Nobuo Tsuda

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy EF. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at EF.

On the magnetism and electronic conduction of itinerant magnetic system Ca1-xSrxRuO3

Electrical resistivity ρ and magnetic susceptibility of sintered Ca 1- x Sr x RuO 3 have been measured. The Weiss temperature changes sign at about x =0.5 in consistent with other studies. Temperature dependence of normalized ρ is almost identical for all the materials in the paramagnetic temperature range irrespective of the sign of magnetic interaction. The Rhodes-Wolfarth ratio is 1.3 for x =1. This ratio and the temperature dependence of ρ suggest that 4 d -electrons in this system are in the intermediately localized state as in Ni. The sign of magnetic interaction is discussed in connection with the orthorhombic lattice distortion, and the antiferromagnetic interaction is attributed to this distortion.

POSITRON LIFETIMES IN OXIDES.

Lifetime spectra of positrons is oxides were measured. Densities of oxygen ions corrected for the ionicities are rather good parameters describing τ 1 -1 and thus positrons may be itinerant in oxides. The annihilation rates τ 1 -1 are about 1.5 times as large as those which are calculated on the assumption that the positrons distribute homogeneously.

Analysis of Phonon Structures in the Tunneling Conductance of Bi-Cuprates

Following a short summary of our tunneling results for Bi 2 Sr 2 Ca n -1 Cu n O y (Bi22( n -1) n ) ( n =1–3), the tunneling phonon structures of Bi2212 are analyzed according to the Eliashberg scheme assuming a certain normal-state conductivity. The McMillan-Rowell spectral function is similar to a model spectral function in which only the Cu–O vibrations are taken into account and the interaction strength is assumed to increase with decreasing frequency. To ascertain whether excitation other than that of phonons contributes to pair formation, we calculated the tunneling conductance for various composite spectral functions. As far as we could discover, the phonon-only spectral function is the function which best explains the tunneling results.

Metal-Insulator Transition in LiTi2-xVxO4

Resistivity, Seebeck coefficient, and the magnetic susceptibility were measured for the sintered pellets to see if a random distribution of Ti and V makes the oxide insulating. The oxide seems to be an insulator in the composition range 0.8 1. It was a sum of the Pauli paramagnetic one of Ti 3+ electrons and the Curie-Weiss one of V 3+ electrons for x <1. A mechanism of the metal-insulator transition was discussed.

Metal-Insulator Transition in LixZn1-xV2O4

Thermoelectric power and electrical resistivity were measured for sintered Li x Zn 1- x V 2 O 4 (0≤ x ≤1). From the temperature and the compositional dependences of the Seebeck coefficient, it is inferred that the metal-insulator transition occurs at 0.3< x <0.4. However, there was no abrupt change in the resistivity. To explain the temperature dependence of the Seebeck coefficient, random distribution of low excited localized states was postulated and the transition seems to be of Mott-Anderson type.

Electronic structure of TiH2

Abstract The electronic structure of TiH2 has been studied using the augmented-plane-wave method and the LCAO interpolation. The density of states and its orbital components show that the conduction band is Ti d-like and that the valence band is largely derived from the hydrogen orbitals with small Ti 3d hybridization. The electronic charges on the hydrogen atom are ∼ 1.5 as compared to 1.6–1.7 of the rare-earth metal hydrides.

Fine Structure in the Tunneling Conductance of a Bi2Sr2CaCu2O8-GaAs Junction

A fine structure was observed above the gap voltage in the tunneling conductance of a Bi 2 Sr 2 CaCu 2 O 8 -GaAs junction at 4.2 K. It is due to electron-phonon interaction, suggesting that this superconductivity is essentially due to phonons.

Metathesis of propylene over unsupported rhenium trioxide

Abstract The metathesis of C3H6proceeds over unsupported ReO3 and the activity is enhanced by the slight reduction of the catalyst with CO. As the extent of reduction is increased, the activity becomes lower and the (co)dimerizations among C2H4 and C3H6 become pronounced. The stereoselectivity of metathesis is found over nonreduced ReO3 and the cis/trans ratio of 2-C4H8 extrapolated to a zero reaction period decreases when the catalyst is reduced.

Temperature dependence of the tunneling conductance of Bi2Sr2CaCu2O8 : phonon contribution to high-Tc superconductivity

Temperature dependence of the tunneling conductance of a Bi 2 Sr 2 CaCu 2 O 8 –GaAs junction was investigated both experimentally and theoretically. The spectral function deduced from the experimental result at 4.2 K is applied to the Eliashberg equations at higher temperatures. The theory well explains the observed smearing of the phonon structures, but there is a small discrepancy between the experimental and the theoretical gap and the former decreases more rapidly than the latter as temperature increases.