Optical conductivity of d-wave superconductors
  We study theoretically the optical conductivity of d-wave superconductors like in high temperature cuprates in the presence of impurities. We limit ourselves at T=0K and focus on the frequency dependence of both sigma_1(omega) and \omega*\sigma_2(omega) for omega<2Delta. When the impurity scattering is in the unitary limit, we find a peak in sigma_1(omega) with omega/Delta\simeq 0.1\sim 0.5$, which may account for the peak seen by Basov et al. in Zn-substituted YBCO. 
Bal\'azs D\'ora, Kazumi Maki and Attila Virosztek