We present detailed results for the susceptibility $\chi$, correlation length $\xi$, and specific heat $c_{\nu }$ for the $\mathrm{XY}$ model. The best fit to $\chi$ and $\xi$ data is obtained by use of the Kosterlitz-Thouless scaling form with $\nu =0.500\mathrm{}\left(1\right)\mathrm{}$, but $\eta$ shows considerable deviation from ¼ down to $T=1.03\mathrm{}$. The critical temperature is estimated to be $T_c=0.898\mathrm{}\left(2\right)\mathrm{}$. The simulations are done on ${64}^2$, ${128}^2$, ${256}^2$, and ${512}^2$ lattices with use of an overrelaxed algorithm which decorrelates as $\tau \approx 0.15{\xi }^{1.2}$. Similar reduction in critical slowing down is anticipated for all continuous-spin models.

The change of the crossing parameter (essentially the phase) between sequential slow separatrix crossings is calculated for Hamiltonian systems with one degree of freedom. Combined with the previous separatrix crossing analysis, these results reduce the dynamics of adiabatic systems with separatrices to a map. This map determines whether a trajectory leaving a given separatrix lobe is ultimately captured by the other lobe. Averaging these results over initial phase yields the reaction probability, which does not asymptote to the fully phase-mixed result even for arbitrarily long times between separatrix crossings.

It is shown that chaotic trajectories in volume-preserving flows, ṙ${=\mathrm{u}}_{\mathrm{\epsilon }}$(x,y,z,t), which are arbitrarily close to integrability, 0<ε≪1, can be either trapped or diffusive throughout the available space. A classification of these flows is proposed which both distinguishes and predicts the appropriate type of behavior. In the unbounded case, a new mechanism of diffusion is found which combines motion on the resonances with an adiabatic drift. This process is reminiscent of Arnold diffusion.

The periodic-orbit quantization approach of Gutzwiller, which is a semiclassical approximation of Feynman’s path-integral formalism, is used to calculate finite-resolution densities of states for a Hamiltonian system whose classical motion is dominated by chaos, viz., the hydrogen atom in a uniform magnetic field. In spectral regions where the resolution obtained is larger than the mean level spacing, it is possible to extract approximate eigenvalues of the quantized system. The particular role of stable periodic orbits and their direct quantization are elaborated. Remarkable agreement with exact quantum calculations results.

We point out the existence of reasonable models in which cosmic strings contain charged-vector-boson fields, and show that these give rise to vector-meson superconductivity analogous to Witten’s scalar-boson superconductivity. In addition, in such models the electromagnetic field is distorted within the string in such a way that it couples to the otherwise neutral Higgs field, giving a new mechanism for scalar superconductivity.

The result of a search for the neutrino oscillation mode ν${¯}_{\mu }$→ν${¯}_e$ in a fine-grained tracking detector is reported. The average neutrino energy is 40 MeV and the average detector distance from the neutrino source is 26.8 m. No evidence for neutrino oscillations through this mode is observed. Limits on δ$m^2$ and ${\sin }^2$(2θ) are presented.

An analysis of electroweak radiative corrections to τ decays is presented. Precise predictions for leptonic and some hadronic decay rates are given. The total hadronic decay width is shown to be relatively enhanced by 2.36% due mainly to short-distance loop effects. Implications for the extraction of ${\Lambda }_{\mathrm{MS}\mathrm{¯}}$ from the τ lifetime or leptonic branching ratios are discussed. (MS¯ denotes the modified minimal-subtraction scheme.)

Measurements of inclusive transverse-momentum spectra for charged particles produced in proton-antiproton collisions at √2 of 630 and 1800 GeV are presented and compared with data taken at lower energies.

The unexpected energy dependence of high-energy fixed-angle pp elastic scattering in a nuclear target can be intrepreted in terms of interference between two perturbative QCD subprocesses. By proposing the attenuation of Landshoff-type contributions in nuclei, we obtain a parameter-free relation that matches the energy dependence of the data of Carroll et al. The approximations improve with larger A and higher energy, leading to a prediction of oscillatory energy dependence for the transparency ratio at higher energy.

We have observed temporal instabilities in the polarizations of counterpropagating laser beams in atomic sodium vapor. For intensities slightly above the instability threshold, the polarizations fluctuate periodically. For higher intensities, the fluctuations are chaotic and the system evolves on a strange attractor whose fractal dimension increases with increasing laser intensity.

The lifetime of positrons contained in an electrostatic trap is studied when small quantities of alkanes (i.e., hydrocarbon molecules of the form ${\mathrm{C}}_{\mathrm{n}}$${\mathrm{H}}_{2\mathrm{n}+2\mathrm{}}$) are added. As n is increased from 4 to 16, the annihilation rate per molecule is found to increase by a factor of 300 and then to level off. These data are interpreted as evidence for the existence of long-lived resonances and bound states of positrons and neutral molecules, and a model of the formation process is discussed.

Computer simulations demonstrate that the entrainment (or advection) of magnetic field with the flow of cathode-emitted electrons can constitute a dominant mechanism for the magnetic field penetration of erosion switch plasmas. Cross-field drift in the accelerating electric field near the cathode starts the penetration process. Plasma erosion propagates the point for emission and magnetic field injection along the cathode toward the load—for the possibility of rapid switch opening.

Time evolutions of the soft-x-ray tomography of a tokamak plasma have been obtained during sawtooth oscillations by use of the Fourier-Bessel expansion technique. The colder plasma is carried to the center by a hydrodynamic flow, and becomes a new hot spot during the crash phase. The old hot core is pushed out and the new hot spot remains at the center after the crash. The observed sawtooth crash is consistent with neither the Kadomtsev model nor the Wesson model. The crash time is shorter as the current profile is broader.

Novel features of solids that may be formed from cold, condensed plasmas confined in external fields (storage rings or ion traps) have been investigated with molecular-dynamics calculations. Such plasmas have well defined outer surfaces and interior layers that are parallel to the outer surface. In each layer the particles are arranged in an approximate hexagonal pattern of a two-dimensional Coulomb lattice. The interactions between layers have also been investigated: They are relatively stronger in the straight (commensurate) and virtually absent in the curved (incommensurate) directions.

The zero-temperature superfluid onset transition for a system of repulsively interacting bosons in a random potential is studied. The associated quantum critical behavior is characterized in d dimensions by exponents ν≥2/d, η, and a dynamic exponent z. Static and dynamic scaling laws are derived: Some measureable exponents, such as z=d, are predicted exactly. Applications to experiments on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ absorbed in porous media are discussed.

Orientational distributions of the rod-shaped tobacco-mosaic-virus particle in magnetically aligned nematic solutions ($0.05M$ borate buffer, $p\mathrm{H} 8.5$) were measured by analysis of the angular spread of x-ray diffraction patterns. We measured Gaussian orientational distributions with order parameters varying between 0.77 in a solution with coexisting nematic and isotropic phase, to 0.95 in the single-phase nematic solution at higher concentration. Our experimental results are in good agreement with model calculations based on a modified Onsager theory for suspensions of hard rods.

Solid solutions of ${\mathrm{Ar}}_{1\mathrm{-}\mathrm{x}}$(${\mathrm{N}}_2$${)}_{\mathrm{x}}$ (0.5<x<1) have been investigated by x-ray powder diffraction and molecular-dynamics simulations. The boundaries between the dynamically disordered hcp phase, the ordered ${\mathrm{Pa}}_3$ phase, and the quadrupolar glass have been determined. The glass state is characterized by broadened diffraction lines. Indications for an elastic instability have been observed which suggest that the formation of the glass state is analogous to that for the mixed cyanides.

The early-stage ordering kinetics in ${\mathrm{Cu}}_3$Au has been investigated by in situ time-resolved x-ray scattering. At temperatures above the first-order transition point, $T_{\mathrm{tr}}$, short-range order fluctuations reeequilibrate in a few seconds after a quench from high temperature. At temperatures just below $T_{\mathrm{tr}}$ the fluctuations relax to a metastable state which then decays by relatively slow nucleation and growth. At lower temperatures, the time scales of the two processes become similar. The ordering becomes continuous well above the classical spinodal temperature.

A trilayer thin film of amorphous alloys ${\mathrm{Ni}}_{40}$${\mathrm{Zr}}_{60}$/${\mathrm{Ni}}_{73}$${\mathrm{Zr}}_{27}$/${\mathrm{Ni}}_{40}$${\mathrm{Zr}}_{60}$ has been deposited on nitrided Si surface, annealed at 200, 250, and 300 °C in He, and analyzed by Rutherford backscattering spectroscopy and cross-sectional transmission electron microscopy. Interdiffusion occurred without crystallization. When Ni was the dominant diffusing species, no voids were seen. When both Ni and Zr diffuse, submicron size voids were observed within the alloy. These observations indicated that self-diffusion in the amorphous alloy can be mediated by localized vacancylike defects as well as by nonlocalized free-volume defects. Also, void formation as a result of the slower diffusing species is opposite to Kirkendall effect in diffusion couples of crystalline solids.

The interaction between F atoms and crystalline Si, which is essential for etching processes in semiconductor device fabrication, is investigated with state-of-the-art theoretical techniques. A comprehensive picture of F reactions with the Si surface, the bulk, and the near-surface region is obtained, in terms of which the etching process is elucidated. Insertion of F into Si-Si bonds becomes possible because of relaxed steric constraints in the near-surface region. Dependence of the etch rate on doping follows naturally, in agreement with observations.