A two-dimensional lattice-gas model with repulsive interactions periodically infinite in one dimension and finite in the other is driven into a mass-transporting steady state by asymmetric chemical potentials applied at the open edges. By computer-simulation techniques the steady-state current, the mass profile, and the order-parameter profile are calculated. The driven system is found to display a nonequilibrium phase behavior controlled by the mass flux.

We have observed both triggered and spontaneous energy releases in tritiated solid hydrogens at temperatures above 1.2 K in several different experiments. These energy releases, which can be triggered by a temperature increase, were observed by monitoring the temperature excursion (‘‘heat spike’’) versus time, the atom spin density, and nuclear-magnetic-resonance signal heights. The heat spikes correlate with a disappearance of free-atom spin density so that fast atomic recombination is the probable cause. The spontaneous heat spikes may be suppressed by improved heat extraction.

Scanning tunneling microscopy has repeatedly resolved individual atoms on a number of metal surfaces with atomic distances 2.5–3 Å. This is in sharp contradiction to the resolution limits previously predicted, 6–9 Å. We present a theory of such atomic resolution in terms of actual tip states, for example, ${\mathit{d}}_{\mathit{z}}^2$ tip states on tungsten tips. Quantitative interpretation of the observed images is obtained with no adjustable parameters. We predict that to achieve atomic resolution, the tip material should be either a d-band metal or certain semiconductor.

We present scanning tunneling microscopy images of the As-rich (2×2) and the Ga-rich (√19 × √19 ) reconstructions of the GaAs(1¯ 1¯ 1¯) surface produced in situ by molecular-beam epitaxy. From the experimental results and total-energy calculations we show that the (2×2) surface consists of As adatom trimers bonded to the underlying surface. The (√19 × √19 ) unit cell is dominated by a two-layer hexagonal ring.

Surface bias voltages induced on a scanning-tunneling-microscope junction illuminated with laser radiation are spatially measured for both metal and semiconductor samples. A surface photovoltage of ∼0.3 eV is observed for Si(111)-(7×7), with large reductions in the vicinity of surface (subsurface) defects having midgap states. These reductions, attributed to a change in the recombination rate, have a typical surface screening distance of 15–25 Å. A small, atomically varying signal of 3–5 mV is observed on both metal and semiconductor samples and demonstrated to arise not from variation in photovoltage but from spatial variations in rectification efficiency.

In Ising-like systems, there is a temperature ${\mathit{T}}^{\mathrm{*}}$, strictly between zero and the usual critical temperature, above which any region of arbitrary size is completely contained within a domain which prefers energetically to be flipped.

Defects in amorphous Si:H are identified by a new atomic model encompassing built-in defects and light-induced defects. The model consists of a metastable localized center having an atom with two possible sites, as in the Chadi and Chang model for DX centers in crystalline GaAs. Specific rehybridizations for two-site atoms produce changes in charge state and bonding that explain a wide range of effects, including n-type or p-type doping or compensation, the Staebler-Wronski effect and its variations with doping, quench-induced defects, and defect spin.

Angle-resolved photoemission and inverse photoemission have been used to measure the two-dimensional dispersion of the energy bands of a single dense layer of Li grown on Be(0001). These two techniques show excellent agreement for both the Fermi-level crossing and the effective mass of the Li valence band. This band is parabolic, with an effective mass (${\mathit{m}}^{\mathrm{*}}$/m) of 1.8, which is larger than that predicted for an isolated atomic layer. The large effective mass may be due to hybridization with the substrate bands. An avoided band crossing of the Li band and the Be surface state is observed.

The transition from adiabatic to nonadiabatic small polaron hopping has unique characteristics among hopping theories; an activation energy in the mobility that changes from being monotonically increasing with the mean distance between hopping sites, ρ, to being independent of ρ, and, simultaneously, an Arrhenius prefactor that changes from being independent of ρ to exponentially decreasing with ρ. We have observed such a transition for the first time in the hole mobilities of the molecularly doped polymer, tri-p-tolylamine:polycarbonate.

We present self-consistent local-spin-density calculations of the static electric dipole polarizability tensor for several isomers of sodium clusters with up to nine atoms. We show how the comparison of the calculated polarizabilities with the experimental data can be used to identify which isomer is observed in the experiments. Our results indicate that sodium clusters with six atoms or less are planar and that the drop in the polarizability of ${\mathrm{Na}}_7$ is related to the change from two-dimensional to three-dimensional geometries. We also present an analysis of recent measurements of photoabsorption resonance frequencies.

Surface infrared reflection-absorption measurements for CO adsorbed on Cu(100) in the frequency range 200–500 ${\mathrm{cm}}^{\mathrm{-}1}$ reveal, in addition to the Cu-CO stretch mode at 345 ${\mathrm{cm}}^{\mathrm{-}1}$, a sharp derivativelike feature at 285 ${\mathrm{cm}}^{\mathrm{-}1}$, assigned to the CO frustrated rotation mode which is usually dipole forbidden. Its infrared activity and Fano line shape arise from coupling to substrate electronic transitions associated with CO-induced surface states.

Reflectance measurements on solid hydrogen to 177 GPa (1.77 Mbar) have been performed from near-infrared to ultraviolet wavelengths (0.5 to 3 eV). Above 150 GPa characteristic free-electron behavior in the infrared region is observed to increase sharply with increasing pressure. Analysis of volume dependence of the plasma frequency obtained from Drude-model fits to the spectra indicates that the pressure of the insulator-metal transition is 149 (±10) GPa at 295 K. The measurements are consistent with metallization by closure of an indirect gap in the molecular solid.

Stern-Gerlach deflections of cold iron clusters in a molecular beam, with from 15 to 650 atoms per cluster, have been measured. It is found that the clusters deflect uniquely in the direction of the increasing field, indicating spin relaxation within the isolated clusters. The measured average magnetic moments increase with increasing cluster temperature and with increasing field, and in all cases they are found to be below the bulk value.

We report on the novel effect of magnetic x-ray dichroism in core-level photoemission from ferromagnets with circularly polarized radiation. Depending on the relative orientation of photon spin and sample magnetization (parallel or antiparallel), a single emission line may be resolved into two lines, due to exchange splitting of the core level. The mechanism is explained in terms of spin-selective dipole transitions in the presence of spin-orbit coupling. Possible applications of magnetic x-ray dichroism in photoemission from magnetic samples are pointed out.

The spectral density of the single-impurity Anderson model is calculated by a combination of quantum Monte Carlo method to provide data on the Matsubara Green’s function, the maximum-entropy method of image reconstruction to invert numerically the spectral representation, and perturbation theory to provide informative default models. The Kondo central peak of the spectral density is shown to be a universal function of ω/${\mathit{T}}_{\mathit{K}}$ and T/${\mathit{T}}_{\mathit{K}}$ at low frequencies, where ${\mathit{T}}_{\mathit{K}}$ is the Kondo temperature. The higher-frequency side peaks are nonuniversal. With decreasing T/${\mathit{T}}_{\mathit{K}}$ the Kondo peak grows as the screened local moment disappears.

The magnetic susceptibility, Hall effect, thermopower, specific heat, and plasma frequency are calculated as a function of doping and temperature for ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ and related compounds. There is good quantitative agreement with experiment. The calculation is based on quasiparticles in a Mott insulator, whose dispersion ε(k) is calculated variationally in the t-t’-J model (and the t-J model). The quasiparticles are assumed to form a weakly interacting Fermi liquid, which is treated in the relaxation-time approximation.

Ab initio molecular-orbital theory is used to unravel the mechanism of HF etching leading to hydrogen-passivated silicon surfaces as observed experimentally. Total-energy calculations on transition states of model silicon compounds suggest that the activation barriers for HF attack of the Si surface determine the resulting surface termination. In particular, the H passivation results from efficient removal of fluorine-bonded surface silicon as ${\mathrm{SiF}}_4$ leaving behind hydrogen.

The properties of electric double layers near closed-curved surfaces of arbitrary shape and genus are obtained exactly within the Debye-Hückel approximation by means of multiple-scattering expansions. Geometric and topological features of the electrostatics and thermodynamics emerge in a straightforward way through convergent expansions in powers of the ratio of the screening length to the principal radii of curvature. Some consequences of these results for the electrostatic contribution to the stability of structures of various shapes are considered.

The effects of short-range interparticle correlations on the multiple scattering of light in colloidal suspensions are studied. Our measurements, and comparisons with theory, of the transport mean free path and the width of the coherent backscattering cone demonstrate the validity of the diffusion approximation of light in correlated suspensions. Dynamic multiple-light-scattering measurements allow determination of the short-time self-diffusion constant of the spheres. The data agree with hydrodynamic theory up to 45% volume fraction with no adjustable parameters.

We study the effects of hydrodynamic interactions on the diffusion of hard spheres in concentrated suspensions. Using a multiple-light-scattering technique that measures the early-time behavior, we find ${\mathit{D}}_{\mathrm{eff}}$/${\mathit{D}}_0$=1-(1.86±0.07)φ, where φ is the volume fraction of spheres, ${\mathit{D}}_{\mathrm{eff}}$ is the effective diffusion coefficient, and ${\mathit{D}}_0$ is the free-particle diffusion coefficient. This agrees with the linear φ term calculated theoretically for short-time self-diffusion. The short-time diffusion coefficient is also found to be continuous across the freezing transition.