Terahertz-frequency carrier dynamics and spectral weight redistribution in the nearly magnetic metalCaRuO3

We present measurements of the low-frequency optical conductivity of epitaxial thin films of $\mathrm{Ca}\mathrm{Ru}{\mathrm{O}}_{3}$, obtained with time-domain terahertz spectroscopy. We resolve a narrow conductivity peak near $\ensuremath{\omega}=0$ that deviates from Drude dynamics down to very low frequencies. By determining the spectral weight of this feature we estimate that the carrier kinetic energy changes by $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ per site upon cooling from room temperature and, where measurements are available, the strength of this change correlates well with magnetism in three-dimensional ruthenium oxides. We show that the time-domain terahertz technique provides a precise measure of the optical mass renormalization in extended Drude analysis and we show that this turns up sharply as the temperature is reduced below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We express our results in terms of a distribution of scattering rates and show how this description relates to other approaches in the literature. Our measurements are consistent with an anomalous power law description over a narrow range of frequencies and temperatures but significant deviations are observed and discussed. We show that our results are not described by a $\ensuremath{\omega}∕T$ scaling form.