Optical-conductivity sum rule in cuprates and unconventional charge   density waves: a short review
  We begin with an overview of the experimental results for the temperature and doping dependences of the optical-conductivity spectral weight in cuprate superconductors across the whole phase diagram. Then we discuss recent attempts to explain the observed behavior of the spectral weight using reduced and full models with unconventional $d_{x^2-y^2}$ charge-density waves. 
L. Benfatto, S. Sharapov