A mean-field theory for the ordering of hydrogen molecules on a triangular net is derived in terms of the quadrupole coupling constant $\Gamma$, the crystal field $V_c$, and the temperature $T$. The phase diagram consists of six regions, separated by first- and second-order transitions. For almost all values of $\frac{V_c}{\Gamma }$ for which ordering occurs, the disordered to ordered phase transition is continuous. At $T=0\mathrm{}$ K, all phases have energy gaps except for a ferrorotational phase which behaves like an $\mathrm{XY}$ model.

Strong anomalies in the velocity and attenuation of longitudinal ultrasound in the megahertz range were observed for the first time in the vicinity of the nematic-smectic-$A$ phase transition in a liquid crystal (terephthal-bis-$p-p^{\prime }$-butylaniline). Results are in strong contradiction with theoretical predictions and are qualitatively consistent with a picture involving both critical fluctuations and relaxation of the smectic order parameter, similar to that observed near the $\lambda$ transition in liquid helium.

The diffraction model is generalized to incorporate a postulate on the electron-phonon interaction; viz., phonons with wavelength exceeding the electron mean free path are ineffective electron scatterers. This postulate leads to a limiting resistivity when the electron mean free path is of the order of the interatomic spacing. The generalized diffraction model is also shown to explain many anomalous features common to high-resistivity metals. Crystalline Nb, $A15\text{'}\mathrm{s}$, and glassy metals are discussed.

de Haas-van Alphen (dHvA) oscillations have been observed in ${\mathrm{Nb}}_3$Sn in the (110) plane and in ${\mathrm{V}}_3$Si at two orientations using single crystals of high resistance ratio and magnetic fields in excess of $H_{c2\mathrm{}}$. The dHvA frequency data for ${\mathrm{Nb}}_3$Sn can be interpreted as a series of nested ellipsoids centered at $M$, as suggested by a recent high-precision calculation of the ${\mathrm{Nb}}_3$Sn Fermi surface.

A self-consistent pseudopotential calculation for an unrelaxed Mo(001) surface reveals strong surface states and resonances. The results are used to interpret the sharp peak structure near the Fermi level and other structures measured by recent angle-resolved photoemission experiments. The structure is attributed uniquely to surface states and resonances. Contrary to other proposals, relativistic, many-body, and surface-contraction effects are not necessary to explain the observed spectra.

Energy-dependent conductivity is studied in the Anderson model of a disordered system by a direct simulation of particle diffusion on a two-dimensional square lattice. In the vicinity of the mobility edges, the results indicate a continuous variation of conductivity, consistent with classical percolation theory. Critical behavior of the participation ratio in the extended regime, which follows from this analysis, also supports this interpretation.

A systematic quantitative study of local-field effects in the plasmon line shape of semiconductors of the diamond (Si, Ge) and zinc-blende (GaP, GaAs, GaSb, InAs, InSb) structures is presented and is contrasted with the behavior in Al and In. Because $\frac{V_G}{\hslash {\omega }_p}\ll 1$, the effects of the periodic potential on the dielectric properties can be accounted for by a perturbation expansion to second order in $\frac{V_G}{\hslash \omega }$. We find quantitative agreement with electron-energy-loss experiments when local-field corrections are included.

It is shown that quantum mechanical barrier penetration (tunneling) is related to classical escape over a related potential barrier, and that therefore the WKB approximation for the former and Kramers approximation for the latter are equivalent.

We propose a gauge theory of weak and electromagnetic interactions based on SU(3) ⊗ U(1). The muonic and electronic doublets are classified differently. As a consequence, we get the same predictions of the Weinberg-Salam SU(2) ⊗ U(1) theory for the ${\nu }_{{\mu }^{-}}\left({\overline{\nu }}_{{\mu }^{-}}\right)$ initiated processes and parity conservation in atoms. The muon number is exactly conserved. Predictions are made that will be tested in the future high-energy colliding-beam experiments.

Inelastic scattering to the lowest $2^{+}$, $3^{-}$, and $5^{-}$ states in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ was studied for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at $E_{\mathrm{lab}}\left({}_{}{}^{16}{}_{}{}^{}\mathrm{O}\right)=60\mathrm{}$ MeV. The angular distributions are compared to distorted-wave and coupled-channel Born-approximation predictions. The results show that the effect of the $3^{-}$ inelastic excitation on the elastic scattering wave function has to be treated explicitly in order to reproduce the angular distributions. This is particularly evident for the $5^{-}$ transition, despite the fact that direct coupling between the $3^{-}$ and $5^{-}$ states is negligible.

Inelastic ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ scattering experiments have been performed on ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ at $E_{{}_{}{}^{12}{}_{}{}^{}\mathrm{C}}=120\mathrm{}$ MeV, and on ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$, ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ at $E_{{}_{}{}^{14}{}_{}{}^{}\mathrm{N}}=161\mathrm{}$ MeV. Giant resonances are observed in all spectra. An angular distribution has been measured on a ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ target, for which distorted-wave Born-approximation analysis is presented.

Realistic breakup states with spin, based on Reid's nucleon-nucleon potential, are used to calculate the correction to elastic deuteron-nucleus scattering matrix elements $S$, to second order in the distorted-wave Born approximation. There are no free parameters. The breakup corrections move the values of $S$ obtained with a simple folding model towards those obtained by Goddard and Haeberli in optical-model fit to observed elastic cross-section and polarization data.

Excitation functions for the $0^{+}$, 6.049-MeV and $3^{-}$, 6.130-MeV states were measured separately at ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}\sim 15.0°$, in steps of 86 keV from $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=22.7-33.0\mathrm{}$ MeV. Enhancement of the cross section by a factor of 2-5 above the constant cross section is clearly observed at 23.60, 24.60, 28.40, and 32.20 MeV, for the lowest three of which the most probable total spins, $J^{\pi }$, are found to be ${14}^{+}$, ${14}^{+}$, and ${15}^{-}$, respectively. These resonance energies are quite different from those for the $3^{-}$, 6.130-MeV state observed at 25.2 and 29.8 MeV.

We report experimental measurements on a new type of vacuum-ultraviolet radiation source. It is shown that the maximum source brightness, within its narrow linewidth, is that of a blackbody at the temperature $T$ of a metastable storage level. The laser-induced emission at 569 Å from a He glow discharge corresponded to a metastable temperature of 22 700°K and was over 100 times brighter than the 584-Å He resonance line.

We report the observation of a reverse electron current excited when fast ions are trapped in a toroidal plasma. The induced electron current was measured in the Culham superconducting Levitron as a function of electron temperature and found to be larger than that predicted assuming a shifted Maxwellian electron distribution.

The resistive drift tearing mode is shown to be stabilized by the ion motion along magnetic field lines. The effects of an electron temperature gradient are included in the discussion of the results.

Geometric focusing of proton and deuteron beams extracted from pinched-beam diodes to current densities above 70 kA/${\mathrm{cm}}^2$ is demonstrated by a variety of experimental techniques. Time-dependent computer simulations of the electron and ion flow show good agreement with experiment.

Neutron-scattering behavior and other properties of the mercury chains in ${\mathrm{Hg}}_{3-\delta }\mathrm{As}{\mathrm{F}}_6$ are calculated from a simple microscopic model. The independent chains satisfy a quantum sine-Gordon equation. The theory accounts for existing observations and makes a number of testable predictions about properties of the disordered phase, the nature of the phase transition, evolution of long-range order, and the excitation spectrum at low temperatures.

A new electrochemical measurement applied to the Li/Ti${\mathrm{S}}_2$ cell reveals subtle changes in slope on the voltage-composition relationship. These anomalies occur at compositions where ordered structures among intercalated lithium are predicted from lattice-gas theories. ${\mathrm{Li}}_x\mathrm{Ti}{\mathrm{S}}_2$ appears to be a nearly ideal model for the study of composition- and temperature-dependent properties of the two-dimensional lattice gas.

Using angle-resolved photoemission and synchrotron radiation, we have determined the energy-versus-momentum valence-band dispersion relations for a Ni(111) crystal. The temperature-dependent ferromagnetic exchange splitting has been directly observed. Both the $d$-band width (∼3.4 eV at $L$) and exchange splitting (0.31 eV) are much smaller than theoretical estimates (∼4.5 eV wide at $L$ with ∼0.7-eV splitting, respectively, at 293 K).