We explain from an intuitive renormalization-group perspective how "collapse of the wave function" in quenched quantum electrodynamics leads to coupling-constant renormalization and an interacting ultraviolet stable fixed point. A diagrammatic expansion in $N_f$, the number of fermion species, suggests that vacuum polarization leads the fixed point of the quenched model stable, and computer-simulation data support this possibility. The scaling region of the quenched lattice model is discovered numerically.

We propose some new models with spontaneous lepton-number breaking where the neutrino masses and the couplings of the Goldstone boson (majoron) to charged fermions are naturally small because they arise in quantum loops. Thus, unlike other majoron models, we do not have to invoke any new scale in the model which is very different from the electroweak scale.

Local deviations from the Grodzins systematics for the product $E\left(2^{+}\right)B\left(E 2\right)$ are observed in several light Ra-Th nuclei. We note an abrupt increase to 5.5% of the isoscalar $E 2$ energy-weighted sum rule, almost a factor of 2 above the value typically found near shell closures and equal to the value found in the very deformed actinides. This indicates mixing of a low-lying deformed state into the ground state of the light Ra-Th nuclei, as expected for a low-lying cluster state. Evidence for mixing of a deformed state is also present in the rms radii and binding energies of these nuclei.

We have measured the Au $L$-shell ionization-cross-section ratio for incident electrons and positrons having kinetic energies $T\simeq 25-55$ keV. In this energy region, $\frac{\sigma _L^{}{}_{}{}^{-}}{\sigma _L^{}{}_{}{}^{+}}$ is observed to be smaller than unity, reflecting the fundamental difference between the Møller and Bhabha interactions for large energy transfers. We have modified Kolbenstvedt's inner-shell ionization description to include (i) positive particles $e^{+}$, (ii) $L$ shells, and (iii) Coulomb-deflection effects, and find reasonable agreement with the data.

We report the first observation of rf-induced transitions of trapped neutral atoms. An rf resonance curve for a sample of trapped Na atoms has been obtained by measuring the relative population of two trapped magnetic substates as a function of applied rf frequency. The shape of the resonance curve has been used to determine the energy distribution of the trapped atoms.

The angular distributions of electrons in multiphoton multichannel ionization of hydrogen for the case of elliptically polarized laser light are calculated within a nonperturbative theoretical model taking into account the Coulomb interaction in the final state. It is found that the ellipticity of the radiation not only modifies the shape but also lowers the fourfold rotational symmetry occurring in linear polarization to a twofold one.

We show that the use of scaled variables in data recording leads to a simple but powerful parametrization of complicated spectra. In the case of the atomic Stark effect the parametrization is complete for all experimentally resolved states and emphasizes the similarities between different atoms, e.g., sodium and hydrogen. Theoretically, the parametrization is linked, in a natural way, to the classical periodic orbits of the system. We found excellent agreement between measurements, calculated quantum spectra, and classical calculations.

New results for quantum limits to light amplifiers are derived which hold in both linear and nonlinear domains of operation. The relative advantages of squeezing the pump and idler modes of the amplifier are discussed.

We present evidence of soliton propagation by 185-fsec dark pulses at a wavelength of 0.62 μm in a 1.4-m length of single-mode optical fiber. Our experiments utilize specially shaped, antisymmetric input pulses, which closely correspond to the form of the fundamental dark soliton. At appropriate power levels the dark pulses propagate without broadening. Our measurements are in quantitative agreement with numerical solutions to the nonlinear Schrödinger equation and constitute the first clear observation of the fundamental dark soliton in optical fibers.

It is demonstrated that traveling waves lose stability with respect to standing waves under the influence of a spatially homogeneous temporal modulation in three different experimental systems. A phase diagram consistent with a recent theoretical prediction is presented.

A two-dimensional Fokker-Planck code SPARK has been constructed to study the process of nonlocal thermal smoothing in planar laser-produced plasmas with no ion motion. The results from SPARK are compared with those from a standard fluid code with flux-limited Spitzer-Härm conductivity. It is found that even when the one-dimensional transport is well modeled by fluid theory the thermal smoothing arising from nonuniform laser energy deposition may be severely overestimated for scale-length modulations shorter than about 80 electron mean free paths.

Recent improvements in the operation of the CTX spheromak device have produced discharges containing evidence for a pressure-driven instability. The instability leads to a distinct event in the discharge, which can be studied in detail. Data are presented which reasonably discount Taylor relaxation of the current profile as the cause of the event. The critical value of the normalized pressure gradient has been measured and is compared with the Mercier limit.

A density-functional theory, based on the smoothed density approximation, is presented in order to study phase transitions in systems of hard spherocylinders with full translational and orientational degrees of freedom. We find both the nematic-isotropic and the $\mathrm{n}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}-\mathrm{S}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-A$ phase transition in a wide range of length-to-width ratios $\frac{L}{D}$. The predictions of our theory are in good agreement with results of computer simulations and we recover Onsager's limit of $\frac{L}{D}\to \infty$. We also make some comments about application of the theory to systems of hard ellipsoids.

We present neutron-spin-echo data on the structural relaxation of glass-forming polybutadiene near the glass transition. Microscopic dynamics on the scale of an interchain distance and the monomeric friction coefficient derived from macroscopic viscosity relaxation obey the time-temperature superposition principle following a common scale. The dynamic structure factor exhibits strong similarities with recent mode-coupling predictions.

Loading a cantilevered bar allows studying the effect of a uniform, externally applied, and continuously variable strain field on the properties of surfaces. Straining a nominally flat Si(100) surface produces unequal populations of the 2 × 1 and 1 × 2 domains. The domain compressed along the dimer bond is favored. The effect depends on the strain and not the strain gradient. The effect saturates at a strain of 0.1% when 90% of the surface is in the favorable domain. The kinetics of developing and annealing away the asymmetry are identical and thermally activated.

The temperature and magnetic field dependences of the critical-current density of superconducting Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films have been determined from transport measurements across individual [001] tilt boundaries and within single [001] grains. The results provide evidence that the basal-plane $J_f$ is limited by flux creep. In contrast, the $I-V$ characteristics and the temperature magnetic field dependences of the grain-boundary $J_c$ are consistent with a superconductor-normal-superconductor-type weak-link behavior. These results imply that the transport properties of these materials can be treated within a BCS framework.

We present calculations of electron-positron momentum density in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ based on band theory in the local-density approximation. Theoretical predictions are in semiquantitative accord with the corresponding two-dimensional positron-annihilation angular-correlation measurements. These results indicate that the conventional local-density-approximation band-theory picture provides a reasonable description of the ground-state electronic structure and the Fermi surface of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Several Fermi-surface-related features are suggested in the positron spectra for the first time.

We have developed a general scheme for carrying out systematic perturbation expansions for groundstate properties of quantum lattice models. As an application, we study the onset of spontaneous Néel order in $S=\frac{1}{2}$ Heisenberg antiferromagnets by expanding around dimerized Hamiltonians. In one dimension (1D) we recover accurately the known exact results. On the square lattice we find novel critical points separating Néel ordered and disordered phases; the estimated critical exponents are consistent with those of the 3D classical Heisenberg model.

Photoemission spectra from potassium adsorbed on an Al{111} surface show a K $4s$-derived state just below the Fermi level, $E_F$, which is first observed just prior to the onset of ordering in the overlayer. The feature disperses upwards with increasing ${\mathbf{k}}_{\parallel }$ and crosses $E_F$ before reaching the Brillouin-zone boundary. Its coverage dependence is indicative of a transition from strongly ionic to metallic bonding.