We present results on the photoproduction of 10 000 charmed particles from the ${10}^8$ recorded triggers of Fermilab experiment E691. The total cross section for the photoproduction of $D^0$ and $D^{+}$ particles (and antiparticles) for $x_F>0.2\mathrm{}$ is measured to be 3.88±0.06±0.40 μb/Be nucleus at $〈E_{\gamma }〉=145\mathrm{}$ GeV. We have also measured the relative production of different charmed particles, their $p_T^2$ and $x_F$ distributions, and the energy dependence of the total charm cross section. The mean $p_T^2$ is 1.16±0.04 Ge${\mathrm{V}}^2$/${\mathit{c}}^2$ and the ratio of charm cross sections at 200 and 100 GeV is 1.96±0.24. Results of fits to the $x_F$ distribution are also reported.

The first evidence is presented that the phase pattern of the planar-transverse optimal reaction amplitudes found previously for elastic-scattering strong-interaction reactions also holds for nonelastic reactions. The pattern is observed in the reaction $p+p\to d+\pi$ in the energy range between 300 and 800 MeV.

The evolution of the $\gamma$ quasicontinuum spectrum with neutron number has been investigated in the sequence of isotopes ${}_{}{}^{152,154,156}{}_{}{}^{}\mathrm{Dy}$. The three nuclei display a pronounced collective $E2\mathrm{}$ component. In ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$ this component shows a splitting into two distinct parts, signifying a structural change along the $\gamma$ cascade above the yrast line. The $E2\mathrm{}$ and statistical components are reproduced in simple $\gamma$-cascade calculations; in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{156}{}_{}{}^{}\mathrm{Dy}$ only rotational bands are included, whereas in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$ additional vibration-like transitions are required to reproduce the two $E2\mathrm{}$ peaks.

The relative strengths of harmonic intensities, generated in xenon by a strong infrared laser, are calculated from the time-dependent dynamics of the electronic charge density. The (nonperturbative) solution of the time-dependent Schrödinger equation for a single-electron model of the atomic system is found to predict line strengths in reasonable agreement with the observed intensities. A single time-dependent calculation provides the intensity predictions for all orders for a given laser wavelength and intensity.

Energy spectra of electrons ejected in the forward direction from 75-keV/amu ${\mathrm{He}}^0$-He,Ar collisions have been measured in coincidence with the charge-state-selected outgoing projectiles ${\mathrm{He}}^0$, ${\mathrm{He}}^{+}$, and ${\mathrm{He}}^{2+}$. The appearance of the cusp-shaped peak in the electron spectrum in coincidence with ${\mathrm{He}}^0$ gives the first clear experimental evidence that electron capture into the continuum states (ECC) occurs in collisions where the projectile is a neutral atom. The ECC for ${\mathrm{He}}^0$ impact is characterized by a considerably smaller width of the cusp peak than for ${\mathrm{He}}^{+}$ impact.

We have observed the formation of an optical intensity shock and the subsequent wave breaking in the nonlinear propagation of 1-psec pulses in an optical fiber. The wave breaking manifests itself as the appearance of oscillations trailing the shock, which are due to the beating of widely separated frequency components which bridge the shock. The experimental results are in good agreement with numerical solutions of the nonlinear Schrödinger equation.

Magnetic turbulence ($\nu <400\mathrm{}$ kHz) has been measured both internal and external to a tokamak plasma as the edge $q$ is varied from 0.6 to 3. The radial and poloidal spatial dependences of the coherence are investigated. The spatial and frequency dependences of the fluctuations are similar over the wide range of $q$.

It is shown that a population of high-energy trapped particles, such as that produced by ion cyclotron heating in tokamaks, can result in a plasma completely stable to both sawtooth oscillations and the fishbone mode. The stable window of operation increases in size with plasma temperature and with trapped-particle energy, and provides a means of obtaining a stable plasma with high current and high $\beta$.

Exact direct and usual pair correlations for a fluid of anisotropic hard cores with orientational as well as one-dimensional translational degrees of freedom are obtained. These results indicate that previous approximations for the direct correlations in anisotropic hard-core fluids have overlooked the effect of orientational configurations for which the hard-core overlap is not a smooth function of particle separation beyond the point of hard-core contact.

A solution to the phase problem is described which gives the phase invariant $\Psi =2\mathrm{}{\phi }_h-{\phi }_g$ for electron structure factors with phases ${\phi }_h$ and ${\phi }_g$ in noncentric crystals. The method exploits the high-voltage dependence of a minimum in convergent-beam transmission-electron-diffraction patterns in the systematics three-beam geometry. For CdS, with $h=\left(00\mathrm{}\overline{2}\right)$ and $g=\left(00\mathrm{}\overline{4}\right)$ we find $\Psi =54.4\mathrm{}°+0.9\mathrm{}°$. The error in the derived ($00\overline{2}$) x-ray structure-factor phase is 0.069°. The method is accurate enough to provide information on the bonding charge distribution in noncentric crystals.

We report evidence of a new long-range order in single-crystal ${\mathrm{Zn}}_3$${\mathrm{As}}_2$ grown epitaxially on lattice-mismatched InP and GaAs. The Raman activity of the layers confirm, in contrast to the bulk $C_{4v}^{12}$ symmetry, a space group of $D_{4h}^{15}$ for the epilayers. Thermodynamic considerations show that the dependence of the free energy on the lattice parameters may be sufficient to account for the stability of the new structure in the epilayer.

An amorphous carbon structure is obtained with a computer simulation based on a first-principles molecular-dynamics method, in which the interatomic potential is constructed directly from the electronic ground state, using density-functional techniques. From our results, which agree well with the limited experimental information available, we analyze the short-range order, particularly the fractions of $s{p}^2$ and $s{p}^3$ sites, together with the electronic properties.

Step-by-step structural information has been obtained for the growth of a metallic film on a semiconductor substrate: Pb/Ge(111). Complete three-dimensional atomic coordinates have been determined for each equilibrium phase formed in the growth process, from submonolayer to multilayer coverages. The quantitative structural model explains for the first time the observed 30° rotation in the overlayer basis vectors of the 1×1 multilayer epitaxial Pb film from those of the two low-coverage phases.

Synchrotron x-ray diffraction analysis of GaAs(001) epilayers grown in situ by molecular-beam epitaxy shows clear evidence of As-As dimers on top of the outermost As layer. These dimers are ordered in a variable way between two structures which both show a $c(4\mathrm{}\times 4)$ symmetry but have different As content. Both structures have twofold symmetry although the unit cell is square.

A new mechanism for the thermal desorption of molecular hydrogen from the monohydride phase on Si(100) has been identified. The unusual first-order desorption kinetics that are observed are due to the irreversible excitation of a hydrogen adatom into a delocalized, two-dimensional band state on the surface with an activation energy of 47 kcal/mol. The desorption reaction occurs between this excited hydrogen adatom and a second, localized hydrogen adatom.

Recombinative desorption of ${\mathrm{D}}_2$ from clean Pd(100) was studied with rotational-state selectivity by using resonantly enhanced two-photon ionization. The amount of vibrationally excited ${\mathrm{D}}_2$ molecules increased exponentially with the surface temperature $T_s$, associated with an activation energy, in addition to that for desorption of ${\mathrm{D}}_2(v^{\prime \prime }=0)$, of 210±60 meV, considerably less than the vibrational energy of free ${\mathrm{D}}_2$ molecules. This activation energy can be accounted for by a temporary trapping of ${\mathrm{D}}_2$ in a molecular chemisorption state with a softened D-D bond, in qualitative agreement with a recently proposed model.

An exact mapping of the classical wave problem to that of electronic motion is utilized together with extensive numerical results to examine the question of the existence of genuine localization (i.e., one occurring when both components have real positive dielectric constants) of classical waves in random binary alloys $A_{1-x}{B}_x$. We find that scalar waves do exhibit localization. We have also developed a coherent potential approximation which for $x<\mathrm{}0.2\mathrm{}$ gives results not that much different from the numerical ones. This result can be easily generalized to electromagnetic fields as well.

The effect of a magnetic field on potential scattering is investigated microscopically. Analytic, exact renormalization-group, and numerical solutions show that the classical scattering dynamics of a charged particle is extremely complex. In one case, the dynamics is described by a discontinuous one-dimensional Hamiltonian map. Preliminary results indicate complex quantum dynamics as well. Consequences include an explanation of the observed breakdown of the quantum Hall effect at large currents and possible $\frac{1}{f}$ noise.

By use of x-ray lithography Si inversion layers have been fabricated with width ∼25 nm and mobility ∼15 000 ${\mathrm{cm}}^2$/V s. These display oscillations in their conductance that are periodic in the number of electrons per unit length, even in zero magnetic field. The oscillations reflect an oscillatory activation energy of the conductance and are accompanied by unusual nonlinearities suggestive of pinned charge-density waves.

Persistent currents in multichannel normal-metal rings threaded by a magnetic flux are treated by a Green's-function approach. The diffusive region is analyzed in detail and the typical current predicted as a function of disorder, temperature, and number of channels. The amplitude and temperature sensitivity of the effect are found to be governed by correlations in the energy spectrum of the ring, of range $E_c$ proportional to the Thouless energy. The results are relevant to persistent-current experiments in mesoscopic metal rings.