A pseudopotential local-density calculation is performed for YH3 to study the unusual hydrogen displacements previously found in neutron diffraction. These displacements are identified as Peierls distortions associated with (hydrogen) lattice instability in this 3D system. The wave vector of these displacements is close to the vector connecting the electron and hole pockets in the undistorted system. With other electron and hole pockets at GAMMA that still overlap after distortion, the possibility of the existence of an excitonic insulator phase will be discussed.

Energy- and angle-dependent photoelectron cross sections from surface and bulk W(110) 4f7/2 core levels are measured and compared with dynamical multiple scattering calculations. The agreement between experimental and theoretical results is found to be significantly better than corresponding previous studies, permitting a determination of the first layer atomic plane distance: d12 = 2.26 +/- 0.05 angstrom. Forward-scattering enhancements along bond directions are observed under selected scattering conditions. In all cases, final-state multiple scattering accounts for the principal energy and angle dependencies in the cross section. Typical variation of bulk and surface 4f photoelectron intensities with kinetic energy or emission angle resulting from final-state effects is observed to be a factor of 2. This result suggests that previous core-level spectra for stepped W(110) surfaces have been incorrectly interpreted.

The magnetic ordering temperatures T(m) of Pr ions in (R1-xPrx)Ba2Cu3O7 systems (R = Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, Nd and Y) with x = 0.5 - 1.0 were measured. We observe that T(m) decreases monotonically with increasing R concentration. At constant x, T(m) is R ion-size dependent. The slope in the T(m) vs. x is steeper for ion with smaller ionic radius. In comparison with the ion-size effect on the superconducting transition temperatures T(c) in these systems, the observed results can be qualitatively interpreted in terms of the hybridization between the local states of Pr ion and the conduction band states of the CuO2 planes.

By accurately fitting tight-binding parameters to ab initio band structures from 14 different tetrahedral volumes, tight-binding parametric formulas have been developed for silicon and germanium. The distance dependences for these orthogonal, nearest-neighbor parameters range from r-2.5 to r-3.3. Repulsive potentials are added in order to reproduce the total energies for a number of bulk structures. It is found that the repulsive potential needed has the simple form of a pairwise interaction multiplied by a structure-dependent constant. Transferability is shown with good bulk and cluster results.

The structural and electronic properties of hydrogen in yttrium are studied using the pseudopotential method within the local-density-functional approximation (LDA). Different concentration regions are considered for the alpha and beta phases. The binding energies associated with different interstitial sites are evaluated as well as the diffusion energy barrier and local vibrational modes. It is found that the occupation of the tetrahedral site is energetically more favorable than that of the octahedral site in the alpha phase. The calculated vibrational frequency is in excellent agreement with the value observed in neutron scattering experiments. Possibility of pairing is also examined from the consideration of energetics.

We present a method to calculate the full phonon spectrum using the local-density approximation and Hellmann-Feynman forces. By a limited number of supercell calculations of the planar force constants, the interatomic force constant matrices are determined. One can then construct the dynamical matrix for any arbitrary wave vector in the Brillouin zone. We describe in detail the procedure for elements in the diamond structure and derive the phonon dispersion curves for Si. The anharmonic effects can also be studied by the present method.

The structural properties of hexagonal-close-packed yttrium are studied by using the plane-wave basis within the pseudopotential method and local-density-functional approximation. By employing a "soft" pseudopotential proposed by Troullier and Martins, satisfactory convergence is achieved with a plane-wave energy cutoff of 30-40 Ry for this early-transition-metal element. The overall results for the structural properties are in good agreement with experiment. It is found that the charge overlap between core and valence electrons has a substantial effect on the accuracy of the calculated structural properties. Two different calculations are performed with and without the outer-core 4p orbital included as a valence state. In addition, as found in some other local-density calculations, the uncertainty in the results due to different exchange-correlation energy functionals may not be negligible in transition metals.

We have performed tight-binding calculations on a model of an amorphous silicon sample generated previously by a molecular-dynamics simulation employing the Stillinger-Weber potential. The sample consists of 588 atoms and contains a high density of floating-bond defects. Two tight-binding calculations are presented, one using the widely accepted Chadi parameters, which include only nearest-neighbor interactions, and the other using the parameters recently proposed by Allen, Broughton, and McMahan (ABM) [Phys. Rev. B 34, 859 (1986)] for a nonorthogonal basis set. Comparison of the densities of states shows similar behavior in the valence band, but the electron density near a defect is less localized with the ABM parameters. It is also found that the projected density of states on the fivefold-coordinated atoms is very close to that on the fourfold-coordinated atoms, while the projected density of states on the threefold-coordinated atoms is distinctly different and has more states in the gap.