Photoemission and the Origin of High Temperature Superconductivity
  The condensation energy can be shown to be a moment of the change in the occupied part of the spectral function when going from the normal to the superconducting state. As a consequence, there is a one to one correspondence between the energy gain associated with forming the superconducting ground state, and the dramatic changes seen in angle resolved photoemission spectra. Some implications this observation has are offered. 
M. R. Norman, M. Randeria, B. Janko, and J. C. Campuzano