We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound $\mathrm{Sm}{\mathrm{B}}_6$, a prototype Kondo insulator. Temperature-dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at $\sim T^{*}=20\mathrm{K}$, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20 K) suggests the emergence of a surface state with an effective electron mass of $0.1m_e$. The conductivity dynamics following optical excitation is also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20 K, indicative of another channel opening up in the low-energy electrodynamics. Taken together, these results are consistent with the onset of a surface state well below the crossover temperature (100 K) after long-range coherence of the $f$-electron Kondo lattice is established.

• Received 15 September 2015
• Revised 22 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.155119