Pseudogap phase in cuprates: Oxygen orbital moments instead of circulating currents

Abstract

Circulating current (CC) loops within the cuprate unit cell are proposed to play a key role in the physics of the pseudogap phase. However, main experimental observations motivated by this sophisticated proposal and seemingly supporting the CC model can be explained in frames of a simple and physically clear microscopic model. We argue that instead of a well-isolated Zhang-Rice (ZR) singlet $^1A_{1g}$ the ground state of the hole center [CuO$_4$]$^{5-}$ (cluster analog of Cu$^{3+}$ ion) in cuprates should be described by a complex $^1A_{1g}$-$^{1,3}B_{2g}$-$^{1,3}E_u$ multiplet, formed by a competition of conventional hybrid Cu 3d-O 2p $b_{1g}(\sigma)\propto d_{x^2 -y^2}$ state and {\it purely oxygen nonbonding} O 2p$\pi$ states with $a_{2g}(\pi)$ and $e_{ux,y}(\pi)$ symmetry. In contrast with inactive ZR singlet we arrive at several novel competing orbital and spin-orbital order parameters, e.g., Ising-like net orbital magnetic moment, orbital toroidal moment, intra-plaquette's staggered order of Ising-like oxygen orbital magnetic moments. As a most impressive validation of the non-ZR model we explain fascinating results of recent neutron scattering measurements that revealed novel type of magnetic ordering in pseudogap phase of several hole-doped cuprates.