What Tc can teach about superconductivity
  We compare the Tcs found in different families of optimally-doped High-Tc cuprates and find, contrary generally accepted lore, that pairing is not exclusively in the CuO2 layers. Evidence for additional pairing interactions, that take place outside the CuO2 layers, is found in two different classes of cuprates, namely the charge reservoir and the chain layer cuprates. The additional pairing in these layers suppresses fluctuations and hence enhances Tc. Tcs higher than 100K, are found in the cuprates containing charge reservoir layers with cations of Tl, Bi, or Hg that are known to be negative-U ions. Comparisons with other cuprates that have the same sequence of optimally doped CuO2 layers, but have lower Tcs, show that Tc is increased by factors of two or more upon insertion of the charge reservoir layer(s). The Tl ion has been shown to be an electronic pairing center in the model system (Pb,Tl)Te and data in the literature that suggest it behaves similarly in the cuprates. A number of other puzzling results that are found in the Hg, Tl, and Bi cuprates can be understood in terms of negative-U ion pairing centers in the charge reservoir layers. There is also evidence for additional pairing in the chain layer cuprates. Superconductivity that originates in the double "zigzag" Cu chains layers that has been recently demonstrated in NMR studies of Pr-247 leads to the suggestion of a linear, charge 1, diamagnetic quasiparticle formed from a charge-transfer exciton and a hole. Other properties of the chain layer cuprates that are difficult to explain using models in which the pairing is solely confined to the CuO2 layers can be understood if supplementary pairing in the chain layers is included. Finally, we speculate that these same linear quasi-particles can exist in the 2-dimensional CuO2 layers as well. 
Theodore H. Geballe and Gertjan Koster