Visualization of Intra-Unit-Cell Orbital Ordering in Bi₂Sr₂CaCu₂O₈₊

The CuO₂ unit cell is the essential element of superconductivity in cuprate. The Coulombic interactions dominating the 3d⁹ and 3d¹⁰ configurations of each copper ion have been the focus of virally all theoretical investigations. However, if Coulombic interactions exist between the electrons of the 2p⁶ orbitals of each planar oxygen atom, the energy degeneracy may be lifted due to orbital ordering. The consequence would be symmetry breaking inside the CuO₂ unit cell and an electronic nematic phase in which the charge transfer energy ε separating each oxygen 2p⁶ orbital from the relevant copper 3d¹⁰ orbital configuration is different for the two oxygen atoms. We developed sublattice resolved imaging of charge transfer energy ε(r) to CuO₂ and successfully detected powerful rotational symmetry breaking of ε(r) inside the unit cell. In fact the energy splitting between the two oxygen atoms on the energy scale of circa 50 meV. Hence a powerful orbitally ordered state occurs in Bi₂Sr₂CaCu₂O₈ at the charge-transfer energy scale. We show that, spatially, this state is arranged in Ising domains that are pinned by the dense dispersal of dopant-ions which render the material superconducting. These data pinpoint the microscopic mechanism for the cuprate nematicity as due to orbital ordering. And such orbital ordering bears striking analogies to the iron d_zx and d_zy orbitals (orbital ordering) well known in iron-based superconductors.