We report here small-angle absolute elastic-scattering cross sections for 25-keV electrons scattering from helium in a crossed-beam apparatus. Our results disagree with those reported by Geiger and Morón-León, but agree with relativistic first Born cross sections obtained with an explicitly correlated wave function. Our results are in agreement with those of Coffman, Fink, and Wellenstein, who used a different approach to measure the elastic differential cross sections. Our experimentally measured x-ray incoherent-scattering factors also agree with the results obtained with the explicitly correlated wave function.

The luminous region of the plasma cloud surrounding deuterium pellets injected into a tokamak is studied spectroscopically. At the time of peak luminosity the electron density is 2.4×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ to within 30% and the temperature is at most 2.0 eV. The intensity ratio of the Balmer-alpha and -beta light from the pellets, the total number of emitted photons, and the apparent size of the radiating region are consistent with local thermodynamic equilibrium at this temperature and density.

The first coherent, single-crystal, magnetic rare-earth superlattices of Gd-Y were grown by metal molecular-beam epitaxy. X-ray structural analysis showed that the superlattices have a composition modulation of two atomic-layer-sharp interfaces with the modulation amplitude approaching 100%. The magnetic properties are consistent with a model in which the central Gd layers in each array behave like ideal ferromagnetic thin films even for as few as five atomic layers. The two interfacial Gd layers at the sides appear not to order ferromagnetically and have a susceptibility ${\chi }_g$ independent of the number of atomic layers.

We describe a simple theory of diffusion-limited-aggregation cluster growth which relates the large-scale shape of the cluster to its fractal dimension. We present results of computer simulation for DLA clusters grown with anisotropic sticking rules which provide strong confirmation of our model in two dimensions. New universal exponents are predicted and found. We are also able to obtain good estimates for the fractal dimension of ordinary DLA.

We have measured the flow-induced shift in the NMR spectrum of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$$B$. Above a temperature-independent critical velocity, the absorption line is observed to shift to higher frequencies as the flow is increased, in qualitative agreement with the theory of textural deformation in the presence of flow due to Fetter.

Since transition-metal impurity levels are found to be aligned with respect to each other within a group of isovalent semiconductor compounds, as if a common reference level existed for them, we propose to use this fictitious level for the band alignment in semiconductor heterojunctions. This rule leads to a valence-band discontinuity in ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}/\mathrm{GaAs}$ heterojunction of $\Delta E_v=(0.34\mathrm{}\pm 0.05)x\Delta E_g$ (AlAs/GaAs) in agreement with the most recent measurements. Predictions of several III-V on III-V and II-VI on II-VI heterojunction band-edge discontinuities are also given.

We present a first-principles theory for the quasiparticle energies of semiconductors and insulators. The full dielectric matrix is used to evaluate the nonlocal, energy-dependent electron self-energy operator. Both local-field effects and dynamical screening are found to be essential for understanding quasiparticle energies. Results for the band gaps, optical transitions, and band dispersions for silicon and diamond are in excellent agreement with existing experimental data.

The rate of neutrino-induced events with observed energy below 100 MeV in the Kamiokande proton-decay detector is much higher than expected from atmospheric neutrinos. However, it is shown here that this rate is consistent with a cosmological background of relic neutrinos that was formed by gravitational stellar collapses that have been taking place since galaxy and star formation.

We show that for a pure gauge theory in a general curved space-time, the effective value of the gauge coupling constant can become small as a result of high curvature.

A simple computational technique related to Feynman path integrals and linear control systems is derived for the dynamical properties of $N$-level model quantum mechanical systems in arbitrary classical fields: periodic, pulses, or sequences of pulses. Since the Hamiltonian is not required to be Hermitian, the effects of natural lifetimes and other relaxation parameters can be determined. The time-dependent Hamiltonian is approximated by a sequence of time-independent Hamiltonians ${\mathrm{H}}^{\mathrm{}\left(n\right)\mathrm{}}$.

We have measured the inclusive branching ratio for $B\to \psi X$ to be (1.10 ± 0.21 ± 0.23)%. The momentum distribution of the $\psi$ mesons suggests a significant fraction of two-body decays.

Electron antineutrinos with energy ∼ 7 × ${10}^6$ GeV have much-enhanced cross sections due to $W$-boson production off electrons. Possible signals due to cosmic-ray sources are estimated. Higher-energy antineutrinos can efficiently produce a $W$ accompanied by radiation. Another possibility, which could lead to shadowing at modest depths, is resonant production of a charged Higgs particle. The importance of muon production by charm showers in rock is pointed out.

The tensor, spin-spin, and spin-orbit terms in the heavy-quark potential have been calculated, via Monte Carlo methods, in SU (3) lattice gauge theory on a $6^3$ × 12 lattice, at $\beta =6, 7, 8, 9, \mathrm{and} 10$. The signs and dependence on quark separation of all terms are generally consistent with one-gluon exchange. There is some evidence for nonperturbative behavior in the spin-orbit and spin-spin potentials at $\beta =6.0\mathrm{}$.

We exposed a stack of CR39 track detectors containing Ag foils to a 1.7-GeV/nucleon ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ beam and investigated the anomalous mean-free-path effect. Neither the whole set of 7517 nor a subset of 2542 interacting fragments produced probably in the Ag target show an effect. By combining the data of this and an earlier experiment we can also exclude an effect for 3219 interacting fragments produced in $\Delta Z=1\mathrm{}$ collisions.

The ratio of cross sections for the $E1\mathrm{}$ and $E2\mathrm{}$ capture amplitudes in the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\alpha , \gamma ){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ has been determined from $\gamma$-ray angular distributions obtained at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=1.7-2.8\mathrm{}$ MeV. The results favor a nonnegligible contribution of the $E2\mathrm{}$ amplitude to the reaction rate at stellar energies.

If a colored plasma is produced in ultrarelativistic collisions of heavy nuclei, its presence can be inferred by observing that the $\rho$-meson peak in the mass spectrum of emitted dileptons disappears as their transverse energy increases.

The electric quadrupole moments of ${}_{}{}^{211}{}_{}{}^{}\mathrm{Rn}$ isomers were determined as $Q\left({\frac{17}{2}}^{-}\right)=19\mathrm{}\left(2\right)\mathrm{}$ e.fm2 and $Q\left({\frac{63}{2}}^{-}\right)=160\mathrm{}\left(22\right)\mathrm{}$ e.${\mathrm{fm}}^2$ via measurements of the quadrupole interaction in a Bi single-crystal host. The results indicate that the spherical shape is preserved in the lead region even at the core-excited very-high-spin regime, in contradiction to theoretical predictions.

The angular distribution of the differential cross sections for the transition to the double-isobaric analog state in the reaction ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}({\pi }^{+}, {\pi }^{-}){}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ was measured at six angles from 20° to 120°. Extra-polation of the differential cross section results in 5.3±0.5 μb/sr at 0° and an angle-integrated cross section of 16.7±1.2 μb. The forward peaking and magnitude of the cross sections differ from those predicted by simple sequential models and the trends of data at higher energies. The results are compared to recent data for the same transition on ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$ at 50 MeV and to a calculation in the six-quark bag model.

We have studied the response of atoms in a dilute collimated atomic beam to transient phase-controlled resonant excitation fields. We demonstrate that the use of such excitation makes it possible to place the atoms into stationary states of the coupled atom-field system. We show that these states, which are easily visualized in the two-level-atom vector model, are equivalent to the dressed states of resonance fluorescence theory. Studies of atoms in pure dressed states should allow interesting tests of basic atom-field interactions.

We have obtained oscillator strengths for transitions between a laser-excited initial state and autoionizing final states. In the case of sodium, a laser was used to populate the $3p$ initial state, and synchrotron radiation was used to excite the autoionizing resonances. These transitions are of the type $2p^{6}3p{}_{}{}^2{}_{}{}^{}P\to 2p^5\left({}_{}{}^2{}_{}{}^{}P\right)3s3p\left({}_{}{}^{1,3}{}_{}{}^{}P\right){}_{}{}^{2,4}{}_{}{}^{}S,{}_{}{}^{2,4}{}_{}{}^{}P,{}_{}{}^{2,4}{}_{}{}^{}D$; they decay to $2p^6{}_{}{}^1{}_{}{}^{}S+e$. The sum of the oscillator strengths for all the observed transitions between the $2p^{6}3p$ initial-state and the $2p^{5}3s3p$ final-state configurations was found to be equal to 0.22(4).