QCD in 2+1 dimensions with N species of fermions, analyzed in the 1/N expansion, is shown to exhibit a chiral and confinement phase transition as N approaches a critical value ${\mathrm{N}}_{\mathrm{c}}$. For N>${\mathrm{N}}_{\mathrm{c}}$, the theory is chirally symmetric and deconfined. For N<${\mathrm{N}}_{\mathrm{c}}$, chiral symmetry is broken and confinement sets in at momentum scales less than the fermion mass. The reliability of the 1/N expansion when N≃${\mathrm{N}}_{\mathrm{c}}$ is discussed, and the behavior of this theory is contrasted with QCD in 3+1 dimensions.

Principles of minimality and unifiability lead us to a unique family of mutants of the electroweak theory which are parametrized solely by the mass of a heavy Z’ boson. These models generate an additional vectorial current-current interaction at low energy, and modify the properties of the observed Z boson in a way that would appear (in the context of the standard model) as an apparently fractional number (exceeding three) of light neutrino species. Such alternative theories offer a foil against which to test our confidence in the orthodox theory.

Absolute rate coefficients for radiative recombination between fully stripped carbon ions and free electrons have been measured for relative energies between 0 an 1 eV. The experiment was performed with a merged-beam technique. A fast-pulsing technique was used to turn on and off the electron source to allow subtraction of background signals stemming from electron capture in the rest gas. The data are compared with the theory for radiative recombination, as derived by Stobbe in 1930 and by Bethe and Salpeter in 1957. The present data constitute the first direct test of the theory for radiative recombination.

Femtosecond laser pulses are applied to the study of the dynamics and the pathways of multiphoton-induced ionization, autoionization, and fragmentation of ${\mathrm{Na}}_2$ in molecular-beam experiments. In particular, we report on first results obtained studying electronic autoionization (leading to ${\mathrm{Na}}_2^{+}$+${\mathrm{e}}^{\mathrm{-}}$) and autoionization-induced fragmentation (leading to ${\mathrm{Na}}^{+}$+Na+${\mathrm{e}}^{\mathrm{-}}$) of a bound doubly excited molecular state. The final continuum states are analyzed by photoelectron spectroscopy and by measuring the mass and the released kinetic energy of the corresponding ionic fragments with a time-of-flight arrangement.

State-resolved cross sections for the dielectronic recombination of hydrogenlike oxygen ions with free electrons have been measured for the first time using the electron cooling device in the heavy-ion Test Storage Ring in Heidelberg. Energies and cross sections for individual terms of the configuration 2l2l’ were determined. Two-electron resonances 2lnl’ with n≥3 contribute 90% to the total dielectronic-recombination cross section. Energies and cross sections are in reasonable agreement with available calculations.

Nonequilibrium continuous and discontinuous phase transitions in the laser with a saturable absorber have been observed. A novel feature of these measurements is the observation of the tricritical behavior. The adequacy of the perturbative approach to describe saturation effects is discussed in light of these measurements.

Wavelet transformations (WT’s) of two-dimensional fractal aggregates are carried out using coherent optical spatial frequency filtering. The ability of optical WT’s to reveal the construction rules of fractals and to resolve local scaling properties through the determination of local pointwise dimensions is demonstrated. This fast method paves the way to time-resolved studies of dynamical phenomena.

Uniformly translating solutions of the one-dimensional complex Ginzburg-Landau equation are studied near a subcritical bifurcation. Two classes of solutions are singled out since they are often produced starting from localized initial conditions: moving fronts and stationary pulses. A particular exact analytic front solution is found, which is conjectured to control the relative stability of pulses and fronts. Numerical solutions of the Ginzburg-Landau equation confirm the predictions based on this conjecture.

Strong Alfvénic turbulence containing a number of solitonlike structures propagating at super-Alfvénic speeds is generated self-consistently and studied by means of computer simulation. A one-dimensional hybrid (kinetic ions, fluid electrons) code is used to investigate the nonlinear evolution of an electromagnetic ion-beam instability that generates low-frequency Alfvén-like waves. As the instability develops, the field-aligned hydromagnetic waves steepen, forming a soliton that bifurcates several times, leading to a fully turbulent state.

A simple physical picture of plasma transport in magnetic fusion devices allows us to deduce the decreasing energy confinement time with added heating power and the increasing confinement time with added plasma current. The first is due to Bohm-like diffusion and the second is due to the characteristics of Z pinches and a decrease of deposited power due to enhanced radiation loss. The addition of momentum into a plasma by flowing current that includes noninductive current drive may lead to a Z-pinch effect that compensates for the decrease in confinement time caused by the heating.

A new approximation to the density-functional theory of classical nonuniform systems is proposed and worked out for the case of the hard-sphere solid. The theory satisfies all the formal properties of the free energy and requires only the direct correlation function of the uniform system as input. the agreement with the computer simulations of the fcc hard-sphere solid is excellent: The resulting free energies, pressures, and fluid-solid coexistence data are reproduced to within the error bars of the simulations. The theory also predicts stable bcc and sc phases which could facilitate the final nucleation into the equilibrium fcc phase.

Inelastic neutron spectra of the commensurate √3 ×3 herringbone monolayer phase of nitrogen physisorbed on the graphite (002) surface at low temperature have been compared with lattice dynamics calculations of the polycrystalline-averaged one-phonon coherent neutron cross section. The observed zone-center energy gap of ∼0.4 THz in the acoustic-phonon branches is a factor of 2 larger than calculated from central atom-atom potentials. We conclude that current models of the corrugation in the adatom substrate potential greatly underestimate the lateral restoring forces in this relatively simple molecular film.

The phonon structure of monolayer graphite formed on some transition-metal carbides has been investigated by using electron-energy-loss spectroscopy. The dispersion relations of the phonons in the monolayer graphite have indicated peculiar softening in intralayer chemical bonds and considerable bonding between the overlayer and the substrate. This modification can be ascribed to the electron transfer from the substrate into the ${\mathrm{\pi }}^{\mathrm{-}}$ band of the graphite.

We present a scaling theory for the time evolution of the morphology of amorphous films, based on the Huygens principle growth algorithm. During the coarsening stage of the growth, the time-dependent correlation length diverges with time as ξ(t)∝${\mathrm{t}}^{\mathrm{p}}$. We calculate p for a range of random and self-similar starting surfaces. When the effect of noise is taken into account, the exponent p reaches a universal value (3/4, in good agreement with experiments.

The molecular dynamics with a first-principles interatomic potential is used to study the phase transition of quartz from the α to β phase. The phase transition, successfully reproduced at about 850 K, is shown to have a peculiar dynamical character in that, although the probability distribution for atomic positions mimics, when averaged over a long time, a displacive shift to the β structure, atoms in fact hop between the two equivalent ${\alpha }_1$ and ${\alpha }_2$ structures by anharmonic motions with a temperature-dependent correlation time.

We report measurements of the valence- and core-level photoemission spectra of mass-selected, monodispersed Pt clusters. For individual samples consisting of the platinum atom through the six-atom cluster deposited onto a silica substrate, we have followed the development of the individual valence-band electronic structure and the Pt 4f core-electron binding energy, and conclude that such small metal clusters do not yet exhibit metallic character.

We present a novel subpicosecond optical transient-reflection method, which employs the surface-plasmon resonance of a thin silver film as a very sensitive probe to study small transients of the effective electron and lattice temperature. By measuring under surface-plasmon resonant and nonresonant conditions, we can discriminate between lattice and electron heating. We find a change of the lattice and electron temperature of respectively, Δ$T_l$=0.10(5) K and Δ$T_e$=30–90 K for an intensity of 180 W ${\mathrm{cm}}^{\mathrm{-}2}$ and an electron-phonon energy relaxation time of 670(70) fs.

Very small quantum-dot structures containing 210 to 25 electrons per dot have been prepared starting from modulation-doped AlGaAs/GaAs heterostructures. The far-infrared response consists of a set of resonances which split, in a magnetic field B, into branches with negative and positive B dispersion. The intersection of these resonances, in classical analogy edge magnetoplasmons, leads to an anticrossing of the dispersions. This resonant coupling is induced by nonlocal interaction which becomes important at small dimensions.

Time-resolved demonstration of picosecond photon echoes due to intrinsic optical excitations of a semiconductor is presented. Phase relaxation times up to several hundred picoseconds are found for excitons localized by chemical disorder in ${\mathrm{CdS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathrm{x}}$ mixed crystals. These long dephasing times are attributed to reduced scattering of the localized excitons as compared to free excitons. Dephasing is identified as due to energy relaxation of the excitons.

Anisotropic propagation of excitonic polaritons in GaAs single-quantum-well waveguide structures is evidenced by means of time-of-flight measurements. This anisotropy suggests that the depolarization effect of two-dimensional excitons enhances the polariton propagation. The observed optical nonlinearity of the group velocity supports the existence of polaritons and reflects the effect of a polariton-polariton interaction on their propagation.