A recent result in continuum Yang-Mills theories that exposes the "essential instability" of the perturbative ground state is shown to be reproduced by a Monte Carlo SU(2) lattice calculation. A discussion is given of the adverse consequences of our findings upon the validity of asymptotic freedom and the reliability of the lattice strategy for computation of physics of continuum gauge theories.

We present a formulation for the lattice-fermion derivative which is both local and explicitly chiral. Provided that the continuum and infinite-volume limits are taken properly, the formulation avoids spectrum doubling, and gives a satisfactory weak-coupling perturbation theory for the case of Yukawa-coupled scalars. For gauge theories we find that a sum over long string paths must be introduced in order to maintain the correct weak-coupling perturbation theory in the continuum limit.

We present evidence from numerical simulations for a first-order chiral-symmetry-restoration transition in QCD at finite temperature. The transition appears only for small quark masses. We use an exact algorithm to incorporate the dynamical quarks.

A simple proof of the nonrenormalization theorems in string theory is given. We prove (with the assumption that space-time supersymmetry is nonanomalous) that the space-time superpotential is not renormalized by string loops. As corollaries of this result we argue that in string perturbation theory, Calabi-Yau vacua (or any other classical solution with space-time supersymmetry) are solutions of the quantum equations of motion with zero cosmological constant, massless particles remain massless, and Yukawa couplings are not renormalized, to all orders.

We have observed a signal of 55 isolated $\gamma$-conversion pairs produced in a wide-band neutrino beam using the Fermilab 15-ft bubble chamber filled with a heavy Ne-${\mathrm{H}}_2$ mixture. The signal is consistent with coherent neutral-current single-${\pi }^0$ production followed by decay of the ${\pi }^0$ with one of the decay $\gamma \text{'}\mathrm{s}$ being lost. This signal corresponds to a rate of (2.0 ± 0.4) × ${10}^{-4\mathrm{}}$ of the total ${\nu }_{\mu }$ charged-current cross section at our average $E_{\nu }$ of 20 GeV. From this result, we obtain a value of $\beta =0.98\mathrm{}\pm 0.24$ for the weak-neutral-current isovector axial-vector coupling.

By use of the electromagnetic form factors for ${\Xi }^{-}$, ${\Sigma }^{-}$, ${\Sigma }^{+}$, and ${\Omega }^{-}$ computed recently from lattice quantum chromodynamics, the Chou-Yang model has been applied to explain and/or predict the differential cross section and other characteristics of ${\Xi }^{-}p$, ${\Sigma }^{\pm }p$, and ${\Omega }^{-}p$ elastic reactions.

The first coincidence measurements of the (${\pi }^{+},{\pi }^{0}p$) reaction are reported. These were performed on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ at $T_{{\pi }^{+}}=245\mathrm{}$ MeV for ${\theta }_{{\pi }^0}=50\mathrm{}°-130°$. Events corresponding to removal of $p$-shell nucleons were identified by their ${\pi }^0$ and $p$ energies. The ${\pi }^0$ energy spectra for such events are compared to predictions of $\Delta$-hole model calculations; the calculations generally underestimate the (${\pi }^{+},{\pi }^{0}p$) cross sections. At forward $\pi$ angles experimental ratios of the (${\pi }^{+},{\pi }^{+}p$) and (${\pi }^{+},{\pi }^{0}p$) reactions are shown to differ significantly from estimates based on free $\pi -N$ cross sections.

Nuclear magnetic resonance of on-line-oriented nuclei is reported for the first time, in the cases of mass-separated ${}_{}{}^{106}{}_{}{}^{}\mathrm{In}$, ${}_{}{}^{108}{}_{}{}^{}\mathrm{In}$, and ${}_{}{}^{104}{}_{}{}^{}\mathrm{Ag}$. Combination of the extracted $g$ factors with independent information on the magnetic-moment values leads to unambiguous spin assignment for the high-spin ground states of ${}_{}{}^{106}{}_{}{}^{}\mathrm{In}$ and ${}_{}{}^{108}{}_{}{}^{}\mathrm{In}$. The high-precision magnetic moments derived for these nuclei as well as the result on ${}_{}{}^{104}{}_{}{}^{}\mathrm{Ag}$ point to strong discrepancies from the additivity relation.

A method of obtaining very large nuclear-spin polarizations is proposed and illustrated by density-operator calculations. The prediction is that chemical reaction and rf irradiation can convert the scalar parahydrogen state into polarization of order unity on the nuclear spins of the products of molecular-hydrogen addition reactions. A means of extending the resultant sensitivity enhancement to other spins is proposed in which the transfer of order occurs through population differences not associated with magnetization.

We present the first numerical results of a multichannel theory for resonant charge exchange in ion-surface scattering at grazing incidence. The time-dependent Schrödinger equation describing resonant exchange can be solved in the adiabatic approximation for the slow perpendicular motion. As an initial application the state dependence of the distance of formation in the $\mathrm{H}\mathrm{}(n=2\mathrm{})\mathrm{}$ following specular reflection of protons at a gold surface is determined.

We present new theoretical results on the carbon-carbon triple bond which suggests that such bonds may be better described in terms of "bent bonds" than by the traditional combination of $\sigma$ and $\pi$ bonds. By use of correlated wave functions such behavior is explicitly demonstrated for difluoroacetylene. These represent the first quantitative calculations which document a case where a triple "bent-bond" description is energetically favored over a $\sigma -\pi$ description.

Different crises, i.e., abrupt qualitative changes in the properties of attractors, have been observed in a C${\mathrm{O}}_2$ laser with internal modulation. They are shown to be related to crossing between a stable or unstable periodic cycle and the strange attractor resulting from the period-doubling cascade observed at low modulation levels. Depending on the operating conditions, boundary and interior crises have been observed.

Large phase shifts that arise during the quantum initiation of stimulated Raman scattering are shown to induce solitons in the pump intensity. These phase shifts are associated with phase waves, which were previously discovered in quantum superfluorescence theory.

Positron lifetime spectra in helium gas, subjected to a uniform dc electric field, are calculated by a Monte Carlo simulation procedure which is based on an earlier scheme developed by Farazdel and Epstein. These calculations clearly show that introduction of a positron mobility edge $E_c^{}{}_{}{}^{+}$ alone produces a striking improvement in the agreement between theory and experiment. By visual inspection it is estimated that $E_c^{}{}_{}{}^{+}={10}_{-2\mathrm{}}^{+5\mathrm{}}$ meV at an electric field of 52 V/cm.

Magnetized electron transport under local heating by laser light is shown to lead to bifurcation: Two solutions are possible, a hot-spot solution and a weak-field solution. In the hot-spot solution strong magnetic fields are generated inhibiting thermal conduction. However, because of the Nernst effect the size of the hot spot is much larger than the laser beam size. In the weak-field solution heat conduction smooths the hot spot. This bifurcation takes place above a threshold laser intensity.

We present investigations of the spreading behavior of nonvolatile liquids (high-molecular-weight polydimethylsiloxane) on smooth horizontal surfaces, and establish the following: (1) the existence of a thin precursor film extending progressively ahead of the macroscopic front of the drops, and that we directly visualize by polarized reflection microscopy; (2) the universality of the spreading kinetics, followed by means of simultaneous size and contact-angle measurements, which appears insensitive to the spreading parameter $S$. Both points give strong experimental support to recent theoretical developments.

The orientation dependence of the nonequilibrium partition coefficient of Bi in Si at constant solid-liquid interface velocity has been measured. The partition coefficient, measured with pulsed-laser melting techniques on a series of Si wafers cut at 5° increments from (110) through (111) to (001), is sharply peaked at (111) and decreases monotonically with increasing inclination from (111). The results suggest that crystal growth and solute trapping occur by the lateral motion of {111} ledges.

A model for describing the dyanmics of glassy systems is presented. The model incorporates the effects of frustration caused by the imperfect packing of icosahedral clusters in flat three-dimensional space and the presence of non-Abelian defect lines. The model is studied by use of Monte Carlo simulations. The addition of uniform frustration causes the dynamics to be glassy.

The first direct observation of the melting transition of a uniformly flat (20-Å-thick) Pb film, sandwiched between two amorphous Ge films, is reported. The transition was observed by transmission electron microscopy with dark-field imaging. The melting transition occurred over a broad temperature range, and was continuous and reversible. It is concluded that the transition proceeds by a disordering mechanism rather than the formation of a liquid-solid interface. The transition was not homogeneous but proceeded at a rate of disordering that was greater from the Pb-Pb grain boundaries than from the Pb-Ge interfacial region.

Using atom-probe spectroscopy we measured the activation energy for desorption and the position of He on W for fields between 3 and 5 V/Å. A tight-binding cluster model with local fields from jellium calculations reproduces the data and gives the image plane as 0.8 Å above the topmost lattice plane. The desorption energy increases by a factor of 20 and the equilibrium position reduces from 3.3 Å in $F=0\mathrm{} \mathrm{to} \sim 2$ Å in $F>\mathrm{}3\mathrm{}$ V/Å. This is interpreted as a transition from physisorption to weak field-induced chemisorption.