The never ending search for high temperature superconductivity
  A brief history of the discovery of new superconductors is given. Different types of pairing mechanisms are considered. By comparing Tcs in different cuprate families it is concluded that the pairing in the CuO2 layers must be supplemented by interactions elsewhere in the unit cell. This conclusion is reached simply by considering the significant variations in Tc that are found in structures that have the same sequence of CuO2 layers within the unit cell but have different intervening layers. A quasi-particle is postulated to account for pairing found in the double chain layer of the Pr247 cuprate and may also exist in the CuO2 layers of all the cuprates. 
T.H. Geballe