The recent rekindling of interest in the mixed valent Kondo insulator
SmB6​ as candidate for a first correlated topological insulator has
resulted in a wealth of new experimental observations. In particular,
angle-resolved photoemission experiments have provided completely new insights
into the formation of the low temperature Kondo insulating state starting from
the high temperature correlated metal. Here, we report detailed temperature and
energy dependent measurements of the optical constants of SmB6​ in order to
provide a detailed study from the point of view of a bulk sensitive
spectroscopic probe. We detect a previously unobserved infrared active optical
phonon mode, involving the movement of the Sm ions against the boron cages. The
changes taking place in the free carrier response with temperature and their
connection to changes in optical transitions between different bands are
discussed. We find that the free charge density starts to decrease rapidly
below approximately 200 K. Below 60 K a small amount of spectral weight begins
to accumulate in low lying interband transitions, indicating the formation of
the Kondo insulating state; however, the total integrated spectral weight in
our experimental window (∼4.35 eV) decreases. This indicates the
involvement of a large Coulomb interaction (> 5 eV) in the formation of the
Kondo insulator.Comment: 10 pages, 7 figures (including supp.). Accepted for publication in
New Journal of Physic