Spin-resolved photoelectron spectroscopy on cuprate systems

We review the basic electronic structure of cuprate systems, and discuss th e importance of strong electron correlation effects. Using resonant photoel ectron spectroscopy it is demonstrated that the undoped cuprates belong to the class of charge-transfer insulators. As a consequence, the first ioniza tion 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-pola rized light, we are able to unravel the different spin states of the valenc e band spectrum of the undoped cuprates and show that the top of the band i s of pure singlet character, which provides strong support for the existenc e and stability of Zhang-Rice singlets in high-T-C superconductors. (C) 200 1 Elsevier Science B.V. All rights reserved.