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VOLUME 96 (2012) | ISSUE 6 | PAGE 424
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 1A1g the ground state of the hole center [CuO4]5- (cluster analog of Cu3+ ion) in cuprates should be described by a complex 1A1g-1,3B2g-1,3Eu multiplet, formed by a competition of conventional hybrid Cu 3d-O 2p b_{1g}(\sigma)\propto d_{x^2 -y^2} state and purely oxygen nonbonding O 2pπ states with a2g(π) and eux,y(π) 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.