The orientational order in simple systems of dipolar hard-core molecules is determined by Monte Carlo simulations. Dipolar hard spherocylinders are shown to form a monolayer smectic-A phase with unpolarized layers. In the columnar phase of dipolar cut spheres, evidence is given for a transition, with increasing dipole moment, from a phase with unpolarized columns to a phase with completely polarized columns arranged with antiferroelectric order. Ferroelectric orientational order is demonstrated for strong dipolar hard spheres.

The initial stages of growth of Pt on Pt(111) have been investigated as a function of the substrate temperature. The main result is that, while between 200 and 400 K the cluster density decreases with increasing temperature in accordance with nucleation theory, in the range of 400–500 K the density increases by more than 1 order of magnitude in obvious contradiction to expectations. At coverages below 0.1 we find at temperatures above 500 K a high density of small clusters (possibly heptamers) which appear to be stable with respect both to decomposition and to further growth.

A computational method has been developed for generating graphitic carbon structures on an arbitrary smooth surface and with a given number of carbon rings. Using both periodic and random surfaces for constraint, many extended graphitic carbon structures have been generated. The energy relative to graphite and the bulk elastic properties have been calculated. Like their periodic counterparts, the random structures are found to be exceptionally stable. Their radial distribution functions match closely those of films of amorphous carbon grown on NaCl substrates from sublimated graphite.

Scanning tunneling microscopy (STM) on a sputtered and annealed ${\mathrm{Pt}}_{25}$${\mathrm{Ni}}_{75}$(111) single crystal reveals a network of subsurface lattice mismatch dislocations caused by platinum enrichment due to preferential sputtering and recoil mixing. Atomically resolved STM topographs are compared with simulations of these dislocations using embedded atom potentials. This allows one to estimate the depth of the dislocations, and thus the thickness of Pt enrichment, which is three monolayers on the 500 eV ${\mathrm{Xe}}^{+}$ sputtered and five monolayers on the ${\mathrm{Ar}}^{+}$ sputtered surface, compatible with the depth of radiation damage.

We study the fluctuations of a stretched string, e.g., a vortex line, moving in a random medium. A pair of nonlinear equations are proposed to describe the evolution of longitudinal and transverse coordinates. The dynamic scaling of the fluctuations is studied analytically (by renormalization group) and numerically. In most cases the fluctuations are superdiffusive, governed by a dynamic exponent z=3/2.

We have built an apparatus capable of attaching a molecule to a liquid-helium cluster and obtaining its infrared spectrum. Clusters made of a few thousand atoms and containing an ${\mathrm{SF}}_6$ molecule show two absorptions located at 945.8 and 946.1 ${\mathrm{cm}}^{\mathrm{-}1}$ which indicate that the guest molecule is located near the surface of the cluster. If the cluster contains two ${\mathrm{SF}}_6$ molecules, the spectrum obtained is that of a slightly perturbed ${\mathrm{SF}}_6$ dimer.

We have studied the adsorption of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on cesium in the temperature range from 1.3 to 4.1 K. The ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ film undergoes a first-order phase transition known as prewetting. The prewetting line intersects the coexistence curve at the wetting temperature, ${\mathit{T}}_{\mathit{w}}$=1.95 K, and terminates at ${\mathit{T}}_{\mathit{c}}$=2.5 K, the prewetting critical point. There are dramatic hysteresis effects when the temperature of the system is varied at coexistence in the vicinity of ${\mathit{T}}_{\mathit{w}}$. This is the first experimental realization of a complete prewetting phase diagram.

An x-ray scattering study of the morphology of the Pt(001) surface reveals that below 1820 K it is atomically smooth on length scales exceeding 5000 Å. However, above 1820 K the Pt(001) surface is rough. Specifically, the scattering near the specular condition is diffuse, and may be modeled using a height-height correlation function which diverges logarithmically at large distances. Our data suggest that the average separation between thermally generated steps is many lattice constants.

A simple reaction-diffusion model for the CO oxidation on Pt(110), exhibiting among other features bistability and oscillations, is studied in an excitable parameter region where traveling solitary pulses with high O coverage may be formed on an otherwise CO-covered surface. If it is assumed that small patches are structurally imperfect, effects of wave splitting of a pulse, as well as partial annihilation and solitonlike behavior, result upon collision of two pulses, in agreement with experimental observations.

We show that in the three-dimensional interacting electron liquid in a magnetic field, with more than one Landau subband occupied, the Fermi momenta of the various subbands differ from those of the noninteracting electron gas. We calculate the interacting Fermi momenta including exchange and correlation in the random-phase approximation. It is shown that the change of Fermi momenta leads to novel behavior of the spin polarization as a function of magnetic field, which should be observable in experiments on wide parabolic quantum wells.

We discuss some new quantum numbers (spin vector) for the Hall fluid, representing orbital spin degrees of freedom. We show that the spin vectors are quantized. In the absence of impurites, two Hall fluids with different spin vectors cannot change into each other without a phase transition and closing of the energy gap. In principle the spin vector can be measured through its coupling to the curvature of space. Our formalism may be described picturesquely as a unification of electromagnetism and ‘‘gravity’’ in condensed-matter physics.

We estimate the Coulomb pseudopotential ${\mathrm{\mu }}^{\mathrm{*}}$ for a model of undoped ${\mathrm{C}}_{60}$, renormalizing away all subbands but the ${\mathit{t}}_{1\mathit{u}}$ conduction band by summing ladder diagrams in the screened interaction. The renormalization of ${\mathrm{\mu }}^{\mathrm{*}}$ is small. For doped ${\mathrm{C}}_{60}$ the screened U is calculated. A very efficient metallic screening is found for the intraband U, but not for the interband U, suggesting a large renormalization of ${\mathrm{\mu }}^{\mathrm{*}}$ within a ladder diagram approach. Exact results for a two-band model suggest, however, that the ladder diagrams strongly overestimate the renormalization.

We have carried out a strong coupling calculation of spin-fluctuation-induced superconductivity for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ using the experimentally determined spin excitation spectrum. We find that it is extremely important to take full account of the momentum and frequency dependence of the interaction and that while lifetime effects substantially reduce ${\mathit{T}}_{\mathit{c}}$ from its value calculated in a weak coupling approximation, it is still possible for spin fluctuations to give rise to high-temperature superconductivity.

Doping states in a two-dimensional three-band Peierls-Hubbard model are investigated with inhomogeneous Hartree-Fock and random phase approximations. They are sensitive to small changes of electron-lattice and electron-electron interactions. For parameters relevant to the insulating copper oxides a small ferromagnetic polaron is found. Moderate intersite electron-lattice coupling triggers a nonlinear feedback mechanism resulting in a rapid crossover from a Zhang-Rice regime to a covalent molecular singlet state in which the local magnetic moment is quenched and local lattice distortion is large. Various states are characterized by distinct optical and infrared absorption spectra.

A strong oscillatory exchange coupling has been discovered in epitaxial (111) and (110) Co/Cu/Co sandwiches with wedge-shaped Cu layers, deposited by molecular-beam epitaxy (MBE) on single-crystal substrates. This finding proves the intrinsic nature of the strong antiferromagnetic coupling in (111)-textured sputtered Co/Cu multilayers. Combined with recent results on (100) Co/Cu/Co MBE-grown samples, it becomes evident that the nature of the oscillations in Co/Cu/Co depends markedly on growth direction, agreeing with predictions invoking the topology of the Fermi surface of the interlayer Cu.

We report on the biaxial response of photoluminescence to electric fields in GaAs quantum-well structures. For fields along [001], emission due to nominally forbidden excitons exhibits substantial differences between [110] and [11¯0] polarizations. Transitions which are allowed within the envelope-function formalism show no noticeable anisotropy. In terms of symmetry, our observations relate to the linear electro-optic (Pockels) effect. The field dependence of the anisotropy is unusual in that it decreases with increasing field. Results can be qualitatively accounted for by perturbation analyses.

A generalized Monte Carlo procedure for the ultrafast dynamics of photoexcited carriers in a semiconductor is presented, where the coherence in the carrier system, as well as band renormalization and excitonic effects in the Hartree-Fock approximation are fully taken into account. The details of the coherent generation process, the energy relaxation, and dephasing of the carriers are analyzed. The approach presents a numerical method for the investigation of phenomena occurring close to the band gap and those typical for the relaxation of hot carriers.

We report the formulation of a solid gel network from purely liquid emulsion droplets. The gel remains rigid at droplet volume fractions as low as ${10}^{\mathrm{-}3}$. The gelation process is identified as diffusion-limited cluster aggregation. We find a surprising order in the gel structure. This ordering is induced by the aggregation kinetics, which result in an unexpected spatial correlation between the growing clusters, ensuring that the network is formed from clusters of nearly equal size and spacing.

We prove that the temporal autocorrelation function C(t) for quantum systems with Cantor spectra has an algebraic decay C(t)∼${\mathit{t}}^{\mathrm{-}\mathrm{\delta }}$, where δ equals the generalized dimension ${\mathit{D}}_2$ of the spectral measure and is bounded by the Hausdorff dimension ${\mathit{D}}_0$≥δ. We study various incommensurate systems with singular continuous and absolutely continuous Cantor spectra and find extremely slow correlation decays in singular continuous cases (δ=0.14 for the critical Harper model and 0<δ≤0.84 for the Fibonacci chains). In the kicked Harper model we deomonstrate that the quantum mechanical decay is unrelated to the existence of classical chaos.

The rotation of the reference frame of a particle is known to lead to a phase change of its wave function proportional to its angular momentum. This can manifest itself as an angle-dependent phase shift of a photon scattered by a fixed target, when the photon state is an eigenstate of the component of total angular momentum perpendicular to the scattering plane. This phase shift has been observed in the quantum beat pattern resulting from the transient excitation of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei by synchrotron radiation.