UNCONVENTIONAL OXYGEN DOPING BEHAVIOR IN HGBA2CA2CU3O8+DELTA

Local density approximation supercell calculations for (HgBa2Ca2Cu3O8)2O are used to elucidate the effects of oxygen doping on HgBa2Ca2Cu3O8. The CuO2 planes are found to be hole doped by the addition of O in the O(4) site. The doping level is considerably lower than would be predicted assuming that O enters as O2- ions. Rather, strong covalency is found between the dopant O(4) and Hg atoms. States associated with the neighboring Hg atoms are pushed up in energy by occupation of the O(4) site to the extent that some antibonding O(4)-p Hg-d states occur above the Fermi energy. Thus the hole doping of the CuO2 layers is reduced by nearly half from that expected from ionic considerations, and this is accompanied by a coalescence of the three plane derived Fermi surfaces.