Persistent photoconductivity in Si-doped molecular-beam epitaxial ${\mathrm{Al}}_{0.28}$${\mathrm{Ga}}_{0.72}$As was studied. The carrier concentration decreases with time according to a simple power law. The decay is explained semiquantitatively by an effective-mass-like model of the impurity with a wave packet centered near but not at the $\Gamma$ point in the Brillouin zone. The shift of the impurity wave function in k space may be explained in terms of long-range composition ordering in the AlAs-GaAs system. Other physical properties may also be influenced by periodic composition fluctuations observed in many semiconducting alloys.

When light from small, distant sources in the Universe is gravitationally focused by an intervening galaxy, the gravitational lens can be influenced by the granularity of the matter distribution which is caused by the stellar (or other compact) masses in the galaxy. A largely analytic, statistical calculation for a gravitational lens due to a collection of compact masses—valid for sources of finite size and for large (as well as small) "optical depths" for the lens—is developed to treat fluctuations in the light caused by such "microfocusing" effects. Previous treatments have been either numerical simulations of the Monte Carlo type or limited to single-star (i.e., low-optical-depth) effects.

Super Riemann surfaces are important in superstring theories as the generalization of the bosonic world sheet. In one approach to their study, one introduces two-dimensional supergravity, subject to certain conditions on the field strengths. Another approach builds super Riemann surfaces from superconformal patching data with no mention of the constraints. We show the equivalence of these two approaches and in particular interpret the torsion constraints as integrability conditions of a certain geometrical structure.

The center frequency and linewidth of the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ resonance line of barium atoms placed near the center of a concentric optical resonator are studied as functions of cavity tuning. Shifts in the transition center frequency, due to radiative level shifts, and changes in linewidth, due to enhanced and suppressed spontaneous emission, are observed. A QED calculation which explicitly includes the resonator mode density gives good agreement with the data.

The possibility of formulating a density-functional theory for time-dependent systems is studied in the linear-response regime. Two results are obtained: (i) The work of Runge and Gross is generalized to a finite time interval with an arbitrary form of the external potential; and (ii) there exists no zero in the mapping $n(\mathrm{r}, \omega )=\int d^3{r}^{\prime }\chi (\mathrm{r}, {\mathrm{r}}^{\prime };\omega )V({\mathrm{r}}^{\prime }, \omega )$ for all $\omega$ under a very general thermodynamic condition.

We report the confinement and cooling of an optically dense cloud of neutral sodium atoms by radiation pressure. The trapping and damping forces were provided by three retroreflected laser beams propagating along orthogonal axes, with a weak magnetic field used to distinguish between the beams. We have trapped as many as ${10}^7$ atoms for 2 min at densities exceeding ${10}^{11}$ atoms ${\mathrm{cm}}^{-3\mathrm{}}$. The trap was ≃0.4 K deep and the atoms, once trapped, were cooled to less than a millikelvin and compacted into a region less than 0.5 mm in diameter.

Ablative implosions of glass microballoon targets driven by twelve 0.53-μm laser beams have been studied by use of secondary DT and D ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ fusion reactions in an initially pure deuterium fuel. The tritons and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei are products of primary DD fusion reactions. Comparisons with results of a hydrodynamic simulation indicate that collisional energy loss for these primary fusion products is strongly enhanced by fuel-pusher mixing taking place during the implosion.

A polarization change of localized and finite-amplitude Alfvén waves is found by use of both MHD theory and numerical simulation. One of the transverse components of the circularly polarized Alfvén wave is stable as it propagates, but the other component is unstable under both self-focusing and diffraction effects. Consequently, the wave changes its polarization from circular to linear.

Particle orbits in a rotating tokamak plasma are calculated from the equation of motion in the frame that rotates with the plasma. It is found that heavy particles in a rotating plasma can drift away from magnetic surfaces significantly faster, resulting in a diffusion coefficient much larger, than for a stationary plasma. Particle simulation is carried out and the results offer a qualitative explanation for some experimental data from the Tokamak Fusion Test Reactor.

The nonlinear saturated state of the $m=1\mathrm{}$, $n=1\mathrm{}$ ideal MHD instability is calculated for a large-aspect-ratio tokamak. When the $q\left(r\right)\mathrm{}$ profile is nonmonotonic with $\Delta q=q_{min}-1>0\mathrm{}$, the amplitude $\xi$ of the nonlinear state is given by $\frac{{\xi }^2{q}^{\prime \prime }}{\Delta q}=7.9[{\left(\frac{\Delta q_c}{\Delta q}\right)}^{\frac{3}{2}}-1]$, where $\Delta q_c$ is the critical value of $\Delta q$ at which the system is marginally stable. This nonlinear state is similar to that seen during the sawtooth crash in large tokamaks and may be related to the steady-state oscillations seen when sawteeth are suppressed by lower-hybrid current drive.

In a tokamak, if the safety factor $q$ becomes equal to unity in the plasma, the kinked plasma equilibrium which is accessible from the initial axisymmetric equilibrium through an internal-kink instability is itself unstable with respect to ballooning modes. This suggests a two-step mechanism for internal disruptions. First an internal-kink-type instability begins. Then the plasma pressure is released through the ballooning instability.

An outstanding problem in the physics of nematic films is the stripe phase which appears in the bend Fréedericksz geometry at temperatures slightly above that of the nematic to smectic-$A$ phase transition in a sufficiently strong magnetic field. It is argued, by use of linear stability analysis, that in fact two transitions occur: the usual Fréedericksz transition to a uniformly distorted state, followed by a second transition to a stripe phase at a slightly higher field. Qualitative agreement is found between experimental results and theoretical predictions.

High-resolution x-ray scattering studies of thin smectic-$C$ ($\mathrm{Sm}C$) samples prepared between solid plates by cooling from the smectic-$A$ phase reveal a surprising "chevron" structure of tilted layers. This structure is formed as a response to the shrinking of the $\mathrm{Sm}C$ layers while anchored to the solid plates. The layer tilt is independent of surface treatment. Our results provide fundamental new information on the structure and surface interactions of SmC layers and provide evidence for a new SmC defect.

We propose a new scheme to calculate the elastic constants of solids which is based on linear-response theory. Elastic constants are given directly by a single self-consistent calculation (i.e., they are not obtained by numerical differentiation of total energies or stresses). As an illustration, we apply our procedure to the determination of the equilibrium lattice constant, bulk modulus, and pressure derivative of the bulk modulus of silicon.

The embedded-atom method, a semiempirical theory of metal bonding, is investigated as a method to calculate the bonding in a covalent material. A simple first-neighbor embedded-atom method model is sufficient to explain the geometry and structure of many metastable phases of silicon, but not its shear behavior. To obtain realistic shear behavior of silicon, an angle-dependent electron density is included to model the effects of bond bending. This model uses only the experimental bulk properties of silicon. Calculated properties of metastable phases and point defects are presented.

Pressure-induced structural phase transitions have been observed in solid xenon by high-pressure x-ray diffraction methods up to 137 GPa (1.37 Mbar). The face-centered cubic structure is unstable above 14 GPa and transforms to an intermediate, close-packed phase. Above 75 GPa a further transition takes place to the hexagonal close-packed structure which remains stable to at least 137 GPa. At this pressure, where $V=10.5\mathrm{}\pm 0.2$ ${\mathrm{cm}}^3$/mol ($\frac{V}{V_{01}}=0.30\mathrm{}$), there is no evidence for band-gap closure.

A preaveraged topological parameter, $\tilde{N}$, is introduced to provide a criterion for the presence of entanglements in polymer melts. The theory predicts a geometrical transition from the entangled to the unentangled state in agreement with experimental data. A generalized Rouse theory is used to describe polymer dynamics in both states.

Ion-shadowing and -blocking experiments on a cylindrical single crystal of Pb reveal a strongly orientation-dependent disordering (melting) of the surface with increasing temperature. The thickness of the disordered surface layer is found to diverge logarithmically as the bulk melting point is approached. The process of disordering is shown to be driven by an orientation-dependent difference in free energy between the surface in its ordered and liquid states.

We report a light-scattering study of the development of bond-orientational (BO) order in a thick, monodomain, freely suspended film. Our static results are consistent with a three-dimensional Landau free energy, and show an approximately linear increase of the BO elastic constant through the ordered phase. Our dynamic results show a hydrodynamic mode, which exhibits critical slowing down at the liquid to BO phase transition, and a nonhydrodynamic mode which is associated with the BO phase.

We have studied the onset and evolution of instabilities at the solid-liquid interface of tin undergoing directional solidification, as a function of growth velocity, temperature gradients, and impurity content. The initiation of an instability that we identify as "cellular" growth, and its passage to pseudosteady state, have been investigated. Within this steady state a new oscillatory phenomenon has been found, indicative of changes in the interface configuration that are quasiperiodic in time.