ELECTRONIC-STRUCTURE AND FERMI-SURFACE TOPOLOGY OF THE INFINITE-LAYERED SUPERCONDUCTOR SR1-XCAXCUO2

SuPerconductivity reported at 110 K, and recently at 150-170 K, in the infinite-layered (SrxCa1-x)CuO2 system is investigated by means of th e full potential linear muffin-tin orbital (FLMTO) method. As in other high-T(c) cuprates, the electronic structure of the parent compounds, CaCuO2 and SrCuO2, and of the separately calculated composition Sr0.7 Ca0.3CuO2 using the experimental lattice parameters, displays strong 2 D features including a low density of states at E(F) (lower than in th e other cuprates) and a simple 2D Fermi surface (rounded square) with strong nesting due to a single hybridized Cu d-O p band. As in La2CuO4 , a major van Hove saddle-point singularity exists near E(F). The dras tic changes of the Fermi surface when Ca/Sr vacancies shift the Fermi energy to the van Hove singularity may have a strong influence on the superconducting properties of the compounds and indicate the need for Sr/Ca vacancies in inducing the high T(c).