Optical Conductivity of the t-J model within Cluster Dynamical Mean   Field Theory
  We study the evolution of the optical conductivity in the t-J model with temperature and doping using the Extended Dynamical Cluster Approximation. The cluster approach results in an optical mass which is doping independent near half filling. The transition to the superconducting state in the overdoped regime is characterized by a decrease in the hole kinetic energy, in contrast to the underdoped side where kinetic energy of holes increases upon superfluid condensation. In both regimes, the optical conductivity displays anomalous transfers of spectral weight over a broad frequency region. 
Kristjan Haule and Gabriel Kotliar