Van Hove singularity scenario and hole concentration in high- Tc HgBa 2CaCu 2O 6+δ superconductor. Pressure effects

Abstract

The electronic structure and the hole concentrations in the recently discovered high- T c superconductor HgBa 2CaCu 2O 6+δ ( δ=0, 1) have been investigated by employing a first-principles full potential self-consistent linear muffin-tin orbital (LMTO) method with the local density functional theory. The scalar relativistic effects have been considered. The hole concentrations of the Cu d and O p x , p y orbitals are seen to be smaller for the HgBa 2CaCu 2O 7 system than those of the HgBa 2CaCu 2 O 6 solid. The van Hove singularity (vHS) induced Cu d and O p mixed peak which is seen to lie above the Fermi level ( E F) at the X(R) points in the δ=1 system shifts below E F in the δ=0 system and thus needs hole doping either by oxygenation or by other methods for making the δ=0 system superconducting. For the δ=0 system, the performed calculation reveals that the hole concentration always increases with the pressure on the crystal showing no maximum value or a saturation value. On the other hand, the vHS peak approaches the Fermi level with increased pressure on the crystal (or decrease in the crystal volume) and is pinned at E F for a relative crystal volume of V/ V 0⋍0.77 ( V 0 is the experimental volume at normal pressure). One observes the Fermi surface nesting area to become maximum at this particular volume. On further reduction of the lattice volume, the vHS peak moves above and away from the E F. This result correlates with the observed behaviour of T c with increased pressure and leads to the conclusion that T c is maximum when the vHS peak is pinned at E F.