In-Plane Spectral Weight Shift of Charge Carriers in $YBa_2Cu_3O_{6.9}$
  The temperature dependent redistribution of the spectral weight of the $CuO_2$ plane derived conduction band of the $YBa_2Cu_3O_{6.9}$ high temperature superconductor (T_c = 92.7 K) was studied with wide-band (from 0.01 to 5.6 eV) spectroscopic ellipsometry. A superconductivity - induced transfer of the spectral weight involving a high energy scale in excess of 1 eV was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states. 
A.V. Boris, N.N. Kovaleva, O.V. Dolgov, T. Holden, C.T. Lin, B.   Keimer, and C. Bernhard