Intrinsic gapless superconductivity in overdoped (Y,Ca)Ba2Cu3Oy: Study of in-plane optical spectra

In this study we present the temperature dependent E//a-axis optical conductivity spectra of detwinned Y_1?xCa_xBa₂Cu₃O_y single crystals (x = 0, 0.11 and 0.16) in the optimally doped and overdoped region. The optical conductivity spectra show Drude like residual conductivity, in the far-infrared region, even well below the superconducting transition temperature, which indicates the increase of the non-superconducting component at the superconducting regime. The origin of the residual conductivity can be either carrier-overdoping or pair-breaking effects due to Ca-disorder. Comparing the spectra for various Ca-content at the optimum doping, we found that Ca-substitution enhances the residual conductivity. In addition, from comparison of the spectra in the optimally doped and overdoped samples at a fixed Ca-content, we can conclude that there is an intrinsic overdoping effect of the residual conductivity.