Single-crystal silicon films grown at 400 °C on Si(111):B(√3 × √3 ) are rotated 180° about the surface normal with respect to the substrate. We discuss a mechanism based on chemical effects due to the boron (√3 × √3 ) reconstruction that favors the film to grow with a B-type (twin) orientation. Films grown on the Si(111)-(7×7) reconstruction under identical conditions have the A-type (untwinned) orientation.

We calculate scanning-tunneling-microscope images of graphite, for various one-atom tips. The results deviate from the s-wave tip model, in some cases severely, depending sensitively upon the tip atom. For relevant atoms, the corrugation of the image is less than 0.3 Å. The small size of the corrugation suggests that mechanical distortions, which enhance the apparent corrugation in experiments with graphite, are even more ubiquitous than has been recognized.

Raman scattering from square and cubic site-percolation lattices has been calculated and a linear scaling for the Raman coupling coefficient is found. Our results show the fluctuation origin of Raman scattering, thereby invalidating the models used so far which are based on scaling properties of the local strain and on the existence of a smooth vibrational wave function.

The X-band optically detected magnetic resonance of poly(3-hexylthiophene) and poly(3-dodecylthiophene) films and solutions are described and discussed. The photoluminescence-enhancing features include (i) a strong, ∼15-G-wide, asymmetric peak at g≃2.003; (ii) a weak peak at g≃4.07; and (iii) a weak, ∼800-G-wide structure around g≃2. It is believed that feature (i) results from polaron recombination, and the others from the decay of two distinct intrinsic triplet polaronic excitons. The levels of all are apparently close to the band edges.

The Sn DX center in GaAs, a deep donor state of Sn, has been observed by Mössbauer measurements at high pressure. The size of the pressure-induced Sn DX Mössbauer resonance compared to the net conduction-electron concentration at zero pressure provides evidence that the Sn DX center localizes two or three electrons in the ground state.

We present a method for performing fully self-consistent, ab initio, self-interaction–corrected, local-spin-density calculations. We demonstrate that the formalism correctly predicts that MnO, FeO, CoO, NiO, and CuO are antiferromagnetic insulators, and that VO is a nonmagnetic metal. The band gaps and moments are drastically improved compared with the local-spin-density approximation.

We analyze the bulk and surface spin-wave modes of a semi-infinite Fe/Gd superlattice. If an externally applied magnetic field is increased from zero, one surface mode is driven soft, indicating a surface phase transition which occurs at a field value about 5 times lower than that necessary to cause a bulk phase transition. The phase transition is shown to nucleate at the surface and has a large penetration depth (on the order of a few hundred angstroms) even for fields just slightly above the value needed to induce the transition.

A new phase of Pd with an expanded lattice (5%) was grown on Fe(001). Ultrathin Fe/Pd and Pd/Fe bilayers and Fe/Pd/Fe trilayers were studied using ferromagnetic resonance and Brillouin scattering. Pd interlayers always couple the Fe layers ferromagnetically. For thicknesses up to four monolayers the Pd is ferromagnetic, in agreement with theory. One additional atomic layer of Pd destroys the ferromagnetism, but the Pd still exhibits a fluctuating magnetic moment partly polarized by the exchange field from adjacent Fe layers. The magnetic moments in the interfaces were determined from the ferromagnetic-resonance absorption intensities.

We have fabricated epitaxial, nonsymmetric M×N superlattices in which ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ (YBCO) layers either M=1, 2, 3, 4, or 8 c-axis unit cells thick are separated by insulating ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ (PrBCO) layers N unit cells thick (N=1 to ∼16). The zero-resistance transition temperature, ${\mathit{T}}_{\mathit{c}0}$, initially decreases rapidly with increasing PrBCO thickness but saturates at ${\mathit{T}}_{\mathit{c}0}$∼19 K or ${\mathit{T}}_{\mathit{c}0}$∼54 K for 1- and 2-cell-thick YBCO layers, respectively. We conclude that although intercell coupling may enhance ${\mathit{T}}_{\mathit{c}0}$, it is necessary for superconductivity of single-cell-thick YBCO layers in PrBCO matrix.

Flux pinning in Y-Ba-Cu-O crystals is studied as a function of fluence of 3-MeV protons, which create random local defects. Order-of-magnitude increases in the critical current density are deduced from magnetic hysteresis loops, with values up to 2×${10}^5$ A/${\mathrm{cm}}^2$ observed at 77 K and 1 T. However, the irreversibility line in the field-temperature plane and the pinning potentials deduced from flux-creep studies are hardly changed. These results are compared to melting and pinning models.

Two crossed microwave pulses are used to generate plasma in their intersection region inside a chamber. It is shown that the frequency of each output pulse is up-shifted. The percentage of frequency shift is consistent with the theory of Wilks, Dawson, and Mori. Besides frequency shift and some attenuation, the similarity between the spectral distributions of the output and input pulses indicates that there is very little distortion to the pulse shape.

A buckling instability has been discovered in monolayer network structures. The instability, which can be seen by fluorescence microscopy, occurs when a network formed in the gas–liquid-expanded (LE) two-phase region is heated. It is interepreted as an Euler instability that arises because of constraints on the monolayer imposed by the network and by the interactions between domains of the LE phase. An analysis in terms of a Langevin equation with a Ginzburg-Landau free energy shows how the instability is related to the equilibrium and transport properties of the monolayer.

Combining neutron and Raman spectroscopies, we identify a succession of regimes in the vibrational dynamics of two aerogels of different microstructures. Aside from the phonon-fracton and fracton-particle mode crossovers, we find an additional crossover that suggests two distinct elastic regimes in the fracton range, which we associate with bending and stretching. Greatly different Debye-Waller factors are found in the two regimes, distinguishing them unambiguously in neutron spectra.

We demonstrate three-dimensional reconstructions of the relative positions of atoms in a crystal by holography with atomically localized incoherent electron sources. Example reconstructions are shown using experimentally measured reflection electron Kikuchi-scattering angular distributions.

We have investigated the behavior of nonconservative (100) antiphase boundaries in L$1_2$-ordered alloys, close to the congruent first-order order-disorder transition. Very accurate inhomogeneous cluster-variation-method calculations in the tetrahedron approximation show that wetting does occur through an infinite series of layering transitions with average logarithmic divergences for the width of the wetting layer and for the excess entropy.

The transition from 2D to 3D growth of Ge on Si(001) has been investigated with scanning tunneling microscopy. A metastable 3D cluster phase with well-defined structure and shape is found. The clusters have a {105} facet structure. Results suggest that these clusters define the kinetic path for formation of ‘‘macroscopic’’ Ge islands.

The relative contributions to the stopping power of the different charge states for a beam of helium ions moving with velocity v in aluminum are shown, for the first time, from a first-principles calculation. Combining dielectric and density-functional results in the appropriate velocity range with the energy lost in the capture and loss processes we have obtained good agreement with experiment.

We study a class of classical wave-propagation phenomena in which a multiply scattered wave in a random medium is directed along a certian spatial direction. Such a situation arises in the random scattering systems in which the scattering potential is slowly varying in the z direction. In 2D, we find a superdiffusive transverse displacement of the center of the transmitted beam ${\mathrm{\chi }}_{\mathrm{c}}$∼${\mathrm{z}}^{\mathrm{v}}$ with v≈14, while the width of the beam scales as Δχ∼${\mathrm{z}}^{\mathrm{v}\prime }$ with v′≈12. In 3D, we find a transition: v≈v′≈12 for weak disorder, while v′≈0.46 and v≈0.66 for strong disorder. © 1990 The American Physical Society

We study a two-dimensional Fermi gas with an arbitrary short-range repulsive interaction. We show that the vertex part has an unusual singularity in the particle-particle channel for all momenta q<2${\mathit{k}}_{\mathit{F}}$, which is not found in higher dimensions. We interpret this as a collective mode representing a bound excitation of two holes. This mode, however, does not lead to an instability, or to a breakdown of Fermi-liquid theory in the low-density limit. The resulting Fermi-surface phase shift vanishes, and a ‖ω${\mathrm{\Vert }}^{5/2}$ correction to Σ’’(${\mathit{k}}_{\mathit{F}}$,ω) is obtained. The 2D case is also contrasted with a similar calculation in 1D.

Consider a local exchange potential v${\mathrm{¯}}_{\mathit{x}}$(r) which depends on a set of orbitals {${\mathrm{\phi }}_{\mathit{i}}$}. It is proven that v${\mathrm{¯}}_{\mathit{x}}$ is the exact Kohn-Sham density-functional exchange potential, when {${\mathrm{\phi }}_{\mathit{i}}$} is optimum, if and only if ${\mathit{v}}_{\mathit{x}}$ satisfies three specified conditions. Exchange potentials are presented which satisfy two of the conditions, one of which is satisfied neither by the original Slater potential nor by the local-density approximation.