Relationship between planar-copper--apical-oxygen bond length and charge transfer in YBa2Cu3O7-δ superconductors

The variation of the hole density and hence the superconducting transition temperature Tc has been widely correlated with a variation in the planar-copper--apical-oxygen bond length. A decrease in this bond is usually taken to reflect an increase in the hole density in the CuO2 planes. Results of electronic-structure calculations are presented that show that a decrease in the planar-copper--apical-oxygen bond does enhance the hole density in the CuO2 planes in the YBa2Cu3O7 superconductor. However, this enhancement is relatively small. A major fraction of the hole density in this compound can thus be attributed to the oxygen atoms in the chains