Optical spectral weight distribution in d-wave superconductors
  The distribution in frequency of optical spectral weight remaining under the real part of the optical conductivity in the superconducting state of a d-wave superconductor depends on impurity concentration, on the strength of the impurity potential as well as on temperature and there is some residual absorption even at T = 0. In BCS theory the important weight is confined to the microwave region if the scattering is sufficiently weak. In an Eliashberg formulation substantial additional weight is to be found in the incoherent, boson assisted background which falls in the infrared and is not significantly depleted by the formation of the condensate, although it is shifted as a result of the opening of a superconducting gap. 
J.P. Carbotte and E. Schachinger