The change in the average surface-atom core-level shift (SCS) produced by submonolayer coverages of alkali adsorbates is quite small for both Ta(110) and W(110), less than 45 meV in magnitude. The small change of the measured SCS’s as a function of coverage decisively supports the covalent-bonding picture of alkali adsorption proposed by Ishida and Terakura.

Although most low-energy-electron-diffraction experiments show that transition-metal surfaces usually present a contraction of the first interlayer spacing, exceptional outward relaxation has been observed for the (001) surfaces of Pd and Rh which both have a fcc structure. These experimental data are not yet understood since it is generally believed, on the basis of simplified tight-binding models, that d valence electrons, like sp electrons, act to produce an inward relaxation. Using an accurate tight-binding theory, we show here that on the (001) face of fcc transition metals with a nearly filled d band, the role played by the d electrons is reversed and thus competes with that of sp electrons. This provides an interpretation of experimental results on Pd and Rh(001) surfaces.

The dielectric behavior of hydrogenated amorphous carbon samples, prepared by keV-Ar-ion bombardment of a polystyrene matrix, in the energy range 0–100 eV has been studied in connection with the change in hydrogen content. The wide-ranging results obtained by means of electron-energy-loss spectroscopy in reflection mode have been successfully compared with the optical constants available in the 1–6 eV range, resulting from the combined use of transmittance and reflectance experimental spectra. A strong rearrangement of the σ→${\mathrm{\sigma }}^{\mathrm{*}}$ transitions is evidenced in the bulk loss function Im(-1/ɛ̃) together with a change in the relative oscillator strengths, on going from pure amorphous carbon to the most hydrogenated (${\mathit{x}}_{\mathit{H}}$=40%) α-C:H sample.

Configuration-interaction (CI) calculations are performed on Na clusters of up to 20 atoms within the spherical jellium model, with particular attention paid to the magic clusters with N=2, 8, and 20. The interacting valence electrons are assumed to move in the Coulomb field of the jellium core. The numerical work is carried out by the nuclear-structure code oxbash modified to handle LS coupling. The many-particle bases are constructed of harmonic-oscillator single-particle states extending over 11 major shells and, alternatively, of single-particle states generated by the local-spin-density approximation (LSDA). The calculated quantities include ground- and excited state energies, ionization potentials, and photoabsorption cross sections. The microscopic structure of the dominating plasmon excitation is studied in neutral and ionized clusters. Results of the LSDA, where applicable, are found to be in substantial agreement with the CI results. Convergence problems are addressed and are found hard to overcome for the larger clusters studied. Tamm-Dancoff approximation and random-phase approximation results are simulated by suitable restrictions of the CI space.

The conversion between polarization states upon reflection on the weakly corrugated interface between uniaxial crystals and isotropic media is studied by means of a model based on the Rayleigh hypothesis. We deal with sinusoidal gratings with the wave vector of the incident light being perpendicular to the grating grooves. Two situations are considered: (i) incidences from an isotropic, lossless dielectric onto a crystal and (ii) from a crystal onto a metal. The results show that in the first case total reflection plays an important role in polarization conversion, even for flat interfaces, where nearly 54% of s-to-p conversion can be attained, and that a weak corrugation can increase this value slightly. On the other hand, for metal-crystal interfaces illuminated under conditions of resonant excitation of surface plasmons, corrugation has a much stronger effect in the change of polarization of the incident wave. It is shown that the efficiency of conversion between ordinary and extraordinary modes can increase more than 250 times when the value of the groove height-to-period ratio is varied from 0 to 0.2. This effect is accompanied by a strong absorption in the metal and by narrow dips in the curves of copolarized efficiencies as functions of the angle of incidence.

Using ab initio electronic structure calculations, we study a cluster-assembled form of carbon which is based on ${\mathrm{C}}_{28}$ units arranged in a diamond crystal. We investigate how the charge density and electronic spectrum of an isolated ${\mathrm{C}}_{28}$ unit is altered in the infinite lattice. We analyze the nature of electronic states near the Fermi level of both an isolated ${\mathrm{C}}_{28}$ cluster and its solid form. A tight binding model, with parameters fitted to reproduce energy eigenvalues calculated from first principles, provides insight on how the states of thin structure are derived from those of ${\mathrm{C}}_{28}$.

The collective excitations associated with density fluctuations of a system of electrons on nanoscale tubules when a constant magnetic field is present along the tube axes are presented. Several types of acoustic and optical magnetoplasma modes are found. The Kohn anomalies associated with the static density response for appropriate particle-hole excitations across the Fermi surface are also discussed. When the spin-paramagnetic contribution is ignored, it is found that the Fermi energy, magnetization, magnetic susceptibility, and the collective properties are periodic in the magnetic flux enclosed by the cylinder. For these to be relevant to graphitic tubules, which are thought to be obtained by rolling two-dimensional graphite sheets into cylinders, their inherent helical structures must be incorporated into the theory. It is shown that a transformation of the cylindrical wave states to helical wave states allows these results to be made relevant to nanoscale tubules with helical symmetry.

Photoelectron spectroscopy has been used to determine the ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ phase diagram for 0≤x≤6 and to investigate the electronic properties of the stable phases. Core-level analysis and vacuum distillation experiments indicate equilibrium ${\mathrm{K}}_1$${\mathrm{C}}_{70}$, ${\mathrm{K}}_4$${\mathrm{C}}_{70}$, and ${\mathrm{K}}_6$${\mathrm{C}}_{70}$ phases at 300 K. They show a eutectoid transformation from a high-temperature ${\mathrm{K}}_3$${\mathrm{C}}_{70}$ phase into ${\mathrm{K}}_1$${\mathrm{C}}_{70}$ and ${\mathrm{K}}_4$${\mathrm{C}}_{70}$ at 440 K. Studies in the low-doping regime demonstrate that K incorporation raises the ordering transition temperatures relative to those observed for pure ${\mathrm{C}}_{70}$. These stabilizations are discussed in terms of peritectoid transformations. The K-${\mathrm{C}}_{70}$ results are then examined in the context of the alkali-metal fullerides of ${\mathrm{C}}_{60}$ to obtain a broader view of the thermodynamics of both systems. Valence-band photoemission results demonstrate that the singly and doubly degenerate lowest unoccupied states of ${\mathrm{C}}_{70}$ are filled upon doping to x=6. They also demonstrate that none of the K-${\mathrm{C}}_{70}$ phases are metallic, despite odd-integer filling, with band splittings that are related to reduced crystal symmetry and electron-electron correlation. Finally, the deposition of small quantities of ${\mathrm{C}}_{70}$ onto a potassium film at 40 K demonstrates that the ionization state for ${\mathrm{C}}_{70}$ molecules can be increased beyond 6.

Electronic structures of metallic ${\mathrm{LaNiO}}_3$ and insulating ${\mathrm{NdNiO}}_3$ are investigated using electron spectroscopies as well as band-structure and model-Hamiltonian calculations. It is shown that there is an increase in the transfer integral, ${\mathit{t}}_{\mathit{p}\mathit{d}}$ in ${\mathrm{LaNiO}}_3$ compared to ${\mathrm{NdNiO}}_3$ though ${\mathit{U}}_{\mathit{d}\mathit{d}}$ and Δ are very similar. A phase diagram obtained with a multiband Hubbard model explains the metal-insulator transition as well as the anomalous resistivity of ${\mathrm{NdNiO}}_3$ under pressure while indicating various interesting possibilities near the metal-insulator phase line.

We analyze the two-dimensional potential around a coated cylinder placed in a nonuniform field. In two special cases the system behaves as if the core of the cylinder were enlarged and the shell absent. These are when the shell dielectric constant is the negative of either the core dielectric constant or the matrix dielectric constant. When the shell dielectric constant has a small imaginary part, the field can exhibit large fluctuations which remain localized near the surface of the coated cylinder.

Conventional molecular-beam epitaxy of Si(001) at low temperatures proceeds epitaxially up to a finite thickness followed by a crystalline-to-amorphous transition. Concurrent low-energy ${\mathrm{Ar}}^{+}$ ion irradiation during deposition results in an increase in epitaxial thickness. Surface smoothing is shown to be the primary effect of ${\mathrm{Ar}}^{+}$ ion irradiation. A possible pathway to the formation of amorphous silicon is the nucleation of twin boundaries on {111} planes. The intersection of a twin boundary with other {111} or {001} planes results in the formation of five- and seven-member rings which leads to the crystalline-to-amorphous transition.

Within the framework of the effective-mass approximation, we have used an improved three-parameter trial function to calculate the exciton binding energy for both the ground state and the excited states in a symmetric double quantum well. When comparing our results with those derived with the commonly used one-parameter trial function, we found that the binding energies are improved significantly, especially for the excited states. Our calculated exciton transition energies agree well with the measured photoluminescence-excitation (PLE) spectra. We have also calculated the oscillator strengths. By comparing our results with the oscillator strength derived from the PLE spectra, we find good agreement for heavy holes, but less satisfactory agreement for light holes.

Girifalco’s intermolecular potential between two ${\mathrm{C}}_{60}$ molecules has been used, in conjunction with molecular dynamics, to study the stability of small (${\mathrm{C}}_{60}$${)}_{\mathit{N}}$ clusters up to N=25. In agreement with experiment, 13-membered clusters were found to be particularly stable. The melting behavior in these clusters differs from that characteristic of inert gas clusters.

Using the Voter and Chen version of the embedded-atom model, we have carried out molecular-dynamics simulations to study the structures and melting of Ni, Pd, Pt, Cu, Ag, and Au clusters in the size range N=2–23. The model predicts that all these clusters have structures based on icosahedral packing and similar thermodynamic behavior. Salient results are that all the 13-atom clusters have high melting temperatures, and that a premelting phenomenon occurs in the 14-atom and 20-atom clusters, which have a single atom outside a very stable structural core. Specific studies for Ni demonstrate that the premelting phenomenon also occurs in ${\mathrm{Ni}}_{15}$, ${\mathrm{Ni}}_{16}$, and ${\mathrm{Ni}}_{17}$, which have, respectively, two, three, and four atoms outside the ${\mathrm{Ni}}_{13}$ core.

In a recent paper [Phys. Rev. B 46, 15 586 (1992)], Hayden et al. found a weak in-plane dispersion of the lowest valence subbands in (311)A quantum-well systems by magnetotunneling spectroscopy. There is only qualitative agreement between the experimental and calculated dispersions. We point out that this actually implies that the observed interface corrugations govern the unusual electronic properties of non-(100)-oriented heterostructures and hence have to be taken into account in the interpretation of the experimental data and especially in the calculations.

The criticism of Nötzel and Ploog is refuted.

We present results from ab initio calculations of the full inverse dielectric matrix of beryllium, from which we obtain the dynamic scattering cross section. This is shown to be in excellent agreement with recent inelastic x-ray-scattering experiments, both in overall shape and in precise details of the fine structure. As the calculations have been performed entirely within the random-phase approximation, this presents conclusive evidence that band-structure effects are responsible for the observed structure.

We present a tight-binding model for silicon which incorporates two-center intra-atomic parameters. The model is fitted to density-functional-theory band structures for silicon in the diamond structure over a number of volumes. It is shown that with only a two-center, orthogonal basis, reasonable total energies can be obtained for many different structures. Thus it eliminates the need to use structure-dependent terms in the total-energy model.

Antidot lattices with a finite number of periods are fabricated such that the size of the total system is smaller than the phase-coherence length of the electrons at low temperatures. The magnetoresistance is dominated by reproducible quantum fluctuations as well as by classical commensurability oscillations. We attribute these fluctuations to electrons that travel phase coherently over the entire system and interfere with each other under the influence of the geometry of the potential landscape. The presence of Aharonov-Bohm oscillations is identified and related to the existence of classically pinned electron orbits.

Quantized conductance is observed in an InAs/AlSb ballistic constriction with channel length of 1.0 μm, at 4.2 K. In this structure eight conductance steps are detected, all within 20% of their theoretical values for purely ballistic transport. Compared to a shorter constriction (L=0.20 μm) located on the same mesa, relatively little degradation of the quantization is observed for the longer device. Possible reasons for the enhanced length performance of InAs/AlSb constrictions compared to similar structures fabricated in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures are discussed.