It has been generally accepted that a strong on-site Coulomb repulsion of the Cu 3d electrons dominates the electronic structure of the high-transition-temperature (high-Tc) superconductors. The on-site Coulomb repulsion has been evaluated as 6-7 eV, comparable with the valence-band width1,2. This strong correlation is thought to cause the Cu 3d electrons to be localized as in a Mott insulator, and doped holes may be transferred to oxygen sites, as the charge transfer energy is small compared with the correlation energy. These doped holes yield a substantial density of states at the Fermi level, characteristic of metals. There has been great effort to find and characterize the electronic states at the Fermi level, because these states relate directly to the mechanism of the high-Tc superconductivity by providing Cooper pairs below Tc. Here we report the first direct evidence for the dominant oxygen-2p nature of the Fermi-liquid state in the high-Tc superconductor, obtained using the technique of angle-resolved resonant photoemission.
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