Charge dynamics of Ca_2-xNa_xCuO₂Cl ₂ as a correlated electron system with the ideal tetragonal lattice

We report the reflectivity and the resistivity measurement of Ca _2-xNa_xCuO₂Cl₂ (CNCOC), which has a single-CuO₂-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical conductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Γ(ω) with doping. It was also found that Γ(ω) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.