Direct inversion of measured multiple-energy Kikuchi electron patterns from a Si(001) (2 x 1) surface with glancing and normal-incidence geometry shows clear images of the surface dimer and the bulk atoms. The three-dimensional artifact-free real-space images of the atoms contributed from different local emitters are resolved clearly. The observations demonstrate that Kikuchi electron holography has the surface sensitivity and can reveal the atomic structures of complicated multiemitter systems. By changing the collecting angle of Kikuchi electrons, one can selectively image the atoms behind the emitter in the backward direction; thus the surface and the bulk information can be obtained with different collecting angles. Therefore, the potential of Kikuchi electron holography to solve the local atomic structure of the unknown surfaces is high.

We show that the continuous feedback approach is highly effective for controlling chaotic systems. The control design for the Lorenz system is presented as an example to demonstrate the strength of this approach. The proposed control is able to eliminate chaos and bring the system toward any of the three steady states. Two different control input locations are considered. Only one system variable is used in the feedback. The control scheme can tolerate both measurement noise and modeling uncertainty as long as they are bounded.

Direct inversion of measured Kikuchi and simulated photoelectron diffraction patterns shows clear images of the neighboring atoms within the range of the electron mean free path. More than ten nearby atoms are obtained for the Ag(100), Si(100) and (2X1) Na/Si(100) systems by the integral-energy phase-summing method. The key point in removing artifacts is a correct role of background subtraction. When this is achieved, the three-dimensional images are essentially high fidelity and artifact free. This demonstrates that electron-emission holography can be used as a direct local structural probe.

Single differential and total cross sections of the electron-impact ionization of the hydrogen atom are calculated numerically in the two-potential distorted-wave approximation for excess energies from 0 to 1 eV. Partial-wave contributions to the cross sections are also investigated. The near-threshold cross section is parametrized by the power gamma and the proportionality constant a(0) for models with various asymptotic charges, and the dependence of a(0) on the asymptotic charge is studied. The validity range of the threshold power law is also discussed.

The solid-solution phase of hydrogen in hexagonal close-packed yttrium (a-YH(x)) is studied using the pseudopotential method within the local-density-functional approximation with a plane-wave basis. The binding energies associated with different interstitial sites are evaluated for several ordered structures: YH0.5, YH0.25, and YH0.167. It is found that the occupation of the tetrahedral site is always energetically favorable. The hydrogen potential-energy curves around the tetrahedral sites along the c axis and along the path connecting the adjacent octahedral sites are also calculated for YH0.25. In particular, the local vibrational mode along the c axis is estimated to be 100 meV, in excellent agreement with that measured in neutron-scattering experiments. Finally, the intriguing pairing phenomenon is investigated by calculating the total energy for various pairing configurations. The possibility of pairing between nearest-neighbor tetrahedral sites is excluded due to the high energy. It is found that the pairing of hydrogen across a metal atom is indeed energetically favorable compared with other kinds of pairs considered and also with isolated tetrahedral hydrogen atoms. The connection with the electronic structure of the system is also examined.

The phase stability is studied for the beta-phase YH2+x system based on first-principles total energy calculations. Our study predicts that the D0(22), ''40'', and D1a structures are stable near x = 0. 25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the D0(22) and ''40'' ordered phases is calculated by the cluster variational method. The calculated order-disorder transition temperature at x = 0.1 for the D0(22) structure is around 280 K, which is consistent with the recent observation of the metal-semiconductor transition near 230-280 K and resistivity anomalies near 200-250 K for the system with x near 0.1 [Daou and Vajda, Phys. Rev. B 45, 10 907 (1992)].

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.

We conducted a detailed study of the structure and magnetic properties of (R1-xPrx)Ba2Cu3O7 sintered samples, where R = Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm, and Nd for x = 0.5-1.0. We found that the temperature dependence of the dc susceptibility follows the Curie-Weiss law in the temperature range 20-300 K and the paramagnetism of the Pr and R sublattices exist independently of one another. The antiferromagnetic ordering temperature T(N) of Pr ions decreases monotonically with increasing R concentration (1-x). At a given x, T(N) is R-ion-size dependent. The slope in the T(N) vs x curve is steeper for ions with smaller ionic radii. The observed results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO2 planes.

We obtained highly resolved and artifact-free 3D holographic images reconstructed from measured Kikuchi electron (quasi-elastic electron) diffraction patterns with contributions from different emitters. Direct inversion of Kikuchi patterns with glancing incidence geometry shows clear images of the surface dimer and the bulk atoms of Si(001)(2 x 1) surface. This observation demonstrates the applicability of electron-emission holography to complicated systems that contain more than one emitter. This work also demonstrates the surface sensitivity of Kikuchi electron holography.

We present the calculation of the full phonon spectrum for silicon and germanium with the pseudopotential method and the local-density approximation without using linear-response theory. The interplanar-force constants for three high-symmetry orientations [(100), (110), and (111)] are evaluated by supercell calculations using the Hellmann-Feynman theorem. By considering the symmetry of the crystal, three-dimensional interatomic-force-constant matrices are determined by a least-squares fit. Interactions up to the eighth nearest neighbors are included. The dynamical matrix, which is the Fourier transform of the force constant matrix, is hence constructed and diagonalized for any arbitrary wave vector in the Brillouin zone, yielding the phonon dispersion. In this paper we will present the calculation details and discuss various aspects of convergence. Phonon dispersions of Si and Ge calculated are in excellent agreement with experiments.

We demonstrate a direct surface structural tool with high resolution of approximately 1 angstrom in all directions by direct Fourier transformation of measured Kikuchi patterns using a multiple-energy phase-summing method. In this method, with an integral over continuous energy spectra in each direction, both the forward- and backward-scattering oscillations are selected for Fourier transformation by varying the energy range and size of the grid. High-fidelity and artifact-free three-dimensional images of Ag atoms for (100) and (111) single-crystal surfaces are obtained.

We propose a simple method to evaluate the energies of ideal metal surfaces as a function of orientation based on cluster energy expansion. By symmetry only clusters with even-number corners will be present. It is found that the energy expansion converges rapidly and in most cases can be truncated at the pair interaction level. The parameters can be determined from a limited number of low-index surface energies obtained from first-principles calculations. The equilibrium crystal shape at T = O is then predicted and the step energy on major facets is derived for some fee metals.

The cubic YH2+x system with an extended hydrogen composition is studied using the pseudopotential method and the local-density-functional approximation with a plane-wave basis. The study focuses on the beta phase with the metal atoms forming a face-centered-cubic lattice and the octahedral sites partially occupied by hydrogen for 0 < x < 1. The self-consistent total-energy calculation is performed by employing the supercell modeling method. The structural property, in particular, the volume contraction with increasing x, is investigated by analyzing the energy changes for different site occupation. It is found that the lattice contracts mainly to increase the interaction of the additional electron and the metal d potential. In addition, the (420)-plane ordering of the x-excess hydrogen is examined for YH2.25 and is confirmed by energetics studies.

The asymmetry in the phase diagram of the H/Ru(001) system is studied by assuming a lattice gas model for the chemisorbed hydrogen and using the cluster variation method. Ground state analysis of the ordered structures shows that the effective pair interaction for the next-nearest neighbors has to be repulsive. We also found that the order-disorder transition temperatures and hence the phase diagram are very sensitive to upsilon3, the ratio of the effective next-nearest to nearest neighbor interactions of H adatoms. The asymmetry in the phase diagram, which cannot be accounted for by the adsorbate relaxation model by Persson [Surf. Sci. 258 (1991) 451], is attributed to the coverage dependence of the effective pair interactions. By assuming a simple piecewise linear dependence of upsilon3 on the chemical potential, we constructed an asymmetric phase diagram which is in excellent agreement with the experimental data. The model studied can be applied to the H/Pd(111) system directly and can be easily generalized for other close-packed metal surfaces.

Using tight-binding models, the energies of a number of silicon and germanium (111) surfaces are studied. These include reconstructed surfaces with dimers and stacking faults (DS), simple adatom surfaces such as 2x2 and c(2x8), and more complicated cases with dimers, adatoms, and stacking faults (DAS). For reconstructed surfaces containing adatoms, it is found that a simple correction term dependent on the adatom concentration is needed in the present total-energy model to account for the unusual geometry. Similarities between the silicon and germanium reconstructions are seen and compare well with ab initio results. There are also some differences between silicon and germanium, for example, the DS surfaces are lower in energy than the relaxed (1x1) for silicon, but higher for germanium. Si(111) reconstructs into the DAS structure while Ge(111) goes to the simple adatom c(2x8) surface. The c(2x8), 7x7 DAS, (1x1), and 7x7 DS surface reconstructions of Ge(111) were studied with in-plane strain. For these surfaces, a strain of about 2% was sufficient to make the 7x7 DAS/DS surface lower in energy than the c(2x8)/(1x1) surface. An analysis of the energy per atom showed that the dimer-row and associated first-layer atoms played a major part in the differing energy behavior, in agreement with an earlier proposal. An expansive strain was applied to the 2x2, 7x7 DAS, (1x1), and 7x7 DS surface reconstructions of Si(111). With a strain of about 2.5% the adatom surfaces switched relative energies, while the adatom free surfaces required only about 1.5% strain. As for germanium, the dimer-row and associated atoms were of major importance in the differing energy change.