Mechanism of metallization and superconductivity suppression in YBa2(Cu0.97 Zn0.03)3 O6.92 revealed by 67Zn NQR

Abstract

We measure the nuclear quadrupole resonance signal on the Zn site in nearly optimally doped YBa2Cu3O6.92, 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 BaxCuO4, 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-Tc cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.