Optical conductivity of a superfluid density wave
  We present a calculation of the low frequency optical conductivity of a superconductor in the presence of quenched inhomogeneity in both the superfluid and normal fluid densities. We find that inhomogeneity in the superfluid density displaces spectral weight from zero frequency to a frequency range that depends critically on the spatial correlation of normal and superfluid density fluctuations. 
Joseph Orenstein