The interacting-boson-model parameters are examined from a shell-model viewpoint with use of the generalized seniority scheme in two nondegenerate $j$ shells. Predictions for the interacting-boson-model parameters are obtained by constructing the boson image of the corresponding fermion operator and are found to be in good overall agreement with the empirically determined values.

The polarizations, $P_{\mathrm{av}}\equiv \frac{〈\overrightarrow{\mathrm{J}}·{\overrightarrow{\sigma }}_{\mu }〉}{J}$ and $P_L\equiv \frac{〈\overrightarrow{\mathrm{J}}·\hat{v}〉}{J}$, of the recoils ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mu }^{-}$,$\nu$)${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$(g.s.) were measured simultaneously by selective recoil implantation. Their ratio $R^{\prime }$ is largely immune to the systematics of $P_{\mathrm{av}}$ and $P_L$ and more dependent on the dynamics than either. Our result (normalized to unit ${\overrightarrow{\mathrm{P}}}_{\mu }$), $R(\mathrm{g}.\mathrm{s}.\mathrm{})=-0.506\mathrm{}\left(41\right)\mathrm{}$, yields $\frac{g_P}{g_A}=9.0\mathrm{}\left(1.7\right)\mathrm{}$ (impulse approximation) and $\frac{F_P}{F_A\left({q_m}^2\right)}=-1.03\mathrm{}\left(14\right)\mathrm{}$ (elementary-particle treatment), to be compared to partial conservation of axial-vector current (PCAC) predictions of 7 and - 0.99, respectively. Thus PCAC is quantitatively verified.

Experimental cross sections for the electron-attachment processes for S${\mathrm{F}}_6^{-}$/S${\mathrm{F}}_6$ and ${\mathrm{Cl}}^{-}$/CF${\mathrm{Cl}}_3$ are reported in the energy range 0-200 meV by normalizing each attachment line shape to measurement of a thermal rate coefficient. When the same ion states are detected, good agreement is found between present values, for which a monoenergetic electron source is used, and swarm-unfolded results. The present data constitute a new limit for cross sections reported at high resolution at the lowest electron energy.

A broad spectrum of unstable parasitic waves can arise from the interaction between the trapped electrons and the radiation generated by a free-electron laser. Imposing a dc electric field with appropriate strength at the onset of trapping can substantially narrow the unstable spectrum and allows considerable enhancement in the radiation intensity. The nonlinear mechanisms which limit the enhancement process are observed to be due to nonlinear frequency shift and detrapping.

The results of hybrid simulations of the formation and the evolution to equilibrium of field-reversal experiment (FRX) configurations are presented. The observed rapid reconnection is explained in terms of forced reconnection driven by a Kruskal-Schwarzschild instability and the self-consistent production of toroidal magnetic field.

The nonlinear behavior of the $m=1\mathrm{}$ instability is investigated within two-fluid theory. In contrast to the purely resistive case, the mode is found to saturate at a finite amplitude if diamagnetic effects are sufficiently strong. The main stabilizing process is a nonlinear azimuthal shear flow.

It is shown that very-low-frequency magnetic modes may be unstable for systems such as tandem mirrors which contain plasma trapped in regions of unfavorable curvature. Onset of the instability occurs when the diamagnetic plasma pressure is sufficient to reverse particle drift velocities.

We present experimental evidence for crystal-face specificity in surface segregation at CuNi alloy surfaces using angular x-ray photoemission spectroscopy, secondary-ion mass spectroscopy, and "chemisorptive titration" techniques. More copper enrichment is found on the (100) surface than the (111) surface of a 17% Cu-83% Ni alloy, in agreement with predictions from current segregation theories.

We report a high-resolution synchrotron x-ray scattering study of the commensurateto-incommensurate transition of monolayer krypton physisorbed on $\mathrm{ZYX}$ graphite. At the transition the substrate-limited coherence of the commensurate monolayer (2300 Å) deteriorates to less than 100 Å, demonstrating that atomic positional disorder is a dominant feature of the weakly incommensurate system. The peak position follows a $\frac{1}{3}$ power law versus reduced temperature.

A differential anomalous x-ray scattering technique has been developed for structural studies of disordered and amorphous systems. The results on amorphous Ge${\mathrm{Se}}_2$ are consistent with the twofold coordination of Se and the fourfold coordination of Ge. The results on amorphous GeSe are consistent with threefold-coordinated models of the structure but not with the fourfold-twofold models.

The (220) lattice plane spacing of an almost perfect crystal of silicon was measured by means of a combined scanning ($\mathrm{LLL}$) x-ray interferometer and a two-beam optical interferometer. From 170 measurements, a value $d_{220}=(192\mathrm{}015.560\pm 0.012)$ fm results in vacuum at 22.50°C. This value is smaller by $1.8\times {10}^{-6\mathrm{}}{d}_{220}$ than that reported by Deslattes et al. for another crystal. Generic variabilities of the two crystals account only for a part of this difference.

A powerful, new extension of the Monte Carlo renormalization-group (MCRG) method is used to accurately determine the tricritical point and exponents in two very different two-dimensional models: an Ising antiferromagnet and a Blume-Capel model. We find four relevant tricritical eigenvalues which are essentially identical for both models. We also demonstrate that subtle warning signals appear in a standard Monte Carlo calculation when a second-order transition is being misinterpreted as first order.

I present Monte Carlo evidence for a first-order phase transition in pure SU(5) lattice gauge theory with Wilson's action in four space-time dimensions. Although less clear, a similar transition is strongly suggested already with SU(4).

Total muon capture rates on nuclei are calculated for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The distribution of muon capture strength is calculated with use of random-phase-approximation and Tamm-Dancoff-approximation methods in a response-function approach. The randomphase-approximation calculation is in excellent agreement with data.

Differential cross sections for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ($\pi$,${\pi }^{\prime }$) were measured between 100 and 300 MeV for momentum transfers of $1.1\hslash$ ${\mathrm{fm}}^{-1\mathrm{}}$ and $1.4\hslash$ ${\mathrm{fm}}^{-1\mathrm{}}$. In this energy range the different energy dependences of the spin-dependent and spin-independent parts of the pion-nucleon interaction provide a very sensitive method of discriminating between transitions that proceed with a spin transfer ($\Delta S=1\mathrm{}$) or without a spin transfer ($\Delta S=0\mathrm{}$). Five transitions in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ were found to be dominated by the $\Delta S=1\mathrm{}$ transition density amplitude.

The classical-trajectory Monte Carlo method has been used to investigate collisions of ions and Rydberg atoms in strong dc electric fields. Cross sections are presented for $n=10\mathrm{}$ and $n=20\mathrm{}$ Rydberg atoms at velocities $1<~\frac{v}{v_e}<~10$ where $v_e=n^{-1\mathrm{}}$ a.u. Electric fields which ionize product Rydberg atoms in states $n^{\prime }=n+\Delta n$ with $\Delta n=1, 2, \mathrm{and} 4$ were used. The electric field caused the cross sections for electron capture to decrease by up to fourfold while the ionization values increased by up to two orders of magnitude.

Interference effects have been observed when coherently related x-ray beams are Compton scattered from an atomic system. We use dynamical diffraction methods to prepare the initial x-ray state and observe the coherent Compton effect in both crosssection and profile measurements.

Results of Monte Carlo studies of the $q$-state Potts model on a square lattice with antiferromagnetic nearest-neighbor coupling and ferromagnetic next-nearest-neighbor coupling are presented for $q>~3$. The three-state Potts model shows a variety of unusual transitions whereas for $q>~4$, we find two first-order phase transitions separated by a broken sublattice symmetry state.

We report detailed observations of the propagation of third sound in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture films as a function of temperature and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration. The data are consistent with a simple model for the film and we conclude that thin ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture films exhibit nearly complete phase separation.

We report the first determination of critical exponents for a chemisorbed overlayer, using low-energy electron diffraction. We examine the order-disorder transition of $p(2\mathrm{}\times 2)$ oxygen on the (111) surface of nickel. This study is the first of critical behavior of a two-dimensional system in the four-state Potts universality class. From measured diffraction-beam profiles, we extract the exponents $\beta$, $\gamma$, and $\nu$. Discussion of disparity between our results and predicted exponents considers several possibilities, including logarithmic corrections.