Wang Lab of Quantum Matter

Discovery of Orbital Ordering in Bi2Sr2CaCu2O8+x

The primordial ingredient of cuprate superconductivity is the CuO₂ unit cell. Here, theoretical attention usually concentrates on the intra-atom Coulombic interactions dominating the 3d9  and 3d10  configurations of each copper ion. However, if Coulombic interactions also occur between electrons of the 2p6  orbitals of each planar oxygen atom, spontaneous orbital ordering may lift their energy degeneracy. This long predicted intra-unit cell symmetry breaking should then generate an orbital ordered phase, for which the charge-transfer energy E   separating the 2p6  and 3d10 orbitals is distinct for the two oxygen atoms. Here we introduce sublattice resolved E(r)  imaging techniques to CuO₂ studies and discover powerful 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 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 lifts the energy degeneracy of p_x/p_y oxygen orbitals at separate CuO₂ oxygen sites, in striking analogy to the ordering of d_zx/d_zy iron orbitals of the iron-based superconductors.

Read more about our research →  Nature Materials, 23, 492–498, 2024.