Destabilization of the Zhang-Rice singlet at optimal doping

The construction of the Zhang-Rice singlet is revisited in the light of recent understanding of high-temperature superconductors at optimal doping. A minimal local model is derived which contains the physical regime found relevant for ARPES experiments, characterized by significant direct oxygen-oxygen hopping. For the values of orbital parameters indicated by experiment, the Zhang-Rice singlet is strongly mixed with a pure oxygen singlet of the same symmetry. The destabilization of the Zhang-Rice ground state is due to the oxygen singlet having twice as large a coherence factor with respect to oxygen-oxygen hopping. An analogous quantum phase transition is identified in the t-t'-J model. The orbital-antisymmetric copper-oxygen singlet is confirmed to be irrelevant, as found originally. The usual symmetry analysis is extended to include dynamical symmetries.