Defects that control the properties of Tl- and Hg-based superconductors

Defects that affect T-c in Tl2Ba2CuO6+delta and HgBa2CuO4+delta have been characterized by neutron powder diffraction In Tl2Ba2CuO6+delta, the dominant defect is interstitial oxygen between the two Tl-O planes, but Cu substitution on the Tl site also affects properties and there is evidence for a second oxygen defect for compositions in the vicinity of maximum T-c. Zn HgBa2CuO4+delta, there are two competing oxygen defects in the Hg layer. The relative concentrations of these defects switches upon passing from the underdoped region, through the maximum T-c, to the overdoped region. This remarkable behavior could result from a change in the topology of the Fermi surface upon passing through the van Hove singularity.