ANGLE-RESOLVED PHOTOEMISSION-STUDY OF SR2RUO4

We present high-resolution (HR) angle-resolved photoemission spectrosc opy (ARPES) measurements of the noncuprate layered perovskite supercon ductor Sr2RuO4. ARPES spectra of the whole valence-band region obtaine d 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)) s how narrower Ru4d epsilon(xy,yz,zx)-O2p pi antibonding bands than thos e predicted by the band calculation. More significantly, there is an e xtended van Hove singularity very close to E(F) (E(B)=11 meV) along th e Ru-O bonding direction, which is known to exist in cuprate high-temp erature superconductors. The Fermi-surface topology obtained by HRARPE S (one electronlike Fermi surface sheet centered at the Gamma point an d two holelike sheets centered at the X point) is different from the b and calculation (two electronlike sheets centered at the Gamma point a nd one holelike sheet centered at the X point), although the electron count is the same in both cases. These results suggest that electron-e lectron correlations cause the modification of the Fermi-surface topol ogy, and is thus necessary for understanding the electronic structure and properties of Sr2RuO4.