First-principles molecular-dynamics simulation is carried out for covalent systems with arbitrary (i.e., tetrahedral and nontetrahedral) coordination in a nonorthogonal, tight-binding basis, generalizing previous successful applications to tetrahedral solids. The generalized method is applied to study various aspects of the ${\mathrm{C}}_{60}$ molecule, including the effect of doping on vibrational modes, and interaction with heteroatoms. Considerable increase in the double-bond length and a much smaller increase in the single-bond length are found. Oxygen chemisorption occurs over the bridge site along the carbon-carbon double bond, weakening the latter.

The single-quantum annihilation of positrons has been studied experimentally with a positron beam and a thin lead target, at energies 1 MeV and higher. Spectral peaks corresponding to the K, L, and M shells have been resolved and observed distinctly for the first time. The shell ratios L/K and M/K have been determined. An analysis of the L peak has yielded the (LII+LIII)/L ratio. The first measurements of the directional distributions of the annihilation quanta of the three individual electron shells are also reported. The results are in agreement with theory. They also point out the potential for applying the phenomena to the development of a tunable, highly directional gamma-ray source.

A relativistic scattering calculation based on the Dirac R-matrix method has been performed within a two-electron model-potential approach for negative-ion binding energies as well as elastic and inelastic scattering cross sections. We find multiplets of narrow shape resonances with J=0, 1, and 2, and widths of a few meV. Their J-averaged energies for even and odd parity are 119.0 meV below the 6${\mathit{p}}_{1/2}$ threshold and 9.1 meV above the 6${\mathit{s}}_{1/2}$ threshold.

We examine the kinematics of quantum vortex configurations in three-dimensional space for an ideal incompressible fluid. The permitted quantum configurations are shown to be surfaces of vorticity, characterized topologically not only by their knottedness, but also by additional global properties such as twists and holes. Our results, obtained by representing an infinite-dimensional group of diffeomorphisms, contradict conclusions reached by approximating the infinite-dimensional system with finitely many degrees of freedom.

The probability distribution of passive temperature fluctuations is experimentally studied in grid-generated wind tunnel turbulence. It is shown that exponential tails occur for a mean temperature gradient but do not in the absence of a mean gradient. The velocity probability distribution is Gaussian.

The effective thermal diffusivity ${\mathit{D}}^{\mathrm{*}}$ and the probability distribution of temperature fluctuations are measured in a stirred fluid across which a temperature gradient is maintained. A distinct mixing transition is observed for ${\mathit{D}}^{\mathrm{*}}$ as a function of Reynolds number R. Above the transitions, the distribution is strongly non-Gaussian and approaches an exponential exp(-‖δT‖/βξ), where β is the local temperature gradient and ξ the correlation length.

We investigate numerically the question of blowup in finite time for the ‘‘swirling flow’’ of the three-dimensional incompressible Euler equations. Using rotational symmetry, the Euler equations reduce to a two-dimensional problem which is numerically solved by finite differences. The elliptic equation relating vorticity to velocity is solved with the multigrid method. Calculations were performed with 896×640 mesh points.

The wavelet decomposition is used to generalize the multifractal formalism to singular signals. The singularity spectrum is directly determined from the scaling behavior of partition functions that are defined from the wavelet transform modulus maxima. Illustrations on fractal signals with a recursive structure, e.g., devil’s staircases, are shown. Applications to fully developed turbulence data and Brownian signals are reported.

New numerical evidence for a transition to hard turbulence in 2D Boussinesq convection is presented. These 2D simulations agree with some, but not all, experimental results for the scaling properties of 3D hard turbulence. The transition to 2D hard turbulence, as measured by a change in the Nusselt-Rayleigh scaling law, coincides with a gradual change in the velocity probability distribution from Gaussian to exponential form and with the development of a ‘‘well-mixed’’ central region.

The new plasma diagnostic technique of spectral blueshifting of femtosecond pulses is used for the first time to analyze quantitatively the ionization of noble gases under the influence of intense, femtosecond illumination. The two processes of strong-field tunneling ionization and electron impact ionization are found to play competing roles on these time scales.

High-resolution electron cyclotron emission and x-ray image reconstructions have been made simultaneously during the sawtooth crash in the fast rotating plasma with neutral beam injection heating on TFTR. The measured x-ray emission is identical as metal impurity radiation. The results suggest that the sawtooth crash is a full reconnection process of the TFTR sawteeth. The crescent shaped ‘‘hot spot’’ in the x-ray emissivity is found not to present flux surfaces. New features are observed during the sawtooth crash, such as an oval-shaped hot spot and a cooler region between the island and the hot spot.

The dynamical properties of randomly cross-linked polymer melts, far above the vulcanization threshold, are studied using molecular-dynamics simulations. The mean-square displacements of the middle and end monomers of the chains as well as the cross-links are considered. The dynamics scattering function S(q,t) as measured by neutron spin echoes is also calculated. The average strand length between two cross-links is about 1/4 of the entanglement length of the un-cross-linked melt. By introducing a new intermonomer potential, we can explicitly test the influence of trapped entanglements. For the present system, the frozen entanglements change the plateau modulus by about 25%.

The heat capacity of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on graphite has been precisely measured over a fine grid of coverages for temperatures extending down to 100 mK and for coverages up to five atomic layers. The data indicate that the two-dimensional liquid is self-bound in each of the first three layers with an areal density of about 0.04 atom/Å ${}_{}{}^2{}_{}{}^{}$. There is also evidence suggesting that this liquid undergoes a Kosterlitz-Thouless transition.

We have studied the spreading of liquid drops on a solid surface by molecular-dynamics simulations of Lennard-Jones systems of liquid, vapor, and solid. As the attraction between liquid and solid increases we observe a smooth transition from partial wetting to terraced wetting, with distinct molecular layers spreading with different velocities. In the terraced case the layers are ordered but not solid, with substantial molecular diffusion both within and between layers, and a growth rate that disagrees with experiments with nonvolatile liquids as well as recent calculations.

By using the scanning tunneling microscope we have determined the probability distribution for the distances between steps on the Cu(100) surface and the vicinal surfaces (11n) with n=7 and 19. The results indicate a repulsive short-range step-step interaction, however, unexpectedly, an attractive interaction at intermediate distances of 3–5 atoms.

The temperature-dependent phase behavior of the reconstructed Au(100) surface is studied using molecular-dynamics simulation based on a well tested many-body force scheme. We present the first theoretical description of the three distinct surface structural phases identified by recent x-ray-scattering measurements. Our calculation predicts critical temperatures of the rotational and surface-disordering transitions in good agreement with experimental results, and sheds light on the nature of these phases.

A new form of CO is found for adsorption on an oxide surface, where an ordered array of CO is found by angle-resolved photoemission to be bound in a tilted or lying-down mode. This new bonding configuration has very strange features which are discussed, including strange ordering of valence bands and abnormal binding energies. A simple model is presented.

Practical information on higher-order correlations in a metallic glass, ${\mathrm{Ni}}_{81}$${\mathrm{B}}_{19}$, is derived from x-ray-absorption near-edge-structure (XANES) data. For the structure of amorphous ${\mathrm{Ni}}_{81}$${\mathrm{B}}_{19}$, a statistical evaluation of XANES calculations revealed that a minority of Ni-Ni-B triangles with peculiar shapes and sizes is to a significant degree responsible for the appearance of the experimental XANES features. The two most relevant types of triangles have one Ni-Ni and one Ni-B distance of approximately 0.25 and 0.20 nm each, identical with the average atomic distances of diffraction data. The second Ni-B distance was found to be sharply defined as 0.195±0.005 or 0.275±0.015 nm in either type.

Pressure-induced amorphization in α-quartz has been investigated using constant-pressure molecular-dynamics calculations with the two-body potential of van Beest, Kramer, and van Santen. Both the static properties and the crystalline-to-amorphous phase transition were very well reproduced. Through an analysis of the elastic moduli, the mechanism for the transformation is shown to be a mechanical instability driven mainly by a cooperative twisting and compression of the helical tetrahedral silicate units with an abrupt decrease in the ${\mathit{C}}_{12}$, ${\mathit{C}}_{23}$, ${\mathit{C}}_{13}$, ${\mathit{C}}_{14}$, and ${\mathit{C}}_{33}$ elastic moduli.

The 5f element thorium has been studied in a diamond-anvil cell to pressures of 300±8 GPa (volume fraction V/${\mathit{V}}_0$=0.40) at 300 K by energy-dispersive x-ray diffraction using a synchrotron source and platunum pressure standard. Thorium undergoes a phase transition from its ambient fcc phase to a body-centered-tetragonal (bct) phase near 100 GPa. This bct high-pressure phase of thorium is similar to the one found previously in the 4f lanthanide element cerium above 12 GPa. We suggest that the 5f band which lies above the Fermi energy in thorium at ambient conditions is occupied at megabar pressures and significantly contributes to bonding.