Shuqiu Wang | Planar-Oxygen Orbital Ordering Mechanism of the Nematic State in Cuprates

Shuqiu Wang, Claredon Laboratory – Oxford

Unforeseen among the quantum matter states of hole-doped CuO₂ was the electronic nematic phase.  The quantum mechanical mechanism of this intra-unit-cell symmetry breaking state, which emerges concurrently with the enigmatic pseudogap phenomenon, is unknown. However, within the three-orbital Emery model of the CuO₂ charge-transfer insulator, theoretical analyses¹ now predict a nematic mechanism based on lifting the energy degeneracy of the two oxygen 2p⁶ orbitals in each unit cell. Such orbital ordering should be identifiable as a difference in charge-transfer energy separating oxygen 2p⁶ orbitals from the relevant copper 3d¹⁰orbital. Here we introduce sublattice resolvedimaging^2,3 of charge transfer energy and discover a splitting of between at the two oxygen 2p⁶ orbitals within a CuO₂ unit cell. Visualising the domains of this charge-transfer nematic finds them spatially on the 100 Å scale. Further, we observe the breaking of inversion symmetry in this charge-transfer nematic, as might be anticipated when time-reversal symmetry breaking is also present.  These data reveal that the mechanism of nematicity in the canonical cuprate Bi₂Sr₂CaCu₂O₈ is intra-unit-cell orbital ordering of the oxygen 2p⁶orbitals in the CuO₂ plane.

References 

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Hamidian, M. H. et al. Nat. Phys. 12, 150–156 (2016).
Lawler, M. J. et al. Nature 466, 347–351 (2010).

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