Dr Shuqiu Wang

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.

An unidentified quantum fluid designated the pseudogap (PG) phase is produced by electron-density depletion in the CuO₂ antiferromagnetic insulator. Current theories suggest that the PG phase may be a pair density wave (PDW) state characterized by a spatially modulating density of electron pairs. Such a state should exhibit a periodically modulating energy gap Δ_P(r) in real-space, and a characteristic quasiparticle scattering interference (QPI) signature Λ_P(q) in wavevector space. By studying strongly underdoped Bi₂Sr₂CaDyCu₂O₈ at hole-density ~0.08 in the superconductive phase, we detect the 8a₀-periodic Δ_P(r) modulations signifying a PDW coexisting with superconductivity. Then, by visualizing the temperature dependence of this electronic structure from the superconducting into the pseudogap phase, we find the evolution of the scattering interference signature Λ(q) that is predicted specifically for the temperature dependence of an 8a₀-periodic PDW. These observations are consistent with theory for the transition from a PDW state coexisting with d-wave superconductivity to a pure PDW state in the Bi₂Sr₂CaDyCu₂O₈ pseudogap phase.