For any Hamiltonian $H$ having vanishing Poisson brackets with the total momentum $\overrightarrow{\mathrm{P}}$ and angular momentum $\overrightarrow{\mathrm{J}}$, it is always possible to construct boost operators $\overrightarrow{\mathrm{K}}$ having the correct Poisson brackets with all the other generators of the Poincaré group. Thus the existence of dynamical variables satisfying the algebra of the Poincaré group generators is not in itself a guarantee of Lorentz invariance.

A wide class of exact solutions of the stationary Einstein-Maxwell equations characterized by a flat "background" three-space is obtained. The solutions can be interpreted as the external gravitational and electromagnetic fields of one or more spinning sources with unit specific charge in stationary configuration.

We report measurements of the ${\omega }^0$ spin-density matrix elements in the reaction ${\pi }^{-}p\to {\omega }^{0}n$ at incident momenta of 3.65, 4.5, and 5.5 $\frac{\mathrm{GeV}}{c}$. In the Gottfried-Jackson frame the data exhibit nonzero values of ${\rho }_{00}$, indicating the presence of unnatural-parity exchange as well as a large value of ${\rho }_{11}+{\rho }_{1-1\mathrm{}}$ which does not show a dip in the momentum transfer distribution. The unnatural-parity contribution decreases with respect to the natural-parity contribution as the momentum increases.

Dips are observed at $t\sim -0.45\mathrm{}$ Ge${\mathrm{V}}^2$ in the differential cross sections of the reactions ${\pi }^{\pm }p\to {\rho }^{\pm }p$ and a break at $t\sim -0.25\mathrm{}$ Ge${\mathrm{V}}^2$ in ${\pi }^{-}p\to {\rho }^{0}n$ at 6 $\frac{\mathrm{GeV}}{c}$. The $I=0\mathrm{}$ $\omega$-exchange contribution in the $t$ channel is isolated by calculating $R^2\mathrm{}\left(t\right)=\frac{1}{2}\left[\frac{d\sigma }{\mathrm{dt}}({\pi }^{-}p\to {\rho }^{-}p)+\frac{d\sigma }{\mathrm{dt}}({\pi }^{+}p\to {\rho }^{+}p)-\frac{d\sigma }{\mathrm{dt}}({\pi }^{-}p\to {\rho }^{0}n)\right].$ The function $R^2\mathrm{}\left(t\right)\mathrm{}$ vanishes at $t\sim -0.45\mathrm{}$ Ge${\mathrm{V}}^2$ and is well parametrized by a Bessel function of the first kind, ${J_1}^2\left(r\sqrt{-t}\right)$, with $r\sim 1$ F.

We have measured the branching ratio $\frac{({K_S}^0\to {\pi }^{+}{\pi }^{-})}{({K_S}^0\to {\pi }^0{\pi }^0)}$ to be 2.22 ± 0.095, using a total of ∼ 32 000 ${\overline{K}}^0$'s produced by $K^{-}$ charge exchange.

An analysis of the double differential cross section of the inclusive $\overline{p}p$ interaction is presented. A comparison of various forms of the distribution function has been made. The best fit is obtained using a Bose-type distribution with two parameters and ordinary phase space for the cross section.

The inclusion of SU(3) symmetry breaking in the $\mathrm{PVV}$ vertices and the use of the "simplest" Hamiltonian consistent with $\Delta I=\frac{1}{2}$ for nonleptonic $\Delta S=1\mathrm{}$ transitions bring the calculation of ${K_2}^0\to \gamma \gamma$ decay mode in agreement with experiment, vis-à-vis the disagreement of two orders of magnitude previously reported.

A previously proposed theory of leptonic weak and electromagnetic interactions is found to be free of the divergence difficulties present in conventional models. The experimental implications of this theory and its extension to hadrons are briefly discussed.

We report the generation of 0.3547-μm radiation by tripling 1.064-μm radiation in a phase-matched mixture of rubidium and xenon. We find $\chi \left(3\mathrm{}\omega \right)=1.4\mathrm{}\times {10}^{-32\mathrm{}}$ esu, which is about ${10}^6$ times greater than that of He. Phase matching occurs for Xe and Rb atoms in the ratio 412:1.

The current-driven ion-sound instability is investigated by two-dimensional computer experiments. It is found that, in contrast to the one-dimensional case, the instability produces an anomalous resistivity.

From the magnitude and angular dependence of the intensity of light scattered from xenon near its critical point we have measured the Ornstein-Zernike long-range correlation length $\xi$ and the reduced compressibility ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T$. We find $\xi =(3.0\mathrm{}\pm 0.10){\epsilon }^{-0.58\mathrm{}\pm 0.05}$ Å and ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T=(1.43\mathrm{}\pm 0.06)\times {10}^{-9\mathrm{}}{\epsilon }^{-1.21\mathrm{}\pm 0.03}$ ${\mathrm{g}}^2$/erg ${\mathrm{cm}}^3$ along the critical isochore, and ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T=(0.346\mathrm{}\pm 0.01)\times {10}^{-9\mathrm{}}{\epsilon }^{-1.21\mathrm{}\pm 0.02}$ ${\mathrm{g}}^2$/erg ${\mathrm{cm}}^3$ along the coexistence curve, with $\epsilon \equiv \frac{|T-T_c|}{T_c}$. Using our results, we have compared the predictions of the Kawasaki-Kadanoff-Swift mode-mode coupling theories with measurements of the thermal diffusivity in xenon.

The complete valence band in amorphous Si${\mathrm{O}}_2$ has been examined by photoelectron spectroscopy at photon energies of 21.2, 26.9, 40.8, and 1486.6 eV. The spectra show emission from an 11.2-eV-wide $p$-derived valence band and from the oxygen $2s$ level at 20.2 eV below the valence-band edge. Four pieces of structure in the $p$ bands are related to the single bonding and the two nonbonding orbitals of the ${\mathrm{O}}^{--}$ ion. A narrow, nonbonding level found at the valence-band edge may cause lattice trapping of valence-band holes.

Measurements of thermoreflectance have been made in the $E_0$ region of germanium at temperatures 17, 85, 204, and 291°K. Elliott's formulas for excitonic absorption with Lorentzian broadening have been used to obtain analytic expressions for the derivatives of the dielectric constant. These give a good fit to the spectra, resulting in values across the temperature range of 2.0±0.2 meV and 1.1-3.7 meV for the exciton's binding energy and the broadening parameter, respectively.

Oscillatory structure in the absorption of thin potassium films has been observed for energies just above the plasmon energy in a wavelength-modulated photoemission experiment. This structure is shown to be due to resonant excitation of longitudinal plasmons. Observation of up to ten resonances allows the evaluation of the dispersion relation, which shows a quadratic behavior in agreement with a theoretical formula based on the Boltzmann equation.

The behavior of sound waves coupled to cyclotron waves is analyzed. We show that a measurement of the dispersion properties of these coupled modes leads to an unambiguous determination of the as yet unknown Fermi-liquid parameters $A_0$ (compressibility) and $A_1$ (current).

The pressure-volume relationship in YbTe has been studied by high-pressure x-ray diffraction up to 300 kbar. This relationship is anomalous in the region 150-190 kbar indicating a continuous electronic collapse with pressure, in which the ${\mathrm{Yb}}^{2+}$ changes to ${\mathrm{Yb}}^{3+}$ state. This must involve $4f-5d$ electron promotion and must result in a semiconductor-to-metal transition. The behavior of YbTe suggests that in rare-earth chalcogenides with large band gap a continuous transition would be favored.

Linear combination of atomic orbital—molecular orbital cluster calculations using extended Hückel theory suggest that the interstitial carbon atom in diamond prefers an "interstitialcy" configuration. The predicted minimum-energy configuration changes with charge state, providing a possible example of the Bourgoin mechanism for "athermal" migration of the interstitial in the presence of ionizing radiation.

Quasielastic neutron scattering experiments on hydrogen in niobium single crystals have been performed in the $\overrightarrow{\mathrm{K}}$ range from 1.7 to 4.3 ${\mathrm{\AA }}^{-1\mathrm{}}$ in the three directions [100], [110], and [111]. The experimental width functions show a significant anisotropy. The results are not in agreement with the theoretical half-widths calculated for tetrahedral and octahedral positions of the hydrogen.

The bond polarizability in Si${\mathrm{O}}_2$ polymorphs increases as the Si-O-Si bond angle increases and as the density decreases. This increase and systematic variations in other properties are due to $\pi$ bonding between Si and O. Delocalization of $\pi$ electrons in Si-O ring configurations and bond strain determine the properties of polymorphs of density under 2.33 g ${\mathrm{cm}}^{-3\mathrm{}}$; they are also responsible for the pressure and irradiation behavior of Si${\mathrm{O}}_2$.

The density of states of the valence and conduction bands of Si and Ge are discussed using a simple tight-binding Hamiltonian. It is shown that certain features of the density of states for the case of the diamond structure are related to the short-range order and hence should be retained in the amorphous state while others depend on long-range order and are expected to disappear.