Mean-Field Analysis of Intra-Unit-Cell Order in the Emery Model of the   CuO$_2$ Plane
  Motivated by recent experiments on high-$T_c$ cuprate superconductors pointing towards intra-unit-cell order in the pseudogap phase, we investigate three distinct intra-unit-cell-ordering possibilities: nematic, nematic-spin-nematic, and current-loop order. The first two are Fermi-surface instabilities involving a spontaneous charge and magnetization imbalance between the two oxygen sites in the unit cell, respectively, while the third describes circulating currents within the unit cell. We analyze the three-band Emery model of a single CuO$_2$ layer including various on-site and nearest-neighbor interactions within a self-consistent mean-field approach. We show how these on-site and further-neighbor repulsions suppress or enhance particular IUC orders. In particular, we show that the attractive interactions necessary for nematic and nematic-spin-nematic orders in one-band models have their natural microscopic origin in the O-O on-site and nearest-neighbor repulsions in the three-band model. Finally, we find that while the nematic and nematic-spin-nematic orders cannot coexist in this framework, the loop-current order can coexist with nematic order. 
Mark Hannes Fischer and Eun-Ah Kim
cond-mat.str-el cond-mat.supr-con