Van Hove singularities and the role of doping in the stabilization, synthesis and superconductivity of HgBa₂Ca_n-1Cu_nO_2n+2+δ

The electronic structure and Fermi surface of the recently discovered HgBa₂Ca_n-1Cu_nO_2n+2+δ superconductors have been determined using the full potential linear muffin-tin orbital method and precise structural information determined with neutrons by Radaelli et al. Whereas for stoichiometric HgBa₂CuO₄ (Hg-1201) the only band crossing the Fermi energy derived from the CuO antibonding state is half-filled, an additional HgO band that crosses E_F exists in the case of HgBa₂CaCu₂O₆ (Hg-1212) and HgBa₂Ca₂CuO₄O₈ (Hg-1223). Thus, stoichiometric HgBa₂CuO₄ is expected to be a Mott insulator with dopants essential for forming the normal metallic state that leads to superconductivity at 95 K, in contrast to two other members of the Hg family that are expected to be "self-doped" to a metallic normal state. As in Hg-1201, the electronic structure is two-dimensional and is dominated by van Hove singularities (vHS's) to which E_F is pinned by dopants whose calculated concentration is found to agree well with that determined by Radaelli et al. for Hg-1212 for their maximum T_c = 128 K sample. Finally, predicted doping levels for stabilizing a large volume of the high-T_c Hg-1223 phase - and hence its highest T_c - are made on the basis of pinning E _F to the vHS.