Electronic Band Structure of the Pyrochlore Ruthenium Oxides A₂Ru₂O₇ (A = Bi , Tl and Y)

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A local-density electronic-band-structure calculation was performed for pyrochlore ruthenium oxides A₂Ru₂O₇ (A = Bi, Tl and Y). As a general feature, these pyrochlores have band structure composed of antibonding states of Ru 4d (t_2g) and O 2p orbitals near the Fermi energy (E_F) analogous to the layered perovskite superconductor Sr₂RuO₄. It was found that effects of A ion's orbitals to the band structure near E_F are quite important in the pyrochlores. Specifically in Tl₂Ru₂O₇, bands which consist of antibonding states of Tl 6s and O 2p orbitals exist with nearly half-fillings. A tight-binding model shows an energy-gap formation of the Tl-O bands with an orthorhombic lattice distortion. The calculated result suggests importance of coupling between electronic and lattice instabilities, which may be a clue to understand metal-insulator transition observed in Tl₂Ru₂O₇.