Surface-phonon focusing is examined with laser generation of pulsed surface acoustic waves (SAW’s). Before presentation of the experimental results, the theory of the effect is summarized including both the ray approach and the stationary-phase analysis. To create a point source of SAW’s the focused beam of a Q-switched Nd:YAG (yttrium aluminum garnet) laser is employed. To visualize the SAW amplitude distribution we use dust patterns arising as a result of SAW-induced dust-particle removal from the surface under investigation. The SAW amplitude measurements with the probe beam deflection technique show that the above patterns reflect adequately the SAW amplitude angular dependence. The strong SAW focusing is observed on cubic crystals such as Si, Ge, GaAs, as well as on ${\mathrm{LiNbO}}_3$ crystal. The results obtained are discussed in comparison with the theory.

The structure of epitaxial films of Fe, Co, and Cu grown at 80–300 K on Cu(100) has been investigated using a spot-profile-analyzing low-energy electron-diffraction (LEED) instrument. In all three systems rings appear around the substrate LEED spots, although the rings differ in intensity and in diameter depending upon the variables of film thickness and deposition temperature. Rings of this type have been studied extensively by Henzler et al. and correlated with the mean separation between islands. Much can be inferred about the growth mechanism through a study of these Henzler rings. The rings contract radially with increasing deposition temperature or with increasing annealing temperature as thermally activated diffusion permits the formation of larger islands with greater distances between them. For all three systems studied here, the onset of thermal diffusion becomes apparent as the ring contracts radially for deposition temperatures above about 150 K. However, for deposition at 80 K, where thermally activated diffusion should be completely suppressed, rings are observed with a radius corresponding to a mean separation between islands on the order of ten atoms. Of the three elements, Fe gives the strongest rings and Cu the weakest. The value of the mean separation suggests that upon condensation these atoms do not come to rest at the immediate site of impact but instead experience some very transient type of mobility associated with the impact and accommodation process.

A high-resolution core-level-photoemission investigation of the (100) surface of TiN and TiC is reported. A surface-shifted component is revealed in both the N 1s level on TiN and in the C 1s level on TiC. Using a curve-fitting procedure a surface shift of -0.57 eV is extracted for the N 1s component on TiN and a shift of -0.26 eV for the C 1s component on TiC. No surface-shifted Ti 2p level could be revealed on either TiN or TiC. These results do not agree with earlier calculated results for TiC which predicted no surface shift in the C 1s level but a small positive shift in the Ti s levels. Using a thermochemical model and accounting only for the loss of coordination at the surface, a shift of -0.07 eV is estimated both for the N 1s level in TiN and for the C 1s level in TiC.

We have investigated the condensation energy and the nature of the high-temperature phase of the reconstruction of W(100) by a combined study of static and dynamic properties based on a model-Hamiltonian approach. We find that the critical dynamics are much more sensitive to the energetics than the static properties. The comparison of the theoretical results with existing experimental data yields insights on the energetics of the √2 × √2 R45° reconstruction on clean W(100) and on the crossover from the disordered √2 × √2 R45° phase to the normal (1×1) phase.

Kinetic energies of ${\mathrm{Ar}}^{+}$ ions desorbed by double photoionization at 100 eV from argon layers (from 2 to 20 monolayers) condensed on platinum are measured and compared with predicted values. Unexpected low kinetic energies are explained by a relaxed Coulomb repulsion mechanism. The weakness of the high-kinetic-energy ion signal is considered to be due to the noncrossing of the bound and repulsive potential-energy curves involved in the desorption model.

A general expression for the energy-loss probability in scanning transmission electron microscopy valid for complex microstructures is presented in the framework of classical dielectric theory. Calculations are carried out for small particles half embedded in a planar interface. We used experimental data for the dielectric functions that characterize the media and we take into account the coupling among different multipolar terms. Resonances, present neither in the planar-interface nor in the isolated-sphere energy-loss spectra, are found. The results agree with many experimental results reported in the literature of the last few years. A different behavior between conducting and insulator supports is found. The effect of an oxide coating around the spherical particle is also discussed.

The intercalation of Fe into the van der Waals gap in the 2H phase transition-metal dichalcogenides ${\mathrm{NbSe}}_2$, ${\mathrm{TaSe}}_2$, and ${\mathrm{TaS}}_2$ produces many interesting electronic, magnetic, and structural effects. The scanning tunneling microscope (STM) and atomic force microscope (AFM) prove to be very sensitive to these changes and we report a wide range of results as a function of Fe concentration. All three materials support similar 3${\mathbf{a}}_0$×3${\mathbf{a}}_0$ charge-density-wave (CDW) structures in the pure state at low temperatures. At low concentrations of Fe the CDW superlattice is still strong at 4.2 K and persists to high concentrations of Fe. At high concentrations, the Fe becomes ordered in the octahedral holes in the van der Waals gaps, and superlattices of the form 2${\mathbf{a}}_0$×2${\mathbf{a}}_0$ and √3 ${\mathbf{a}}_0$× √3 ${\mathbf{a}}_0$ are observed. These can be detected at both 300 and 4.2 K. STM spectroscopy at 4.2 K shows that in 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{NbSe}}_2$ and 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaSe}}_2$ the energy gap in the electronic spectrum is initially reduced, but stabilizes at higher Fe concentrations and remains well defined for the ordered 2${\mathbf{a}}_0$×2${\mathbf{a}}_0$ phase. A transition from a CDW to a mixed CDW and spin-density-wave state is indicated, since these high Fe concentration phases are antiferromagnetic. In 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaS}}_2$ both 2${\mathbf{a}}_0$×2${\mathbf{a}}_0$ and √3 ${\mathbf{a}}_0$× √3 ${\mathbf{a}}_0$ superlattices are observed. The 2${\mathbf{a}}_0$×2${\mathbf{a}}_0$ regions show a large energy gap, while the √3 ${\mathbf{a}}_0$× √3 ${\mathbf{a}}_0$ do not. The latter phase is ferromagnetic and would not be expected to exhibit a gap. The development of the electronic structures over the entire range of Fe concentrations has been followed by STM and AFM and can be tracked in detail.

We present ellipsometric measurements of multilayer adsorption of xenon, krypton, and argon on highly oriented pyrolytic graphite along numerous isotherms spanning the coverage range from completion of the first layer to about twelve layers and the temperature range from below the melting of the top layer to above the bulk adsorbate melting point ${\mathit{T}}_{\mathit{m}}$. The three adsorbates have very similar phase diagrams, and all show reentrant first-order layering. The top layer of three-layer and thicker films disorders at 0.81${\mathit{T}}_{\mathit{m}–}$0.83${\mathit{T}}_{\mathit{m}}$. For films thicker than three layers, first-order layer condensation reappears at shifted coverages and chemical potentials in the range 0.83${\mathit{T}}_{\mathit{m}–}$0.87${\mathit{T}}_{\mathit{m}}$ to 0.92${\mathit{T}}_{\mathit{m}–}$0.94${\mathit{T}}_{\mathit{m}}$. The solid adsorbate films reach a limiting thickness of about 12 layers at saturation, but the limiting thickness increases rapidly just below ${\mathit{T}}_{\mathit{m}}$ and reaches the equivalent of about 24 layers in the liquid region. We discuss implications of these results for roughening and melting of the adsorbate (111) surfaces. Chemical potentials for layer condensation are compared to a simple Frankel-Halsey-Hill theory.

We report high resolution low-energy-electron-diffraction measurements on the Pd(110) surface. We demonstrate that the clean surface consists of semiordered islands. The island structure is stable up to 650 °C at which point the island edges roughen and step-step correlations decrease. Above 1100 °C, surface evaporation becomes important and the surface becomes kinetically rough. A simple model is presented that makes use of step-step interactions to generate the periodic island structure. This model predicts that ordered islands form below the roughening temperature if the step creation energy is small compared to the step-step interaction. The existence of isolated steps is shown to be consistent with embedded atom calculations that predict a small step-formation energy on Pd(110) surfaces.

We compute by numerical transfer-matrix methods the surface free energy τ(T), the surface stiffness coefficient κ(T), and the step free energy s(T) for Ising ferromagnets with (∞×L) square-lattice and (∞×L×M) cubic-lattice geometries, into which an interface is introduced by imposing antiperiodic or plus/minus boundary conditions in one transverse direction. These quantities occur in expansions of the angle-dependent surface tension for either rough or smooth interfaces. The finite-size scaling behavior of the interfacial correlation length provides the means of investigating τ(T), κ(T), and s(T). The resulting transfer-matrix estimates are fully consistent with previous series and Monte Carlo studies, although current computational technology does not permit transfer-matrix studies of sufficiently large systems to show quantitative improvement over the previous estimates.

We calculate the optical response of a crystalline metal with a surface. Our model uses a single Fourier component of bulk potential, an infinite barrier at the surface, and the random-phase approximation to describe the many-body interactions. Within these constraints the calculations determine the full quantum-mechanical response. The results for an applied long-wavelength field parallel or perpendicular to the surface are characterized by the d parameters, ${\mathit{d}}_{\mathrm{\parallel }}$ and ${\mathit{d}}_{\mathrm{\perp }}$. For the latter case we find novel long-ranged oscillations in the induced density near the surface. Although these arise from the simultaneous presence of the surface and lattice scattering, they extend far into the bulk and are incommensurate with the lattice. Using parameters appropriate for Li(110), we find that interband absorption causes both ${\mathit{d}}_{\mathrm{\perp }}$ and ${\mathit{d}}_{\mathrm{\parallel }}$ to have frequency dependences significantly different from the jellium limit.

We present measurements of the low-temperature, T<20 K, conductance, G(T), of amorphous Fe/Si multilayers as a function of the temperature at which they are annealed, ${\mathit{T}}_{\mathrm{ann}}$. We concentrate on two of the multilayers which have 10 Å thick Fe layers and different Si layer thicknesses (32 and 40 Å). At low temperatures, the quantum corrections to G(T) are logarithmic indicating that the individual Fe layers are in the two dimensional limit and are not coupled to one another. Annealing the samples at temperatures as low as 150 °C drives the interdiffusion of the Fe and Si layers. This interdiffusion leads to stronger coupling between the conducting layers and a reduction in their conductances. After an anneal at 350 °C, the conductance of the multilayer sample with the thinner Si layers rises sharply and G(T) acquires a square root temperature dependence. This behavior signals a crossover from two to three dimensional electronic transport.

Several properties of the lowest excited singlet state of the ${\mathrm{C}}_{60}$ molecule are investigated with reference to the Jahn-Teller distortion. We find that the symmetry of the molecular structure in the relaxed excited state is approximately the ${\mathit{C}}_{2\mathit{h}}$ symmetry, though the strict symmetry is that of the inversion group ${\mathit{C}}_{\mathit{i}}$. Also it is found that the most remarkable contribution to the Jahn-Teller distortion is given by the ${\mathit{h}}_{\mathit{g}}$ mode of the lowest frequency and the next by the ${\mathit{h}}_{\mathit{g}}$ mode of the fourth-lowest frequency. Furthermore, we calculate the frequencies of the normal modes in the relaxed excited state. It is shown that the frequency splittings of the ${\mathit{h}}_{\mathit{g}}$ modes of the lowest and the fourth-lowest frequency reach several tens of ${\mathrm{cm}}^{\mathrm{-}1}$. Also it is pointed out that the two ${\mathit{h}}_{\mathit{g}}$ modes are the radial and the tangential mode described by the second-order vector spherical harmonics.

Photoelectron measurements were obtained for Yb-${\mathrm{C}}_{60}$ and the results are compared to those reported for alkali-metal fullerides and Yb isolated in xenon or solid ${\mathrm{NH}}_3$. The valence photoelectron spectra for Yb deposited on ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}$ on Yb show changes in the Yb 4f peaks, and in the ${\mathrm{C}}_{60}$ valence states and a metal-induced state just above the Yb 4f bands. The position of this valence feature is near to that found for an analogous feature in the nonsuperconducting ${\mathrm{K}}_6$${\mathrm{C}}_{60}$ fulleride. Similar features also appear when Yb is solvated in condensed ammonia. The appearance and binding energy of the Yb 4f levels argue that the Yb is largely divalent but has a small trivalent component. A comparison of the data for Yb-${\mathrm{C}}_{60}$ with that for Yb clusters in xenon shows that the changes in the electronic state of the Yb are not those found for small Yb particles in a nonreactive matrix.

The ${\mathrm{C}}_{1\mathit{s}}$ core photoelectron shakeup spectrum of ${\mathrm{C}}_{60}$ has been studied experimentally and theoretically. It is found that the lowest shakeup satellites correspond to global charge-transfer excitations which move charge from the opposite side of the fullerene cage to the core-hole region, while more localized excitations give rise to higher-energy satellites. This finding is in accord with a generalized version of the Mahan-Nozières–de Dominicis model of core excitation, in which a local dielectric response is included with the original adiabatic contribution.

A diffuse-neutron-scattering investigation of the 260 K order-disorder transition in a ${\mathrm{C}}_{60}$ single crystal reveals the presence of ordered clusters within a narrow temperature range above the transition temperature. This cluster state is metastable and gives evidence for heterophase fluctuations. In addition, during an excursion to lower temperatures we investigate regions near reciprocal-lattice points with half-integer indices where recently additional superlattice intensities were reported to have been observed by electron-microscopic methods. At three selected reciprocal-lattice points, i.e., (0.5,0.5,5.5), (2.5,2.5,0.5), and (3.5,3.5,5.5), no superlattice reflections can be found down to 10 K.

The two-body effects for a Mott-Wannier exciton in a magnetic field are investigated. For excitons with nonvanishing pseudomomentum there exists an outer potential well which contains a class of weakly bound and delocalized quantum states. The spectrum and eigenfunctions in this potential well are calculated in a harmonic approximation.

We present a method for the calculation of total energies and forces that scales linearly with the number of atoms in the system. The key points are (i) an unconstrained conjugate gradient minimization of the electronic energy that avoids the need of explicit orthonormalization, and (ii) description of the electrons in terms of localized wave functions, truncated beyond a radius ${\mathit{R}}_{\mathit{c}}$. The method is variational, giving an upper bound to the exact total energy, and is exact as ${\mathit{R}}_{\mathit{c}}$→∞. We test the method for a model tight-binding Hamiltonian, and in full ab initio molecular-dynamics calculations.

The local structural order in chalcogenide network glasses is known to change markedly at two critical compositions, namely, the percolation and chemical thresholds. In the ${\mathrm{As}}_{\mathit{x}}$${\mathrm{Te}}_{100\mathrm{-}\mathit{x}}$ glassy system, both the thresholds coincide at the composition x=40 (40 at. % of arsenic). It is demonstrated that the electrical switching fields of As-Te glasses exhibit a distinct change at this composition.

Hyperfine parameters for nitrogen-pair defects have been calculated using the density-functional theory. The results calculated using a gradient-corrected local-spin-density approximation are in fairly good agreement with the observed ones. It is confirmed that nitrogen-pair defects are the origin of the complicated ESR signal in amorphous silicon nitride prepared at a high deposition rate.