Find in Library

We measure the nuclear quadrupole resonance 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 $\mathrm{La}{}_{2-x}$ Ba _x CuO _4 , similarly well-known to strongly suppress superconductivity (SC). These results show that in-plane oxygen 2p ^5 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.