Probing the singlet character of the two-hole states in cuprate superconductors

Using spin-resolved resonant photoemission we have probed the singlet vs. triplet character of the two-hole state in the layered cuprates Bi₂Sr₂CaCu₂O_8+δ, La_2-xSr_xCuO₄ and Sr₂CuO₂Cl₂. The combination of the photon circular polarization with the photoelectron spin detection gives access to the character of the photoemission final states, which correspond to the two-hole configurations localized at a (CuO₄) site. In particular, the lowest energy state is found to have a very high singlet character in all the measured compounds. This can be considered as a strong indication of the existence and stability of the so-called Zhang-Rice singlets in the layered cuprates.