Angle-resolved photoemission study o

We present high-resolution (HR) angle-resolved photoemission spectroscopy (ARPES) measurements of the noncuprate layered perovskite superconductor (Formula presented)(Formula presented). ARPES spectra of the whole valence-band region obtained along two high-symmetry directions in the Brillouin zone show clear dispersion, generally similar to that of a band calculation. However, HRARPES measurements taken in the vicinity of the Fermi level ((Formula presented)) show narrower Ru4dɛ(xy, yz, zx)-O2pπ antibonding bands than those predicted by the band calculation. More significantly, there is an extended van Hove singularity very close to (Formula presented) ((Formula presented)=11 meV) along the Ru-O bonding direction, which is known to exist in cuprate high-temperature superconductors. The Fermi-surface topology obtained by HRARPES (one electronlike Fermi surface sheet centered at the Γ point and two holelike sheets centered at the X point) is different from the band calculation (two electronlike sheets centered at the Γ point and one holelike sheet centered at the X point), although the electron count is the same in both cases. These results suggest that electron-electron correlations cause the modification of the Fermi-surface topology, and is thus necessary for understanding the electronic structure and properties of (Formula presented)(Formula presented).