A quantum-mechanical Hamiltonian formulation of lattice spin systems is used to search for phase transitions in the O(2), O(3), and O(4) Heisenberg models in two dimensions. Strong-coupling expansions which have been calculated through eighth order indicate a phase transition at nonzero coupling for the O(2) model, but the non-Abelian models are predicted to exist only in the strong-coupling phase.

High-statistics measurements of the absolute differential cross section for $n-p$ scattering have been made over neutron c.m.-system scattering angles $9.5°<{\theta }^{*}<64.5\mathrm{}°$. The statistical error is 1.7 to 3.3% for 2°-wide angular bins, and the systematic error is 2.7 to 3.3%. The cross section is fitted by $\frac{d\sigma }{d{\Omega }^{*}}=A \exp \mathrm{}\left(\mathrm{bt}\right)\mathrm{}$, with $A=10.27\mathrm{}\pm 0.36$ mb/sr, $b=5.00\mathrm{}\pm 0.05$, and $0.01<-t<\mathrm{}0.39\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. For the ratio of the real to the imaginary part of the forward-scattering amplitude we obtained ${\alpha }_n>~-0.43\mathrm{}\pm 0.04$, consistent with other less precise determinations of ${\alpha }_n$.

Vacuum polarization at long distances for confined heavy-quark-antiquark ($Q\overline{Q}$) pairs is considered. The vacuum-polarization-corrected static potential is shown to have a radial dependence which should allow interpolation between charmonium, upsilon states, and other heavy $Q\overline{Q}$ systems. It is argued that the static, confining potential cannot grow faster than a linear potential at large distances, within the framework of this analysis.

The magnetic moment of the ${\Lambda }^0$ hyperon has been measured to be ${\mu }_{\Lambda }=(-0.6138\mathrm{}\pm 0.0047){\mu }_N$.

An attempt is made to understand the characteristic features of angular and energy distributions observed in high-energy hadron-nucleus and nucleus-nucleus reactions in terms of a simple physical picture. Questions in connection with the "slowly moving source," in particular, the possible excistence of velocity-dependent frictional forces between the constituents of interacting hadrons, are discussed.

$\gamma$-ray multiplicities as a function of energy dissipation and mass asymmetry have been calculated from a diffusion model which simultaneously treats energy dissipation and angular momentum transfer on the basis of particle transfer. The good agreement obtained between the model calculations and the experimental multiplicities for ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$-induced reactions on ${}_{}{}^{107,109}{}_{}{}^{}\mathrm{Ag}$, ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ targets lends credence to the one-body dissipation mechanism and $l$ fraction along the mass-asymmetry coordinate.

The thick-target-thick-catcher technique has been used to determine average kinetic properties of a number of target-fragmentation products formed in the reactions of 25-GeV (2.1-GeV/nucleon) ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ions with Au. The forward momentum transferred to the target as a function of product mass is essentially identical to that for 28-GeV protons, but quite different than that for 1.0- and 3.0-GeV protons, suggesting that the total projectile kinetic energy is the significant parameter governing limiting fragmentation rather than the energy per nucleon.

For sodium atoms in an atomic beam, we demonstrate focusing, defocusing, and steering caused by the transverse dipole forces exerted by the radial intensity gradient of a superimposed and co-propagating resonant cw light beam. Dipole radiation-pressure forces differ from the forces due to spontaneous emission and are needed to achieve optical traps for neutral atoms.

We consider the extension of the complex-coordinate technique to the problem of locating molecular resonances. We suggest a method which uses complex normalizable functions and which becomes equivalent to the usual dilatation transformation asymptotically, but is different for small values of the electronic coordinates. The technique is illustrated by application to the bound states of ${\mathrm{H}}_2^{+}$ and to a model nonspherical resonance problem.

A free-electron laser using an intense relativistic electron beam in a rippled magnetic field has been operated in the stimulated-Raman-scattering regime to yield megawatt-level radiation at submillimeter wavelengths; line narrowing to $\frac{\Delta \lambda }{\lambda }\sim 2\%$ is reported.

A new method of nonlinear spectroscopy aiming to probe solids in the highly dispersive and absorbing region is presented. It requires only one laser and uses wave-vector rather than frequency discrimination of the nonlinear signal from the input beams. Application is made to the study of the ${\Gamma }_1$ biexciton level in CuCl.

Analytical and numerical studies of collisionless tearing modes that are wider than the "current channel" are presented. The $m=1\mathrm{}$ type of inertial mode is shown to be strongly unstable for typical tokamak shear and ${\beta }_e$. Large spatial extension and large growth rate make it a possible candidate for explaining plasma disruption.

Results are reported of a molecular-dynamics simulation of a semiclassical two-component plasma in the strong-coupling regime. Static and dynamic properties are discussed, with emphasis laid on the collective motion and its description in terms of the spectra of charge fluctuations and the frequency-dependent dielectric function.

The inward trasport of a neon test impurity injected into the plasma in the Impurity Study Experiment (ISX) tokamak has been significantly reduced by poloidally asymmetric injection of hydrogen gas into the discharge. The result is consistent with the impurity-flow-reversal effect as predicted by neoclassical transport theory.

We propose a mechanism for the major disruption in tokamaks that involves the nonlinear destabilization of tearing modes by the $\frac{\mathrm{}(m=2\mathrm{})\mathrm{}}{\mathrm{}(n=1\mathrm{})\mathrm{}}$ tearing mode, where $m$ and $n$ denote the poloidal and toroidal mode numbers, respectively. The magnetic islands generated can extend across the plasma cross section. For resistivities of the order of magnitude of these in TOSCA and LT-3, the time scale for their appearance is consistent with the time for the major disruption.

It is shown that in monolayer superfluid ${\mathrm{He}}^4$ films, in addition to third-sound modes, small amplitude effects can lead to the existence of gapless solitons made up of superfluid condensate. These nonlinear excitations can be created by localized perturbations in the superfluid density. The conditions are studied under which such initial disturbance evolves into an ordered string of solitons, and the differences to be expected in thicker films are discussed.

Isolated single-layer edge dislocations in smectic liquid crystals are observed by polarization microscopy. The modification of the smectic-$A$—smectic-$C$ transition temperature by the strain field of the dislocations is used to make them visible. Observations of periodic arrays in thin samples, by various polarization-contrast mechanisms, and the measurement of the Burgers vector confirm the nature of the defects observed.

We report on new information obtained on the nature of the nonregistered Si layers at the Si-Si${\mathrm{O}}_2$ interface using MeV backscattering-channeling techniques on thin Si single crystals. Analysis of the scattering intensity from the front and back surfaces of the crystal allows us to conclude that the atoms in the nonregistered monolayers have a displacement from the "bulk" position between 0.1 and 1.0 Å.

We have performed Monte Carlo simulations for correlation functions in various solid-on-solid models, some of which are equivalent to the $F$ model, the two-dimensional Coulomb gas, and the planar model. Our results for the $F$ model can be quantitatively represented using the theory of Kosterlitz and Thouless. We use this theory to help determine the transition temperatures in other systems.

We have carried out neutron inelastic scattering measurements on a Cu-8-at.%-Mn alloy employing different energy resolutions varying over two orders of magnitude. The temperature dependence of the elastic magnetic scattering cross section which is related to the Edwards-Anderson order parameter is found to vary systematically with the energy resolution employed to separate the elastic scattering from the inelastic/quasielastic contributions and hence on the effective time constant of measurement.