MECHANISMS FOR HOLE DOPING IN THALLIUM CUPRATE SUPERCONDUCTORS HAVINGSINGLE COPPER-OXYGEN SHEETS

This article describes some recent studies on the effect of chemical s ubstitution on the superconducting properties of thallium cuprates con taining single Cu-O sheets interleaved with single and double Tl-O lay ers. These studies strongly suggest that, for Tl2Ba2CuO6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from th e Cu 3d x2-y2 bands, i.e., the Tl-O layer creates holes in the CuO2 la yers. This is in contrast to TlBa2CuO5-type phases, where the metal ch emistry forces the oxidation of CuO2 layers. However, studies on TlSr2 -xLaxCuO5 and TlBa1-xSrxLaCuO5 systems indicate that for Tl-O single r ock-salt layer phases with a short in-plane Cu-O distance, such as TlS rLaCuO5, the CuO2 layer x2-y2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO2 layer.