Angle-resolved and resonant photoemission study of Sr₂RuO₄

Resonant and angle-resolved photoemission measurements were performed for the non-cuprate layered superconductor Sr₂RuO₄ (T_c = 0.93 K). Resonant valence band photoemission spectroscopy shows the existence of a correlation induced satellite to the main d band across the Ru 4p-4d threshold. Angle-resolved measurements show a surprisingly narrow band of about 50 meV width and positioned at 26 meV below the Fermi level, along the ΓX and ΓZ directions. This band, arising fom Ru 4d-O 2p antibonding states, is nearly dispersionless over a wide range of k points about the zone boundary along the Γ-Z direction (Ru-O direction), reminiscent of the extended Van-Hove singularity observed in the cuprates. The results emphasize the presence of strong correlations, substantial charge-transfer character to states at the Fermi level and the existence of a singular flat band near the Fermi-level in this low T_c layered superconductor, just like the cuprates.