FERMI-SURFACE AND EXTENDED VAN-HOVE SINGULARITY IN THE NONCUPRATE LAYERED PEROVSKITE SUPERCONDUCTOR SR2RUO4

With high-resolution (approximate to 22 meV) angle-resolved photoemiss ion spectroscopy, the Fermi surface of the first copper free layered-p erovskite superconductor, Sr2RuO4, was determined. We observed three b ands to cross the Fermi energy in qualitative agreement with LDA band structure calculations; one electron-like surface encircling the <(Gam ma)over bar> point in the projected Brillouin zone, and two hole-like surfaces around the (X) over bar point. The most striking aspect of th e measurements is the observation of an extended van Hove singularity. It is located 17 meV below the Fermi energy and extends around the (M ) over bar point for about 0.2 Angstrom(-1) along both the <(Gamma)ove r bar> - (M) over bar - <(Gamma)over bar> and the (X) over bar - (M) o ver bar - (X) over bar directions. These observations demonstrate that van Hove singularities near the Fermi surface are a more generic feat ure of layered oxides, and call for a clarification of their exact rol e in oxide superconductivity.