Local O-delta probing in the high-T-c superconductor HgBa2CuO4+delta

Electric-field gradients on mercury sites of the HgBa2CuO4+delta high-T-c s uperconductor were measured with the perturbed angular correlation techniqu e and interpreted with ab initio calculations. Under oxygen annealing, an a symmetric electric-field gradient has been assigned to the presence of sing le oxygen atoms, O-delta, which are located in the Hg planes. These experim ents provide an atomic scale tool for studying charge-density variations oc curing in the neighborhood of the Hg atoms, which can be induced, particula rly, by pointlike defects.