Role of the Madelung energy in hole conductivity in copper oxides: Difference between semiconductors and high-Tc superconductors

The difference in Madelung site potential, VM, between a hole on a copper site and one on an oxygen site can control the holes site preference and ability to delocalize. Among a variety of copper oxide structures, eVM is shown to vary by 3 eV and to clearly separate two classes of compounds: Those with eVM>47 eV are metallic with high Tc and delocalized holes, whereas those with lower "VM are semiconducting because their holes are self-trapped on oxygen. © 1989 The American Physical Society.