Low energy electrodynamics and a hidden Fermi liquid in the heavy-fermion CeCoIn\(_5\)

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn\(_5\). The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Below the \(\approx\) 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the \(f\) orbital - conduction electron hybridization and the formation of the heavy-fermion state. Via an extended Drude model analysis, we measure the frequency-dependent scattering rate (\(1/ \tau \)) and effective mass (\(m^*/m_b\)). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nonetheless, the width of the low-frequency Drude peak (characterized by a {\it renormalized} quasiparticle scattering rate (\(1 / \tau^* = m_b/ m^* \tau \)) does show a \(T^2\) dependence giving evidence for a hidden Fermi state.