  The infrared optical conductivity of intermediate valence compound TmSe
reveals clear signatures for hybridization of light $d$- and heavy f-electronic
states with m* ~ 1.6 m_0 and m* ~ 16 m_0, respectively. At moderate and high
temperatures, the metal-like character of the heavy carriers dominate the
low-frequency response while at low temperatures (T_N < T < 100 K) a gap-like
feature is observed in the conductivity spectra below 10 meV which is assigned
to be a mobility gap due to localization of electrons on local Kondo singlets,
rather than a hybridization gap in the density of states.

Strongly Correlated Electrons