We estimate the ${\pi }^{+}$-${\pi }^0$ electromagnetic mass difference within the 1/N expansion approach to hadronic matrix elements. Perturbative QCD and a truncated meson theory describe the high- and low-photon-loop-momentum contributions, respectively. The matching between these complementary pictures for strong interactions is discussed.

The harder lepton’s energy spectrum for the decay sequence $Z^0$→${\tau }^{+}$${\tau }^{\mathrm{-}}$→${\mu }^{+}$$e^{\mathrm{-}}$X is sensitive to the τ-couplings parameter ξ${\alpha }_H$ where ξ is the Michel polarization parameter for ${\tau }^{\mathrm{-}}$→$l^{\mathrm{-}}$νν¯, and ${\mathrm{\alpha }}_{\mathit{H}}$≃-2${\mathit{a}}_{\mathrm{\tau }}$${\mathit{v}}_{\mathrm{\tau }}$/(${\mathit{a}}_{\mathrm{\tau }}^2$+${\mathit{v}}_{\mathrm{\tau }}^2$) where $a_{\tau }$ and $v_{\tau }$ describe $Z^0$→${\tau }^{+}$${\tau }^{\mathrm{-}}$ at tree level. Due to a factorization property, radiative corrections to such energy-energy correlations are as tractable as for $A_{\mathrm{LR}}$; however, this test does not require instrumentation of longitudinal beam polarization.

Inclusive electron-scattering measurements of Δ electroexcitation in nuclei are reported. Electrons with energies of 0.96, 1.1, 1.3, and 1.5 GeV were scattered from ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, C Fe, and W at 37.5°, corresponding to $Q^2$=0.20–0.52 (GeV/c${)}^2$ at the Δ peak. The centroid of the Δ-region cross-section peak is above that for the free nucleon and it shifts to higher invariant mass as $Q^2$ increases. The A dependence in the dip region and ratios of nuclear to nucleon integrated cross sections indicate that at these $Q^2$ values there is little specifically nuclear, e.g., quasideuteron, background contribution.

We have demonstrated a new slowing and cooling technique which uses a standing-wave laser field collinear with a thermal Na atomic beam to produce a continuous supply of slow atoms, with most probable velocities ranging from -40 to + 80 m/sec. The velocity spread can be ∼10 m/sec (150 mK), the density ${>10\mathrm{}}^8$, and the peak density in phase space ∼100 times that at the peak of the original Maxwell-Boltzmann distribution.

Hasselbrink, Waldeck, and Zare reported the observation of a strong orientation of the CN fragment produced by photodissociation of ICN molecules with circularly polarized light at 248 nm. We present in this paper a theory of this effect: A vibronic effect creates a coherent motion of the CN fragment and amplifies the photon angular momentum in order to produce a strong CN orientation. This effect is general and should open a new way towards laser control of reactive processes.

A simple model, in which a boundary layer creates spatial turbulence by emitting ‘‘hog plumes,’’ is introduced. In the turbulent regime we find exponentially decaying fluctuations, a characteristic frequency of time signals, and a power-law scaling between analogs of hydrodynamic quantities.

Experiments on the growth of hcp helium crystals from dilute ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He solutions show that a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration on the order of ${10}^{\mathrm{-}6}$ is sufficient to lower the roughening temperature of the c facet by 20%. We interpret this as resulting from the adsorption of up to 0.25 monolayer of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ on the liquid-solid interface, with a binding energy of about 10 K. The presence of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ leads to a growth instability near 1 K.

We reconsider the debated question of the collapse transition of two-dimensional polymers. The solvable model of self-avoiding walks in presence of percolating vacancies (i.e., at the so-called Θ’ point) is shown to be stable against the introduction of further interactions, anisotropy, and change of lattice. This confirms the conjecture of the identity of the Θ’ point with the genuine tricritical Θ point and rules out a higher multicriticality.

Precision heat-capacity and vapor-pressure isotherm measurements indicate that the melting of monolayer Xe on graphite is always first order. This conclusion is consistent with the results of simulation studies but in sharp contrast with the claim advanced in a series of x-ray studies of a crossover from first-order to continuous Kosterlitz-Thouless-Halperin-Nelson-Young melting.

X-ray standing waves generated above a gold mirror surface during total-external reflection are used to unambiguously locate a zinc ion layer which is embedded ∼200 Å above the mirror surface in a Langmuir-Blodgett multilayer with a precision of 2 Å. .AE

We have used elastic neutron scattering to determine the magnetic phase diagram of ${\mathrm{CsMnBr}}_3$, an XY antiferromagnet on a stacked triangular lattice. The application of a field along (100) splits the antiferromagnetic transition and results in an intermediate phase of spin-flop character. This phase is extremely narrow near the zero-field transition at ${\mathrm{T}}_{\mathrm{N}}$=8.32 K which is shown to be a tetracritical point. This suggests that the unusual critical properties of the zero-field phase transition may be due to its tetracriticality as opposed to belonging to a new universality class.

The direct observation of polarization mixing in nuclear Bragg scattering of synchrotron radiation by α${\mathrm{-}}^{57}$Fe${\mathrm{’}\mathrm{’}}_2$${\mathrm{O}}_3$ is reported. Application of a magnetic bias field having the appropriate orientation was shown to produce almost complete rotation of the σ-polarized incident beam to a π-polarized diffracted beam. Rotation of this field about the diffraction vector through 90° caused the diffracted beam polarization to change continuously from σ through elliptical to π, as predicted theoretically.

The nature of the 2.7-eV photoluminescence in as-manufactured oxygen-deficient high-purity silica glasses is studied. Luminescence lifetime measurements and ab initio molecular-orbital calculations are consistent with the luminescence being a triplet-to-ground transition of a neutral oxygen-vacancy defect (?Si-Si?).

The asymptotic decay of the survival probability is calculated for a quantum particle moving at zero temperature on a one-dimensional tight-binding chain possessing randomly placed irreversible traps of strength γ. The survival probability exhibits a decay, P(t)∼exp(-${\mathrm{At}}^{1/4}$), which is slower than that associated with a diffusing particle.

The conduction properties of a two-dimensional array of electrons on a helium film supported by a dielectric substrate are reported. The system undergoes a metal-insulator transition at the melting point as substrate imperfections pin the lattice phase. The frequency and field dependence of the finite-frequency conductivity and the noise characteristics are analogous to those of sliding charge-density waves.

The line shape of plasma resonances in small sodium clusters (8≤N≤20) is calculated taking into account quadrupole fluctuations of the cluster shape. At room temperatures, an average value of Γ/${\omega }_M$∼0.1 for the damping factor is found. In the case of deformed clusters, these fluctuations are not able to wash out the multipeaked structure associated with the dipole resonance along the principal axes of the cluster.

An electronically driven lattice deformation which explains qualitatively the supermodulus effect in metallic superlattices is derived. It is shown in the Thomas-Fermi approximation that the total energy of a metallic superlattice is lowered by uniform deformations of the constituent materials. The theory gives the correct order of magnitude for changes in lattice constants, explains why the supermodulus effect is not observed in systems in which at least one constituent is nonmetallic, and predicts the absence of an effect when the bulk Fermi energies of the constituent metals are equal.

Theoretical arguments and numerical calculations are used to discuss the phase diagram of the two-dimensional negative-U Hubbard model. Our results are consistent with (1) a vanishing transition temperature at half-filling but with a ground state having both superconducting and charge-density-wave long-range order, and (2) a Kosterlitz-Thouless transition at a finite temperature into a superconducting state with power-law decay of the pairing correlations away from half-filling.

The specific heats of two samples of ${\mathrm{UPt}}_3$ have been measured in the vicinity of the transition to the superconducting state. In both cases the specific-heat anomalies are sharper than any previously observed, and two maxima are clearly resolved. The results are interpreted as evidence of a splitting of the transition and unconventional pairing. A model that is consistent with the known sample dependence of the superconducting-state specific heat is used to derive ‘‘intrinsic’’ values of the related parameters.

The possibility of a new thermodynamic phase in the mixed state of bulk, disordered, type-II superconductors is suggested: a vortex-glass superconductor. This phase lacks conventional off-diagonal long-ranged order, yet is argued to be a true superconductor with vanishing dc resistance. In this phase metastable currents are predicted to decay as (lnt${)}^{\mathrm{-}1/\mathrm{\mu }}$ with μ(≤1) a universal exponent. Relevance to experiments on bulk high-${\mathrm{T}}_{\mathrm{c}}$ oxides is mentioned.