Discovery of orbital ordering in Bi₂Sr₂CaCu₂O_8+x

The primordial ingredient of cuprate superconductivity is the CuO2 unit cell. Theories usually concentrate 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 generate an orbitally ordered phase, for which the charge transfer energy epsilon separating the 2p(6) and 3d(10) orbitals is distinct for the two oxygen atoms. Here we introduce sublattice-resolved epsilon(r) imaging to CuO2 studies and discover intra-unit-cell rotational symmetry breaking of epsilon(r). Spatially, this state is arranged in disordered Ising domains of orthogonally oriented orbital order 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 oxygen energy levels by similar to 50 meV, in underdoped CuO2.