MECHANISM OF HOLE DOPING IN HG-BASED CUPRATE SUPERCONDUCTORS

The new Hg-based cuprate superconductors are considered important materials for technological applications. These compounds can be obtained, for instance, from the double-layer Bi- or Tl-based superconductors by the replacement of a (Bi2O2)2+ or (Tl2O1)2+ layer by a single Hg2+ layer. Results of electronic structure calculations for the hole densities in the three Hg-based cuprate superconductors HgBa2CuO4+delta, HgBa2CaCu2O6+delta, HgBa2Ca2Cu3O8+delta are presented in this paper for two oxygen stoichiometries delta=0 and delta=0.125. We find that for delta=0 these compounds are practically undoped due to the absence of oxygen atoms in the plane of Hg. The non-stoichiometry (delta>0) in these superconductors plays a crucial role and provides the only source of hole doping. Our calculations show that for delta=0.125 the total carrier density in the CuO2 Planes per formula unit's essentially the same in all three compounds with a value approximately 0.32 hole/formula unit. This value is larger than that expected on the basis of purely ionic considerations, due to the lowering of Hg states arising from Hg-O interactions. We also find that in the Hg 1223 superconductor the hole density is approximately uniformly shared between the three CuO2 planes.