Deviations from Drude response in CaRuO$_3$

We report on the low frequency optical conductivity measurements of epitaxial thin films of CaRuO$_3$, obtained with time-domain terahertz spectroscopy. We find that the conductivity does not follow a simple Drude form, even at low temperatures and frequencies; alternative power-law forms also fail to describe it adequately. To achieve a good fit to the experimental data, we find it necessary to use a multi-component model consisting of multiple {\em Drude} terms and a finite frequency Lorentzian. This parameterization allows us to set an upper bound on the free-carrier spectral weight, from which we can set a lower bound on the renormalized mass of $m^*/m\cong 10$. If the scattering is assumed to be isotropic, our results imply a frequency dependence in the mass enhancement and scattering rate that extends well below terahertz frequencies. However, our results are also consistent with frequency-independent carrier dynamics that vary strongly over the Fermi surface.