Electronic structure of hole centers in CuO2 planes of cuprates

Both theoretical considerations and numerous experimental data point to a more complicated nature of the valence hole states in doped cuprates than it is predicted by the simple Zhang-Rice model. Actually we deal with the competition of conventional hybrid Cu 3d-O 2p b1g ∝ dx2-y2 -state and purely oxygen nonbonding state with a2g- and eux, y ∝ px, y -symmetry. Accordingly, the ground state of such a non Zhang-Rice hole center CuO45- as a cluster analog of Cu3+ ion should be described by a complex 1A1g-1,3B2g-1,3Eu multiplet with several competing charge, orbital, and spin order parameters, like conventional ones (e.g., spin moment or Ising-like orbital magnetic moment) and unconventional, or hidden ones (e.g., intra-plaquette's staggered order of Ising-like oxygen orbital magnetic moment or combined spin-quadrupole ordering). The non Zhang-Rice hole CuO45- centers should be considered as singlet-triplet pseudo-Jahn-Teller centers prone to strong vibronic coupling. The complex non Zhang-Rice structure of the ground state multiplet of hole centers manifests itself in many unconventional properties of doped cuprates, in particular, in a pseudo-gap regime.