Angle-resolved photoemission study of Sr2RuO4

We present high-resolution (HR) angle-resolved photoemission spectroscopy (ARPES) measurements of the noncuprate layered perovskite superconductor Sr2RuO4. 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 (E(F)) show narrower Ru4d epsilon(xy,yz,zx)-O2p pi antibonding bands than those predicted by the band calculation. More significantly, there is an extended van Hove singularity very close to E(F) (E(B)=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 Gamma point and two holelike sheets centered at the X point) is different from the band calculation (two electronlike sheets centered at the Gamma 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 Sr2RuO4.