In a recent publication [Phys. Rev. Lett. 68, 2011 (1992)] Noble and Burke applied the R-matrix method to low-energy electron scattering by oxygen molecules. They found that resonant contributions were made to the ${\mathit{a}}^1$${\mathrm{\Delta }}_{\mathit{g}}$ and ${\mathit{b}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ scattering cross sections, via a ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$ resonance, which significantly modified previous theoretical predictions and which were not observed in previous experiments. Here we present new results which provide the first definitive set of experimental data to confirm Noble and Burke’s prediction of resonance enhancement in the electron impact excitation of the a and b electronic states.

A highly accurate calculation is supplemented by an adiabatic approximation to explore the resonance spectrum of positronium ${\mathrm{Ps}}^{\mathrm{-}}$. Surprisingly, the spectrum can be understood and classified with ${\mathrm{H}}_2^{+}$ quantum numbers by treating the interelectronic axis of ${\mathrm{Ps}}^{\mathrm{-}}$ as an adiabatic parameter. We report and interpret the existence of ${}_{}{}^1{}_{}{}^{}$S shape resonances, a phenomenon so far unknown in three-body Coulomb systems. The new results on ${\mathrm{Ps}}^{\mathrm{-}}$ combined with previous results for ${\mathrm{H}}^{\mathrm{-}}$ suggest the existence of a resonance spectrum and its similarity for all ABA Coulomb systems with charges ‖${\mathit{Z}}_{\mathit{A}}$/${\mathit{Z}}_{\mathit{B}}$‖=1 and masses ${\mathit{m}}_{\mathit{A}}$/${\mathit{m}}_{\mathit{B}}$≥1.

Optical vortex-soliton filaments are observed in a bulk self-defocusing Kerr nonlinear refractive medium. The dark cylindrical core, located at the axis of a 2π helical phase ramp, is stationary and stable, with a size that depends inversely on the field strength. Wave guiding of a weak probe beam within the core is reported. A single optical vortex soliton was experimentally created using a quasihelical phase mask. Pairs having opposite topological charge were experimentally and numerically investigated using a convective Kelvin-Helmholtz instability of dark soliton stripes.

Exchange of angular momentum between photons and atoms cannot only cause optical pumping, but also a lateral displacement of a laser beam traveling through a homogeneous atomic vapor. We present experimental data from the ground state of atomic sodium demonstrating this effect.

The transition to turbulence in plane Couette flow was studied experimentally. The subcritical aspect of this transition is revealed by the stable coexistence of laminar and turbulent domains. By perturbing the flow, a critical Reynolds number has been determined, above which an artificially triggered turbulent spot can persist. The study of the spatiotemporal evolution of these spots shows, among other things, the existence of waves traveling away from the turbulent regions.

Rotating convection near onset is studied using a three mode model but with the inclusion of spatial dependence. An apparently chaotic dynamics is found, consistent with experiment, without the necessity of adding ad hoc noise terms as in previous models. There is no phase transition approaching the point of linear instability of straight rolls from the disordered side, and the characteristic domain size and switching time remain finite here. This behavior is discussed in terms of the velocity of walls between the different domains.

I report observations of a new state of traveling-wave convection which consists of pairs of localized wave packets which propagate around an annular cell in opposite directions. During their periodic collisions, the wave packets experience a loss in amplitude due to nonlinear interaction which exactly compensates the linear gain they experience at other times. This leads to a regular, small-amplitude state with high phase and group velocities. This state offers the promise of being quantitatively explainable on first principles using the simplest model of coupled complex Ginzburg-Landau equations.

Smith-Purcell (S-P) radiation is produced when electrons graze the surface of a grating. Calculations based on the theory of diffraction radiation show that, given severe restrictions on ${\mathit{e}}^{\mathrm{-}}$-beam quality, S-P radiation is highly efficient. Efficient S-P x-ray generation requires relativistic ${\mathit{e}}^{\mathrm{-}}$ beams having a transverse momentum and dimension whose product approaches the Heisenberg uncertainty limit.

We show here that the experimental field-emission electron spectra from W nanoprotrusion tips ending in a single atom consist solely of well-separated peaks. Each spectrum shifts linearly with the applied field-emission voltage. The peaks in the spectra strongly suggest that the tunneling occurs only through a localized band structure at the topmost atom. A charge confinement and a field penetration over the top atom is introduced to explain the linear shifts.

Siloxene (${\mathrm{Si}}_6$${\mathrm{O}}_3$${\mathrm{H}}_6$) and its derivatives show strong visible luminescence. By applying semiempirical quantum chemical calculations to the siloxene structure, we demonstrate that the presence of oxygen in a planar array of silicon atoms results in molecular orbitals confined to Si rings or chains which are responsible for the luminescence properties. Our results on the optical and vibrational properties of siloxene support earlier experimental results identifying siloxenelike structures as the luminescent agent in porous silicon.

We relate anharmonic equilibrium thermal fluctuations of smectics A to fluctuations of the Kardar-Parisi-Zhang (KPZ) dynamical model for a growing interface. The KPZ model in 1+1 dimensions is one to one related to a 2D smectic elastic model whose scaling behavior is then obtained exactly. The KPZ model in 2+1 dimensions maps into an elastic critical point of 3D smectics A with broken inversion symmetry (head-to-tail packing of layers). We discuss the elasticity and fluctuations of these novel smectic-A phases.

X-ray diffraction was used to monitor the in situ reaction of Pd deposited on Si(111) at room temperature. An ordered silicide forms spontaneously beneath a poorly ordered overlayer. It is commensurate and strained at low coverage, but relaxes to an unstrained state above a critical thickness of 18 Å. During both phases of growth sustained intensity oscillations are seen that correspond to a layerwise consumption of the substrate at the buried interface.

The Si(111):H surface is studied within the framework of density-functional theory using the local-density approximation. In particular, the Si-H stretch mode is investigated in detail. The phonon-phonon interaction strength is found to be 2Γ=-76.6 ${\mathrm{cm}}^{\mathrm{-}1}$ and the phonon bandwidth to be 7.2 ${\mathrm{cm}}^{\mathrm{-}1}$. Two-phonon bound states are found to exist with a binding energy of 86.4 ${\mathrm{cm}}^{\mathrm{-}1}$, in excellent agreement with the recent experimental result of 90 ${\mathrm{cm}}^{\mathrm{-}1}$.

The disordering of the uniaxial Cs/Cu(110) (1×3) phase was studied by scanning tunneling microscopy as a test case for the theoretical predictions of a two-step disordering mechanism. Lowering the Cs coverage first causes the formation of domain walls in the (1×3) phase, indicating a transition from a two-dimensional commensurate solid into an incommensurate floating solid. Dislocations in this wall pattern lead to progressing disordering into an incommensurate fluid. Further coverage reduction stabilizes fluctuating domains of a (1×4) phase, which signal a close by continuous solid to fluid transition.

A new approach to the Peierls distortion is described, where the lattice distortion is induced by some means external to the system. This nonequilibrium technique removes the temperature limitations on the Peierls distortion and the limitation of phonon localization, and so can generate charge-density waves at an arbitrary temperature. A method is considered for coherent phonon generation and subsequent use with a quantum wire array to induce a charge-density-wave transition.

A variational method shows that for a range of densities the compensated low density interacting electron system possesses a ground-state crystalline phase with L=0 spin-singlet pairs and with an energy lower than the Wigner crystal. The physical origin of the additional binding is intrapair exchange resulting from wave-function overlap and cooperative anharmonic behavior.

We report on measurements of complex ac conductance of a random system of carbon-wax mixtures using both frequency (f) and amplitude (V) as measuring probes. The latter gives rise to nonlinear effects which necessitate modification of the usual two-variable scaling law. We propose a three-variable scaling form that reproduces our key experimental results summarized in the form of a R-f-V diagram (R is the inverse of the real part of the ac conductance). The results are also discussed in the context of some existing models.

We examine the nature of the ground state of a two-dimensional interacting electron gas in a self-induced gauge field. We show that for a realistic form of such a many-body gauge field in two dimensions and increasing inhomogeneity of the electron density, a spontaneous current-carrying state develops. Its relation to other time-reversal broken-symmetry ground states is discussed.

We show how the breakdown of Kramers degeneracy for spin-orbit coupled bands in asymmetric double quantum wells may give rise to a new mechanism for tunneling which does not require resonant conditions. The most interesting aspect of this finding is that, in this case, there is a ‘‘spin’’-dependent delocalization of the wave function, leading to preferential tunneling of one ‘‘spin’’ state. We relate this effect to the symmetry properties of the wave functions of both single and asymmetric double quantum wells, and provide a qualitative explanation in terms of perturbation theory.

Far-infrared magnetotransmission and photoconductivity measurements combined with visible-light, photon-dose experiments on wide-barrier GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well samples planar doped with Si donors in both well and barrier centers have provided unambiguous evidence for the existence of ${\mathit{D}}^{\mathrm{-}}$ ion states and the evolution of the occupancy of the donor-ion electronic states in the quantum wells with excess electrons in this quasi-2D system.