The authors construct and analyze a lattice field theory of two $Z_2$ gauge fields which interact in a minimal gauge-invariant fashion. Although the theory presented here, a generalization of the two-dimensional Ashkin-Teller spin system, has no formal continuum limit, it is found that it has an electrodynamicslike phase similar to that observed in general $Z_N$ theories for $N>~4$. This model is probably the simplest generalization of the conventional $Z_2$ pure gauge theory which has a massless phase separated from the strong- and weak-coupling regions by lines of second-order phase transitions.

The deep-inelastic electromagnetic structure functions of deuterium and aluminum nuclei have been measured. The kinematic dependence of the ratio of aluminum and deuterium structure functions is similar to the dependence of the ratio of steel and deuterium structure functions, and provides further evidence for the distortion of the quark momentum distributions of nucleons bound in a nucleus.

A new rotationally invariant Hamiltonian method, formulated on a four-dimensional hyperspherical surface, is proposed for the numerical study of quantum gauge field models. It is shown that in the Coulomb gauge it is sufficient, as well as necessary, to restrict transverse potentials to the zero-free domain of the Faddeev-Popov determinant. Numerical studies of an SU(2) model support Gribov's suggestion that the zeros provide a natural way to understand the origin of confinement.

The average internal-conversion probabilities as a function of spin for continuum $\gamma$ transitions are deduced from the energy dependence of the evaporation-residue atomic-charge-state distributions. Strongly enhanced internal-conversion probabilities are observed at high spin. The implications of these results for nuclear shapes at high spin are considered.

A variational method for fitting the Hartree-Fock exchange potential is investigated. The method has properties similar to an earlier method that successfully treats the direct part of the electron-electron Coulomb interaction in density-functional calculations. The present method is tested on atomic neon and should permit reliable linear combination of atomic orbital Hartree-Fock molecular calculations that grow in complexity only as the cube of the number of basis orbitals.

The spontaneous-emission spectrum of an atom in an ideal cavity is calculated.

The first high-resolution measurement of the ${\nu }_3$ vibration (doubly degenerate asymmetric stretch) of the hydronium ion (${\mathrm{H}}_3$${\mathrm{O}}^{+}$) is reported. An analysis of sixty transitions in the symmetric and antisymmetric inversion states yields effective band origins of 3530.165(55) and 3513.840(47) ${\mathrm{cm}}^{-1\mathrm{}}$, respectively.

Recently, large discrepancies have appeared between theoretical predictions and experimental measurements of dielectronic recombination cross sections (${\sigma }^{\mathrm{DR}}$) for ${\mathrm{Mg}}^{1+}$ targets. This disagreement has provoked new work aimed at understanding, more fully, the effect of applied fields upon the process. This Letter reports calculations of ${\sigma }^{\mathrm{DR}}$ for ${\mathrm{Mg}}^{1+}$ in a small applied electric field. The field effect is large enough to bring the experimental ${\sigma }^{\mathrm{DR}}$ into much closer agreement with the theory.

Two-round-trip-time modulation of the cavity field has been observed in a passive ring resonator, containing an ammonia cell, pumped by smooth 100-ns C${\mathrm{O}}_2$ laser pulses. The results are in excellent accord with the theory of Ikeda instability instability in a two-level system, which is generalized to include reservoir kinetics.

An experimental study of a two-stage millimeter-wave source in which the same intense relativistic electron beam first produces powerful (500-MW) radiation at 12.5 GHz and then uses that radiation as a "pump" for a free-electron-laser interaction at frequency $f>\mathrm{}140\mathrm{}$ GHz is described. Implication for two-stage free-electron-laser experiments with reduced electron energy requirements are discussed.

A double-shell-target implosion was studied with the GEKKO IV glass-laser system. The inner-shell trajectory, recorded for the first time by a newly developed two-frame x-ray shadowgraphy technique, was analyzed by a self-similar flow model, implying more than 20% outer-inner kinetic-energy conversion. The trajectory also agreed with a one-dimensional code result including a vacuum insulation effect. A two-dimensional particle-in-cell simulation analyzed the illumination nonuniformity printed on the inner shell.

For one-dimensional three-wave parametric instabilities with oppositely directed group velocities for the decay waves in unbounded plasmas, it is known that for uniform coupling amplitude and constant wave-vector mismatch ($\frac{d\Sigma k_i}{\mathrm{dx}}=\mathrm{const}$) there cannot be an absolute instability. It is shown that this is an exceptional case, and that in general an absolute instability can be obtained in an inhomogeneous plasma with a sufficiently strong pump. A simple criterion is given for the threshold of the absolute instability.

High-resolution time-of-flight measurements of 17-meV He atoms scattered from an Ag(111) surface along the $〈112〉$ and $〈110〉$ azimuths are reported. Most of the spectra reveal two sharp peaks and from their locations dispersion curves have been determined out to the zone boundary. The agreement of the lower-frequency mode with the theoretical Rayleigh dispersion curve is excellent. The other mode is significantly lower (35%) than the calculated longitudinally polarized surface mode.

Low-energy electron diffraction and thermodynamic measurements are reported for molecular nitrogen weakly chemisorbed on a Ni(110) surface in the temperature region between 87 and 150 K. A commensurate (2×1) phase transforms with increasing coverage into a sequence of smoothly varying incommensurate solid structures via a liquid-like intermediate phase. This is apparently the first experimental example of the commensurate-fluid-incommensurate phase sequence in uniaxial systems predicted by several recent theories.

At high coverages and low temperatures oxygen on exfoliated graphite takes the form of a bilayer film plus bulk solid clusters. A transition at 47 K is interpreted as prewetting in which a thick film is formed. Evidence for this transition is seen in heat-capacity and magnetic-susceptibility measurements.

Experiments are described in which superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ is cooled to sufficiently low temperatures that the normal-fluid fraction becomes so small ($\frac{{\rho }_n}{\rho }<{10}^{-6\mathrm{}}$) that the quasiparticle properties become undetectable. The liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is then effectively a thermal vacuum. In this regime nonlinear effects are observed in the behavior of a vibrating wire, effects which are not observed in an actual vacuum and which are evidently associated with the mechanical response of the $B$-phase superfluid texture.

The magnitude of the plasma-resonance contribution to surface-enhanced Raman scattering on silver-island films has been measured with use of an experimental arrangement in which the resonance can be suppressed. The measured reduction in Raman scattering is ∼ 300, but theoretical analysis indicates that there may remain an additional unsuppressed factor ≲3. The data show that plasma resonances are not essential for observation of Raman scattering by molecular monolayers using standard detection techniques.

This Letter presents a general approach to calculation of the quasiparticle excitation energies of semiconductors which includes the energy dependence of the self-energy with a local density-functional approach. Both direct and indirect band gaps in Si are in much better agreement with experiment than are the bands from the ground-state theory. The relatively large corrections arise only after the dielectric screening of the electron gas is modified to account for a gap in the excitation spectrum.

Pressure-induced superconductivity in Eu${\mathrm{Mo}}_6$${\mathrm{S}}_8$ is sensitive to the systematic addition of oxygen. As oxygen content increases, the critical pressure above which the sample is superconducting ($P\simeq 11$ kbar) decreases, the onset is less abrupt, the maximum $T_c$ (11.8 K) decreases, and $\frac{d{T}_c}{\mathrm{dP}}$ at high pressures (-0.29 K/kbar) is less negative. The $\frac{c}{a}$ ratio and the unit cell volume vary linearly with the extrapolated zero pressure $T_c$. The results are discussed in terms of an oxygen-induced defect at the Eu site.

With use of spherical geometry, a translationally invariant version of Laughlin's proposed "incompressible quantum fluid" state of the two-dimensional electron gas is formulated, and extended to a hierarchy of continued-fraction Landau-level filling factors $\nu$. Observed anomalies at $\nu =\frac{2}{5},\frac{2}{7}$ are explained by fluids deriving from a $\nu =\frac{1}{3}$ parent.