Tatsuya Shishidou

Electronic structure properties and BCS superconductivity in β-pyrochlore oxides: KOS2O6

We report a first-principles density-functional calculation of the electronic structure and properties of the recently discovered superconducting b-pyrochlore oxide KOs2O6. We find that the electronic structure near the Fermi energy EF is dominated by strongly hybridized Os 5d and O 2p states. A van Hove singularity very close to EF leads to a relatively large density of states at EF, and the Fermi surface exhibits strong nesting along several directions. These features could provide the scattering processes leading to the observed anomalous temperature dependence of the resistivity and to the rather large specific-heat mass enhancement we obtain from the calculated density of states and the observed specific-heat coefficient. An estimate ofTc within the framework of the BCS theory of superconductivity taking into account the possible effects of spin fluctuations arising from nesting yields the experimental value.

Overlayer and superlattice studies of metal/ceramic interfaces: Fe/TiC

Adhesion and magnetism at the Fe(001)/TiC(001) interface, studied by first-principles calculations using the full-potential linearized augmented plane wave method, show that the interfacial Fe and C atoms can form significantly strong covalent bonding, which makes the interface structure with Fe sitting on top of C as the most stable structure for both overlayers and superlattices. Due to this strong bonding, the first layer of Fe at the interface shows a considerably reduced magnetic moment (−20%), while the second layer almost recovers its bulk value. The interface C atom has a negative spin polarization, while the interface Ti atom has a positive magnetic moment.