Mechanisms for hole doping in thallium cuprate superconductors having single copper-oxygen sheets

This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-O sheets interleaved with single and double Tl-O layers. These studies strongly suggest that, for Tl2Ba2CuO6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x2-y2 bands, i.e., the Tl-O layer creates holes in the CuO2 layers. This is in contrast to TlBa2CuO5-type phases, where the metal chemistry forces the oxidation of CuO2 layers. However, studies on TlSr2-xLaxCuO5 and TlBa1-xSrxLaCuO5 systems indicate that for Tl-O single rock-salt layer phases with a short in-plane Cu-O distance, such as TlSrLaCuO5, the CuO2 layer x2-y2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO2 layer. (orig.)