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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.

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.

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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.

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