An experimental upper limit on the branching ratio for the decay ${\mathrm{\pi }}^0$→νν¯ is set at 8.3×${10}^{\mathrm{-}7}$ (90% C.L.). This decay is forbidden if neutrinos are purely left handed. The limit also applies to any decays of the ${\mathrm{\pi }}^0$ to weakly interacting neutrals.

Results of the first pion single-charge-exchange experiment on a polarized nuclear target are reported. Analyzing powers ${\mathit{A}}_{\mathit{y}}$ for the (${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^0$) reaction at ${\mathit{T}}_{\mathrm{\pi }}^{+}$=163 MeV on a polarized ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ target were measured over an angular range between 20° and 60° in the laboratory system. Calculations in the distorted-wave impulse approximation do not reproduce these new data.

The two-photon bremsstrahlung emitted in collisions of 7–15-keV electrons with free (gaseous) argon, krypton, and xenon atoms was observed. The agreement with recent theoretical calculations is poor.

We present a completely ab initio calculation of harmonic generation in xenon exposed to a strong laser field. The time-dependent Schrödinger equation for the atomic response and the propagation equation are numerically integrated yielding excellent agreement with experiment. The weaker variation with pump intensity of the induced dipole in the high-field regime leads to an enormous enhancement in phase matching compared to the perturbative limit and a defocusing of the generated harmonics.

We present a geometric estimate from below on the growth rate of a small perturbation of a three-dimensional steady flow of an ideal fluid and thus we obtain effective criteria for local instability for Euler’s equations. We use these criteria to demonstrate the instability of several simple flows and to show that any flow with a hyperbolic stagnation point is unstable.

We present a new, rigorous treatment of Poisson’s equation in solids with no shape approximation to the potential. In contrast to existing techniques, this method allows the unified treatment of periodic solids, as well as materials containing imperfections such as substitutional or interstitial impurities and disordered alloys. Explicit expressions for the potential and the Coulomb part of the energy are given. The validity and utility of this approach is demonstrated by numerical calculations.

We represent ring polymers in free space with the rod-bead model and show through unbiased computer simulations that the probability of observing a trivial self-entanglement (P) has a decreasing exponential dependence on the contour length (N) of the polymer, or that P=exp(-N/${\mathit{N}}_0$). The characteristic length (${\mathit{N}}_0$) varies by many orders of magnitude depending on chain flexibility and solvent quality. We also suggest that sufficiently large knots are always composite, not prime.

On the basis of thermodynamically self-consistent integral equations for the pair structure it is shown that dense binary mixtures of hard spheres, of diameters ${\mathrm{\sigma }}_1$ and ${\mathrm{\sigma }}_2$, phase separate when the size ratio ${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$ is less than 0.2, and the partial packing fractions of the two species are comparable.

Equilibrium structures of small (3–29)-atom Cu clusters are determined by simulated annealing, and finite-temperature ensembles are simulated by Monte Carlo techniques using the effective-medium theory for the energy calculation. Clusters with 8, 18, and 20 atoms are found to be particularly stable. The equilibrium geometrical structures are determined and found to be determined by a Jahn-Teller distortion, which is found to affect the geometry also at high temperatures. The ‘‘magic’’ clusters retain their large stability even at elevated temperatures.

We present the first analysis of high-resolution x-ray-diffraction spectra of finite-size Thue-Morse GaAs-AlAs superlattice heterostructures, which show essential properties of singular continuous measures. The implications for future investigations of disordered systems are considered.

The evolution of the (√3 × √3 )R30° domain-size distribution with coverage for Ag on Si(111) has been studied by high-resolution low-energy electron diffraction. Scaling of the size distribution, which can be fitted by a gamma distribution, is observed. A temperature dependence of the exponent n describing a power growth law for mean size versus coverage is found where n decreases with decreasing deposition temperature. By introducing a simple relation, q+nd=1, where the dimensionality d=2 and q is an exponent describing the power law for domain density versus coverage, the behavior of n is explained in terms of the experimentally observed behavior of q.

We have fabricated metal-insulator-semiconductor structures with poly(3-hexyl thienylene) as the active semiconductor. Optical-absorption bands due to injected carriers seen near 0.4, 1.80, and 2.16 eV are assigned to the optical transitions of the singly charged polaron, which is expected to be the thermodynamically favored excitation under these experimental conditions. Additional absorption bands at 0.5 and 1.18 eV are assigned to optical transitions of doubly charged bipolarons. We contrast the strong confinement deduced for the polaron (gap states separated by 1.80 eV) with the weak confinement observed for bipolarons (gap states separated by 0.7 eV).

We report on the first in situ study of the electronic state and the structure of underpotentially deposited submonolayers of Cu on Au(111) by x-ray-absorption spectroscopy. We found (i) various ordered adlayer structures, dependent on coverage, time, and direction of the potential scan, and (ii) strong interactions of Cu and oxygen and charge transfer from Cu to Au. During the deposition, the initial (√3 × √3 ) structure at 0.3 monolayer evolved to an equilibrium with the c(5×5). The latter and the (1×1) phases were observed at 0.6 and 1 monolayer, respectively. The three structures are present at 0.6 monolayer during the stripping scan.

A strong emission arising from states lying higher in energy than those optically pumped is observed under interband excitation into partially filled Landau levels of a modulation-doped GaAs quantum-well structure in a magnetic field. The intensity of this emission depends quadratically on the excitation power and is strongly influenced by magnetic field through the occupancy of the Landau levels. This effect demonstrates the high efficiency of Auger processes in partially filled Landau levels of a two-dimensional electron gas.

The electronic structures of clean InAs(100) surfaces have been investigated by in situ high-resolution electron-energy-loss spectroscopy. Intrinsic electron accumulation layers with carrier densities strongly depending on the surface reconstruction are formed on both As-stabilized and In-stabilized surfaces. The correlation between the surface electron densities and the surface reconstructions suggests that electrons in the accumulation layers are induced by the donorlike intrinsic surface states of InAs whose energy spectrum is determined by the surface reconstructions.

Time-resolved and cw photoluminescence of excitons in coupled quantum wells with an applied electric field is modeled using a Fermi-Dirac distribution. This distribution can result from inhomogeneous broadening due to interface roughness and the strong, short-range electric dipole repulsion between excitons. The model quantitatively explains the striking temperature dependence of the luminescence linewidth and peak position previously interpreted as a phase transition to an ordered state [T. Fukuzawa, E. E. Mendez, and J. M. Hong, Phys. Rev. Lett. 64, 3066 (1990)]. At very low temperatures (≲6 K), the excitons are in a metastable distribution.

We present theoretical calculations of exchange asymmetries in the transmission of electrons through ultrathin films of ferromagnetic Fe. The results account nicely for the magnitude of the asymmetries observed by Pappas et al. in photoemission studies of Cu covered by an ultrathin film of Fe. We argue that exchange asymmetry in the transmissivity of the Fe film, rather than the spin dependence of the electron mean free path, is responsible for the effects reported by these authors.

We report a systematic experimental study of the superconducting resistive transition in one-dimensional Al wires of length 0.6 to 110 μm. Shorter wires show a peak in resistance as a function of temperature near ${\mathit{T}}_{\mathit{c}}$, with a value above the normal-state resistance. Near the peak, the resistance decreases sharply in a magnetic field of only a few Oe. In the same regime, the current-voltage characteristic resembles that of a superconductor-insulator-normal tunnel junction. These new results may be a manifestation of coherent effects in small superconducting samples.

The 2D t-J model is studied with exact-diagonalization techniques applied to clusters of sixteen to twenty sites with various hole doping levels. Already for two holes (≥10% doping), a large electronic Fermi surface, compatible with Luttinger’s theorem, is observed. The full single-particle Green’s function is examined, and is shown to exhibit quasiparticlelike behavior, with dispersion consistent with the band structure of the noninteracting limit, and bandwidth scaling approximately as J for J smaller than t.

The spin-hole coherent-state path integral is used to generate a systematic large-spin expansion of the t-J model on the square lattice. The single hole’s classical energy is minimized by small polarons with short-ranged interactions. Intersublattice hopping of polarons is forbidden by a tunneling selection rule. We derive the low-energy Lagrangian which reduces to the model of Wiegmann, Wen, Shankar, and Lee of Néel-gauge-field induced superconductivity.