Mechanism of metallization and superconductivity suppression in YBa₂(Cu_0.97 Zn_0.03)₃ O_6.92 revealed by ⁶⁷Zn NQR

We measure the nuclear quadrupole resonance signal on the Zn site in nearly optimally doped YBa₂Cu₃O_6.92, when Cu is substituted by 3% of isotopically pure ⁶⁷Zn. We observe that Zn creates large insulating islands, confirming two earlier conjectures: that doping provokes an orbital transition in the CuO₂ plane, which is locally reversed by Zn substitution, and that the islands are antiferromagnetic. Also, we find that the Zn impurity locally induces a breaking of the D₄ symmetry. Cluster and DFT calculations show that the D₄ symmetry breaking is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in Ba_xCuO₄, similarly well-known to strongly suppress superconductivity (SC). These results show that in-plane oxygen 2p⁵ orbital configurations are principally involved in the metallicity and SC of all high-T_c cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.