We demonstrate that instabilities in a Hamiltonian system can occur via deformations that reduce the symmetry of the system. The movement of eigenvalues at an equilibrium point of a family of Hamiltonian systems is constrained by the symmetry type of the system. If deformations of a family change the symmetry type, then instabilities can appear at multiple eigenvalues that produce large amplitude changes in the system dynamics. We illustrate this phenomenon in the context of a low-dimensional Hamiltonian normal form, and then analyze the instability of a vortex filament in a strain field.

The Wigner distribution of the electric fields in a quasimonochromatic light wave is equivalent, in the paraxial approximation, to the cross-spectral density function of the wave. The intensity distribution in a plane may be described as a projection across this Wigner distribution. Three-dimensional intensity information can be used to generate a set of projections from which it is possible to reconstruct the second-order statistics of the partially coherent wave field.

We study the phase space for arrays of coupled Josephson-junction oscillators which have coexisting in-phase and antiphase attractors. Arrays with four or more oscillators have significantly different attracting sets from those with two or three: Continuous families of out-of-phase attractors can exist instead of isolated ones. A new mechanism in the presence of small amplitude noise is proposed to account for the observed transitions among different stable antiphase states, and the rarely observed transitions between an in-phase and an antiphase attractor.

We describe a quantum many-body system undergoing multiple resonant tunneling which exhibits chaotic behavior in numerical simulations of a mean-field approximation. This phenomenon, which has no counterpart in the classical limit, is due to effective nonlinearities in the tunneling process and can be observed in principle with a heterostructure.

We consider the gluon fusion process gg→ggH in the limit that the top quark is much heavier than the Higgs boson. At high energy hadron colliders such as the CERN large hadron collider and the Superconducting Super Collider, this process is an important source of Higgs bosons at large transverse momentum.

Using self-consistent field molecular-orbital theory, we show that the interaction of hydrogen molecules with a ${\mathrm{Ni}}^{+}$ ion is characteristically different from that with a neutral Ni atom. While hydrogen chemisorbs dissociatively on the neutral metal atom, it is bound to the cation in its molecular form. The atomic bonding is a consequence of the Pauli exclusion principle whereas the bonding of the molecular hydrogen results from an electrostatic interaction. We predict that a ${\mathrm{Ni}}^{+}$ ion can bind at least six hydrogen molecules.

It was only recently found that a stable negative ion ${\mathrm{Ca}}^{\mathrm{-}}$ ${}_{}{}^2{}_{}{}^{}$P exists below the Ca${(}^1$S) ground state. An electron affinity of 43±7 meV was obtained experimentally, and numerous theoretical calculations have now found values between 0 and 100 meV. We report the first photodetachment threshold measurements on ${\mathrm{Ca}}^{\mathrm{-}}$, which display a p-wave threshold above Ca 4s4p ${}_{}{}^3{}_{}{}^{}$P and a large p-wave shape resonance above Ca 4s4p ${}_{}{}^1{}_{}{}^{}$P. Both threshold energies obtained from the data agree with a new electron affinity of 18.4±2.5 meV for Ca${(}^1$S).

Combining the linearly polarized high-flux photon beam of a dye laser and the synchrotron radiation of a vacuum-ultraviolet undulator, lithium atoms have been aligned and excited to Li 1snln’l’ core resonances. The angular distribution of the electrons emitted upon the decay of these resonances strongly depends on their symmetry character. Exploiting this dependence, the symmetry of Li 1snln’l’ core resonances has been determined. The results allow for stringent tests of theoretical predictions and demonstrate the great potential of angular-resolved photoelectron spectroscopy on aligned atoms.

A novel experiment on elastic electron scattering from free, spatially oriented molecules has been performed. ${\mathrm{CH}}_3$I molecules state selected by an electrostatic hexapole lens were oriented in a homogeneous electric field. The differential scattering cross sections were measured at an electron energy of 1 keV in the angular range of 4°–14° with the orientation switched on and off. From these data the orientation-dependent contribution to the molecular interference was determined. The results show a distinct oscillatory pattern as a function of momentum transfer as predicted by theory.

We have made an absolute measurement of the Cu K-shell impact ionization cross section by 5.1-GeV electrons, which demonstrates directly a density effect predicted by Fermi in 1940. By determining the ratio of the K x-ray yield from a thin front and back layer of the target by a novel grazing emission method, we have verified the effect of transition radiation on the x-ray production, suggested by Sorensen and reported by Bak et al.

Double differential electron emission yields following the impact of fast (3.95×${10}^7$cm/sec)${\mathrm{Ar}}^{9,12,18+}$, ${\mathrm{Ne}}^{9+}$, ${\mathrm{Xe}}^{30,44+}$, and ${\mathrm{Th}}^{70,75+}$ ions on Cu and Au targets have been measured. The electron emission is dominated by low-energy electrons (<50 eV). It is found that the total yield, which rises to about 100 electrons per ion, is a nonlinear function of the total potential energy of the incident ion (up to about 200 keV).

We have studied the vibrations in a granular material and have found that they are qualitatively different from those found in solids or liquids. In response to an external oscillation, the vibrations experienced by a single particle in the material are exceedingly noisy with a power spectrum S(f)∝${\mathit{f}}^{2.2}$ over more than 5 decades in frequency. The frequency response depends on the detailed packing of the beads, reminiscent of conductance fluctuations in mesoscopic metals.

Absolute-intensity measurements of luminous emissions from shocked silicon show significant disagreement with predictions that assume thermal equilibrium between electrons and ions is reached immediately behind the shock front. Taking into account both electron-ion equilibration and electron thermal conduction, we have estimated an electron-ion coupling coefficient of ${10}^{16}$ W/${\mathrm{m}}^3$ -K This finding will also have important ramifications in pyrometric determinations of shock temperatures.

Experimental studies examining a anew relativistic regime of high-intensity short-pulse propagation in plasmas have been performed which present evidence for the formation of a stable mode of spatially confined (channeled) propagation. For an electron density of ∼1.35×${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and a power of ∼3×${10}^{11}$ W, the results indicate a channel radius ,1 μm and a peak intensity ∼${10}^{19}$ W ${\mathrm{cm}}^2$. Comparison of these findings with a dynamical theory yields agreement for both the longitudinal structure and the radial extent of the propagation observed.

Displacive adsorption of As on vicinal Si(001) surfaces has in the past been attributed to reduction of step-related (extrinsic) surface stress. Here we show images of As adsorption on flat Si(001), obtained with in situ low-energy electron microscopy, and with scanning tunneling microscopy. We find displacive adsorption even on micrometer-sized terraces, on which steps play no significant role. These new results reveal the role of intrinsic surface stress as a driving force for interfacial mixing.

We have first shown a long-range self-organization of a pseudo-one-dimensional single-crystal island array in an early epitaxial-growth stage of NiAl on (001) AlAs at 790 K. Up to the coverage θ=0.4, the dominant growth feature is an elongation along the [11¯0${]}_{\mathrm{AlAs}}$ direction with constant island height. As a result, there appears a quasiperiodic array of NiAl wires over a few micrometers long running along the [110${]}_{\mathrm{AlAs}}$ direction at θ=0.4. After that, the dominant growth behavior changes from the lateral growth to a growth in height.

We present calculations of the electronic structure for higher-order rational approximations to quasicrystals up to 12 380 atoms in the unit cell, based on a tight-binding linear-muffin-tin-orbital technique. The dispersion relations of the electronic states of quasicrystals are related to those of quasiperiodic free-electron model. Highly degenerate free-electron states are found close to the Fermi level. Their wave vectors define the analog of a Brillouin zone; the ‘‘Brillouin-zone’’–Fermi-sphere interaction leads to the formation of a pseudogap at the Fermi level.

We have used a first-principles molecular-dynamics technique to find the relaxed atomic geometries and the corresponding electronic structures of three of the simplest novel forms of three-coordinated solid carbon. We compare the energies of these structures to those of graphite and of the icosahedral fullerene ${\mathrm{C}}_{60}$. All of these forms of carbon contain only one kind of carbon atom. One of these, which we name polybenzene, is found to have a substantially lower energy per atom (0.23 eV) than ${\mathrm{C}}_{60}$, yet contains only 24 atoms per unit cell.

The fractal dimension of the loop-erased self-avoiding walk is shown to be identical to that of the chemical path on equally likely spanning trees. This result is valid for arbitrary lattices and in all dimensions. In particular, in two dimensions, using the spanning tree correspondence and the known results for the Potts model from conformal field theory, the fractal dimension is shown to be exactly 5/4 as conjectured by Guttmann and Bursill.

The regions where the three colors meet (the ‘‘tricords’’) are studied in percolation with three colors in three dimensions (3D). The tricords undergo a percolation transition as the proportions of the three colors are varied. Our Monte Carlo simulations indicate that this transition is not in the same universality class as the usual percolation transition in 3D.