Inclusive proton spectra from reactions induced by ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ projectiles have been decomposed by use of a refined multisource analysis. Yields from centrallike collisions have been extracted and compared with Boltzmann-master-equation predictions. The comparison yields the initial number of degrees of freedom, $n_0$. The excitation energy per initial degree of freedom, $\frac{E^{*}}{n_0}$, is found to be essentially independent of the colliding masses, depending only on the per-nucleon energy of the projectile. An empirical relation connecting $\frac{E^{*}}{n_0}$ with the available incident energy is given.

The 0-0 band of the $d{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}-b{}_{}{}^1{}_{}{}^{}\Pi$ electronic transition of SiC was detected near 6100 ${\mathrm{cm}}^{-1\mathrm{}}$. The SiC molecule was observed by high-resolution Fourier-transform emission spectroscopy from a composite-wall hollow cathode. Ab initio quantum chemical calculations confirm the identity of the carrier of the spectrum as SiC and the $d{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}-b{}_{}{}^1{}_{}{}^{}\Pi$ assignment. Combination of the observed $r_0$ for SiC $b{}_{}{}^1{}_{}{}^{}\Pi$ with theoretical shifts in bond lengths from $b{}_{}{}^1{}_{}{}^{}\Pi$ to $X{}_{}{}^3{}_{}{}^{}\Pi$ gives the astrophysically important bond lengths $r_0=1.723\mathrm{}$ Å and $r_e=1.719\mathrm{}$ Å for $X{}_{}{}^3{}_{}{}^{}\Pi$ to an estimated accuracy of 0.001 Å.

A novel theory for the self-consistent evolution of a nonlinear trapped-ion temperature-gradient-driven instability, based on the turbulent trapping of resonant ions in the electrostatic potential of the waves, is proposed. Threshold-dependent, non-steady-state turbulence (nonlinear instability) is shown to develop. The resulting anomalous thermal and particle transports act to reconfigure the equilibrium temperature and density profiles in such a way as to return the system towards marginality. Implications of the theory for present and future-generation fusion experiments are discussed.

An amorphous silicon structure is obtained with a computer simulation based on a new moleculardynamics technique in which the interatomic potential is derived from a parameter-free quantum-mechanical method. Our results for the atomic structure, the phonon spectrum, and the electronic properties are in excellent agreement with experiment. In addition we study details of the microscopic dynamics which are not directly accessibly to experiment. We find in particular that structural defects are associated with weak bonds. These may give rise to low-frequency vibrational modes.

We study the effect of Coulomb interactions on the onset of superconductivity with increasing intergrain coupling in granular systems at $T=0\mathrm{}$. We show that in three dimensions (3D) the onset transition is described by Euclidean scalar electrodynamics, and so can be either first or second order, depending on parameters. A renormalization-group analysis indicates that in 2D the transition can either be first order, or second order with $3\mathrm{D}-\mathrm{XY}$-like critical behavior. In 1D Coulomb forces are shown to destroy the superconducting state.

In situ Fourier-transform infrared spectra of polyaniline give clear evidence for an insulator-to-metal transition in the first oxidation step and a metal-to-insulator transition in the second oxidation step. The former originates from a pure protonic mechanism without intercalation of counter ions whereas in the latter counter-ion insertion without protonic activity occurs. In the metallic form of the polymer a free-carrier absorption peak is explicitly observed and found to be in agreement with an effective-medium-type dielectric function for a granular metallic system.

Frequency-dependent conductivity measurements are reported in the dc to 150-GHz range in the moderately heavy-fermion compound ${\mathrm{CePd}}_3$. A Drude expression is found with a renormalized relaxation time ${\tau }^{*}$, and a renormalized plasma frequency ${\omega }_p^{*}$ at low temperatures. This experiment represents the first direct measurement of these quantities, and the enhanced mass obtained from the frequency-dependent conductivity compares favorably with that determined from thermodynamic measurements.

We show that the application of an electric field $F$ along the growth axis of a semiconductor superlattice results in a strong localization of the eigenstates, a blue shift of the optical-absorption edge, and the presence of oscillations periodic in $F^{-1\mathrm{}}$. These unique electro-optical properties are derived here within the framework of a tight-binding description of the envelope functions and also from numerical solutions of the Schrödinger equation for a finite set of coupled quantum wells.

The ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ hyperfine structure of the anomalous muonium center in silicon has been resolved in muonspin-rotation spectra. The spectra of the weak ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ satellite lines show that there are two equivalent Si neighbors on the <111> symmetry axis with large positive $p$-like spin densities. These results, which are confirmed by level-crossing-resonance spectroscopy, establish that anomalous muonium in the group-IV semiconductors is an interstitial muonium located at the bond center.

By generalizing Onsager's reaction-field approach, we calculate the local filed in random dielectric media characterized by uniform or percolative correlations. The distribution of the local field is found to display a double-peak character in general. We show that the variance of the distribution is linearly correlated to the deviation of the mean from the Lorentz value, where the latter quantity is also identified as the correlation to the Clausius-Mossotti relation. Structural correlations are demonstrated to have a significant effect in determination of the distribution characteristics.

Cyclotron-resonance measurements of surface-state electrons on liquid helium are reported. By independently controlling the electron density and electric field, we identify the contribution to the conductivity from many-electron effects. The linewidth increases with density, in conflict with existing theories. The onset of electron crystallization is not observed with cyclotron resonance.

We present evidence for the excitation of the two-photon Doppler-free $1S-2S$ transition in muonium. Thermal muonium atoms were produced in vacuum by a 27.5-MeV/c pulsed positive-muon beam incident on a powdered Si${\mathrm{O}}_2$ target. Transitions were induced by the light from a frequency-doubled pulsed dye laser and were detected by photoionization of the $2S$ state. The $1S(F=1\mathrm{})\mathrm{}\to 2S(F=1\mathrm{})\mathrm{}$ transition frequency is within 300 MHz of the QED prediction.

Laser-induced continuum structure with a Fano-type line shape has been observed in xenon by the coupling of a bound [$5p^{5}10p(J=0\mathrm{})\mathrm{}$] state to the ionization continuum with a tunable dye laser. The continuum was probed by three-photon ionization, two-photon-resonantly enhanced by the $5p^{5}6p(J=0\mathrm{})\mathrm{}$ state.

We predict that anomalously long localization lengths—which in some special cases diverge—can be expected for $p$-polarized light propagating through one-dimensional disordered structures under certain conditions. We give a physical argument for the existence of such anomalies, and support it by a long-wavelength theory and Monte Carlo calculations.

Ion bombardment of polycrystalline Ge, Si, and Au films results in grain-boundary migration rates which are weakly temperature dependent, and which greatly exceed thermal migration rates. The enhanced migration rate is proportional to the rate of nuclear collisions at or very near the grain boundary. We present a transition-state model which accounts for the observed kinetics of grain-boundary migration during bombardment. The ratio of atomic jumps at grain boundaries to the local collision-induced Frenkel-defect generation rate is characteristic of each material.

The Si(111)-As surface is imaged by scanning tunneling microscopy under UHV conditions. A threefold-symmetric 1 × 1 structure is observed, consistent with models suggesting substitution of As for Si in the outer half of the top double layer. No evidence for residual stacking faults on the scale of the 7 × 7 mesh is detected, but atomic debris, interpreted as displaced Si atoms, is seen. $I-V$ characteristics of the junction show features separated by 1.9-2.3 eV consistent with an empty surface state in the projected gap and a filled state resonant with the bulk as predicted by self-energy calculations for this surface.

Time-lapse topography has been done with a scanning tunneling microscope on a clean, annealed Au(111) surface showing the effects of surface diffusion of Au atoms at room temperature. Within several minutes, features such as marks made by a gentle touch of the tunnel tip are seen to change as a result of the diffusion of Au atoms over the surface. The diffusion does not depend on the magnitude or sign of the tunneling current and voltage parameters. These experiments demonstrate the unique ability of the scanning tunneling microscope to obtain directly information about surface diffusion on an atomic scale.

An investigation is presented of elasticity percolation in isotropic media with arbitrary elastic constants. A model which combines the central-force potential energy and a triangular lattice with a large unit cell allows us to vary the ratio between the two Lamé coefficients. Results of numerical simulations indicate that the critical properties of the percolation transition do strongly depend on the elastic constants.

New molecular-dynamics data are reported for the shear viscosity of a Lennard-Jones fluid as a function of the shear rate $\dot{\epsilon }$, $\eta \left(\dot{\epsilon }\right)$ is deduced from the transient behavior of the system suddenly subjected to a planar Couette flow. $\eta \left(\dot{\epsilon }\right)$ is found insensitive to $\dot{\epsilon }$ for $\dot{\epsilon }<1\mathrm{}$ (in reduced units) and in complete agreement with the Green-Kubo value $\eta \mathrm{}\left(0\right)\mathrm{}$. At higher $\dot{\epsilon }$, the expression $\eta \left(\dot{\epsilon }\right)\approx 1.29-0.013{\dot{\epsilon }}^2$ reproduces our data fairly well.

The "normal" transport properties of the high-$T_c$ superconductors, especially tunneling and anisotropic resistivity, are extraordinarily anomalous. We show that these properties can be explained, perhaps uniquely, by a two-dimensional resonating-valence-bond state.