We demonstrate that it is possible to obtain spin-resolved valence band spectra with a very high degree of spin polarization from antiferromagnetic transition metal materials if the excitation light is circularly polarized and has an energy close to the cation ${2p}_{3/2}$ ( $L_3$) white line. We are able to unravel the different spin states in the single-particle excitation spectrum of CuO and show that the top of the valence band is of pure singlet character, which provides strong support for the existence and stability of Zhang-Rice singlets in high- $T_c$ superconductors.

• Received 10 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1126