Discovery of orbital ordering in Bi 2 Sr 2 CaCu 2 O 8+ x

The primordial ingredient of cuprate superconductivity is the CuO 2 unit cell. Theories usually concentrate on the intra-atom Coulombic interactions dominating the 3 d 9 and 3 d 10 configurations of each copper ion. However, if Coulombic interactions also occur between electrons of the 2 p 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 ε separating the 2 p 6 and 3 d 10 orbitals is distinct for the two oxygen atoms. Here we introduce sublattice-resolved ε ( r ) imaging to CuO 2 studies and discover intra-unit-cell rotational symmetry breaking of ε ( 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 ~50 meV, in underdoped CuO 2 .