Temperature-dependent scattering rate and optical mass of ferromagnetic metallic manganites

We report on the optical properties of the hole-doped manganites ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3.$ Transmission and reflection of thin films are measured in the infrared at temperatures from 10 to 150 K using Fourier-transform spectroscopy. The scattering rate and optical mass are obtained by fitting the far-infrared transmission data to a Drude model. The scattering rate shows a $T^2$ dependence with temperature. The optical mass enhancement differs only slightly from specific heat results. In addition, we compare the infrared spectral weight to band-structure calculations.