Temperature-induced spectral weight transfer in   Bi_2Sr_2CaCu_2O_{8+delta}: a conventional view
  In contrast with the recent photoemission observations by Shen et al. of anomalous temperature-induced momentum-dependent spectral weight transfer in Bi_2Sr_2CaCu_2O_{8+delta}, we find that in the same material, in spite of the unusual spectral lineshape change between superconducting and normal states, the integrated spectral weight displays minimum variation as a function of temperature, which is in agreement with the sum rule of angle-resolved photoemission: the integrated spectral weight is proportional to the momentum density, n(k). 
J. C. Campuzano, H. Ding, H. Fretwell, J. Mesot, A. Kaminski, T.   Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki