Optical Spectroscopy of Fermi Liquids

Metallic materials are normally characterized as Fermi liquids if their low temperature dc resistivity has a $T^2$ temperature dependence. It was shown by Gurzhi that there is is also a frequency dependence and the overall resistivity is given by $\rho(\omega,T)=C(\omega^2+b\pi^2T^2)$, where the scaling constant $b=4$ for a Fermi liquid with umklapp scattering [1]. A survey of literature shows that where spectroscopic experiments exist, $b=4$ is hardly ever observed [2]. We will present spectroscopic data on LiFeAs, Sr$_2$RuO$_4$ and Sr$_3$Ru$_2$O$_7$, three materials that show $T^2$ resistivity at low temperatures and discuss their excitation spectra.\\[4pt] [1] R. N. Gurzhi, Sov. Phys. JETP {\bf 14}, 886 (1962).\\[0pt] [2] U. Nagel {\it et al.} PNAS {\bf 109}, 19161 (2012).