Mechanism of metallization and superconductivity suppression in YBa${}_2($Cu${}_{0.97}$Zn${}_{0.03}{)}_3$O${}_{6.92}$ revealed by ${}^{67}$Zn NQR

We measure the nuclear quadrupole resonance (NQR) signal on the Zn site in nearly optimally doped YBa${}_2$Cu${}_3$O${}_{6.92}$, when Cu is substituted by 3% of isotopically pure ${}^{67}$Zn. We observe that Zn creates large insulating islands, confirming two earlier conjectures: that doping provokes an orbital transition in the CuO${}_2$ 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 D${}_4$ symmetry. Cluster and DFT calculations show that the D${}_4$ symmetry breaking is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in La${}_{2-x}$Ba${}_x$CuO${}_4$, similarly well-known to strongly suppress superconductivity. These results show that in-plane oxygen $2p^5$ orbital configurations are principally involved in the metallicity and superconductivity of all high-T${}_c$ cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.