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We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn5. Below the ≈40 K Kondo coherence temperature, a narrow Drude-like peak forms, as a result of the 𝑓-orbital–conduction-electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate (1/𝜏) and effective mass (𝑚*/𝑚𝑏). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the renormalized quasiparticle scattering rate (1/𝜏*=𝑚𝑏/𝑚*⁢𝜏) shows a 𝑇^2 dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for a Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass. 

We report terahertz time-domain spectroscopy experiments demonstrating strong light–matter coupling in a terahertz LC metamaterial (MM) in which the phonon resonance of a topological insulator thin film is coupled to the photonic modes of an array of electronic split ring resonators. As we tune the MM resonance frequency through the frequency of the low-frequency α mode of (BixSb1–x)2Te3 (BST), we observe strong mixing and level repulsion between the phonon and MM resonance. This hybrid resonance is a phonon polariton. We observe a normalized coupling strength, η = ΩR/ωc ≈ 0.09, using the measured vacuum Rabi frequency and cavity resonance. Our results demonstrate that one can tune the mechanical properties of these materials by changing their electromagnetic environment and therefore modify their magnetic and topological degrees of freedom via coupling to the lattice in this fashion.