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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 propose in this paper a theoretical model to investigate surface self-diffusion of single adatoms on the face-centered-cubic metals. Calculations are performed on the channeled (110), densely packed (111) and loosely packed (001) surfaces of iridium at temperature T = 800 K. Three realistic model potentials, Embedded Atom method, Sutton-Chen and Rosato-Guillope-Legrand potentials, are applied to describe the interatomic interaction of the adatom/substrate systems. These potentials all involve a few empirical fittings of bulk properties of solid which incorporate with many-body effects. With these potentials, conventional molecular dynamics (MD) is employed to obtain trajectories of the atoms. On the (111) plane, via the Einstein relation, the estimated random walk exponential prefactors and activation energies do exhibit Arrhenius behavior, which are in reasonably good agreement with the experimental results. On the (001) and (110) faces, a number of theoretical evidences for atomic diffusion by exchange mechanism of the adatom with a surface atom are presented, which are again in fairly good agreement with the experiments. In addition, an examination of the exchange diffusion characteristics on several systems (Cu, Rh and Pt) is also presented.

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.