Orbital-selective Mass Enhancements in Multi-band   Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ Systems Analyzed by the Extended Drude Model
  We investigated optical spectra of quasi-two-dimensional multi-band Ca$% _{2-x} $Sr$_{x}$RuO$_{4}$ systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near $x=0.5$. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the $d_{yz/zx}$ bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near $x=0.5$ should come from the renormalization of the $d_{xy}$ band. 
J. S. Lee, S. J. Moon, T. W. Noh, S. Nakatsuji, and Y. Maeno