Terahertz transmission studies of the topological Kondo insulator candidate SmB$_6$*

The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures (T$<10$ K) and high pressures. At low temperatures, a plateau is observed in the resistivity, contrary to the divergence expected for a normal Kondo insulator. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator, a material with a Kondo insulating bulk, but topologically protected metallic surface states.\footnote{M. Dzero \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 106408 (2010)} Here, time domain terahertz spectroscopy (TDTS) is used to investigate the temperature dependent low frequency optical conductivity of single crystals of SmB$_6$. We find evidence for a substantial bulk conductivity at a frequency of a few hundred GHz, which challenges the notion of this material as having a clean gap. The evidence for topological surface states and their properties will be discussed.