The dielectric function $\epsilon (\mathbf{q},\omega )$ of Li-intercalated graphite (Li${\mathrm{C}}_6$) with $\mathbf{q}\parallel c$ axis was deduced from inelastic x-ray scattering data and compared with $\epsilon (\mathbf{q},\omega )$ of pristine graphite. A displacement and broadening of the first peak of $\mathrm{Im}\epsilon (\mathbf{q},\omega )$ upon intercalation was found, which we attribute to a 2-eV shift of the "metal" $\sigma$ state of Li${\mathrm{C}}_6$ to lower energies relative to the $\sigma$-type interlayer state of graphite.

We report pseudopotential calculations of the electronic structure of a ${\mathrm{Si}}_4$${\mathrm{Ge}}_4$(001) superlattice grown on a Si substrate. Our results show that it is possible to account for the observed optical spectra, without invoking indirect transitions, in terms of a model in which the microscopic potential and the bond lengths in the Ge layer are bulklike.

The long-time tail in the diffusive behavior in a strongly chaotic system (stadium billiard) is studied from the point of view of "stickiness" of invariant lines. Diffusion near the invariant line in the phase space can be described by a one-dimensional continuous random-walk problem with pausing time which is inversely proportional to the distance from the line. The problem is studied by numerical methods and it is shown that the first-passage-time distribution has an algebraic decay tail in the form $\sim t^{-v}$ with the estimated exponent $v\simeq 2.933\pm 0.005$.

Wigner statistics for correlations of matrix eigenvalues are shown to be a property of any matrix ensemble with a density of levels and probability distributions for matrix elements that are smooth. This justifies the universality of level correlations in generic quantum systems, while suggesting that level widths and other eigenvector-dependent statistics are system dependent.

I argue that standard descriptions of stellar collapse omit the primary mechanism for dissipative neutrino reactions in nuclear matter, nuclear excitation by neutral-current scattering. The nuclear heating rate, due primarily to muon- and tauon-neutrino excitation of giant resonance states, is on the order of 90 MeV/nucleon sec at a radius of 100 km. I discuss possible effects of both neutral- and charged-current neutrino heating in models of stellar collapse.

Conventional quarks and leptons of any one generation are assigned to the chiral-color representations of strong- and electroweak-interactions symmetry ${\mathrm{SU}\mathrm{}\left(3\right)\mathrm{}}_L\otimes {\mathrm{SU}\mathrm{}\left(3\right)\mathrm{}}_R\otimes {\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}_L\otimes {\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}_Y$. The anomalies are canceled by addition of exotic quarks and leptons that are all singlets under the weak-interactions group ${\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}_L$. The model requires only two Higgs representations to break the gauge symmetry and to give masses to all the quarks and leptons. All quarks and leptons acquire masses through the seesaw mechanism of Gell-Mann, Ramond, and Slansky.

The coincidence cross section for the two-body breakup of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been determined up to recoil momenta of 350 MeV/c with the ($e,e^{\prime }p$) reaction. The momentum probability distribution for the $t+p$ final state deduced from the cross section is compared to theoretical predictions that include correlations. A substantial deviation from the impulse approximation is observed, which cannot fully be explained by final-state interaction effects and contributions due to charge exchange.

Nonlinear optical balance between a dissipative and a nonlinear parametric process, which is known to suppress the resonant excitation at large laser intensities, can be quenched if the nonlinear medium is placed inside a Fabry-Perot cavity. This property is demonstrated by the comparison of results obtained from extracavity and from intracavity experiments on nearly degenerate four-wave mixing resonantly enhanced by stimulated Brillouin scattering.

The Lamb shift ($n=2\mathrm{}$) in ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ has been measured in a beam of metastable helium ions by means of the microwave resonance method. Our result is 14.0420(12) GHz which is in good agreement with the theoretical result based on the binding correction of Mohr, 14.0423(5) GHz.

The differential cross sections for double and single ionization and the corresponding charge-state fractions of He resulting from impact of fast protons have been measured as functions of both the scattering angle (0.25-4.1 mrad) and the impact energy (300-1000 keV) of the proton. The differential cross sections smoothly decrease with the scattering angle, but the fraction of doubly charged ions, $F_2$, exhibits a distinct peak at ≅0.9 mrad. This peak is not easily explained in terms of the single- and double-ionization mechanisms thought to operate at these energies.

We report the observation of shell structure in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ nonneutral plasmas (ion clouds) confined in a Penning trap. Clouds containing up to 15000 ions (density ≅ ${10}^8$ ${\mathrm{cm}}^{-3\mathrm{}}$) were laser cooled to temperatures of about 10 mK. Under these conditions, the ions are strongly coupled and exhibit liquidlike and solidlike behavior through the formation of concentric shells. The shells were observed by direct imaging of the laser-induced ion fluorescence for values of the Coulomb coupling constant $\Gamma$ ranging from about 20 to 200. The shell structure is compared with theoretical predictions.

Angular distributions of 3-9-MeV protons scattered in singly and doubly ionizing collisions with He have been measured. A sharp shoulder is seen at 0.55 mrad, a feature attributed to binary encounters between the projectile protons and quasifree target electrons. The ratio of double to single ionization of He is found to be nearly independent of scattering angle between 0.25 and 0.55 mrad and to have a value near 2%, substantially lower than the ratio found in photoionization producing fast ejected electrons with the same energies.

We present an experimental observation of convective transport in oscillatory convection of a binary mixture. The results show the existence of mass transfer in the direction of propagation of nonlinear traveling waves (TW) as well as that of a backflow. Good agreement with a simple model for particle trajectories in TW was found. We found that the two previously observed oscillatory states differ in mass transfer. In linear counterpropagating waves no mass transfer is detected, and the mass-transport onset coincides with the transition to nonlinear TW.

We present what we believe are the first experimental observations of ion correlation effects in a dense plasma. The plasma is produced in aluminum by colliding shocks driven by high-power laser beams. Computer simulations suggest that a peak temperature of over 1 eV and densities of several times that of solid are produced. Short-range order within the plasma is observed by means of the extended x-ray-absorption fine-structure (EXAFS) spectrum of the alminum $K$ absorption edge. Densities measured from the EXAFS spectrum are in reasonable agreement with the computer predictions.

Recent experiments involving high-power radio-frequency heating of a tokamak plasma show strong suppression of the sawtooth oscillation. A high-energy trapped-particle population is shown to have a strong stabilizing effect on the internal resistive kink mode. Numerical calculations are in reasonable agreement with experment.

Numerical simulations of capillary displacement in 2D porous media indicate a dynamical critical transition as the contact angle $\theta$ of the invading fluid varies. In the nonwetting limit ($\theta =180\mathrm{}°$), growth patterns are fractal as in the invasion percolation model. As $\theta$ decreases, cooperative smoothing mechanisms involving neighboring throats become important. The typical width of invading fingers appears to diverge at a critical angle which depends on porosity. Below this angle the fluid floods the system uniformly.

We report x-ray scattering studies of liquid layers on the Pb (110) and (111) surfaces at temperatures substantially below the bulk melting point of Pb. Surprisingly, the initial liquid scattering appears at $\approx 220 \mathrm{K}\mathrm{or}<\frac{1}{2}{T}_m$ on both the (110) and the (111) surfaces. On the (110) surface, analysis of the scattered intensity yields 2-3 monolayers of liquid from 575 to 599 K while substantially less than 1 monolayer of liquid is present on the (111) surface.

We have explicitly tested, using pseudopotential total-energy techniques, the effect of two representative embrittling impurities on the grain-boundary cohesion of a metal. We find that substitutional Ge and As impurities increase the Al[111] interlayer energy of cohesion both at the impurity-doped layer and one layer into the bulk. Our calculations do not support either the first-layer or the second-layer decohesion models of impurity-promoted grain-boundary embrittlement.

The superfluid transition of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed in porous media is analyzed with use of a generalization of the Kosterlitz-Thouless theory appropriate to connected surfaces. Topological excitations with quanta of circulation around loops which enclose the pore space are identified. On large length scales these excitations are described by a Villain model with coupling constant determined by the geometry of the pore space and the effect of vortex excitations. For a range of geometries the onset of the 3D critical region is determined by the Kosterlitz-Nelson universal jump line.

A self-consistent phonon calculation was used to calculate the phonon spectrum and density of states of ${\mathrm{D}}_2$ and ${\mathrm{H}}_2$ adsorbed on graphite. The adsorbed molecules are assumed to be in a free-rotor state and the molecular solid is taken to be the $\sqrt{3}\times \sqrt{3}R30\mathrm{}°$ commensurate phase. The theoretical results are compared to recent neutron-scattering results for the phonon modes of ${\mathrm{D}}_2$ and with older data for ${\mathrm{H}}_2$. The effect of the use of coordinates both parallel to and perpendicular to the surface in the functional form for the wave function was found to be significant. Thus these are not purely two-dimensional systems.