased (up to 196 MHz). In contrast to earlier torsional-oscillator measurements on this system, we observe a dissipation peak at the transition. We discuss the information which these results provide about the role of two-dimensional vortices in the superfluid transition in films adsorbed on porous substrates.

We suggest a unifying perspective of various manifolds subject to quenched correlated disorder. The new picture incorporates seemingly disparate problems as domain walls in impurity-stricken Ising magnets and directed walks upon random lattices. Using Flory-style arguments and a functional nonlinear renormalization group, we obtain new results, including the random-bond interfacial roughening exponent ${\zeta }_{\mathrm{RB}}=\frac{2}{9}\epsilon$, where $\epsilon =5-d$, which we believe to be exact, as well as a conjecture for the (2+1)-dimensional directed polymer index ${\zeta }_{2+1}=\frac{30}{49}$.

We study both multivalued discrete and continuum spin chains in a binary random field. The discrete spin model with $N$ values is mapped to a ($N-1\mathrm{}$)-dimensional dynamical system. The system shows a drastic dimensional reduction with the important consequence that the local magnetization is a fractal. For small field, for the Ising and the continuum spin-$O\left(n\right)\mathrm{}$ chains, the multifractal properties of the magnetization can be described by a universal $f\left(\alpha \right)$ curve.

Neutron-diffraction data were obtained from single-phase icosahedral powder of the system ${\mathrm{Al}}_{74}$${\mathrm{Si}}_5$${\mathrm{Mn}}_{21}$ and its modification by isomorphous substitution on the Mn sites. Amplitudes and phase differences of the partial structure factors ($F_{\mathrm{Al}}, F_{\mathrm{Mn}}$) were determined. From their $Q_{\perp }$ dependences, and within a strip-projection approach, phases were reconstructed. Atomic densities calculated in the physical 3D space show that Mackay icosahedra, the structural units usually invoked for quasicrystal models, do not emerge as the basic ingredients of the atomic arrangements.

An icosahedral quasiperiodic zero-temperature phase with lower enthalpy than in Bravais and close-packed crystal structures has been found for a system of particles interacting via a square-well potential. This remains true after small changes in the potential, pressure, and even after small changes within a class of icosahedral structures. Above a certain critical pressure, the icosahedral phase transforms into the bcc Bravais crystal structure.

Plasmons in small protrusions of an otherwise flat gold film were studied with a scanning near-field optical microscope (SNOM). Individual protrusions were selected by approaching them with another medium in a controlled way. Extremely narrow resonances in terms of scattering intensity versus distance were observed. The results open new opportunities for local plasmon studies as well as for SNOM with enhanced sensitivity and resolution, e.g., operation in a novel topographic mode (optical tunnel microscopy).

Electric-field-dependent measurements are reported in the spin-density-wave state of the organic conductor ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$N${\mathrm{O}}_3$. Nonlinearity appears in the longitudinal conductivity above a finite threshold field of about 40 mV/cm. No temperature dependence is detectable up to at least half of the transition temperature. The excess conductivity is smaller in samples with a lower resistance ratio. The sliding spin-density-wave mode might be responsible for the observed electric-field-dependent response.

A photoluminescence study of an (AlGa)As-GaAs quantum-well-wire array directly grown by molecular-beam epitaxy on a tilted substrate is described. A strong anisotropy was observed in the ratio of the electron-light-hole-exciton peak intensity to the electron-heavy-hole-exciton peak intensity. A theory incorporating the optical selection rule for two-dimensional quantum confinement is found to agree very well with the measured data. These results constitute the first evidence of two-dimensional quantum confinement in artificial wire structures having cross-sectional dimensions in the nanometer range.

Major progress has been made in understanding the complicated magnetic structures exhibited by dhcp neodymium metal. New evidence for the 2-q model of the structure between 19.1 and 9 K has been discovered in neutron-diffraction studies of intermodulation harmonics of the type $2{\mathrm{q}}_i\pm {\mathrm{q}}_j$. We have applied the same technique at 4.5 K and below. From an analysis of the harmonic combinations observed, we propose a new model for the lowest-temperature phase of Nd; it is characterized by a multidomain 4-q structure in which the magnitude and direction of each component wave vector is different.

The approximate mapping of the antiferromagnetic Heisenberg chain onto a (1+1)-dimensional quantum field theory indicates that the ground state is disordered with a finite gap for integer spin. We study a lattice of coupled (1+1)-dimensional field theories as a model for weakly coupled spin chains. This model offers a possible resolution to the mystery of the upper mode in the low-temperature ordered phase of CsNi${\mathrm{Cl}}_3$, whose polarization is inconsistent with spin-wave theory.

We report the observation of spin-polarized photoelectron diffraction for antiferromagnetic MnO(001). Data at kinetic energies near 100 eV for the spin-split Mn $3s$ multiplet show abrupt changes in the ${}_{}{}^5{}_{}{}^{}S:{}_{}{}^7{}_{}{}^{}S$ intensity ratio at a temperature of 530±20 K that is 4.5 times the Néel temperature. An additional temperature-dependent study done at higher electron kinetic energies, along with azimuthal diffraction measurements, support the conclusion that this abrupt change is due to a new type of high-temperature short-range-magnetic-order transition.

Chaotic diffusion in periodic Hamiltonian maps is studied by the introduction of a sequence of Markov models of transport based on the partition of phase space into resonances. The transition probabilities are given by turnstile overlap areas. The master equation has a Bloch band spectrum. A general exact expression for the diffusion coefficient $D$ is derived. The behavior of $D$ is examined for the sawtooth map. We find a critical scaling law for $D$, extending a result of Cary and Meiss. The critical scaling emerges as a collective effect of many resonances, in contrast with the standard map.