Charge dynamics of Ca 2 − x Na x CuO 2 Cl 2 as a correlated electron system with the ideal tetragonal lattice

We report the reflectivity and the resistivity measurement of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ (CNCOC), which has a single-${\mathrm{CuO}}_{2}$-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 $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 $\ensuremath{\Gamma}(\ensuremath{\omega})$ with doping. It was also found that $\ensuremath{\Gamma}(\ensuremath{\omega})$ shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.