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

Abstract

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 Tl 2Ba 2CuO 6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x 2− y 2 bands, i.e., the Tl-O layer creates holes in the CuO 2 layers. This is in contrast to TlBa 2CuO 5-type phases, where the metal chemistry forces the oxidation of CuO 2 layers. However, studies on TlSr 2− x La x CuO 5 and TlBa 1− x Sr x LaCuO 5 systems indicate that for Tl-O single rock-salt layer phases with a short in-plane Cu-O distance, such as TlSrLaCuO 5, the CuO 2 layer x 2− y 2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO 2 layer