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We review the basic electronic structure of cuprate systems, and discuss the importance of strong electron correlation effects. Using resonant photoelectron spectroscopy it is demonstrated that the undoped cuprates belong to the class of charge-transfer insulators. As a consequence, the first ionization states are likely to consist of bound states with a compensated local spin structure rather than the usual high spin states expected from Hund's first rule. Using spin-resolved resonant photoemission with circularly-polarized light, we are able to unravel the different spin states of the valence band spectrum of the undoped cuprates and show that the top of the band is of pure singlet character, which provides strong support for the existence and stability of Zhang-Rice singlets in high-T-C superconductors. (C) 2001 Elsevier Science B.V. All rights reserved.
