The role of apical halogen coordination on the Tc of superconducting cuprates in the effective hole model

An effective hole model describes quantitatively the aspects of the doping curves of cuprates (e.g. optimal Tc). When subtraction of holes to the apical O is made, the remaining effective holes in the CuO2 plane scale quantitatively with Tc in a range up to the optimum. Here we investigate a similar effect of apical halogen in selected related materials. It is found that apical halogen analogs show generally similar phenomena to their apical O counterparts. However, their potentially large lattice parameters and different distances to the apical halogen provide unique opportunity to study the influence on pair formation of changing the plane dimensions, and Tc. In the effective hole model the number of holes at optimum is a function of the CuO2 plane dimensions. These properties have the potential to achieve unusual superconductivity, as exemplified by the unusually high Tc of Sr2CuO2F2+z. New work, especially in the absence of important data on hole counts and structural parameters, should be of interest in the light of the effective hole model.