The equivalence between the body-centered solid-on-solid model and the asymmetric six-vertex model is used to study the surface free energy and the equilibrium shape of bcc crystals in the solid-on-solid approximation. The thermal evolution of the (001)- and (011)-type facets is investigated analytically and illustrated for different values of the anisotropy of the next-nearest-neighbor couplings. It is shown that the (001) facet undergoes a roughening transition of the Kosterlitz-Thouless universality class and that the asymptotic regime below the critical point T=${\mathit{T}}_{\mathit{R}}$ is large. The occurrence of a universal jump in surface stiffness at the point where this facet vanishes at ${\mathit{T}}_{\mathit{R}}$ is reconfirmed.

Using the self-consistent surface-Green-function method we study the chemisorption of isolated adatoms bonded to the (111) surface of aluminum. In order to identify chemical trends, a group-I, group-IV, and group-VII adsorbate is considered. Our study is analogous to that of Lang and Williams with the difference that their substrate was jellium, whereas we take the substrate atomic structure fully into account. The influence of the atomic structure is clearly noticeable in the electronic density of states, charge density, and atomic geometry, but the main (qualitative) conclusions of Lang and Williams are supported by our results. The discussion of the group-I adsorbate is complemented by calculations for periodic adlayers, in order to demonstrate the influence of adsorbate-adsorbate interactions.

LO phonons in polar material can tunnel resonantly through a rigid barrier provided that they can excite an interface polariton. A theory of phonon tunneling based on a macroscopic continuum model is presented. The ability of LO phonons to be transmitted is shown to be dependent upon there being sufficient dispersion for the LO frequency band to overlap significantly the ‘‘classical’’ dispersion of interface modes. The interaction with electrons in the barrier is discussed. It is also shown that interface modes appear in their own right only in the frequency range below the LO band but above ${\mathrm{\omega }}_{\mathrm{TO}}$.

Field emission from localized narrow bands above the Fermi level (${\mathit{E}}_{\mathit{F}}$) are obtained in two nanometer-scale systems: gold nanoclusters deposited on W support tips and nanotips that are W nano-scale protrusions formed on W support tips. The shift of localized bands was used as a probe of the tunneling process. The emission above ${\mathit{E}}_{\mathit{F}}$ is explained by considering the role of the tail of the Fermi function in supplying electrons to these localized levels. Conservation of energy is consistent with this process. The emission of hot electrons opens the possibility of local cooling of nanostructures.

The surface-embedded Green-function method has been successfully used to perform self-consistent calculations of the electronic structure of various metal surfaces. Although the method is quite general, the original implementation treats only surfaces having two-dimensional inversion symmetry. We have now modified the technique to remove this restriction. As a test, the electronic structure of Al(111) has been calculated. The calculated work function of 4.22 eV and the energy and dispersion of the calculated surface state/surface resonance bands along high-symmetry directions in the surface Brillouin zone are all in excellent agreement with experiment. Results are compared with other theoretical work.

We have measured the adsorbate-induced change in the infrared reflectance of a Cu(100) surface upon adsorption of oxygen at room temperature, at four oxygen coverages, over the frequency range from 100 to 2000 ${\mathrm{cm}}^{\mathrm{-}1}$. The observed frequency dependence provides an independent test of a model of conduction-electron scattering from disordered adsorbates that incorporates nonlocal electrodynamics. We find good agreement with theory for coverages below 0.25 ML. The value of the characteristic roll-off frequency ${\mathrm{\omega }}_{\mathit{p}}$${\nu }_{\mathit{F}}$/c, however, which is predicted to be independent of the adsorbate, is different from that found for CO on the same crystal. At 0.35-ML coverage, the frequency dependence deviates strongly from the predicted form and the magnitude of the reflectance change decreases. It is possible that these deviations are related to an incipient ordering and reconstruction of the surface.

A simple quantum-sphere model for small metallic particles of arbitrary size embedded in nonabsorbing dielectric media is used to compute the real and imaginary parts of the dielectric function. The quantum confined electrons can be considered as nearly free electrons but having discrete energy levels. The dielectric function contributed from transitions between energy levels is calculated via the momentum transition matrix under the zero-temperature approximation. Consequently, we have derived the size dependence of the absorption spectra which clearly indicate the blueshift and the broadening of the spectra as the particle sizes are reduced. The theoretical simulation is in satisfactory agreement with the experimental data for particle sizes below 10 nm.

Another method has been proposed to evaluate the frictional force in the stationary state. This method is applied to the one-dimensional model of clean surfaces. The kinetic frictional force is seen to depend on velocity in general, but the dependence becomes weaker as the maximum static frictional force increases and in the limiting case the kinetic friction becomes only weakly dependent on velocity as described by one of the laws of friction. It is also shown that there is a phase transition between a state with vanishing maximum static frictional force and a state with finite static frictional force. The role of randomness at the interface and the relation to the impurity pinning of the sliding charge-density wave are discussed.

The localized character of the 3d electrons in the antiferromagnetic oxides NiO(100) and CoO(100) has been studied with electron energy-loss spectroscopy at primary energies between 20 and 1200 eV. The spectra of both charge-transfer compounds exhibit weak but sharp loss structures within the insulating gap region due to crystal-field excitations of the 3${\mathit{d}}^{\mathit{n}}$ configuration (${\mathit{d}}^{\mathit{n}}$→${\mathit{d}}^{\mathit{n}\mathrm{*}}$). These parity forbidden d-d excitations are strongly enhanced at low primary energies due to exchange scattering. Their observed loss intensity depends on the momentum transfer at the scattering process as is shown by angle-dependent measurements. At NiO(100) a surface d state is found that is due to the lower symmetry of the ligand field at the surface. The measurements demonstrate the existence of spin-forbidden exchange excitations ${}_{}{}^3{}_{}{}^{}$${\mathit{A}}_{2\mathit{g}}$→${(}^1$${\mathit{E}}_{\mathit{g}}$${,}^1$${\mathit{T}}_{1\mathit{g}}$) in NiO(100) and reveal their excitation energies. A resonant enhancement of all intra-atomic d-d transitions in NiO(100) is found at the Ni 3s excitation threshold. Temperature-dependent measurements up to the Néel point show only little influence of the antiferromagnetic ordering on the EELS spectra of NiO(100). The different shapes of the absorption edges due to interatomic d-d transitions across the insulating gaps of NiO(100) and CoO(100) are discussed.

Dispersion measurements of second-harmonic generation from a clean Cu(111) surface show a resonance feature at the second-harmonic (SH) photon energy of 4.1 eV, consistent with an image-potential-state to surface-state transition in agreement with polarization selection rules. The sensitivity of the SH efficiency to surface defects on Cu(111), observed by Bloch et al. [Phys. Rev. B 45, 12 011 (1992)], is explained in terms of an inherent energy broadening of the surface-state wave function.

C:H films in the thickness range <1 to about 20 monolayers have been prepared by hydrocarbon-ion-beam deposition at Pt(100) and Pt(111) single-crystal surfaces covered with a graphite monolayer. The films were investigated by Auger electron, electron-energy-loss (EELS), and high-resolution electron-energy-loss spectroscopies to investigate their electronic and vibronic characteristics. The hydrogen content of the films was determined at [H]/[C]=0.05–0.5, depending on deposition parameters such as feed gas and target temperature. AES and EELS identify the films deposited at 350 K as being of a distorted graphitic nature with a carbon [${\mathit{sp}}^2$]/[${\mathit{sp}}^3$] ratio of about unity. Vibrational spectroscopy reveals that at the film surfaces H atoms are bound to C atoms in sp, aromatic ${\mathit{sp}}^2$, and ${\mathit{sp}}^3$ hybridization states. From the vibrational spectra, the presence of -C≡CH, (aromatic)=CH, and -${\mathrm{CH}}_{\mathit{x}}$, x=1,2,3 groups at the surface is inferred. Upon annealing, the films at higher temperatures, 500–1400 K, sp, ${\mathit{sp}}^3$, and ${\mathit{sp}}^2$ groups get destroyed sequentially, paralleled by evolution of hydrogen (major product) and hydrocarbon species (minor product) from the films. After annealing at 1040 K, vibrational spectra exhibit only a ${\mathit{sp}}^2$ CH-related C-H stretch band and, accordingly, from EELS increased ${\mathit{sp}}^2$ bonding in the annealed films is obtained. This stability sequence of ${\mathrm{CH}}_{\mathit{x}}$ groups at the carbon network is in accordance with the expectations drawn from kinetic and thermodynamic data of hydrocarbons. The chemical structure of the deposited C:H films is insensitive to the nature of the deposition ion, e.g., methane, ethane, ethylene, or benzene, from which it is suggested that specific C-H bonding in the deposition ions does not act as a precursor for the film structure. Films deposited in the submonolayer range exhibit primarily ${\mathit{sp}}^3$ C-H bonding in accordance with a simple picture of the initial growth of the films.

The wave-vector and directional dependence of the Rayleigh elastic wave localized in epitaxial films of bcc cobalt are measured by Brillouin scattering. The Rayleigh velocities are well described by elastic continuum theory and yield values for the ${\mathrm{C}}_{11}$ and ${\mathrm{C}}_{44}$ elastic constants that are reduced to about 65% of the corresponding values in the thermodynamically stable hcp configuration. While the reduction in ${\mathrm{C}}_{11}$ in these supported films (several hundred angstroms thick) is consistent with recent calculations for bulk bcc cobalt, the ${\mathrm{C}}_{44}$ and ${\mathrm{C}}_{12}$ values we have derived are significantly smaller than those predicted by the theory. The ${\mathrm{C}}_{\mathit{i}\mathit{j}}$’s determined independently from the Rayleigh wave dispersion along distinct crystallographic directions in films of different orientations agree well.

The possibility of observing a Coulomb gap due to carrier-carrier interactions in granular metallic systems is invesitgated, with particular emphasis on porous carbon structures such as carbon aerogels and activated carbon fibers (ACF’s), both of which contain ultrafine pores and grains, the size of which can be controlled by the sample preparation procedures. The exp[-(${\mathit{T}}_0$/T${)}^{1/2}$] temperature dependence observed at low temperature in the dc electrical conductivity of both materials is characteristic of granular metallic systems. A variable-range hopping mechanism near a Coulomb gap around the Fermi level is proposed to circumvent the difficulties of previous tunneling and hopping models. The temperature dependence of the magnetoresistance in fields up to 15 T is measured for both aerogels and ACF’s to further confirm the effect of the Coulomb gap. The results not only qualify porpous materials as a medium for studying localization phenomena, but also shed some light on the complex problem of transport near the percolation threshold in granular metallic systems in general.

The strain in Co layers of multilayers with largely different structural mismatch (Co/Ni, Co/Ag, and Co/Cu) has been measured with nuclear magnetic resonance and compared with a model for strain in multilayers that is based on the model of Van der Merwe and Jesser. For the Co/Ni multilayers, the measured strain depends on both the Co and the Ni layer thicknesses over a large range and follows the behavior expected from the model in the coherent regime. The strain in the Co/Ag multilayers is proportional to 1/${\mathit{t}}_{\mathrm{Co}}$ and independent of the thickness of the Ag layers. This behavior resembles that for the incoherent regime. For Co/Cu multilayers the results indicate that the multilayers are in the transition region from coherent to incoherent behavior.

We present an analytical solution for the dielectric response of infinite and semi-infinite superlattices with the constituent dielectric function ε(ω,z) being an arbitrary periodic function of one coordinate z. The long-wave limit in the local theory is used. All results are expressed in terms of the two bulk quantities, namely, the average over one period of the functions ε(ω,z) and 1/ε(ω,z). The damping of the bulk and the surface plasmon modes specific for superlattices with a continuously varying constituent dielectric function is obtained and discussed. Our theory provides deeper insight into the role of the local-field effects in the dielectric response of a superlattice.

The structural properties of Co/Cr(001) superlattices grown by molecular-beam epitaxy have been studied by means of grazing-incidence x-ray-scattering techniques. From the in-plane diffraction pattern, we find that hcp-Co(11.0) is epitaxially grown on Cr(001) with the Co[00.1] axis parallel to Cr[110]. This epitaxial relation is equivalent to the Pitsch-Schrader orientational relationship. The Co layers exhibit a precursive structural phase transition from hcp to bcc with decreasing Co layer thickness ${\mathit{t}}_{\mathrm{Co}}$. First, the Co in-plane unit cell changes its shape from rectangular (hcp) to square (bcc), due to the epitaxial strain. Second, the intensity I of the hcp-Co{1¯1.1} spots shows a continuous decrease with decreasing ${\mathit{t}}_{\mathrm{Co}}$. The intensity I is proportional to the square of the displacement δ of the Co atoms from the bcc center sites, which is the order parameter for this structural transition. This intensity decrease therefore implies that the Co lattice continuously changes its symmetry from hcp to bcc. The experimentally determined ${\mathit{t}}_{\mathrm{Co}}$ dependence of δ is in good agreement with a model calculation describing the competition of the surface potential on Cr(001) and the bulk hcp potential of Co. Finally, we find that the observed anomalous out-of-plane expansion of the Co layers is a precursor of the structural transition. The non-Poisson-like behavior can be explained by a rigid-atom model which takes into account the atomic displacement during the structural transition.

The electronic and geometrical structure of Cu microclusters is investigated utilizing a parametrized tight-binding linear-muffin-tin-orbital (TB-LMTO) method in combination with the real-space recursion technique. The parameters of the TB-LMTO Hamiltonian are derived from ab initio self-consistent k-space TB-LMTO calculations for the corresponding cluster material in its bulk phase for varying lattice constants. It is found that Cu clusters exhibit certain similarities to simple-metal clusters with respect to the gross electronic features: The calculated density-of-state (DOS) spectra exhibit exceptionally large gaps for sizes, corresponding to the well-known magic numbers of the spherical jellium model. Likewise, this shell structure shows up as pronounced drops at the magic sizes in calculated ionization potentials, in agreement with recently published experimental data. For the geometrical structure of Cu clusters we find high-symmetry arrangements to be Jahn-Teller unstable. The geometry of the magic ${\mathrm{Cu}}_{20}$ cluster, obtained by means of the simulated annealing strategy, exhibits a high sphericity, but low overall symmetry. The calculated root-mean-square bond-length fluctuation in ${\mathrm{Cu}}_{20}$ qualitatively matches the results from molecular dynamics simulations for other materials. Due to the dramatically growing computational difficulties with increasing cluster size, for larger clusters with sizes N>55, only uniformly relaxed fcc structures were examined. For such geometries, the DOS spectra in the range N≊500 are found to be almost fully converged to the corresponding k-space result of solid Cu.

We study the phonon-focusing effects of the Rayleigh and pseudosurface waves propagating ballistically on the (100) surface of cubic crystals. Specifically, solving the wave equations for lattice displacements, we consider the finite-wavelength effect at the MHz-frequency range and for 1-cm propagation distance. The amplitude and polarization of lattice displacements at the surface associated with surface waves are sensitive to the propagation direction. Owing to this characteristic of surface waves, the focusing factor defined in the ray picture does not necessarily describe correctly the intensity of surface waves excited from a point source. The angular dependences of the calculated displacement amplitudes explain an important feature of the recent focusing experiment of Rayleigh waves, which the simple theory based on the focusing factor fails to predict.

Ultrathin films of Co have been grown by slow vapor deposition on an FeAl{001} substrate (lattice constant ${\mathit{a}}_0$=2.904 Å). Up to a thickness of about 13 Å the films are found to be pseudomorphic with the substrate, but contain 5–10 at. % Al. Quantitative low-energy electron diffraction (LEED) finds the best-fit model for the films to be pure Co with body-centered-tetragonal structure, bulk interlayer spacing 1.40±0.03 Å, and first interlayer spacing 1.25±0.03 Å, suggesting that the Al impurities were disordered and contributed only to the background of the LEED pattern. Strain analysis gives ambiguous results with regard to the equilibrium phase, as both fcc and bcc Co (metastable phases) have strain ratios within acceptable limits. The strains themselves are unusually large for fcc Co and fairly small for bcc Co, but the Poisson ratio has normal values for fcc Co and unusually small values for bcc Co. The contraction of the first interlayer spacing is abnormally large (∼11%) for magnetic materials.

The c(2×2) low-energy electron-diffraction (LEED) pattern observed on Co films thicker than about 13 Å on FeAl{001} is shown to be caused by a strained hexagonal Co(112¯0) film. The film is strained differently in the two orthogonal directions within the plane of the interface: it is expanded by 0.91% in one and compressed by 5.4% in the other direction. As a consequence, the bulk interlayer spacing, as determined by quantitative LEED, is expanded by 2.9% with respect to the spacing in the unstrained hexagonal-close-packed phase (1.2535 Å). This is a more general case of pseudomorphic epitaxy than when film and substrate have similar unit meshes, and requires a more elaborate strain analysis. Such strain analysis finds an interlayer strain of 3.35%, in satisfactory agreement with the quantitative LEED result. It is noted that the strained Co(112¯0) film does not grow directly onto the FeAl{001} substrate, but rather on a strained Co{001} film, 8–10 layers thick, which grows first on the substrate and is analyzed in a previous paper.