  The primordial ingredient of cuprate superconductivity is the CuO2 unit cell.
Here, theoretical attention usually concentrates on the intra-atom Coulombic
interactions dominating the 3d^9 and 3d^10 configurations of each copper ion.
However, if Coulombic interactions also occur between electrons of the 2p^6
orbitals of each planar oxygen atom, spontaneous orbital ordering may split
their energy levels. This long predicted intra-unit cell symmetry breaking
should then generate an orbital ordered phase, for which the charge-transfer
energy E separating the 2p^6 and 3d^10orbitals is distinct for the two oxygen
atoms. Here we introduce sublattice resolved E(r) imaging techniques to CuO2
studies and discover intra-unit-cell rotational symmetry breaking of E(r), with
energy-level splitting between the two oxygen atoms on the 50 meV scale.
Spatially, this state is arranged in disordered Ising domains of orthogonally
oriented orbital order that appear bounded by dopant ions, and within whose
domain walls low energy electronic quadrupolar two-level systems occur.
Overall, these data reveal a Q=0 orbitally ordered state that splits the energy
levels of the oxygen orbitals by ~50 meV, in underdoped CuO2.
