A systematic procedure for deriving the guiding-center Hamiltonian $K$ is described, valid for arbitrary ratio of particle gyro-scale length to perpendicular magnetic scale length. Practial examples are given where the method permits an explicit evaluation of $K$ for cases where the small-gyroradius approximation used for standard guiding-center theories is invalid.

The hyperfine interaction is shown to be at the origin of the finite lifetime measured for the metastable level $A_{1\mathrm{u}}\left({}_{}{}^3{}_{}{}^{}T_{1\mathrm{u}}^{}{}_{}{}^{}\right)$ of ${\mathrm{Tl}}^{+}$-like ions in alkali halides.

Light scattering on potassium dihydrogen phosphate demonstrates that the ordering electric field reduces the isothermal microscopic response. At the continuous transition occurring at the critical field no microscopic critical fluctuation exists (no diverging correlation range) and only macroscopic thermodynamic quantities can diverge classically. This general effect for symmetry-nonbreaking elastic transitions is caused in potassium dihydrogen phosphate by higher than bilinear strain-polarization couplings in the free energy.

The first evidence for Si${\mathrm{H}}_1$-like centers in crystalline Si ($c$-Si) is presented from infrared measurements of H (D) implanted at 80 K. In contrast to Si${\mathrm{H}}_1$ centers in amorphous Si ($a$-Si) which are stable to ≈ 700 K, the crystalline band anneals below 300 K with an activation energy and illumination enhancement that are characteristic of the Si vacancy. These results relate specific defects for implanted H in $c$-Si to previous observations for H in $a$-Si.

High-resolution heat-pulse experiments reveal that transverse, ballistic phonons in Ge are focused into sharply defined beams along the [001] and [110] axes. Angular distributions within these beams display fine-structure representing focusing singularities. It is shown that these features are dependent in a sensitive way on the topology of the phonon-frequency surface.

A mechanism, based on anharmonic elasticity and the critical behavior of the strains around dislocations, is proposed for the ciritical static central peak seen in light scattering from K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and K${\mathrm{H}}_3$${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$. It is shown that the main source of the anomaly is the ferroelasticity occurring in both crystals, though ferroelectricity in K${\mathrm{H}}_2$PO also enhances the scattering.

A self-consistent pseudopotential band-structure calculation for the recently determined high-pressure (above 38 kbar) monoclinic tellurium structure is presented. The density of states and charge densities for representative regions within the unit cell are calculated. Our results show that high-pressure monoclinic tellurium is metallic, and that a crystal bond-length asymmetry is caused by an electronically driven distortion. The results also predict a highly anisotropic conductivity.

This Letter reports the first experimental observation of heavy and light excitons by resonant Brillouin scattering of polaritons in ZnSe. The number of the Brillouin peaks as well as their variations with the excitation light energy are well accounted for on the basis of a three-branch polariton dispersion curve in which the longitudinal-transverse splitting $E_{\mathrm{LT}}$ and the exchange energy $\Delta$ mix both kinds of excitons. The observation of the three branches allows us to determine $E_{\mathrm{LT}}$, $\Delta$, and the translational masses $M_h$ and $M_1$ of heavy and light excitons.

The in situ optical properties of the interface between Si and its thermally grown oxide, deduced over the spectral region between the visible and the near-ultraviolet by analysis of spectroscopic ellipsometric data, are characteristic of a (7±2)-Å region of atomically mixed Si and O of average stoichiometry Si${\mathrm{O}}_{0.4\pm 0.2}$. The results are incompatible with either microroughness or an abrupt transition from Si to Si${\mathrm{O}}_2$, but rather support a graded transition region.

Outside a surface of sufficiently low work function, incident 10-eV ${\mathrm{He}}^{+}$ ions convert to excited atoms which, in turn, eject electrons in the process of Auger deexcitation (Penning ionization). The electron spectroscopy thus demonstrated for ${\mathrm{He}}^{+}$ incident on Ni(100) with adsorbed ${\mathrm{K}}^{+}$ is the essential equivalent of that employing incident metastable atoms. Energy and wave-function considerations governing the partition between neutralization and deexcitation processes are discussed and evidence of the surface sensitivity of these processes is presented.

It is pointed out that scaling relations suggested previously between the critical and ground-state properties of electron-hole droplets lack theoretical justification; by detailed calculations for model systems with widely varying band structure, these relations are shown not to be satisfied. New scaling relations are proposed, and their existence is traced to systematic trends in the band structure and the exchange-correlation energy of the electron-hole system.

Molecular-dynamics simulations of the two-dimensional, classical electron gas yield indications for the existence of a self-diffusion constant, in contrast to the case of shortrange interactions. The velocity autocorrelation function exhibits marked oscillations in the strong-coupling regime. In order to avoid the Ewald summations of the Coulomb interactions in a periodic system, the electrons are confined on a spherical surface.

Measurement was made of $\frac{d\sigma }{\mathrm{dt}}$ for $n_{\uparrow }+p_{\uparrow }\to n+p$ at ${P_{\perp }}^2=0.8\mathrm{} \mathrm{and} 1.0$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ at 6 GeV/c. The 6-GeV/c 53%-polarized neutrons from the 12-GeV/c polarized deuteron beam at the Argonne zero-gradient synchroton were scattered from our 75%-polarized proton target. Both spins were oriented perpendicular to the scattering plane. We found large unexpected spin-spin effects in $n-p$ elastic scattering which are quite different from the $p-p$ spin-spin effects.

Angular distributions for dileptons formed in hadronic collisions at large mass $Q^2$ and large transverse momentum $Q_t$ are calculated, assuming various two-body quantum chromodynamic subprocesses. The main results are the following: (a) The polar and azimuthal distributions are correlated. (b) For fixed ${Q_t}^2$, the distributions scale in the variable $w=\frac{{Q_t}^2}{Q^2}$, and thus are independent of total energy. (c) Each subprocess exhibits a characteristic function of $w$, independent of the constituent distributions.

We have measured the linear polarization, $P_{\gamma }$, of continuum $\gamma$ rays for ${}_{}{}^{151,152}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{156-158}{}_{}{}^{}\mathrm{Er}$ produced through (${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$,$\mathrm{xn}$) reactions. We find that $P_{\gamma }\approx 0.35$ for $E_{\gamma }<~1.2$ MeV in the rotational Er nuclei and, in contrast, that $P_{\gamma }\approx 0.05$ for $E_{\gamma }<~1.0$ MeV in the oblate Dy isotopes. This, taken together with the spectrum shapes of the contiuum $\gamma$ rays, suggests a rather direct feeding of the yrast states by statistical cascades. The Dy data for $1.0<~E_{\gamma }<~1.5$ MeV indicate collective rotations at spin $I>\mathrm{}40\mathrm{}\hslash$.

The characteristic features of the energy distributions of the 180° -emitted protons, deuterons, and tritons from 0.6 to 400 GeV incident energy, the extremely weak angular dependence of single-particle inclusive cross sections in the backward directions, and the existence of forward-backward $\mathrm{pp}$ correlation are shown to be due to fragmentation of the effective target (that is, the system of nucleons in the target nucleus along the axis of the incident proton) in proton-nucleus collisions. Correlation effects in violent processes are also briefly discussed.

Measurements of $K$-shell ionization yields for ($\alpha$,$\mathrm{xn}$) and (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$,$\mathrm{xn}$) evaporation residues exclude the existence of a large $M1\mathrm{}$ component in the quasicontinuum spectrum of well-deformed Dy nuclei below 500 keV. Upper intensity limits are deduced for $M1\mathrm{}$, $E1\mathrm{}$, and $E2\mathrm{}$ components.

I discuss the effect of flavor mixing on proton-decay Hamiltonian in SU(5) grand unified theories with arbitrary number of fermion families and show that in the simplest model with only one {5} Higgs multiplet, all fermion interactions (with Higgs as well as gauge bosons) are describable in terms of a single mixing matrix. For the more complicated case of several {5} multiplets, the above feature persists for gauge-boson interactions though not for Higgs bosons.

We examine the possibility that the $\tau$ neutrino is sufficiently massive to require it to be unstable ($m_{{\nu }_{\tau }}\gtrsim 50$ eV). A combination of experimental and astrophysical bounds requires a mass greater than 10 MeV. For larger masses, we bracket the lifetime and decay modes of ${\nu }_{\tau }$, and we give concomitant limits on the leptonic mixing angles.

This paper reports on the first results of the study of $e^{+}{e}^{-}$ collisions at $\sqrt{s}=27.4\mathrm{}$ GeV and $\sqrt{s}=27.7\mathrm{}$ GeV at PETRA, using the $4\pi$-sr electromagnetic and calorimetric detector MARK-J. We obtain an average $R=\frac{\sigma (e^{+}{e}^{-}\to \mathrm{hadrons})}{\sigma (e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-})}=3.8\mathrm{}\pm 0.3 \left(\mathrm{statistical}\right)\pm 0.6 \left(\mathrm{systematic}\right)$ and a relative $R=1.0\mathrm{}\pm 0.2$ between the two energies. The $R$ values, the measured thrust distribution, and average spherocity show no evidence for the production of new quark flavors.