We have observed, in exact numerical solutions of small systems, the microscopic precursors of superconductive pairing of fermions and semions (half-statistics quasiparticles) in two dimensions. We recognize the paired state by flux quantization at intervals of hc/2e. We find that the fermions pair only for values of an interparticle potential u which is large and negative, while the semions pair for a wide range of u, including strong repulsion. We also find that the semions, in the paired state, prefer quantized flux in odd multiples of hc/4e.

Unusual properties of the Hubbard chain of arbitrary degeneracy N and excluded-site occupations of more than two electrons are presented: (i) In zero field and in the limit of large N the ground state resembles an interacting Bose gas. The 1/N contributions vanish identically, so that corrections to leading order are 1/${\mathrm{N}}^2$. (ii) The susceptibility at T=0 shows logarithmic singularities as H→0. (iii) The Hubbard model and the Heisenberg chain have the same low-energy spin excitations for all U. (iv) The specific-heat coefficient γ is singular. These properties hold for arbitrary band filling and (ii)–(iv) hold for all N≥2.

The nature of flux-creep phenomena in the case of collective pinning by weak disorder is discussed. The Anderson concept of flux bundle is explored and developed. The dependence of the bundle activation barrier, U, on current j is studied and is shown to be of power-law type: U(j)∝${\mathrm{j}}^{\mathrm{-}\mathrm{\alpha }}$. The values of exponent α for the different regimes of collective creep are found.

Measurements are reported of the conductivity and dielectric constant at frequencies up to 10 MHz for a single crystal of ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{y}}$ with y∼0.001 to 0.01. The reported behavior is typical of conventional doped semiconductors, with dielectric constant (at y=0) and effective mass that are not exceptionally large. The dielectric constant grows as the insulator-to-metal transition is approached, but only for the electric field in the ${\mathrm{CuO}}_2$ layers, indicating that the transition is two dimensional. The crossover to weak localization occurs close to the antiferromagnetic phase boundary.

We have used resistance measurements at pressures to 95 kbar and magnetic fields to 8 T to probe the establishment of Kondo coherence in the heavy-fermion Kondo lattice ${\mathrm{UBe}}_{13}$. With increasing pressure, the negative magnetoresistance characteristic of noninteracting Kondo impurities evolves into the positive magnetoresistance expected for a coherent Kondo lattice. We show that coherence is established by the suppression of inelastic scattering, over a temperature range which expands as pressure decreases the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$ and increases the Kondo temperature ${\mathrm{T}}_{\mathrm{k}}$.

The dielelctric breakdown model (DBM) is generalized to include lower cutoffs, which prevent growth at low fields. The new models may represent realistic situations in some DBM and some viscous fingering experiments. Multifractal theory is shown to provide quantitative predictions for the crossover from the usual DBM patterns (at small finger sizes) to a new, spiky, behavior (at large sizes). For one of the models, the theory also predicts when growth will stop. The predicted crossover scaling function is confirmed by numerical simulations.

We numerically study the quantized verion of the Frenkel-Kontorova model, using Feynman quantization and the Metropolis algorithm. The gross structure of the quantum ground state above the critical parameter value mirrors the classical equilibrium configuration, provided that ħ is not too large; nevertheless, its fine structure is significantly different, and can be related to a sawtooth map rather than to the standard map. The dependence of energy on temperature is also investigated and is explained in terms of the quantum phonon spectrum for linearized motion.

We have studied the behavior of the Floquet spectrum for A Fokker-Planck equation describing a nonlinear Brownian rotor driven by an angle-dependent dynamic external force consisting of two traveling sine waves with amplitudes ${\epsilon }_1$ and ${\epsilon }_2$. For the case ${\epsilon }_2$=0, the Fokker-Planck equation is separable (in the sense that is has two well defined eigen-numbers) and the nearest-neighbor spacing distribution appears to be Poisson random for small spacings. For both ${\epsilon }_1$≠0 and ${\epsilon }_2$≠0, we observed nonlinear resonance and level repulsion, indicating that the spectrum, at least locally, exhibits universal random-matrix-type behavior and that information about the underlying dynamics of the Brownian particle is lost from the spectrum.

Gravitational waves produce a time delay between the different images of a gravitational lens. The measurements of the time delay in the gravitational lens 0957±561 put new limits on the amplitude of low-frequency gravitational waves h<2×${10}^{\mathrm{-}5}$[ν/(3×${10}^{\mathrm{-}18}$ Hz)] in the frequency range 3×${10}^{\mathrm{-}18}$ Hz<ν. Future measurements of the time delay in other gravitational lenses may permit this limit to be improved.

We explain the relationship between global strings of the Abelian Higgs model and vortices in a superfluid. We show that the nonrelativistic Magnus force law for vortices can be derived from global-string dynamics, but only when an external background field has a special Lorentz-noninvariant configuration ${\mathrm{H}}^{\mathrm{ijk}}$∝${\mathrm{\epsilon }}^{\mathrm{ijk}}$. We present a self-consistent classical theory for relativistic Higgs vortices in a superfluid, and show that superfluid vortices can be described as a system of spinning global strings.

A recent, careful analysis of the world data on pp scattering concluded that the pp${\pi }^0$ coupling constant was 3%–4% less than expected from charge independence and the known pn${\pi }^{+}$ coupling constant. We propose a much simpler interpretation of the analysis of the pp data which involves no unreasonable level of charge dependence. Instead one must recognize that the pion-nucleon form factor needed in a boson-exchange model of the nucleon-nucleon force differs significantly from that relevant to a free nucleon.

We report the first observation of the disappearance of flow in heavy-ion collisions. This is accomplished by measuring the excitation function of the average in-plane transverse momentum for the symmetric system ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$, using beam energies of 130, 70, and 50 MeV/nucleon. The observation is indicative of a change from dominantly repulsive to attractive scattering. We also present the results of calculations performed with the Boltzmann-Uehling-Uhlenbeck equation which support the concept of vanishing flow for this system in the energy region between 30 and 50 MeV/nucleon.

Our investigation of the energy dependence of dispersive effects in elastic electron scattering from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ has been extended to a beam energy of 690 MeV. The cross section observed in the minimum of the form factor is larger than that predicted using a static charge density obtained by fitting lower-energy data with the customary phase-shift analysis. This ‘‘filling-in’’ of the minimum at 690 MeV is even larger than that observed previously at 430 MeV. This indicates an increase in the relative strength of higher-order processes in the minimum with an increase of beam energy; the presence of dispersive effects is the most likely explanation for this effect.

Four-body integral equations are used to calculate dd→dd and dd→${\mathrm{p}}^3$H amplitudes using a single-term separable nucleon-nucleon potential in channels ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$${\mathrm{-}}^3$${\mathrm{D}}_1$. First-order perturbation theory is used to include in the four-body kernel the contribution of d-wave two- and three-nucleon channel components that result from the tensor force. Cross sections and analyzing powers are calculated using all positive- and negative-parity four-body amplitudes with total angular momentum J≤4. Comparison with data is presented.

For the first time and in contrast to a recent experiment, a pronounced nonspherical angular distribution for the fragment ions following double photoionization of ${\mathrm{H}}_2$ has been observed. In addition, the accompanying cross section has turned out to be surprisingly small in comparison to the prediction of a recent theoretical investigation. Both observations suggest that the dynamics of double photoionization for the simplest two-center two-electron system remains an open problem.

The observation of C-H bending, and C-C and C-H stretching vibrational levels in high-resolution C 1${\mathrm{s}}^{\mathrm{-}1}$${\mathrm{\pi }}^{\mathrm{*}}$ photoabsorption spectra of ethylene and benzene demonstrates that, within the validity of the Franck-Condon principle, both molecules have broken symmetry in this core-excited state. A simple model based on the antibonding character of the ${\mathrm{\pi }}^{\mathrm{*}}$ electron, and on the local rehibridization of the bonding orbitals on the excited C atom, is utilized to explain our observations and to show that both core-excited molecules become nonplanar. Our results also show that the core hole is localized on the excited carbon atom.

The bifurcation diagram corresponding to the Eckhaus stability curve has been constructed for the one-dimensional Swift-Hohenberg equation in a finite domain. Finite amplitude solutions with particular spatial wavelength recover linear stability, as predicted by the Eckhaus curve, after a sequence of secondary bifurcations from the branch of solutions with this wavelength. No connectivity between the primary solution branches is admissible if the stability predicted by this bifurcation diagram is to correspond to the prediction of the Eckhaus analysis. The Eckhaus curve does not exist if nonlinear couplings destroy this bifurcation pattern. This is demonstrated by analysis of a coupled pair of swift-Hohenberg equations which allow new types of secondary bifurcation from each primary family.

Ion orbits are followed in phase space by means of a new scheme of the optical tagging plasma diagnostic technique. An orbit distortion for ions interacting resonantly with an ion acoustic wave is clearly observed even in the low excitation regime, where the plasma response and the corresponding macroscopic fields exhibit linear behavior.

The effect of an energetic ion population on the nonlinear stability of a tearing mode of single helicity is determined using kinetic theory. A dynamical equation is derived for the magnetic-island width. It is shown that the island growth can be suppressed in a tokamak when the energetic ion-density profile peaks just outside the rational surface. Effects due to resistive interchanges and bootstrap currents are included in the model.

Polarimetry measurements of the Doppler-shifted ${\mathrm{H}}_{\mathrm{\alpha }}$ emission from a neutral hydrogen beam on the PBX-M tokamak have been employed in a novel technique for obtaining q(r) and magnetic field pitch-angle profiles using the Stark effect. The resulting q(r) profile is very broad and its central value, q(0), is significantly below 1, which has important implications for theoretical models of sawteeth.