A model of stimulated Raman scattering from underdense plasmas in which the laser intensity profile and plasma density have been corrupted by the filamentation instability is described. The model accounts in a unified way for inhomogeneity in the density, for Landau damping, and for local enhancements in light-wave intensities. In shallow filaments the concentration of the light gives rise to modest increases in growth. On the other hand, for deeper filaments the inhomogeneity and Landau damping dominate to suppress the instability. In addition, backscatter is enhanced relative to sidescatter.

Experiments with a second-generation collective ion accelerator (IFA-2) are reported. Results demonstrate that ion acceleration with fields of 33 MV/m over 30 cm has been achieved with a controlled collective accelerator for the first time.

We present the results of a high-resolution x-ray-scattering study of the pretransitional fluctuations above the nematic-to-smectic-$C$ phase boundary in the ¯7S5/8OCB system. The fluctuations are well described by the Lifshitz-point model of Chen and Lubensky with minimal adjustable parameters. We have located the line along which the x-ray cross section $\sigma \left(\mathrm{q}\right)$ exhibits pure $q_{\perp }^{-4\mathrm{}}$ fluctuations. Close to the nematic-smectic-$A$-smectic-$C$ point this line is simply related to the smectic-$A$-smectic-$C$ transition boundary.

We report light-scattering measurements of nematic director fluctuations near the nematic-smectic-$A$-smectic-$C$ multicritical point in the ¯7S5-8OCB system. Our data analysis corrects for the effects of phase-boundary curvature and elucidates the evolution in the critical behavior as the nematic-smectic-$A$-smectic-$C$ point is approached from both the $\mathrm{N}-{\mathrm{S}}_A$ and $\mathrm{N}-{\mathrm{S}}_C$ side of the multicritical point. The results agree closely with x-ray scattering data and the bend elastic-constant behavior differs from any current theoretical predictions.

Critical and tricritical wetting transitions are studied in a simple-cubic nearest-neighbor Ising model. The magnetization $m_1$ in the surface layer and the susceptibilities ${\chi }_{11}=\frac{\partial m_1}{\partial H_1}$ and ${\chi }_1=\frac{\partial m_1}{\partial H}$, where $H_1$ is the surface field and $H$ the bulk field, are calculated by use of Monte Carlo methods. The critical behavior of these quantities is found to be consistent with simple mean-field behavior; the singularities predicted by various renormalization-group treatments are not observed.

Icosahedral quasicrystals of Al-Cu-Li are formed by solid-state reaction at the grain boundaries of host alloy. Convergent-beam electron-diffraction measurements confirm that the symmetries of the quasicrystals belong to the $m\overline{3}\overline{5}$ point group. The icosahedral grains are formed by a nucleation and growth process and are strongly oriented at the grain-matrix interface. The stability of the icosahedral phase is compared with that of metallic glasses produced by a similar technique.

Experimental measurements of the superconducting-to-normal phase boundary for a Sierpenskigasket wire network are presented. The fractal phase-boundary curve, $T_c\mathrm{}\left(H\right)\mathrm{}$, shows four orders of dilational invariance and is in excellent quantitative agreement with theoretical predictions. The fracton dimension, $\bar{\overline{d}}$, has also been measured and is found to be 1.35 ± 0.02, consistent with the calculated value of 1.365 for the planar Sierpinski gasket.

The NMR line shape and spin-echo behavior of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ NMR in powder samples of K${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_{0.5}$${\mathrm{Br}}_{0.5}$ are reported. The line shape and spin-echo data are best explained by the molecules' developing increasingly preferred orientations as the temperature is decreased. Models in which the molecular reorientations simply slow down with decreasing temperature do not fit our data. The quadrupolar freezing is nearly complete at temperatures well above the dielectric freezing temperature. The NMR results from KCN-KBr are compared to results from the orientational glasses $o-{\mathrm{H}}_2-p-{\mathrm{H}}_2$ and ${\mathrm{N}}_2$-Ar.

We propose a new set of hydrodynamic equations for flow of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ through superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. They differ from earlier work by including both a mutual friction force between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and a viscous force. Both the mechanical vacuum model, in which mutual friction is neglected, and an earlier model by the authors, without viscous force, are limiting cases of this new description. An experimental setup is devised in which both forces play a significant role. The results of this experiment are in agreement with the theory.

The bonding geometry, fundamental H vibrational frequencies, and dynamic dipole moments for the saturated H coverage phase on W(001) are calculated with the ab initio linear augmented plane wave method. All three mode frequencies agree well with experiment and support earlier electron-energy-loss spectroscopic mode assignments. The present results indicate that nearest-neighbor central-force-constant models at surfaces are inadequate for quantitative analysis of adsorbate vibrations.

The structural properties of the $\mathrm{W}\mathrm{}\left(001\right)p(1\mathrm{}\times 1)-2\mathrm{H}$ system are predicted by use of all-electron total-energy calculations. Both the vibrational frequency (133 meV) and the work-function change (0.95 eV) are in excellent agreement with experiment at the calculated equilibrium H-W interlayer spacing of 1.12 Å. A proposed electronic driving mechanism, which includes local bonding effects and depends on the character of the ${\overline{\Sigma }}_2$ surface-state wave functions, explains the various observed reconstructions of the clean-W(001) and H/W(001) systems, including symmetries and coverage dependence.

We have developed and tested a practical and accurate version of the linearized muffin-tin orbital (LMTO) technique for surface calculations. We have done this by extending the standard LMTO basis with a small number of independent plane-wave orbital functions. The basic advantage of the LMTO method, small basis size, is thus retained. This implies sufficiently great computational advantages of time and memory so as to extend the range of feasibility for high-quality surface electronic-structure calculations to larger problems (more atoms per two-dimensional cell) than hitherto possible.

A theoretical analysis is presented of the problem of the onset of global phase coherence in granular superconductors and Josephson-junction arrays. It is shown that the ratio $\frac{\left(\frac{h}{e^2}\right)}{R}$, where $R$ is the resistance, plays an important role in the thermodynamics. This is due to quantum fluctuations of the order parameter and occurs in superconductivity only because the phase of the order parameter is both a statistical as well as a dynamical variable. The theory provides a natural qualitative explanation of the recent experiments of Orr, Jaeger, Goldman, and Kuper.

dc conductivity measurements have been performed at low temperature in amorphous ${\mathrm{Si}}_x{\mathrm{Sn}}_{1-x}$ semiconductor alloys. It is shown that the experimental results are correctly explained by the variable-range-hopping theory, whereas the present form of the small-polaron model is unable to account for the low-temperature experimental results. The values of the physical parameters extracted from the variable-range-hopping theory are physically reasonable and give some information about the electronic energy levels within the semiconductor band gap.

Optical detection of magnetic resonance is reported for three distinct zinc-vacancy-zinc-interstitial Frenkel pairs in ZnSe produced by 2.5-MeV electron irradiation at 4.2 K. One is detected as distant-donor-to-Frenkel-pair recombination in luminescence at ∼600 nm. The other two are detected as localized-triplet recombination luminescence at ∼800 nm. The emitting excitonic state is analyzed as a hole on the zinc vacancy with an exchange-coupled electron on the nearby interstitial zinc. From this, the EPR properties of the isolated interstitial zinc donor are deduced.

Magnetic and magnetotransport measurements on ${\mathrm{Ni}}_{79}$${\mathrm{Mn}}_{21}$ reentrant alloy are reported. When the specimen is cooled in a field $H$, both the magnetoresistance and its field derivative exhibit a sharp jump at the same field as magnetization. Evidence is given that (1) magnetization reversal is dominated by 180°-domain-wall propagation and (2) this is a direct consequence of the triad symmetry of Dzyaloshinsky-Moriya anisotropy which generally stabilizes the 180° domain wall and destabilizes other wall configurations.

Simple, nearest-neighbor $\mathrm{XY}$ models on unfrustrated lattices with competing ferromagnetic $\left[cos\right(\delta \theta \left)\right]$ and nematic $\left[cos\right(2\mathrm{}\delta \theta \left)\right]$ interactions are studied. For sufficiently strong competition, this model exhibits four phases in spatial dimension $d=3\mathrm{}$ and five in $d=2\mathrm{}$, including in both cases a new phase with extensive zero-point entropy. As a result of this zero-point entropy the system does not acquire perfect order even in the zero-temperature limit. The model has vortices of both irrational and integer winding number; in $d=2\mathrm{}$ their unbinding mediates the phase transitions, which are all of the Kosterlitz-Thouless type.

The lifetime spectra of positrons subjected to a pulsed electric field while slowing down in low-temperature gaseous helium reveal a mobility threshold $E_c^{}{}_{}{}^{+}$, below which the positrons exhibit no observable mobility. Energy-loss rates for collisions of a positron with a single helium atom provide an estimate of $E_c^{}{}_{}{}^{+}=15\mathrm{}\pm 3$ meV and an upper limit on the width of this threshold of 1.5 meV. Possible connections between $E_c^{}{}_{}{}^{+}$ and Anderson localization are discussed.

The ionization of a lithium atom immersed in liquid ammonia has been studied by quantum path-integral Monte Carlo calculations. Before it eventually becomes self-trapped (solvated), the lithium $2s$ electron passes through a transition state in which it is unsolvated or quasifree. Dramatic changes occur in the lithium-ion inner solution shell during the ionization process.

A model of diffusion $\psi (\mathrm{x},t)=\Delta \psi (\mathrm{x},t)+V\left(\mathrm{x}\right)\psi (\mathrm{x},t)$ is studied, where $V\left(\mathrm{x}\right)$ is the Gaussian random potential. It is found that the probability distribution function is concentrated (localized) at some metastable potential attractor, while the localization center hops discontinuously in search of a better metastable attractor. The sample-averaged localization-center displacement is found as ${\chi }_c\sim \frac{t}{2\ln t}$, i.e., it is sub-ballistic.