Dopant sites and structure in high T-c layered cuprates

EXAFS experiments on La2-xSrxCuO4 (LSCO) have revealed that the Sr dopants occupy two kinds of sites, a magnetic Cu one with long Sr-apical O bonds, and a metallic Cu one with short Sr-apical O bonds. Very surprisingly, the fraction of metallic bonds decreases as x(-0.9) through the insulator-metal transition, which takes place in two stages. Homogeneous high-temperature superconductivity occurs only in the intermediate phase between these two transitions. The author's earlier filamentary theories of the impurity band (Si:P) and cuprate (YBCO) metal-insulator transitions are extended to discuss the phase diagrams of pseudogaps and superconductivity in LSCO in the light of the EXAFS data. Differences between LSCO and YBCO are also explained naturally by differences in nanodomain structures. The recent observation of superconductivity in ferromagnetic RuSr2GdCu2O8-delta, with a T-c similar to that of LSCO, is explained without additional assumptions by the filamentary theory.