Microwave Conductivity Measurements of URu$_2$Si$_2$

Narrow, free-carrier spectra occur in the heavy fermion metals, in which the high quasiparticle effective mass renormalizes the width of the Drude conductivity peak from infrared frequencies, where it is found in more typical metals, down to the microwave region. This narrow peak has been observed [1] in a thin film of UPd$_2$Al$_3$, occurring with a Lorentzian lineshape due to the dominant defect-scattering in the film. We have taken microwave conductivity measurements on URu$_2$Si$_2$ in a temperature range in which the sample has entered into its hidden order state but above the superconducting transition. These measurements exhibit a narrow Lorentzian lineshape, indicating high sample purity and considerably reduced impurity scattering. If the inelastic scattering is sufficiently low, there is the possibility for an observation of electron-electron scattering. Results to date will be presented. \\[4pt] [1] Scheffler, M. $\textit{et al.}$, $\textit{Nature}$ $\textbf{438}$, 1135 (2005).