We have performed the first excitation of high-n states of positronium using resonant, two-photon excitation applied to the n=1 to 2 and 2 to n transitions. Absolute values for the line positions were quantitatively determined for n=14 and 15 and compared well with calculated predictions. The prediction of the ${\mathit{n}}^{\mathrm{-}3}$ scaling of the relative transition rates was observed for n=12 to 19.

We report results on the transition from soft to hard turbulence in simulations of 2D Boussinesq convection. The computed probability densities for temperature fluctuations are exponential in form in both soft and hard turbulence, unlike what is observed in experiments; in contrast, we obtain a change in the Nusselt number scaling on Rayleigh number in good agreement with the 3D experiments.

The lifetime of the circulating electron beam in the Naval Research Laboratory modified betatron has been increased by more than 2 orders of magnitude with the addition of strong focusing windings to the device. The injected beam consistently spirals from the injector to the minor axis and is trapped. The ∼0.5-kA trapped electron ring has been accelerated above 10 MeV from the injection energy of 0.5 to 0.6 MeV. The beam acceleration has been confirmed not only by the x-ray attenuation technique but also with the detection of photoneutrons.

The phase diffusion and mean drift equations which describe the behavior of a convection pattern are derived, a step which is essential for obtaining quantitative comparisons between theory and experiment. The theory recovers the boundaries of the Busse Balloon, agrees closely with the dominant wave numbers observed by Heutmaker and Gollub [Phys. Rev. A 35, 242 (1987)] and Steinberg, Ahlers, and Cannell [Phys. Sci. 30, 534 (1985)] in natural and target patterns, predicts a new instability which is important in facilitating wave-number adjustment in circular target patterns and in initiating time dependence, and predicts the Rayleigh numbers at which loss of spatial correlation due to global defect nucleation will occur.

Thermal boundary layers in turbulent Rayleigh-Bénard convection are studied experimentally using a novel system in which the convecting fluid is sheared from below with a flowing layer of mercury. Oscillatory shear substantially alters the spatial structure and frequency of the eruptions, with minimal effect on the heat flux (<5%). The temperature probability distribution function (PDF) just above the lower boundary layer changes from Gaussian to exponential, without significant changes in the interior PDF. Implications for theories of ‘‘hard’’ turbulence are discussed.

We have measured the heat capacity of submonolayers of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on Ag films between 1.7 and 3.3 K. Good fits to the results are obtained with a model of a noninteracting two-dimensional Bose gas. The sticking probability for room-temperature ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms on cold Ag has been measured as a function of substrate temperature and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ coverage. The sticking probability is 4% at low coverage, and abruptly drops to 1% for coverages above 0.5 monolayer.

A three-dimensional random-tiling icosahedral quasicrystal is studied by a Monte Carlo simulation. The hypothesis of long-range positional order in the system is confirmed through analysis of the finite-size scaling behavior of phason fluctuations and Fourier peak intensities. By investigating the diffuse scattering we determine the phason stiffness constants. A finite-size scaling form for the Fourier intensity near an icosahedral reciprocal wave vector is proposed.

The energy of each phonon mode is fluctuating around its equilibrium value, and its power spectral density was derived with a quartz specimen which was illuminated by a laser light. Fluctuations of the Brillouin scattered light intensity were measured by means of the photon-counting technique. The power spectral density has been derived to be 8×${10}^{\mathrm{-}5}$/f per phonon mode provided that phonon-number fluctuations are independent from mode to mode.

We report high-resolution angular-resolved electron-energy-loss measurements of the cross section of surface-plasmon excitation. The intensity shows a pronounced minimum as a function of exchanged momentum near ${\mathit{q}}_{\mathrm{\parallel }}$=0 whose position depends on impact energy and angle of incidence of the electron beam and is determined by the value of the phase shift occurring in the scattering process. The effect has been taken into account in the determination of energy and dispersion of the surface plasmon of Ag(100). A linear dispersion is found in the range of ${\mathit{q}}_{\mathrm{\parallel }}$ between 0 and 0.15 A${\mathrm{̊}}^{\mathrm{-}1}$ in contrast to the quadratic behavior found for Ag(111) and Ag(110).

Dislocations in GaAs induced by plastic deformation have been studied by scanning tunneling microscopy. Atomically resolved images of perfect and partial dislocations penetrating the cleaved {110} surface were obtained. A Burgers-vector analysis of the observed dislocations was performed. During observation the dislocations were found to be mobile over nm distances. No band bending was observed around the dislocation cores indicating that they are essentially electrically neutral.

The phase diagram of vicinal Si(100) as a function of misorientation angle and temperature is calculated. Contrary to previous suggestions that only double-layer steps should appear on the equilibrium surface, it is predicted that the single-layer stepped surface is at equilibrium for small misorientation angles. This structure is stabilized by strain relaxation and by the thermal roughening of the steps. For annealed surfaces the critical angle at which the transition between the single- and double-layer stepped surface occurs is calculated to be ${\mathrm{\theta }}_{\mathit{c}}$≊2°.

We have used high-resolution low-energy electron diffraction to study the disordering of the (3×1) reconstruction on Si(113). We find that the reconstruction disorders via a continuous transition at approximately 850 K. Above the transition temperature the positions of the superlattice diffraction beams shift away from their commensurate positions. The shift is proportional to the broadening of the beams, as expected from basic scaling arguments for a system in the chiral three-state Potts-model universality class. Also consistent with numerical studies of this class is the observation of critical exponents α, β, γ, and ν approximately equal to those of the pure three-state Potts model.

Deviations in atomic positions from those of the average crystallographic structure of superconducting ${\mathrm{Tl}}_2$${\mathrm{ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ were studied by pair-distribution function (PDF) analysis of pulsed-neutron-scattering data. The PDF shows a clear change in the local structure at the onset of superconductivity. In addition to the previously observed local displacements of Tl and O within the Tl-O sheets, correlated displacements of O and Cu perpendicular to the Cu-O plane have been found. The arrangement of the O atoms appears to be different above and below ${\mathit{T}}_{\mathit{c}}$.

It is shown that in dielectrics exhibiting a complete photonic band gap, quantum electrodynamics predicts the occurrence of bound states of photons to hydrogenic atoms. When the atomic transition frequency lies near a photonic band edge, the excited atomic level experiences an anomalous Lamb shift and splits into a doublet. One member of this doublet exhibits resonance fluorescence whereas the other level is dressed by the emission and reabsorption of near-resonant photons whose amplitude decays exponentially from the vicinity of the atom.

Charge accumulation in the quantum-well region of a double-barrier AlGaAs/GaAs resonant-tunneling-diode structure is studied by using photoluminescence and photoluminescence-excitation spectroscopy. The observed optical spectra are found to change systematically with an applied bias and make it possible to directly determine the charge accumulation under various biasing conditions. At the resonant condition, a charge accumulation of 5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ is obtained. The importance of band-gap normalization in resonant-tunneling process is also indicated.

We report on the observation of photoluminescence from the second and third electronic subbands in undoped GaAs-AlAs superlattices under application of an electric field perpendicular to the layers. The occupation of the higher subbands is achieved by sequential resonant tunneling of electrons. We determine the relative occupation of the second subband at resonance. Together with the electrically measured transport time between adjacent wells this novel spectroscopic effect provides a new method to determine the intersubband relaxation time.

Charge-carrier spin scattering in diluted-magnetic-semiconductor multiple quantum wells is directly observed through the circular polarization of luminescence in a femtosecond time-resolved measurement. This represents a new short-time regime for direct spin-relaxation studies. Separate observations of electron and hole relaxation are accomplished by varying the applied magnetic field. Surprisingly, the spin-flip scattering rates in these systems appear to be independent of the extremely strong spin-spin–exchange interaction between the charge carriers and the magnetic ions.

We have investigated the influence of a two-dimensional electron gas on interband optical transitions in an (In,GaAs) modulation doped single quantum well under conditions where the Fermi level is very near the n=2 conduction subband and a many-body exciton is observed from this subband. Strong variations occur in external magnetic fields in both the exciton spectrum and its amplitude of which the latter correlates with the longitudinal Hall resistance in the quantum Hall regime.

We have used high-quality ${\mathrm{Cu}}_{92}$${\mathrm{Mn}}_8$/${\mathrm{Al}}_2$${\mathrm{O}}_3$ multilayers to investigate the shift in temperature and the change in scaling behavior of the spin-glass transition as a function of the layer thickness. In the thinnest CuMn layers (60 and 88 Å), the spin-glass exponents determined from the nonlinear susceptibility shift to new values, distinctly different from those for the bulk. These exponents are consistent with numerical studies of 2D spin-glass systems, suggesting that a 3D to 2D crossover occurs.

We have performed angle-resolved photoemission experiments and density-functional band calculations for NiO and CoO. The measured oxygen bands are in excellent agreement with the calculation, but the measured cation 3d dispersion is only about 25% of the calculated one. We have also demonstrated the effects of the antiferromagnetic order on the electronic structure of these materials.