Donor electrons in (Cd,Fe)Se show no bound magnetic polaron effects in the spin-flip light-scattering spectra, that are usually observed in manganese-based materials. The absence of thermal magnetic fluctuations demonstrates that the ground state of the ${\mathrm{Fe}}^{2+}$ ion has no permanent magnetic moment—a feature of the Van Vleck ions. The spin-flip linewidth is accounted for by consideration of compositional alloy fluctuations. A sizable magnetic anisotropy produces a similar optical anisotropy.

A model for the velocity of a wave front having a free edge and moving in an excitable medium is proposed. It contains three main ingredients: a curvature effect, a normal velocity profile effect at a free edge, and a finite width effect. The model allows one to study the process in which arbitrary initial conditions evolve into rotating spiral waves. Two new scaling laws, for the time to form a spiral’s core and for the distance between the cores of a mirror image pair of rotating spiral waves, are numerically derived.

The assumptions underlying the revised Enskog kinetic theory are used to determine the dynamics of correlations between time-displaced fluctuations in an equilibrium hard-sphere fluid, leading to a new expression for the spectrum observed in the depolarized interaction-induced light scattering.

The behavior of the two-loop four-point graviton amplitude is investigated in the type-II superstring theory, where the genus-two Riemann surface degenerates into two tori. The amplitude turns out to be finite in this limit and it is shown explicitly to the lowest order of the pinching parameter that it produces the product of two one-loop amplitudes, as is expected from the factorization property.

Recent Letters and Comments discuss the quantization of self-dual two-dimensional Lagrangeans which describe systems that are not explicitly canonical. We make some remarks on the most efficient method for exhibiting the canonical structure.

A new beam line and detector were constructed to increase statistical precision and greatly reduce systematic uncertainty on the ratio of the CP-nonconserving parameters ε’/ε. Major improve- ments are discussed together with a result from a first run: ε’/ε=0.0032±0.0028 (statistical) ±0.0012 (systematic). The precision is better than earlier measurements yet the result is still consistent with the superweak mechanism (which predicts zero) and that due to Kobayashi and Maskawa. Significantly more data are being collected.

The first measurements of the transverse momenta ($p_T$) of heavy projectile fragments produced by fragmentation of 1.2-GeV/nucleon La are presented. The momentum distributions of the fragments (Z≃26–56) are Gaussians and broader than predicted by models based on internal momenta of the projectile nucleons. The larger $p_T$ observed do not appear to be due solely to Coulomb effects, but are consistent with a simple model whereby nucleons from the (hot) overlap region recoil collectively into the (colder) projectile fragments. Thus, $p_T$ and the fragment mass may provide a good indication of the vector impact parameter b.

Electron-scattering cross sections for the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(e,e’p) have been measured for recoil momenta between 300 and 600 MeV/c and for missing energies up to 90 MeV. Proton momentum distributions in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, corrected for final-state–interaction and meson-exchange effects, have been obtained from 318 to 600 MeV/c for the pd channel and from 290 to 515 MeV/c for the ppn channel. Explicit evidence for nucleon-nucleon correlations is presented.

The elastic magnetic form factor of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ has been measured for effective momentum transfers 2.47≤${\mathrm{q}}_{\mathrm{eff}}$≤3.65 ${\mathrm{fm}}^{\mathrm{-}1}$ by electron scattering. The form factor drops by almost three decades in this range. The data follow an extreme single-particle Woods-Saxon shell-model calculation. Many-body effects such as meson-exchange currents or core polarization do not seem to play a significant role in the q range of this experiment.

In an active ‘‘microcavity’’ the condition of maximum ‘‘enhancement’’ of spontaneous emission corresponds to resonant coupling of atoms with a single field mode. We demonstrate that this exceptional condition results in the effective cancellation of spontaneous emission, in the onset of stimulated emission at exceedingly low excitation levels, and in an anomalously high stimulated-emission gain. In the context of phase-transition theory the active microcavity behaves as a statistical ensemble undergoing an order-disorder transition at an extremely high value of the critical temperature.

We report the first measurements of electron-impact–excitation cross sections for very highly charged ions (${\mathrm{Ba}}^{46+}$), and introduce a powerful new technique for studying these ions. Approximately 2×${10}^4$ ${\mathrm{Ba}}^{46+}$ ions/cm were trapped inside the space charge of an ≃120-mA electron beam and their x-ray emission spectra observed with Si(Li) and Bragg-crystal–diffraction spectrometers. Cross sections for several N=3 levels excited from the neonlike ${\mathrm{Ba}}^{46+}$ ground state were measured relative to radiative recombination on the same ions.

We present a procedure for obtaining optimized trial wave functions for use in quantum Monte Carlo calculations that have both smaller statistical errors and improved expectation values, compared to commonly used functions. Results are presented for several two-electron atoms and ions (including some excited states) and for the Be atom.

We report experimental observations of traveling-wave convection in a binary fluid mixture in an annular container—a flow which approximates a large, one-dimensional dynamical system. In this geometry, the convective rolls align themselves radially and propagate azimuthally. We observe competition between several non- equilibrium states. This leads to a rich variety of dynamical behavior, including coexisting regions of conduction and convection, sources and sinks of convective rolls, and more complex spatiotemporal defects.

Self-diffusion at surfaces can be studied with quasielastic scattering of low-energy He atoms. This is demonstrated for the Pb(110) surface close to the melting point, ${\mathrm{T}}_{\mathrm{m}}^{\mathrm{Pb}=600.7\mathrm{}}$ K. From the width of the quasielastic energy distribution of scattered He atoms it is inferred that at T≥450 K the surface atoms are anomalously mobile. At T≥550 K, surface mobilities are found to exceed the bulk liquid value. These results complement recent ion-scattering measurements on surface disorder of Pb(110).

Layer-dependent core-level shifts have been studied for Yb epitaxially grown on Mo(110). Clearly resolved Yb 4f positions are simultaneously distinguished for the interface, the free Yb surface, and the intermediate layers. The measured shifts provide new and detailed quantitative information on the energetics of adhesion and interface segregation; this type of data is otherwise most difficult to obtain. The determined energies are in good agreement with results from semiempirical calculations.

Aggregated Co particles were produced by inert-gas evaporation in Ar. A fractal dimension ${\mathrm{D}}_{\mathrm{f}}$ was extracted from electron micrographs; it was strongly dependent on the mean particle radius R¯. We found 1.9≲${\mathrm{D}}_{\mathrm{f}}$≲2.05 at R¯≲8 nm and 1.35≲${\mathrm{D}}_{\mathrm{f}}$≲1.6 at R¯≳8 nm. The low ${\mathrm{D}}_{\mathrm{f}}$ can be understood as an effect of magnetic dipole interaction, leading to chainlike aggregates when the critical size for ferromagnetism in Co particles is exceeded. The experimental values are in good agreement with recent computer simulations.

Using the technique of impact-collision ion-scattering spectrometry, we showed that the geometry of the Si(111)-(√3 × √3 )Ag surface consists of Si honeycomb on top of Ag trimers. The separation of the Si and Ag layers is 0.7±0.3 Å and the Si-Ag bond length is 2.73±0.3 Å. We also observed, for the first time, type-A and type-B arrangements of the Si and Ag atoms on the surface, with type B being type A rotated 180° about the surface normal.

With the aid of electronic structure calculations an explanation has been found for the experimentally observed difference in geometric structure between ${\mathrm{CoSi}}_2$/Si(111) and ${\mathrm{NiSi}}_2$/Si(111) interfaces.

The Laughlin-Haldane prediction that the charge $e^{\mathrm{*}}$ of quasiparticles excited across the energy gap of fractional quantum Hall-effect ground states at ν=p/q is $e^{\mathrm{*}}$=±e/q, a new fundamental quantum of nature, is found to be consistent with experiment. The experimental probe of $e^{\mathrm{*}}$ is ${\sigma }_{\mathrm{xx}}^c$, obtained from the extrapolated 1/T=0 intercept of the activated region of the Arrhenius plot, which is shown to be constant for p/q fractions of the same q and scale as 1/$q^2$ for q=3, 5, 7, and 9.

We observe for the first time the anomalous upturn in the parallel upper critical field predicted for metallic multilayers composed of two materials which have different diffusion coefficients but which have the same superconducting transition temperature. We observe this upturn, which reflects the shift of the nucleation field from the clean to the dirty layers, in multilayers of Nb/${\mathrm{Nb}}_{0.6}$${\mathrm{Ti}}_{0.4}$ having composition wavelengths Λ≥420 Å. The temperature at which the upturn occurs increases with Λ, as predicted by theory.