Electronic structure of hole centers in CuO2 planes of cuprates

Abstract

A theoretical analysis and a large amount of experimental data indicate that the structure of the valence hole states in doped cuprates is more complicated than assumed in the simple Zhang-Rice singlet model. In fact, we are dealing with a competition between a hybrid Cu3d–O2pb1g∝dx2−y2-state and purely oxygen nonbonding states with a2g- and eux,y∝px,y-symmetries. Thus, as a cluster analog of a Cu3+ ion, the ground state of a non-Zhang-Rice CuO45− hole center of this sort should be described by complicated A1g1−B2g1,3−Eu1,3 multiplet with a set of charge, orbital, and spin order parameters, some of which are well known (e.g., spin moment or “ferromagnetic” Ising orbital momentum localized on oxygen ions) while others are unconventional or hidden (e.g., “antiferromagnetic” ordering of Ising orbital momenta localized on four oxygen atoms or a combined spin-orbital-quadrupole ordering). The non-Zhang-Rice CuO45− centers are actually singlet-triplet pseudo-Jahn-Teller centers with strong vibron coupling to the lattice. The complicated structure of the ground-state multiplet of the hole centers shows up in many of the unusual properties of doped cuprates, in particular, their pseudo-gap phase.