Hisashi Yamazaki

Structure analysis of oxygen-adsorbed tungsten (001) surface

Abstract Structure of the tungsten (0xa00xa01) surface annealed at 1200 K for the saturation coverage of oxygen, W(0xa00xa01)-2×1–O, has been studied by low-energy electron diffraction (LEED) and the symmetrized automated tensor LEED program. The optimized structure is that with missing rows of tungsten atoms and double rows of oxygen atoms. The layer separation between the oxygen atom and the topmost tungsten atom layer is 0.50±0.06 A, the distance between oxygen rows is 2.4±0.3 A and all other separations between tungsten atoms are the same as bulk values. The distance between the oxygen atom and the tungsten atom agrees well with sum of atomic radii of oxygen and tungsten. Oxygen atoms are located in the threefold hollow site at the tungsten (0xa01xa01) facet appearing on the tungsten (0xa00xa01) surface with missing rows.

Structure analysis of W(0 0 1)2 × 1–O surface at room and liquid nitrogen temperatures

Abstract Surface structure of O-adsorbed W(0xa00xa01) surface after annealing to 1200 K has been analyzed by low energy electron diffraction at 77 K as well as at room temperature. The optimum structure has tungsten missing rows and oxygen double rows. Furthermore, the R-factor is minimized at the structure that O atoms are adsorbed on one of the two different threefold hollow sites of the (1xa01xa00) facet appearing on the W(0xa00xa01)2xa0×xa01 with missing row. However, the results suggest that two domains of O atoms adsorbed on both the two different threefold hollow sites coexist. Then, I–V curves have been analyzed as a function of the mixing ratio of the two domains having different O adsorption sites at room and low temperatures. The energy difference between these two sites has been estimated to be 6.5 meV from the temperature dependence of the mixing ratio.

Graphite formation on natural diamond (111) surfaces by electron irradiation and heat treatment

Abstract The surface structure of a natural diamond (111) surface has been investigated by using low-energy electron diffraction (LEED) and electron energy-loss spectrometry (EELS). The sample was irradiated by an electron beam of 100 eV and of 2000 eV for 24 h in sequence prior to heating at 1400 K. At each stage of these procedures, observation and measurement were performed. The EELS spectrum was analysed by a nonlinear least-squares curve-fitting method. At the final stage, the spectrum showed peaks characteristic graphite (6 and 27 eV) while those of diamond were diminished to a negligible height (23 and 34 eV), and the LEED pattern became very complicated with a close similarity to that of monolayer graphite on metal carbide. We conclude from these facts that monolayer graphite is formed on the diamond (111) surface by the electron irradiation and the subsequent heat treatment.

Electronic structure of B 2 p σ and p π states in MgB 2 , AlB 2 , and ZrB 2 single crystals

The effect of electron correlation (EC) on the electronic structure in MgB 2 , AlB 2 , and ZrB 2 , is studied by examining the partial density of states (PDOS) of B 2pσ and pπ orbitals using the polarization dependence of x-ray emission and absorption spectra. The discrepancies between observed and calculated PDOSs cannot be attributed to EC effects. The present results suggest that the EC effect is less than the experimental error (∼0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.

Molecular orbital study of nitrogen chemisorption on tantalum (100) and (110) surfaces

Abstract The electronic structures and binding energies of a nitrogen atom adsorbed at three symmetry sites on the Ta(100) and (110) surfaces are obtained by means of the extended Huckel molecular orbital method. The Ta surfaces are represented by finite arrays of Ta atoms and the repulsive energy between nitrogen and Ta is calculated by using Andersons approximation [A.B. Anderson and R. Hoffmann, J. Chem. Phys. 60 (1974) 4217]. The most stable site for nitrogen adsorption is found to be the five coordination number site for the (100) surface and the two coordination number site for the (110) surface. The calculated binding energies for the most stable sites are close to the value evaluated from the observed chemisorption heat. The difference in the binding energy between the adsorption sites on the (110) surface is fairly small as compared with the difference on the (100) surface. The charge of adsorbed nitrogen and the equilibrium bond distance between adatom and the nearest neighbor Ta atom do not vary sensitively with adsorption site: the charge is about −0.25¦e¦ and the bond distance is about 3.1 A.

Electronic Structure of B-2

The effect of electron correlation (EC) on the electronic structure in MgB 2 , AlB 2 , and ZrB 2 , is studied by examining the partial density of states (PDOS) of B 2pσ and pπ orbitals using the polarization dependence of x-ray emission and absorption spectra. The discrepancies between observed and calculated PDOSs cannot be attributed to EC effects. The present results suggest that the EC effect is less than the experimental error (∼0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.