Local O δ probing in the high- T c superconductor HgBa 2 CuO 4 + δ

Electric-field gradients on mercury sites of the ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ high-${T}_{c}$ superconductor were measured with the perturbed angular correlation technique and interpreted with ab initio calculations. Under oxygen annealing, an asymmetric electric-field gradient has been assigned to the presence of single oxygen atoms, ${\mathrm{O}}_{\ensuremath{\delta}},$ which are located in the Hg planes. These experiments provide an atomic scale tool for studying charge-density variations occuring in the neighborhood of the Hg atoms, which can be induced, particularly, by pointlike defects.