Constraints on the Fermi liquid scaling of the optical conductivity in MnSi

We will present time-domain terahertz spectroscopy measurements of the optical conductivity of MnSi thin films. The measurements cover a temperature range $T = $ 5--300 K and a frequency range $\nu =$ 0.1--4.0 THz, with high accuracy and precision. Below $T\approx 35$ K and $\nu \approx 2$ THz, the conductivity is consistent with the prediction of Fermi-liquid theory, $\rho(\omega,T) = [\sigma(\omega,T)]^{-1} = \rho_0 + A[(\hbar\omega)^2 + (p\pi k_B T)^2]$, with $p=2$. We observe deviations from this scaling at higher frequencies and temperatures, which allows us to establish the boundary of the Fermi-liquid scaling regime. As the temperature increases further, the system loses quasi-particle coherence, while the plasma frequency inferred from a Drude fit decreases dramatically.