The electronic structure and the hole concentrations in the recently discovered high-To superconductor HgBa2CuO4+6 (~= 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 dx2_y2 and O Px.y orbitals are seen to be larger for the HgBaCuO5 system than those of the HgBaCuO4 solid. However, the van Hove singularity (vHS) induced Cud and O p peak which is seen to lie above the Fermi level in the fi= l system shifts below the Fermi level in the g= 0 system. Thus, the occurrence of the superconducting behaviour observed in the HgBa2CuO4+z with = 0.1 appears to originate by the hole filling of the g= 0 system by oxygenation to achieve the pinning of the vHS peak at the Fermi level. The Fermi surface nesting area in the g= 0 compound is seen to be larger than that seen in the J= 1 compound. The calculation reveals that the vHS peak approaches towards the Fermi level and the hole concentrations increase with the pressure on the crystal predicting a possibility of enhancement of T¢ at high pressures, a result which is in agreement with the experiments.
The vHS is pinned at the Fermi level for a critical volume of V/Vo = 0.65, Vo being the normal volume. This volume may be achieved for a pressure of 24 GPa, a value in very good agreement with the experimental data. * Corresponding author. pounds HgBa2Can_lCu2nO2n+2+~ (n= 1, 3) have been seen to show values of Tc comparable with the thallium-bearing compounds. Earlier [ 5 ], the two CuO2 layers compound HgBa2RCu206+~ (with R as a rare-earth element) was seen to be non-superconducting. However, later on, the compound Hg-Ba2CuO4+~ (~=0.1) has been seen [6] to be superconducting with Tc=94 K. On the other hand Schilling et al. [7] have observed a T¢= 133 K in a material containing HgBa2Ca2Cu308÷~ and Hg-Ba2CaCu206+~ in an ordered superstructure composed of a sequence of the unit cells of these two phases. The value of Tc in this material is distinctly higher than the previous established record value of 125-127 K in TI2Ba2Ca2Cu30~o.