We have investigated persistent flow of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ with an ac gyroscope filled with 20-μm powder. In ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$, currents circulate undiminished for 48 h at least; this implies a viscosity 12 orders of magnitude lower than in the normal fluid. In ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$A$, the current does not persist. The observed critical velocity in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ at $P<~12$ bars is independent of temperature. At $P>\mathrm{}12\mathrm{}$ bars there are two regimes in the $B$ phase: For example, at 29.3 bars the ultimate critical velocities are 5.4 and 7.8 mm/s, respectively.

Hydrogen shows no measurable backscattering for photoelectrons in extended x-ray absorption fine structure. When a H atom is located between an absorber and a backscatterer, however, it affects the photoelectron wave in magnitude and phase. This is demonstrated on Ni and Cr hydrides. The radial and angular information of the hydrogen position provided by this effect has a wide range of applicabilities such as the structure of metal-hydrogen systems, the trapping of hydrogen by impurities, and the determination of hydrogen bonds in molecules.

An approximate analytic solution to the scattering problem of atoms by a weakly corrugated Morse potential is presented. In the classical limit the resulting scattering amplitude coincides with that given by the Kirchhoff approximation for the corrugated hard-wall model with a corrugation function identical to that of the Morse potential. Quantum mechanical effects appear as deviations from the Kirchhoff formula. For a general potential it is shown that it is the potential corrugation near vanishing potential which dominates the scattering process and not that at the classical turning point.

The authors have used NMR to determine the structure of ${\mathrm{C}}_2$${\mathrm{H}}_2$ adsorbed at room temperature on small platinum particles by probing the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$-${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$-${}_{}{}^1{}_{}{}^{}\mathrm{H}$, and ${}_{}{}^1{}_{}{}^{}\mathrm{H}$-${}_{}{}^1{}_{}{}^{}\mathrm{H}$ dipolar interactions among the nuclei in the adsorbed molecules. They find a model of (77 ± 7)% CC${\mathrm{H}}_2$ and (23 ± 7)% HCCH. The C-C bond length of the CC${\mathrm{H}}_2$ is 1.44 ± 0.02 Å, midway between a single and double bond, suggesting that both C atoms bond to the surface.

We present direct measurements of the optical absorption in undoped and ammoniatreated polyacetylene utilizing the technique of photothermal deflection spectroscopy. Our spectra show an Urbach-type edge and a broad, N${\mathrm{H}}_3$-sensitive sub-band-gap absorption. Quantitative comparisons of absorption and spin-resonance results do not show the predicted midgap absorption from neutral solitons.

A novel technique, based on Grassmann path integrals, is shown to provide a powerful new tool in the theory of phase transitions in two-dimensional Ising-spin models. Accurate phase boundaries as well as exact critical singularities are produced for two representative models of disorder: the dilute ferromagnet and the $\pm J$ spin-glass.

Depolarization of muonium due to superhyperfine interaction, which until now has severely impeded the study of muonium in solids, can be quenched in a high transverse magnetic field (1.2 T). We report the observation of muonium precession in single crystals of KCl, Ca${\mathrm{F}}_2$, GaP, and ${\mathrm{Al}}_2$${\mathrm{O}}_3$. The high-field frequencies are of order 2 GHz and provide a direct and accurate (60 ppm) measure of the hyperfine interval. Several novel features of the muonium centers are observed.

An internal photoemission experiment which yields the energy-dependent momentum-and energy-loss rates of hot electrons is designed. The results on solid hydrocarbon films are analyzed in terms of the appropriate Boltzmann equation. Band mobilities are obtained for hot electrons. Their energy-loss rate has a maximum at the LO phonon energy associated with the C-H stretching mode.

The in situ electron-spin resonance of a voltage-biased NbAuNb Josephson junction is reported. The Au barrier is doped with Gd or ${}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$ ions. Sharp resonances appear in the $I-V$ curves at frequencies equivalent to the crystal-field splitting of $\mathrm{Au}\mathrm{Gd}$ (1.0 and 1.7 GHz) and to the hyperfine splitting of $\mathrm{Au}{}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$ (2.87 GHz). The principle of this new type of ESR-Josephson-junction spectrometer, as well as its application, is discussed.

It is shown that in order to determine all reaction amplitudes for a reaction containing particles with arbitrary spins without even a discrete ambiguity (except for an overall phase factor), one must measure at least one observable with spin polarization along each of three noncoplanar directions. The result has an impact on experimental programs measuring polarization quantities in elementary particle, nuclear, atomic, and molecular physics.

The problem of the early-time decay rate of a model system is addressed by examining the evolution of the probability current at a detector located a macroscopic distance from the source. If one interprets this current as the flux of decay products, early-time departures from Fermi's "golden rule" are not, in principle, observable.

The WKB cross section for backward glory scattering of a massless polarized wave propagating in curved space-time is given explicitly.

The Kohn-Sham equations are derived from the diagrammatic structure of many-body perturbation theory for the exact exchange-correlation energy, $E_{\mathrm{xc}}$, without explicit recourse to a minimum or extremal principle, thus providing new support for this implementation of density-functional theory in a most common situation. Furthermore, it is shown that for a large class of approximate expressions for $E_{\mathrm{xc}}$, there exist approximate Kohn-Sham equations which give the same density and energy as other means of calculation within the same diagrammatic approximation.

A general method for optimizing real-space renormalization-group transformations is presented. It is applicable to both spin systems and lattice gauge theories, with use of Monte Carlo computer simulations. It provides substantial improvement in the convergence of critical exponents for the $d=3\mathrm{}$ Ising model.

We report on a pure gauge SU(3) lattice theory computation performed on an array of 64 microprocessors interconnected as a $2^6$ hypercube and having the cumulative power of approximately eight V AX11/780's. The availability of a substantial number of computer cycles, coupled with an improvement in the algorithm, made possible a high-statistics determination of the heavy-quark potential on a ${12}^3$ × 16 lattice.

The results of a high-statistics calculation of the SU(3) heavy-quark potential are presented. The validity of scaling is tested quantitatively. New results are given for the string tension and the strength of the long-range Coulomb term in the potential.

In a search for heavy $Q=\frac{2}{3}e$ and $Q=\frac{1}{3}e$ particles produced in $e^{+}{e}^{-}$ collisions at a center-of-mass energy of 29 GeV, no candidate events were found in 77 ${\mathrm{pb}}^{-1\mathrm{}}$ of data collected by the time projection chamber at PEP. Upper limits are established on the inclusive cross section for the production of $Q=\frac{2}{3}e$ and $Q=\frac{1}{3}$ particles in the mass range 1-13 GeV/${\mathit{c}}^2$, improving upon previously established limits.

By supplementation of the standard SU(5) Lagrangrian with nonrenormalizable (dimension five) terms which are needed in any case to obtain acceptable masses for the first two fermion generations, the proton decay predictions can be neatly reconciled with the ongoing experiments. We present a class of such models in which the dominant decay mode of the proton is ${\mu }^{+}{K}^0$. A triplet of colored Higgs bosons of mass ∼ ${10}^{12}$ GeV, required for successfully implementing the inflationary scenario, mediate this and other decays.

The excitation functions and energy distribution of the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}+A\to {\pi }^0+X$ for beam energies of several tens of megaelectronvolts per nucleon are analyzed. Statistical-decay theory yields good agreement with the data under the assumption that a hot spot is formed. Its size is determined from the velocity of the emitting source. The cross section for forming a hot spot shows a systematic behavior which fits well with the systematics recently observed in fragmentation reactions.

We consider the nuclear mass-number ($A$) dependence of the nuclear structure functions. We restrict ourselves to a two-component model, in which we assume the existence of an exotic component other than the normal nucleon component in nuclei. We find that there exist scaling phenomena and that the structure function of the exotic component should be independent of $A$. We also show that the $A$ dependence of the nuclear structure function can be attributed to that of the surface-to-volume ratio which is incorporated in the probability of finding the exotic component in the nucleus.