It has recently been observed that thin film AgBr(111) may exhibit well-pronounced superstructures which are visualized by surface decoration, i.e., by deposition of a very small quantity of metal. Depending on the growth conditions of the AgBr films, these superstructures establish superlattices with 〈110〉 and 〈321〉 directions, respectively, of the AgBr lattice. They are thought to result from surface reconstruction of the otherwise highly polar surface. A fractal model of AgBr(111) superstructures is presented which for the first time, to our knowledge, enables us to assign the experimentally observed superlattices to surface configurations of proper dimensions and directions. By applying a simple procedure of covering the surface lattice sites so as to approach a half coverage, superstructures are formed which may stabilize the surface by coarsening on the atomic scale. The contribution of the subsurface space-charge layer, which is mainly due to mobile silver ion interstitials in a near surface region, as well as its possible interaction with neutral metal clusters formed at the surface during vapor deposition, are emphasized.

Identities and approximations are developed for the first and second moments of the spectral density of center-of-mass oscillations of a commensurate quantum monolayer solid. This corresponds to the one-phonon approximation for inelastic scattering at a reciprocal lattice vector. The results are applied with parameters for monolayers of helium/graphite and hydrogen/graphite. Estimates of the zone-center gap with averages in an optimized Jastrow trial function are internally consistent to better than 10%. There is a marginal level of agreement between the zone-center gap for helium/graphite observed with inelastic neutron scattering and the gap calculated with the Fourier component of the helium/graphite potential derived from atom-surface scattering. The application for hydrogen/graphite amounts to a reanalysis of previous calculations and is in good agreement with them.

The electronic structure of monolayer graphite on a TiC(111) surface is investigated by first-principles band calculations. Occupied π bands of the graphite layer are similar to those of bulk graphite with a constant shift to a lower-energy region. Unoccupied ${\mathrm{\pi }}^{\mathrm{*}}$ bands are drastically deformed by hybridization with the substrate. The calculated band structure reproduces well an experimental band dispersion. Calculated electronic charges show no charge transfer from the substrate to the graphite layer. The lowering of the π bands is explained by a change of electronic occupation in the graphite layer from the occupied σ and π states to the unoccupied ${\mathrm{\pi }}^{\mathrm{*}}$ states due to hybridization with the substrate. The calculated results are consistent with the observed anomalous expansion of the lattice constant of monolayer graphite on metal substrates. The calculated band dispersion and electronic charges do not depend on the lateral position of the monolayer graphite relative to the substrate, which is explained by an averaging effect in incommensurate systems. Scanning-tunneling-microscopy (STM) images calculated from the results of the band calculation do not show the atomic structure of the graphite but show superstructures, which explain the experimental STM images.

Using a molecular-dynamics simulation, we have investigated the mechanisms of single dimer self-diffusion on the (111), (001), and (110) surfaces of fcc iridium. We use a realistic many-body potential, namely, the Rosato-Guillope-Legrand model potential, which involves empirical fittings of bulk properties of the solid. On the (001) and (110) surfaces, evidence of atomic diffusion by exchange mechanisms of the dimer atoms with substrate atoms are found. This is in good agreement with field-ion microscope observations. In addition, a preliminary investigation of the mechanisms of self-diffusion of iridium trimer on the (110) plane has also been carried out.

We report the results of surface electronic-structure calculations for the three low-index faces of elemental Pb. To our knowledge, these are the first calculations for the Pb(110) and Pb(111) surfaces addressing their electronic structure. The underlying bulk crystal is described by a realistic second-nearest-neighbor empirical tight-binding Hamiltonian which includes s and p orbitals and takes spin-orbit coupling into account. The resulting 6s- and 6p-derived bands are entirely decoupled. Our Hamiltonian yields the bulk density of states and the occupied bulk energy bands in excellent agreement with the data from x-ray and angle-resolved ultraviolet photoelectron spectroscopy measurements. The electronic structure of the (001), (110), and (111) surfaces is calculated for semi-infinite systems employing the scattering theoretical method. Our calculations predict a number of occupied as well as empty surface states or resonances in the energy regions of both the s- and p-band projections. All three surfaces show a pronounced resonance around -8 eV and a band of bound surface states near -2 eV below ${\mathit{E}}_{\mathit{F}}$. There are no surface states in the gap between the occupied s and p bands. For exemplary cases we highlight the origin and nature as well as the spatial localization of characteristic surface features.

The magnetic ordering of vanadium overlayers on Fe(100) was studied using spin-polarized electron-energy-loss spectroscopy. Net magnetic moments of the V surface were observed for one and two monolayers of vanadium, while no magnetic ordering of the surface was observed for thicker films. The vanadium monolayer couples antiferromagnetically to the Fe substrate, while the surface of two monolayers of vanadium aligns ferromagnetically with the substrate. The magnetic moment of the V monolayer is estimated to be less than 1${\mathrm{\mu }}_{\mathit{B}}$.

Using a tight-binding, total-energy model, we predict the atomic and electronic structure of the relaxed (1×1) corundum (0001) surface. The surface shows a large, bond-length-conserving relaxation, which is allowed by the topology of the surface. The relaxation is driven by a rehybridization of the surface Al atoms to ${\mathit{sp}}^2$, and an accompanying drop in the energy of occupied surface states, during the relaxation. Displacements of surface atoms from bulk positions are as large as 0.7 Å, and should be observable using a low-energy electron diffraction intensity analysis. Full relaxed atomic positions are reported, as well as a wavelength-resolved surface band structure, including orbital characters of surface states.

The local density of states (LDOS) of a single disclination and a disclination pair of various configurations in the monolayer graphite is calculated by the recursion method introduced by Haydock [J. Phys. C 5, 2845 (1972)]. The LDOS shows the existence of some resonant states near the Fermi energy. At the Fermi level, the value of the LDOS vanishes for a single disclination of five- and seven-membered rings, and remains a finite value for four- and eight-membered rings. On going away from the disclination center, the shape of the LDOS approaches that of the perfect lattice. At the Fermi level, a sharp peak structure, which corresponds to a localized state, appears in the LDOS of fused disclinations consisting of a four-membered ring and a seven-membered ring, and the value of the LDOS vanishes in the case of fused disclinations consisting of two five-membered rings. But these features are drastically changed if the configuration of the disclination pair satisfies a certain simple condition. Using the fact that the Fermi level of these systems with disclinations is the same as that of the perfect monolayer graphite, the charge and the stability energy at each site are calculated from the LDOS. We show that the disclination center of a five-membered ring has a negative charge, that of a seven-membered ring has a positive charge, and that of an even membered ring is neutral.

We present self-consistent ab initio total-energy calculations on ${\mathrm{TiO}}_2$ (110) surfaces for which the atomic geometry has been relaxed to equilibrium. For stoichiometric supercells we find no surface states in the bulk band gap, in accord with experiment. Oxygen vacancies on the surface give rise to Ti 3d-like surface states at the edge of the bulk conduction band. The relaxations of the atoms at the surface were found to be substantial, and those induced by a vacancy were localized in its vicinity. Bulk vacancies introduce states about 0.3 eV below the edge of the bulk conduction band.

Critical properties of the two-dimensional order-disorder phase transitions of the p(2×2) and the (√3 × √3 )R30° superstructures of sulfur chemisorbed on Ru(001) were determined by spot-profile analysis using high-resolution low-energy electron diffraction. Both transitions are continuous, as evident from the power-law behavior observed for 0.01≤‖t‖≤0.1 (t=T/${\mathit{T}}_{\mathit{c}}$-1) and from the values obtained for the critical exponents. For smaller ‖t‖ the phase transitions are finite-size rounded by an interaction of the superstructure domains with steps (average distance between steps ∼275 Å). The values of the effective exponents β of the order parameter, ν of the correlation length, γ of the susceptibility, and the exponent η [only determined for p(2×2)] fall close to the values theoretically predicted for the four-state and three-state Potts universality classes, respectively. Deviations of experimental values from Potts values, found for β and γ, are attributed to corrections to scaling which in part might be specific of the lattice gas. Additional experiments on vicinal surfaces with higher step densities show that pinning of the superstructure domains at monoatomic steps limits the correlation length to values below the average terrace width. The finite-size-induced effects are quantitatively compatible with predictions from finite-size scaling theory.

By a combination of experimental data [from the Ni (111) (2×2)-K structure], model calculations, and simple formal theory, it shown that a strong initial-state effect exists in backscattering photoelectron diffraction, which can be ascribed to the parity of the emitted photoelectron source wave field. Unlike the initial-state effect recently discussed in forward scattering photoelectron (and Auger electron) diffraction, which is a spherical wave effect only present due to the close proximity of the emitter and scatterer, this parity effect in the backscattering geometry exists even in the lowest order approximation of the scattering, i.e. the plane wave approximation.

The unoccupied surface electronic structure of Gd(0001) was investigated with high-resolution inverse-photoemission spectroscopy. An empty surface state near ${\mathit{E}}_{\mathit{F}}$ is observed at Γ¯. Two other surface-sensitive features are also revealed at 1.2 and 3.1 eV above the Fermi level. Hydrogen adsorption on Gd surfaces was used to distinguish the surface-sensitive features from the bulk features. The unoccupied bulk-band critical points are determined to be ${\mathrm{\Gamma }}_3^{+}$ at 1.9 eV and ${\mathit{A}}_1$ at 0.8 eV.

The relative stabilities of various geometries of 20-atom carbon cluters are investigated to determine the ground state, using a generalized tight-binding molecular dynamics technique. The 20-atom fullerene cage (relaxing to ${\mathrm{C}}_{\mathit{s}}$ symmetry due to Jahn-Teller distortion) is found to be the most stable structure even when compared to the recently proposed bicyclic ring structure.

Identically prepared fullerite films were exposed to different oxygen environments, including supersonic atomic and molecular beams as well as ambient pressure. Only in atomic oxygen does mass loss occur, likely due to fragmentation of the ${\mathrm{C}}_{60}$ cage by extraction of ${\mathrm{C}}_2$O or CO. In other cases, an epoxide overlayer is formed, involving up to five oxygen atoms interacting with the π electrons of a single cage. Electron-energy-loss spectra of the oxygenated films are compared with in situ UHV-deposited fullerite. The marked difference in the spectra is used to isolate the loss spectrum of the 18-eV plasmon of fullerine epoxide, and a Kramers-Kronig analysis of the loss function is used to estimate the epoxide band gap to be 5.5±0.5 eV.

Scanning tunneling microscopy was used to study Au-overlayer formation on the first and second monolayers of ${\mathrm{C}}_{60}$ on a Au(111) substrate. At 300 K, Au overlayers were clustering on top of the first monolayer of ${\mathrm{C}}_{60}$, and the stronger Au-Au bonding did not break the underlying long-range close-packed order of ${\mathrm{C}}_{60}$ molecules. However, Au diffused under the second ${\mathrm{C}}_{60}$ layer and formed two-dimensional clusters. These clusters provided multiple nucleation sites and the second ${\mathrm{C}}_{60}$ monolayer was rearranged around them. As a result, the second monolayer granulated. Bonding between the second monolayer and underlying Au clusters, characterized by charge transfer, was evidence that density of states near the Fermi level became higher. Our real-space images clarified some details presented in a recent photoelectron spectroscopy study.

We have conducted a study of the adsorption, thin-film growth, and the reaction of ${\mathrm{C}}_{70}$ on the Si(100)2×1 surface. It is found that the crystalline thin film can be grown with good quality by keeping the substrate at approximately 120 ° C during deposition. ${\mathrm{C}}_{70}$’s above the first layer can desorb at temperatures above approximately 250 °C, which is similar to the situation in the bulk phase, but the first layer of the ${\mathrm{C}}_{70}$ film which is directly bonded to the Si(100) surface does not desorb or decompose upon heating to 1000 °C. It reacts with the Si(100) substrate at temperatures above 1000 °C to form the SiC(100)3×2 islands. The reaction products can finally desorb at temperatures above 1300 °C, resulting in surface roughening.

An asymptotically exact partition function of the multichannel resonant-level model is obtained through Tomonaga-Luttinger bosonization. A Fermi-liquid vs non-Fermi-liquid transition, resulting from a competition between the Kondo resonance and the x-ray edge singularity, is elucidated explicitly via renormalization-group theory. In the strong-coupling limit, the model is renormalized to the Toulouse limit.

We study the exactly solvable strongly interacting electron models recently introduced by Brandt and Giesekus and further generalized by other authors. For a very general class of models, including the Hubbard, the periodic Anderson, and the Emery models with certain hopping matrices and infinitely large on-site Coulomb repulsion on d sites, we prove that the known exact ground state is indeed the unique ground state for a certain electron number. The uniqueness guarantees that one can discuss physics of various strongly interacting electron systems by analyzing the exact ground states.

We show that, for doped semiconductor structures at nonzero temperatures, processes which scatter electrons into a state ‖k〉 can contribute strongly to the decay of a nonequilibrium electron occupation of ‖k〉. For electrons, the decay rate γ(k) is given by the sum of the total scattering-out and scattering-in rates of state ‖k〉. The scattering-in term is safely neglected in the nondegenerate and highly degenerate limits, but it increases γ(k) of low-energy electrons injected into semidegenerate systems (between these limits) by the factor [1-${\mathit{f}}_0$(k)${]}^{\mathrm{-}1}$. We show that the degree of degeneracy of a system of fermions depends strongly on the system dimension, so that doped systems of reduced dimension can be semidegenerate, with the two scattering rates comparable, even at high temperature.

The photoionization spectrum of a level in the upper half of the band gap of zinc selenide has been measured. There are two thresholds, at 0.46 and 1.72 eV. These are assigned to transitions from the level to the Γ valley and to a set of higher valleys, giving an intervalley spacing of 1.26 eV in agreement with the pseudopotential calculations of J. R. Chelikowsky and M. L. Cohen [Phys. Rev. B 14, 556 (1976)]. The level concentration is 3×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}3}$. Thermal-emission experiments give an apparent activation energy 0.33 eV and an apparent capture cross section 1.4×${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$, values similar to those obtained by other authors for a level in ZnSe by deep-level trap spectroscopy. In the zinc selenide sample used here, there are no other levels with similar concentration in the upper half of the band gap.