A wide variety of numerical models of gravitational clustering are used to assess error in estimates of the mass density of the Universe from Virgo infall. The mean estimate of density is too low in both critical-density and low-density models, and it has a large dispersion in all models. Biased cold-dark-matter models produce an estimate of mass density in good agreement with observational estimates from infall.

Numerical calculations for a model anharmonic system interacting with a laser are used to analyze the quantum mechanical implications of classical structure in stochastic regions due to cantori (associated with the breakup of invariant Kolmogorov-Arnol’d-Moser surfaces). The numerical results show that a quantum wave packet may remain localized, even though classical orbits are strongly chaotic. Consequently, the quantum dynamics continues to exhibit ‘‘tunnelinglike’’ behavior even when diffusion is not classically forbidden.

A new vacancy-complex model for the (2×2) reconstructed As-stabilized (1¯ 1¯ 1¯) surface of GaAs in which the surface bilayer contains two As and three Ga atoms per unit cell is proposed. Half the surface As atoms occupy stacking-fault sites, leading to nearly optimal rehybridizations of surface-bonding orbitals. The surface As coverage is consistent with both mass spectrometry and Auger data. The calculated surface electronic structure is nonmetallic and provides a satisfactory description of polarization-dependent angle-resolved photoemission data.

The (1¯ 1¯ 1¯) surface of GaAs exhibits three stable reconstructions. Two are (2×2), As stabilized and Ga stabilized respectively, and the third is (√19 × √19 ). Transitions between these structures are obtained by variation of the experimental conditions. We propose new models for all of the above reconstructions, based on ab initio total-energy calculations and experimental information regarding surface composition.

Dark-field imaging of charge-density-wave superlattice reflections in α-U by transmission electron microscopy reveals a multiple-domain structure with a single wave vector q in each domain. It is found that the nearly commensurate charge-density wave occupies most of the area (≳80%) at temperatures below 23 K. The domains of the incommensurate charge-density wave start to expand very rapidly as temperature approaches 23 K. Thermal evolution of this phase transition is also studied. Striations similar to that observed in many martensitic transformations are also found at T<43 K.

The total and differential atomic pair-correlation functions of an icosahedral (quasicrystalline) solid were determined for the first time, directly by the differential anomalous-x-ray-scattering technique using synchrotron radiation. The observed atomic distances involving uranium atoms in icosahedral ${\mathrm{Pd}}_{58.8}$${\mathrm{U}}_{20.6}$${\mathrm{Si}}_{20.6}$ suggest the presence of a quasicrystalline sublattice with vertex decoration. The pair distribution functions in the icosahedral and amorphous phases are similar up to the second-nearest neighbors, or up to 6 Å.

We show that in anisotropic superconductors in which impurity scattering is the dominant relaxation process at low temperatures, the viscosity and ultrasonic attenuation in the hydrodynamic regime, when calculated in the Born approximation, tend to constant values while the thermal conductivity, κ, varies as T. These results are in conflict with experiment. We show that if multiple-scattering effects are taken into account, the ultrasonic attenuation and κ/T fall with decreasing temperature.

We have performed neutron-scattering measurements on a single crystal of ${\mathrm{CeCu}}_6$ in applied magnetic fields from 0 to 6.4 T. The observed momentum-dependent magnetic scattering implies the existence of short-range antiferromagnetic correlations. While it induces no resonances, the field broadens the spectra considerably at T=0.4 K. An effective Hamiltonian which incorporates both single-impurity Kondo properties and antiferromagnetic Ruderman-Kittel-Kasuya-Yosida interactions describes our data.

When the quantized Hall effect occurs at a semiconductor surface such as Si(110), where the carriers have a time-reversal valley degeneracy, there should be a spontaneous valley polarization at appropriate values of the filling factor ν. There can then be dissipation at T=0 due to radiation of Goldstone bosons (‘‘valley waves’’) at impurity sites, provided that the current density exceeds a critical value ${\mathrm{j}}_{\mathrm{c}}$ determined by the intervalley electron-electron scattering or other terms which modify the valley-wave dispersion at long wavelengths. The dissipation above ${\mathrm{j}}_{\mathrm{c}}$ is described by a constant resistivity ${\mathrm{\rho }}_{\mathrm{xx}}$, which should be small but measureable, and sensitive to the density of neutral impurities.

It is pointed out here that a coherent state propagating through an amplitude dispersive medium will, under suitable conditions, evolve into a quantum superposition of two coherent states 180° out of phase with each other. The response of a homodyne detector to this superposition of macroscopically distinguishable states is calculated. Signatures which an experimentalist might look for in the homodyne detector’s output in order to verify the generation of such states are described.

The elementary excitations at (1/3)-Landau-level filling have been studied for various spin polarizations by finite-size calculations in a periodic rectangular geometry. For low magnetic fields, the spin-reversed quasiparticles carrying fractional electron charge are found to be important. The Zeeman energy due to the spin flip explains qualitatively the linear magnetic field dependence of the activation energies observed experimentally.

The electrical conductivity of Si:As has been measured from 0.1 to 2 GHz in the temperature range 1 to 4 K. At donor densities ≲6.4×${10}^{18}$ As/${\mathrm{cm}}^3$, the conductivity approached a temperature-independent value σ(ω) which exhibited a superlinear frequency dependence [σ(ω)∞${\mathrm{\omega }}^{\mathrm{s}}$, s>1] for sufficiently low donor density. The superlinear dependence is not in agreement with a Shklovskii-Efros prediction in the regime ħω≪${\mathrm{e}}^2$/ε${\mathrm{r}}_{\mathrm{\omega }}$. Although in qualitative agreement with the Mott ${\mathrm{\omega }}^2$ law, the density variation of s(n) is much more rapid than theoretical predictions. Photon-assisted correlated multiple-electron absorption of large clusters is considered as a possible explanation of the results.

Measurements of the temperature and carrier-density dependence of the strongly localized conductance of short silicon inversion layers are reported. At the lowest temperatures we observe well-isolated, large conductance peaks whose width and temperature dependence are only consistent with resonant tunneling and are inconsistent with Mott hopping. Several new features are observed which we believe may be the result of Coulomb interactions.

The electronic and magnetic character of epitaxial Fe films on Ag(001) has been studied as a function of Fe coverage by spin- and angle-resolved photoemission with synchrotron radiation. At coverages well below a monolayer, the spectra exhibit a local spin-split electronic state. Although spectra for films in the monolayer coverage range display electronic structure in close agreement with calculated monolayer film critical-point energies, no spin polarization is observed up to 2.5 monolayers coverage. Thicker films approach the spin-split electronic structure and spin polarization of bulk Fe(001).

A phase transformation in graphite irradiated by visible picosecond laser pulses is observed above a well-defined pump threshold. Time-resolved reflectivity measurements at different wavelengths indicate that the high-temperature phase is not metallic. At low irradiation levels, a fast decaying plasma component is observed.

Inelastic-neutron-scattering measurements with the constant-q technique have been used to characterize the energy response of the scattering in paramagnetic nickel. At small wave vectors q, the scattering peaks at zero energy, indicative of spin diffusion, but at larger q the scattering maximizes at finite ±E demonstrating that the spectral response has propagating character. The relative width ΔE/E of the excitation decreases with increasing ‖q‖ as expected, with the excitation becoming well defined as its energy becomes comparable with kT.

First-principles pseudopotential total-energy and electronic-structure calculations are reported for two Si(111)(√3 × √3 ):Si adatom geometries. Based on these calculations, a theory of the electronic structure of Si(111)(7×7) is formulated, and the binding energy of Si adatoms on Si(111) is determined.

Dynamic behavior of Brownian particles close to a wall is studied by photon correlation from an evanescent wave with variable penetration depth. Surface correlation spectra strongly differ from the bulk measurements and are completely interpreted in terms of the combined wall’s mirror effect and evanescent-wave geometry. Relevance of these new effects to the light-scattering study of surface dynamics is discussed.

A limit is established for the violation of the equality of passive and active gravitational mass. Our test is based on an asymmetry in the composition of the moon. We hypothesize that the 2-km offset between the moon’s center of figure and center of mass indicates an asymmetry in the distribution of Fe and Al. Unless the Fe on one side and the Al on the other attract each other with the same force, the moon will not be in the orbit predicted by classical mechanics. Using the results from laser ranging and a model for the moon’s interior we find that the ratios of active to passive mass for Fe and Al are the same to a precision of 4×${10}^{\mathrm{-}12}$.

We develop a scenario in which feeble intermediate-range forces emerge as an effect resulting from the compactification (à la Kaluza-Klein) of a five-dimensional model. It is conjectured that this general strategy will prove applicable to more realistic multidimensional theories such as the ten-dimensional superstring. We speculate that deviations from the standard gravitational force—similar to the ones reported recently as a ‘‘fifth force’’—may be interpreted as evidence for higher dimensions.