ELECTRONIC-STRUCTURE AND FERMI-SURFACE OF THE HGBA2CUO4+DELTA SUPERCONDUCTOR - APPARENT IMPORTANCE OF THE ROLE OF VANHOVE SINGULARITIES ON HIGH-TC

The electronic structure and Fermi surface of the recently discovered HgBa2CuO4+6 superconductor with T(c)=95 K is calculated making use of the full-potential linear muffin-tin orbital (FLMTO) method. Similarly to the other high-T(c) cuprates, the main feature of the electronic structure of undoped HgBa2CuO4 is a single free-electron-like two-dimensional dpsigma band crossing E(F). A(s) for the ''infinite layered'' compound, (Sr1-xCax)1-yCuO2 with T(c) = 110 K, the Fermi sur-face has the shape of a rounded square. and a major van Hove saddle-point singularity (vHs) exists near E(F). Drastic changes of the density of states and Fermi surface are found when the hole doping moves the Fermi energy precisely onto the vHs, which is now seen to have a strong influence on the superconducting properties of this compound. These striking results also call attention to and provide possible support for vHs based excitonic pairing mechanisms for high T(c).