We mapped the Fermi surface of the first copper-free layered perovskite superconductor, S${\mathrm{r}}_2$Ru${\mathrm{O}}_4$, by high-resolution $\left(\approx 22\mathrm{meV}\right)$ angle-resolved photoemission. Three bands cross the Fermi energy, consistent with band structure calculations; one around $\overline{\gamma }$ and two around $\overline{X}$. The highlight is the observation of an extended van Hove singularity located 17 meV below the Fermi level. It extends around $\overline{M}$ for $\approx 0.2{Å}^{-1\mathrm{}}$ along $\overline{\gamma }-\overline{M}-\overline{\gamma }$ and $\overline{X}-\overline{M}-\overline{X}$ in the projected Brillouin zone. This raises important questions related to the possible role of a van Hove singularity for oxide superconductivity.

• Received 22 December 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.4845