Electron plasma waves excited by optical mixing of two antiparallel C${\mathrm{O}}_2$ laser beams have been detected by ruby-laser Thomson scattering. The density resonance is confirmed by two separate methods.

We have used temporally and spectrally resolved Thomson scattering in a C${\mathrm{O}}_2$-laser-plasma interaction experiment to identify electron plasma waves driven by stimulated Raman scattering in a low-density preformed plasma. Plasma waves were observed in a Gaussian-shaped plasma whose peak density was in the range $\mathrm{}(0.01-0.05)\mathrm{}{n}_c$, in qualitative agreement with threshold calculations using a convective amplification model. The plasma waves were observed only during the early part of the pump pulse, and disappeaŕed coincidentally with the onset of ion waves driven by stimulated Brillouin scattering.

Test-particle acceleration is studied in a two-dimensional, turbulent magnetohydrodynamic simulation whose initial magnetic field configuration is a periodic sheet pinch. Some test particles become trapped near large electric field in the reconnnection region for approximately an Alfvén transit time. The maximum speed attained is consistent with an analytic estimate which depends on the reconnection electric field, the Alfvén speed, and the ratio of Larmor period to the Alfvén transit time.

The thermal wave of a minor disruption can initiate the H phase of a neutral-beam-heated divertor tokamak discharge. Its propagation is used to probe the plasma edge conditions at the H transition. The results show the existence of a transport barrier which forms at the plasma edge and impedes the flow of particles and energy across the plasma surface, giving rise to improved confinement properties. Location and extension of the barrier coincide with the edge zone of increased shear specific to the divertor configuration.

Pinch-point stability and unstable—pulse-shape analyses for relativistic-electromagnetic instabilities in three dimensions are presented. The time-asymptotic, unstable pulse shape is determined by $\frac{{\omega }_{\mathrm{pi}}^{\prime }\left(\overrightarrow{\mathrm{V}}\right)}{\gamma }$ as a function of $\overrightarrow{\mathrm{V}}$, where ${\omega }_{\mathrm{pi}}^{\prime }\left(\overrightarrow{\mathrm{V}}\right)$ is the observer's frame pinchpoint growth rate and $\gamma ={(1-\frac{V^2}{c^2})}^{-\frac{1}{2}}$. The differential equations for the observer's frame pinch points are established. The results are illustrated for instabilities in an anisotropic, relativistic electron plasma.

Laser-plasma interaction has been numerically simulated with a Fokker-Planck code, including inverse-bremsstrahlung absorption and ion motion. The results were compared with the hydrocode LILAC, with adjustable flux limiter $f$. The coronal temperature and density profile agree with $f=0.15\mathrm{}$, but heat penetration into the cold dense plasma is better fitted by $f=0.08\mathrm{}$. Thus, there is still a factor of 3 to 5 to be explained between classical kinetic theory and the usual ad hoc value $f=0.03\mathrm{}$.

The maximum amplitude of the $m=2\mathrm{}$ tearing mode in tokamaks is found to increase rapidly with rising safety factor at the magnetic axis, $q\left(0\right)\mathrm{}$. The resulting large magnetic islands encompass virtually the entire plasma cross section for $1.5\lesssim q\left(0\right)\mathrm{}\lesssim 1.8$ and are shown to be related to the vacuum bubbles formed during the saturation of the ideal $m=2\mathrm{}$ kink mode. On the basis of these results, a mechanism for major distruptions is proposed along with a possible method for their elimination.

The electromigration in the steady-state mode and anisotropic diffusion of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Ni}$ in single-crystal $\beta$-tin have been measured by a tracer-sectioning technique. For the first time the solubility of Ni in Sn has been observed. The unusual features are extremely rapid mobility, low activation energies, and extraordinary anisotropy of diffusion ($\frac{D_{\parallel }}{D_{\perp }}=7\mathrm{}\times {10}^4$). The strength of the electromigration driving force in the basal plane of Sn shows a surprisingly strong temperature dependence.

We report on the first spatially resolved study of the chemical and electronic properties of microdomains in molecular monolayers. Analytical electron-microscope measurements of stage-4 Sb${\mathrm{Cl}}_5$-intercalated graphite show Sb-rich islands of lateral dimension 500-1000 Å. Energy-loss spectra yield a 6-eV graphite $\pi$ plasmon modulated by the domain structure, but the 25-eV $\pi +\sigma$ plasmon and the carbon core excitations are spatially invariant. The derived island and background compositions are ${\left(\mathrm{Sb}{\mathrm{Cl}}_3\right)}_{\frac{1}{8}}{\left(\mathrm{Sb}{\mathrm{Cl}}_4^{-}\right)}_{\frac{1}{32}}$ and ${\left(\mathrm{Sb}{\mathrm{Cl}}_5\right)}_{\frac{9}{16}}{\left(\mathrm{Sb}{\mathrm{Cl}}_6^{-}\right)}_{\frac{9}{32}}$.

We demonstrate that anharmonic phonon-phonon coupling constants can be extracted from frozen-phonon total-energy calculations. The method is applied to the optical modes of diamond. The zone-center coupling constants are completely determined through fourth order, and are used to compute an effective four-phonon vertex including virtual optical-phonon exchange. The interaction is found to have the wrong sign to allow formation of a two-phonon bound state.

The first direct determination of the conduction band of hydrogenated amorphous silicon has been performed by means of x-ray inverse photoemission. We found a feature 1.2-4 eV above the Fermi level which may be associated, on the basis of its annealing behavior and energy position, with the Si-H antibonding orbital. Comparison with data on crystalline silicon clearly shows that the amorphous-silicon conduction-band density of states does not exhibit critical-point structures.

In a two-dimensional electron gas with a spatially modulated charge density we observe a splitting of the two-dimensional-plasmon dispersion. The charge-density modulation is induced in a metal-oxide-silicon capacitor with a modulated oxide thickness of submicrometer periodicity. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum and depends strongly on the Fourier expansion coefficients of the charge-density profile.

The first measurements of Josephson tunneling into both the paramagnetic and antiferromagnetic phases of Sm${\mathrm{Rh}}_4$${\mathrm{B}}_4$ are reported. The results show that the superconducting order parameter of Sm${\mathrm{Rh}}_4$${\mathrm{B}}_4$ is enhanced by the antiferromagnetic order occurring below the Neél temperature of about 0.87 K. On the basis of this observation, the applicability of various proposed theories of coexistent superconductivity and antiferromagnetism can be determined for Sm${\mathrm{Rh}}_4$${\mathrm{B}}_4$.

New features are observed in normal-emission photoelectron spectra from Ni(100) in a narrow range of photon energies around 25 eV. These features are inconsistent with either direct transitions from the bulk or emission from occupied surface states or resonances. We suggest that they are a consequence of interference between the ordinary direct transition emitting an electron in the normal direction and the excitation from the same initial state into a final state that would normally be emitted from the surface at $\bar{\Gamma }$ in the second surface Brillouin zone, but at this energy is trapped in a surface wave.

We consider the Landau theory of an almost-localized Fermi liquid with a very large $\frac{m^{*}}{m}$ ratio. We argue that this system has a superconducting transition to a $p$-wave state. We estimate $T_c$, calculate the specific-heat discontinuities at $T_c$, and discuss phase stability. Using available experimental data for $\gamma$ and $k_{\mathrm{F}}$, we find that our results for this model are in good agreement with the known experimental values for heavy-fermion compounds.

A splitting of the Cu-related 1.36-eV luminescence in GaAs into a set of lines in the $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$ alloy system is interpreted in a model with only the nearest-neighbor group-V shell influencing the defect levels. Experiental peak intensities are in good agreement with simple calculations and the binding energies of the defects are found to follow calculated chemical trends for defect pairs. Results for the ternary alloys are extrapolated to and compared with the case of Cu in GaP.

We show that a previous model for ($\mathrm{GaAs}$,) - $\mathrm{Ge2x}$, xwhen more accurately analyzed, does not fit the observed composition dependence of the direct energy gap. A simple model for the lattice disorder is proposed. This model fits the composition dependence of the gap with no adjustable parameters. Our results show that antisite disorder and the transition from zinc-blende to diamond symmetry have no significant influence on the energy gap of this material.

Following continuous-time random-walk dynamics, we consider particle diffusion and trapping on fractals, and study the interplay of the spatial ($\alpha$) and temporal ($\gamma$) stochastic aspects. At long times and for broad waiting-time distributions the mean squared displacement behaves as $〈r^2\mathrm{}\left(t\right)\mathrm{}〉\sim t^{\alpha \gamma }$, whereas the particle decay is algebraic, $\Phi \mathrm{}\left(t\right)\mathrm{}\sim t^{-\gamma }$.

An example is presented of a model of an amorphous quantum mechanical system, a liquid of quantal Ising spins, which can be solved exactly within certain many-body theories. Analytical solutions of the model in mean-field theory are shown to reveal a decrease in the extent of the ferromagnetic region (compared to an equivalent classical system) and the occurrence of some degree of quantum localization. Both phenomena are analyzed as a competition between quantum mechanics and the condensed phase.

We have searched for flavor-changing neutral-current weak decays of the $b$ quark and find $R_B(b\to l^{+}{l}^{-}X)<\mathrm{}0.31\%$ at 90% confidence level. This limit excludes models with the $b$ quark in a left-handed isosinglet. We also place limits on the ratio of the $B^{\pm }$ and $B^0$ semileptonic branching fractions and on $B^0{\bar{B}}^0$ mixing.