Zhang-Rice singlet

The calculations at the electron correlation level do not reveal the unpaired spin in the Cu02 planes indicating the formation of the singlet spin state. This supports the surmise [54] that due to a strong Cu-O hybridization, all spins in the Cu02 planes are coupled (the, so-called, Zhang-Rice singlet). 

Figure 1. Illustration of the copper-oxygen singlet formation on the square lattice. The additional doped hole cannot go to the copper site due to strong on-site Coulomb repulsion and is distributed among four oxygen sites forming together with the copper spin a Zhang-Rice singlet.

The index pd corresponds to a Zhang-Rice singlet formation with one hole placed on the copper site whereas the second hole is distributed on the neighboring oxygen sites [12], Namely, T/ " " = -2= is the copper-oxygen creation operator in terms of copper (X) and oxygen... 

---