A simple model of restructuring for loopless clusters is presented in which single-bonded particles may escape from the cluster and diffuse in space until they reach the cluster again. In its steady-state regime, this model defines a new statistics of clusters at equilibrium with a fractal dimension $D=1.54\mathrm{}\pm 0.08$ in two dimensions estimated by direct simulations. Comparisons are made with out-of-equilibrium diffusion-limited aggregation and with at-equilibrium statistics of animals.

We study interacting particle (spin) systems on a lattice under the combined influence of Glauber (spin flip) and simple exchange (Kawasaki) dynamics. We prove that when the conserving exchanges occur on a microscopically fast scale the macroscopic density (magnetization) evolves according to an autonomous nonlinear diffusion-reaction equation. Microscopic fluctuations about the deterministic macroscopic evolution are found explicitly. They grow, with time, to become infinite, when the deterministic solution is unstable.

The Krylov-Bogoliubov method of averaging is applied to the time-dependent quantum anharmonic oscillator. A regular perturbation expansion contains secular terms. The averaging approximation does not, and as a result has a validity over larger time intervals. A new variant of the usual averaging transformation is used and rigorous error bounds are derived. Rigorous averaging methods have been applied extensively to ordinary differential equations but our work appears to be the first generalization to partial differential equations.

We report a measurement of the inclusive charged-particle distribution for gluon jets derived from nearly threefold-symmetric three-jet events taken at center-of-mass energy of 29 GeV in $e^{+}{e}^{-}$ annihilation. The charged-particle spectrum for these jets is observed to fall off more rapidly than those of quark jets of the same energy.

We present a large-scale Monte Carlo calculation of the deconfining phase-transition temperature in lattice quantum chromodynamics without fermions. Using the Wilson action, we find that the transition temperature as a function of the lattice coupling $g$ is consistent with scaling behavior dictated by the perturbative $\beta$ function for $\frac{6}{g^2}>6.15\mathrm{}$

We present new data on the Feynman-$x$ dependence of $\Upsilon$ production in proton-tungsten collisions at a proton-beam momentum of 400 GeV/c. Comparing these data with the well-understood Drell-Yan dimuon continuum, we conclude that the $\Upsilon$ production mechanism must be qualitatively different.

Analysis of underground muon events from the direction of Cygnus X-3 shows evidence for large time variability of the flux in addition to the 4.8-h modulation. Our data support earlier suggestions that high fluxes occur with a 34.1-d cycle. Events measured during high-rate periods show increased statistical support for the hypothesis linking underground muons with this x-ray binary.

We have determined the electroweak parameters ${{sin}^2\theta }_{\mathrm{W}}$ and $\rho$ by a measurement of deep-inelastic neutrino-nucleon scattering used a fine-grained neutrino detector exposed to a narrowband neutrino beam at Fermilab. The sampling properties of our detector have permitted neutralcurrent and charged-current events to be well separated over a wide kinematic range, thereby allowing a determination of ${{sin}^2\theta }_{\mathrm{W}}$ and $\rho$ to be made with good statistics and small systematic errors. We have found ${{sin}^2\theta }_{\mathrm{W}}=0.246\mathrm{}\pm 0.012\pm 0.013$ in a single-parameter fit.

As nuclear temperature increases, the surface-tension coefficient decreases and the Coulomb repulsion is effective in pushing the nuclear matter outward, leading to the formation of toroidal and bubble nuclei. We obtain the threshold temperatures above which these nuclei are stable against symmetry-preserving distortions. They are found to decrease with the mass number.

New and more accurate energy profiles for interstitial muonium in diamond clusters (${\mathrm{C}}_{10}$${\mathrm{H}}_{16}$ to ${\mathrm{C}}_{30}$${\mathrm{H}}_{40}$) have been determined by ab initio and approximate ab initio calculations, and have been critically compared to previous theoretical work. The energy profiles are shown to be extremely sensitive to the position of the surface atoms used to terminate the cluster. It is established that if the equilibrium carbon-hydrogen bond length is used to terminate the cluster the energy profiles converge rapidly with cluster size, but the use of any other value for $d_{\mathrm{C}-\mathrm{H}}$ leads to qualitatively incorrect energy profiles for small clusters.

We have measured the linewidths of ${\left[5\mathrm{}{d}_{\frac{3}{2}}{n}_2{d}_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ autoionizing Rydberg states of barium with principal quantum numbers ranging between $n_2=14\mathrm{}$ and $n_2=100\mathrm{}$. Dramatic deviations of the autoionization rates from the normal $n_2^{*-3\mathrm{}}$ scaling law have been observed. Whereas the ${\left[5\mathrm{}{d}_{\frac{3}{2}}60d_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ level exhibits a linewidth $\Gamma$ (FWMH) as low as 6 MHz, higher ($n_2\approx 80$) members of this series are broadened up to 5 GHz. Autoionization is inhibited for principal quantum numbers $n_2\approx 60$ because of configuration interaction between the ${\left[5\mathrm{}{d}_{\frac{3}{2}}{n}_2{d}_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ and ${\left[5\mathrm{}{d}_{\frac{5}{2}}{n}_3{d}_{\frac{5}{2}}\right]}_{J=0\mathrm{}}$ Rydberg series. This causes the state at $n_2=60\mathrm{}$ to be metastable.

We have measured a secondary electron spectrum at ${\theta }_L=0\mathrm{}°$ produced in ${\mathrm{H}}^{-}$-He collisions in which the detected electrons are time correlated with the detection of protons. This electron energy spectrum does not show the double-peaked structure usually seen in ${\mathrm{H}}^{-}$-He collisions and associated with the dominant single-electron-loss process which produces neutral hydrogen rather than protons. The measured electron energy spectrum is asymmetric and implies a nonisotropic angular distribution in the projectile frame. A possible source of this asymmetry is discussed.

For the first time a nonadiabatic (all-particle) calculation has been carried out for the electric polarizabilities and hyperpolarizabilities of ${\mathrm{H}}_2^{+}$, H${\mathrm{D}}^{+}$, and ${\mathrm{D}}_2^{+}$ in their lowest rovibronic states. The value of the hyperpolarizability $\gamma$ is dramatically different from that which would be assumed from calculations based on the Born-Oppenheimer approximation unless account is taken of a vibrational contribution which (unlike its counterpart for the $\alpha$ polarizability) is nonzero even for a homonuclear diatomic molecule. For ${\mathrm{H}}_2^{+}$ we find that ${\alpha }_{\mathrm{zz}}=5.827\mathrm{}$ a.u. and ${\gamma }_{\mathrm{zzz}}=2.2\mathrm{}\times {10}^3$ a.u.

The output power of a laser containing an elasto-optic modulator exhibits period-doubling bifurcations culminating in chaos and periodic windows. The bifurcation diagram of the frequency-modulated laser appears quite similar to that of the usual quadratic mapping. In addition, it demonstrates generalized bistability and depends on the rate of change of the control parameter. The experimental results show excellent agreement with those provided by a simple model of the laser.

We have observed a new phenomenon in the pinched phase of an argon collapsing-gas-shell $Z$-pinch. An x-ray streak camera was used to observe the hard-x-ray (>1 keV) emission as a function of $Z$ position and time. The streak photographs show that at a given $Z$ location the x-ray emission "winks." We have developed a model based upon runaway electrons interacting with ions to explain this phenomenon.

Realistic potentials are used to calculate the properties of dislocation dipoles in krypton and xenon monolayers physisorbed on graphite. The results are not as predicted by elasticity theory. Short vacancy dipoles are favored but are immobile. The xenon monolayer is similar to its floating counterpart. The commensurate krypton monolayer is dominated by the substrate and favors small defects. In incommensurate krypton the vacancy dipoles are coupled to the domain walls but can move freely; in this case a novel continuous-melting mechanism is indicated.

We develop two- and three-body classical interatomic potentials that model structural energies for silicon. These potentials provide a global fit to a data base of first-principles calculations of the energy for bulk and surface silicon structures which spans a wide range of atomic coordinations and bonding geometries. This is accomplished by use of a new "separable" form for the three-body potential that reduces the three-body energy to a product of two-body sums and leads to computations of the energy and atomic forces in $n^2$ steps as opposed to $n^3$ for a general three-body potential.

New and accurate light-scattering measurements near the critical point of the nonionic micellar solution $n$-dodecyl octaoxyethylene glycol monoether in ${\mathrm{H}}_2$O in the reduced temperature range $6\times {10}^{-5\mathrm{}}<\epsilon <2\mathrm{}\times {10}^{-2\mathrm{}}$ confirm that this binary system presents anomalous critical exponents $\gamma$ and $\nu$. The values obtained are $\gamma =0.88\mathrm{}\pm 0.03$ and $\nu =0.43\mathrm{}\pm 0.03$. The substitution of ${\mathrm{D}}_2$O for ${\mathrm{H}}_2$O changes the exponents to $\gamma =1.20\mathrm{}\pm 0.03$ and $\nu =0.59\mathrm{}\pm 0.03$. Intermediate exponents are obtained by use of a mixture of ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O as the solvent. Both the osmotic compressibility and the correlation range closely follow a power-law behavior as function of $\epsilon$ without showing any crossover effect or correction-to-scaling contribution.

We have measured the longitudinal spin-relaxation functions of positive muons injected into trans-${\left(\mathrm{CH}\right)}_x$ at various temperatures and external fields. They showed non-single-exponential functions and were fitted by a model which takes into account both the on-chain diffusion and the disappearance of the muon-produced soliton. It was found that the on-chain diffusion rate $D_{\parallel }$ is almost constant from 29 to 290 K, while the disappearance rate $D_3$ slightly increases with temperature.

The attenuation and the variation of the phase velocity of high-frequency ultrasonic waves have been measured in ${\left(\mathrm{KBr}\right)}_{1-x}{\left(\mathrm{KCN}\right)}_x$ crystals for $x=0.25\mathrm{} \mathrm{and} 0.50$ in the temperature range 0.1 to 8 K. For both crystals we observe the low-temperature acoustic anomalies characteristic of structural glasses. The results are interpreted in terms of a tunneling model with a broad spectrum of energy splittings and relaxation times. Spectral density and phonon-coupling constants are determined and discussed with respect to their dependence on $x$.