The cooperative emission process in KCL;${\mathrm{O}}_2$${\mathrm{}}^{\mathrm{-}}$ has been studied as a function of the dephasing rate of the transition dipole. As the temperature of the sample is increased from 10 to 30 K, the emission evolves continuously from that characteristic of superfluorescence to that of amplified spontaneous emission. These results are in qualitative agreement with the predictions of current theories, but quantitative agreement is obtained only when current theories are modified so that the noise source that initiates the emission process is allowed to act continuously during the superfluorescent buildup.

Stimulated Brillouin scattering spectra have been measured from plasmas produced by spatially and temporally incoherent laser light. The measured backscatter spectra and intensities differ substantially from measurements of Brillouin scattering with coherent illumination. Backscatter is strongly suppressed and the Brillouin signature in the spectra disappears. This quenching of stimulated Brillouin scattering is observed even when the laser coherence time is much larger (10×) than the predicted convective saturation time of the stimulated Brillouin scattering instability.

We derive a simple model which predicts the observed lower-hybrid current-drive density limit for all major experiments to within a factor of 2. The model is based on (i) the experimentally observed upshift in $k_{?}$ and (ii) the $k_{?}$ dependence of the mode coalescence condition of the linear mode conversion of a lower-hybrid wave into a hot-plasma wave.

Excitation, propagation, and absorption of directly launched ion Bernstein waves were studied with ${\mathrm{CO}}_2$ laser scattering in the Alcator C tokamak. Optimum excitation is obtained when an ion-cyclotron harmonic layer is located just behind the antenna. Wave absorption at the ω=3${\Omega }_{\mathrm{D}=1.5\mathrm{}{\Omega }_{\mathrm{H}}}$ layer was observed. 3 It may be necessary to control the edge density fluctuation level in order to minimize scattering of the ion Bernstein waves and optimize heating.

A simple model of a three-component mixture, representing oil, water, and surfactant, displays many of the properties of microemulsions. Proximity to critical phenomena is responsible for the occurrence of low and ultralow surface tensions and of short-range structure. The increase of the amphiphilic nature of the surfactant, known to reduce the amount of it needed to solubilize oil and water, also alters the wetting behavior and introduces liquid-crystal phases at lower temperatures. It has no strong effect on the surface tensions.

The first observation of the smectic ${\mathrm{A}}_{\mathrm{d}}$–smectic ${\mathrm{A}}_2$ critical point is reported. Detailed x-ray studies show that a first-order phase transition between the partially bilayer smectic ${\mathrm{A}}_{\mathrm{d}}$ and the bilayer smectic ${\mathrm{A}}_2$ phases which have the same symmetry terminates in a critical point in the temperature-concentration plane.

The low-frequency vibrational density of states, Z(ω), in fractal silica aggregates has been measured by inelastic, incoherent neutron scattering. The samples were compressed powders of a hydroxylated, silica smoke-particle product, Cab-O-Sil. In the regime, ħω<0.5 meV Z(ω) was found to be of the form ${\mathrm{\omega }}^{\mathrm{p}}$ with p=0.8±0.1 at T=136 K and p=1.1±0.05 at T=265 K. The values of p are larger than the current theoretical predictions for excitations on fractal networks. The observed temperature dependence of Z(ω) indicates an important role of anharmonicity.

Three-beam diffraction is analyzed in a geometry appropriate for high-energy electron diffraction. An expression for the lowest-order correction to the kinematic intensity is derived from which a prescription for measuring crystallographic-phase invariant is obtained. This analysis is a good approximation for the diffraction observed in large convergence angle electron-diffraction patterns. Phase invariants are measured in the noncentrosymmetric crystal InP to an accuracy of ±15°. The technique provides a general and practical method for measuring phases.

The kinetics of ordering are studied in ${\mathrm{SbCl}}_5$-intercalated graphite by utilization of x-ray synchrotron radiation and position sensitive detection. Analysis of the time-dependent scattering function (Lorentzian) confirms that the ordering obeys an algebraic growth law until defect pinning sets in. We obtain evidence for a novel kinetic phenomenon in which thermal roughening is suppressed in the unpinned state.

We present an in situ x-ray scattering study showing the existence of the lattice-gas lattice-liquid phase transition for H dissolved in Nb/Ta superlattices where it is found that critical fluctuations with wavelengths shorter than the superlattice periodicity are suppressed. Since fluctuations of only long wavelengths develop, phase separation into gas and liquid phases cannot occur. These results demonstrate that the coherent phase transition is sensitive to the microscopic properties of the H-H interaction.

A density-functional theory for the structure and surface free energy γ of solid-liquid interfaces is presented which avoids perturbation and square-gradient approximations by the utilization of (i) the weighted-density approximation for the Helmholtz free-energy functional and (ii) a physical, two-parameter density profile of the interfacial region. The hard-sphere [100] fcc-liquid interface is found to be only 4–5 layers wide with γ=0.66kT/${\mathrm{\sigma }}^2$, consistent with simulations on the similar ε(r/σ${)}^{\mathrm{-}12}$ interface.

Granular aluminum near the metal-insulator transition has been found to have a range of metal concentrations over which it is metallic (M) but not superconducting (S). For higher concentrations it is superconducting and at lower concentrations insulating (I). There is a striking resemblance of the temperature dependence of resistivity in the S,M, and I ranges of granular aluminum to the temperature dependence of sheet resistance in very thin quench-condensed films. However there is as yet no theory that gives a satisfactory explanation of this behavior in both two- and three-dimensional systems.

A small decrease in ${\mathrm{T}}_{\mathrm{c}}$ of 0.3 K to 0.5 K is observed when as much as 90% of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ is substituted with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. This result is consistent with our observation that there is an oxygen isotope effect in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$, but in contrast with previous reports that there is no isotope effect for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. This new result suggests that phonons play an important role in the electron-pairing mechanism in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.

The possibility of an electric quadrupole Kondo effect for a non-Kramers doublet on a uranium (U) ion in a cubic metallic host is demonstrated by model calculations showing a Kondo upturn in the resistivity, universal quenching of the quadrupolar moment, and a heavy electron anomaly in the electronic specific heat. With inclusion of excited crystal-field levels, some of the unusual magnetic response data in the heavy-electron superconductor ${\mathrm{UBe}}_{13}$ may be understood. Structural phase transitions at unprecedented low temperatures may occur in U-based heavy-electron materials.

By application of the perturbed γ-ray distribution method following heavy-ion reactions, we have found an experimental method of producing and investigating strong local 4d magnetism in metals. Mo ions recoil implanted into alkali metals show a large local moment and an extremely small 4d spin-relaxation rate. The data along with an analysis using a Born-Haber cycle are consistent with a localized 4$d^5$ shell behavior for isolated Mo ions in Rb and Cs.

In a fcc lattice it is possible to have conserved staggered magnetization if the spin coupling constants are very anisotropic and fulfill certain conditions, which are almost realized in CeAs. The result is a spin-wave spectrum that is quadratic in q with a very small gap. Other consequences are the degeneracy of single and triple q antiferromagnetic states, sensitivity to random strains and diffusive modes above ${\mathrm{T}}_{\mathrm{N}}$. .AE

The theory of forming a coherent twinning array in a parent phase is studied for a tetragonal to orthorhombic displacive transition. We find that this structure can be stabilized without the use of dislocations by a long-range interaction between the twin boundaries which is mediated via the parent phase. The dynamics of the twin boundary lattice consists of elementary excitations with surprisingly low frequency and a limiting ∼ (wave vector${)}^{1/2}$ dispersion.

The motion of a long DNA molecule in a gel is examined when an applied field is suddenly rotated 90°. First there is a nucleation stage where kinks above a certain length will appear. Then these kinks coarsen as a result of competition between them, with the longer kinks growing at the expense of the shorter ones. A regime is reached which is self-similar in time, with the number of kinks decreasing inversely with time, analogous to droplet growth in supersaturated solution.

A hexapole focusing field provides a general method for determining the orientation of polar polyatomic molecules. In the first application of this method to surface scattering we find that at 620 K ${\mathrm{CHF}}_3$ is 27% more likely to desorb with the fluorine end of the symmetry axis pointing directly away from the surface than with the opposite orientation. As the temperature is lowered, the desorption anisotropy decreases. Direct scattering gives preferential orientation opposite that of desorption.

An experiment has recently been proposed to measure the splitting of a photon into two photons in an intense plane-wave field. Coherent adsorption and emission of several plane-wave photons may play an important role at these high intensities. We make an approximate calculation of these nonlinear effects which shows that they become important at intensities of 2×${10}^{13}$ V/cm, about twice the intensity in the proposed experiment.