Spectroscopic evidence for strongly correlated electronic states in La-Sr-Cu and Y-Ba-Cu oxides

Photoemission studies of (${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${)}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.7}$ have revealed that their electronic structure is essentially that of a Mott insulator, i.e., Cu 3d electrons are localized and the density of states at the Fermi level is very low. This result combined with the magnetic properties suggest a fluctuation of the local moment due to intersite exchange, which may be consistent with a bipolaron state or a resonating-valence-bond state.