Temperature dependence of the optical spectral weight in the cuprates:   Role of electron correlations
  We compare calculations based on the Dynamical Mean-Field Theory of the Hubbard model with the infrared spectral weight $W(\Omega,T)$ of La$_{2-x}$Sr$_x$CuO$_4$ and other cuprates. Without using fitting parameters we show that most of the anomalies found in $W(\Omega,T)$ with respect to normal metals, including the existence of two different energy scales for the doping- and the $T$-dependence of $W(\Omega,T)$, can be ascribed to strong correlation effects. 
A. Toschi, M. Capone, M. Ortolani, P. Calvani, S. Lupi and C.   Castellani