In-plane optical spectra of   Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-{\delta}}$: Overdoping and disorder   effect on residual conductivity
  We measured the temperature dependence of the in-plane polarized reflectivity spectra of twin-free Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-{\delta}}$ single crystals with different Ca-concentrations (x=0,0.11 and 0.16) from optimally doped to heavily overdoped region. Low energy optical conductivity spectra showed a Drude-like residual conductivity at temperatures far below the superconducting transition temperature, which indicates the presence of unpaired-normal carriers in the superconducting state. Comparing the spectra at a fixed Ca-content or at a fixed doping level, we have revealed that the carrier overdoping increases unpaired carriers in addition to those induced by the Ca-disorder. We also found the superconducting behavior of the one-dimensional CuO chains for the Ca-free samples. 
E. Uykur, K. Tanaka, T. Masui, S. Miyasaka, S. Tajima
cond-mat.supr-con