Very unstable viscous fingers moving in a linear channel are investigated as well as diffusion-limited aggregates grown in a strip between two reflecting walls. In both cases a large number of independent runs are performed and the cell occupancy distribution is measured. It is shown that the zone of large occupancy has the width and shape of the Saffman-Taylor finger λ=0.5. Similarly, in sector-shaped cells, the width of the large occupancy region is the limit width of the stable fingers. In the case of a 90° cell, its shape is that of a predicted analytical solution.

The liquid state of carbon at low pressure is investigated wtih a first-principles molecular dynamics simulation. Its controversial electronic properties are elucidated in terms of density of states and conductivity calculations, showing that the system is a metal, in agreement with experiments reported last year. Furthermore, an accurate analysis of the atomic structure indicates that the liquid is composed of two-fold, three-fold, and four-fold coordinated atoms, which display different bonding properties.

Hot-electron transport in a 2D electron gas is investigated as a function of the electron’s excess energy. The inelastic mean free path at energies below the longitudinal-optical-phonon energy is found to be an order of magnitude longer than theoretical predictions. For higher injection energies, LO-phonon emission is found to be the main scattering mechanism. This, together with the ballistic motion for energies below the phonon energy, leads to previously unobserved periodic oscillations in the maximum energy of hot electrons as a function of their injection energy.

We have observed quenching of the Hall resistance in a GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructure Hall junction containing four narrow constrictions leading into a quantum dot. Backgated junctions show quenching within a broad but finite electron density range. In contrast, junctions containing fewer constrictions show little or no quenching behavior. These results show conclusively that the nature of the junction region is crucial in producing quenching and can be explained in terms of junction scattering using Buttiker-Landauer formulas.

A numerical technique for the characterization of the chaotic regime of dissipative maps through unstable periodic orbits is presented. It is shown that although the maps are dissipative their trajectories can be derived from a Hamiltonian, which allows us to calculate unstable periodic orbits of arbitrary length finding all points to any desired accuracy. Applying the method to the Hénon map we found that in a previously unexplored region of parameter space the topological entropy exhibits plateaus on which it is constant while the dynamics is characterized by a strange repeller.

We demonstrate the utility of the periodic-orbit description of chaotic motion by computing from a few periodic orbits highly accurate estimates of a large number of quantum resonances for the classically chaotic 3-disk scattering problem. The symmetry decompositions of the eigenspectra are the same for the classical and the quantum problem, and good agreement between the periodic-orbit estimates and the exact quantum poles is observed.

We prove the rigorous lower bound $z_{\mathrm{sw}\ge \mathrm{\alpha }/\nu }$ for the dynamic critical exponent of the Swendsen-Wang algorithm. For two-dimensional q-state Potts models with q=2,3,4, this implies $z_{\mathrm{sw}\ge 0}$,(2/5,1. We present numerical data indicating that $z_{\mathrm{sw}=0.55\mathrm{}\pm 0.03}$, 0.89±0.05 for q=3,4 (95% confidence limits, statistical errors only). The discrepancy for q=4 appears to be caused by multiplicative logarithmic corrections.

A line-charge–neutron composite is proposed as a new model for fractional spin. Considering time-varying line-charge configurations, the kinetic angular momentum is a constant of motion while the canonical angular momentum is not. This result is contrary to the case with magnetic flux tubes. The fractional angular momentum (spin) is, therefore, determined without ambiguity.

We present a gauge theory of two-dimensional gravity which is derived from a generalization of the Jackiw-Teitelboim model. Using canonical quantization, we construct the exact quantum solution of this model.

The observation of solar neutrinos at Kamiokande rules out the Mikheyev-Smirnov-Wolfenstein (MSW) theory with a ‘‘large’’ mass difference between the neutrinos, Δ${\mathrm{m}}^2$≃${10}^{\mathrm{-}4}$ ${\mathrm{eV}}^2$. There remain the MSW theory with a small Δ${\mathrm{m}}^2$≃${10}^{\mathrm{-}7}$ ${\mathrm{eV}}^2$ or the assumption of unexpectedly low central temperature in the Sun. A decision between these two alternatives will be possible on the basis of the future gallium experiment, and the Sudbury experiment with heavy water.

The production of cosmic-ray positrons from the annihilation of weakly interacting massive particles (WIMP’s) is considered. Conventional supersymmetric-neutralino annihilation generally yields an unobservably small $e^{+}$ flux. However, a massive WIMP (≳20 GeV) with a large annihilation cross section into a single $e^{+}$$e^{\mathrm{-}}$ pair produces a distinctive and observable shelf in the cosmic-ray $e^{+}$ spectrum. Only Dirac neutrinos obviously generate such a feature, but it may also appear in more elaborate neutralino models. Such models are constrained by upper limits on the low-energy antiproton flux.

In some composite models of quarks and leptons, leptons are expected to develop strong interactions at high energies. This hypothesis can be tested by studying the absorption of high-energy particles emitted by point sources in the sky on the cosmic microwave background. Data on Cygnus X-3 exhibit an anomaly in the absorption. A plausible interpretation of the anomaly is that some of the interactions are caused by neutrinos. We infer that the cross section of neutrino-nucleon interactions is approximately 6 mb at energies of a few PeV, significantly exceeding the prediction of the standard model.

In order to model positronium-positronium scattering in QED or meson-meson scattering in QCD, we consider ${\mathrm{QED}}_{1+1}$, which is a gauge theory and confines single fermions. We present first numerical results of a lattice calculation on scattering of two composite particles. The composite particles are taken as neutral, fermion-antifermion, lowest-mass eigenstates of the Hamiltonian. We use the light-cone momentum representation on a lattice and employ a nonperturbative time-dependent method to compute the S matrix.

The quantization of meson fluctuation arount a two-soliton system is discussed. A new consistency equation which must be satisfied by the classical two-soliton configuration is derived. We demonstrate the existence of solutions to this equation in (1+1)-dimensional soliton models and show how they lead to covariant one-meson exchange.

Cross sections were determined in the Δ(1232) excitation region for the reactions ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(γ,p)pn and ${}_{}{}^2{}_{}{}^{}\mathrm{H}$(γ,p)n at several photon energies and proton emission angles. Inclusive proton spectra were measured with a magnetic spectrometer, using quasimonochromatic $e^{+}$-annihilation photons. The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ data can be mainly accounted for by predictions assuming photon absorption by a pair of nucleons, except in the high-momentum part of the proton spectra. They clearly indicate the need for additional contributions, such as three-body absorption mechanisms.

Microsecond isomers in the very proton-rich N=82 nuclei ${}_{}{}^{153}{}_{}{}^{}\mathrm{Lu}$ and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Hf}$ have been identified by γ-ray spectroscopy following mass analysis of ${}_{}{}^{102}{}_{}{}^{}\mathrm{Pd}$ +245 MeV ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ reaction products using the Daresbury Recoil Separator. The decays of the isomers, interpreted as (π$h_{11/2}$${)}^n$ states, are characterized. Examination of reduced E2 transition rates between (π$h_{11/2}$${)}^n$ states in the N=82 isotones indicates that the half-filling of the π$h_{11/2}$ subshell occurs just below Z=71.

A comparative analysis of wave profile modification effects in Raman free-electron lasers is presented. The analysis is based on a 3D theoretical model that is valid in both Raman and Compton regimes. We study two companion effects, the optical guiding and the excitation of space-charge waves with transverse field components. Both effects are compared through exemplary parameters based on previous free-electron laser experiments. We conclude that transverse field profile modification due to space-charge waves may be significant in comparison to the optical guiding effect.

Orbital angular momentum of the coherently aligned Cooper pairs in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{A}$ is encountered by an object immersed in the condensate. We evaluate the associated quasiparticle-scattering asymmetry experienced by a negative ion; this leads to a measureable, purely quantum-mechanical reactive force deflecting the ion’s trajectory. Possible hydrodynamic Magnus effects are also discussed.

We have studied the structure and surface energetics of vicinals of Au(100) and Au(111), using a well-tested many-body force model for gold and molecular dynamics. Surface reconstruction by in-plane contraction—known to occur on Au(100) and Au(111)—takes place on terraces between steps, causing the appearance of particularly favored ‘‘magic’’ orientations whenever the terrace width roughly contains an integer number of reconstruction cells. This phenomenon has implications on the faceting behavior of these surfaces, and explains recent observations of ‘‘locked’’ distances between steps.

We present a theory for the atomic force microscopy (AFM) of deformable surfaces and apply it to graphite with and without intercalated atoms. Using continuum elasticity theory for graphite layers, with parameters obtained from ab initio calculations, we determine quantitatively local distortions in the vicinity of a sharp AFM tip as a function of the applied force. Our calculations show that AFM should be a unique tool to determine local surface rigidity and to measure the healing length of graphite in the vicinity of intercalated impurities or steps.