Electronic structure of superconductors without apical oxygen:Sr2CuO2

The electronic structures of ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{F}}_{2}$, ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$, and ${\mathrm{Ca}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ are investigated by means of the full-potential linear muffin-tin orbital method. As in other high-${\mathit{T}}_{\mathit{c}}$ cuprates, the electronic structures of these compounds display strong two-dimensional features including a low density of states at ${\mathit{E}}_{\mathit{F}}$ and a simple Fermi surface of the form of a rounded square. A major van Hove saddle-point singularity exists near the Fermi level. We predict the optimum superconductivity properties in these materials to occur when doped by 0.35--0.38 holes per unit cell.