Direct observation of particle-hole mixing in the superconducting state   by angle-resolved photoemission
  Particle-hole (p-h) mixing is a fundamental consequence of the existence of a pair condensate. We present direct experimental evidence for p-h mixing in the angle-resolved photoemission (ARPES) spectra in the superconducting state of Bi_2Sr_2CaCu_2O_{8+\delta}. In addition to its pedagogical importance, this establishes unambiguously that the gap observed in ARPES is associated with superconductivity. 
J.C. Campuzano, H. Ding, M.R. Norman, M. Randeria, A.F. Bellman, T.   Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku,...