Kavli Affiliate: J. C. Seamus Davis
| First 5 Authors: Shuqiu Wang, Niall Kennedy, Kazuhiro Fujita, Shin-ichi Uchida, Hiroshi Eisaki
| Summary:
The primordial ingredient of cuprate superconductivity is the CuO$_2$ 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 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 $varepsilon$ separating the $2p^6$
and 3d$^{10}$ orbitals is distinct for the two oxygen atoms. Here we introduce
sublattice resolved $varepsilon(r)$ imaging techniques to CuO$_2$ studies and
discover powerful intra-unit-cell rotational symmetry breaking of
$varepsilon(r)$, 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 CuO$_2$ oxygen
sites, in striking analogy to the ordering of $d_{zx}/d_{zy}$ iron orbitals of
the iron-based superconductors.
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