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

Photoemission studies of (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${)}_{2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ and ${\mathrm{YBa}}_{2}$${\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.