Understanding aggregation phenomena is fundamental in colloid science. Recent experimental work has shown that colloid aggregation is universal, independent of the chemical details of the particular colloid system. Although it had been possible to obtain this universality in the diffusion limit via computer simulations, the simulations, the reaction limit remained elusive to simulators. Here is described how the results of different algorithms have led us to consider the correct one, for which the universality is obtained. The important mechanisms of reaction-limited colloid aggregation are therefore fully identified.

In a careful Raman spectroscopic study on the high pressure δ phase of nitrogen we have observed an anomalous break in the slope of the vibrational Raman shift as a function of temperature. We interpret this as a second-order phase transition due to an orientational localization of the molecules, a phenomenon which has not previously been detected experimentally. Our results explain the change above 12 GPa in the experimental compressibility at room temperature measured by Olijnyk. The interpretation is consistent with the results of computer simulations.

We present a microscopic theory of the transition to and properties of the amorphous solid state of a system of vulcanized (i.e., randomly crosslinked) macromolecules. Our approach invokes a variational hypothesis for the random static density fluctuations characterizing this solid state. The variational parameter is the inverse monomer localization length, which is zero in the liquid state and increases continuously as the mean number of crosslinks exceeds a critical value. The emergent solid is a homogeneous isotropic elastic medium, whose elastic moduli we compute near the transition.

The dynamics around the liquid-glass transition in ${\mathrm{B}}_2$${\mathrm{O}}_3$ have been investigated by photon correlation spectroscopy. A two-step relaxation is for the first time observed in a ‘‘strong’’ network forming glass. The behavior is similar to the fast β and slower α relaxations observed for ‘‘fragile’’ glasses and proposed in mode coupling theory (MCT) for simple glass forming systems. We find that the time decays of the two processes obey power laws consistent with MCT. It seems that a two-step relaxation is a more general relaxation pattern than has been discussed so far.

We have observed a series of oscillations in the transverse acoustic response of superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$, which, in analogy with earlier observations involving longitudinal sound, suggest an evolving standing-wave pattern. These oscillations were observed with 60.8 MHz transverse sound at pressures between 1 and 3 bars. The change in the reciprocal of the phase velocity can be obtained from this data, and it is compared with a recent theoretical prediction [G. F. Moores, and J. A. Sauls, J. Low Temp. Phys. 91, 13 (1993)].

When a ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixture is placed inside porous aerogel with an open volume fraction of 0.98, the coexistence region is found to be detached from the superfluid transition line, giving rise to a miscible superfluid mixture at high ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration and low temperature with intriguing properties.

Single-crystal x-ray diffraction has been performed on solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ from 1 to 58 GPa over the temperature range 46–400 K. The high-density properties of helium are pinned down: the hcp structure is stable apart from an fcc loop along melting in between 15 K and around 285 K; comparison of the 304 K hcp equation of state with a series of calculations demonstrates there are significant attractive many-body interactions; and the thermal pressure has been measured between 95 and 304 K and it differs from self-consistent phonon calculations.

Formation energies for a variety of GaAs(100) surface structures have been calculated as a function of the atomic chemical potentials using the first-principles pseudopotential density-functional approach. We find that the surface transforms through four phases as the chemical potential varies across its accessible range. As the Ga chemical potential increases the surface transforms from an As-rich c(4×4) through two distinct (2×4) structures and finally to a Ga-rich (4×2) phase. The predicted structures account for most scanning tunneling microscopy observations for the c(4×4), (2×4), and (4×2) phases.

Polarized angle-resolved Raman spectra of the Si-H stretching vibrations on stepped H-terminated Si(111) surfaces confirm the constrained orientation of the step dihydride derived from ab initio cluster calculations. They further show that the step normal modes involve little concerted motion of the step atoms, indicating that step relaxation reduces the steric interaction much further than predicted.

Vicinal Si(111) surfaces misoriented towards the [2¯11] direction facet reversibly upon cooling through the (1×1) to (7×7) reconstructive phase transition. We have used low-energy electron microscopy to examine the kinetics of this phase separation. Nucleation and growth of facets occur when the surfaces are quenched just below the phase boundary. However, at lower temperatures we find evidence for the existence of a spinodal which divides the faceting kinetics into unstable and metastable regions. The location of the spinodal is consistent with previous measurements of Si(111) step energetics.

We study the two-dimensional interacting electrons in magnetic field. It is found that for systems with completely filled Landau levels, when the electron density and magnetic field are large enough, the total energy is lowered by magnetic field. The exchange Coulomb interaction is responsible for such a transition.

In the quantum Hall regime the influence of edge channels on the evolution of the current through the device has been analyzed by time-resolved transport measurements. The observed nonlinear dependence of the time-resolved current on applied voltage is qualitatively in agreement with the model describing edge channels as compressible strips separated by incompressible regions.

H adsorption on Ni(110) at sample temperatures ${\mathit{T}}_{\mathit{S}}$>200 K is known to result in a one-dimensionally disordered surface exhibiting a missing/added row reconstruction with irregular row-to-row spacing but long range periodicity along the rows. Inverse photoemission shows an unoccupied surface state with wave vector perpendicular to the rows being completely quenched due to the one-dimensional disorder. In contrast, an unoccupied surface state propagating along the rows is not attenuated at all. This indicates that the latter is a one-dimensional chain state, localized to individual atom rows.

A model which can account for the experimentally observed variations of the giant magnetoresistance in thin magnetic multilayers with mean free path, interface roughness, magnetic layer, and normal layer thickness has been developed. The model requires the existence of quantum-well states wihin individual layers or groups of layers, depending on the magnetic state of the film. The calculated results are obtained by the application of quantum size effect transport theory to these individual layers.

The optical conductivity is obtained exactly within BCS theory as a function of frequency, temperature, and disorder parameter, by simplifying the general impurity scattering mechanism to backward scattering. The familiar features of infrared measurements are recovered. In the dirty limit, the ratio of superconducting to normal absorption rate approaches the Mattis-Bardeen result.

We study the T=0 critical properties of the superfluid-insulator transition in 2D hard core Bose systems with disorder. Using quantum Monte Carlo simulations and finite size scaling on 64×64 lattices we find the dynamical exponent z=0.5±0.1 and the correlation length exponent ν=2.2±0.2. At the transition, the system is metallic with a conductivity ${\mathrm{\sigma }}_{\mathit{c}}$=(1.2±0.2)(${\mathit{e}}^{\mathrm{*}}$${)}^2$/h and a nonzero compressibility κ≠0. These conclusions differ from the existing scaling theory as well as from simulations on simplified models argued to be in the same universality class. Our results are suggestive of new low lying collective excitations (modified from usual phonons) in the disordered system.

We have observed quantum interference of vortices in a Josephson-junction array. When vortices cross the array along a doubly connected path, the resultant resistance oscillates periodically with an induced charge enclosed by the path. This phenomenon is a manifestation of the Aharonov-Casher effect. The period of oscillation corresponds to the single electron charge due to tunneling of quasiparticles.

${\mathit{H}}_{\mathrm{c}2\mathrm{}}$(T) has been measured for thin BSCO films over a larger combined range of magnetic field and reduced temperature than for any other superconductor. ${\mathit{H}}_{\mathit{c}2}$(T) diverged anomalously as the temperature decreased: At the lowest temperature, it was 5 times that expected for a conventional superconductor. Such a strong divergence cannot be explained by any conventional model.

Magnetic moment measurements on niobium/copper multilayers by means of a torque magnetometer exhibit a series of maxima at specific field strengths ${\mathit{H}}_{\mathit{N}}$ when the applied magnetic field makes a small angle with the plane of the superconducting layers. These maxima are shown to be closely related to rearrangements of the parallel vortex system, during which an increasing number of vortex chains is formed in the sample.

The magnetic phase diagram of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ in the antiferromagnetic regime (x≤0.02) has been derived from ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ nuclear quadrupole resonance studies from 4 to 250 K. The data demonstrate freezing of the doped holes’ effective spin degrees of freedom below ${\mathit{T}}_{\mathit{f}}$≊(815 K)x into a spin-glass-like state which is superimposed on the antiferromagnetic background. These and previous results allow a detailed magnetic phase diagram to be constructed for x≤0.05 and reveal a distinct crossover at x≊0.02 in the nature of the spin-glass transition.