The heteroepitaxial growth of ${\mathrm{C}}_{60}$ on GeS(001) has been studied using low-energy electron diffraction, selected-area electron diffraction, high-resolution electron microscopy, x-ray diffraction, and x-ray and ultraviolet-photoelectron spectroscopy (UPS). The simultaneous observation of diffraction spots characteristic of the substrate and the ${\mathrm{C}}_{60}$(111) overlayer allows us to specify the geometry of the epitaxy. The shape of the intensity curves of the C 1s and Ge 3d photoemission lines strongly suggests a layer-by-layer-type growth, confirmed by the observation in the synchrotron x-ray diffraction spectrum of finite-size oscillations on the (111) Bragg reflection peak of a thin ${\mathrm{C}}_{60}$(111) film. From a theoretical simulation of the C 1s and Ge 3d line-intensity curves, the mean free path of a C 1s and Ge 3d photoelectron in solid ${\mathrm{C}}_{60}$ is estimated to about 15.4 and 17 Å, respectively. The plot of the film thickness versus deposition time shows evidence for a small difference in sticking coefficient between the first monolayer and the upper ones. A detailed analysis of the C 1s line shapes for normal and grazing emission suggests the existence of inequivalent carbon sites at the interface. The first valence-band feature of the substrate presents a downward band bending of about 200 meV with increasing ${\mathrm{C}}_{60}$ coverage. From the shift of the cutoff in the UPS spectra we deduce a work function increase of about 100 meV upon monolayer adsorption. The characteristic spectral features of ${\mathrm{C}}_{60}$ observed in the UPS spectra for bulk fullerite are slightly broadened and shifted to lower binding energies at submonolayer coverages and show no direct evidence for significant hybridization, indicating that the ${\mathrm{C}}_{60}$-substrate interaction is mainly dominated by van der Waals bonding. All these observations can be explained by a positive effective dipole of about 8×${10}^{\mathrm{-}31}$ C m induced on the ${\mathrm{C}}_{60}$ molecule upon adsorption onto the GeS substrate.

As most technologically important metals will form oxides readily, any complete study of adhesion at real metal surfaces must include the metal-oxide interface. The role of this ubiquitous oxide layer cannot be overlooked, as the adhesive properties of the oxide or oxide-metal system can be expected to differ profoundly from the adhesive properties of a bare metal surface. We report on the development of a computational method for molecular-dynamics simulations, which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal oxides accurately. We discuss in detail results using this method for α-alumina and several of its low-index faces.

The effects of different assumptions made in the simulation of surface segregation free energies are considered. Thirty face-centered-cubic dilute binary alloys are investigated using embedded-atom method potentials. First, it is demonstrated that the inclusion of local atomic relaxations in lattice statics simulations strongly affects the segregation free energy at (111) free surfaces in over half of the alloys. Second, a Monte Carlo technique, namely, the overlapping distributions method (ODMC), is used to determine the effect of the vibrational entropy term on the surface segregation Helmholtz free energy for simulations at elevated temperatures (1000 K). It is determined that the vibrational entropy term is important for a quarter of the alloys. We conclude that for accurate calculations of surface segregation free energies, both local atomic relaxations and the vibrational entropy must be included in nearly all cases. Since the ODMC method is very computer-time intensive, a technique based on a simplification of the quasiharmonic approximation for calculating free energies is investigated. Results from this free-energy minimization (FEM) method using the local harmonic approximation are compared to results from the ODMC method. It is found that the FEM method calculates the segregation free energies at (111) free surfaces accurately for most of the 30 alloys. Segregation free energy profiles are also calculated as a function of distance from (111) free surfaces employing both methodologies. The agreement is found to be poor for alloys that have a large solvent atom and a small solute atom. Other problems and sources of error with the FEM method are also discussed.

The periodic Hartree-Fock approach is used in all-electron calculations on bulk rutile (${\mathrm{TiO}}_2$) and various slabs modeling the (001), (100), and (110) surface to study possible geometric and electronic surface relaxations, convergence of properties with slab thickness, and the relative stability of the different surfaces. Results are discussed by means of a Mulliken population analysis, band structures, and the density of states. We find displacements of surface atoms ranging from 5 to nearly 40 pm; the (001) surface appears to be significantly unstable with respect to the other two [(100) and (110)], in accord with experimental findings. Specific features of the density of states projected onto different Ti sites and the relative stability of the different surfaces agree with a possible dissociation behavior of the rutile structure into ${\mathrm{TiO}}_2$ chains, which are calculated to be stable.

The unoccupied electronic states of the Al(111) surface have been studied using k-resolved inverse-photoemission spectroscopy (KRIPES). In addition, a first-principles calculation of the bulk Al electronic structure has been performed to facilitate interpretation of the experimental data. The KRIPES spectra obtained along the [1¯10], [112¯], and [1¯ 1¯2] azimuths of the surface Brillouin zone are characterized by well-defined features within 5 eV of the Fermi level, and broad, weak features at higher energies. In general, surface states and resonances appeared as strong spectral features while bulk transitions were weak for this surface. First-principles electronic-structure calculations were necessary to obtain a qualitative account of the bulk features, and semiquantitative agreement was obtained when excitation effects were considered. Dispersion of an unoccupied surface resonance along the [112¯] azimuth is consistent both with an occupied surface resonance found by an earlier photoemission study and with the predictions of surface electronic-structure calculations in the literature. A strong feature observed in the [1¯12] direction is identified as an odd surface state occurring in a symmetry gap and may account for earlier electron-energy-loss data.

Raman-scattering and infrared reflection measurements on misfit-layer compounds (RS${)}_{\mathrm{n}}$${\mathrm{NbS}}_2$ (n=0.6,1.2; R=La,Ce) have been carried out in order to investigate the interlayer interaction and its relation to the charge transfer from the RS layer to the ${\mathrm{NbS}}_2$ layer. It is found that the intralayer modes of the ${\mathrm{NbS}}_2$ layer in (RS${)}_{\mathrm{n}}$${\mathrm{NbS}}_2$ are considerably upshifted from those of a pristine ${\mathrm{NbS}}_2$ crystal. Further, some extra modes are observed. These facts indicate a strong interlayer interaction in these compounds. The modes corresponding to the intralayer vibrations of the ${\mathrm{NbS}}_2$ layer are less perturbed for the stage-2 (n=0.6) compounds than for the stage-1 (n=1.2) compounds owing to the difference of the amount of the charge transfer. However, the ${\mathrm{A}}_2^{\prime }$ (421 ${\mathrm{cm}}^{\mathrm{-}1}$) mode, which is Raman inactive in an isolated ${\mathrm{NbS}}_2$ layer, appears strongly for the stage-2 compounds. This result is interpreted in terms of the breakdown of the mirror symmetry of the ${\mathrm{NbS}}_2$ layer in the stage-2 compounds caused by the intercalation of the RS layer.

We present the results of comparative studies of photoinduced electron transfer from semiconducting polymers onto ${\mathrm{C}}_{60}$, focusing on the effect of strong Coulomb correlations, which lead to excitons (bound electron-hole pairs) as the primary photoexcitations. For this purpose, we investigated two different soluble polydiacetylene (PDA) derivatives in which the lowest-energy photoexcitations are excitons with a binding energy of approximately 0.5 eV. The experimental studies (absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spectroscopy, and picosecond transient and near-steady-state photoconductivity) were carried out in pristine materials and in composites of PDA with ${\mathrm{C}}_{60}$. The PDA data are compared with the results of similar studies on semiconducting conjugated polymers such as poly(para-phenylene-vinylene) and its derivatives, the PPV’s, and polythiophene and its derivatives, the P3AT’s. The results show clearly that in contrast to the high quantum efficiency photoinduced electron transfer discovered earlier in the PPV’s and the P3AT’s, the excitonic bound state of the electron and hole inhibits photoinduced electron transfer from PDA onto ${\mathrm{C}}_{60}$. We conclude that in the PPV’s and the P3AT’s, the Coulomb interaction is sufficiently well screened that upon photoexcitation free carriers are created, thereby making possible facile electron transfer to a nearby acceptor.

We have observed enormous shifts of the optical-absorption peak during the reduction of gold-sulfide particles (${\mathrm{Au}}_2$S) to gold particles. A two-step colloidal method is used for the nanoparticle synthesis. We can explain our findings by assuming the colloidal particles have a gold coating on the surface. This is also consistent with our transmission-electron-microscopy figures, displaying a core-shell structure, and electron-diffraction data. The optical-absorption peak initially shifts toward the red and at later times toward the blue wavelengths. By controlling the initial size of the gold-sulfide particles, the resonance shift is correlated with a theoretical model that includes both quantum confinement and the resonance effects (the so-called surface-plasmon resonance). The use of metal-coated particles with a nonmetallic core material offers two advantages for studying quantum confinement. First, the particles are initially large, and have a large polarizability and consequently a large absorption cross section, and second, the thin metal layer confines the electron in one dimension and can extend itself in the other two dimensions.

A modified elastic theory is given by introducing the coupling between the charge-density wave and the distortions in the top layer into the model of McTague and Novaco. It is found that the continuous rotational transformation observed on the (001) surfaces of Au and Pt can be described by this theory.

We have studied the electronic properties of the Cu/${\mathrm{TiO}}_2$(110) and Fe/${\mathrm{TiO}}_2$(110) interfaces using in situ ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). The chemical interaction between these two transition metals and the substrate was monitored by examining O 2p and band-gap emission in UPS, and the unoccupied Ti 3d band in IPS. For Cu film thicknesses ranging from 0.1 to 80 Å, there is no sign of overlayer oxidation or substrate reduction in either spectroscopy indicating that there is no significant charge transfer between Cu and the substrate. In contrast, strong interaction between Fe and ${\mathrm{TiO}}_2$(110) has been found, especially at low coverages. Charge transfer between the Fe overlayer and the substrate takes place at the very initial stage of deposition resulting in oxidation of the adsorbed Fe atoms and reduction of the Ti ions on the surface. Although the band-gap emission due to the formation of Ti 3d states is usually obscured by the Fe 3d band in UPS, IPS clearly shows a ${\mathrm{Ti}}^{3+}$ induced state at ∼3 eV above the Fermi level as seen on defective ${\mathrm{TiO}}_2$(110). The UPS spectrum from an oxygen-exposed Fe overlayer indicates formation of an iron-oxide layer composed of FeO and ${\mathrm{Fe}}_2$${\mathrm{O}}_3$ phases.

We present effective-medium calculations of the optical properties of chemical vapor deposition-grown diamond films, under the assumption that these films are contaminated by various kinds of defects. Two types of defective films are considered: (i) a distribution of graphite crystallites in diamond, with each crystallite small compared to the wavelength of light; and (ii) a rough diamond-air interface. The calculations are carried out by treating the film in case (i) and rough surface layer in case (ii) as homogeneous media with effective dielectric properties. In both cases, we find characteristic modifications of the frequency-dependent reflectivity. The experimentally observed reflectivity of a diamond film is very well reproduced by our calculation over a broad frequency range between 2 and 10 eV.

Electron-momentum spectroscopy [binary encounter (e,2e)] experimental results are presented for the core state of an amorphous carbon allotrope. The (e,2e) cross section has two identifiable regions. One is a narrow-energy-width core-band peak that does not disperse with momentum. At higher binding energies there is an energy-diffuse multiple-scattering continuum, which is a consequence of (e,2e) collisions with core electrons that are accompanied by inelastic scattering of one or more of the incoming or outgoing electrons. Comparisons of experimental momentum distributions with the Hartree-Fock atomic carbon 1s orbital are presented for both regions.

Recent observations on various low-index metallic surfaces of an extraordinary thermal attenuation of diffracted peak intensities suggest that anharmonic effects are important at high temperatures, though such observations have also been interpreted in terms of disordering processes (premelting and roughening). In order to address this problem, we have carried out an extensive molecular-dynamics investigation of the evolution in temperature of the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom method potentials. We observe both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration; the latter are found in both cases to be larger in the plane of the surface than out of the plane. An analysis of our simulated low-energy electron-diffraction intensities supports the experimental interpretation of the measured ones in terms of enhanced mean-square displacements. Indeed, anharmonicities are found to appear at temperatures well below the onset of disordering, which occurs at 1200 and 1000 K for the (100) and (110) surfaces, respectively (via in-plane diffusion and adatom-vacancy formation). The onset of disordering proceeds differently on the two surfaces: while it involves essentially only the outermost layer on the (100) surface, both the first and second layer participate in the process on the (110) surface. We also calculate the phonon spectra at high-symmetry points of the surface Brillouin zone, which we find to be in good agreement with experimental values at room temperature. We predict, further, that the temperature-dependent frequency shifts are not significantly softened in comparison with bulk phonons, which suggests that surface phonons are affected by anharmonicity in a nontrivial way. Several of the observed properties can be understood in terms of the larger displacements that surface atoms can afford, because of their reduced coordination compared to bulk atoms.

We present a molecular-dynamics simulation of adatom diffusion along the two close-packed steps on the (111) surfaces of gold and silver. Both metals are modeled by employing many-body potentials derived within the second-moment approximation to the tight-binding model. The simulation predicts very different behaviors for the two metals. For Au, the diffusion is much faster along the step with (111) microfacets (step B), whereas for Ag the diffusion is faster along the step with (100) microfacets (step A). The difference between the diffusion coefficients along the steps is more marked in gold and, for both metals, the Arrhenius plots show a dynamical lowering of the activation barriers with respect to the static potential barriers; no evidence of an inversion of the anisotropy of diffusion is obtained. As the diffusion along steps is quasi-one-dimensional, the results of the simulations have been compared to those based on the Fokker-Planck equation in a one-dimensional periodic potential. The agreement between the model and the simulations is remarkable for B steps both in gold and silver; the model predicts the temperature dependence of the rate, the correct proportion of long jumps, and the details of the behavior of the mean-square displacement. In A steps, the agreement is satisfactory for Ag and qualitative for Au; in the latter case, the diffusion path is rather different from a straight line and the application of a one-dimensional model may be questionable.

We have performed an ab initio study of the surface core-level binding energy shift (SCLS) for 11 of the simple metals by means of a Green’s-function technique within the tight-binding linear-muffin-tin-orbitals method. Initial- and final-state effects are included within the concept of complete screening, whereby a SCLS becomes equivalent to the surface segregation energy of a core-ionized atom, a quantity we obtain by separate bulk and surface impurity calculations. The results are in good agreement with experiment in most of those cases where the data originates from single-crystal measurements. We discuss the surface shifts of the electrostatic potentials and the band centers in order to trace the microscopic origin of the SCLS in the simple metals and find that the anomalous subsurface core-level shifts in beryllium are caused by charge dipoles, which persist several layers into the bulk. We furthermore conclude that the unexpected negative sign of the SCLS in beryllium is predominantly an initial-state effect and is caused by the high electron density in this metal.

We used Brillouin scattering in p-s polarization to study shear horizontal (SH) acoustic surface phonons in a silicon on insulator structure made of a Si/${\mathrm{SiO}}_2$ bilayer on a Si substrate. Two main peaks were measured, the first belonging to the discrete part and the second to the continuous part of the spectrum. The latter feature originates from a type of pseudosurface wave, or quasiresonance, typical of the investigated structure [Bottani et al., J. Phys. Condens. Matter 6, L85 (1994)]. Because of the SH polarization of the phonons considered Brillouin scattering takes place only via the elasto-optic mechanism, that is, by modulation of the bulk dielectric function of the material. To fully explain the experimental data we performed a computation of the p-s Brillouin cross section. The SH phonon spectrum, both discrete and continuous, was computed numerically using a slab approximation. The guided-wave nature of the discrete mode and the strong surface localization of the pseudomode are illustrated by means of the layer projected phonon density of states. We computed the incident p electric field transmitted in the medium and the fluctuating polarization vector radiating Brillouin light. The p-s cross section was evaluated using a Green-function method introduced some time ago by Laks and Mills [Phys. Rev. B 20, 4962 (1979)] [2]. The computed intensity of the scattered field is in good agreement with the experimental finding.

A lattice-gas model for the description of equilibrium surface segregation of interstitials on (001)-oriented body-centered-cubic surfaces is introduced. Assuming realistic nearest and more distant pairwise interactions between adjacent M-X and X-X atoms, the model is well suited to reproduce the experimentally observed c(2×2) ordering of segregated nitrogen and carbon on α-Fe(001). With increasing range of the repulsive X-X interactions ${\mathit{cphi}}_{\mathit{X}\mathit{X}}$(d), different orderings such as (2×1), (2×2), and c(4×2) occur at θ=1/2 and θ=1/4, 3) / 4 . The coverage dependence of the excess Gibbs free energy of segregation is discussed for the present lattice-gas model.

Electron-correlation effects in the fullerene molecule and its ions are investigated in the framework of the Hubbard model. The variational Monte Carlo method and the Gutzwiller wave function are used. Most attention is paid to the case of intermediate interactions, but also the strong coupling limit, where the Hubbard model reduces to the antiferromagnetic Heisenberg model, is considered for the fullerene molecule. In this case we obtain a very low variational ground state energy. Futher, we have calculated the main spin correlation functions in the ground state. Only short-range order is found. The pairing energy of two electrons added to a fullerene molecule or to a fullerene ion is also calculated. Contrary to the results obtained by second-order perturbation theory, pair binding is not found.

Within the tight-binding approximation, the consequences on the band spectrum of the lack of orthogonality between orbitals are fully investigated for a one-dimensional monoatomic crystal with one orbital per atom. The limitations of the nearest-neighbor approximation are first exposed and then, through a set of reasonable assumptions, a general analytical result is obtained for this system. It is seen that orbital nonorthogonality destroys the symmetry of the band spectrum with respect to the corrected atomic level, decreasing the effective mass at the top of the band and increasing it at its bottom. As an application, the errors involved in neglecting the multicenter integrals in the interaction matrix elements are evaluated.

Utilizing the grand canonical partition function as well as the cumulant summation formula, we consider a systematic approximation scheme for a strongly correlated fermion system. As an example, we investigate the single-impurity Anderson model. We are motivated by the fact that for this model there are physical aspects to the approximations used that are simply understood. In particular the lowest-order truncation yields the Kondo temperature as well as a many-particle understanding of the approximation of Zwicknagl, Zevin, and Fulde to the f spectral function.