Turbulence produced by a temperature gradient in a collisional plasma is investigated. The system evolves to a state in which highly elongated streams of plasma move up and down the temperature gradient. The resulting transport greatly exceeds estimates based on mixing-length arguments. It is argued that such streams are the preferred nonlinear state of turbulent fluctuations driven by both $\nabla T_e$ and $\nabla T_i$.

The power spectral densities of turbulent fluctuations computed from Zakharov's model are shown to have unexpected structure which results from the coherent temporal evolution of cavitons. These spectra have features in common with the spectra measured during ionospheric heating.

We solve in mean-field approximation a model describing the lamellar phases of lyotropic liquid crystals as stacks of interacting bilayer membranes. The model is a synthesis of a Landau-Ginzburg theory of membrane-melting transitions and a continuum treatment of the molecular forces acting between bilayers. The resulting phase diagrams include both planar and modulated phases and are in semiquantitative agreement with experimental observation on neutral lipids. The approach highlights the role of interactions in the thermodynamic behavior of membrane systems.

We report the first measurements of elastic coefficients and viscosities that clearly demonstrate the distinction between rigid and semiflexible behavior of a main-chain polymer nematic liquid crystal, a solution of poly-$\gamma$-benzyl glutamate (PBG) in mixed organic solvents. Quasielastic Rayleigh scattering studies show a crossover which occurs at a molecular chain length near the persistence length of PBG as the chain length increases. These results are in qualitatively good agreement with recent theoretical predictions for semiflexible polymers. It is seen that bending distortions of an individual polymer play an important role in the fundamental nature of nematic elasticity and viscosity.

Phenomena of adsorption in nonstandard geometries and, in particular, wetting on curved surfaces display new and unexpected features. Contrary to recent expectations adsorption phase transitions on cylindrical substrates can continue to exist when the bulk correlation length $\xi$ of the adsorbate becomes much larger than the cylinder radius $r_1$. The global phase diagram for wetting on cylinders features two regimes, "surface" transitions ($\xi \lesssim r_1<\infty$) and "line" transitions ($0<\mathrm{}{r}_1\lesssim \xi$), which become connected at a critical double point. For spherical substrates, the global wetting phase diagram is qualitatively the same as for cylinders.

We examine the dynamics of the development of interparticle spatial correlations during the later stages of a first-order phase transformation in which the second-phase particles generate a stress field. Through an analysis of the diffusional and elastic interactions between two particles, it is shown that the long-ranged motion of the centers of mass of the particles within the solid is an important component in the development of the experimentally observed spatial correlations.

Small-angle neutron-scattering data, obtained from a near-symmetric isotropic diblock copolymer, demonstrate non-mean-field behavior over a wide temperature range (≥ 57°C) above the weakly first-order microphase-separation transition. These results are quantitatively explained by a fluctuation theory based on the Hartree approximation employed by Brazovskii to describe such phase transitions.

Recent measurements on the diffuse and Bragg scattering of neutrons from a polycrystalline sample of a ${\mathrm{Kr}}_{0.9}$-${\mathrm{Ar}}_{0.1}$ binary alloy indicate the presence of argon microclusters dissolved in a krypton-rich matrix, contradicting previous indications that argon and krypton are soluble in all proportions. On the other hand, measurements on a ${\mathrm{Ar}}_{0.9}$-${\mathrm{Kr}}_{0.1}$ sample are consistent with the picture of an ideal solid solution.

Quantitative x-ray diffraction techniques have been used to estimate the mean square thermal displacements of atoms in the interface region, and to measure the thermal expansion coefficients parallel and normal to the interface plane for the large-angle $\Sigma =13\mathrm{}$ ($\theta =22.6\mathrm{}°$) [001] twist grain boundary in Au. The thermal properties of the grain boundary are shown to be much different from those of the bulk material, which indicates that the effective interatomic potential for an atom in the grain boundary is different from that in bulk Au.

The growth of Ge on Ge(111) has been monitored by in situ x-ray reflectivity and diffraction. For suitably chosen geometries, the scattered x-ray intensity is extremely sensitive to atomic-scale surface morphology; dramatic intensity changes occur upon deposition of a fraction of a monolayer. For substrate temperatures up to 200°C oscillations are observed in the scattered intensity, indicating growth by two-dimensional nucleation. Reflectivity curves reveal the detailed surface atomic geometry. All observations can be quantitatively understood by use of kinematical diffraction theory.

We report the first demonstration of positron-induced Auger-electron spectroscopy. A beam of low-energy (${10}^1$ eV) positrons was used to create core holes at the surface of Ni and Cu by matterantimatter annihilation. Estimates are developed for the probability of positrons annihilating with a $3p$ electron found to be as high as 3.7(7)×${10}^{-2\mathrm{}}$ in Ni. The implications of the extremely high signal to background are discussed and several important advantages of this process for surface analysis are suggested.

The electronic excitations of potassium clusters embedded in a MgO matrix have been investigated as a function of cluster size and momentum transfer by means of electron-energy-loss spectroscopy. The experimental results show deviations from predictions of classical electrodynamics but they are in qualitative agreement with a calculation for excitations of a jellium sphere made with use of the time-dependent local-density approximation.

The effect of measurement-probe geometry on the weak-localization magnetoresistance of short silver wires is investigated. The wires have lenghts comparable to the electron phase coherence length $l_{\phi }$, which is a few microns at 1 K. The influence of different probe geometries is seen directly in the magnitude and shape of the low-field magnetoresistance of these wires. We have extended the theory of weak localization to include the effects of the measurement geometry, and find good quantitative agreement with experiment.

Spin-split adsorbate-induced bands have been observed for $p(1\mathrm{}\times 1)$ oxygen and $c(2\mathrm{}\times 2)$ sulfur on Fe(100) by use of spin-polarized angle-resolved photoemission. For the $p_z$-symmetry orbitals, the exchange splitting is considerably larger at the Brillouin-zone center (1.3 eV for O, 0.5 eV for S) than at the zone edge (0.25 and 0.2 eV, respectively). This is attributed to enhanced hybridization with substrate $d_z^{}{}_{}{}^2$ orbitals when the adsorbate $p_z$ band overlaps the bulk continuum. Our observation of an exchange splitting provides evidence of a small magnetic moment on the adsorbate atoms. Our results also indicate that the adsorbate layer is ferromagnetically aligned with the substrate.

Far-infrared magneto-optical experiments have been performed on a $p$-type HgTe/CdTe superlattice with a small effective band gap. The angular dependence of the hole-cyclotron-resonance spectra reveals that the effective mass is 2 orders of magnitude heavier in the growth direction than transverse to it. These results provide evidence for a large valence-band offset. In addition, a tentative identification of hole spin resonance has been made. Evidence is presented which shows that the superlattice band structure can be substantially modified by the application of modest magnetic fields.

We present experimental and theoretical evidence for impurity-assisted off-diagonal (i.e., $\Delta n>\mathrm{}0\mathrm{}$ transitions between conduction- and valence-band Landau levels ($n=0,1,\dots$) for magnetoluminescence in modulation-doped degenerate semiconductor quantum wells. This new selection rule is employed to determine simultaneously the optical effective masses of the electrons and in-plane light holes in $n$-type strained GaAs/InGaAs/AlGaAs quantum wells.

We show that resonant dipole-dipole interactions are suppressed at all interatomic or intermolecular separations in periodic dielectric structures in which spontaneous emission is inhibited at the resonant optical transitions. This profoundly modifies molecular properties including donor-acceptor energy transfer, collisional dynamics, molecular spectra, and dissociation energies.

Highly excited hydrogen atoms have been exposed to an intense pulse of microwaves with frequency near the initial classical electron orbit frequency. Measurements were made of the probability distribution for different bound states after the pulse. Structure in such distributions is found to be near classical, and arises from the presence of a classical resonance island in action-angle space. The measured distributions also yield values of electron localization length in action space that are in agreement with quantum localization theory and with numerical values based on quantum mechanics.

We report the first measurements of microwave excitation and ionization of excited hydrogen atoms for scaled frequencies $n_0^3\omega$ up to 2.8. Classical 3D calculations which directly model this 36.021-GHz experiment agree quite well for $n_0^3\omega <1\mathrm{}$, agree less well for $1\le n_0^3\omega \le 2$, and do not agree for $n_0^3\omega >2\mathrm{}$. This supports theoretical predictions that as $n_0^3\omega$ rises above 1, quantal ionization threshold fields rise above those for the onset of classical chaos; however, the data continue to reveal local stability near certain rational frequency ratios that recalls classical behavior.

A two-dimensional condensed-matter lattice model is presented which exhibits a nonzero quantization of the Hall conductance ${\sigma }^{\mathrm{xy}}$ in the absence of an external magnetic field. Massless fermions without spectral doubling occur at critical values of the model parameters, and exhibit the so-called "parity anomaly" of (2+1)-dimensional field theories.