We investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to ≅20 Å. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.

We report the first systematic study of the frequency dependence of optical nonlinearities of bulk GaAs at room temperature. In contrast to the previous understanding, band filling and plasma screening of Coulomb enhancement of continuum states are found to be the dominant contributions to the dispersive optical nonlinearities under quasi steady-state excitations. The partly phenomenological semiconductor plasma theory is in good agreement with the experimental data.

The first experimental evidence for a thermal-equilibrium defect density in undoped hydrogenated amorphous silicon, a metastable material, is presented. The defect density is in thermal equilibrium at temperatures above 200°C; the defects measured at room temperature are largely those frozen in during cooling from the equilibrium regime. Rapid quenching reversibly increases defect density. The defect density can be reversibly decreased by shunting of the defect-generation process during cooling. This latter technique has important implications for $a$-Si:H-based solar cells.

A mean-field theory is presented for the coarsening of a two-dimensional soap-film network. This theory explains (1) the correlation between area and number of sides of the cells, and (2) the anomalous absence of dynamical scaling laws, which is shown to have its origin in the topological constraint of a space-filling network.

Martensitic transformations are shown to be driven by the interplay between two fluctuating strain components. No soft mode is needed, but a central peak occurs representing the dynamics of strain clusters. A two-dimensional magnetic-analog model with the martensitic-transition symmetry is constructed and analyzed by computer simulation and by a theory which accounts for correlation effects. Dramatic precursor effects at the first-order transition are demonstrated. The model is also of relevance for surface reconstruction transitions.

Using lithographic techniques, we have prepared two-dimensional pores with widths controllable in the range 10- 100 Å. These materials are formed from amorphous superlattices consisting of alternating layers of silicon and silicon dioxide by use of a selective etch. The micropores adsorb hydrocarbons from solution in a size-selective manner. The effects of confinement in these molecular-scale structures are observed by fluorescence spectroscopy: We demonstrate that pyrene molecules trapped in 20-Å pores lack the mobility necessary to form dimers.

In a recent article Schoen and Yau develop an inequality which shows that there exists a relationship between the minimum energy density ${\rho }_0$ in a star and the size $R$ of the star, of the form ${\rho }_0{R}^2\le \frac{\pi }{6}$. This article shows that this inequality is valid for two different measures of the size of the star and conjectures that the inequality can be improved to ${\rho }_0{R}^2\le \frac{3\pi }{32}$.

In problems such as the anharmonic oscillator and the H-atom Stark effect, the energies of bound states and resonances can be formally expressed in terms of series with a radius of convergence zero. We propose a method of rational approximation to sum such series and illustrate it by application to the anharmonic oscillator. In a certain sense this method is close to the best one possible.

A Hamiltonian formulation of general relativity based on certain spinorial variables is introduced. These variables simplify the constraints of general relativity considerably and enable one to imbed the constraint surface in the phase space of Einstein's theory into that of Yang-Mills theory. The imbedding suggests new ways of attacking a number of problems in both classical and quantum gravity. Some illustrative applications are discussed.

The asymptotic $\frac{D}{S}$ ratio of the deuteron, $\eta$, is deduced from new measurements of tensor analyzing powers for sub-Coulomb ($d,p$) reactions. Values of $\eta$ obtained under various experimental conditions (i.e., corresponding to different analyzing powers, bombarding energies, target nuclei, and final states) are consistent within their statistical uncertainties. A weighted average of the individual measurements gives $\eta =0.0256\mathrm{}\pm 0.0004$. This result is compared with predictions obtained from a variety of theoretical models.

Relative elastic differential cross sections are measured for 100- to 300-eV positrons and electrons scattering at 30° to 135° angles from argon atoms in the first crossed-beam experiment to be performed with a positron beam. Good agreement is found with prior electron work and the few available calculations for positrons. The shapes of the positron and electron differential cross-section curves at 100 eV are quite different, but appear to become more similar at 300 eV.

We present measurements of the angular distribution of fast hydrogen atoms formed by electron capture of 2.8- and 5.0-MeV protons in atomic hydrogen. In the angular region of the Thomas peak (0.47 mrad) the experimental results obtained with this pure three-body collision system are in reasonable agreement with a strong-potential Born calculation and the impulse approximation, but not with other higher-order theories.

We report the first measurement of the angular distribution of the 304-Å ${\mathrm{He}}^{+}\mathrm{}(n=2\mathrm{})\mathrm{}$ radiation following photoionization. This distribution reflects the alignment of the ion, which is related to the fraction $\xi =\frac{\sigma \mathrm{}(2p,kd)\mathrm{}}{\left[\sigma \mathrm{}\right(2p,ks)+\sigma \mathrm{}(2p,kd\left)\right]}$ of $d$ component in the electron wave. The experimental angular distributions correspond to alignments of -0.62 ± 0.03 and -0.62 ± 0.02 at photon energies of 65.5 and 66.5 eV, respectively. These translate into ratios $\xi =0.25\mathrm{}\pm 0.04 \mathrm{and} 0.25\pm 0.03$, in good agreement with close-coupling calculations.

The rate of radiated photons per electron passing through a helium-filled Ĉerenkov counter is observed to oscillate as a function of the gas pressure when a weak, static magnetic field is applied. The data agree well with an analysis which treats Ĉerenkov and synchrotron radiation as limiting manifestations of a unified process.

Supercontinua extending from the ultraviolet to the infrared are observed from high-pressure (1-40 atm) Ar, Kr, Xe, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, or C${\mathrm{O}}_2$ illuminated with 2-psec or 70-fsec, 0.6-μm pulses with an energy ≲500 μJ. The blue spectral component is shown to display a nearly universal behavior for all gases and pulse durations. Although the maximum intensity of the focused, femtosecond pulse in an evacuated cell was ∼${10}^{13}$ W/${\mathrm{cm}}^2$, continuum generation was only observed with the femtosecond pulse when the threshold for self-focusing was exceeded.

The onset of electromagnetic oscillations that are observed in magnetically confined plasmas where beams of fast neutrals are injected is associated with the excitation of a mode with poloidal wave number $m^0=1\mathrm{}$ and phase velocity equal to the core-ion diamagnetic velocity. The resonant interaction of the mode with the beam ions is viewed as a form of dissipation that allows the release of the mode excitation energy, related to the gradient of the plasma pressure.

Fast waves with frequencies near the mean gyrofrequency [$\omega \sim {\left({\omega }_{\mathrm{ci}}{\omega }_{\mathrm{ce}}\right)}^{\frac{1}{2}}$] were excited in a toroidal, magnetized plasma. Experimental measurements were made of wavelengths and phase velocities perpendicular to the toroidal magnetic field, and wave energy trajectories in the plasma, with varying wave frequency, plasma density, and magnetic field values. Experimental results agree with predictions from the cold-plasma dispersion relation.

The formation of $F$ centers by monochromatic soft x-ray radiation was studied in the range of photon energies 50 to 1500 eV by use of a sensitive laser-induced luminescence technique. Production efficiencies of the order of $\frac{1 \mathrm{keV}}{\left(F \mathrm{center}\right)}$ are obtained throughout the upper end of the above energy range in KCl and KBr at 77 K. Saturation densities of about ${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ occur, with the total number of centers depending upon the depth of penetration of the radiation. Reduced efficiency is noted at the lowest energies and highest absorption coefficients, and is possibly related to the observed photodesorption of neutral and ion species from the surface.

Channeling and blocking effects of electrons and positrons emitted from ion-implanted radioactive In dopants in silicon have been utilized for lattice location of In in parts-per-million concentrations. A majority of In atoms occupies substitutional sites after implantation at room temperature. Defect recovery, which is observed after annealing at 700 K, increases the channeling effects. These results corroborate conclusions from previous Mössbauer studies on the same system under identical implantation conditions. The nature of the damage cascades of the individual implanted impurity probe atoms is discussed.

We present a novel concerted exchange mechanism for the diffusion of substitutional atoms in semiconductors that does not involve any defects. For self-diffusion in Si we show that this mechanism is energetically favorable and is able to account for a number of experimental observations. Implication for the diffusion of substitutional dopants is discussed.