THz conductivity measurement of MnSi

We present measurements of the low-frequency optical conductivity of a thin film of MnSi, using time-domain terahertz spectroscopy. At low temperatures and low frequencies, we extract the DC resistivity, scattering life time and plasma frequency from a Drude fit. We obtain a value of $\omega_p\simeq1.0$ eV, which can be used to estimate the renormalization coefficient through comparison with band theory. At higher temperatures, a deviation from Drude behavior is observed, suggesting a loss of quasi-particle coherence. In the region of low temperatures and high frequencies, we see evidence for a crossover to the anomalous power law dependence observed by Mena \textit{et al.}\footnote{F.P. Mena \textit{et al.} Phys. Rev. B. {\bf67}, 241101(R) (2003).} As the temperature increases, the anomalous frequency dependence becomes more pronounced, and the plasma frequency inferred from a Drude fit increases dramatically. Above T$\approx 50$ K, $\sigma_2(\omega)$ develops a negative slope that is inconsistent with both a Drude model and the anomalous power law observed earlier, indicating a sharp pseudogap in the conductivity spectrum.