Frustrated kinetic energy, the optical sum rule, and the mechanism of   superconductivity
  The theory that the change of the electronic kinetic energy in a direction perpendicular to the CuO-planes in high-temperature superconductors is a substantial fraction of the condensation energy is examined. It is argued that the consequences of this theory based on a rigorous $c$-axis conductivity sum rule are consistent with recent optical and penetration depth measurements. 
Sudip Chakravarty, Hae-Young Kee, and Elihu Abrahams