Fluid-displacement experiments in Hele-Shaw cells filled with a viscoelastic fluid show a novel transition between a viscous fingering (VF) regime producing fractal patterns of ‘‘fingers’’ and a viscoelastic facturing (VEF) regime producing fractal patterns of ‘‘cracks.’’ VEF patterns are characterized by branching angles of 90° with respect to the main crack, behind the tip, and by a lower fractal dimension than VF. The transition is controlled by several parameters, including the Deborah number and the system deformability.

We have measured autocorrelation functions for light scattered from capillary waves at the interface in a near-critical binary liquid mixture both at two-phase equilibrium and after abruptly raising the temperature to drive the system toward one-phase equilibrium. Use of the capillary-wave dispersion relation allows the extraction of an effective ‘‘surface tension.’’ This surface tension represents a dynamical integrity of the nonequilibrium interface which may affect pattern formation when the interface is driven.

We have calculated and measured the properties of X-band microwaves propagating in a 2D array of low-loss high-dielectric-constant cylinders. Transmission bands and photonic band gaps are conclusively identified in excellent agreement with the theoretical predictions. Detailed data on the properties of isolated defect states are also presented. We conclude that studies of this model scattering system allow the quantitative evaluation and testing of ideas regarding wave propagation and localization in strongly scattering media.

Stimulated backscattered harmonic radiation generated by the interaction of an intense pump-laser field with an electron beam or plasma is analyzed using a fully nonlinear, relativistic, fluid theory valid to all orders in the pump-laser amplitude. The backscattered radiation occurs at odd harmonics of the Doppler-shifted incident laser frequency. The growth rates, saturation levels (efficiencies), and thermal limitations are calculated for the backscattered harmonic radiation. This mechanism may provide a practical method for producing coherent radiation in the XUV regime.

Orientational fluctuations in long chain molecules at the air/water interface were detected by second-harmonic spectroscopy. The flucutations depend on the polarization of the incident (ω) and detected (2ω) light showing that the fluctuations are orientational rather than density in origin. The phenomenon is attributed to an orientational phase transition that is dependent on chain length, head group, and charge. Scaling experiments based on increasing the laser beam area indicated a correlation length (domain size) greater than 6 μm near the transition point.

Combining optical microscopy and atomic force microscopy, a complete characterization of Langmuir-Blodgett bilayers made of arachidic acid has been performed. At macroscopic scales, both techniques can be used to control the homogeneity of the transferred films. By atomic force microscopy, roughness and thickness of the bilayers could be determined. These are compatible with a normal orientation of the fully extended aliphatic chains with respect to the substrate. Molecular-resolution images give the first direct experimental evidence for the presence of pairs of dislocations in those systems.

Using the evanescent-wave ellipsometry technique, we have observed the growth of a disordered liquid-crystal layer adjacent to a ${\mathrm{SiO}}_{\mathit{x}}$-coated surface near the nematic-isotropic transition. We have also observed the pretransitional growth of a planar-oriented liquid-crystal layer in the isotropic phase of the same system, with a negative orientational order parameter. Both behaviors can be explained quantitatively by the Landau–de Gennes theory.

High-resolution x-ray scattering experiments to study the critical smectic-A fluctuations in the nematic phase have been performed on a mixture of two nonpolar, two-benzene-ring compounds. The ratio ${\mathit{T}}_{\mathit{N}\mathit{A}}$/${\mathit{T}}_{\mathit{N}\mathit{I}}$ (=0.885) for this mixture is much smaller than for previously studied similar materials. The smectic correlation lengths parallel and perpendicular to the smectic-layer normal are found to diverge anisotropically with characteristic exponents ${\nu }_{\mathrm{?}}$=0.75±0.03, ${\nu }_{\mathrm{\perp }}$=0.65±0.03, and the susceptibility with γ=1.24±0.05.

The temperature-pressure phase diagram was determined for a crystal of squaric acid which consists of molecular sheets of [${\mathrm{O}}_4$${\mathrm{O}}_4$${]}^{2\mathrm{-}}$ skeletons and intervening protons. Under hydrostatic pressures, the paraelectric phase composed of the polar (${\mathit{C}}_{1\mathit{h}}$-distorted) molecules is realized even near 0 K due to a joint quantum motion of the protons and deforming molecular skeletons. Further application of pressure causes a disappearance of the π-bond alternation of the constituent molecule at all temperatures caused by enhanced motion of the protons.

The electronic and optical properties of ${\mathrm{C}}_{60}$ in the fcc lattice have been studied by a first-principles method. It is shown that ${\mathrm{C}}_{60}$ has a low dielectric constant and an optical spectrum rich in structures; this is drastically different from diamond and graphite. The spectrum shows five disconnected absorption bands in the 1.4 to 7.0 eV region with sharp structures in each band that can be attributed to critical-point transitions. This is a manifestation of the localized molecular structure coupled with long-range crystalline order.

We study a transition from the localized to the delocalized phase in a novel sparse-random-matrices (SRM) ensemble. Within the scope of a supersymmetric approach it can be described by an order-parameter function. We attribute a clear physical meaning to this function providing a link between our approach and other methods. Equations for a ‘‘mobility edge and inverse participation ratio’’ are derived. Some analogies between the SRM model and mean-field theory of spin glass as well as possible applications to the theory of Anderson localization are discussed.

The evolution of Wannier excitons in free argon clusters is investigated using fluorescence excitation spectroscopy with synchrotron-radiation excitation. The energetic positions of the absorption bands are analyzed as a function of the cluster size from their appearance in small clusters up to the solid-state limit in crystallites with a radius R≊200 Å. A three- and two-dimensional confinement of bulk and surface states is observed.

Hydrogenic species (protons and deuterons) are removed efficiently from single crystals of ${\mathrm{LiNbO}}_3$ during electron irradiation in vacuum near room temperature. Irradiation in air produces no significant change in the concentration of the hydrogenic species.

The quantum Hall effect is due to discontinuities in the chemical potential at certain bulk electron densities. It has been conjectured that the resulting edge states in some fractional Hall gaps can have an intricate composite structure which reflects the correlations in the incompressible bulk fluid. We present direct numerical evidence obtained from exact-diagonalization studies in favor of this conjecture. We show that the states on the ν=2/3 fractional quantum Hall edge constitute a two-component system in which a ν=1/3 droplet of holes is contained within a ν=1 droplet of electrons.

Fully self-consistent band-structure calculations have been performed for palladium and platinum. The effective Hamiltonian contains exchange, correlation, and relativistic effects as well as the external magnetic field. The anisotropy of the cyclotron-orbit Zeeman splitting agrees well with experimental data from de Haas–van Alphen measurements. The calculated total susceptibilities also agree with the experimental values.

Study of the electrical conductances of a series of ultrathin Bi films as a function of increasing thickness has revealed behavior which can be correlated with the onset of superconductivity, even in the insulating state. The conductances of these films scale empirically with a single parameter which falls to zero when the films become superconductive. These observations suggest a direct transition between insulating and superconducting behavior. This transition occurs at a normal-state sheet resistance close to the quantum resistance for pairs, ${\mathit{R}}_{\mathit{q}}$=h/4${\mathit{e}}^2$.

We report the direct observation of magnetic-field-dependent energy levels in an anisotropic two-dimensional electron gas. The experiment was carried out by far-infrared-reflectivity measurements on a well studied member of the Bechgaard salts, (TMTSF${)}_2$${\mathrm{ClO}}_4$, at low temperatures. We find two distinct bands above a threshold field and a number of unusual features which we interpret in terms of the underlying quantized field-induced spin-density-wave subphases. We also observe in the optical data details associated with anomalies in this material in high magnetic fields.

We study the effects of a transverse magnetic field on the dynamics of the randomly diluted, dipolar coupled, Ising magnet ${\mathrm{LiHo}}_{0.167}$${\mathrm{Y}}_{0.833}$${\mathrm{F}}_4$. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the introduction of quantum fluctuations.

We study a class of variational wave functions for strongly interacting one-dimensional lattice fermions in which correlations among the particles are specified by a single variational parameter. We find that the wave functions describe the ground-state properties of the one-dimensional t-J model remarkably well over the entire phase diagram in which interaction strength and density are varied. Specifically the wave function describes a Tomonaga-Luttinger liquid at low J/t, a phase-separated state at large J/t, and a stable state with infinite compressibility between these phases at low densities.

We present a model of superconductivity with the BCS-like pairing due to Aharonov-Bohm forces. The gap is proportional to the first power of the small parameter (in which the self-consistent perturbation scheme is developed), as opposed to the BCS class of models where the gap is exponentially suppressed with the small parameter.